CA1116835A - Solvents for acidic gas removal and process therefor - Google Patents

Abstract

SOLVENTS FOR ACIDIC GAS REMOVAL AND PROCESS THEREFOR

ABSTRACT OF THE DISCLOSURE

Disclosed is an improved process for removing carbon dioxide containing acidic gas from a normally gaseous mixture wherein the gaseous mixture is contacted with a liquid anhydrous solvent which physically absorbs said acidic gas but which is chemically unreactive with said acidic gas, said solvent being the reaction product of an alkylating agent and a solvent which is chemically reactive with said acidic gas for its removal from said mixture.
The preferred solvent is selected from the group consisting of N,N-dialkylaminoalkanol, N,N-dialkoxyalkylaminoalkanol, N-alkyl-N-alkoxyalkylaminoalkanol, N,N-dialkylaminoalkyl-ether, N,N-dialkoxyalkylaminoalkylether, N-alkyl-N-alkoxy-alkylaminoalkylether, dialkyl carbonate, di(alkoxyalkyl) carbonate, mono-alkyl-moncalkoxyalkyl carbonate, and mixtures thereof.

Description

D~SCRIPTION
SOLVENTS FOR_C:tDIC GAS tl~l~O_L A/:D l'D~/cE~Ss THE~EFOR
Back~_und ol the Invention The present invention relates to the removal Or carbon dioxide containing acidic gases from gaseous mixtures con-taining same and more particularly to an improved process ror such acidic ~as removal and an improved solvent for such process.
It is well known to contact carbon dioxide containing acidic gas mixtures ~such as mixtures containing acidic gases including CO2, H2S, SO2, SO3, CS2, HCN, COS, and oxy-gen and sulfur derivative Or Or Cl-C4 hydrocarbons) with a liquid solvent to remove these acidic gases. Two general classes of solvents are used in such scrubbing pro-cesses: "physical" solvents which physically absorb the acidic gas, and "chemical" solvents which chemically react with the acidic gas ror removal of same. Chemical solvents invariably are provided in aqueous form and efrectively operate on gaseous mixtures containlng low concentrations Or acidic gas, whlle physical solvents in-variably are provided in anhydrous form and operate very effectively on gaseous mixtures containing a high concentra--- tion of acidlc gas. Physlcal solvents are known to provide low solvent circulatlon rates and low regeneration energies for their recovery, but have very low selectivitles for . .
^;~ preferentially absorbing the acidic gas over the remainder ~, of the gaseous mixture.
Prior art processes for removing acid gas from gaseous mixtures include that Or Sartori et al (U.S. Pat. No.
4,100,257) wherein the solvent is a mixture of sterically hindered amine and a t-aminoalcohol dispersed in an organic solvent, optionally admixed with water; and that of Sartori et al (U.S. Pat. No. 4,101,633) wherein the solvent is an amine mixture of sterically hindered amine and a t-amino-alcohol dispersed in water. Yet another prior art proposal ' ' 1~16835 is that Or Renault (U.S. Pat. No. 3,516,793) wherein ele-mental sulrur is recovered from a gaseous mixture by contact-ing the gaseous mlxture wlth an alkanolamine or morpholine dispersed in a solvent of a monoalkylether of a polyhydric alcohol, optionally containing water, followed by contacting the resultingsolutionswith molecular oxygen to form the elemental sulfur. Note, that Example l of Renault shows no carbon dioxide absorption with such solvent mixture.
Other proposals show various other solvents usually in aqueous form and more often being chemically reactive with the acidic gas for removal Or same from the feed gaseous ~ mixture. Evencombinations of physical and chemical solvents -~ have been proposed, but usually such solvent mixtures t contain water.
;~
~ 15 Advantages of the present invention include the F': effective treatment of high acidic gas containing gaseous - mixtures, low solvent clrculation rates, and low regeneration energies, while achievlng relatively high selectivity of the solvent for acid gas absorption. These and other advantages will become readily apparent from the description of the F~ invention herein contained.

