DD202129A5 - METHOD FOR REMOVING CARBON DIOXIDE AND, IF ANY, SULFUR HYDROGEN FROM A GAS MIXTURE - Google Patents

Abstract

A method for removing carbon dioxide and, if present, hydrogen sulfide from a gas mixture in which a) the gas mixture is contacted under elevated pressure and in countercurrent with a solvent containing a tertiary amine and a physical absorbent; b) the resulting laden solvent is flashed to a pressure which is higher than the total partial pressure of the carbon dioxide and hydrogen sulphide present in the loaded solvent at the prevailing temperature; c) the laden solvent obtained in step b) is flashed to a pressure which is below the total partial pressure of the carbon dioxide and hydrogen sulphide present in the loaded solvent at the prevailing temperature, and the semi-regenerated solvent obtained in step c) - optionally after all or a part of it has been completely regenerated - is used as a solvent in step a).

Description

"Process for the removal of carbon dioxide and, if present, hydrogen sulphide from a gas mixture"

claimed

Priority: 15 June 19 81 - United Kingdom - No. 81182 88

Field of application of the invention;

The present invention relates to a process for removing CO and, if present, hydrogen sulfide from a gas mixture.

Characteristic of the known technical solutions In many cases it is necessary to remove from gas mixtures carbon dioxide and, if present, hydrogen sulfide as well as other sulfur-containing impurities, for example carbon oxysulfide. The removal of hydrogen sulphide and / or other sulphurous impurities from gas mixtures may be required to render these gas mixtures suitable for catalytic conversions using sulfur sensitive catalysts or, in order to reduce environmental pollution, if the resulting gas mixtures or incineration

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gases are released into the atmosphere.

Examples of CO ₂ -containing gas mixtures from which H "S and / or other sulfur-containing compounds in general must be removed are gases obtained by the 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.

The removal of carbon dioxide from gas mixtures, either as such from a gas mixture containing no or virtually no hydrogen sulphide (for example natural gases) or mixed with hydrogen sulphide in the event that the latter compound is present in the gas mixture, is often required in order to bring the gas mixtures to a desired calorific value and / or to avoid corrosion in delivery lines and / or freezing in refrigeration devices and / or transport of worthless carbon dioxide in the gas mixture eventually to be used for certain purposes.

In many cases, the carbon dioxide and, if present, the hydrogen sulfide are removed from said gas mixtures using liquid solvents, which are often basic. At least a portion of the carbon dioxide present in the gas mixtures is absorbed in the liquid solvent along with at least a portion of the optional hydrogen sulfide present. The hydrogen sulphide and carbon dioxide (which are also referred to herein as acidic gases) are removed from said gas mixtures at the pressure of the gas mixture concerned, that is, in many cases at elevated pressure.

The laden solvent obtained after the absorption of CO_ and, if present, H_S from the gas mixture must be partially or completely regenerated, releasing H "S, if present, and CO 2.

If hydrogen sulphide is present in the product after regeneration of the

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If the solvent gas obtained is charged in substantial quantities, then this gas can not be released into the atmosphere until at least the major part of the hydrogen sulphide has been removed therefrom. This hydrogen sulfide is very conveniently removed from this gas by converting it to elemental sulfur, which is separated. The conversion of hydrogen sulphide into elemental sulfur is generally carried out according to the prior art by means of the Claus process in which some of the hydrogen sulphide is oxidized to sulfur dioxide and forms by the reaction of H ₂ S with SO ₂ sulfur and water with or without the aid of a suitable catalyst. In order to perform a Claus process, the molar percentage of hydrogen sulfide in a mixture with CO_ must be at least about 15. If the percentage is between about 15 and about 40, the Claus process may be carried out by separating one-third of the gas, burning the hydrogen sulphide contained therein to sulfur dioxide, and then recovering the resulting SO ₂ -containing gas with the residue of H ₂ S-containing gas mixed, whereupon the Claus reaction can be carried out further at elevated temperature and preferably in the presence of a catalyst. If the gas contains about 40 mol% H_S or above, the Claus process can be carried out by burning the gas with an amount of air sufficient to convert one third of the hydrogen sulfide to sulfur dioxide, and then the hydrogen sulfide and sulfur dioxide react with each other to form sulfur and water.

