RU2824360C2 - Method of producing elementary sulphur and sulphuric acid - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: present invention relates to chemistry, specifically to a method and apparatus for producing elementary sulphur from feed gas and sulphuric acid. Method includes stages a-g. At step a), a Claus reaction furnace feed stream containing from 15 to 100 vol.% of H₂S, a sub-stoichiometric amount of oxygen and, optionally, a fuel stream is introduced. At step b), a Claus reaction furnace feed stream is fed into a reaction furnace section operating at elevated temperature, for example above 900 °C. At step c), the product of the reaction furnace section and the flow of sulphuric acid are fed into the sulphuric acid evaporation section downstream of the reaction furnace section, thus providing the feed gas for the Claus converter. Then, at step d), the feed gas for the Claus converter is cooled to provide a cooled feed gas for the Claus converter, and optionally, removal of elementary sulphur from the gas. At step e) cooled feed gas for Claus converter is supplied for contact with Claus reaction catalyst, at step f) residual Claus reaction gas and elemental sulphur are removed, optionally cooling the effluent from the catalytically active substance in the Claus reaction and at step g), directing the stream containing the residual Claus reaction gas to treat the residual Claus reaction gas.

EFFECT: providing a method of converting H₂S into elementary sulphur with high efficiency, which consists in intensive decomposition of impurities, reduced additional dependence of the process on fuel and reduced cooling as a result of evaporation of sulphuric acid.

16 cl, 5 dwg, 2 tbl, 2 ex

Description

The present invention relates to a process for converting _H2S into elemental sulfur and sulfuric acid, optionally at a specified ratio between elemental sulfur and sulfuric acid.

_H2S is a common byproduct in many processes, including hydrodesulfurization of refinery streams, natural gas processing, and viscose fiber production. It is desirable to convert _H2S before releasing it to the atmosphere because _H2S is highly toxic, has an unpleasant odor, and is a hazard to the environment.

Oil refining processes, in addition to producing the well-known highly concentrated _H2S gas, can often also produce so-called sour water stripper gas, which contains _H2S , _H2O and _NH3 in approximately equal amounts.

In particular, in oil refineries, the Claus process, which has been known and optimized for over 8 decades, has been the process of choice for reducing _H2S emissions. The Claus process operates by substoichiometric combustion of _H2S to form _SO2 in a Claus reaction furnace, providing feed gas for the Claus converter. The subsequent Claus process converts _H2S and _SO2 to form elemental sulfur, which can be condensed and discharged.

Typically, the Claus process achieves a removal rate of 95% to 98%, which is insufficient to meet environmental regulations. Therefore, it is common practice to treat the Claus process tail gas to ensure that the sulfur content is reduced to above 99%. Tail gas treatment is often performed in a sulfuric acid plant, which requires handling of sulfuric acid.

It has now been established that sulfuric acid can be recycled to the Claus reaction furnace where it can contribute to the formation of elemental sulfur and, in addition, provide opportunities for optimization of the size and operating costs of the Claus process and Claus tail gas.

WO 2012/152919 A1 presents a sulfuric acid process for treating Claus reaction tail gas, which describes the conversion _{of H2S} to _H2SO4 in the Claus reaction tail gas. The process steps are as follows:

1. Substoichiometric oxidation.

2. Transformation according to Klaus's reaction.

3. Oxidation of reduced sulfur compounds ( _H2S ) in the residual gas of the Claus reaction in an oxygen-enriched atmosphere at 400-700°C.

4. Catalytic oxidation of SO ₂ to SO ₃ .

5. Condensation of H ₂ SO ₄ .

It is recognised that the _H2SO4 product is not always desirable and it has been suggested that the sulphuric acid be recycled to the preceding Claus reaction stage or to the _H2S oxidation stage as described above. However, the recirculation of _sulphuric acid is seen simply as a reduction in sulphuric acid emissions and the implications of _H2SO4 recirculation for the wet sulphuric acid or Claus process have not been assessed, i.e. it is not recognised that _H2SO4 recirculation requires a reduction in the amount _{of O2} sent to the Claus reaction furnace and also has a positive effect on _the Claus reaction and sulphuric acid processes.

WO 2012/152919 A1 also discloses that auxiliary fuel may be required in both the Claus reaction furnace stage and the _H2S oxidation stage to achieve the desired operating temperature, without indicating the realization of beneficial effects of using feed gases as auxiliary fuel for _H2S oxygenation in the sulfuric acid process.

Thus, the proposal to _{recycle H2SO4} into the Claus reaction furnace does not in itself provide a working method, but requires additional modifications to the method.

According to the present invention, a method for converting _H2S into elemental sulfur with increased efficiency is provided, in which the Claus process is combined with a sulfuric acid process. According to this method, sulfuric acid obtained in the sulfuric acid process for treating the residual gas of the Claus reaction is recycled to a sulfuric acid evaporation section in or after the Claus reaction furnace for decomposition and obtaining elemental sulfur.

For the purposes of the present invention, the stoichiometric amount of oxygen shall be determined assuming that the products derived from N, H, C, S and O in the feed gas are _N2 , _H2O , _CO2 and _SO2 . If sub-stoichiometric (also referred to as sub-stoichiometric) amounts of oxygen are present, this means that not all components of the feed are fully oxidized. For Claus reaction feed gas, this means that the process gas after sub-stoichiometric combustion/reaction may contain unreacted _H2S , _NH3 and hydrocarbons from the feed stream(s), as well as _H2 , CO, COS and _CS2 formed in an _O2 deficient environment.

For the purposes of this application, a fuel is defined as a composition that, upon oxidation of _O2, produces _N2 , _H2O , _CO2 , and _SO2 as reaction products and releases a significant amount of energy as a result of the reactions. A mixture _of hydrocarbons (e.g., natural gas with _CH4 and _C2H6 ) and _H2S is a typical fuel gas, but the fuel gas may also contain CO, _NH3 , and _H2 .

For the purposes of the present invention, oxygen ( _O2 ) is understood to be a stream containing _O2 , such as air, enriched air, and pure oxygen, but may also be a process gas containing _O2 .

For the purposes of the present application, the sulfuric acid section is understood to mean a plant for converting a process gas containing sulfur (such as _H2S , _SO2 , COS, _CS2 and elemental sulfur) into sulfuric acid. The sulfuric acid section may be implemented as a catalytic or thermal section oxidizing sulfur compounds to _SO2 , followed by a section oxidizing _SO2 to _SO3 , followed by either a condenser providing sulfuric acid by condensing hydrated _SO3 or an absorber providing sulfuric acid by absorbing _SO3 in sulfuric acid, but those skilled in the art are also aware of other means for producing sulfuric acid, such as _{H2O2 scrubbers} .

For the purposes of this application, sulfur oxidation means shall be understood to mean any process equipment that receives reduced or elemental sulfur and converts it into SO ₂ . Such sulfur oxidation means may be a combustion chamber, catalytic equipment, or combinations thereof.

For the purposes of the present application, the Claus reaction furnace may be described as comprising two sections: a reaction furnace section and a sulfuric acid evaporation section. The names of these sections are simply designations of the individual sections and are not exclusive descriptions of the chemical reaction occurring therein. The two sections are to be understood as non-overlapping, with the sulfuric acid evaporation section being located after the reaction furnace section.

The sulfuric acid vaporization section may be either a separate chamber or part of a separate chamber of the Claus furnace, defined as the volume downstream of the uppermost sulfuric acid inlet and terminating at the Claus furnace outlet, typically through the waste heat boiler tube sheet. This volume may be further defined as the volume of the Claus furnace downstream of a plane defined by the uppermost sulfuric acid inlet and perpendicular to the axis defined by the product flow from the Claus furnace.

For the purposes of this application, pressure given in units of mbarg shall be understood as gauge pressure relative to atmospheric pressure.

According to a broad aspect, the present invention relates to a method for producing elemental sulfur from a feed gas containing from 15%, 20%, 30 vol.%, 40 vol.% or 50% to 99 vol.% or 100 vol.% H ₂ S and a sulfuric acid stream, comprising the steps of

a. providing a Claus reaction furnace feed stream comprising a quantity of said feed gas, a quantity of oxygen and optionally a quantity of fuel, wherein the quantity of oxygen is sub-stoichiometric with respect to the Claus reaction,

b. directing said Claus reaction furnace feed stream to a section of the reaction furnace operating at an elevated temperature, such as above 900°C, providing a product to the section of the reaction furnace,

c. directing said reaction furnace section product and a quantity of sulfuric acid to a sulfuric acid evaporation section downstream of said reaction furnace section, providing feed gas for the Claus converter

d. cooling said feed gas for the Claus converter to provide cooled feed gas for the Claus converter and optionally removing elemental sulfur from the gas

e. directing said cooled feed gas to a Claus converter for contact with a substance catalytically active in the Claus reaction,

f. removing residual Claus reaction gas and elemental sulfur, optionally by cooling the outlet stream, from said Claus reaction catalytically active substance,

g. directing the flow containing said Claus reaction residual gas to the treatment of the Claus reaction residual gas,

with the attendant advantage of a method comprising injecting sulphuric acid into the sulphuric acid evaporation section, providing high temperature combustion of said feed gas including impurities, without cooling from evaporation and decomposition of the sulphuric acid.

According to another embodiment, the temperature at the outlet of the reaction furnace section is higher than 50°C or 100°C and less than 200°C or 500°C than the temperature at the outlet of said sulfuric acid evaporation section, with the attendant advantage of obtaining energy, efficient cooling, with less need for heat exchange equipment. In particular, if some of said feed gas is diverted for reaction in the sulfuric acid evaporation section, this temperature difference may be small.

