NZ200951A

NZ200951A - A process for the removal of co2 and, if present,h2s from a gas mixture

Info

Publication number: NZ200951A
Application number: NZ200951A
Authority: NZ
Inventors: M W Mcewan; Der Pas-Toornstra H M Van
Original assignee: Shell Int Research
Priority date: 1981-06-15
Filing date: 1982-06-14
Publication date: 1985-02-28
Also published as: IT8221849A0; PL236913A1; DE3222281C2; ES513052A0; DK162192C; DE3222281A1; NL193746B; KR840000263A; DK162192B; CA1205276A; BE893386A; NL8202061A; AU546704B2; NL193746C; NO154785C; JPS57209626A; DD202129A5; DK266382A; SU1577685A3; NO821968L

Description

<div class="application article clearfix" id="description"> New Zealand Paient Spedficaiion for Paient Number £00951 2 009 5 t Priority Births): . .'SV.fc;?.' Cow^fet® Specification Filed: ft.1 a IV .. 28FEB 1985 &j8I0: t • • ia^7... p.Q, Journal, ftio: • 82 NEW ZEALAND PATENTS ACT, 1953 No.: Date: COMPLETE SPECIFICATION PROCESS FOR THE REMOVAL OF C02 AND, IF PRESENT, H2S FROM A GAS MIXTURE Vj We, SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ B.V., Carel van Bylandtlaan 30, The Hague, the Netherlands, a Netherlands Company hereby declare the invention for which K/ we pray that a patent may be granted to x&e/us, and the method by which it is to be performed, to be particularly described in and by the following statement: - - 1 - (followed by page la) la The invention relates to a process for the removal of CO2 and, if present, from a gas mixture. In many cases it is necessary to remove CC>2 and, if present, and other sulphur-containing impurities 5 such as COS from gas mixtures. The removal of HgS and/or other sulphur-containing impurities from gas mixtures may be necessary in order to render these gas mixtures suitable for catalytic conversions using sulphur-sensitive catalysts, or to reduce environmental 10 pollution if the said gas mixtures or combustion gases obtained therefrom are discharged to the atmosphere. Examples of CO^-containing gas mixtures from which HgS and/or other sulphur-containing compounds generally have to be removed are gases obtained by 15 partial combustion, or complete or partial gasification, of oil and coal, refinery gases, town gas, natural gas, coke-oven gas, water gas, propane and propene. Removal of CO2 from gas mixtures, either as such 20 from a gas mixture which contains no or virtually no ?009 5 1 H2S (e.g. natural gases), or in admixture with H2S in case the latter compound is present in the gas mixture, is often necessary to bring the gas mixtures on a desired calorific value and/or to avoid corrosion in 5 transport lines and/or to avoid freezing in cryogenic equipment and/or to avoid transport of CC^ which is of no value in the gas mixture ultimately to be used for a certain purpose. In many cases the CC^ and, if present the HpS, 10 will be removed from the said gas mixtures using liquid solvents, which will often be basic. At least part of the CO2 present in the gas mixtures will be absorbed in the liquid solvent together with at least part of the E^S, if present. The f^S and CO2 (which 15 in this specification are also indicated as acid gases) will be removed from the said gas mixtures at the pressure of the gas mixture concerned, i.e. in many cases at elevated pressure. The loaded solvent obtained after absorption of 20 CO2 and, if present, I^S from the gas mixture is to be regenerated partly or totally, during which regeneration H2S, if present, and CO2 are set free. In case HgS is present in an appreciable content in the gas obtained after regeneration of the loaded 25 solvent, this gas cannot be discharged to the atmosphere before at least most of the HgS has been 3 removed therefrom. The H.2S is very suitably removed from this gas by converting it to elemental sulphur, which is separated off. The conversion of H2S into elemental sulphur is generally carried out in the art 5 by means of a Claus process in which some of the H2S is oxidized to S02, and sulphur and water are formed by reaction of H2S with S02, with or without the assistance of a suitable catalyst. In order to be able to carry out a Claus process, the molar percentage of h2S in a mixture with C02 must, be at least about 15. If this percentage is between about 15 and about 40, the Claus process can be carried out by separating one third part of the gas, combusting the H2S therein to S02, and subsequently mixing the resultant S02~con-15 taining gas with the balance of the H2S-containing gas, after which the Claus reaction can be further carried out at elevated temperature and preferably in the presence of a catalyst. In case the gas contains about 40$ mol H2S or more, the Claus process 20 can be carried out by combusting