Patent Application
20130303811
This application claims the benefit of priority to French Patent Application No. 1201362, filed on May 10, 2012, and is incorporated herein by reference in its entirety. The entire disclosures of all applications, patents and publications, cited herein are incorporated by reference herein in their entirety.
The invention relates to a process for treating gas that contains in particular sulphur-containing compounds as impurities to be removed. In particular the process applies to the treatment of gaseous effluent discharged from furnaces of a carbon black production unit.
Various industrial processes exist that emit gaseous effluents containing sulphur. These effluents or residual gases have to be treated before being discharged into the atmosphere in order to reduce the amount of these polluting sulphur-containing compounds to acceptable levels, so as to comply with the increasingly stringent environmental standards.
The sulphur found in industrial residual gases, for example in the gases emitted by coal, petcoke (petroleum coke) or also biomass gasification units, or in the gases emitted by calcination furnaces of carbon black production units, is generally in the form of sulphur dioxide, hydrogen sulphide and carbon sulphides such as carbon disulphide (CS2) and carbonyl sulphide (COS). The carbon sulphides are relatively inert compounds and are therefore difficult to remove efficiently from the gaseous effluent.
Various methods are known in the prior art for removing sulphur. These methods are generally based on the principle of forming hydrogen sulphide (H2S), which is a reactive compound and can easily be removed.
Thus, for example, the document U.S. Pat. No. 5,466,427 discloses a process for treating a residual gas containing sulphur, particularly in the form of carbon sulphides, which consists in contacting the said gas with a catalyst so as to effect a hydrolysis of carbon disulphide (CS2) and carbonyl sulphide (COS) to form hydrogen sulphide. The catalyst used in this document comprises:
As noted by the applicant, the prior art process operates satisfactorily provided that the gas to be treated contains a relatively small amount of unsaturated hydrocarbon products (in particular alkyne and diene type compounds), that is to say in a maximum concentration of 50 ppm, or even 30 ppm by volume.
An object of the invention is to provide a process for treating a gas capable of converting the sulphur-containing compounds present in particular in the form of CS2 and/or COS that is operational even if the feedstock to be treated contains unsaturated hydrocarbon compounds, for example in a concentration greater than 30 ppm by volume.
To this end the process according to the invention comprises the following stages:
It has been found that the stage a) involving hydrogenation of the feedstock, prevents the progressive deactivation, or even an obstruction of the hydrolysis catalyst of stage b), particularly as a result of the formation of polymerisation gums on the surface of the catalysts. In fact, thanks to the pre-treatment of the feedstock in stage a) the unsaturated hydrocarbon compounds are converted by hydrogenation into compounds that are not liable to polymerise and therefore poison or lead to coking or obstruction of the pores of the hydrolysis catalyst in the subsequent stage b), the result being an improved efficiency of the treatment process compared to the prior art.
In the context of the invention the term “unsaturated hydrocarbon compounds” covers in particular alkene and alkyne type compounds and also diene type polyunsaturated compounds.
According to one embodiment the stages a) and b) are carried out in the same reactor, in which two catalyst beds, namely a hydrogenation catalyst and a hydrolysis catalyst, are arranged in succession. The catalyst beds are disposed with respect to one another in the reactor in such a way that the feedstock to be treated comes into contact with the hydrogenation catalyst bed before the hydrolysis catalyst bed.
According to another alternative embodiment, the process employs two specific reactors (i.e. a hydrogenation reactor and a hydrolysis reactor), in which the hydrogenation reactor is installed downstream of the hydrolysis reactor.
According to a preferred embodiment the process according to the invention comprises a treatment stage of the gas leaving the hydrolysis stage, which consists for example in trapping the H2S that is formed or converting the H2S into elementary sulphur.
According to an advantageous embodiment a liquid/gas separation stage of the gas to be treated may be performed before carrying out the hydrogenation stage.
Likewise it is possible to carry out a liquid/gas separation of the gas leaving the stage b) before passing it to an H2S treatment unit.