In many cases, the gas released during the regeneration of the loaded solvent is not suitable for use in a Claus process because the hydrogen sulfide content is too low and such process requires further hydrogen sulfide content raising procedures.

If the gas released during regeneration has a hydrogen sulfide content that is sufficiently high for use in a Claus process, it may nonetheless be

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-4-essential that the hydrogen sulfide content is increased, since in the latter case the total amount of gas to be used in the Claus process is lower and accordingly the devices used can be made smaller.

The increase of the hydrogen sulfide content of the gas released during the regeneration of the loaded solvent may, of course, be effected by preferentially absorbing the hydrogen sulfide from this gas in a suitable solvent, and regenerating this solvent after loading. However, in view of the additional equipment required and the energy required to regenerate this loaded solvent, such a second absorption process is unattractive.

Object of the invention

The invention provides a method of removing carbon dioxide and, if present, hydrogen sulfide from a gas mixture in which the amount of energy required to regenerate the solvent laden with carbon dioxide and, if present, hydrogen sulfide is very low and in which high-gaseous gases are present Hydrogen sulfide content, which are suitable for use in a Claus process, can be obtained in a single absorption step from H ^ S-containing gas mixtures

Explanation of the essence of the invention

Accordingly, the present invention provides a process for removing carbon dioxide and, if present, hydrogen sulfide from a gas mixture characterized in that

a) the gas mixture is contacted under elevated pressure and in countercurrent with a solvent containing a tertiary amine and a physical absorbent;

b) subjecting the resulting laden solvent to flash or short-path distillation at least once while depressurizing to a pressure which is greater than the total carbon black pressure and hydrogen sulfide present in the loaded solvent at the prevailing temperature;

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c) subjecting the loaded solvent obtained in step b) to flash or short-path distillation at least once while depressurizing to a pressure which is below the total partial pressure of the carbon dioxide and hydrogen sulphide present in the loaded solvent at the prevailing temperature; in step c) obtained solvent - optionally after all or a part of which has been completely regenerated - is used as a solvent in step a).

The solvent includes a tertiary amine, a physical absorbent, and preferably water.

Acid gases can react with tertiary amines.

These tertiary amines are very suitably aliphatic, especially those having at least one hydroxyalkyl group per molecule. Examples of these are triethanolamine, tripropanolamine, triisopropanolamine, ethyldiethanolamine, dimethylethanolamine and diethylethanolamine.

The preference is given to methyldiathanolamine.

A physical absorbent is a compound in which acidic gases are soluble but without reacting with it. Very suitable physical absorbents are sulfolane and substituted sulfolanes, alcohols having 1 to 5 carbon atoms per molecule (for example methanol), tetraethylene glycol dimethyl ether, N-methyl pyrrolidone, alkylated carbamides (for example dimethylformamide). The preference is given to sulfolane. The word "sulfolane" refers to the compound "tetrahydrothiophene-1,1-dioxide".

The content of tertiary amine and physical absorbent (and, if present, water) in the solvent can vary within wide limits. Very suitably this contains

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Solvent 10 to 60 weight percent tertiary amine, preferably methyl diethanolamine, 15 to 55 weight percent physical absorbent, preferably sulfolane, and 5 to 35 weight percent water.

It is essential that the solvent used in the process of the invention comprises a tertiary amine and a physical absorbent. In other procedures, the

differ from the method according to the invention only in that the solvent has a secondary and / or primary amine instead of a tertiary amine or a tertiary amine but no physical Absorbtionsmittel, in the short path distillation according to step c) less carbon dioxide is released, and accordingly contains the in the partial regeneration in step c) obtained half-regenerated solvent in the event that was present in the original gas mixture hydrogen sulfide, the hydrogen sulfide and carbon dioxide in a lower molar ratio than in the inventive method. Moreover, in the abovementioned process, the complete regeneration of the semi-regenerated solvent (which is generally done by stripping with steam) requires more steam and, in terms of a Claus process, provides a mixture of carbon dioxide and hydrogen sulfide in one less favorable molar ratio than in the case of the method according to the present invention.

