NO154785B - PROCEDURE FOR THE REMOVAL OF CO 2 AND EVENTLY PRESENT HUMAN H2S FROM A GAS MIXTURE. - Google Patents

Description

The present invention relates to a method for removing C02 and any H2S present from a gas mixture.

In many cases, it is necessary to remove CC>2 and possibly present H2S and other sulfur-containing pollutants such as COS from gas mixtures. The removal of H2S and/or other sulfur-containing contaminants from gas mixtures may be necessary to make these gas mixtures suitable for catalytic transfers using sulfur-sensitive catalysts, or to reduce environmental pollution if such gas mixtures or combustible gases obtained from them are released into the atmosphere.

Examples of CC^-containing gas mixtures from which

I^S and/or other sulphur-containing compounds must generally be removed, are gases obtained by 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 C02 from gas mixtures, either as such from a gas mixture containing no or practically no H2S (e.g. natural gases), or in mixture with H2S in case the latter compound is present in the gas mixture, is often necessary to bring gas mixtures on a desired calorific value and/or to avoid corrosion in transport lines and/or to avoid freezing in cryogenic equipment and/or to avoid the transport of C02 which is of no value in the gas mixture which is ultimately to be used for certain purposes .

In many cases, C02 and any H2S present will be removed from the aforementioned gas mixtures using liquid solvents, which will often be basic. At least part of the carbon dioxide that is present in the gas mixtures will be absorbed in the liquid solvent together with at least part of the possibly present H2S. H2S and C02 (which in this description are also designated as

acid gases) will be removed from the gas mixtures at the pressure the gas mixture has, i.e. in many cases at elevated pressure.

The loaded solvent obtained after absorption of CO2/ and possibly present H2S from the gas mixture must be partially or completely regenerated, during which regeneration any present f^S and CO2 are released.

In the event that H2S is present in considerable quantity in the gas obtained after regeneration of the charged solvent, this gas cannot be released into the atmosphere until at least most of the hydrogen sulphide has been removed from the solvent. H-^S can very conveniently be removed from this gas by transferring it to elemental sulphur, which is separated. The transfer of H2S to elemental sulfur is generally carried out in the art by means of a Claus process in which some of the hydrogen sulphide is oxidized to sulfur dioxide, and sulfur and water are formed by reacting H2S with SO2, with or without the aid of a suitable catalyst. To enable a Claus process to be carried out, the molar percentage content of H2S in a mixture with C02 must be at least approx. 15. If this percentage is between approx. 15 and approx. 40, the Claus process can be carried out by separating out a third of the gas, burning the hydrogen sulphide in it to sulfur dioxide, and then mixing the obtained sulfur dioxide-containing gas with the rest of the hydrogen sulphide-containing gas, after which the Claus reaction can then be carried out at an elevated temperature and preferably in the presence of a catalyst. If the gas contains approx. 40 mol% H2S or more, the Claus process can be carried out by burning the gas with an amount of air sufficient to transfer one-third of the hydrogen sulfide to sulfur dioxide, and then reacting the hydrogen sulfide and sulfur dioxide to form sulfur and water.

In many cases, the gas released during the regeneration of the charged solvent is unsuitable for use in a Claus process as its hydrogen sulphide content is too low and further treatments to increase the hydrogen sulphide content must be carried out with such a gas.

In the event that the gas released during regeneration should have a hydrogen sulfide content that is sufficiently high for use in a Claus process, it may still be important to increase the hydrogen sulfide content thereof, because in the latter case the total amount of gas to be used in

The Claus process, lower, and consequently the installations can be

of smaller size.

The increase in hydrogen sulphide content of the gas that free-

is done during regeneration of the loaded solvent,

can of course be achieved by preferential absorption of the hydrogen sulphide from the gas in a suitable solvent and regeneration of this solvent after loading.

However, such a different absorption process is undesirable in light

of the additional installations that must be built and the additional amount of energy needed for the regeneration of this charged solvent.

A method has now been found for removing CC>2 and possibly present H^S from a gas mixture,

in which the energy required for the regeneration of the solvent loaded with CC^ and possibly present t^S is very low, and in which gases with a high hydrogen sulphide content suitable for use in a Claus process are obtained by means of a simple absorption step from gas mixtures containing H2S.