According to another embodiment, the feed gas is divided into a reaction furnace section feed directed to the reaction furnace section and a sulfuric acid vaporization section feed directed to the sulfuric acid vaporization section, with the attendant advantage of allowing independent temperature control of the reaction furnace section and the sulfuric acid vaporization section. Such temperature control may be manual, perhaps with the aid of a suitable representation of the process parameters, but it may also be automatic, based on process measurements.

According to another embodiment, the ratio between said feedstock for the reaction furnace section and the feedstock for the sulfuric acid evaporation section is controlled to maximize thermal destruction of impurities present in the reaction furnace section, typically by maximizing the temperature in the reaction furnace section.

According to another embodiment, the average residence time of the process gas in the Claus reaction furnace is less than 5 seconds, or less than 2 seconds, with the corresponding advantage of such a reaction furnace having an appropriate size, while providing sufficient time for the evaporation of sulfuric acid, as well as the necessary partial chemical conversion of H ₂ S to SO ₂ .

According to another embodiment, the reaction furnace section and/or the sulfuric acid evaporation section includes a turbulence enhancer with the attendant benefit of reducing the residence time, distribution in the reaction furnace section and the sulfuric acid evaporation section.

According to another embodiment, the sulfuric acid evaporation section comprises an impaction means, such as an impact wall or a chamber filled with inert material, with the corresponding advantage of breaking up the droplets upon impact to ensure that there is no _{liquid H2SO4} in the feed gas for the Claus converter.

According to another embodiment, said treatment of the residual gas of the Claus reaction comprises the steps of

directing a flow containing said Claus reaction tail gas, oxygen and fuel as feed gas into a Claus reaction tail gas combustion chamber operating at a temperature above 900°C or a catalytic means for oxidation, providing feed gas for a _SO2 converter,

h. directing said feed gas to an SO2 converter for contact with a substance catalytically active in the oxidation of _SO2 to _SO3 , providing a gas enriched in _SO3 ,

i. converting said _SO3 -rich gas into concentrated sulphuric acid, either by absorption of _SO3 by sulphuric acid or by hydration of _SO3 , cooling and condensation of sulphuric acid,

j. recycling at least a portion of the resulting sulfuric acid to a sulfuric acid evaporation section,

with the attendant advantage of such a process having high conversion and thermal efficiency and avoiding the undesirable formation of sulfuric acid. The use of a reaction furnace section and a Claus tail gas combustor operating at a temperature above 900°C ensures complete conversion of the components present and this may not necessarily require the presence of a fuel in addition to the feed gas. Furthermore, homogeneous Claus reactions will take place in the reaction furnace section and the sulfuric acid evaporation section so that elemental sulfur can be removed while cooling the feed gas to the Claus converter. The feed stream to the Claus tail gas combustor may typically include _H2S as a fuel from the Claus tail gas and/or a separate stream containing _H2S , a hydrocarbon or other fuel. In addition to the aforementioned process steps, the process may include additional steps such as heat exchange to change the temperature (or otherwise condition the process streams) to a range appropriate to the processes occurring.

According to another embodiment, the feed gas of the reaction furnace section contains less than 0.1 vol.% non-elemental nitrogen, such as _NH3 , with the attendant benefit of preventing the formation of, for example, ammonium salts that can plug the Claus condenser(s).

According to another embodiment, the feed stream of the reaction furnace section contains less than 50%, 20%, 10% or 1% elemental nitrogen with the attendant benefit of providing a high temperature process in the reaction furnace section and a reduced volume of process gas due to the reduced presence of N ₂ . This can be achieved by using pure O ₂ or oxygen-enriched air as the oxygen source.

In an alternative method, steps d, e and f are performed sequentially 2-5 times, which has the advantage of providing higher conversion in the Claus process.

According to another embodiment, the _H2S : _SO2 ratio of said Claus converter feed gas is below 4:1, 3:1 or 2:1, with the attendant advantage of such feed gas providing a Claus reaction tail gas containing _H2S for the Claus reaction tail gas combustor, minimizing the need for adding fuel gas, since the oxidation of _H2S releases a significant amount of energy, whereas _SO2 does not release energy in the Claus reaction tail gas combustor.

According to another embodiment, the _H2S : _SO2 ratio of said gas for the Claus converter is lower than 1.6:1, 1.8:1 or 2:1, with the attendant advantage of having a substantially _H2S -free Claus reaction tail gas. In a downstream sulphuric acid production unit, this may be an advantage, since _SO2 will not be oxidised without an _SO2 conversion catalyst, and thus the Claus reaction tail gas may be preheated with a combination of catalytic _SO2 oxidation (controlled bypass of the feed gas containing _H2S ) and recirculation of the process gas around the catalytic _H2S oxidation catalyst, so that the temperature rise across the _H2S oxidation catalyst may be precisely controlled. With an unknown and/or changing _H2S concentration in the Claus reaction tail gas, there is a high risk of overheating the _H2S oxidation catalyst.

According to another embodiment, the method further comprises the step of directing a quantity of additional feed gas to said Claus reaction tail gas combustor with the attendant advantage of providing additional sulphur and fuel for the sulphuric acid process. The additional feed gas may contain impurities that can be burned prior to treatment with the sulphuric acid process and/or hydrogen sulphide and other fuels that can contribute to the production of sulphuric acid and the combustion of the Claus reaction tail gas in the combustor. If the additional feed gas contains a large quantity of inert gases or sulphur-free fuel, the method also has the advantage of avoiding an increase in the size of the Claus converter due to the secondary stream. The additional feed gas may originate from the same source as the feed gas or from a different source.

According to another embodiment, said additional feed gas comprises greater than 5 vol. % of non-elemental nitrogen, such as ammonia, with the attendant advantage of providing a process in which non-elemental nitrogen components that may be difficult to oxidize in the sub-stoichiometric atmosphere of the reaction furnace section can be directed to the Claus reaction tail gas combustor. Such a process can be particularly advantageous if the additional feed gas is a sour water stripper (SWS) gas containing _H2S , _NH3 and _H2O - where only _H2S is desired in the Claus process and _NH3 is problematic in the Claus process due to potential plugging with ammonium salts. Such SWS gas can instead be directed to a sulfuric acid plant, where _NH3 handling is well known.

According to another embodiment, the amount of sulfur in the additional feed gas is at least 1%, 2%, or 5% of the total amount of elemental sulfur removed from the process, with the attendant benefit of such feed gas being able to provide thermal energy as well as contribute to sulfur reduction.

According to another embodiment, the substance catalytically active in the Claus reaction comprises activated aluminum (III) oxide or titanium (IV) oxide, with the attendant advantage of such substance providing an efficient process for the production of elemental sulfur.

According to another embodiment, step (d) is carried out at a pressure of 200 mbarg to 700 mbarg, a temperature of 200°C to 350°C and a space velocity of 800 ^Nm3 /h/ ^m3 to 3000 ^Nm3 /h/ ^m3 , with the attendant advantage of such conditions being effective for the production of elemental sulphur.

According to another embodiment, step (d) is carried out at a temperature of from 100°C to 150°C, and step (e) comprises a step of periodically heating said substance catalytically active in the Claus reaction to ensure the removal of condensed elemental sulfur into the liquid or gas phase, with the concomitant advantage of low temperature, favorable for achieving very high conversion of _SO2 and _H2S into elemental sulfur, both due to the favorable equilibrium at low temperature and due to the shift of the equilibrium by removing the reaction product.

According to another embodiment, said substance catalytically active in the conversion of _SO2 to _SO3 comprises vanadium, with the attendant advantage of such substance providing an efficient method for producing sulfuric acid.

According to another embodiment, said step i) is carried out at a pressure of from 50 mbarg to 200 mbarg, a temperature of from 380°C to 520°C and a space velocity of from 800 ^Nm3 /h/ ^m3 to 1500 ^Nm3 /h/ ^m3 , per catalyst bed, with the concomitant advantage of conditions which are effective for oxidising _SO2 to form _SO3 .

According to another embodiment, the amount of sulfur in the recycled sulfuric acid stream is greater than 1%, 3% or 5% and less than 17%, 21% or 25% of the total amount of elemental sulfur removed from the process. Recycling above the lower limits has the advantage of providing a process gas volume reduction effect, while recycling less than the upper limits avoids a situation where additional fuel must be added to the reaction furnace section, which results in additional process volume and operating costs.

According to another embodiment, the concentration of said sulfuric acid is at least 80 wt.% or 90 wt.%, with the attendant advantage of such acid providing for the addition of oxygen to the Claus process while minimizing the amount of water that needs to be evaporated in the sulfuric acid evaporation section.

According to another embodiment, the sulfuric acid in the recirculated sulfuric acid stream is sprayed in said sulfuric acid evaporation section using compressed air or injection nozzles, wherein the residence time in the sulfuric acid evaporation section is at least 0.5 seconds, 1 second or 1.5 seconds, wherein the associated advantage of such residence time is that it is sufficient for complete evaporation of the sulfuric acid droplets.

According to another embodiment, the molar ratio of _H2S : _O2 components fed to the reaction furnace sections is at least 2.5, with the attendant benefit of such low oxygen content providing sub-stoichiometric partial conversion of _H2S to _SO2 , via the contribution of thermal dissociation _{of H2SO4 ,} adding the remaining _O2 to obtain the desired _H2S : _SO2 ratio of 2.0 in the Claus feed gas.