the gas with a quantity of air which is sufficient to convert one-third part of the H2S into S02, and subsequently reacting the H2S and S02 to form sulphur and water. In many instances the gas set free during re-25 generation of the loaded solvent is unsuitable for use in a Claus process, the H2S content thereof being too 1 low and. further processes for increasing the H2S content are to be carried out with such a gas. In case the gas set free during regeneration should have an H2S content which is sufficiently high 5 for use in a Claus process it may nevertheless be of importance to increase the H2S content thereof, because in the latter case the total amount of gas to be used in the Claus process is lower, and accordingly the installations can be of smaller size. 10 The increase in H2S content of the gas set free during regeneration of the loaded solvent can, of course, be achieved by preferential absorption of the H2S from that gas in a suitable solvent, and regeneration of that solvent after loading. However, such a 15 second absorption process is unattractive in view of the extra installations to be built and the extra amount of energy needed for regeneration of that loaded solvent. The invention provides a process for the re-20 moval of C02, and if present, HgS from a gas mixture, in which process the energy needed for the regeneration of the solvent loaded with C02 and, if present, H2S, is very low, and in which process gases with high H2S contents suitable to be used in a Claus 25 process are obtained with a single absorption step from gas mixtures containing f^S. ') r> r •0 200951 Accordingly, the invention provides a process for the removal of C02, and, if present, H2S from a gas mixture, which process is characterized in that: a) the gas mixture is contacted at elevated pres-5 sure countercurrently with a solvent which comprises a tertiary amine and a physical absorbent; b) the loaded solvent obtained is flashed at least once by pressure release to a pressure which is above the total partial pressure of the C02 and 10 H2S present in the loaded solvent at the pre vailing temperature; c) the loaded solvent obtained in step b) is flashed at least once by pressure release to a pressure which is below the total partial pressure of the 15 C02 and H2S present in the loaded solvent at the prevailing temperature, and semi-lean solvent obtained in step c) is, optionally after all or part thereof has been totally regenerated, used as solvent in step a). 20 The solvent comprises a tertiary amine, a physical absorbent and preferably water. Acid gases are able to react with tertiary amines. Very suitable tertiary amines are aliphatic, in particular those which contain at least one hydroxy- 25 alkyl group per molecule. Examples are triethanolamine, tripropanolamine, triisopropanolamine, ethy 1 diethar^o.1--amine, dimethylethanolamine, diethylethanolamirie. ^\\ ■\ 'Qctv. Preference is given to methyldiethanolamine. A physical absorbent is a compound in which acid gases are soluble, but without undergoing a reaction therewith. "Very suitable physical absorbents are 5 sulfolane and substituted sulfolanes, alcohols with 1-5 carbon atoms per molecule (e.g. methanol), tetra-ethylene glycol dimethyl ether, N-methylpyrrolidone, alkylated carboxylic acid amides (e.g. dimethylform-amide). Preference is given to sulfolane. The word 10 "sulfolane" denotes the compound "tetrahydrothiophene 1,1-dioxide". The contents of tertiary amine and physical absorbent (and, if present, water) in the solvent may vary between wide limits. Very suitably the solvent 15 contains in the range of from 10 to 60%w of tertiary amine, preferably methyldiethanolamine, in the range of from 15 to 55$w of physical absorbent, preferably sulfolane, and in the range of from 5 to 35#w of water. It is essential that the solvent used in the 20 process according to the invention comprises a tertiary amine and a physical absorbent. In modified processes solely differing from the process according to the invention in that the solvent contains a secondary and/or a primary amine instead of a tertiary amine or 25 contains a tertiary amine but no physical absorbent, less CO2 is set free in the flashing in step c), and ^ ^ (0s' r-' ' ' V Y Y I 7 accordingly, in case was present in the original gas mixture, the semi-lean solvent obtained in the partial regeneration in step c) contains. K^S and CC^ in a lower molar ratio than obtained with the process 5 according to the invention. Moreover, total regeneration (which in general is carried out by stripping with steam) of the said semi-lean solvent requires in the case of the above modified processes more steam and yields a mixture of