By means of the invention, a method is provided for the removal of CC^ and possibly present H^ S

from a gas mixture, whereby, as already known in itself from BRD publication no. 22 26 215, the following steps are carried out.-a) the gas mixture is brought into countercurrent contact at elevated pressure with a solvent comprising a physical absorbent ;

b) the resulting charged solvent is flashed at least once upon depressurization to a higher pressure

than the total partial pressure of the CC>2 and H2S present in the charged solvent at it

prevailing temperature;

c) the charged solvent obtained in step b) is flashed at least once upon depressurization to a pressure

which is lower than the total partial pressure of that C02

and H2S present in the loaded solvent at the prevailing temperature, so that a semi-lean solvent is obtained.

The new and characteristic feature of the method is that a solvent is used which, in addition to the physical absorbent, includes a tertiary amine.

Acid gases are capable of reacting with tertiary amines. Very suitable tertiary amines are aliphatic, especially those containing at least one hydroxyalkyl group per molecule. Examples are triethanolamine, trippropanolamine, triisopropanolamine, ethyldiethanolamine, dimethylethanolamine, diethylethanolamine. Methyldiethanolamine is preferred.

A physical absorbent is a compound in which acidic gases are soluble, without reacting with the compound.

Very suitable physical absorbents are sulfolane and substituted sulfolanes, alcohols with 1-5 carbon atoms per molecule (e.g. methanol), tetraethylene glycol dimethyl ether, N-methyl-pyrrolidone, alkylated carboxylic acid amides (e.g. dimethyl formamide). Sulfolane is preferred. The word "sulfolane" denotes the compound "tetrahydrothiophene-1,1-dioxyd".

The content of tertiary amine and physical absorbent (and water if present) in the solvent can vary within wide limits. Very suitably, the solvent contains from 10 to 60% by weight of tertiary amine, preferably methyldiethanolamine, from 15 to 55% by weight of physical absorbent, preferably sulfolane, and from 5 to 5% by weight of water.

It is essential that the solvent used in the present method contains a tertiary amine and a physical absorbent. In modified methods which only differ from the present method in that the solvent contains a secondary and/or a primary amine instead of a tertiary amine or contains a tertiary amine but no physical absorbent, less CO 2 is released in "flashing" step c ), and consequently, when H2S is present in

the original gas mixture, the semi-lean solvent obtained by the partial regeneration in step c) contains H2S and CO2 in a lower mole ratio H2S:CO2 than that obtained by the present method. Moreover, requiring total regeneration (which is generally carried out by steam stripping) of the said semi-lean solvent, in the case of above-

standing modified processes, more steam and gives a mixture of CO 2 and H 2 S in a less favorable molar ratio for a Claus process than in the case of the present process.

A modified process that differs from the invention in that the solvent comprises only one or more physical absorbents, as amines are not present, in many cases requires more solvent and more absorption trays in an absorption column used in step a) to obtain the same amount of absorbed acid gases than the present method. Moreover, more non-acidic gases are absorbed in the solvents comprising only one or more physical absorbents than in the solvents used in the present method, which acid gases are released during the "flashing" in step b). In case a solvent comprising only one or more physical absorbents is used, this amount of non-acidic gases becomes so large that it is not attractive to use the gases as fuel (which can be done with the non-acidic gases released in stages

b) in the present method), and consequently these non-acidic gases again need to be pressurized (by means of

expensive compressors) before recycling to step a).

Bringing the gas mixture into contact with the solvent in step a) is carried out at elevated pressure, by which is meant a pressure of at least 5, and in particular at least 10 bar. Pressures in the range from 20 to 100 bar are well suited.

The contact between the gas mixture and the solvent is most conveniently achieved in a contact zone, e.g. an absorption column with from 15 to 80 contact plates, such as valve plates, bubble cap plates, splash plates etc. Surprisingly enough, it has been shown that by using the solvent in the present method, H2S can be practically removed from the gas mixture used as feed while regulating the amount of C02 that remains in the purified gas. This regulation can be achieved by regulating the solvent circulation, i.e. the ratio between the amount of solvent fed to the extraction zone and the amount of gas mixture fed to it. In the event that no or almost no H 2 S is present in the gas mixture, the amount of CC 2 removed therefrom can also be regulated by solvent circulation. If desired, the carbon dioxide can be removed to a very large extent. The solvent circulation can be further reduced, if desired,

by removing charged solvent at an intermediate point from the contact zone at the lower part thereof, cooling the removed charged solvent outside, and reintroducing it to the lower part of the contact zone for further contact with the gas mixture to be purified, e.g. as described in British Patent No. 1,589,231.