According to another embodiment, said amount of sulfuric acid is obtained from a source other than the Claus tail gas treatment plant, with the attendant advantage of providing additional conversion of the sulfuric acid to elemental sulfur.

According to another embodiment, a quantity of SWS gas consisting of 20-50% _NH3 , 20-50% _H2S and at least 80% of the balance being _H2O is sent as additional feedstock to one or both of said reaction furnace and Claus residual gas treatment sections.

According to another embodiment, the amount of gas in the process is cooled and directed upstream to control the temperature of the process, which allows for active temperature control of highly exothermic processes.

According to another embodiment, one or more streams directed to said Claus reaction furnace are preheated by heat exchange with a hot process stream, which allows to minimize or avoid the requirements for auxiliary fuel to achieve the desired temperature for evaporation of sulfuric acid and conversion of the feedstock.

According to another embodiment, one or more streams directed to said Claus reaction tail gas combustion chamber are preheated by heat exchange with a hot process stream with the corresponding advantage of minimizing or eliminating the requirement for auxiliary fuel to achieve the desired combustion temperature and subsequent oxidation of SO ₂ .

According to another embodiment, at least one of said catalytically active substances for the oxidation of _SO2 to _SO3 or _H2S to elemental sulfur and/or at least one product removed from one of said catalytically active substances is cooled by heat exchange, such as an interlayer heat exchange or an internally cooled catalytic reactor, with the attendant advantage of allowing active control of the temperature of highly exothermic processes by means of an interlayer heat exchange or an internally cooled catalytic reactor, such as a boiling water reactor having a tubular or plate cooling circuit.

Another aspect of the present invention relates to a process plant comprising a reaction furnace section, a sulphuric acid evaporation section, a Claus waste heat boiler, a Claus conversion section, a means for oxidising sulphur and a sulphuric acid section, in which the reaction furnace section has an inlet and an outlet, the sulphuric acid evaporation section has a gas inlet, a sulphuric acid inlet and an outlet, the Claus waste heat boiler has a gas inlet, a gas outlet and an elemental sulphur outlet, the Claus conversion section has a gas inlet, a gas outlet and an elemental sulphur outlet, the means for oxidising sulphur has an inlet and an outlet, and the sulphuric acid section has a gas inlet, a gas outlet and a sulphuric acid outlet, where the inlet of the reaction furnace section is configured to receive a feed gas and an oxidiser, where the outlet of the reaction furnace section is configured to be in liquid communication with the gas inlet of the sulphuric acid evaporation section, the sulphuric acid inlet of the sulphuric acid evaporation section is configured to receive a sulphuric acid stream, and the outlet of the sulphuric acid evaporation section is configured to be in liquid communication with the inlet of the Claus waste heat boiler, where the outlet of the Claus waste heat boiler is configured to be in liquid communication with the inlet of the Claus conversion section, and wherein the inlet of the sulphur oxidation means is configured to be in liquid communication with the outlet of the gas outlet of said Claus conversion section, the outlet of the sulphur oxidation means is configured to be in liquid communication with the inlet of the sulphuric acid section, characterized in that the outlet of the sulphuric acid from the sulphuric acid section is in liquid communication with the inlet of the sulphuric acid of said sulphuric acid evaporation section, with the accompanying advantage that the process includes injecting sulphuric acid into a separate sulphuric acid evaporation section, providing high temperature combustion of said feed gas, including impurities, without cooling from evaporation and decomposition of the sulphuric acid.

According to another embodiment, the reaction furnace section and the sulfuric acid evaporation section are two separate mechanical units, with the attendant advantage of providing independent optimization and positioning of the two sections.

According to another embodiment, the reaction furnace section and the sulfuric acid evaporation section are a single mechanical unit with a corresponding cost reduction advantage.

The present invention describes a combination of the Claus process and the sulfuric acid process, which can efficiently produce the amount of sulfuric acid required for a process plant, or even avoid producing sulfuric acid and convert excess sulfuric acid into elemental sulfur that can be transported to other places.

For maximum conversion to elemental sulfur, 1/3 of the H ₂ S must be converted to SO ₂ .

The stoichiometric ratio between _H2S and _SO2 is controlled by controlling the amount of oxygen in the reaction furnace section. Oxygen is usually supplied with atmospheric air, but can also be _O2 enriched air or even pure _O2 .

The addition of oxygen to the reaction furnace section must also take into account the amount of _NH3 , CO, _H2 and hydrocarbons in the feed streams.

If the combustion temperature in the reaction furnace section is less than 1100°C, the conversion of, for example, NH ₃ may be incomplete. The consequence of this will be a feed gas for the Claus converter that has the potential to form ammonia salts such as (NH ₄ ) ₂ SO ₄ and (NH ₄ ) ₂ S ₂ O ₃ , which may clog the Claus condenser.

The partially oxidized feed gas for the Claus converter is then converted to elemental sulfur by the following reactions at temperatures typically above 200°C in the presence of a catalytically active substance such as activated aluminum(III) or titanium(IV) oxide.

Often 3-4 Claus converters are operated in series to increase the conversion to the maximum, which increases the cost of the Claus installation.

Temperature control in the Claus process is important to ensure that the elemental sulfur formed in the catalytic converter remains gaseous until the desired process position for condensation. Another limitation is related to the fact that since the Claus process is exothermic, it is advantageous to operate at low temperatures.

An alternative to the above process is the so-called sub-dew point Claus process, in which the catalytically active substance operates at temperatures at which elemental sulfur is not in the gas phase. Such a sub-dew point Claus process would require an appropriate scheme for removing condensed elemental sulfur, i.e. by pulsating the temperature and purging the elemental sulfur with an inert gas.

Even with 3-4 Claus converters/condensers/superheaters in series, it is not possible to achieve a sulfur recovery of more than ~98%, which is insufficient to meet most environmental regulations. Therefore, a Claus plant is usually equipped with a so-called Claus tail gas reaction unit, one example of which is the below-dew point process mentioned above. There are many tail gas processes with different characteristics. To achieve very high removal efficiencies, these Claus tail gas reaction units become more complex and approach the same cost as the Claus plant itself.

The resulting elemental sulfur generally has no direct use in plants producing a waste stream containing _H2S , but elemental sulfur is easily transported to other facilities and stored for long periods of time.

A common alternative to the Claus process is the conversion of _H2S into sulphuric acid, the so-called wet sulphuric acid process. The sulphuric acid produced can be used in other chemical processes in the plant. The wet sulphuric acid process can also be used to clean the residual gases of a Claus process plant. A similar process using dry sulphuric acid can also find application in this respect.

Sulfuric acid processes oxidize _H2S to _SO2 and _SO2 to _SO3 and then hydrate the _SO3 to sulfuric acid, either by reaction with water in the gas phase in the so-called wet sulfuric acid process or by absorption in concentrated sulfuric acid in the so-called contact process or dry process. The reaction temperature for the oxidation of _SO2 to _SO3 will be in the range of 400-500°C in the presence of a catalytically active substance, usually including vanadium. Typically, wet sulfuric acid processes produce sulfuric acid with a concentration in the range of 92%-98%, while dry sulfuric acid processes can also produce sulfuric acid with a concentration above 98%.

In addition, it may also be attractive to collect high pressure steam in the range of 30 barg to 80 barg from highly exothermic sulphuric acid processes, whereas the Claus process will only produce lower pressure steam and in much lower quantities.

However, producing large quantities of sulfuric acid may be less attractive, even if the sulfuric acid is sold commercially, because transporting sulfuric acid is complex and regulated.

The reactions that occur in the sulfuric acid process (dry and wet) are

The general reaction of the sulfuric acid process can be described in accordance with

The wet sulfuric acid process as a conventional solution for Claus tail gas provides a solution that meets environmental regulations at lower capital and operating costs than alternatives. The only drawback of the wet sulfuric acid process so far has been the production of sulfuric acid, which is not always desirable.

It has now become clear that the integration of the Claus process and the sulfuric acid process can also be accomplished by recycling all or substantially all of the sulfuric acid produced to the sulfuric acid evaporation section. With the new invention, the integrated Claus/wet sulfuric acid process eliminates the disadvantage of the inconvenient sulfuric acid product and at the same time reduces the plant size of both the Claus process and the wet sulfuric acid process.

Combustion of sulfuric acid is known in the regeneration of spent sulfuric acid in a wet sulfuric acid plant, but has not been used in practice in a Claus reaction furnace or under Claus process conditions.

When sulfuric acid is introduced into the sulfuric acid evaporation sections, the following general reaction occurs

To complete this overall reaction, the following reactions must be completed:

Reaction (8) is a normal evaporation reaction in which the energy required to heat the liquid and evaporate the water and sulfuric acid is provided by the hot ambient process gas. The effect of complete evaporation _of the sulfuric acid is that the gaseous _H2SO4 is much less aggressive than the droplets _of liquid _H2SO4 .

Reaction (9) is an endothermic dissociation reaction that occurs almost instantaneously at temperatures above 600°C. At this point in time, some SO ₃ will begin to react with H ₂ S to form SO ₂ , H ₂ O, and elemental sulfur.

Reaction (10) is an endothermic decomposition reaction occurring rapidly at temperatures above 900°C. In oxygen-rich atmospheres, chemical equilibrium prevents complete dissociation, but in reducing atmospheres, removal of the _O2 product (by reaction with _H2S ) allows complete decomposition. The reaction between _H2S and _O2 occurs very rapidly at these elevated temperatures.

