FR2846959A1 - Storing liquid sulfur comprises contacting it with a hot inert gas selected from nitrogen, carbon dioxide and argon - Google Patents

Abstract

Storing liquid sulfur comprises contacting it with an inert gas selected from nitrogen, carbon dioxide and argon at a temperature of at least 120 degreesC.

Description

- 1 The present invention relates to a tank inerting process

of

storage of liquid sulfur.

  The refining and natural gas industry processes large quantities of acid gases containing hydrogen sulfide (H2S). These acid gases come mainly from the production units of petrol and diesel cuts (catalytic and hydrocracker cracking), but also from hydrotreatment units such as hydrodesulfurization. The sulfur content is an essential property of fuels 10 and is subject to increasingly strict regulations, which leads refiners to

  having to deal with more and more acid gases.

  This treatment is carried out in sulfur recovery units of the Claus type. In these units, the sulfur contained in the acid gases is recovered in the form of liquid elemental sulfur in condensers. This liquid sulfur is then directed to a sulfur pit and / or stored in large tanks from 500 to 5000

  tonnes before its re-shipment to industries consuming sulfur, such as sulfuric acid production plants. Liquid sulfur is stored either in metal tanks or in pits lined with antacid concrete. It is maintained in liquid form at an optimal viscosity by heating in a range from 120 to 20 1400C.

  In storage tanks, liquid sulfur can emit different types of gas. It first releases hydrogen sulfide absorbed during the condensation step. The quantities of hydrogen sulfide emitted into storage tanks can be significant and reach levels dangerous for the safety and health of operators (hydrogen sulfide is toxic, even at low content, and the mixture H2S- air is explosive (explosive limit value = 3.4%) In addition, it has been observed that sulfur storage emits sulfur dioxide. This sulfur dioxide comes from the oxidation of elemental sulfur in the liquid phase or gaseous by the oxygen in the air, which is often present in the gaseous air of the storage tanks. The evacuation of sulfur dioxide and hydrogen sulphide in the atmosphere by the vents of the storage tanks causes nuisance, including odors, and is sometimes made impossible by regulations

severe environmental conditions.

  if To avoid these problems, some manufacturers recycle the gases from the vents in the storage tanks to their sulfur recovery unit (either to the combustion furnace or to the incinerator). However, this recycling of gases from the vents to the incinerator only displaces the problem and in any case contributes to the increase in SO2 emissions to the chimney. To reduce the hydrogen sulfide content of liquid sulfur to very low levels (up to 10 ppm), and therefore the emissions of this hydrogen sulfide, one solution is to degas the liquid sulfur, as indicated for example in US 4, 612,020 or US 5,080,695. However, this degassing step 10 does not reduce the quantities of SO 2 that can be produced in the storage tank.

  by interaction of gaseous oxygen with liquid sulfur.

  To limit the formation of SO 2, it has been proposed to sweep the free space of the liquid sulfur storage tank with a stream of cold inert gas, generally nitrogen. This solution has the advantage of stopping the formation of sulfur dioxide since it eliminates the entry of oxygen molecules into the tank. However, this inerting does not solve the third common problem associated with the storage of liquid sulfur and which is due to the formation of iron sulfides. Indeed, due to the use of steel tanks, iron sulphides can appear on the cold upper parts of the storage tanks. When opening tanks - for example, during maintenance phases or when drawing off liquid sulfur - these pyrophoric iron sulphides can be driven by gravity from the cold parts to the warmer parts of the tanks. - ignite and cause the explosion of residual sulfur vapors. Some sulfur storage tanks in a refinery have already suffered explosions due to this problem. To avoid this

  problem, it can be proposed to treat the internal walls of steel storage tanks with an organic coating: this solution seems to eliminate the problems of corrosion and the risks of formation of iron sulfide. However, this treatment is expensive and is difficult to apply to existing tanks. In addition, it does not solve the other emission problems of hydrogen sulfide and sulfur dioxide.

  The aim of the present invention is to propose a new process for the storage of liquid sulfur making it possible to limit the emission of sulfur dioxide gases and the

formation of iron sulfides.

  To this end, the invention relates to a process for storing liquid sulfur in which the liquid sulfur is brought into contact with an inerting gas comprising at least one inert gas chosen from nitrogen, carbon dioxide and argon and presenting a

temperature of at least 1200C.

  The invention therefore consists in bringing the liquid sulfur into contact with an inert gas. This contacting is generally done by covering the surface of the liquid sulfur with the inert gas. The surface of the liquid sulfur can be swept by the inerting gas, for example by introducing a flow of the inerting gas into the upper part of the liquid sulfur storage tanks. The inerting gas can be brought into contact with liquid sulfur in two ways. According to a first mode, the liquid sulfur is contained in a tank and the inerting gas is injected into the tank with a constant flow rate (so-called dynamic mode). According to a second mode, the liquid sulfur is contained in a tank and the gas headspace of the tank made up of the inerting gas is static

(so-called static mode).

  According to an essential characteristic of the invention, the inerting gas must

  have a temperature of at least 120'C, preferably at least 140'C. A temperature of at most 170 ° C is recommended. Generally, the inerting gas is heated before being introduced into the liquid sulfur storage tank.

