DE2123773A1 - Process for the separation and recovery of acid gases from gas mixtures - Google Patents

Description

The invention relates to a method for separating and recovering acidic gases, in particular from Carbon dioxide from gas mixtures containing such gases.

In known processes for the separation and recovery of acid gases from gas mixtures, the acid gases are in absorbs alkaline solutions, which are regenerated to extract the acidic gases from them. Known absorbent Solutions are solutions of alkaline salts such as carbonates, phosphates, borates and phenates of sodium, potassium and Ammonium.

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The effectiveness of the procedures using this Absorbent solutions is due to the relatively low speed or amount (rate) of the transition of molecules of the acidic gas, e.g. carbon dioxide or hydrogen sulfide, is limited from the gas phase to the liquid phase. They are different Improvements to increase the rate of transition by adding various substances to the absorption solutions in amounts which are small compared to the major components of these solutions. Many Additives have been suggested including arsenic oxides, alkanolamines such as monoethanolamines and diethanolamine unc, such alkanolamines as ethylaminoethanol, the use of which is described in British Patent 1,218,083, as well as amino acids such as glycine and such amino acids as N-ethyl-β-alanine, its use in the British patent application No. 37277/69.

For most promising additives, the most important property is a high level of activity for promotion the rate of transition of the acid gas molecules from the gas phase to the liquid phase. Other desired Li ^ er » stocks are:

1. Volatility: The additive preferably has a

low volatility.

2. Toxicity: The additive preferably has a

low toxicity and, in particular, is non-toxic.

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3. Stability:

4. Corrosion:

5. Foaming:

b. Deposition;

7. Solubility

The additive is preferably for long periods of time, e.g. Stable for about a year or even longer in the presence of alkaline solutions at temperatures up to about 130 ° C. The auditory is preferably essentially non-corrosive to the material the plant for the removal of acid gases.

The additive preferably causes only slight foaming of the absorption material.

The additive preferably does not cause any deposits to form within the plant for the removal of acid gases.

The additive is preferably highly soluble in the absorption solution.

Many of the previously proposed additives that are extremely effective in improving the transition rate, bring problems related to their practical use, since they are not one or the other of the desired ones Have properties to a sufficient extent. Arsenic cycle are toxic and cause the formation of solid deposits within the acid gas removal system. One problem is with the use of alkanolamine additives

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be avoided for toxicity, which are associated with the use of Arsenoxyden, many alkanolamines are j including some of which are used with the best efficiency to improve the transition rate _J highly volatile and can be effective only in systems that are specifically set up for the reduction of Alkanolaminverlusten or have been modified. Some of the more successful alkanolamine additives have somewhat limited solubility in the absorption solutions. Some amino acids are very effective in improving the rate of transition, are non-toxic, do not form solid deposits, and have satisfactorily low volatility and high solubility in absorption solutions. However, some of these compounds most effective in improving the transition rate develop a tendency to decompose to some extent with the lapse of time under the conditions required for acid gas removal while being stable compounds under normal conditions.

According to the invention there is provided a method for the separation and recovery of acidic gases from gas mixtures containing them created by absorption in an absorbent solution, which is a solution of an alkaline salt of a Alkali metal contains and by regenerating the absorbent solution, wherein the absorbent solution further a contains a minor amount of an amino acid or an ester or salt thereof and wherein the amino acid has the formula

H 0

R ¹ - Nh - C ^ C ^ where R is a hydrogen atom;

R ^v OH

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or represents an alkyl group and R ^{1 represents} an n-alkyl group having 2 to 7 carbon atoms, an isopropyl, isobutyl or cyclohexyl group.

Of course, in the present context, an ammonium salt is also used as a salt of an alkali metal consider.

The amino acid is preferably one in which R is a hydrogen atom or an n-alkyl group having 1 to 6 Represents carbon atoms.

R ¹ is preferably an n-alkyl group, in particular an n-alkyl group having 2 to 6 carbon atoms.

The amino acid is very suitable, a substituted glycine, preferably one in which R 'is an n-alkyl group and especially one in which R' is an n-alkyl group having 2 to 6 carbon atoms. Particularly suitable acids are N-ethylglycine (C ₂ H ₅ NH - CH ₂ - COOH), N-butylglycine CnC ₄ II ₉ Kh - CK ₂ -COOH) and N-pentylglycine CnC ₅ H ₁₁ NH - CK ₂ - COOK)

Examples of other suitable amino acids are N-ethyl

axaiiiii, ii-Äthyj-- -aminobutyric acid, h-'at hylvaline and N-ethylamirio-r.-caproiic acid e.