Broad Statement of the Invention ~' The present inventionis an improvement in a cyclic process for removing carbon dioxide containing acidic gas from a normally gaseous mixture containing same, wherein -~ said gaseous mixture is contacted with a solvent under acidic gas/solvent contact conditions, said acidic gas bearing solvent is stripped of said acidic gas, and said -~ 30 stripped solvent is recycled for contact with additional -~ gaseous mixture. The improvement comprises contacting said gaseous mixture containing on a dry molar basis between about 5% and 95% carbon dioxide with a solvent which is liquid under said contact conditions. The solvent is restricted to an anhydrous solvent which physically absorbs said acidic gas but which is chemically unreactive with - said acidic gas under said contact conditions. The solvent is the reaction product of an alkylating agent and a chemical solvent, said reaction product being said physical ~16~335 solvent. Prererably, the solvent i8 selected from the group consisting OrN,~-dlalkylamlnoalkanols and certain ether derivatives thereor, N,N-dialkylaminoalkylethers and certain ether derivatlves thereof, and dialkylcarbon_ ates and certain ether derivatives thereof.

Detailed Description Or the Invention The normally gaseous mixture to be treated in accord-ance with the process of the present lnvention can be natural gas, coa] gas, biomass-derived gas, re~lnery gas, or any other normally gaseous mixture containing acidic gases.
The solvents Or the present invention are physical solvents whlch means that such solvents will physically absorb the acidic gas but such solvents are chemically un-reactive with the acidic gas under gaseous mixture/solvent contact conditions.' The solvent is anhydrGus because water generally decreases the solvent's capacity for absorbing carbon dioxide even though improved selectivitles generally result by including water as a cosolvent. The solvent of the present invention retains advantages common to most physical solvents, including the ability to effectively scrub high acid gas containing gaseous mixtures(e.g. 15-95%
by volume acldic gaR and usually 15-60%), providing a low solvent circulation rate (high ratio of gaseous mixture per unit of solvent), and low energy for regenerating the sol-vent (stripping absorbed acidic gas from the solvent for reuse of the solvent). Further, the novel physical sol~
vents of the present invention have a high selectivity for acidic gas absorption which heretofore was an advantage which chemical solvents possessed. The selectivity of the instant solvents can approach the high selectivities of the chemical solvents from which they are derived.
Selectivity for present purposes means the preference of the solvent to absorb acidic gas (measured and referred .~16835 to in terms Or carbon dioxlde for purposes of thls appllca-tion) and not absorb the deslred fuel gas (ror example, methane, ethane, and the like) in admixture with the acidic gas. Selectlvity can be determined experimentally by con-tacting, for example, a me~hane/carbon dioxide gas mixturewith the solvent at a given temperature and pressure, and measuring the resulting molar or volume concentration o~
carbon dioxide and methane both in the solvent and in the product gas phase. Selectivity then is calculated as ; 10 follows:
, SELECTIVITY = 4 , . Gco2 : GCH4 where SC2 is the moles of C02 in the solvent, SCH4 is the moles Or CH4 in the solvent, Gco2 ls the moles of C02 in the product gas, and ~ 15 ECH4 is the moles of CH4 in the product gas.
The preferred solvents are N,N-dialkylaminoalkanols, N,N-dialkylaminoalkylethers, and dialkylcarbonatès. The alkyl substituents Or the preferred solvents can contaln ether linkages and the resulting ether derlvatives are preferred solvents for the present invention also. The preferred solvents can be represented conventionally by the following general structures:
Rl (I) ~ N R3 - 0 - R4, and : R2 O
(II) R5 - O - C - O - R6, h Rl, R2, R5 and R6, independently is a Cl-C8 alkyl group or alkoxyalkyl group, R3 is a Cl-C6 divalent alkyl group, and R4 is hydrogen or a Cl-C8 alkyl group.
The preferred solvents can be selected from the group con-sisting of 2-dimethylaminoethanol, 2-diethylaminoethanol,

