FR2711323A1 - Process for the removal by adsorption of hydrocarbons contained in the air. - Google Patents

Abstract

The invention relates to a process for adsorbing hydrocarbons contained in the air entering an adsorption reactor for at least one inlet E and leaving via at least one outlet S characterized by the fact that an adsorbent is inserted into unit volumes: cones, tetraeds, cylinders, making it possible to considerably reduce the pressure losses in the adsorption reactor without harming the expected performances expressed in relation to the volume of adsorbent used in the operation.

Description

The present invention relates to a method of elimination by

  adsorption of hydrocarbons contained in the air and more particularly in the atmosphere of certain workshops from which they must be removed to comply with the health and safety rules imposed by legislation. By extension, the present invention also relates to volatile organic compounds which in addition to carbon and oxygen can

  contain O S and N as atoms of constitution.

  The fact of selectively absorbing organic compounds from the air at a suitable adsorbent mass has been known and practiced for decades, as has the practice of desorption by scanning the adsorbent mass with a flow of desorption gas, air for example, brought to a temperature 100 to 200 C higher than the temperature of the absorption operation. The adsorbent masses most frequently recommended today are high surface active charcoals, silicalites and oleophilic zeolites obtained by advanced dealumination of zeolites with a high Si 02 / AI203 ratio including zeolites of the ZSM type, the 1 5 mordenite and especially the faujasite; these silicic compounds have the advantage of being regenerable with hot air without incurring the deterioration that may be feared with active carbon. A final adsorbent capable of being used in this type of operation can be prepared from a suitably dealuminated amorphous silica-alumina. The performance of these alumina silicas is slightly lower than that of dealuminated zeolites in terms of absorption speed and quantities of hydrocarbons absorbed per gram of solid at saturation, but the process of

  dealumination is much simpler and the cost of the solids obtained much lower.

  The present invention relates to the use of the adsorbents described above and more particularly the silicic compounds under original conditions which make it possible to make the most of their physicochemical characteristics. This implementation is designed to make the most of the activity of the adsorbent (in terms of adsorption speed), its adsorption capacity, performance in terms of removal of VOCs (volatile organic compounds) while minimizing pressure losses within the adsorption bed. The pressure losses to overcome between the inlet and the outlet of the unit, both during the adsorption period and during the desorp: ion, constitute the most important economic item among the various

operating costs.

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  The prior art describes adsorption processes using adsorbents implemented in the form of axial fixed beds, fluidized beds or even mobile beds (Handbook of separation process technology - 1987) describing the various types of reactors 'adsorption. Among these reactors, none recommends the implementation recommended in the present invention. Starting from adsorbents of the activated carbon type: silicas, dealuminated silica-aluminas, silicalites, dealuminated molecular sieves capable of adsorbing hydrocarbons and more generally VOCs in the presence of water, the object of the present invention relates to a setting in original work of adsorbent materials 1 0 making it possible to reduce the pressure drop across the adsorbent bed while ensuring good retention of the hydrocarbons (or even VOCs) to be removed from the solid, therefore making it possible to considerably reduce the pressure losses in the adsorption reactor without harming the expected performance expressed relative to the volume of adsorbent put

implemented in the operation.

  i 5 In a first variant of the process, the adsorbent used in the form of granules whose equivalent average diameter is between 0.5 and 5 mm is encapsulated inside volumes of fabrics woven or knitted in metallic, silicic or made of temperature resistant composites. The mesh is smaller than the diameter of the particle and the equivalent diameter of the elementary volume relating to the particles is adjusted to the dimensions of the reactor. The pressure drops observed are therefore closely linked to the dimensions of the elementary volume and within the adsorption reactor, each of the elementary volumes behaves like a mini adsorption reactor obeying, in terms of performance, the general laws of chemical engineering. . Under these conditions, the shape of the bed can conform to one or other of the subsequent geometries: axial bed, radial bed, insertion into a rotary cellular cylindrical bed. The geometries of these three types of beds are shown in Figures la, lb, lc (E designating at least one inlet and S designating at least one outlet). The elementary volumes of adsorbents can have various geometric shapes (drawn next to FIG. 1b): cylinders, spheres, cubes or preferably tetrahedra, as described in European patent application No. 494550- AT. In all cases, care is taken that the diameter of the adsorption bed is approximately 50 times greater than the equivalent diameter of the elementary volume. The