The residence time in the sulphuric acid evaporation section is typically 1-2 seconds, it is necessary to ensure that reactions 8, 9, 10 and 1 are completed within this time. When the process gas is cooled, i.e. in the Claus boiler, the reaction rates are sharply reduced and further conversion will be insignificant.

The presence of _O2 and/or _SO3 in the process gas contacting the catalyst in downstream Claus reactors will result in catalyst deactivation due to a "sulfation" reaction in which the catalytically active alumina or titanium oxide is converted to catalytically inactive aluminum sulfate or titanium sulfate and, consequently, the formation of elemental sulfur in these reactors will be reduced, leading to an increase in the amount of unreacted sulfur compounds in the tail gas of the downstream sulfuric acid plant and to increased emissions to the atmosphere.

In addition, _SO3 in the process gas may combine with water when passing through sulfur condensation units, condense and form sulfuric acid in the elemental sulfur product, which may lead to corrosion of process equipment.

To eliminate the risk that any unreacted _SO3 from the sulphuric acid evaporation section will cause operational problems in the Claus converters and elemental sulphur condensers, it is preferable to install a catalytic reactor between the waste heat boiler outlet and the first elemental sulphur condenser or the first catalytic Claus reactor. The catalyst effectively reduces the _SO3 content to harmless _SO2 and/or elemental sulphur at the expense of _H2S and _H2 present in the reaction furnace off-gases.

If all the sulfuric acid produced in the Claus tail gas sulfuric acid process is sent to a sulfuric acid evaporation section, the Claus process can be used, which uses the very high removal efficiency and thermal efficiency of the sulfuric acid plant to reduce _H2S emissions, but whose only product is elemental sulfur, which is easy to handle and transport.

In addition, by recycling the _sulfuric acid, _O2 is released by decomposition of _H2SO4 , so that the amount of combustion oxidizer added will be reduced, which, if the oxidizer is atmospheric air, allows a dramatic reduction in the process volume, since atmospheric air contains about 80% inert _N2 , i.e. 4 volumes of _N2 per volume of _O2 .

The overall reaction based on the use of air as the O ₂ carrier for the Claus reaction furnace is as follows:

Similarly, the general Claus reaction based on _H2SO4 as _{the O2} carrier for the Claus reaction furnace is as follows:

Comparing the two reactions, it becomes evident that _H2SO4 is _an excellent carrier of _O2 and has the (theoretical) potential to reduce the Claus reaction residual gas volume flux by 67% compared to atmospheric air.

It may also be advantageous to redirect some of the feed gas to the Claus tail gas combustor, since the feed gas has a heating value that can be used in the Claus tail gas combustor, thus reducing the need for auxiliary fuel addition. This may be even more advantageous if there are two sources of feed gas, such as one _NH3- free feed gas and one _NH3 -containing feed gas, since the sub-stoichiometric conditions in the reaction furnace section prevent complete oxidation of the _NH3 . The so-called sour water stripper (SWS) gases are an example of such an _NH3 -containing feed gas.

In the Claus processes for SWS gas treatment _, complete destruction of _NH3 in the reaction furnace section is critical, otherwise ammonia salts such as ( _NH4 ) _2SO4 and ( _NH4 ) _{2S2O3 are} formed, which plug the final elemental sulfur condenser. Special high intensity (two-stage) burners are capable _of achieving the high temperatures required for thermal destruction of _NH3 , but require precise control of the acid gas in two separate streams.

However, it is well known to treat SWS gas in a sulfuric acid plant since complete oxidation of _NH3 to _N2 and NO is achieved with excess oxygen at temperatures around 1000°C. Thus, it may be desirable to configure an integrated Claus/sulfuric acid process with two combustors to direct a first feed containing _H2S and little or no _NH3 to a reaction furnace section, with a gas containing _NH3 , such as SWS gas, being directed to a Claus reaction tail gas combustor. In such a configuration, it may be desirable to design the sulfuric acid plant to include a selective catalytic reduction (SCR) section for _NOx with _NH3 in the presence of a vanadium based catalyst.

When designing a Claus burner and reaction furnace, the breakdown of hydrocarbons (e.g. _CH4 and BTX) and, if present, _NH3, typically determines the operating temperature and residence time. Breakthrough of these species into the Claus catalyst section can lead to catalyst fouling and deactivation, as well as plugging of elemental sulfur condensers, resulting in unscheduled shutdowns.

Reactions involving _H2S (reactions 1 and 2) are generally considered fast and will not be rate determining reactions.

Typically, temperatures of 1000-1050°C are required for complete destruction of hydrocarbons, taking into account the effects of slow mixing and temperature/composition gradients. Similarly, 1200-1250°C is required to ensure adequate destruction of _H2S in the reaction furnace.

To achieve such high temperatures, especially in the case of low calorific value feed gases, a choice of feed gas preheating, oxygen enrichment, staged acid gas combustion and/or fuel gas co-firing may be required.

With the introduction of sulfuric acid, the proper design and operation of the Claus burner and reaction furnace will become more important, because the overall effect of injecting sulfuric acid into the Claus reaction furnace is to reduce the temperature, the exact value depends on the energy value of the feed gas and the relative amount of sulfuric acid fed to the reaction furnace. Considering that about 5% of the total elemental sulfur production is supplied as sulfuric acid, the overall temperature reduction in the Claus reaction furnace is usually 50-100 °C.

The simplest scheme is to configure the Claus burner to accept all feedstock and adjust the temperature in the Claus reaction furnace by adding fuel gas and/or using, for example, feed gas preheating and O ₂ enrichment.

However, in some cases this can significantly increase fuel gas consumption, combustion air consumption and overall process gas consumption to the point that the Claus and Claus exhaust gas plants become too large, resulting in an uncompetitive solution. In addition, the increased fuel consumption will lead to increased operating costs.

The disclosed invention describes a method of adding sulfuric acid to a Claus reaction furnace in such a way as to provide the required high operating temperatures for the destruction of hydrocarbons and _NH3 and to reduce the size of the plant and operating costs.

It has been found that establishing a high temperature in the reaction furnace section with a lower stoichiometric O ₂ content than that required for the Claus reaction and directing the sulfuric acid to a downstream sulfuric acid flash section which cools the process gas through endothermic reactions associated with the vaporization, dehydration and dissociation of sulfuric acid provides the necessary temperature for the destruction of hydrocarbons and NH ₃ while maintaining the benefits of the O ₂ enriching properties of sulfuric acid. This is achieved by recognizing that while hydrocarbons and NH ₃ are destroyed in the high temperature reaction furnace section, the sulfuric acid flash section which includes the sulfuric acid feed does not require as high a temperature as both the reactions associated with H ₂ SO ₄ (7-10) and the direct reactions associated with H ₂ S (1-2) will occur at these lower temperatures. The lower temperature will result in lower equilibrium conversion of _H2S and _SO2 to elemental sulfur, however, subsequent catalytic converters can compensate for this somewhat lower yield of elemental sulfur in the reaction furnace.

The effects of _O2 enrichment and process gas cooling by sulphuric acid injection result in both lower process gas consumption and lower process gas temperature in the waste heat boiler, and thus a smaller and cheaper waste heat boiler can be used.

According to one embodiment, the Claus reaction furnace is designed such that the high-temperature section of the reaction furnace, including the Claus burner, receives the full volume of combustion air, raw gas containing NH ₃ , and at least a portion of the acid gas, and the sulfuric acid evaporation section is designed to obtain the full amount of sulfuric acid and, optionally, a portion of the acid gas. This allows NH ₃ to be decomposed without co-combustion of the fuel gas, since the temperature-reducing effect of introducing sulfuric acid is delayed until NH ₃ is destroyed.

The reaction furnace section operates under very rich fuel conditions, i.e. the O ₂ input is less than that required to completely convert all combustibles to CO ₂ , SO ₂ and H ₂ O. This is normal practice for the Claus reaction furnace, since typically only about 1/3 of the required O ₂ is fed. However, when introducing sulfuric acid into the sulfuric acid vaporization section, the reaction furnace section operates with an even smaller O ₂ input, since a significant amount of O ₂ is supplied via H ₂ SO ₄ and to a lesser extent via the atomization medium in the sulfuric acid vaporization section. The heat generated in the reaction furnace section is directly related to the O ₂ input, and the process gas temperature is directly related to the heat generated and the total amount of process gas in the reaction furnace section. To increase the temperature in the reaction furnace section, the total amount of process gas can be reduced by diverting a portion of the acid gas to the sulfuric acid vaporization section. The amount of acid gas that can be removed depends on several factors such as the heating value of the inlet streams, the degree of preheating of the feed streams, the required temperature of the reaction furnace section, the presence of impurities in the acid gas, and the temperature of the sulfuric acid evaporation section.

For example, in petroleum refineries it is common to use concentrated acid gas (>80 vol.% _H2S ) and a gas containing _NH3 and _H2S (sour water stripper (SWS) gas), where most of the inlet sulfur is present in the acid gas. The reaction furnace section can then be designed to receive all of the SWS gas and combustion air, plus some of the acid gas, so that the temperature in the reaction furnace section will be around 1200-1250°C and complete destruction of the _NH3 will be achieved prior to the sulfuric acid flash section, where the sulfuric acid, any spray media, and the remaining acid gas are introduced. In such a setup, the temperature in the sulfuric acid flash section will be of secondary importance since the acid gas contains low concentrations of impurities (such as hydrocarbons) that would otherwise require a certain temperature for complete destruction.