Another method, which differs from the method of the invention in that the solvent has only one or more physical absorbents, wherein amines are absent, often requires more solvent and more absorbent bottoms in an absorption column used in step a) to cause the same amount of acidic gases is absorbed as in the method according to the invention. Moreover, in solvents having only one or more absorbent agents, more non-acidic gases are absorbed than in the solvents used in the process of the present invention, which then undergo short path distillation

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in step b) are released. If a solvent is used which has only one or more physical solvents, then this amount of non-acidic gases is so high that it is not attractive to use them as fuel (as with the non-acidic gases liberated in step b) of the process according to the invention is possible; these non-acidic gases require repressurization (using capital intensive compressors) before being returned in step a).

The contacting of the gas mixture with the solvent in step a) takes place under elevated pressure, which is at least 5 bar and in particular at least 10 bar. Pressures in the range of 20 to 100 bar are very suitable.

The contacting of the gaseous mixture with the solvent takes place in a very suitable manner in a contact zone, that is to say in an absorption column which has 15 to 80 contact points.

For example, valve base, bubble-cap, baffles, and the like, it has surprisingly been found that by using the solvent of the present invention, the hydrogen sulfide can be substantially removed from the gas mixture used as the feed while the amount of carbon dioxide remaining in the purified gas is controlled. This regulation can be carried out by appropriately controlling the solvent cycle, that is, the ratio of solvent fed to the extraction zone to the amount of gas mixture fed thereto. If there is no or only very little hydrogen sulphide in the gas mixture, the amount of carbon dioxide removed therefrom can likewise be regulated by the solvent circuit. If necessary, the carbon dioxide can be removed to a large extent. The solvent circuit may optionally be further reduced by

stripping the loaded solvent from this zone at an intermediate point in the lower part of the contact zone, cooling the withdrawn loaded solvent outside and allowing it to be further contacted with the gas to be purified.

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mixture - as described, for example, in British Patent Specification No. 1,589,231 - fed back into the lower part of the contact zone.

The temperature during the contacting of the gas mixture and the solvent in step a) may vary within wide limits. Temperatures in the range of 15 to 110 ° C are very suitable, temperatures in the range of 20 to 80 ° C are preferred.

In step a), if present, all carbon oxysulfide or most of it is removed from the gas mixture.

The loaded solvent obtained from step a)

contains C0_, H ₂ S (if any) and in general proportions of dissolved non-acidic components from the gas mixture to be purified, that is, hydrocarbons and / or hydrogen and / or carbon monoxide. These non-acidic gases are at least partially removed from the loaded solvent by short path distillation according to step b) under a pressure which is higher than the total partial pressure of the acid gases present in the loaded solvent. In this way, only very small amounts of acidic gases are released from the solvent together with the non-acidic gases, for example hydrocarbons and / or hydrogen and / or carbon monoxide. Optionally, the gas mixture obtained in the short path distillation in step b) can be recycled in step a); However, in order to avoid recompression, this gas mixture is preferably used for other purposes, for example as heating gas (optionally after removal of all hydrogen sulphide present or a part thereof, for example by contacting said gas mixture with a small amount of regenerated solvent ). Non-acidic gases must be removed from the loaded solvent before this solvent dissolves

a pressure is flashed which is lower than the total partial pressure of the acid gases, otherwise the hydrocarbons and / or the hydrogen and / or carbon monoxide released together with a significant amount of acidic gases

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would. Since, in many cases, these acidic gases or combustion gases obtained therefrom must be released to the atmosphere, the hydrocarbons and / or the hydrogen and / or the carbon monoxide would be simultaneously discharged or burned, which would be a waste of these valuable compounds.

