FR2918386A1 - Mercaptans e.g. methyl mercaptans, eliminating method, involves extracting mercaptans by setting in contact liquid to be treated with regenerative aqueous solution such as deacidifcation solvent, and regenerating solvent - Google Patents

Abstract

The method involves extracting mercaptans by setting in contact an effluent e.g. liquid to be treated (11), with a regenerative aqueous solution e.g. deacidification solvent (12) having amine, to realize an acid-base reaction with mercaptans present in the effluent to be treated, where acid dissociation constant of the amine is greater than or equal to 10. The deacidification solvent is regenerated. Hydrocarbonated fractions (14) obtained from the extraction of the mercaptans are washed.

Description

The present invention relates to the elimination of mercaptans in

  gaseous and liquid effluents, and in particular the effluents from the natural gas processing lines.

  In the case of natural gas, three main treatment operations are considered: deacidification, dehydration and degassing. The first step, which is deacidification, aims to eliminate acidic compounds such as carbon dioxide (CO2), hydrogen sulphide (H2S), carbon oxysulfide (COS) and mercaptans, mainly carbon dioxide. methyl mercaptan, ethyl mercaptan and propyl mercaptans. The generally accepted specifications for the deacidified gas are 2% CO2, 4 ppm H2S, and 20 to 50 ppm volume of total sulfur. The dehydration step then controls the water content of the deacidified gas against transport specifications. Finally, the degassing stage of natural gas makes it possible to guarantee the hydrocarbon dew point of the natural gas, again depending on transport specifications. Deacidification is therefore often done first, for various reasons. For safety reasons, it is best to remove toxic acid gases such as H2S in the first stage of the process chain. Another advantage of first of all achieving the deacidification is to avoid the pollution of the various unit operations by these acidic compounds, in particular the dehydration section and the condensation and separation section of the heavier hydrocarbons. Acidic compounds such as CO2, H 2 S and COS are generally removed by washing the gas with a solvent allowing these compounds to be absorbed preferentially to the hydrocarbons. This absorption is preferably promoted by a chemical reaction. The use of an aqueous alkanolamine solution is the most common solution. At the end of the deacidification step, the gas meets the specifications as to the content of CO2, typically less than 2 mol%, and of H2S, typically 4 molar ppm.

  The case of sulfur impurities such as mercaptans is, on the other hand, significantly different. Indeed, there is no or little reaction between mercaptans and alkanolamines conventionally used. The absorption is then limited to the physical solubilization of these compounds in the solvent.

  Thus, only a portion of the light mercaptans, especially methyl mercaptan, is removed during this deacidification step, by physical solubilization in the aqueous solvent. Heavier mercaptans, such as ethyl, propyl and butyl mercaptans, or containing more than four carbon atoms, are not sufficiently soluble in an aqueous solution, nor acidic enough to react significantly in the aqueous solution with the amines conventionally used. , and therefore remain largely in the gas. Most of these acid gas absorption processes therefore have a mercaptan extraction efficiency of less than 50% (Kohl and Nielsen, R. Gas Purification, 5th Edition, Gulf Publishing Company, Houston, 1997). However, it is necessary to cite some technical solutions using solvents with a strong physical component that make it possible to achieve under specific conditions 90% elimination of sulfur compounds at the cost of significant energy consumption, particularly because of the importance of solvent flow required by these performances.

  The dehydration step, following the deacidification step, can be carried out, for example, by a glycol process, in particular by using triethylene glycol (TEG), which makes it possible to lower the water content of the gas to a value close to 60 molar ppm. However, mercaptans are practically not eliminated during this stage.

  A final fractionation step finally makes it possible to separate the treated gas into its various constituents so as to valorize each of the cuts produced: Cl section, C2 section, or C1 + C2 section, C3 section, C4 section, and C5 + heavier section, optionally still separated into various complementary fractions. The majority of the sulfur compounds will concentrate in the liquid phases which will have to be further processed in order to meet the sulfur specifications generally imposed.