CC>2 and I^S in a less favourable 10 molar ratio for a Claus process than in the case of the process according to the invention. A modified process differing from that according to the invention in that the solvent comprises one or more physical absorbents only, amines being absent, 15 requires in many cases more solvent and more absorption trays in an absorption column used in step a) to achieve the same amount of acid gases absorbed than the process according to the invention. Moreover, more non-acid gases are absorbed in solvents which com-20 prise one or more physical absorbents only than in the solvents used in the process according to the invention, which non-acid gases are set free during the flashing in step b). In case a solvent is used which comprises one or more physical absorbents only, this 25 amount of non-acid gases is so great that it is not attractive to use it as fuel (as can be done with the 2 009 5 1 8 non-acid gases, set free in step b) in the process, according to the invention, and accordingly these non-acid gases need repressurizing (with the aid of capital intensive compressors) prior to recycling to 5 step a). The contacting of the gas mixture with the solvent in step a) is carried out at elevated pressure, which is considered to be a pressure of at least 5S in particular of at least 10 bar. Pressures in the 10 range of from 20 to 100 bar are very suitable. The contacting of the gas mixture with the solvent is very suitably carried out in a contacting zone, e.g. an absorption column which comprises in the range of from 15 to 80 contacting layers, such as 15 valve trays, bubble cap trays, baffles and the like. It has surprisingly been found that by using the solvent in the process according to the invention the I^S can substantially be removed from the gas mixture used as feed while regulating the amount of CO2 which 20 is left in the purified gas. This regulation can be achieved by regulating the solvent circulation, i.e. the ratio of solvent fed to the extracting zone and the amount of gas mixture fed thereto. In case no or hardly no is present in the gas mixture, the 25 amount of CO2 removed therefrom can also be regulated by the solvent circulation. If desired, 9 the CO2 can be removed to a very great extent. The solvent circulation can still be further reduced, if desired, by removing loaded solvent at an interm&diate point from the contacting zone at the lower part there-5 of, externally cooling the removed loaded solvent, and reintroducing it to the lower part of the contacting zone for further contacting of the gas mixture to be purified, e.g. as'described in British patent specification 1,589,231. 10 The temperature during the contacting of the gas mixture and the solvent in step a) may vary between wide limits. Temperatures in the range of from 15 to 110°C are very suitable, temperatures in the range of from 20 to 80°C are preferred. 15 In step a) all or the greater part of COS, if present, is removed from the gas mixture. The loaded solvent obtained from step a) contains CO2, H2S (if any) and, in general, amounts of dissolved non-acid components from the gas mixture to be 20 purified, e.g. hydrocarbons and/or hydrogen and/or carbon monoxide. These non-acid gases are to be removed at least partially from the loaded solvent by flashing in step b) to a pressure which is higher than the total partial pressure of the acid gases present 25 in the loaded solvent. In this way only very small amounts of acid gases are released from the solvent «7) (7=5! ^ © - <r //- , • /, / i 10 together with, the non-acid gases, e.g. hydrocarbons and/or hydrogen and/or carbon monoxide. If desired, the gas mixture obtained from the flashing in step b) i may be recirculated to step a), but in order to avoid 5 recompressing this gas mixture is preferably used for any other purpose, e.g. as fuel gas (if desired after removal of all or part of the H2S present, e.g. by contacting the said gas mixture with a small amount of lean solvent). Non-acid gases have to be removed 10 from the loaded solvent before this solvent is flashed to a pressure which is lower than the total partial pressure of the acid gases, because otherwise the hydrocarbons and/or hydrogen and/or carbon monoxide would be set free together with an appreciable amount 15 of acid gases. As in many cases these acid gases or combustion gases obtained therefrom are to be discharged to the atmosphere, the hydrocarbons and/or hydrogen and/or carbon monoxide would be discharged, or burnt simultaneously, which would be a waste of these valuable 20 compounds. Although the loaded solvent may be flashed in step b) several times, each time at a lower pressure, in most cases the greater part of dissolved non-acid components will be removed in one flash step, and for 25 that reason it is preferred to flash the loaded solvent once in step b). 