CA1154939A - Process for the purification of nonreacting gases - Google Patents
Process for the purification of nonreacting gasesInfo
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- CA1154939A CA1154939A CA000351897A CA351897A CA1154939A CA 1154939 A CA1154939 A CA 1154939A CA 000351897 A CA000351897 A CA 000351897A CA 351897 A CA351897 A CA 351897A CA 1154939 A CA1154939 A CA 1154939A
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
- Y02P20/00—Technologies relating to chemical industry
- Y02P20/50—Improvements relating to the production of bulk chemicals
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Abstract
ABSTRACT OF THE DISCLOSURE
A process is disclosed for the purification of an inert or non-reacting gas such as lower mono-olefins by the removal of trace amounts of reacting impurities such as carbon dioxide therefrom, comprising passing said gas through an essentially non-aqueous liquid solution of alkali or alkaline earth metal hydroxides and/or weak acid salts and certain selected liquid polyhydric alcohols.
A process is disclosed for the purification of an inert or non-reacting gas such as lower mono-olefins by the removal of trace amounts of reacting impurities such as carbon dioxide therefrom, comprising passing said gas through an essentially non-aqueous liquid solution of alkali or alkaline earth metal hydroxides and/or weak acid salts and certain selected liquid polyhydric alcohols.
Description
11,952-Z
The present in~ention relates to the purification of c~rtain gas streams and, m~re particularly~ to such purlfication of such gas streams by the removal of minute quantities of impuri~ies therefrom~
The reaction of basic alkali or alkaline earth metals ~ se, or as hydroxides or weak actd salt~, with acids is ~now~ chemistry. Such chemistry, as elementary as it may seen~ has become the foundatio~ of s~gnificant commercial practices. For example, in the field o~ puri-fying indust~ial gas streams, such as hydroge~, carbonmonoxide~ air~ o~ygen, nitrogen~ argon, helium, lower mono-oleins, lower diolefins, and the like, beds or suspensions of ~olid particles of sodium or potassium hydroxides have been employed to remove gross ~mounts of acid gases. Such prucesses were discovered ko be relatively ineficient in removing ~race amounts of impurities, such as hydrogen sulfide, carbon dioxide, carbonyl sulftde9 sulfur dioxide and mercaptans and ~he like from such streams. Indeed, such alkali metal hydroxides have been taught for use as absorbents to separate acetylene from ethylene, see U,SO
The present in~ention relates to the purification of c~rtain gas streams and, m~re particularly~ to such purlfication of such gas streams by the removal of minute quantities of impuri~ies therefrom~
The reaction of basic alkali or alkaline earth metals ~ se, or as hydroxides or weak actd salt~, with acids is ~now~ chemistry. Such chemistry, as elementary as it may seen~ has become the foundatio~ of s~gnificant commercial practices. For example, in the field o~ puri-fying indust~ial gas streams, such as hydroge~, carbonmonoxide~ air~ o~ygen, nitrogen~ argon, helium, lower mono-oleins, lower diolefins, and the like, beds or suspensions of ~olid particles of sodium or potassium hydroxides have been employed to remove gross ~mounts of acid gases. Such prucesses were discovered ko be relatively ineficient in removing ~race amounts of impurities, such as hydrogen sulfide, carbon dioxide, carbonyl sulftde9 sulfur dioxide and mercaptans and ~he like from such streams. Indeed, such alkali metal hydroxides have been taught for use as absorbents to separate acetylene from ethylene, see U,SO
2,74~,517O
There is described in U.S~ Patent NoO 2,~85,332, patented January 2, 1940, a process for dehydrating and acid seques~ering of refrigeration systems by supplying to the refrigeration systems of an alkali alcoholate~ parti-cularly sodium methylola~e. The alcoholate is supylied to the sys~em to remove water by a methathysis reaction which yields the alkali hydroxide, such as sodium hydroxide.