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  reduction in the pressure drop created by a bed of free grains of the same thickness is greater than a factor of 10. It will therefore be possible to use bed thicknesses two to five times greater than that authorized by the pressure drop when a bed of free grains is used. In addition, these grain beds tightened in elementary volumes force the gas flow to random paths which, at comparable head loss and spatial speed, give purification performances much better than those obtained on adsorbent of the monolith type with parallel channels.

adsorbent phase coatings.

  In a second variant of the process, the grains are free grains either in the form of solid extrudates, or in the form of hollow extrudates introduced into a mini radial bed as presented in FIG. 2; these elementary reactors have a length of between 100 and 200 cm, a thickness of between 10 and 30 cm and are arranged inside a collection volume for the treated gas, like the tubes of an exchanger (Figures 2a and 2b ). Each adsorption volume is presented as a mini radial reactor fed by the center, the treated gases being recovered outside, in the shell where the mini-reactors are housed. This type of arrangement has the advantage of creating low pressure drops and of being made up of identical metallic wire elements which can be filled before inserting them into the

  collection grille. The capacity of each element is between 50 and 2001.

  for an element length of 100 cm; the thickness of the bed is linked to the equivalent diameter of the adsorbent grains and to the authorized pressure drop in the unit. Two reactors are arranged in parallel, one being in the adsorption phase, the other in the desorption phase (Figure 2d). Each reactor can contain, for example, from 4 to 36 mini-beds and, for large capacities, it is possible to join two mini-beds 25 so as to double the capacity of the installation. It should be noted that this type of thin catalyst bed is particularly suitable for the use of oleophilic dealuminated molecular sieves (mordenites, ZSM, faujasite) for which the characteristic curve QA. f (PA) (where QA is the quantity of A absorbed on the solid and PA its partial pressure) reaches its plateau (saturation of the solid) for low PA values, A being the hydrocarbon to be adsorbed. In this case, a well-cut adsorption front is created, allowing the use of a small thickness.

of bed.

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  This type of arrangement can also involve unitary elements

  parallelepipedic as shown in Figure 2c.

  Whether cylindrical or parallelipiped unitary elements, the walls of the element for the gas inlet and outlet are made of rigid stainless steel fabrics (or stiffened by reinforcing bars) whose width

  mesh remains smaller than the smallest dimension of the adsorbent particles.

  In a third variant, the adsorbent is deposited by coating on a knitted support either of aluminum wire or of glass fibers to form a mattress in

  which the gases will take on random paths.

  1 0 These elements, however, offering a low pressure drop will find applications in air treatments where the flow rates are high and where the

available pressures are low.

  On the other hand, the low volume adsorption capacity, due to the low mass of adsorbent deposited by coating (of the order of 100 Kg per m3) requires a

  1 5 implementation of such a mass by a continuous regenerative process (cf. fig. Lc).

  For the adsorptions of ordinary volatile organic compounds (VOCs), little or not polymerizable, adsorption takes place under ordinary conditions of temperature and pressure, that is to say at a temperature between 0 and 120 C

  and preferably between 20 and 60 C at the rejection pressure of the gases to be eliminated.

  Generally, VOCs are contained in available air at a relative pressure between 0.1 and 1 MPa and generally between 0.1 and 0.2 MPa. The spatial velocity applied depends on the adsorbent considered. For activated carbon, it will be between 1,000 and 5,000 m3 / h of gas to be treated per m3 of adsorbent and for dealuminated zeolites between 5,000 and 50,000 m3 / h / m3. Spatial speed, especially for activated charcoal, will depend on the rate

of planned purification.