2-diisopropylaminoethanol, 2-dibutylaminoethanol, l-dimeth-ylamino-2-propanol, 3-dimethylamino-1-propanol, 2-(N-methyl -N-ethylamino)ethanol,methyl 2-tdimethylamino)ethyl ether, ethyl 2-(diethylamino)ethyl ether, methyl 2-(diisopropyl-amino)ethyl ether, ethyl 2-(dibutylamino)ethyl ether, meth-yl (1-methyl-2-dimethylamino)ethyl ether, ethyl 3-dimethyl-amino-l-propyl ether, methyl 2-(N-methyl-N-ethylarnino)ethyl ether, dimethyl carbonate, diethyl carbonate, dipropyl -~ carbonate, bis-(2-ethoxyethyl) carbonate, bis-(methoxyethyl) carbonate, bis-t3-methylbutyl)carbonate~ monoethylmono-2-butoxyethyl carbonate, and mixtures thereof.
The preferred solvents can be derived from correspond-ing chemical solvents and the preferred physical solventæ
retain high selectivities (for fuel gas/acidic gas separa-tion) of the chemical solvents from which they are derived.
For example, N,N-dialkylaminoalkanols can be derived from ~ primary and secondary aminoalkanols (for example mono-`~ ethanolamine, diethanolamine, and the like) by the addition o~ an alkylating agent to such chemical solvent. Alkylat-ing agents include alkyl halides, alkenes (with ethylenic unsaturation such as vinyl, allyl, or the like, or with olefinic unsaturation), alkyl epoxides, alkyl carboxylates, alkyl sulfates, and a wide variety of other alkylating a-gents well known in the art. For present purposes the al-kylating agents optionally can contain ether linkages (al-

3 kyl ethers) and such alkoxyalkyl alkylating agents are in-cluded herein. Similarly, diaklyl carbonates can be deriv-ed from alkali metal carbonate salts and di-alkali metal carbonate salts by similar reaction techniques. The reac-tion product of the alkylating agent and the chemical sol-vent is a physical solvent which retains a high selectivityofthe chemical solvent from which the physical solvent was 1~1683~

derived J and possesses the advantages whlch conventlonal physical solvents (e.g. methanol or the llke) generally have.
A variety Or alcohols and glycol ether cosolvents can be admixed with the physical solvents Or this invention up to about 50% by weight Or the physical solvent. Such co-solvents can aid in maintaining a practical viscosity of the solvent especially at lower temperatures of operation.
Such cosolvents also are chemically unreactive with the acidic gas. Representative alcohol cosolvents include alk-anols such as methanol, ethanol, and the like and mixtures thereof. Representatlve glycol ether cosolvents include monoaklylethers of glycols (for example 2-methoxyethanol, 2-ethoxyethanol, 3-methoxypropanol, and the like), dialkyl-ethers of glycols (for example, dimethoxymethane, 1,2-dl-methoxyethane, and the like), dicarboxylic acid esters of glycols (for example ethylene glycol diacetate and the like), and the like and mixtures thereof.
In practicing the present invention, the gaseous mix-ture is contacted with the solvent, the solvent regenerated (the acidic gas stripped from the solvent), and the regen-erated solvent recycled for additional absorption of acidic gas. While the gaseous mixture/solvent contact can be a ; batch operation, efriciency and economy dictate that the contact be a continuous operation using, for example, ; countercurrent gas/liquid absorption columns. Contact or acidic gas removal conditions include temperatures of about -50 to 200F, advantageously about 0 to 100F; and pres-sures of about 1 to 2000 psig, advantageously about 100 to lO00 psig. Absorption columns are conventional in construc-tion and in materials of construction, and preferably are packed columns, plate columns (for example, bubble cap, sieve, or valve types), spray columns, and like convention-al equipment.