The Gaseous Feedstock to be Treated
The gas that can be treated by the process according to the invention may be obtained from coal or petcoke or even biomass gasification units or from calcination furnaces of carbon black production units. Typically the gas to be treated may contain COS and/or CS2 in an amount between 10 ppm by volume and 0.5 volume %. The gas thus generally includes COS in an amount that is most often between 10 ppm by volume and 0.3 volume %, CS2 in an amount between 10 ppm by volume and 0.3 volume % and possibly HCN in an amount between 20 ppm by volume and 0.2 volume %. The gas may also contain hydrogen, CO, SO2, CO2, H2S and water.
The gas generally contains unsaturated hydrocarbons in an amount between 30 ppm by volume and 5 volume %, preferably between 0.05 and 3 volume %. Generally the unsaturated hydrocarbon compounds basically include short-chain, typically C2, C3 or C4, hydrocarbon products of the group of alkenes, alkynes and polyunsaturated compounds, such as for example ethylene, acetylene and butadiene.
The Hydrogenation Stage (Stage a):
Within the scope of the invention the hydrogenation may be selective, that is to say it involves only alkynes and diene type polyunsaturated compounds but not mono-olefins. Nevertheless, even though this is not generally necessary, there is no disadvantage in carrying out a total hydrogenation, that is to say hydrogenating all the unsaturated compounds, including mono-olefins, into paraffins.
The hydrogenation catalyst employed in the stage a) comprises a metal chosen from platinum, palladium, nickel and cobalt individually or as a mixture, and deposited on a porous support.
According to a first variant of the process according to the invention, the hydrogenation catalyst comprises platinum, and the content of platinum, expressed as metal, is normally between 0.02 wt. % and 4 wt. % with respect to the total weight of the catalyst. Preferably the content of platinum is between 0.05 and 3 wt. %, more preferably between 0.1 wt. % and 2.5 wt. % with respect to the total weight of the catalyst.
According to a second variant of the process according to the invention, the hydrogenation catalyst comprises palladium and the content of palladium, expressed as metal, is between 0.05 wt. % and 5 wt. % with respect to the total weight of the catalyst. Preferably the content of palladium is between 0.05 and 3 wt. %, more preferably between 0.1 wt. % and 1 wt. % with respect to the total weight of the catalyst.
According to a third variant of the process according to the invention, the hydrogenation catalyst comprises nickel and the content of nickel is generally between 0.5 wt. % and 15 wt. % of nickel oxide with respect to the total weight of the catalyst. Preferably the content of nickel oxide is between 4 wt. % and 12 wt. %, more preferably between 6 wt. % and 10 wt. % with respect to the total weight of the catalyst.
According to a fourth variant of the process according to the invention, the hydrogenation catalyst comprises cobalt and the content of cobalt is generally between 0.5 wt. % and 15 wt. % of cobalt oxide with respect to the total weight of the catalyst. Preferably the content of cobalt oxide is between 1 wt. % and 10 wt. %, more preferably between 2 wt. % and 4 wt. % with respect to the total weight of the catalyst.
According to a particular embodiment the hydrogenation catalyst contains either platinum, or palladium, or nickel, or cobalt, and may also include molybdenum. In this case the molybdenum content, expressed as molybdenum oxide, of the said catalyst is between 1 wt. % and 20 wt. % with respect to the total weight of the catalyst, preferably between 6 wt. % and 18 wt. %, and more preferably between 8 wt. % and 15 wt. %.
The catalyst of stage a) is a catalyst that also comprises a porous support on which are deposited the metal(s) or precursor(s) of the metals or oxides active in hydrogenation. The support may be chosen from aluminas, silicas, titanium oxide, silicon carbide or their mixtures.
The porous support is preferably chosen from alumina, nickel or cobalt aluminate, silica, silica-aluminas, silicon carbon, titanium oxide or their mixtures. Pure alumina or titanium oxide is preferably used.
According to a very preferred variant, the support consists of cubic gamma alumina or delta alumina.