Although the loaded solvent in step b) can be flashed several times, namely

each time at a lower pressure, most of the greater part of dissolved non-acidic components is usually removed in a short path distillation step, and it is therefore preferred to subject the loaded solvent to a one-time short path distillation in step b).

The laden solvent obtained in step b), which contains only small amounts of other dissolved compounds in addition to acid gases, is flashed off in step c) to a pressure which is below the total partial pressure of the acid gases in the loaded solvent at the prevailing temperature. It has been found that in the process according to the invention (in which a tertiary amine and a physical absorbent solvent is used) the amount of carbon dioxide released is substantially higher than in other processes which differ from the process according to the invention only in that the solvent instead of a tertiary amine contains a secondary and / or primary amine or a tertiary amine but no physical absorbent. If the loaded solvent also contains hydrogen sulfide, then the gas released after the expansion in step c) has a significantly higher molar ratio of CO ₂ t H ₂ S than the molar ratio of the gases originally present in the loaded solvent. It is advantageous to heat the loaded solvent before or during short path distillation in step c), for example to a temperature in the range of 45 to 110 ° C, because in this case the molar ratio of CO: H ₂ S in the after released the relaxation

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Gases is increased even further. The above-mentioned modified process using a solvent having a secondary or primary amine and a physical absorbent or a solvent having a tertiary amine and no physical absorbent provides a substantially lower molar ratio of CO- : H_S in the released after relaxation gas. Since the gas released in step c) has a higher molar ratio of CO: H_S than originally; loaded solvents, the molar ratio of sulfur pulp to carbon dioxide remaining in the solvent after relaxation in step c) is higher than originally.

Since in each relaxation in step c) the molar ratio of H-S to CO- in the remaining solvent increases, it may be advantageous to remove the loaded solvent in stage c)

at least twice,.

at

each time to a lower pressure or higher temperature in the event that the original gas mixture contains hydrogen sulphide. In general, the loaded solvent after relaxation in step c) is at about atmospheric pressure.

In step c) a significant amount of carbon dioxide is released, and accordingly, the loaded solvent is regenerated to a considerable extent, so that a semi-regenerated solvent is formed. In the case where the original gas mixture was substantially free of hydrogen sulfide, the half-regenerated solvent obtained in step c) contains carbon dioxide as the sole acid gas, and it is preferably used as such at least partially as the solvent in step a). In many cases, the proportion in the semi-regenerated solvent of carbon dioxide obtained in step c) is so low that it is preferred to use this half-regenerated solvent as the sole solvent in step a). If appropriate, a part of the half-regenerated solvent obtained in step c) or else all of it can be completely regenerated (in particular

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For example, by stripping with steam) and used as a solvent in step a). In the case where the fully-regenerated solvent and the half-regenerated solvent are both used as the solvent in step a), the former is preferably fed into the contact zone at a position farther from the inlet of the gas mixture than that for the gas stop »regenerated solvents.

In the case where the original gas mixture contains not only carbon dioxide but also hydrogen sulfide, the acid gases released during short path distillation in step c) often contain small amounts of hydrogen sulfide so that they can be released to the atmosphere after combustion. Optionally, or if necessary, the hydrogen sulphide present in the gases released during the short path distillation in step c) can be removed therefrom by contacting these gases with a solvent under conditions which preferably remove hydrogen sulphide from carbon dioxide favor. With regard to such a removal, for example, a mixture comprising an amine and, optionally, a physical absorbent may suitably be used, but in particular the solvent to be used according to the invention. To ensure high selectivity to hydrogen sulfide removal, contacting is conveniently done in a column with less than 20 touch bottoms and at high gas velocities - for example as described in British Patent No. 1,362,384.

In the case where hydrogen sulfide was present in the original gas mixture, the semi-regenerated solvent obtained in step c) contains hydrogen sulfide and carbon dioxide in a high molar ratio. In view of the high content of hydrogen sulphide, this semi-reduced solvent is not suitable for use as a solvent in step a), and the acid gases present in it must be removed therefrom. The acidic gases are made from said semi-regenerated solvent to give a fully regenerated solvent by complete regeneration

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away. The regeneration takes place in

very suitably by heating in a regeneration column (for example to a temperature in the range of 80 to 160 ° C), wherein the heating is preferably carried out by stripping with steam.