~116835 Followlng such absorptlon step, the spent solvent (solvent plus absorbed acidic gas) is transported to a regeneration operation for regeneration orthe solvent. Re-generation typically lnvolves heating of the solvent usual-ly under reduced pressure and/or inert gas sparging of thesolvent, and varlous combinations and variations of these methods. Depending upon the boiling point Or the physical solvent, regeneration temperatures of around 100 to about 350F and higher can be used optionally under reduced pressures of as low as about 1 psig. Regeneration of such spent solvent is a well known and common commerci~l process.
The regenerated solvent then is recycled tothe absorp-tion zone, optionally with fresh solvent when necessary, 1" desirable or convenient, for additional absorption of acid-ic gas. The solvent circulation rate of such a cyclic proces~ is the rate at which the solvent is cycled through the process,pervOlume of gas treated. An advantage of the physical solvent of the present invention is that typically the solvent circulation rate is low compared to the circu-; lation rate required of most chemical solvents. Usually, sulfur-containing acid gas can be treated for recovery of elemental sulfur and the C02 vented to the atmosphere, ?5 though other uses of the acidic gas may be practiced in conventional fashion.
The following examples show in detail how the inven-tion can'be practiced and should not be construed as limit-,;' ing. In this application, all temperatures are in degrees Fahrenheit ~nd all gas percentages are molar or volume per-centages, unless otherwise expressly indicated.

EXAMPLES
Example 1 Three preferred N,N-dialkylaminoalkanols were tested in order to determine their selectivity for C02 absorption 35 and their capacity for absorbing C02. These tests were ~1683~;

conducted by contactlng a CH4-CO2 g~seous~m~xture (50:50 by volume) with the variou9 solvents at 32F and 350 psig in a batch equili~rium cell (3OO ml) immersed in a constant-temperature bath. The rollowing results were obtained.

CtD2 SOLUBILITY
~` SOLVENT SELECTIVITY ¦ SCF CO2 I_AL. SOLVENTI
2-Dimethylaminoethanol 43.1 1.28 -~ 2-Diethylaminoethanol188 l.9O
Dimethylamino-2-propanol 216 5.87 *Per atmosphere of CO2 partial pressure For comparison purposes, lt should be noted that methanol (a conventional physical solvent) has a selectiv-ity of only 6.5 and a CO2 solubility of only o.66 SCF
CO2/gallon Or solvent. The exceptionally high selectivi-ties of the physical solvents of the present invention clearly are demonstrated by the above-tabulated results.
Heretofore, conventional physical solvents did not possess as high of selectivities as chemical solvents possessed. The present physical solvents retain the high selectivities of conventional chemical solvents while possessingthe desirable properties Or physical solvents (e.g. low regeneration en-` ergy, and low solvent circulation rate).
Example 2 Several solvents were tested in order to confirm their physical absorption of CO2 and their chemical unreactivity with CO2. Some Or the solvents also were combined with monoalkyl ethçrs Orglycol as cosolvents. Each solvent was contacted with CO2 and then subjected toconventional desorp-tion (or regeneration) with heating and/or nitrogen sparg-ing of the solvent. The solvent prior to CO2 absorption, after CO2 absorption, and after desorption was subjected to - 1~16835 inrrared spectroscopy test:lng and the resulting infrared spectra analyzed to determine whether any C02 had chemical-ly reacted wlth the solvents and to conrirm C02 desorption.
The rollowing table details the C02 absorption conditions and the results obtained.

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Thus, the physical absorption of C02 and chemical un-reactivity with C02 by the novel physical solvents isdemon-strated.
Example 3 - 5 Several solvents were tested in order to determine the solubility Or C02 in the solvents at various temperatures and to determine the heat of solution oI' CO2 in the sol-vents. Monoalkyl ether o~ glycol cosolvents were added in equal weight proportions to the solvents in some of the tests. In each test pure C02 was contacted with the sol-vent at 200 psig at the temperatures indicated in the following table which displays the results obtained.