  A first alternative for gaseous effluents is the removal of the mercaptans upstream of the fractionation step. Complementary treatment to lower the residual mercaptan content may consist of an adsorption step, for example using a 13X zeolite, to carry out the desulfurization, the pore sizes of these zeolites permitting selective adsorption of the mercaptans. The processes used are then methods of the TSA (Thermal Swing Adsorption) type for which the adsorption is carried out at ambient or moderate temperature, typically between 20 and 60 C. The desorption is carried out at high temperature, typically between 200 and 350.degree. C, under purge gas, which may be in particular a part of the purified gas, generally between 5 and 20% of the feed gas flow, or the light fraction Cl, C2 or C1 + C2 of the purified gas after splitting. The desorption gas, containing a large quantity of mercaptans, must then be treated before being recycled, for example by treatment with a basic solution (sodium hydroxide or potassium hydroxide) with the well-known constraints of management, treatment and elimination of formed salts and aqueous phases. The recycling is preferably carried out upstream of the glycol dehydration unit and / or upstream of the adsorption purification units. The pressure is either kept substantially constant throughout the cycle, or lowered during the regeneration phase so as to promote regeneration. At the end of this adsorption purification step, the gas is at the total sulfur specifications. The disadvantage of these adsorption sieves lies partly in the production of a gaseous effluent rich in mercaptans, which in turn requires treatment.

  Another alternative for the gaseous effluents consists of carrying out a treatment on the liquid cuts resulting from the fractionation as a function of the distribution of the mercaptans in the different sections obtained. This operation is usually a caustic wash (Kohl and Nielsen, R. Gas Purification, 5th Edition, Gulf Publishing Company, Houston, 1997). The countercurrent contact in a tray column of the hydrocarbon feedstock with a concentrated sodium hydroxide solution, between 10 and 20% by weight, ensures the elimination of all the sulfur compounds such as COS and mercaptans. The mercaptans react with sodium hydroxide to give mercaptides which are then oxidized in the presence of a catalyst present in the solvent to give disulfides while regenerating the caustic solution. The latter are then separated by decantation of the aqueous phase. In the case of liquid effluents, this problem of elimination of mercaptans also exists, particularly in the case of the softening of hydrocarbons. Many patents already offer solutions for the elimination of these mercaptans in the liquid phase.

  US Pat. No. 4,029,589 recommends mixing the hydrocarbon fraction with halides (iodides and bromides) and complexing agents such as amines or carboxylic acids. US Patents 4,207,173 and US 4,490,246 use a phthalocyanine-based catalyst in the presence of base and oxygen. The base used is teta- alkylguanidine to convert mercaptans to disulfides. Likewise, US Pat. No. 4,383,916 uses an oxide catalyst in the presence of methanol to eliminate mercaptans. U.S. Patents 4,459,205 and 4,466,906 use a polyaminoalkylpolycarboxylic acid metal complex deposited on an ion exchange resin to convert mercaptans to disulfides. No. 4,514,286 claims the use of peroxides of the hydroxide type and cumene hydroperoxide in the presence of a strong base such as an amine. There is also a method based on a wool burn catalyst described in US Pat. No. 4,794,097 on which cobalt phthalocyanine is deposited. The disadvantage of these various techniques is the production of disulfides, which are generally sent to a hydrotreatment process which then gives back H2S. The management of these disulfides remains problematic. In addition to liquid-liquid extraction techniques, there are other types of processes for the removal of mercaptans from a liquid hydrocarbon phase. One known technique is to treat the liquid hydrocarbon fraction on an adsorbent (Gas Conditioning and Processing, Vo14: Gas Treating and Sulfur Recovery, Fourth Edition, Chapter 4, RN Maddox, DJ Morgan, Campbell Petroleum Series 1998). The operation of this method is very close to the TSA type described above. The liquid charge is sent from bottom to top on an adsorbent bed. This step is carried out at ambient or moderate temperature, typically between 20 and 60 C. Once the bed is saturated with sulfur compounds, the liquid therein is drained. The desorption stage is carried out at a high temperature, typically between 200 and 300 ° C., under the purge from the top to the bottom of a purge gas. At the end of the desorption stage, the bed is cooled using the purified liquid. The treatment of the desorption gas has the same disadvantages as that described above for the treatment upstream of the fractionation. The present invention therefore aims to overcome one or more of the disadvantages of the prior art by providing a method for removing mercaptans gaseous or liquid effluents by the use of a regenerable solvent. The invention relates to a process for the elimination of mercaptans contained in the gaseous and liquid effluents, comprising: a step of extracting the mercaptans by contacting the effluent to be treated with a regenerable aqueous solution comprising at least one amine, the the pKa is greater than or equal to 10, capable of carrying out an acid-base reaction with the mercaptans present in the effluent to be treated, and at least one step of regeneration of the aqueous solution. The pKa of the amine is generally greater than or equal to 10.6 and less than or equal to 13.5, the aqueous solution possibly comprising one or more other amines. The total amine concentration of the aqueous solution is between 2 mol and 5. The process may also comprise a step of washing the hydrocarbon cuts obtained at the end of the mercaptan extraction step.

  The step or the regeneration steps are by relaxation and / or thermal regeneration. The thermal regeneration step is carried out at a pressure of between 1 bar and 3 bars.