The gas obtained in this regeneration has a hydrogen sulphide content which is suitable for use in a Claus process for the production of sulfur.

The solvent obtained after complete regeneration can very suitably be reused both in step a) and optionally brought into contact with the gas obtained by the short path distillation in step b) or c).

It should be understood that in order to keep the amount of energy required in the process as low as possible, it is advantageous to carry out heat exchange of product streams where appropriate. embodiments

example 1

There are 10 000 kmol / h of a gas mixture (composition: 80 Vo1-% methane, 5% by volume of ethane, 3% by volume of propane, 1 vol% butane, 1% by volume of hydrogen sulfide and 10% by volume of carbon dioxide) in the lower part of a 30 DOüen having absorption column at a temperature of 40 ° C and a pressure of 50 bar fed. This gas mixture is contacted in countercurrent with 300 m / h of a regenerated solvent consisting of methyl diethanolamine (50% by weight), sulfolane (25% by weight) and water (25% by weight). Purified gas leaves the upper part of the absorption column in an amount of 9511.4 kmol / h. This gas contains 644 kmol / h of carbon dioxide and less than 4 volumes per million of hydrogen sulfide.

The loaded solvent (300 m / h) is withdrawn at the bottom of the absorption column; it contains 99.7 kmol / h of sulfur

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hydrogen and 356 kmol / h of carbon dioxide. This loaded solvent is flashed to a pressure of 15 bar at a temperature of 69.2 ° C. The flashed gas (45 kmol / h) contains 1.4 kmol / h of hydrogen sulphide and 10.7 kmol / h of carbon dioxide. The rest is made up of hydrocarbons. The loaded solvent obtained after this first short path distillation contains 98.6 kmol / h of hydrogen sulphide and 345 kmol / h of carbon dioxide. It is heated by heat exchange with completely regenerated I-solvent and flashed at a temperature of 70 ° C to a pressure of 1.3 bar. The gas released during this second short path distillation (29 3.6 kmol / h) consists of 35.6 kmol / h of hydrogen sulphide and 258 kmol / h of carbon dioxide. It is contacted in countercurrent with 148 m / h of fully regenerated solvent in a second absorption column with 13 valve bottom at a temperature of 40 C and a pressure of 1.1 bar, resulting in 212 kmol / h of a gas consisting of carbon dioxide with 300 parts / million hydrogen sulphide.

The loaded solvent obtained in the latter absorption column is regenerated together with the half-regenerated solvent obtained after the second short path distillation. The semi-regenerated solvent contains 63 kmol / h of hydrogen sulfide and 87 kmol / h of carbon dioxide. The regeneration is carried out by stripping with steam, resulting in a gas consisting of 98.6 kmol / h H ₃ S and 133 kmol / h CO ₃ . This gas is very suitable for use in a Claus process. The solvent obtained after complete regeneration (448 m / h) is recycled partially (148 m / h) to the second absorption column and partly (300 m / h) (after heat exchange with the loaded solvent from the first short path distillation) as a fully regenerated solvent used in the absorption column.

Example 2 TO 000 kmol / h of a gas mixture (comp

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90.65% by volume of methane and 9.35% by volume of carbon dioxide) at the bottom of an absorption column having 20 valve trays at a temperature of 35 ° C. and a pressure of 91 bar. This gas mixture is contacted in countercurrent with 84 4 m / h of a solvent consisting of methyl diethanolamine (50% by weight), sulfolane (25% by weight) and water (25% by weight). This solvent is a semi-regenerated solvent; it contains 1374 kmol / h CO ₂ and no methane. The emerging at the top of the absorption column gas (9069 kmol / h) consists of 98 vol .-% of methane, balance carbon dioxide.