  Once the adsorbent is saturated under the adsorption and purification conditions applied, the air (or gas) flow to be purified is stopped and replaced by a desorption flow, the characteristics of which depend on the adsorbent used. When the adsorbent is an alumina silica or dealuminated zeolite, the desorption agent

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  is constituted by a stream of air heated between 140 and 300 C and preferably between 160 and 250 C which is passed over the adsorption bed at a space speed between 100 and 500 m3 / h. The desorption stream is cooled to room temperature through an exchanger and the organic vapors partly condensed before being separated from the gas phase in a separator. After separation, the gaseous phase can be recycled on the adsorption bed when a minimum rejection of volatile organic compounds is targeted. The desorption operation is carried out at a pressure close to and slightly

  higher than atmospheric pressure.

  When the adsorbent used is activated carbon, the desorption will also be carried out under the conditions mentioned above, but to avoid degradation of the adsorbent, an inert gas (nitrogen or gas) will preferably be used in air. of

  combustion) close to stoichiometry as desorption flow.

  The recommended process diagram can be presented in two variants depending on the

  type of reactor used: mobile or fixed reactor.

  When the reactor is mobile, of the type in FIG. 1C, it is the adsorbent bed which is displaced from the adsorption zone to that of desorption. The reactor, divided into quarters of identical volumes (6 to 10 volumes for example), rotates periodically around its axis by the value of a circular sector. If the reactor has n identical sectors (6 to 10 sectors for example), n-3 sectors are in the adsorption phase and are traversed from top to bottom by the gas to be treated, as in FIG. 1C for example three other sectors being in the desorption phase involving the following three successive phases: heating, desorption, cooling. In this case, well-partitioned collection zones allow the recovery of gas flows by preventing them from mixing with the gases to be treated. The heating takes place under a stream of desorbent gas heated between 130 C and 250 C; the regenerated solid is cooled

  under a cold air stream (or cold desorbing gas).

  Fixed reactor systems can have two identical adsorption reactors regardless of the number and geometry of the adsorbent beds. One of the reactors is in the adsorption phase while the other operates on the sequence: heating, desorption, cooling. The principle of the operation remains the same as

  these are Figures la, lb, 2a, 2c.

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Examples

Example 1

  A faujasite with an Si / AI atomic ratio equal to 100, in the form of extrudates having an average diameter of 1.8 mm for an average length of 4 mm, is inserted into conical volumes made of stainless steel fabric whose mesh length is 1.2 mm for a wire thickness of 0.3 mm. The cone has a 30 mm long edge and a base diameter of 30 mm also. These cones, at a rate of 85 per liter, are placed in a cylindrical reactor where they constitute an adsorption bed 1.12 m high for 0.07 m2 of base surface. The bed is supported at its base by a perforated canvas of stainless steel 1 0 whose mesh is 5 mm for a wire thickness of 1 mm. Fluids

  run through the adsorption bed from bottom to top.

  The adsorption tests are carried out with air loaded with butylacetate and xylenes at the rate of 200 (parts per million) in air at 30 ° C. at around 70% relative humidity (RH). The applied spatial speed is equal to 10,000, expressed in i 5 volumes of polluted air per volume of adsorbent and per hour, which corresponds to linear speeds in empty barrel of 3.1 m per second. The pressure drop observed is 240 mm of water (24 hPa, approximately 24 mbar). The effluent is analyzed at the outlet and the test stopped for desorption when the content of total hydrocarbon components is approximately 5% that of the gas to be treated. Under the experimental conditions of the test it is found that the zeolite adsorbs 8.4% of its weight of hydrocarbons. This corresponds to

  2 kg of oil during 2.6 hours of operation before drilling the bed.

  For the desorption operation, air heated to 180 ° C. is circulated at a spatial speed of 300 m3 per m3 of bed and per hour. The bed temperature gradually increases from 30 ° C. to 175 ° C. with an equally progressive desorption of the adsorbed hydrocarbons and chromatographic assay of the concentration of the gases. Butyl acetate desorbs faster than xylene, but after 1 hour 30 minutes of circulation of hot air, the product is completely desorbed. Products condensed at - 15 C

  correspond to a weight of around 2 kg.