More preferably the hydrogenation catalyst employed in stage a) comprises palladium. According to another preferred embodiment it comprises nickel and molybdenum.
The catalyst according to the invention may be prepared by any means known to the person skilled in the art, and in particular by impregnating metallic elements on the selected support. This impregnation may for example be realised according to the method known to the person skilled in the art by the term dry impregnation, in which exactly the amount of desired elements is introduced in the form of salts that are soluble in the chosen solvent, for example demineralised water, so as to fill as completely as possible the pores of the support. The support thus filled by the solution is preferably dry. The preferred support is alumina or titanium oxide, which may be prepared from any type of precursors and moulding tools known to the person skilled in the art.
After introducing the precursors of the metallic elements, and possibly moulding the catalyst, the latter is subjected to a thermal treatment comprising a drying stage followed by a calcination. The drying is generally carried out in air between 20° C. and 200° C., preferably between 40° C. and 180° C. The calcination is generally performed in air or in dilute oxygen, and the treatment temperature is generally between 200° C. and 550° C., preferably between 300° C. and 500° C.
The Hydrolysis Stage (Stage b):
The hydrogenation catalyst employed in the stage b) is a catalyst that comprises alumina or titanium oxide, preferably titanium oxide. The catalyst according to the invention may also include at least 1 wt. %, preferably between 0.5 wt. % and 10 wt. %, and more preferably between 1 wt. % and 5 wt. %, of at least one sulphate or silicate of an alkali or alkaline earth metal or of a rare earth.
The said alkali metal is preferably chosen from lithium, sodium, potassium and, more preferably, from sodium or potassium. The said alkaline earth metal is preferably chosen from calcium, barium, strontium and magnesium. The rare earth is preferably chosen from lanthanum, cerium, praseodymium or neodymium. Most preferably the rare earth is lanthanum.
More preferably the catalyst contains a silicate or a sulphate of sodium, potassium, calcium or barium. Most preferably it comprises calcium or barium sulphate and, still more preferably, calcium sulphate.
According to a particular embodiment the hydrolysis catalyst also comprises a metal chosen from nickel, cobalt, molybdenum and tungsten.
According to another variant of the process according to the invention, the hydrolysis catalyst used in stage b) comprises 60 wt. % to 99.8 wt. % of titanium oxide or alumina with respect to the weight of the catalyst, and also a metal chosen from nickel, cobalt, molybdenum and tungsten, individually or as a mixture.
According to a preferred variant the hydrolysis catalyst comprises:
According to an even more preferred variant the said hydrolysis catalyst comprises nickel and molybdenum or cobalt and molybdenum supported on a titanium oxide support.
According to another very preferred variant, the hydrolysis catalyst used in stage b) comprises:
Preferably the titanium oxide has a rutile or anatase crystallographic structure.
The catalyst may be prepared by any technical means known to the person skilled in the art, and in particular by impregnating precursors of nickel, cobalt, molybdenum, sulphate or silicate of an alkali metal or alkaline earth metal or of a rare earth on the previously moulded support.
This impregnation may be realised for example according to the method known to the person skilled in the art by the term “dry impregnation”, in which exactly the amount of desired elements is introduced in the form of salts soluble in the chosen solvent, for example demineralised water, so as to fill the pores of the support as completely as possible.
The impregnated support may then be dried, preferably at a temperature between 20° C. and 200° C., more preferably between 40° C. and 180° C. A calcination is then generally carried out in air or in diluted oxygen, and the calcination temperature is generally between 200° C. and 550° C., preferably between 300° C. and 500° C.
These aspects as well as other aspects of the invention will become clear in the following detailed description of particular embodiments of the invention, and with reference to the accompanying drawings, in which:
The figures are not drawn to scale. Generally the same elements are denoted by identical reference numerals in the figures.