The loaded solvent (844 m / h) is withdrawn at the bottom of the absorption column; It contains 2131 kmol / h of carbon dioxide and 174 kmol / h of methane and has a temperature of 53 ° C. This laden solvent is flashed to a pressure of 24 bar at a temperature of 51 ° C Das

Flask gas (204 kmol / h) consists of 159 kmol / h of methane and 45 kmol / h of carbon dioxide. The loaded solvent (844 m / h) obtained after this first short path distillation contains 2086 kmol / h of carbon dioxide and 15 kmol / h of methane. It is heated to a pressure of 1.3 bar at a temperature of 40 ° C

flashed.

The gas released during this second short path distillation (727 kmol / h) consists of 15 kmol / h of methane and 712 kmol / h of carbon dioxide. The half-regenerated solvent (844 m ³ / h) obtained in the second short path distillation contains 1374 kmol / h of carbon dioxide and no methane, and is fed at the upper end of the absorption column as a solvent for the gas to be purified as described above.

Comparison experiment

For comparison purposes, the same procedure as described in Example 2 is 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 carbon dioxide from the feed gas and the use of non-regenerated

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In the absorption step, a solvent cycle which is five times as high as that required in Example 2 of the process according to the invention is required. In addition, about five times as much methane is absorbed in the absorption column per hour and released during the first short path distillation; the amount of methane released in this first short path distillation is so high that this gas must be recompressed and recycled for economic reasons, necessitating the installation of expensive compressors.

Claims (13)

240719 3 He finds the piuch A process for the removal of carbon dioxide and, if present, hydrogen sulfide from a gas mixture, characterized in that a) the gas mixture is contacted under elevated pressure and in countercurrent with a solvent containing a tertiary amine and a physical absorbent; b) the loaded solvent obtained at least once under Depressurizing to a pressure which is above the tamper pressure of the carbon dioxide and hydrogen sulphide present in the loaded solvent, which is above the prevailing temperature, subjected to a flash relationship or short path distillation; c) the laden solvent obtained in step b) is subjected to flash or short-path distillation at least once while depressurizing to a pressure which is below the total partial pressure of the carbon dioxide and hydrogen sulphide present in the laden solvent at the prevailing temperature. and semi-regenerated solvent obtained in step c), optionally after all or part of it has been fully regenerated, used as the solvent in step a). 2. The method according to item 1, characterized in that the solvent contains water. 3. The method according to item 1 or 2, characterized in that the tertiary amine is aliphatic and contains at least one hydroxy dlkylgruppe per molecule. 4. The method according to item 3, characterized in that the tertiary amine is Methyldiäthanölamiη. 24 0 719 3 A method according to any one of the preceding points, characterized in that the physical absorbent is tetrahydrothiophene-1,1-dioxide. 6. The method according to any one of the preceding points, characterized in that the solvent contains methyl diethanolamine, Te tr ahydrothiophene-1, 1-dioxide and water. 7. The method according to item 6 », characterized in that the solvent contains 10 to 60 weight percent of methyl diethanolamine, 15 to 55 weight percent tetrahydrothiophene-1,1-dioxide and 5 to 35 weight percent water. 8. Method according to any one of the preceding points!, Characterized in that the gas mixture in step a) is brought into contact with a solvent in a contacting zone which has 15 to 80 contact layers. 9. The method according to any one of the preceding points, characterized in that the step a) is carried out at a pressure of 20 to 100 bar. 10. The method according to any one of the preceding points, characterized in that step a) is carried out at a contact temperature of 20 to 80 ° C. 11. Method according to any one of the preceding. Points, characterized in that the loaded solvent is flashed off in step c) to atmospheric pressure. 12. Method according to any one of the preceding! Dots , characterized in that the half-regenerated solvent obtained in step c) is regenerated by stripping with steam before it is used as a solvent in step a). 13. The method according to any one of items 1 to 11, characterized in that the gas mixture is substantially free 240719 3 of hydrogen sulfide and the semi-regenerated solvent obtained in step c) is used as the sole solvent in step a).