Sodium hydroxide, or caustic, is left in ~he systemO The r .-. 1 '`'~, ~ 3 ~ 11,952-2 consequences are obvious. Caustic is a corrosive age~t and its presence in the refrigerant gas cannot be bene-icial in the long run. In add:ition, the pa~en~ speaks of removing acids as well. For example, hydrogen chloride, presumably as hydrochloric acid, is converted to sodium chloride. It may be that HCl ls more corrosive than NaClg bu~ certainly NaCl is not ad~antageous to leave in the refrigerant gas. Thus, this patent utilizes known chemis~
try to solve one problem, but the patent's process creates other problemsO As bad a problem that water may be, caustic formation cannot be perceived as significa~t improvement.
U.S. Patent Mo. 2,177,068, patented October 24, 1939, is concerned with the treatment of natural gas to remove wa~er and acid gases. The patent utilizes poly hydric alcohols in ~ombination wi~h amines, e,g " alkanol amines, or this purpose. The paten ack~owledges that it was old~ even the~, to use such amines to remove acid gases, and polyols have ~ong bee~ know~ as humectants.
In the above two processes 3 the gases beIng treated are sa~urated hydrocarbons whose inertness have been well established. The impurities being removed do not adversely affect the ~unc~ion of these gases, they only adversely affect the environment in which the gases are used.
In any event, the process of this patent suff~rs from the rellance upon amines to extract the acid gases.
These amine systems ar~ known to be corrosiveO
The evolution of industrial gas purification has followed the improvements in impurity detection~ With ~ ~ 5~ ~ 3~ 952-2 improved analytical procedures that can be applied to evaluation of industrial gas streams has come the need for superior processes or the removal of newly detectable impurities.
As employed herein, the terms '~on~reacting"
and "inert gases'l refer to those industrial gases which are capable of treatment in accordance with the process of the invention without reacting with the seLected alkali/
polyhydric alcohol solutions defined below.
It is generally understood that olefins~ due to their manner of production, storage and/or handling, may typically contain in trace amounts in the range of parts per million of some or all of the followlng impurities:
water vapor, hydrogen sulide, carbon dioxide~ carbonyl sulide, sul~ur dioxide, and mercaptans. Many commercial syntheses require an olefin feed having below 1, and preferably below 0.5 ppm o each of the impurities~
For example, in order to make suitable grade ethylene for the production of certain grades of poly-ethylene, small concentrations of CO2 (of approximately10-25 ppm) in the olefin feed ha~e to be reduced to less than 1 ppm. One present practice uses a caustic pellet absorber bed~ Two major problems associated with this procedure are: ~i) only about 3 percent of the NaOH
presPnt in ~he bed is converted to Na2CO3 (via 2NaOH +
C2 = Na2CO3 ~ %2); and (ii) after the outer coat of the pellet is conver~ed to ~he carbonate, ~he w~ter generated can cause particl~ agglomexation and bridging which in tur~ causes cha~neling and finally complete solidification o the bed. The latter is a particularly difficult problem ~ 3 ~ 11,95~-2 because the bed conten~s at times have had to be removed manually. An aqueous caustic solution has been proposed as a~ alternatlve. One problem with this approach is that the wa~er vapor from the solution is introduced into the ethylene, thus necessitating 7eolite water-adsorption beds which: (i) would be large and costly to install; a~d tii) would be e~pensive to regenerateO
The process of this invention employs a liquid, essentially non-aqueous treating medium, that is, the medium typically does not contribute more water to ~he gas undergoing treatment. The liquid treating medium exhibits (i) a high capacity (i.e. 9 solubility~ for the alkali, (ii) a very low vapor pressureg (iii) when combined with the alkali, the system is chemically stable, and (iv~ a sufficien~ly low viscosity to permit proper wetting of the gas being treatedO
The process of this invention is the use of an alkali (or alkaline ear h) metal/liquid polyhydric alcohol solution to r~move the aforedefined trace impurities, and, in particular, to si~ultaneously remove substantially all of the deleterious trace impurities when they are present, rather tha~ ~ust a single impuri~y, as has been prlor art approach.