In natural gas processing plants, the feed gas to the Claus unit will have a lower concentration of _H2S (typically <60 vol.%) and will contain hydrocarbons that require temperatures above 1000-1050°C to ensure complete destruction of the hydrocarbons. In such a situation, the preferred option would likely be to route all of the acid gas to the reaction furnace section.

Turbulence enhancers can be installed in both the reaction furnace section and the sulphuric acid evaporation section so that efficient mixing of the process gas from the reaction furnace section and the sulphuric acid and optionally a portion of the acid gas is completed as quickly as possible. Turbulence enhancers can be, for example, throttle rings, vector walls, tangential inlets, etc., which will provide a narrower distribution of the process gas residence time, improve mixing, increase the evaporation of the sulphuric acid droplets and reduce the probability that a droplet will experience a residence time much lower than the average.

The sulphuric acid evaporation section may also be equipped with impactor means to reduce the number of large droplets which, due to their high inertia, will not be able to follow the flow lines of the gas diverted close to the obstruction in the impactor means. Instead, the large droplets will continue to move forward, collide with the obstruction and evaporate from the surface of the obstruction. As a result, the risk of unreacted _SO3 being carried over into the process downstream of the Claus reaction furnace waste heat boiler is reduced, thereby avoiding condensation of sulphuric acid in elemental sulphur condensers and/or deactivation by sulphation of the catalyst in Claus catalytic converters. Such impactor means may be grate walls, impact walls, vector walls, labyrinth walls and compartments made of (inert) packing material.

According to another embodiment, the feed gases for the Claus unit are acid gas with a high concentration of H ₂ S and SWS gas containing NH ₃ , and the sulfuric acid plant for the Claus residual gas is designed to receive the entire amount of SWS gas, which allows saving fuel in the combustion chamber of the Claus residual gases and eliminates operational problems in the Claus unit associated with incomplete destruction of NH ₃ in the Claus reaction furnace. The reaction furnace section in the Claus unit is designed to receive the entire amount of combustion air, fuel gas (if necessary) and at least a portion of the acid gas, while the sulfuric acid evaporation section is designed to receive at least a portion of the sulfuric acid from the sulfuric acid plant of the Claus residual gas and the remaining acid gas.

According to another embodiment, the evaporation of sulfuric acid occurs in a compartment separated from the reaction furnace section, and the sulfuric acid-saturated gas from this compartment is added to the process gas from the high-temperature reaction furnace section after the evaporation of sulfuric acid is completed. Such an embodiment will provide a special compartment for the evaporation of sulfuric acid, providing sufficient time for the droplets to evaporate before mixing with the main flow of process gas. The advantage of this embodiment is that the special chamber can be much smaller than the reaction furnace, but at the same time have a much longer residence time for the droplets to evaporate, which allows for an economical solution for the reaction furnace.

The energy required to evaporate the sulphuric acid can be provided by taking a portion of the process gas from a section of the reaction furnace and directing the sulphuric acid into this smaller volume of gas. The advantage of this embodiment is that the energy is already present in the process gas and that in the Claus plant there is no dilution of the process gas with combustion products when using auxiliary fuel.

An alternative energy source is a portion of the combustion air, which can be additionally preheated before being introduced into the sulfuric acid stream. The advantage of this solution is that the flow and temperature of the combustion air can be more easily controlled than by diverting a portion of the process gas from the reaction furnace section.

Another option is to combust the fuel gas with air to produce a hot process gas into which sulphuric acid can be injected. The advantage of this solution is that the reaction furnace section is practically untouched and for, for example, retrofit solutions, a sulphuric acid evaporation chamber can be constructed while the existing Claus plant is in operation.

The integrated process according to the present invention may also benefit from the use of oxygen-enriched air or substantially pure oxygen in the reaction furnace section. The advantage of using oxygen-enriched air is the reduction of the amount of inert nitrogen in the process gas and thus the reduction of the volume of the process gas and thus the reduction of the size of the plant. The absence of nitrogen dilution also results in an increase in the combustion temperature, which may be advantageous if impurities requiring complete conversion are present, especially since the amount of oxygen in the Claus reaction furnace is sub-stoichiometric. Since the Claus catalyst is sensitive to impurities such as light hydrocarbons, it may often be advantageous to operate the Claus reaction furnace with oxygen-enriched air to achieve an elevated temperature for complete oxidation of the impurities. This also has the additional advantage of allowing an initial homogeneous non-catalytic Claus conversion, which may occur at temperatures above 900°C.

However, from a thermal efficiency standpoint, high combustion temperatures may be limited by the choice of construction materials in the Claus reaction furnace and subsequent waste heat boiler. For feed gases with high _H2S concentrations _, oxygen enrichment may result in process gas temperatures above the design material temperatures. However, the combination of _H2SO4 recirculation (which cools the process gas by evaporation and acid decomposition) makes it possible to use enriched _O2 in such a scheme.

The Claus tail gas combustor will typically be operated with atmospheric air, and in addition it may be advantageous to direct gases with low concentrations of sulfur compounds into the Claus tail gas combustor to completely burn the sulfur compounds, releasing significantly more energy than the partial oxidation occurring in the Claus furnace.

As a consequence, it may be advantageous to route feed gases containing high concentrations of _H2S to the Claus unit, while diverting feed gases with lower concentrations, as well as feed gases containing _NH3 , to the Claus reaction tail gas combustion chamber.

If the Claus reaction tail gas combustor receives only the Claus reaction tail gas containing only a limited amount of _H2S , the calorific value is too low to maintain stable combustion. In this situation, the addition of auxiliary fuel is required. This auxiliary fuel can be _H2S , SWS gas or hydrocarbon feedstock, but it is preferable to use the amount of existing feed gas for the integrated Claus and sulfuric acid plant.

Integration of the Claus process and the sulfuric acid process offers integration advantages. These include the ability to reduce the volumetric flow rate in the Claus process by feeding oxidant in the form of sulfuric acid, which can replace atmospheric air. In addition, feed gas utilisation can be optimised so that feed gases containing fuels that contribute significantly to the production of elemental sulfur can be sent to the Claus process, while feed gases containing thermal energy and unreacted products such as _CO2 can be sent to the sulfuric acid process. If the process is designed to recycle too much sulfuric acid, additional fuel may be required to provide the heat required to vaporise and dissociate the sulfuric acid.

According to a preferred embodiment, the sulfuric acid produced in the Claus tail gas sulfuric acid plant is cooled and sent to an intermediate storage tank. In principle, a sulfuric acid storage tank can be omitted, since the Claus reaction furnace is designed to receive all the sulfuric acid produced in the sulfuric acid plant. However, in order to ensure high reliability of the entire plant, the installation of a tank allows the Claus plant to be operated (briefly) without injection of sulfuric acid, while the sulfuric acid plant is still in operation. Such situations can occur during startup and shutdown, as well as when maintenance of the sulfuric acid tuyeres/nozzles is required. The tank will also allow the sulfuric acid product to be diverted, if this becomes a desired product, and will also allow the import of sulfuric acid from other sources.

The capacity of the sulfuric acid production tank from 4 to 120 hours is a good compromise between the cost of the tank and the flexibility of the sulfuric acid plant for Claus + Claus tail gas.

According to another embodiment, the sulfuric acid produced in the sulfuric acid plant for the Claus residual gas is further concentrated in an acid concentrator before being reused in the Claus reaction furnace. The operation of such a concentrator would require energy to evaporate water from the sulfuric acid, but most of this energy is then saved in the sulfuric acid evaporation section. The advantage of this embodiment is a smaller decrease in the temperature of the process gas in the sulfuric acid evaporation section and a decrease in the flow rate of the process gas through the Claus plant and the Claus residual gas plant.

In addition to the wet sulphuric acid process, sulphuric acid can also be produced in other sulphur reduction processes. A prime example is the SNOX process, which combines selective catalytic reduction of NOx with a wet sulphuric acid process, a scheme which is particularly favourable for flue gases with an SO ₂ content of less than 1 vol.%. Also relevant are H ₂ O ₂ scrubbers, which are often used for small volumes of flue gases with low SO ₂ concentrations, where the SO ₂ is reacted with H ₂ O ₂ , directly forming H ₂ SO ₄ . In addition, dry sulphuric acid processes based on absorption in sulphuric acid can be considered. Sulphuric acid is also produced in a process in which the feed gas enters a reactor and passes upwards through a bed of activated carbon, in which the sulphur dioxide is converted to sulphuric acid by wet catalysis in the presence of oxygen and water. Sulfuric acid from all of these and other sources can also be advantageously directed to the Claus reaction furnace.

However, the benefit of injecting sulfuric acid into a Claus reaction furnace diminishes as the sulfuric acid concentration decreases due to the additional energy required to evaporate the water and the increase in process gas flow caused by the increased amount of water in the sulfuric acid feed. _Sulfuric acid produced by reaction with _H2O2 and activated carbon processes is usually too low to provide advantageous integration with a Claus unit, but when combined with an acid concentration process, such processes may be advantageous for integration with a Claus process.

Integration of the two processes also allows the process to be implemented in which the Claus process is operated at low conversions, such as 90% or 95%, since it may be cheaper to carry out additional conversion in the sulfuric acid process compared to adding an additional Claus converter stage.

The standard Claus plant design requires >50 vol.% _H2S in the feed gas to be thermally self-sustaining in the Claus reaction furnace. At lower _H2S concentrations, feed gas preheating and a so-called split-flow configuration are required. Claus plants processing feed gases with _H2S contents of <10-20 vol.% are rare. On the other hand, sulfuric acid processes very efficiently treat these so-called lean _H2S gases, producing concentrated sulfuric acid. The sulfuric acid product will have a high sulfur and oxygen concentration.