The temperature during the contact of the gas mixture and the solvent in step a) can vary between wide limits. Temperatures in the range from 15 to 110°C are well suited, while temperatures in the range from 20 to 80°C are preferred.

In step a), all or most of any COS present is removed from the gas mixture.

The loaded solvent obtained in step a) contains CO2, optionally I^S and, in general, amounts of dissolved non-acidic constituents from the gas mixture to be purified, e.g. hydrocarbons and/or hydrogen and/or carbon monoxide. These non-acidic gases must be removed at least partially from the charged solvent by "flashing" in step b) to a pressure higher than the total partial pressure of the acid gases present in the charged solvent. In this way, only very small amounts of acidic gases are released from the solvent together with the non-acidic gases, e.g. hydrocarbons and/or hydrogen and/or carbon monoxide. If desired, the gas mixture obtained from the "flashing" in step b) can be recycled to step a), but to avoid recompression this gas is preferably used for other purposes, e.g. as fuel gas (if desired after removing all or part of the I^S present, e.g. by bringing this gas mixture into contact with a small amount of lean solvent). Non-acidic gases must be removed from the charged solvent before this solvent is "flashed" to a pressure lower than the total partial pressure of the acid gases, because otherwise the hydrocarbons and/or hydrogen and/or carbon monoxide would be released together with a considerable amount of acid gases. As these acid gases or combustion gases obtained from them are in many cases to be released into the atmosphere, the hydrocarbons and/or the hydrogen and/or the carbon monoxide would be released or burned at the same time, which would be a waste of these valuable compounds.

Although the loaded solvent can be flashed in step b) several times, each time at a lower pressure, in most cases the majority of dissolved non-acidic constituents will be removed in the first flash step, and for this reason it is preferred to subject it to loaded solvent flashing once in step b).

The charged solvent obtained in step b),

which besides acid gases contain only small amounts of other dissolved compounds, flashing is subjected in step c) to a pressure below the total partial pressure of the acid gases in the loaded solvent at the prevailing temperature.

It has been found that in the method according to the invention

(in which a solvent comprising a tertiary amine and a physical absorbent is used) the amount of which is released is much higher than in modified methods which differ from the present method only 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 the event that the charged solvent also contains P^S, the gas released after depressurization in

step c\ a much higher molar ratio of C02 to H2S than the molar ratio of these gases originally present in the loaded solvent. It is advantageous to heat the loaded solvent, i.e. to a temperature in the range from 45 to 110°C, before or during flashing in step c), because in that case the molar ratio of CC>2 to H2S is increased in the gases that are released after depressurization, further. The above modified process in which a solvent is used which contains a secondary or primary amine and a physical absorbent or a solvent

solvent comprising a tertiary amine and no physical absorbent, gives a much lower molar ratio of CO 2 to H 2 S in the gas released after depressurization. Because the gas liberated in step c) has a higher molar ratio of C02 to H2S than that in the original loaded solvent, the molar ratio of H2S to C02 present in the solvent after the pressure release in step c) is higher than the original.

As with each pressure relief in step c) the molar ratio of U^S to CC>2 in the remaining solvent is increased, it may be advantageous to subject the loaded solvent to flashing in step c) at least twice, each time to a lower pressure or at a higher temperature in case the original gas mixture contains H2S. In general, the pressure of the charged solvent after the pressure is lifted in step c) will be approx. atmospheric.

In step c) a large amount of CC^r is released and consequently the charged solvent is actually regenerated to a considerable extent and gives semi-lean solvent. In the event that the original gas mixture was essentially free of H2S, the semi-lean solvent obtained in step c) will contain CO2 as the only acid gas and is preferably used as such at least partially as a solvent in step a); in many cases the amount of CO 2 present in the semi-lean solvent obtained in step c) will be so low that it is preferable to use this semi-lean solvent as the only solvent in step a). If desired, some or all of the semi-lean solvent obtained in step c) can be completely regenerated (e.g. by stripping with steam) and used as solvent in step a). in the event that fully regenerated solvent and semi-lean solvent are both used as solvents in step a), the former is preferably introduced into the contact zone at a point further from the inlet of the gas mixture than the semi-lean solvent.