  The adsorbent is then cooled to 30 ° C. by circulation of unpolluted air at a space speed equal to 2000. The pressure drop measured during the cycles

  adsorption is less than 10 mm of water (= 1 hPa = 1 mbar).

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Example 2

  The conditions of this test are identical to those of test 1, except the space speed reduced to 5700 h -1. The pressure drop measured through the adsorption bed is then 137 mm of water (= 13.7 hPa = 13.7 mbar). The maximum content of total adsorbed hydrocarbons rises to 9.8% of the weight of the zeolite used, which corresponds to 2.5 kg of weight gain before drilling the bed. The piercing appears

after 4.5 h of adsorption.

  Desorption is carried out by circulation of a quasi-stoichiometric combustion gas of methane previously cooled to 200 C at a space speed equal to 1 0 to 150 h -1. Under the conditions applied, the operation can be considered as

  completed after 1 hour of desorption gas circulation.

Example 3

  The test is carried out with the same zeolite as test 1 and under the same operating conditions but it is inserted into tetrahedral volumes of 3 cm of edge instead of 1 5 to insert it into conical volumes. The pressure drop observed is slightly lower (190 mm of water or 19 hPa = 19 mbar) and the performance observed is slightly lower, namely that the piercing of the bed appears after only 2.3 h of adsorption.

Example 4

  A test is carried out under operating conditions identical to that of Example 2, that is to say with a space velocity of 5,000, but using 98.7% dealuminated silica-alumina as adsorbent. This silica-alumina is in the form of spheres whose diameter is between 2 and 4 mm. The results obtained show an unexpected but non-negligible adsorption of the two hydrocarbons, but also a very strong adsorption of water. Piercing takes place after 1.2 hrs instead of 4.5 hrs as observed on the dealuminated zeolite. At the time of drilling, the adsorbent captured 1% of hydrocarbon solvents and 35% of water. Such an adsorbent could advantageously be used for the simultaneous adsorption of the compounds

  volatile organic and water vapor.

  Example 5 An adsorption reactor such as that shown in Figure 2b is used as the adsorption reactor, which for large capacities can be arranged as shown in the figure.

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  2C in a parallel or cylindrical enclosure. The walls of the bed are formed by a rigid stainless steel fabric with a mesh equal to 1.4 mm and made up of wires with a diameter of 1 mm, giving the fabric good rigidity. In addition, each element comprises a rigid frame preventing any deformation of the adsorption volume under the effect of gas speeds or temperature rises during the desorption operation. The thickness of the adsorbent layer is equal to 9 cm; the central collector has a diameter of 10 cm, the height of the element is 1 m, hence a bed containing 55 liters of adsorbent crossed radially by the gas flows to be treated. The adsorbent solid is dealuminated zeolite identical to that recommended in Example 1, but used in its original state without insertion into elementary volumes of any form.

geometric whatever.

  At 30 C, for a space velocity equal to 10 000 h -1, a pressure drop equal to 61 mm of water (6 hPa) is observed with a gas identical to that of Example 1. The drilling of the bed occurs when the content of adsorbed hydrocarbons is 2.1%,

  1 5 after 3 hours of walking or operating.

  Desorption carried out with a stream of air heated to 220 C operates in 35 mm

  and restores the adsorbent treated to its original properties.

  A bed of the same volume (55 1) and the same thickness (9 cm) but parallelipiped gives a pressure drop of 50 mm of water and slightly higher performance

  2 0 in terms of quantity of hydrocarbons adsorbed before drilling (10.3%).