The amino acid can be in the solution of the alkaline salt as a free acid or as a solvent or preferably

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imported as salt thereof. When introduced as an ester wirci, ax ester group is preferably an n-alkyl group with 1 to 6 carbon atoms, in particular a MethyloGer ethyl group. Suitable salts are alkali salts, in particular Potassium salts.

The amount of amino acid that makes up the absorption solution. depends on the solubility of the amino acid in the absorption solution and on the temperature at which the absorption step is carried out. The amino acid is preferably added to the absorption solution in amounts of up to 10%, especially 2'-b to 5 % , based on the weight of the solution.

Two of the most commonly used absorbent solutions in acid gas separation and recovery processes of the type to which the invention relates and which are preferred in any process of the invention include aqueous solutions of potassium carbonate. The absorption solution preferably contains up to about 50% by weight, particularly 15 to 35% by weight, of potassium carbonate or another alkaline salt of an alkali metal.

It is assumed that the amino acid ate the rate of transition of acid gases from the gas to the liquid phase aaüurch that it provides an alternative reaction path for the absorption of the acidic gases by the absorption solution offers. If carbon dioxide a by aqueous potassium carbonate solution

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In the absence of an additive, the carbon dioxide reacts with the solution at the gas-liquid interface and converts it to potassium bicarbonate through the reaction: CO ₂ + K ₂ CO ₃ + H ₂ O> 2KHCO ₃

If the amino acid is also present in the absorption solution the carbon dioxide can also react with the amino acid, according to the reaction

C ₀ Hr HOOC CH

H - i \ + CO ₀ »N

\ c cooh c - cooh

r / /

It is believed that through this mechanism the amino acid catalyzes the rate of absorption by that they provide an alternative and faster reaction path for the transfer of carbon dioxide molecules from the gas phase in provides the bulk of the absorption solution.

In carrying out the method of the invention can using the gas mixture from which acidic gases are to be removed in intensive contact with the absorption solution The usual measures can be taken, for example by a tower, e.g. with ceramic rings or saddle fillers or is filled with bubble trays or sieve trays or with the help of a blower reactor.

In the absorption stage, the gas mixture is in the 109848/1800

Base of the tower introduced while fresh solvent in aen headboard is inserted. The gas mixture, which has largely been freed from acid gases, emerges from the head section. Preferably the temperature of the absorption solution during the absorption stage is between 50 ° C. and 120 ° C., in particular between 80 C and 115 C. The pressure of the gas mixture can be atmospheric However, it is preferably higher than atmospheric, as this gives the possibility of reducing the size of the device used and also to a reduction in the concentration of acid gases in the exiting from the top of the tower Gas leads.

The absorption solution loaded with absorbed gases can be regenerated by conventional measures, ζ, 3 .:

(a) Pressure reduction, causing the evaporation of the acidic

Gases is effected,
or (b) by passing the solution into a tower of similar construction to that used in the absorption stage at a location at or near the top of the tower and passing an inert gas, such as air or nitrogen, or preferably steam, up the tower .

The temperature of the solution during the regeneration step can be between 50 and 130 C, preferably between 100 and 125 ° C. The pressure is preferably

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within the range of 0.5 to 3 atmospheres, where atmospheric pressure is very useful.

The rate of transition of the acidic gas from the gas to the liquid phase, which is achieved using a given Concentration of, for example, N-ethyl, n-butyl, n-pentyl-5 cyclohexyl, iso-propyl or n-pentylglycine can be achieved, is comparable to that obtained using an equal concentration of one of the more effective alkanolamine additives, which have been proposed so far is achievable. However, these preferred amino acid additives are im generally several times more soluble in the absorption solution than the alkanolamines. Therefore, higher concentrations can be used of the preferred additives can be employed to effect a rate of acid gas transition, which is often higher is than that achievable with arsenic oxides or the more effective alkanolamine additives.

The preferred additives are considerably less volatile than many of the alkanolamine additives previously suggested were and can therefore be used in plants that are not particularly aimed at reducing losses Additives have been set up or modified.

The toxicity of the absorption solutions according to the invention is considerably lower than that of absorption solutions, which contain arsenoxyae and still is

1 0 9 8 4 H / 1 B 0 0

- -yr -

lower than that of solutions containing alkanolamines as additives.

In addition, there is N-ethylglycine in absorption solutions more stable over longer periods of time under the conditions required for acid gas absorption than some of the previously proposed high activity amino acids.