  Other features and advantages of the invention will be better understood and will become clear from reading the description given hereinafter with reference to FIG. 1, schematizing a method according to the invention, appended and given as a 'example.

  The present invention provides a novel mercaptan removal solvent for the removal of mercaptans contained in a hydrocarbon effluent liquid or gaseous, having the ability to be regenerated by restoring the mercaptans. The process is based on the use of an aqueous amine solution which is different from the solvents conventionally used for the treatment of gaseous or liquid effluents and in particular natural gas, by the strong basicity of the amine function of the molecule. The extraction solvent according to the invention is therefore an aqueous solution containing one or more compounds comprising one or more nitrogen atoms capable of producing, under the conditions of implementation of the invention, an acid-base reaction of the type RSH + B <=> RS- + BH + with the mercaptans present in the feed to be treated. The reversibility of a reaction of this type will promote the regeneration of the solvent. In addition the use of a strong base directly to capture mercaptans avoids the production of disulfides, unlike commonly used methods. These reactive compounds also have a higher affinity for the aqueous phase than for the charge to be treated. The amine used for mercaptan uptake will have a pKa greater than or equal to 10, and generally greater than or equal to 10.6 and less than or equal to 13.5. The total concentration of amine in the solvent is between 2 mol.F1 and 5 mol.r1.

  By way of example, these reactive compounds may belong to the family of guanidines, amidines, aliphatic amines, cyclic amines, aromatic amines, 1,8-bis (dialkylamino) naphthalenes or multiamines. Examples that may be mentioned include guanidine, pentamethylguanidine, butyltetramethylguanidine, tert-butyltetramethylguanidine, 1,5,7-triazabicyclo [4.4.0] dec-5-ene, 7-methyl-1,5,7-triazabicyclo [4.4. 0] dec-5-ene, 1,4,5,6-tetrahydropyrimidine, N-methyl-1,4,5,6-tetrahydropyrimidine, 2-methyl-2-imidazoline, 1,5-diazabicyclo [ 4.3.0] non-5-ene, 1,8-diazabicyclo [5.4.0] undec-7-ene, 6- (dibutylamino) -1,8-diazabicyclo [5.4.0] undec-7-ene, 4-azabenzimidazole, anabasine, sparteine, piperidine, N-methylpiperidine, hexamethyleneimine, pyrrolidine, 3-pyrroline, 2,5-dimethyl-3-pyrroline, cyclohexylamine, diethylamine, diisopropylamine, dipropylamine, methylisopropylamine, dibutylamine, triethylamine, tripropylamine, diisopropylethylamine, 1,4-butanediamine, N, N-dimethyl-1,4-butanediamine, N, N, N'N'- tetramethyl-1,4-butanediamine, 1,5-pentanediamine, hexamethylenediamine and 1,8-bis (dimethyl mino) naphthalene. The techniques required to achieve the liquid-liquid contact are well known to those skilled in the art. It may be a cascade of mixer-settlers, or more conventionally as illustrated in FIG. 1, of a liquid-liquid contact column implementing the contactor technology (column with trays, bubble column). , disc column, membrane contactor, etc.) most adapted to the properties of the two phases. The contact column (10) is thus fed at the bottom by the liquid (11) to be treated, and at the top by the deacidification solvent (12). The liquid-liquid contact is made under the conditions of availability of the liquid effluent to be treated. For example, in the case of the condensate treatment chain of a natural gas, the liquid effluent is available at a pressure of between 1 and 20 bars, and temperatures between 20 C and 60 C. At the end in this contact step, the mercaptan-loaded solvent (13) is recovered at the bottom of the column, and the hydrocarbon fraction (14) at the top of the column.

  A washing section (40) with water (41) of the hydrocarbon fraction (14) obtained at the end of the deacidification section may optionally be provided to remove the traces (42) of solvent that may have been mechanically entrained at the outlet with the hydrocarbon feed treated. An additional liquid-liquid separation step (50) may also be added as a precaution at the end of this washing step, to ensure the absence of entrainment of an aqueous phase (51) dispersed in the phase hydrocarbon (52). These problems are well known to those skilled in the art who will implement the corresponding techniques. The hydrocarbon solution resulting from these deacidification and washing sections meets the sulfur content specifications: the chemical reaction between the mercaptans and the retained amine will promote the transfer of the sulfur compound into the extraction solvent. . The other acid gases present in the solvent are also captured by the solvent, in particular COS, CO2 and H2S. The solvent, loaded with mercaptans (13), obtained at the end of the deacidification section is then regenerated. Depending on the pressure level of the treated hydrocarbon feedstock, the regeneration of the absorbent solution is optionally carried out by a succession of detents associated with a thermal regeneration of the solution. During any relaxation, a hydrocarbon stream which has been coabsorbed in the solvent with the mercaptan gases is recovered in gaseous form. This stream can be recycled in the process or used as "fuel gas". The most suitable technique is thermal regeneration. In this case, the recovered solvent (13) is reheated in a heat exchanger (20) and sent to the thermal regeneration column (30). The mercaptans (31) and the hot regenerated solvent (32) at the bottom of the column are obtained at the top of the column. The hot solvent (32) obtained at the bottom of regeneration is used to heat the solvent to be regenerated and is returned to the contact column (10). Before being injected into the contact column (10), the solvent (32) is regulated in temperature by a control system (exchanger) (60).