A combination of a sulfuric acid plant to treat the lean gas in _H2S (and/or other sulfur compounds) coupled with a Claus plant to treat the high _H2S gas and receive acid from the sulfuric acid plant would be a profitable plant since the feed streams to both the Claus plant and the sulfuric acid plant are optimal in terms of conversion efficiency, thermal efficiency and plant size/cost.

The combination of the Claus process and the sulfuric acid process can also be used to optimize feedstock handling. Sulfuric acid processes, and in particular the wet sulfuric acid process, have the advantage of being well suited to contaminated feedstocks, including SWS gases containing ammonia as discussed above, "dirty sulfur" containing organic impurities and moderate amounts of inorganic impurities, dilute streams of _H2S , _SO2 and other sulfur compounds, including burner flue gases and FCC gas. Similarly, gases with high hydrogen sulfide content that must be diluted before being processed in a wet sulfuric acid plant can instead be sent directly to the Claus process.

Brief description of the drawings

Fig. 1 shows a sequential Claus + sulfuric acid process in which the sulfuric acid is recycled to the Claus unit with one section of the Claus reaction furnace.

Fig. 2 shows an integrated Claus process + sulfuric acid process with the addition of sulfuric acid to the sulfuric acid evaporation section in accordance with the present invention.

Fig. 3 shows the details of the integrated Claus + sulfuric acid process; Schematic diagram of the Claus reaction furnace, where the sulfuric acid evaporation section is separated from the reaction furnace section in the form of two separate chambers, and the hot process gas is used as an energy source for the evaporation of sulfuric acid.

Fig. 4 shows the details of the integrated Claus process + sulfuric acid process; Schematic diagram of the Claus reaction furnace, where the sulfuric acid evaporation section is separated from the reaction furnace section in the form of two separate chambers, and hot combustion air is used as an energy source for the evaporation of sulfuric acid.

Fig. 5 shows the relationship between the temperature in the reaction furnace section and the amount of acid gas supplied to the first section of the reaction furnace.

In Fig. 1, the Claus burner and reaction furnace 6 are configured to receive acid gas (3), optional fuel gas (4), combustion air (2) and sulfuric acid (108). At the rear of the reaction furnace 6, a waste heat boiler is located that cools the off-gases of the Claus reaction furnace, which enter via line 8 into a first elemental sulfur condenser 10, where elemental sulfur is condensed and removed via line 60, while the substantially elemental sulfur-free process gas exits via line 12 for reheating in a heat exchanger 14, which may also be an integral burner. The reheated process gas 16 enters the first catalytic Claus reactor 18, in which an additional amount of elemental sulfur is formed. The converted process gas 20 then passes through a second elemental sulfur condenser 22 where liquid elemental sulfur is withdrawn via line 62 and the substantially sulfur-free process gas exits via line 24. Typically, the process gas passes through 1-2 further heat exchangers (26), a conversion stage(s) (30) and a condensation stage (34) before exiting the Claus unit as residual Claus gas via line 36 and entering a Claus residual gas treatment unit.

Elemental sulfur lines 60, 62 and 64 are combined into a common sulfur well 66. The sulfur well is purged with air 70 to release and remove vapors, and the exhaust gas 72 from the sulfuric acid well can be directed either to the Claus burner 6 or to the residual gas combustion chamber 42. The product of degassing elemental sulfur leaves the Claus unit through line 68.

The Claus residual gas 36 is optionally heated in the heat exchanger 37 and fed via line 39 to the burner of the residual gas combustion chamber 42. Additional fuel is fed via line 44 and preheated combustion air is added via line 100. The so-called sulphur well exhaust gas 72 can also be advantageously added to the combustion chamber. A waste heat boiler is arranged at the rear of the Claus residual gas combustion chamber 42, which cools the exhaust gases of the combustion chamber. The cooled exhaust gas 74 of the combustion chamber can be further cooled in the air heater 75, and the further cooled exhaust gas 80 of the combustion chamber can be mixed with a stream of hot dilution air 98. The mixed process gas 82 then enters the SO ₂ converter 84, which consists of several catalyst beds for oxidising SO ₂ to SO ₃ and heat exchangers arranged between the catalyst beds. Typically, 1-3 catalyst beds are arranged. The converted off-gas 86 is directed to a condenser 88, in which the concentrated sulfuric acid is withdrawn through a lower outlet 104, optionally cooled (not shown) and pumped to an optional intermediate storage tank 106. From the tank, the sulfuric acid is directed to a Claus burner and a reaction furnace 6 via a line 108. The off-gas 90, which is substantially free of sulfuric acid, is directed to a stack 50 or other installation for further recovery of SO ₂ and/or sulfuric acid mist (not shown).

The sulfuric acid condenser 88 uses air 92 as a cooling medium, and hot air leaves the condenser through line 94, and some of this air can be further heated in the air heater 75. Then, the heated air 96 can be directed to the Claus residual gas combustion chamber (42) as combustion air through line 100 and through line 98 added to the process gas 80 to provide sufficient oxygen for the oxidation reaction of SO ₂ to SO ₃ in the SO ₂ converter 84.

Fig. 2 shows a Claus burner and a reaction furnace 6 according to the present invention. The Claus burner receives combustion air 2, at least a portion of the acid gas 3 and, optionally, a fuel gas 4 or an SWS gas containing NH ₃ 1. These gases are ignited and allow a reaction in a section of the reaction furnace 117. At the rear of the section of the reaction furnace 117, a sulfuric acid evaporation section 119 further receives a stream of sulfuric acid 108 and, optionally, a portion of the acid gas 3. At the rear of the sulfuric acid evaporation section, a waste heat boiler is located, cooling the Claus reaction furnace with exhaust gases. The cooled exhaust gas of the Claus reaction furnace 7 is optionally sent to a catalytic converter 9, in which a catalyst for the destruction of SO ₃ is installed. The converter exhaust gas 8, which is substantially free of SO ₃ , enters via line 8 into the first elemental sulfur condenser 10, where the elemental sulfur is condensed and removed via line 60, while the process gas, which is substantially free of elemental sulfur, exits via line 12 for reheating in a heat exchanger 14, which may also be an integrated burner. The reheated process gas 16 enters the first catalytic Claus reactor 18, where additional elemental sulfur is formed. The converted process gas 20 then passes through a second elemental sulfur condenser 22, where liquid elemental sulfur is removed via line 62 and the process gas, substantially free of sulfur, exits via line 24. Typically, the process gas passes through 1-2 further heat exchangers (26), a conversion stage(s) (30) and a condensation stage (34) before exiting the Claus unit as residual Claus gas via line 36 and entering a residual Claus gas treatment unit.

Elemental sulfur lines 60, 62 and 64 are combined into a common sulfur well 66. The sulfur well is blown with air 70 to release and remove vapors, and the exhaust gas 72 from the sulfur well can be directed either to the Claus burner 6 or to the residual gas combustion chamber 42. The product of degassing of elemental sulfur leaves the Claus unit through line 68.

The Claus residual gas combustion chamber 42 is designed to receive additionally heated Claus residual gas 39, fuel 44, combustion air 100 and, optionally, a portion of the SWS gas 1 that is not sent to the reaction furnace section 117. The fuel 44 may also be a portion of the acid gas 3. The exhaust gas from the Claus residual gas combustion chamber is cooled and sent via line 74 to the stages of catalytic oxidation of SO ₂ to SO ₃ and the stages of condensation of sulfuric acid 76, which were described in Fig. 1. The hot air from the stage 100 of condensation of sulfuric acid can be used in the Claus residual gas combustion chamber 42, the exhaust gas of the sulfuric acid condenser 90 is sent to the stack 50 or to an installation for further recovery of SO ₂ and/or sulfuric acid mist. The condensed sulfuric acid 108 is directed to the sulfuric acid evaporation section 119 of the Claus burner and reaction furnace 6, possibly via an intermediate storage tank (not shown).

In Fig. 3 an alternative arrangement of the Claus burner and reaction furnace is shown. The acid gas 3, combustion air 2 and optionally fuel 4 or SWS gas containing NH ₃ 1 are ignited in the Claus burner 6, reacting at high temperature and forming the Claus reaction furnace exhaust gas 110. A portion of the hot Claus reaction furnace exhaust gas 112 is directed to a sulfuric acid vaporization chamber 114, in which a stream of sulfuric acid 108 is sprayed either through hydraulic nozzle(s) or through pneumatic nozzle(s). The vaporization chamber exhaust gas 116 is then mixed with the remaining fraction of the Claus reaction furnace exhaust gas 115 to form a mixed Claus reaction furnace exhaust gas 118. The temperature is controlled by heat exchanger 120 and the cooled exhaust gas 8 is directed downstream to the Claus plant and the tail gas sulfuric acid plant shown in Fig. 1.

Fig. 4 shows an alternative arrangement of the Claus burner and reaction furnace. Acid gas 3, optional fuel 4 and optional SWS gas containing NH ₃ , 1 and a portion of combustion air 2 are directed into the combustion chamber 6 and ignited, forming a hot off-gas 110 of the Claus reaction furnace. The exhaust combustion air is optionally heated in a heat exchanger 113 to form a hot air stream 117. Sulfuric acid 108 is atomized and mixed with the hot air stream in a vaporization chamber 114 to form a vaporized sulfuric acid air stream 116. The air stream 116 is mixed with the Claus reaction furnace exhaust gas 110 to form a mixed Claus reaction furnace exhaust gas 118. The temperature is controlled by a heat exchanger 120 and the cooled exhaust gas 8 is sent to a subsequent Claus unit and a sulfuric acid tail gas unit as shown in Figure 1.