2 0 0 9 a The loaded solyent obtained in step b) - which besides acid gases contains only small amounts of other dissolved compounds: - is. flashed in step c) to a pressure below the total partial pressure of the acid gases 5 in the said loaded solvent at the prevailing temperature. It has been found that in the process according to the invention (in which a solvent comprising a tertiary amine and a physical absorbent is used) the amount of CC>2 set free is much higher than in modified 10 processes differing solely from the process according to the invention in that the solvent contains a secondary and/or primary amine instead of a tertiary amine or contains a tertiary amine, but no physical absorbent. In case the loaded solvent also contains 15 H2S the gas set free after the pressure release in step c) has a much higher molar ratio of CO2 to HgS than the molar ratio of these gases originally present in the loaded solvent. It is of advantage to heat the loaded solvent, e.g. to a temperature in the range of 20 from 45 to 110°C before or during flashing in step c), because in that case the molar ratio of CO2 to H2S in the gases set free after the pressure release is still further increased. The above modified process in which a solvent is used which comprises a secondary or 25 primary amine and a physical absorbent or a solvent which comprises a tertiary amine and no physical 12. ab.s'.or.he.nta gives a much lower molar ratio of CO^ to H^S in the gas .s.et free after the preasure release. Because the gas. set free in step c) has a higher molar ratio of CO^ to than that of the original 5 loaded' solvent, the molar ratio of I^S to C0£ present in the solvent remaining after the pressure release in step c) is higher than originally. Because at each pressure release in step c) the molar ratio of f^S to CO2 in the remaining solvent 10 is increased, it may be of advantage to flash the loaded solvent in step c) at least twice, each time to a lower pressure or at a higher temperature in . case the original gas mixture contains HgS. In general, the pressure of the loaded solvent after the pressure 15 release in step c) will be about atmospheric. In step c) a large amount of CO2 is set free, and accordingly the loaded solvent is in fact regenerated to an appreciable extent, yielding semi-lean solvent. In case the original gas mixture was substantially 20 free from HgS the semi-lean solvent obtained in step c) contains C02 as the only acid gas, and it is prefer- -■entially used as such at least partially as solvent in step a); in many cases the amount of CO2 present in the semi-lean solvent obtained in step c) will be 2 5 so low that it is preferred to use this semi-lean solvent as the only solvent in step a). If desired, 200951 13 part or all of the semi-lean solvent obtained in step c) can be totally regenerated (e.g. by stripping with steam) and used as solvent in step a). In case totally regenerated solvent and semi-lean solvent are 5 both used as solvents in step a) the former is preferentially introduced into the contacting zone at a point further removed from the inlet of the gas mixture than the semi-lean solvent. 10 contain C0£ but also I^S the acid gases set free during the flashing in step c) in a number of cases will contain so small amounts of HgS that they can be discharged to the atmosphere after incineration. If desired or needed the present in the gases set 15 free during the flashing in step c) may be removed therefrom by contacting these gases with a solvent under conditions which favour preferential removal of HjS over CC^. For such removal, e.g. a mixture comprising an amine and optionally a physical absorbent, 20 may suitably be used, in particular the solvent to be used according to the present invention. In order to achieve a high selectivity for HgS removal the contacting is very suitably carried out in a column with less than 20 contact trays and at high gas velocities, e.g. as described in New Zealand patent specification 164284. In case the original gas mixture did not only 009 5 1 14 In cas;e H^S -was present in the original gas. mixture the semi-lean solvent, .ob.tained in step c) contains and CO2 in a high molar ratio. In view pf the high content of HgS this semi-lean solvent is not suitable 5 to be used as solvent in step a) and the acid gases present therein have to be removed therefrom. The acid gases are removed from the said semi-lean solvent by total regeneration to yield lean solvent. The regeneration is very suitably carried out by heating