Thus, in accordance with the present invention, a process is provided for the purification of an industrial gas stream of hydrogen, earbon monoxide, air, nitrogen, oxygen, helium, argon9 mono-olefins having from two to five carbon a~oms per molecule, diolefins having four or ive carbon atoms per molecule, paraffins, or acetylenes~
by the removal of trace amounts of reacting impurities, ~ 3 ~ 11,952-2 i~troduced therein prior to or cluring a process for the production or treatment of said gas, and including, for example, hydrogen sulfidej carbon dioxide, carbonyl sulfide, sulfur dioxide, hydroge~ chloxide, hydrogen cyanide, ~itric a id, and mercaptans. This is accomplish-ed by contacting said gas, at a temperature between about lS and 100C" with an essentially non-aqueous liquid solution comprising from about 0,5 to 15 weight percent of alkali or alkaline earth metal, measured as its hydroxide or weak inorganic acid salts, in a liquid aliphatic poly-hydric alcohol havi~g a ca~bon to oxygen ratio of one (1) to five (5), and at least two (2) oxygens thereof being separated by not more than ~wo (2) sequential carbon atoms.
In addition, the process of this invention com-p~ises continuously feeding an industrial gas streamg as described, to a body of liquid alkali/polyhydric alcohol solution, as described herein, maintained at a temperature between about 15~ and lOO~C., in which the gas s~ream is removed from contac~ wi~h the body into an atmosphere in which the partial pressure of water ther~in is insufficient to significantl~ increase the water concentration in the stream. In the preferred embodiment, the water concen-tration of the s~ream removed from the liquid body and the atmosphere is ~ess than was contained in the stream fed to the body.
The "alkali and alkaline ea~th metals" referred to herein are tho~e from groups IA, and II~ of the Period Table of Elements; specifically they are: lithiumg sodium, potassium, rubidium, ce~ium, magnesium, calci~m, strontium, and barium. As discussed hereinbelow, the preferred 6~
11,~5~-2 ~ ~ 5 ~ ~ 3 alkali metals a~e sodium and po~assium~ wi~h po~assi~
constituting the most preferresl metal ~or the pract~ce of the process o~ the present invention. For purposes of discussion only, the term "alkali" shall mean alkali metal and alkaline earth metal as hereinabove defined.
For brevity in description and for convenience, the process of the invention is primarily discussed here-in~elow ~n connection with the removal of trace amounts of impurities of the group set forth hereinabove from e~hylene gas.
The alkali value pxovided in the polyhydric alco-hol solution may be obtained from the hydroxides and weak inorganic acids of the alkali or alkaline earth metals.
The polyhydric alcohol includes glycerol and alkylene glycols, for e~Emple, those having up to twelve carbon atoms ~such as: ethylene glycol, diethylene glycol, trieth~lene glycol, tetrae~hylene glycol~ propylene glycol, dipropylene glycol, l~-butylene glycol, di-l/2~butylene glycol, and the like), and alkyl ne oxide adductsi Such alkylene oxide adduc~s are made by reacting ~h4 alkylene oxide with a polyol such as the aforementioned glycols, glycerol and other riols (for example, 1,2,6-hexane triol7 monomethyl ether o erythritol~ and the monomethyl ether of pentaerythritol). Ethylene oxide and propylene oxide adducts of the polyols are preferred. Most preferred of the adducts are the ethylene oxide adduct, alone or reacted with propylene oxide~
The process of the present invention is for the purification of a gas stream which contains as its major component an inert gas which does not reac~ chemically ~ 3 3~ ll,952-2 (non-reacting) with the liquid ~lkali/polyhydric alcohol solution selected for the proce'ss and which stream contains, in trace amounts, a minor co~ponent which chemically reacts with ~he same liquid alkali/polyhydrie al~ohol solution~
Examples of the non-reacting ma,jor gas components are:
hydrogen; carbon monoxide; air; oxygen; nitrogen; helium;
~rgon; lower mono-ole~ins, lower diolefins and derivatives ~hereof; parafins', such as methane, ethane and the like;