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~ 12 --1116~33 The above-tabulated results demonstrate the relatively low heats Or solution which the presentphysical solvents have which is a decided benefit Or the process. Also, the increasing solubility of C02 in the solvents at decreasing temperatures is demonstrated. The primary impediment to using very low temperatures is that the viscosity Or the solvent may increase to a value which precludes practical commercial practice of the invention.
In this connection, use oI glycol ether cosolvents can assist in maintaining a useful viscosity of the solvent at lower absorption temperatures.
Example 4 A series of laboratory scale continuous absorption runs were conducted in a Jacketed, continuous, counter-current absorber (1.5 inch inside diameter) packed withGoodloe knitted mesh cartridges to a height of three feet.
The feed gas containing acidic components analyzed to contain by volume: 32.81% C02, 10.20% CH4, 0.027% C2H6, 46.54% H2, and 10.42% C0. The absorption conditions are displayed in Table 4A below and the results obtained are displayed in Table 4B below. A comparative test wlth methanol solvent was run also.

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1116~3 The above-tabulated results clearly demonstrate the excellent acidic gas removal which thepresent physical solvents provide in the process. Note that the CO2 removal and solvent circulation rates (ratio of feed gas to solvent and ratio Or purified gas to solvent) are comparable for the comparative methanol solvent run and run 1 using a pr~ferred solvent of the lnvention. This comparability is unexpected since the absorption temperature for the comparative run was so much lower than the temperature of run 1. It is known that solubility of CO2 increases with decreasing temperature. Thus, lowering the temperature of the preferred solvents will only improve their CO2 absorption.
More remarkable, however, is the dramatic almost two-fold increase in selectivlty of the l-dimethylamino-2-propanol solvent over the selectivity of the methanol solvent. The solvent of run 1 has retained a high selectivity like that of the chemical solvent from which it was derived, while possessing advantageous properties which physical solvents generally possess.
The purified gas from the absorption zone typically goes to a methanation process for converting C0 and un-absorbed CO2 into methane. Usually, about 4-6% CO2 is tolerable in the purified gas for its methanation in ;~ commercial practice of this process.

Claims (10)

-17-WHAT IS CLAIMED IS:

1. In a cyclic process for removing carbon dioxide containing acidic gas from a normally gaseous mixture con-taining same, wherein said gaseous mixture is contacted with a solvent under acidic gas/solvent contact conditions, said acidic gas bearing solvent is stripped of said acidic gas, and said stripped solvent is recycled for contact with additional gaseous mixture, the improvement which comprises:
contacting said gaseous mixture containing on a dry molar basis between about 5% and 95% carbon dioxide with a solvent which is liquid under said contact conditions, said solvent restricted to an anhydrous solvent which physi-cally absorbs said acidic gas but which is chemically unreactive with said acidic gas under said contact conditions, said solvent selected from the group consisting of N,N-dialkylaminoalkanol, N,N-dialkoxyalkylaminoalkanol, N-alkyl-N-alkoxyalkylaminoalkanol, N,N-dialkylaminoalkylether, N,N-dialkoxyalkylaminoalkylether, N-alkyl-N-alkoxyalkyl-aminoalkylether, dialkyl carbonate, di(alkoxyalkyl) carbonate, mono-alkyl-monoalkoxyalkyl carbonate, and mix-tures thereof, said solvent optionally in admixture with an anhydrous cosolvent which also is chemically unreactive with said acidic gas.

2. The process of claim 1 wherein said reaction product is selected from the group represented by the fol-lowing general formulas and mixtures thereof, (I) , and (II) , where each R1, R2, R5, and R6, lndependently is a C1-C8 divalent alkyl group or alkoxyalkyl group, R3 is a C1-C6 alkyl group, and R4 is a hydrogen or C1-C8 alkyl group.