In the event that the original gas mixture contained not only CO2, but also H2S, the acid gases released during the flashing in step c) will in many cases contain such small amounts of E2S that they can be released into the atmosphere after combustion. If desired or necessary, the I^S present in the gases liberated during the flashing in step c) may be removed therefrom by contacting these gases with a solvent under conditions favoring the preferential removal of H^S over CO 2 . For such removal, e.g. a mixture containing an amine and possibly a physical absorbent is suitably used, in particular the solvent to be used according to the present invention. In order to achieve a high selectivity for the removal of H 2 S, the contacting is very suitably carried out in a column

with less than 20 contact plates and at high gas velocities, e.g. as described in British Patent 1,362,384.

In case H 2 S was present in the initial gas mixture, the semi-lean solvent obtained in step c) contains H 2 S and CO 2 in a high molar ratio. In view of the high content of H2S, this semi-lean solvent is not suitable for use as a solvent in step a), and the acid gases present therein must be removed. The acid gases are removed from said semi-lean solvent by total regeneration to obtain lean solvent. The regeneration is very suitably carried out by heating in a regeneration column (eg to a temperature in the range from 80 to 160°C), which heating is preferably carried out by stripping with steam.

The gas obtained during this regeneration has such a content of H2S that it can be suitably used in a Claus process for the production of sulphur.

The lean solvent obtained after regeneration can very suitably be reused in step a), and also to be brought into contact with the flashed gas from step b) or c),

if desired.

It will be clear that in order to keep the amount of energy needed in the process as low as possible, it is advantageous to carry out heat exchange of the process streams where this is appropriate.

Example 1

10,000 kmol/h of a gas mixture (composition:

80% by volume methane, 5% by volume ethane, 3% by volume propane, 1% by volume butane, 1% by volume H2S and 10% by volume C02) was introduced into the bottom of an absorption column containing 30 valve stages, at a temperature of 40°C and a pressure of 50 bar. This gas mixture is brought into countercurrent contact with 300 m 3 /h of a lean solvent consisting of methyldiethanolamine (50% by weight), sulfolane (25% by weight) and water (25% by weight). Purified gas leaves the top of the absorption column in an amount of

9511.4 kmol/h; this gas contains 644 kmol/h C02 and less than 4 volume parts per million (ppm) of H„S.

The loaded solvent (300 m 3 /h) is removed from the bottom of the absorption column. It contains 99.7 kmol/h H2S and 356 kmol/h C02. This charged solvent is flashed to a pressure of 15 bar at a temperature of 69.2°C. The gas that is flashed off (45 kmol/h) contains 1.4 kmol/h H2S; 10.7 kmol/h C02, and the rest consists of hydrocarbons.

The loaded solvent obtained after this first flashing contains 98.6 kmol/h H2S and 3445 kmol/h C02.

It is heated by heat exchange with lean solvent and flashed to a pressure of 1.3 bar at a temperature of 70°C. The gas released during this second flashing

(293.6 kmol/h), consists of 35.6 kmol/h H2S and 258 kmol/h C02. It is brought into countercurrent contact with 14 8 m^/h of lean solvent in another absorption column with 13 valve trays at a temperature of 40°C and a pressure of 1.1 bar, yielding 212 kmol/h of a gas consisting of C02 containing 300 ppm H2S. The loaded solvent obtained in the latter absorption column is regenerated together with the semi-lean solvent obtained after the second flashing. This semi-lean solvent contains 63 kmol/h H2S and 87 kmol/h C02. The regeneration is carried out by stripping with steam and produces a gas consisting of 98.6 kmol/h H2S and 133 kmol/h C02, which gas is well suited for use in a Claus process. The lean solvent obtained after regeneration (448 m 3 /h) is partly (148 m 3 /h) recycled to the second absorption column and partly (300 m 3 /h) (after

heat exchange with the loaded solvent from the first flashing) used as lean solvent in the absorption column.