This · high level of activity experienced in the process of the invention have applied amino acids to promote the transition rate, creates the possibility of gas removal systems to practice so that there is a reduced content of acidic gas in the gas emerging from the systems results.

In addition, the cost of capital can be reduced by using smaller plants which are under generation of the same acid gas content in their source gases as before. Using the method of the invention can improve the reliability of acid gas removal systems due to decreased corrosion, decreased foaming and reduced deposits of solids are increased.

The invention is illustrated in more detail by the following examples.

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example 1

The transition coefficient (<! ") For CO ₂ in 30 cent, ^e (wt./vol.) Potassium carbonate solutions with different concentrations of a number of additives were measured at 70 C and a C0 2 of 0.6 ₉ -Sättigungsverhältnis measured.

The results are shown in Table I which shows the values for K ₂ for a series of percentages (w / v) of
Indicates additive in potassium carbonate solutions with a content of the following additives:

A = glycine
. B = Ar isr. III oxide
C = diethanolamine
D = N-ethylglycine

The transition coefficient was _> over the entire
rtuditive concentration range measured (0 to 5%), for
N-ethylglycine greater than the respective values for diethanolamine and glycine. Over the range from 0 to 2.05%
the transition coefficient for arsenic-III-oxide was just above that of N-ethylglycine. At higher concentrations
However, the transition coefficient for N-ethylglycine aiisteigenc exceeded that for arsenic-III-cxya.

Note: The CO ₉ saturation ratio is 2

(K)

a measure of the total concentration of carbon dioxide in the studied solution and is defined as the ratio of the

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Number of g-moles of carbon dioxide in the solution to the number of g-ions in potassium. The same term can also be expressed as CO ₂ saturation index 2 , which

(K ₂ O) twice the numerical value of the CO ₂ saturation ratio

Has.

Table I.

Additive

* 7 * 5 Λ Λ

Transfer coefficient K "χ 10 mol ¹ cm" see * "atm""

% Additive (w / v)

0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

A glycine ⁶ > ^{7 7} > ° ⁷ > ⁴

^H » ^{5 17} > ³

B Arsen-Ill-oxya ¹ · ¹ » ⁰

Ci) ethanolamine ⁸ > ⁵ l ° i ⁵

D N-ethyl

glycine ⁹ » ^{5 12} > ⁵

⁸ > °

⁸ > ⁷

9.5 10.0

^2O » ^{9 22} » ^{2 23} » ²

1 ⁶ > ⁵

²² > ° ²⁵ »°

.5 22.5 24.5 26.5

31.5 34.4 37.5

Example 2

The transition _{coefficients (K 2} ) for CO ₂ in 30% (w / v) potassium carbonate solutions containing 2% (w / v) of a range of additives were measured at 70 ° C and with a range of CO ₂ -Saturation ratios measured. The results are given in Table II, which shows values for K ₂ for a series of values for the CO ₂ saturation ratio of solutions containing the following additives: N-ethylglycine, arsenic III oxide, diethanolamine, glycine.

The transition coefficient for N-ethylglycine was over

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the entire measured range of values for the C0 ₂ saturation ratio (0.55 to 0.8) is greater than the relevant values for diethanolamine and glycine. Over the middle part of the range, ie from 0.6 to 0.7, the transition coefficients for N-ethylglycine and arsenic-III-oxide were almost the same. The value for N-ethylglycine exceeded that for arsenic-III-oxide at the upper and lower ends of the range examined.

Table II

Additive transition coefficient K ₀ χ 10

-1 -1 -1 mole cm see atm

C0 _o saturation ratio ^(C0 2 ⁾

¹ "TkT"

0.55 0.6 0.65 0.7 0.75 0.8

N-ethylglycine 24.6 19.8 16.3 14.0 12.5 .11.4

Arsenic III oxyo. 21.5 19.0 16.6 14.0 11.6 9.2

Diethanolamine 19.3 14.6 12.0 10.0 8.7 7.7

Glycine 10.0 8.0 6.5 5.7 5.1 4.8

Example 3

The potassium salts of a number of amino acids were prepared and in each case the catalytic activity of the compound was determined by the rate of absorption of CO ₂ in a 30% (w / v) potassium carbonate solution, the axe compound, on a standard laboratory tower contained, measured at 70 C and a flow rate of 10 l / h.

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- • -

The corresponding vapor pressure values were also determined using a conventional technique with a U-tube pressure gauge. The catalytic activity of a 30% (w / v) potassium carbonate solution which did not contain any amino acid as an additive was determined in the same way. The results are shown in Table III which gives the transition coefficients that would be obtained for each additive at a range of CC ^ saturation ratios.