and acetylenes. "olefins" referred to herein are under stood to be those selected from the group consisting of mono-olefins having from two to five carbon atoms per molecule, diolefins having four or five carbon ato~s per molecule and derivatives ~hereof~ Typical of such olefins are ethylene, propylene, the butylenes, the pent~lenes, 1,3-butadiene,lS4-pentadiene and derivatives thereof, . The reacting components are primarily those which form acids in aqueous solutions. These materials react ~i h the alkali of the solvent to form inorganic or organic salts therewith, thereby proYiding an effective means o their removal from the gas. Examples of acidic or reacting components include hydrogen sulfide, carbon dioxide, carbo~yl sulfide, sulur dioxide, mer~aptans9 hydroge~ chloride, hydrogen cyanide, nitric oxide, and the like~
It has been found in practicing this process that water vapor may be simultaneously removed from t~e feed gas m2terial even though it is usually present in quantities much higher than trace quantities~ The removal of water vapor with the alkali1polyhydric alcohol solution is believed to follow a completely different mechanism ~ 3 9 11,952-2 from that followed by the removal of the ~race quantities of the reacting components of the feed material~ The mechanism for wa~er removal may be based on ~he humectant properties of ~he polyhydric alcohols. In addition, i~ is believed that fuxther solution capaci~y can be effected by such metathesis reactions as:
RONa ~ H2O = ROH ~ NaOH
and Na2GO3 + ~ O = ~NaOH ~ ~2 It has additionally been found that water removal by the alkali/polyhydric alcohol solution can ~e effectet with respect to contained water in the industrial gas stream whîch is present up to the limi s of saturation in the stream (viz. dew point). The hygroscopicity q~lity of the polyhydric alcohol which induces water absorption (via hydrogen bonding mechanism) has bePn found not to diminish capacity of the alkali/polyol solution for reaction of the solution wi~h other reacting components in trace æmounts listed above~ Thus, such reactions which involve the reacting component in which water can become an equi-librium component seem not to be adversely affected by the presence of such large amounts of water in ~he solution.
Accordingly, it is understood that any relatively small amount of water vapor contained in ~he non-reacting gas feed (introduced therein prior to or dur~ng a process for the production or treatment of ~he non-reacting gas) is scavenged by the polyhydric alcohol and alkali of the absorbent solution. Similarly, any water of reaction formed in ~itu is also scavenged by the alkali solution components as it is ound. Thus, the absorbent solu~ion is characterized herein as an "essentially non-aqueous" solution during ~ 3~ 11,952-2 the period in the process of the inven~ion when it i5 act-ing to effec~ puri~ication by removal of trace amounts o reac~ing impurities from the non-rP cting gas stream.
It should be noted in passing that the partial pressure of water maintained in the vapor phase above the absorbe~t solution is lower thc~ that in the non-reacting ga~ stream (mole basls), so that water is not reintroduced into ~he non~reacting gas stream.
The typical, non-aqueous absorbent solutions of the inve~tion may initially be prepared by the addition of alkali or alkaline earth metal values to a solution of the selected liquid polyhydric alcohol ~glycerol and glycols of he stated group). This addition to the poly-hydric alcohol solution may be by the direct addition of alkali metal (such as by ~he addition of sodium turnings~, by the ad~ition of ~he alkali or alkaline earth metal ~n the form of i~s hydroxide, or by the addition of the alkali or alkaline ear~h metal in the fonm of its carbonate or bicarbonate, or other similarly weak inorganic acid salts. The addition in the metal or hydroxide or salt form ~ may reac ~n a desired manner to form the poly-hydric alcoholate of the alkali or alkaline earth metal.
It may be that a carbonate form is first hydrolyzed by water to ~he hydroxide before alcoholate formationO
The alkali/polyhydric alcohol solutions are formed by simply mixing the alkali component wi~h the poly~
hydric alcohol component until a solution of the two compo-nents is formedO This can be done at room temperature or at elevated temperatures, viz, temperatures as high as the boiling point of the alcohol, Normally, mixing be ween 10.