  The introduction is generally made in the upper part of the storage tank 20 for liquid sulfur.

  According to the preferred embodiment of the invention, the inert gas of the inerting gas is nitrogen.

  The inerting gas used generally comprises at least 79% by volume of at least

  minus an inert gas, preferably 92 to 100%, even more preferably 92 to 98%. When the inert gas is nitrogen, its complement in the inerting gas can be any type of gas such as oxygen, argon or CO2. It is preferably oxygen.

  It has been observed that the best results have been obtained for an inerting gas comprising 92 to 98% by volume of nitrogen and 2 to 8% by volume of oxygen. This type of inerting gas can be produced by a process of separation of gas from air by permeation using a membrane. It may for example be the FLOXAL process marketed by Air Liquide. The sweeping by the inerting gas and its maintenance of pressure in the storage tank can be obtained using a

  VESTAL control system marketed by Air Liquide.

  The method according to the invention is particularly suitable for the methods of

  storage of liquid sulfur in steel tanks.

  Preferably, the liquid sulfur is subjected to a preliminary degassing step before being stored according to the method of the invention so as to limit the formation of

hydrogen sulfides.

  The inerting gas can be brought into contact with liquid sulfur in two modes. 5 According to a first mode, the liquid sulfur is contained in a tank and the inerting gas is injected into the tank with a constant flow rate (so-called dynamic mode). According to a second mode, the liquid sulfur is contained in a tank and the gaseous sky of the

  tank consisting of the inerting gas is static (so-called static mode).

  The method according to the invention can be implemented for tanks for transporting liquid sulfur and during the transport of this liquid sulfur. Depending on the mode of transport used, a source of inerting gas can also be transported and allow continuous sweeping of liquid sulfur. It is for example possible to place a process for separating gas from air from air using a membrane on a boat. If the source of inerting gas cannot be transported (on a train 15 for example), then the tank is placed under an atmosphere of inerting gas during the

  duration of transport (so-called static mode).

  One of the advantages of the process according to the invention is that the inerting gas can

  be released into the atmosphere through the degassing vents of the storage tank.

EXAMPLE

  Different inerting gases comprising nitrogen as an inert gas and having different oxygen contents have been introduced into the upper space of a steel tank containing previously degassed liquid sulfur. The gas overhead temperature was set at different values and controlled by the use of thermocouples placed in the gas overhead of the tank. A flushing gas flow corresponding to a residence time in the tank of 180 minutes was applied throughout the duration of the dynamic tests. A steel coupon was suspended in this upper space for 48 hours in order to study the formation of iron sulfide on its surface and the corrosion rate. The S02 content of the atmosphere above the

  liquid sulfur was also measured.

  The results are collated in the table below.

  Sky temperature 80 C 140 C gas Nitrogen content of 79% vol 95% vol 79% vol 95% vol inerting gas Moles of FeS formed on the 2.26 10- 3 1.43 10-3 2.27 10 -6 0.57 10-6 steel coupon Corrosion rate (10-3 c704 526 1 2 (10 'cm / year) SO2 content after 24 h (mode - - 150 ppm 0 ppm static) SO2 after 48 h (mode - - 600 ppm 20 ppm static) SO2 content after 24 h - 20 ppm 0 ppm (dynamic mode) SO2 content after 48 h -10 ppm 0 ppm (dynamic mode) In order to determine the number of moles of FeS formed on the steel coupons, the steel coupons are soaked in 6M hydrochloric acid: each mole of FeS produces one mole of H2S. Then, the moles of H2S formed are trapped using a solution of AgNO3, forming Ag2S. The amount of Ag2S obtained is determined by gravimetry, a NH3.5M solution being used to wash the precipitate. The SO2 content has was measured by sampling using a syringe in the

  gas tank and the composition of the sample was analyzed using a gas chromatograph.

  It can be seen that the formation of iron sulphide is greatly lowered when the temperature of the gas overhead is maintained at 140 C. In addition, it is found that the

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  Reducing the oxygen content in the gaseous air also has a beneficial effect on the corrosion rate.

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Claims (7)

  1. A method for storing liquid sulfur in which the liquid sulfur is brought into contact with an inerting gas comprising at least one inert gas, chosen from nitrogen, carbon dioxide and argon and having a temperature d 'at least 120C.   2. Method according to one of the preceding claims, characterized in that the gas inert is nitrogen.   3. Method according to the preceding claim, characterized in that the inerting gas   includes at least 79% by volume of inert gas.   4. Method according to the preceding claim, characterized in that the inerting gas comprises 92 to 100% by volume of inert gas.   5. Method according to one of the preceding claims, characterized in that the sulfur   liquid is subjected to a preliminary degassing step using an inert gas.   6. Method according to one of the preceding claims, characterized in that the sulfur   liquid is contained in a tank and in that the inerting gas is injected into the tank with constant flow.   7. Method according to one of claims 1 to 5, characterized in that the liquid sulfur 25 is contained in a tank and in that the gaseous headspace of the tank consisting of the gas inerting is static.