  This thermal regeneration is generally carried out at a pressure of between 1 and 3 bars. The pressure level of the thermal regeneration is essentially conditioned by the maximum temperature supported by the solvent, the pressure setting the temperature of the reboiler of this regeneration section. The mercaptans (31) obtained in the regeneration section by inversion of the reactions carried out in the extraction column between the mercaptans and the reactive compounds of the solvent are thus discharged at the top of the regeneration column. The gaseous effluent obtained is essentially composed of absorbed mercaptans. It is also saturated with water under the conditions of temperature and pressure set by the condenser of the regeneration column. This gaseous effluent may be recycled to another site facility, such as a Claus process or an acid gas reinjection line. Given the presence of acidic impurities such as COS, H2S or CO2, a second regeneration section may optionally be provided. This section will have the objective of reversing the reactions between these acidic impurities and the reactive compounds of the solvent, insofar as the main regeneration section will operate preferentially under the conditions corresponding to the release of mercaptans absorbed in the solvent. This secondary section, also called "reclaimer" or "reclaiming section" by those skilled in the art, will treat a fraction of solvent corresponding in volume to the fraction of reactive compounds neutralized by these acidic impurities. The following example is used to illustrate the process according to the invention and to demonstrate the reactivity of the amines described in the document with respect to the mercaptans. Methyl mercaptan was contacted with an aqueous solution of 1,8-diazabicyclo [5.4.0] undec-7-ene (DBU). This molecule has a pKa at 25 C of 12.8. The aqueous solution used in the experiment has a concentration of 0.04 mol.l-1. 9.42 mmol of methyl mercaptan gas was contacted in a closed reactor with 177.4 ml of aqueous solution, which corresponds to 7.1 mmol of DBU. The evolution of the system is followed until the liquid-vapor equilibrium is reached. Analysis of the phases when the equilibrium is reached shows that 8.64 mmol of mercaptan was absorbed by the solvent, ie a mercury removal rate of 91.7%. The invention, by the reversible reaction between the mercaptans and the solvent, thus makes it possible to obtain a very good mercaptan elimination rate and to circumvent the problem of elimination of the disulfides produced by the technologies conventionally used. The subject of the present invention applies preferentially to the treatment of a liquid hydrocarbon fraction. It can also be applied in the case of a gaseous hydrocarbon fraction, for example a natural gas or an adsorption sieve regeneration gas, but the risks of mechanical entrainment and losses due to volatility must be taken into account to select the reactive compounds of the solvent. They will be a decisive criterion in the selection of the reactive compounds of the solvent.

Claims (7)

  1) Process for removing mercaptans contained in the gaseous and liquid effluents, characterized in that it comprises: a step of extraction (10) of the mercaptans by contacting the effluent (11) to be treated with a regenerable aqueous solution (12) comprising at least one amine whose pKa is greater than or equal to 10, capable of carrying out an acid-base reaction with the mercaptans present in the effluent to be treated, and at least one regeneration stage ( 30) of the aqueous solution (12).   2) Process according to claim 1, characterized in that the pKa of the amine is greater than or equal to 10.6 and less than or equal to 13.5.   3) Method according to one of claims 1 or 2, characterized in that the aqueous solution (12) comprises one or more other amines.   4) Method according to one of claims 1 or 2, characterized in that the total concentration of amine is between 2 mol.r1 and 5 mol.1-1.   5) Method according to one of claims 1 to 4, characterized in that it comprises a step of washing (40) hydrocarbon fractions (14) obtained at the end of the step of extraction (10) mercaptans.   6) Method according to one of claims 1 to 5, characterized in that the regeneration step is by expansion and / or thermal regeneration.   7) Process according to claim 6, characterized in that the step of 30 ° thermal regeneration is carried out at a pressure between 1 bar and 3 bar. 15 25