With reference to
As shown in
The total amount of hydrogen present in the gas to be treated and possibly added hydrogen is such that the molar ratio between hydrogen and the unsaturated hydrocarbon compounds to be hydrogenated is greater than the stoichiometric amount and is preferably between 1 and 3000 moles per mole and preferably between 300 and 2000 moles per mole.
The hydrogenation stage is generally carried out at a pressure between 0.1 and 5 MPa, preferably between 0.5 and 3 MPa, at a temperature between 100 and 400° C., preferably between 150° C. and 250° C., and a catalyst volume with respect to the amount of gas to be treated with 1 m3 of catalyst for 1000 to 4000 Nm3/h of gas to be treated, i.e. a HSV between 1000 and 4000 h−1.
With reference to
If the water content in the feedstock is not sufficient, then additional water may be introduced via the line 6, so as to carry out the hydrolysis with an excess of water with respect to the hydrolysable molecules (COS, CS2, HCN).
Any afore described hydrolysis catalyst may be used in this embodiment.
The reactions that take place during this stage may be represented by the following conversions:
COS+H2O→CO2+H2S
CS2+2H2O→2H2S+CO2
The hydrolysis stage is typically carried out at a pressure between 0.1 and 5 MPa, preferably between 0.5 and 3 MPa, at a temperature between 100 and 400° C., preferably between 150° C. and 250° C., and a catalyst volume with respect to the amount of gas to be treated of 1 m3 of catalyst for 1000 to 4000 Nm3/h of gas to be treated, i.e. a HSV between 1000 and 4000 h−1.
The hydrolysis is carried out in the presence of an excess of water with respect to the molecules to be hydrolysed. Preferably the reaction is carried out with a molar ratio of water to hydrolysable products of between 5 and 1000 moles per mole, and more preferably between 10 and 500 moles per mole.
The gaseous effluent treated in the hydrolysis reactor 2 is then extracted and led via the line 7 to a heat exchanger 8, for example a cooling tower, so as to cool the treated gas. The treated and cooled gas is transferred by the line 9 to a liquid/gas separator 10. The liquid condensation water is recovered at the bottom of the separator 10, while the gas depleted in H2O and containing H2S is led via the line 11 to a treatment unit 12, which may for example be a unit for trapping H2S or a unit for converting H2S, which for example oxidises H2S to form elementary sulphur:
2H2S+SO2→3S+2H2O
According to the invention the two hydrogenation and hydrolysis reactions of the feedstock to be treated may be carried out in the same reactor comprising a first hydrogenation catalyst bed and a second hydrolysis catalyst bed, the beds being arranged with respect to one another in the reactor so that the gaseous feedstock to be treated comes into contact with the hydrogenation catalyst bed before the hydrolysis catalyst bed, as shown in
With reference to
In order to carry out the hydrogenation of the gaseous feedstock which is introduced via the line 3, an additional supply of hydrogen may possibly be implemented by means of the line 4 situated upstream of the catalyst bed 13. If necessary an internal space separates the catalytic beds 13 and 14, so as to arrange in this space an injection point for adding via the line 6 extra water necessary for the hydrolysis reaction.
The operating conditions used for the two catalytic reactions and described with reference to
In a similar manner to the embodiment of
The third embodiment of the process according to the invention is represented in
In this case, as shown in
As shown in
The Examples 1 (comparative), 2 and 3 (according to the invention) relate to the efficiency of the conversion of the sulphur compounds in the form of H2S of a typical feedstock A whose composition is shown in Table 1. The feedstock A corresponds to a gaseous effluent leaving a carbon black production unit and contains sulphur and nitrogen compounds (COS, CS2 and HCN) and acetylene. Example 1 is carried out according to the process described in the U.S. Pat. No. 5,466,427.
The Examples 2 and 3 are carried out according to the process of the invention, comprising at least one first stage a) for hydrogenating the unsaturated hydrocarbon compounds present in the feedstock A, followed by a stage b) for the catalytic hydrolysis of the sulphur and nitrogen compounds (COS and/or CS2 and HCN) present in the effluent leaving the stage a).