3. The process of claim 1 wherein said solvent is selected from the group consisting of 2-dimethylamino-ethanol, 2-diethylaminoethanol, 2-diisopropylaminoethanol, 2-dibutylaminoethanol, 1-dimetnylamino-2-propanol, 3-dimethylamino-1-propanol, 2-(N-methyl-N-ethylamino)ethanol, methyl 2-(dimethylamino)ethyl ether, ethyl 2-(diethylamino) ethyl ether, methyl 2-(diisopropylamino)ethyl ether, ethyl 2-(dibutylamino)ethyl ether, methyl (1-methyl-2-dimethyl-amino)ethyl ether, ethyl 3-dimethylamino-1-propyl ether, methyl 2-(N-methyl-N-ethylamino)ethyl ether, dimethyl carbonate, diethyl carbonate, dipropyl carbonate, bis-(2-ethoxyethyl) carbonate, bis-(2-methoxyethyl) carbonate, bis-(3-methylbutyl) carbonate, monoethyl-mono-2-butoxyethyl carbonate, and mixtures thereof.

4. The process of claim 1 wherein said gaseous mixture contains a dry molar basis between about 15% and 60% carbon dioxide.

5. The process of claim 1 wherein said contact conditions include a temperature of between about -50° and 200°F and a pressure of between about 0 and about 2000 psig.

6. The process of claim 5 wherein said temperature ranges from about 0° to 100°F and said pressure ranges about 100 to 1000 psig.

7. The process of claim 1 wherein said solvent is admixed with up to about 50% by weight of an alcohol or glycol ether cosolvent.

8. The process of claim 7 wherein said cosolvent is a monoalkylether of a glycol.

9. In a process for removing carbon dioxide contain-ing acidic gas from a normally gaseous mixture containing same, wherein said gaseous mixture is contacted with a solvent at a temperature of between about -50° and 200°F
and a pressure of between about 1 and 2000 psig, the improve-ment which comprises:
contacting said gaseous mixture containing on a dry molar basis between about 15% and 95% carbon dioxide with a solvent which is liquid at said temperature and said pressure, said solvent restricted to an anhydrous solvent which physically absorbs said acidic gas but which is chemically unreactive with said acidic gas at said temperature and said pressure, said solvents selected from the group consisting of N,N-dialkylaminoalkanol, N,N-dialkoxyalkylaminoalkanol, N-alkyl-N-alkoxyalkylaminoalkanol, N,N-dialkylaminoalkylether, N,N-dialkoxyalkylaminoalkylether, N-alkyl-N-alkoxyalkylaminoalkylether, dialkyl carbonate, di(alkoxyalkyl) carbonate, mono-alkyl-monoalkoxyalkyl carbonate, and mixtures thereof, said solvent optionally in admixture with an anhydrous cosolvent which also is chemically unreactive with said acidic gas.

10. The process of claim 9 wherein said solvent is selected from the group consisting of 2-dimethylaminoethanol, 2-diethylaminoethanol, 2-diisopropylaminoethanol, 2-dibutyl-aminoethanol, 1-dimethylamino-2-propanol, 3-dimethylamino-1-propanol, 2-(N-methyl-N-ethylamino)ethanol, methyl 2-(dimethylamino)ethyl ether, ethyl 2-(diethylamino)ethyl ether, methyl 2-(diisopropylamino)ethyl ether, ethyl 2-(dibutylamino)ethyl ether, methyl (1-methyl-2-dimethyl-amino)ethyl ether, ethyl 3-dimethylamino-1-propyl ether, methyl 2-(N-methyl-N-ethylamino)ethyl ether, dimethyl carbonate, diethyl carbonate, dipropyl carbonate, bis-(2-ethoxyethyl) carbonate, bis-(2-methoxyethyl) carbonate, bis-(3-methylbutyl) carbonate, monoethyl-mono-2-butoxyethyl carbonate, and mixtures thereof.