Example 2

10,000 kmol/h gas mixture (composition 90.65 vol.% methane and 9.35 vol.% C02) is introduced into the bottom of an absorption ion column with 20 valve trays at a temperature of 35 C and a pressure of 91 bar. This gas mixture is brought into countercurrent contact with 844 m 3 /h of a solvent consisting of methyldiethanolamine (50% by weight), sulfolane (25% by weight) and water

(25% by weight). This solvent is a semi-lean solvent; it contains 1374 kmol/h C02 and no methane. The gas leaving the top of the absorption column

(9069 kmol/h), consists of 98 volume% methane, and the rest is C09^.

The charged 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 charged solvent is flashed at a pressure of 24 bar and a temperature of 51°C. The gas powered by (204 kmol/h) consists of 159 kmol/h methane and 45 kmol/h CO,,. The loaded solvent obtained after this first flashing

(844 m 3/h), contains 2086 kmol/h C02 and 15 kmol/h methane.

It is heated and flashed to a pressure of 1.3 bar and a temperature of 40°C. The gas released during this second flashing (727 kmol/h) consists of 15 kmol/h methane and 712 kmol/h C0^~. The semi-weak solvent obtained in the second flashing (844 m/h) contains 1374 kmol/h C02 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.

Comparison experiment

For comparison, the same method as described in example 2 was carried out with a solvent consisting of diisopropanolamine (50% by weight), sulfolane (25% by weight) and water (25% by weight) (not according to the invention). For the removal of the same amount of C02 from the feed gas, and the application

of unregenerate semi-lean solvent in absorption

step, a solvent circulation of approx. 5 times as high as that required by the method according to up-

the invention described in example 2. In addition, approx. 5 times as much methane per hour in the absorption column and is released in the first flashing. The amount of methane released in this first flashing is so high that, for economic reasons, this gas must be recompressed and recycled, necessitating the installation of expensive compressors.

Claims (14)

1. Procedure for removing C02 and any present from a gas mixture, where a) the gas mixture is brought into countercurrent contact at elevated pressure with a solvent comprising a physical absorbent; b) the charged solvent obtained is flashed at least once upon depressurization to a pressure higher than the total partial pressure of the present CC>2 and H2S in the charged solvent at the prevailing temperature; c) the charged solvent obtained in step b) is flashed at least once upon depressurization to a pressure lower than the total partial pressure of the CO 2 and H 2 S present in the charged solvent at the prevailing temperature, so as to obtain a semi-lean solvent, characterized in that a solvent is used in addition to the physical absorbent comprises a tertiary amine. 2. Method according to claim 1, characterized in that a solvent containing water is used. 3. Method according to claim 1 or 2, characterized in that an aliphatic tertiary amine containing at least one hydroxyalkyl group is used as the tertiary amine for use in the solvent. 4. Method according to claim 3, characterized in that methyldiethanolamine is used as tertiary amine. 5.. Method according to claims 1-4, characterized in that tetrahydrothio-phen-1,~ 1-dioxyd is used as physical absorbent in the solvent. 6. Method according to claims 1-5, characterized in that a solvent containing methyldiethanolamine, tetrahydro-thiophene-1,1-dioxide and water is used. 7. Method according to claim 6, characterized in that a solvent is used which contains from 10 to 60% by weight of methyldiethanolamine, from 15 to 55% by weight of tetrahydrothiophene-1,1-dioxyd and from 5 to 35% by weight of water. 8. Method according to claims 1-7, characterized in that in step a) the gas mixture is brought into contact with the solvent in a contact zone which has from 15 to 80 contact layers. 9. Method according to claims 1-8, characterized in that step a) is carried out at a pressure of 20 to 100 bar. 10. Method according to claims 1-9, characterized in that step a) is carried out at a contact temperature of 20 to 80°C. 11. Method according to claims 1 - 10, characterized in that the charged solvent in step c) is flashed to atmospheric pressure. 12. Method according to claims 1-11, characterized in that the semi-lean solvent obtained in step c) is used, after all or part of it has been completely regenerated, as a solvent in step a). 13. Method according to claim 12, characterized in that the semi-lean solvent obtained in step c) is regenerated by stripping with steam before it is used as a solvent in step a). 14. Method according to claim 12 or 13, where a gas mixture is used which is essentially free of H^S, characterized in that the semi-lean solvent obtained in step c) is used as the only solvent in step a ).