in a 10 regeneration column (e.g., to a temperature in the range of from 80 to 160°C), which heating is preferably carried out by stripping with steam. The gas obtained during this regeneration has such an i^S content that it can be suitably used in a 15 Claus process for the preparation of sulphur. The lean solvent obtained after regeneration can very suitably be reused in step a), and also for contacting the flashed gas from step b) or c) if desired. It will be clear that in order to keep the 20 amount of energy needed in the process as low as possible, it is of advantage to carry out heat exchange of process streams where appropriate. EXAMPLE 1 10,000 kmol/h of a gas mixture (composition 80$v 25 methane, 5$v ethane, 3$v propane, l$v butane, l%v HgS and 10%v CO2) are introduced into the bottom of 15 an absorption column containing 30 valve traya, at a temperature of- 40°C and a pressure of 50. bar. This 3 gas mixture is countercurrently contacted with 300 m /h of a lean solvent consisting of methyldiethanolamine 5 (50$w), sulfolane (25$w) and water (25%w). Purified gas leaves the top of the absorption column in an amount of 9511-4 kmol/h; this gas contains 644 kmol/h CO2 and less than 4 volume parts per million (ppm) of h2s. 10 The loaded solvent (300 m^/h) is removed from the bottom of the absorption column; it contains 99-7 kmol/h HgS and 356 kmol/h CO2• This loaded solvent is flashed to a pressure of 15 bar at a temperature of 69.2°C. The gas flashed off (45 kmol/h) contains 1.4 15 kmol/h HgS, 10.7 kmol/h CO2, the balance consists of hydrocarbons. The loaded solvent obtained after this first flash contains 98.6 kmol/h I^S and 345 kmol/h CO2. It is heated by heat exchange with lean solvent, and flashed to a pressure of 1.3 bar at a temperature 20 of 70°C. The gas set free during this second flash (293*6 kmol/h) consists of 35 - 6 kmol/h and ;258 kmol/h C02. It is contacted countercurrently with 148 m^/h lean solvent in a second absorption column with 13 valve trays at a temperature of 40°C and a 25 pressure of 1.1 bar, yielding 212 kmol/h of a gas which consists of CO2 containing 300 ppm HgS. ;2 009 51 ;16 ;The loaded solvent, .obtained in the 'last-mentioned absorption column is regenerated together with the semi-lean solvent obtained after the second flash. This semi-lean solvent contains 63 kmol/h H2S and 5 87 kmol/h C02. The regeneration is carried out by stripping with steam, yielding a gas which consists of 98.6 kmol/h H2S and 133 kmol/h C02, which gas is very suitable to be used in a Claus process. The lean solvent obtained after regeneration (448 m^/h) is ;-z ;10 partly (148 m /h) recycled to the second absorption column and partly (300 m /h) (after heat exchange with the loaded solvent from the first flash) used as lean solvent in the absoiption column. ;EXAMPLE 2 ;15 10,000 kmol/h of a gas mixture (composition ;90.65$v methane and 9-35$v C02) are introduced into the bottom of an absorption column containing 20 valve trays at a temperature of 35°C and a pressure of 91 bar. This gas mixture is countercurrently con-20 tacted with 844 m'Vh of a solvent which consists of methyldiethanolamine (50$w), sulfolane (25#w) and water (25%w). This solvent is semi-lean solvent; it contains 1374 kmol/h C02 and no methane. The gas leaving the top of the absorption column (9069 25 kmol/h) consists for 98$v of methane, the balance being C02. ;? 009 ;17 ;The loaded solvent (844 m^/h) is removed from the bottom of the absorption column; it contains 2131 kmol/h CO2 and 174 kmol/h methane and has a temperature of 53°C. This loaded solvent is flashed 5 to a pressure of 24 bar and a temperature of 51°C* The gas flashed off (204 kmol/h) consists of 159 kmol/h methane and 45 kmol/h CO2. The loaded solvent obtained after this first flash (844 m^/h) contains 2086 kmol/h CO2 and 15 kmol/h methane. It is heated 10 and flashed to a pressure of 1.3 bar and a temperature of 40°C. The gas set free during this second flash (727 kmol/h) consists of 15 kmol/h methane and 712 kmol/h CO2. The semi-lean solvent obtained in the second flash (844 m^/h) contains 1374 kmol/h CO2 15 and no methane, and is introduced at the top of the absorption column as solvent for the gas mixture to be purified as described above. Comparative Experiment For comparison the same process as described in 20 Example 2 is carried out with a solvent consisting of diisopropanolamine (50$w), sulfolane (25#w) and water (25$w) (not according to the invention). For the removal of the same amount of CO2 from the feed gas, and the use of non-regenerated semi-lean solvent in the 25 absorption step, a solvent circulation about five times as high as that needed in the process according </div>