Example 6

  The gas of Example 1 is treated, but using as activated mass activated carbon inserted in conical microvolumes identical to those of Example 1. The bed of conical volumes has the same dimensions as those of Example 2 5 1 The operating conditions in terms of temperature and pressure are the same, but the space speed is reduced to 4,000 h'l. The carbon used is in the form of particles having the following characteristics: average diameter: 1.2 mm; specific surface: 937 m2 / g; unfilled filling density: 0.22. Under these conditions, the drilling of the bed takes place after 23 hours of operation and the retention of hydrocarbons from the

  Coal is 35% based on its own weight.

  Desorption is carried out by circulating a gas of combustion gas over the bed

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  natural cooled to 250 C and very poor in oxygen, at a space speed of 200 h -1 Hydrocarbons are condensed after cooling; the gaseous fraction of the outlet separator is recycled in desorption after mixing with the combustion gas which it participates in cooling to 250 C. The desorption operation can be considered to be completed after 3 h of treatment. A second adsorption operation identical to the first achieves a

  capture of hydrocarbons of 34% by weight relative to the weight of coal used.

  A third, then a fourth adsorption operation preceded by desorption cycles carried out under the conditions described above makes it possible to reach 34.5

  10% and 35.3% retention by weight of adsorbed hydrocarbons.

Example 7:

  A mattress is used, either of aluminum wire or of glass fibers, knitted and then coated with a zeolite powder of the same type as that of the preceding examples, according to an art relating to the techniques already used for the

  1 5 production of catalytic converters.

  The mattress thus obtained is in the form of a cylinder of 0 80 mm and mm in length, the meshes of which are 2 to 3 mmn and the 0 of the wire of approximately 8/10 th of a mm. This adsorption mass contains 92 G of faujasite with an Si / AI atomic ratio>

, as in example 1.

  2 0 The adsorption buffer is placed in a 0 80 mm cylindrical reactor in which air is circulated at room temperature with a flow rate of 30 m3 / h, ie at the speed of 1.7 m / s , loaded with 300 ppm of butyl acetate or approximately 1.2 g / m3 of hydrocarbon.

  The output concentration is observed by chromatography every 3 minutes.

  The hydrocarbon concentration is less than 25 ppm for the first four measurements and rises sharply to 300 ppm for the following, indicating a

  breakthrough of the adsorption front after 12 to 15 minutes of operation.

  The pressure drop measured on such an adsorption "bed" is less than 2 hPa (2 mbar) or less than 100 mm water column per meter of bed length at the speed of

3 0 1.7 m / s.

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  The overall VVH is 30,000 h-1 and the volume adsorption capacity is

  around 9 Kg of butyl acetate per m3 of bed.

  The regeneration was carried out as in the previous examples with air

hot at 150 and against the current.

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Claims (6)

  1. Method for adsorption of hydrocarbons contained in the air entering an adsorption reactor by at least one inlet E and leaving by at least one outlet S, characterized in that an adsorbent is inserted into volumes unitary: cones, tetrahedra, cylinders, making it possible to considerably reduce the pressure losses in the adsorption reactor without harming the expected performance expressed compared to   volume of adsorbent used in the operation.   2. Method according to claim 1 characterized in that the adsorbent chosen 1 0 in particular from granules, solid extrudates, hollow extrudates is inserted into unit volumes traversed radially by the gases to be treated and arranged in a heat exchanger type grille.   3. Method according to one of claims 1 and 2 wherein, the gas from the   desorption is cooled before passing through a separator where the desorbed organic compounds are condensed before recycling the fraction from the adsorption head of this separation.   4. Method according to one of claims 1 to 3 wherein the adsorbent used   is a dealuminated zeolite of the faujasite, mordenite, or silicalite type.   5. Method according to one of claims 1 to 4 wherein the adsorbent used   is dealuminated alumina silica, specially recommended when   to simultaneously extract the water and the hydrocarbons present in the gases to be treated.   6. Method according to one of claims 1 to 5 wherein the adsorbent is a carbon   activated whose regeneration is ensured by circulation of an inert gas (nitrogen or combustion gas) poor in oxygen, the gaseous fraction of the separator, after   2 5 condensation, being used as diluent and coolant for desorption gas.