The gaseous feedstock to be treated thus contains a not negligible amount of acetylene of up to 0.3 vol. %.
The feedstock A is passed directly together with water to a reactor for the hydrolysis of the compounds COS and CS2 (reactor dimensions: diameter 2 cm, height 10 cm) containing a catalyst prepared by impregnating a support based on titanium dioxide (TiO2) in the form of extruded cylindrical pellets of diameter 3.2 mm and average length 6.7 mm, on which is deposited nickel and molybdenum. The catalyst has the following composition: 2.5 wt. % of nickel oxide (NiO), 9.0 wt. % of molybdenum trioxide (MoO3) and 88.5% wt. % of titanium dioxide. The weight % are expressed with respect to the total weight of the catalyst.
The operating conditions for the catalytic hydrolysis reaction are as follows:
The composition of the effluent leaving the reactor is analysed (by gaseous phase chromatography). The results obtained after 48 hours' operation are shown in Table 2.
After 48 hours' operation a reduction of the content of CS2 of the order of barely 12% is observed. In contrast, a reduction of the content of COS of 61% is found. Thus, the hydrolysis catalyst exhibits a low hydrolysis activity for carbon disulphide when the reaction is carried out with a gas containing unsaturated organic compounds (in the present case acetylene).
The same feedstock A whose composition was given in Table 1 is first of all passed to a first hydrogenation reactor according to stage a) of the invention. The hydrogenation catalyst used in stage a) consists of 0.28 wt. % of Pd on a support consisting of agglomerated gamma alumina in the form of spheres of diameter 1.7 mm. The stage a) is carried out under the following operating conditions:
The effluent leaving stage a) is analysed after 48 hours' operation according to the method described in Example 1 and has the composition given in Table 3.
Thus, the hydrogenation stage has enabled the concentration of acetylene to be considerably reduced, with the corresponding formation of ethane.
The effluent leaving stage a) is then passed to a second reactor according to stage b) of the invention. The catalyst used in stage b) consists of 91 wt. % of TiO2 and 9 wt. % of CaSO4. The stage b) is carried out under the following operating conditions:
It is found that the hydrogenation stage carried out on the residual gas has a positive effect on the hydrolysis yield of the compound CS2. Thus, a reduction of the CS2 content of the order of 86% is obtained. It may therefore be concluded that a pre-treatment of the gas in order to reduce the concentration of unsaturated organic compounds enables a better hydrolysis activity of carbon disulphide to be maintained.
The feedstock A described in Table 1 is first of all passed to a first reactor according to stage a) of the invention. The hydrogenation catalyst used in stage a) consists of 0.28 wt. % of Pd on a support consisting of agglomerated gamma alumina in the form of spheres of diameter 1.7 mm. The stage a) is carried out under the following operating conditions:
The composition of the effluent leaving stage a) is analysed after 48 hours' operation according to the method described in Example 1. The composition of the effluent leaving stage a) is given in Table 5.
The prior hydrogenation treatment of the feedstock gas thus enables acetylene to be converted into ethane and consequently enables the content of acetylene to be reduced to a value of less than 0.01 vol. %.
The effluent leaving stage a) is then passed to and treated in a second reactor according to stage b) of the invention. The catalyst used in stage b) is that described in Example 1. The catalyst has the following composition (expressed in terms of the total weight of the catalyst): 2.5 wt. % of nickel oxide (NiO), 9.0 wt. % of molybdenum trioxide (MoO3) and 88.5 wt. % of titanium dioxide.
The operating conditions of this stage b) are as follows:
The composition of the effluent leaving stage b) is analysed according to the method described in Example 1. The results are shown in Table 6.
The analyses show that the prior hydrogenation stage intended to saturate the unsaturated organic compounds enables the catalytic performance of the catalyst to be maintained, in particular the hydrolysis activity with regard to carbon sulphides.
Here too, a reduction in the content of CS2 of the order of 87% and of COS of the order of 61% is obtained.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1201362 | May 2012 | FR | national |