Figure 5 shows the temperatures at the outlet of the reaction furnace section 117 as a function of the proportion of acid gas sent to the Claus reaction furnace that is sent to the reaction furnace section (versus that sent to the sulfuric acid evaporation section) for a number of process variants - see Figure 2 for a process diagram.

The Claus unit receives SWS gas 1 and acid gas 3. The Claus unit is designed to divert a portion of the problematic SWS gas to the Claus residual gas sulfuric acid unit, where the SWS gas does not cause problems. The SWS gas of the Claus reaction furnace will be directed to the reaction furnace section. In addition, the Claus reaction furnace is designed to supply sulfuric acid to the sulfuric acid evaporation section 119, and the acid gas can be directed to both the reaction furnace section and the sulfuric acid evaporation section. All combustion air is directed to the reaction furnace section.

The upper solid curve describes the situation where all the SWS gas is sent to the reaction furnace section (100% of SWS gas in RF). The temperature in the reaction section is important for the destruction of NH ₃ in the process gas, and it is seen that reducing the proportion of acid gas in the first section of the reaction furnace increases the reaction temperature and thus enhances the destruction of NH ₃ .

The two middle dotted curves represent operations where a portion of the SWS gas (50% and 60% of the SWS gas for the RF, respectively) is sent to the Claus tail gas sulfuric acid plant, which reduces potential NH ₃ problems in the Claus plant by reducing fuel gas consumption in the sulfuric acid plant and increasing sulfuric acid production and hence sulfuric acid injection into the sulfuric acid flash section. By feeding 100% of the acid gas to the reaction furnace section, the temperature approaches or falls below the minimum temperature for efficient NH ₃ destruction. By diverting a portion of the acid gas to the sulfuric acid flash section, the temperature increases to ensure efficient NH ₃ destruction.

The dotted (bottom) line shows an operation where all of the SWS gas is sent to the Claus residue gas sulfuric acid plant, eliminating the NH ₃ problems in the Claus plant. The increased flow of sulfuric acid to the sulfuric acid evaporation section results in a lower temperature in the reaction furnace section, and thus a portion of the acid gas will need to be diverted to maintain the high temperature in the reaction furnace section. However, the reaction furnace section temperature can be reduced because NH ₃ is not present in the Claus burner feed.

According to this embodiment, a portion of the SWS gas can be diverted to a Claus tail gas sulfuric acid plant to reduce the fuel gas consumption in the Claus tail gas combustion chamber. Diverting a portion of the SWS gas results in an increase in sulfuric acid production and thus an increase in the sulfuric acid input to the sulfuric acid evaporation section, thereby reducing the exhaust gas temperature of the Claus reaction furnace.

Example 1. Two-Section Claus Reaction Furnace for a Refinery Gas Case To document the effect of integrating the Claus process and the sulfuric acid process, four additional examples of the process shown in Fig. 2 were analyzed in comparison with the prior art process as shown in Fig. 1.

These examples are based on the following feed gases:

Feed gas enriched in _H2S (stream 2 in Figs. 1 and 2):

Total gas flow: 8190 ^Nm3 /h

_H2S concentration: 94 vol.%

Concentration of _H2O : 6 vol.%

_H2S rich gas is typical of refinery gas and will also contain varying amounts of light hydrocarbons.

Feed gas enriched in H ₂ S and NH ₃ (stream 70 in Figs. 1 and 2):

Total gas flow: 3669 ^Nm3 /h

_H2S concentration: 28 vol.%

_NH3 concentration: 45 vol.%

_H2O concentration: 27 vol.%

These streams, containing _H2S and _NH3 , are typically off-gases from so-called sour water strippers and are considered SWS gases. They may also contain varying amounts of light hydrocarbons.

The fuel gas is natural gas _CH4 , which is sent to a Claus residual gas treatment plant.

The feed streams, combustion air and Claus tail gas are preheated to the highest possible degree by utilizing the heat released in the combined Claus process + sulphuric acid process.

In this example, the Claus process operates with a sulphur recovery from the feedstock of 95%, i.e. it can be considered a well-functioning Claus unit with only two catalytic stages.

The sulfuric acid injection scheme shown in Fig. 2 is compared with a Claus plant equipped with an amine off-gas unit that captures sulfur compounds in the form of _H2S and returns gaseous _H2S to the Claus burner and reaction furnace. The flue gas emissions of a Claus plant equipped with an amine off-gas unit or a sulfuric acid plant are comparable.

The data for Example 1 are given in Table 1, where the rows represent the amount of SWS gas sent to the Claus furnace, the recycle amount (moles of S atoms relative to S formation), the Claus furnace outlet temperature and the corresponding process gas flows from the Claus furnace, the fuel gas flow rate and the process gas flow from the Claus residual gas combustor

Example 1.A

The first column of Table 1 shows data for a Claus unit equipped with an amine off-gas unit. The 5% of the total sulfur recycled to the Claus reaction (RF) furnace is to be understood as gas containing H ₂ S and not sulfuric acid, H ₂ SO ₄ . The reaction furnace temperature is 1400 °C. The process gas flows are increased by recirculating acid gas from the amine off-gas unit and by additional combustion air required for partial combustion of the recycled H ₂ S. The increased process gas flow requires a larger Claus unit because the process gas flow is increased at the inlet to the Claus unit (RF process gas stream outlet, reaction furnace) and at the outlet of the Claus unit (process gas from the combustor, i.e., Claus residual gas combustor). The increase in fuel gas flow is associated with the increased flow into the Claus residual gas combustor.

Example 1.B

The basic operation of a Claus plant is characterized by the fact that all SWS gas and acid gas flow is directed to the Claus burner and reaction furnace, and all acid produced in the Claus tail gas sulfuric acid plant is directed to the sulfuric acid evaporation section.

Example 1.C-1.E

Examples 1.C through 1.E shown in Table 1 illustrate the effect of directing a portion of the SWS gas to the Claus tail gas combustor in the sulfuric acid plant. First, the SWS gas will increase the production of sulfuric acid, which will increase the proportion of total sulfur as H ₂ SO ₄ sent to the Claus plant. Diverting all of the SWS gas to the sulfuric acid plant will increase the total sulfur as H ₂ SO ₄ to 16%. The overall effect of introducing sulfuric acid into the Claus reaction furnace is a decrease in temperature, which is seen in Row 3 (sulfuric acid vaporization section outlet temperature). The sulfuric acid vaporization section outlet temperature is shown (119 in Fig. 2). Fig. 5 shows that the temperature in the reaction furnace section (117 in Fig. 2) can be increased by diverting a portion of the acid gas to the sulfuric acid vaporization section. If such a two-section Claus furnace configuration were not used, the addition of sulfuric acid to the Claus furnace would be significantly limited because the temperature of the reaction furnace would limit the amount of sulfuric acid sent to the Claus furnace. An alternative would be to add fuel to the Claus burner, but the associated costs may offset the benefits of adding sulfuric acid.

The effect of oxygen enrichment of the sulphuric acid is quantified in line 4, which shows the total volume of process gas in the Claus catalytic section, i.e. the process gas stream RF. The bottom process gas stream can be used either to increase the capacity of an existing plant or to reduce the size of a new plant. By combining the effect of the lower process gas flow rate to the Claus tail gas combustor and the replacement of fuel gas with SWS gas, the overall plant fuel consumption is significantly reduced when the SWS gas bypasses the Claus tail gas combustor. In addition, the process gas flow rate to the sulphuric acid plant for tail gas (process gas from the combustor) is reduced due to the reduced tail gas flow from the Claus unit.

Example 2. Two-section Claus reaction furnace for natural gas processing

To document the effect of two sections in a Claus reaction furnace, calculations were performed for a Claus burner and reaction furnace for a typical gas from a natural gas processing plant.

These gases are characterized by a relatively low concentration of H2S, a high concentration of _CO2 and an amount of hydrocarbons that require complete destruction in the reaction furnace, since hydrocarbons can cause fouling/clogging of the catalyst sections after the Claus reaction furnace.

Sour gas has the following composition

50 vol. % _H2S

40 vol.% _CO2

9 vol.% _H2O

1 vol. % CH ₄

Combustion air is atmospheric air preheated to 400°C. Fuel gas is pure natural gas _CH4 .

The Claus burner, i.e. the reaction furnace section, is designed to receive all of the combustion air and, if used, all of the fuel gas. Acid gas will also be sent to the reaction furnace section, as high temperatures are required to ensure complete destruction of the hydrocarbons in the feedstock. A rule of thumb is that 1050°C is required to completely destroy the hydrocarbons.

The Claus unit is expected to operate at ~95% efficiency in converting sulfur compounds to elemental sulfur, and the tail gas treatment unit captures unreacted sulfur compounds and returns them to the Claus reaction furnace via the sulfuric acid evaporation section.

The results of Examples 2.A-2.G are presented in Table 2. All flows are relative to the base case. RF denotes the reaction furnace (section), and SAE denotes the sulfuric acid evaporation SAE (section).

Example 2.A shows data for a Claus unit with an amine-based tail gas treatment unit. All feed is sent to the Claus burner, so the temperatures in the two sections of the Claus reaction furnace are equal.