Claims

<div class="application article clearfix printTableText" id="claims"> 200951 18 to the invention described in Example 2 is needed. Moreover, about five times as much methane is absorbed per hour in the absorption column, and released in the first flash; the amount of methane set free in 5 this first flash is so high that for economical reasons this gas has to be recompressed and recycled, which makes the installation of expensive compressors necessary. \) o- c 200951 19 WHAT WE CLAIM IS:

1. A process for the removal of CO^ and, if present, H^S from a gas mixture, characterized in that:

a) the gas mixture is contacted at elevated pressure countercurrently with a solvent which comprises

5 a tertiary amine and a physical absorbent;

b) the loaded solvent obtained is flashed at least once by pressure release to a pressure which is above the total partial pressure of the CO2 and H2S present in the loaded solvent at the

10 prevailing temperature;

c) the loaded solvent obtained in step b) is flashed at least once by pressure release to a pressure which is below the total partial pressure of the CO2 and H2S present in the loaded solvent at the

15 prevailing temperature,

and semi-lean solvent obtained in step c) is, optionally after all or part thereof has been totally regenerated, used as solvent in step a).

2. A -process according to claim 1, characterized in

20 that the solvent contains water.

3. A process according to claim 1 or 2, characterized in that the tertiary amine is aliphatic and contains at least one hydroxyalkyl group per molecule,.^.

2 0 0 9

20

4. A process according to claim 3> characterized in that the tertiary amine is methyldiethanolamine.

5. A process according to any one of the preceding claims, characterized in that the physical absorbent

5 is tetrahydrothiophene 1,1-dioxide.

6. A process according to any one of the preceding claims, characterized in that the solvent contains methyldiethanolamine, tetrahydrothiophene 1,1-dioxide and water.

10 7« A process according to claim 6, characterized in that the solvent contains in the range of from 10 to 60%w of methyldiethanolamine, in the range of from 15 to 55$w of tetrahydrothiophene 1,1-dioxide and in the range of from 5 to 35$w of water.

15 8. A process according to any one of the preceding claims, characterized in that in step a) the gas mixture is contacted with the solvent in a contacting zone which comprises in the range of from 15 to 80 contacting layers.

20 9' A process according to any one of the preceding claims, characterized in that step a) is carried out at a pressure in the range of from 20 to 100 bar.

10. A process according to any one of the preceding claims, characterized in that step a) is carried out at 25 a contacting temperature in the range of from 20 to 80°C.

I

21

11. A process according to any one of the preceding claims, characterized in that the loaded solvent in step c) is flashed to atmospheric pressure.

12. A process according to any one of the preceding 5 claims, characterized in that semi-lean solvent obtained in step c) is regenerated by stripping with steam before being used as solvent in step a).

13. A process according to any one of claims 1-11, characterized in that the gas mixture is substantially

10 free of f^S and the semi-lean solvent obtained in step c) is used as the only solvent in step a).

14. A process as claimed in claim 1, substantially as hereinbefore described with reference to the Examples 1 and 2.

15 15. Gas mixtures from which CO2 and, if present, H2S have been removed by means of a process as claimed in any one of the preceding claims.

DATED THJS DAY OF 1903.

A. J. PARK & SON

AGENTS FOR TKc APPLICANTS

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