Table III

7 transition coefficients K „χ

Additive Concentr- ,,, -1 -1. -1

ration ^{Ko1 cm sec atm}

CO ^ saturation ratios ^(C0 2 ⁾ * ■ ~ TkT ~

0.55 0.6 0.65 0.7 0.75 0,

Glycine 0.82 N 13.5 '10.0 8.7 7.8 6.2 Sarcosine

(N-methyl-

glycine) 2.77 w / v 11.5 9.5 9.0 8.3 7.2 U ₅ S N -ethylglycinel, 9th w / v 20.0 "16.0 12.8 11 , 3 10.0 N-ethylglycine 3, 2 w / v 24.8 19.8 16.5 Ih, 5 K-ethylglycine 5.0 w / v 3 2.8 25.5 20, ö 17, 0 N-isopropyl

glycine 1.75 w / v 13.2 10.8 8.5

N-n-butyl

gIyein 2 w / v 32.0 25.0 21.8 19.0 18.0

ΪΜ-tert-butyl-

glycine 2 w / v 11.4 8.4

N-n-pentyl

glycine 1.2 w / v 19.5 17.0 14.6 12.2 9.6 7.3 h-cyclohexyl

glycine 1.9 w / v 22.4 17.9 13.0 8.7 iM-n-heptyl

glycine 0.9 5 w / v 9.9 7.2

Additive 8.0 U, 6.6 5.6 4.6 3.9 3.3

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The catalytic activities became the. "molar activity" calculated for jeae connection as follows. At a CO ^ saturation ratio 0.65 was the transition coefficient of a 30% potassium carbonate solution containing c mol / l additive, assumed as the value K and that of the solution which contains no additive as the value K. The molar activity can be defined will be as:

M = ^K c ~ ^K o ^K oc

Ls values were calculated for the molar activity for each additive. For N-tert-butylglycine and Nn-heptylglycine, the calculation was based on the transition coefficient at a C0 ₂ saturation ratio of 6.0. The results are shown in Table IV which gives the molar activities as the nearest whole number. It can be seen from the results that the additives of the invention give higher molar activities than the other amino acids studied, such as glycine and sarcosine.

Table IV

Additive molar activity

(Lit. Mol " ¹ )

Glycine 1

Sarcosine 2

w-ethyl glycine 11

N-isopropylglycine 9

N-n-Butylglycine 25

N-tert-butylglyein 2

N-n-Pentylglycine 25

N-cyclohexylglycine 2 3

N-n-heptylglycine 6

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Claims (10)

Claims 1. Process for the separation and recovery of acidic gases from a gas mixture containing them by absorption in an absorbent solution comprising a solution of an alkaline alkali metal salt and a lesser amount of a Amino acid or an ester or salt thereof and regeneration of the absorbent solution, characterized in that that an amino acid of the formula H 0
R »- NH - C- ^ C ¹ ^, where R is a hydrogen atom R OH
or an alkyl group and R 'is an n-alkyl group having 2 to 7 carbon atoms, an isopropyl, isobutyl or cyclohexyl group. 2. The method according to claim 1, characterized in that one uses an amino acid of the formula given, in which R is a hydrogen atom or an n-alkyl group having 1 to 6 carbon atoms. 3. The method according to any one of the preceding claims, characterized in that i, ian uses an amino acid of the formula given, wherein R ^{1 is} an n-alkyl group having 2 to 6 carbon atoms. 4. The method according to any one of the preceding claims, Gaaurch that one is used as an amino acid 109848/1800 substituted glycine is used. 5. The method according to claim 4, characterized in that that one .als an amino acid N-ethylglycine, N-n-butylglycine or N-n-pentylglycine is used. 6. The method according to any one of the preceding claims, characterized in that one is used as the amino acid salt Alkali metal salt is used. 7. The method according to any one of the preceding claims, characterized in that adding the amino acid or its ester or salt to the absorbent solution in amounts between 2% and 5%, based on the weight of the solution, are added. 8. The method according to any one of the preceding claims, characterized in that the alkaline alkali metal salt is potassium carbonate in amounts between 15% and 35% on the weight of the absorbent solution. 9. The method according to any one of the preceding claims, characterized in that the absorption stage at a Temperature of the absorption solution between 80 and 115 ° C performs . 10. The method according to any one of the preceding claims, characterized in that the regeneration stage at 109848/1800 a temperature of the absorbing solution between 100 ^c and 125 C. 109848/1800