Example 2.B shows data for a Claus unit equipped with a sulfuric acid plant for tail gas treatment. The sulfuric acid is sent to the sulfuric acid vaporization section. The temperature in the reaction furnace section is lower than in Case A because part of the oxygen (and hence energy) feed is fed via the sulfuric acid to the sulfuric acid vaporization section. However, the temperature is still above the minimum hydrocarbon destruction temperature. The addition of sulfuric acid reduces the temperature due to the endothermic reactions associated with the vaporization, dehydration, and dissociation of sulfuric acid. In this example, the temperature will be below the minimum hydrocarbon destruction temperature, and thus sending the sulfuric acid to the reaction furnace section will result in an increased risk of hydrocarbon carryover to the downstream catalytic section of the Claus unit.

In Example 2.C, additional fuel gas was fed to the Claus burner to achieve the same temperature in the reaction furnace section as in Example 2.A. As can be seen, this is possible without increasing the process gas flow through the Claus unit and the tail gas unit, but the fuel gas consumption has increased.

In Example 2.D, fuel was added to the Claus burner to obtain the same temperature in the sulfuric acid vaporization section as in Example 2.A. An increase in the process gas flow rate and fuel flow rate is observed. The difference between Column C and D describes the difference between having a two-section Claus reaction furnace (Column C) proposed in this description, compared to a reaction furnace with a single section in which sulfuric acid is directed to a Claus burner (Column D). A Claus reaction furnace constructed with a reaction furnace section and a sulfuric acid vaporization section will provide a lower process gas flow rate to both the Claus unit and the Claus tail gas unit, will have a lower fuel gas flow rate, while providing the same conditions for the destruction of NH ₃ and hydrocarbons in the feed gases.

Examples 2.E, 2.F, and 2.G describe a Claus + tail gas configuration in which 5% of the acid gas is sent to a tail gas sulfuric acid plant, saving some fuel in the Claus tail gas combustion chamber and doubling the acid throughput of the base case in Example 2.B.

The effect of sending twice as much sulfuric acid to the Claus reaction furnace is to reduce the temperature in both the reaction furnace and the sulfuric acid vaporization section. In this example, the reaction furnace section temperature is considered too low for complete destruction of the hydrocarbons (Example 2.E), so fuel gas must be added to the Claus burner to raise the temperature. In Example 2.F, the reaction furnace section temperature is increased, resulting in process gas flows comparable to the base case (Example 2.B). A more detailed analysis of the operation, especially of the waste heat boiler after the Claus reaction furnace and especially of the sulfuric acid plant for tail gases, is required to evaluate which arrangement is preferable. Generally, the energy release in a sulfuric acid plant is higher than in a Claus plant because most of the energy released in a sulfuric acid plant can be recovered as saturated or superheated high-pressure steam, while the Claus plant primarily utilizes excess energy as saturated steam at a lower pressure. The G column is fed with fuel gas in such a way that the temperature of the sulfuric acid evaporation section increases, i.e. this corresponds to a configuration with a single section of the reaction furnace. The configuration of a two-section reaction furnace is characterized by lower gas consumption in both the Claus unit and the Claus tail gas unit and lower fuel gas consumption compared to a configuration with a single section.

The data in Table 2 show that the competitiveness of the Claus plant with the sulfuric acid plant is best achieved by using a two-section Claus reaction furnace in which the sulfuric acid is directed to the sulfuric acid evaporation section.

Claims (34)

1. A method for obtaining elemental sulfur from a feed gas containing from 15 vol.% to 99 vol.% or 100 vol.% _H2S and a sulfuric acid stream, comprising the following stages: a. providing a Claus reaction furnace feed stream comprising a quantity of said feed gas, a quantity of oxygen and optionally a quantity of fuel, wherein the quantity of oxygen is sub-stoichiometric with respect to the Claus reaction, b. directing said Claus reaction furnace feed stream to a section of the reaction furnace operating at an elevated temperature, such as above 900°C, providing a product to the section of the reaction furnace, c. directing said product section of the reaction furnace and a quantity of sulfuric acid to a sulfuric acid evaporation section downstream of said section of the reaction furnace, providing feed gas for the Claus converter, d. cooling said feed gas for the Claus converter to provide cooled feed gas for the Claus converter, and optionally removing elemental sulfur from the gas, e. directing said cooled feed gas to a Claus converter for contact with a substance catalytically active in the Claus reaction, f. removing residual Claus reaction gas and elemental sulfur, optionally by cooling the outlet stream, from said Claus reaction catalytically active substance, g. directing the flow containing said Claus reaction residual gas to the treatment of the Claus reaction residual gas. 2. The method according to claim 1, wherein the feed gas contains from 20 vol.% to 99 vol.% or from 30 vol.% to 99 vol.% or from 40 vol.% to 99 vol.% or from 50 vol.% to 99 vol.% _H2S . 3. The method according to claim 1, wherein the temperature of the product of the reaction furnace section is more than 50°C or 100°C and less than 200°C or 500°C higher than the temperature at the outlet of said sulfuric acid evaporation section. 4. The method according to claim 1, wherein the feed gas is divided into a reaction furnace section feedstock sent to the reaction furnace section and a sulfuric acid evaporation section feedstock sent to the sulfuric acid evaporation section. 5. The method of claim 4, wherein the ratio between the feedstock for the reaction furnace section and the feedstock for the sulfuric acid evaporation section is controlled to maximize thermal destruction of impurities present in the reaction furnace section. 6. The method according to claim 1, wherein the reaction furnace section and/or the sulfuric acid evaporation section comprises a turbulence amplifier. 7. The method according to claim 1, wherein the sulfuric acid evaporation section comprises an impaction means. 8. The method according to claim 1, wherein said treatment of the residual gas of the Claus reaction comprises the stages: h. directing a flow comprising said Claus reaction tail gas, oxygen and fuel as feed gas into a Claus reaction tail gas combustion chamber operating at a temperature above 900°C or a catalytic means for oxidation, providing feed gas for a SO ₂ converter, i. directing said feed gas to an SO ₂ converter for contact with a substance catalytically active in the oxidation of SO ₂ to SO ₃ , providing a gas enriched in SO ₃ , j. converting said _SO3 -rich gas into concentrated sulphuric acid, either by absorption of _SO3 by sulphuric acid or by hydration of _SO3 , cooling and condensation of sulphuric acid, k. recycling at least a portion of the resulting sulfuric acid to a sulfuric acid evaporation section. 9. The method according to claim 1, wherein said treatment of the residual gas of the Claus reaction comprises the steps of: i. directing the Claus residual gas into a Claus residual gas combustion chamber operating at a temperature above 900°C or a catalytic oxidizer, providing oxidized Claus residual gas; ii. cooling said oxidized residual Claus gas; iii. contacting the cooled oxidized residual Claus gas with hydrogen peroxide, removing a quantity of less concentrated sulfuric acid; iv. converting the said less concentrated sulphuric acid by evaporation of water; v. recycling at least a portion of the resulting concentrated sulfuric acid to the sulfuric acid evaporation section. 10. The method of claim 1, wherein the amount of sulfuric acid comes from a source other than the Claus residual gas treatment device. 11. The method according to claim 1, wherein the amount of sulfur in the sulfuric acid stream is more than 1%, 3% or 5% and less than 17%, 21% or 25% of the total amount of elemental sulfur removed from the process. 12. The method according to claim 1, wherein the concentration of said sulfuric acid is at least 80 wt.% or 90 wt.%. 13. The method according to any one of claims 1 to 12, wherein an amount of additional process gas consisting of 20-50% _NH3 , 20-50% _H2S and at least 80% of the balance being _H2O is sent as additional feedstock to one or both of said reaction furnace and Claus residual gas treatment sections. 14. A process plant for producing elemental sulfur from a feed gas containing _H2S and a sulfuric acid stream, comprising a reaction furnace section, a sulfuric acid evaporation section, a Claus waste heat boiler, a Claus conversion section, sulfur oxidation means, and a sulfuric acid section, wherein the reaction furnace section has an inlet and an outlet, the sulfuric acid evaporation section has a gas inlet, a sulfuric acid inlet and an outlet, the Claus waste heat boiler has a gas inlet, a gas outlet, and an elemental sulfur outlet, the Claus conversion section has a gas inlet, a gas outlet, and an elemental sulfur outlet, the sulfur oxidation means has an inlet and an outlet, and the sulfuric acid section has a gas inlet, a gas outlet, and an sulfuric acid outlet, wherein the inlet of the reaction furnace section is configured to receive the feed gas and the oxidizer, wherein the outlet of the reaction furnace section is configured to be in liquid communication with the gas inlet of the evaporation section sulfuric acid, the sulfuric acid inlet of the sulfuric acid evaporation section is configured to receive a flow of sulfuric acid, and the outlet of the sulfuric acid evaporation section is configured to be in liquid communication with the inlet of the Claus waste heat boiler, where the outlet of the Claus waste heat boiler is configured to be in liquid communication with the inlet of the Claus conversion section, and where the inlet of the sulfur oxidation means is configured to be in liquid communication with the outlet of the gas outlet of the said Claus conversion section, where the outlet of the sulfur oxidation means is designed with the possibility of liquid communication with the inlet of the sulfuric acid section, characterized in that the outlet of the sulfuric acid from the sulfuric acid section is in liquid communication with the sulfuric acid inlet of the said sulfuric acid evaporation section. 15. The process plant according to item 14, wherein the reaction furnace section and the sulfuric acid evaporation section are two separate mechanical units. 16. The process plant according to item 14, wherein the reaction furnace section and the sulfuric acid evaporation section represent a single mechanical unit.