~'~ 5 ~ 9 3 9 Ll,952-~
the two components is effe~t~d at temperatures o~ from abou~ 30C. to about 250C7, preferably at temperatures of from about 40C. to about 2~QC.
The alkali value supplied to the polyhydric alcohol may be in ~he orm of the hydroxide, and as ~alts of weak inorganic acids 7 ViZ. carbonio acid~ Preferably, the alkali is supplied as the hydroxide, thP carbonate and/or the bicarbonateO A3 pointed out below, the greatest efficiency of impurity removal ~ypically whe~ ~he alkali is employed as the hydroxide~ but that the most stable system is achieved with the carbonate. Illustrative of suitable alkali are, by way of example only, lithium hydroxide, lithi~m carbonate, lith1um blcarbonate~ sodium hydroxide, sodium carbonate~ sodium bicarbonate, potassium hydroxide, potassium carbonate, potassium bicarboDa~e9 rubidium hydroxide, rubidium carbonate, rubidium bicar-bona~e, cesium hydroxide, cesium carbonate, cesium bicar-bonate~ magnesium hydroxideg magnesium carbonate, magne-sium bicarbonate, calcium hydroxide~ calcium carbonate, c~lcium biearbonate~ strontium hydroxide, strontium carbo-nat~, barium hydroxide, barium carbonate, and barium bi-carbonate.
It has been found that addition of from about 0~5 to 15 weight percent of contained alkali or alkaline earth metal values may be employed to form an operable non-aqueous absorben~ solution h~ving a satisactory contained salt concentration3 after reaction with a minor portion of the liquid polyhydric alcohol (glycerol or selected glycol)3 presuming salt formation. It has additionally been found that elevation to a ~emperature o the order of about 200C.
~ 3 ~ 1~,952-2 is desirable to effect an activation of the carbonate to the hydroxide f ormO The alkali, in the hydroxide form, may enter in to aD equilibrium reactiorl which generate the alcoholate and a small amo~t of water~ Whe~her ~uch alcoholates contribute to the process of this invention is hard to determine, but it is believed alcoholate formation plays a significan~ role.
TG develop process design data and to study the charac~eristics of the column operation, a high-pressure pilot unit was constructed, An Oldershaw type glass column, 28 mm ID, containing 20 trays was employed. The glass column was mounted in a pressure gauge (Jergusen Co.) for high-pressure operations. The feed gas (ethylene3 was preheated to the desired column temperature in a shell-and-tube heat exchangerO Another heat exchanger between the d rculating pump and the column was used to ~ontrol the temperature of ~he liquid entering the top of the columnu Caus ic/glycol solutions were prepared by feed-ing the desired amoun~q of sodium hydroxide into the glycol in a mix tank. The solution was stirred at 50-75C. under nitrogen purge, Three hours of stirring was sufficient to dissolve the raustic pellets completely. The solution was then transferred ~o ~he reservoir section of the Oldershaw column for water s~ripping. The solution was circulated at S0CO~ 50 psig, ~th nitrogen gas flowing eountercurrent to the liquid flow. The s~ripping was continued until the water concentration in the purge nitrogen decreased below 10 ppm.
The Oldershaw colu~n contained three sample taps to analyze the gas entering and leaving the column. The 12~
~ 3 ~ 11,952 2 gas samples were continuously analyzed for C02 and ~O.
Also periodic liquid samples were taken for viscosity de~ermina~ions.
The column was tested under a varlety of operat ing conditions. These tests can be summari7ed as follows:
Solvents: Ethylene Glycol Triethylene Glycol (TEG) Reactan~s:* 2% (wt) NaOH
or 5% (wt~ NaOH
270 Pyrogallol 2% Mn~12 Feed Gas: N2 or C2H4 Column Pres ure, psig: 100-365 Column Temperature, G.: 38-60Co Liquid-to-Gas M~ss Flow Ratio: 0.023-0.167 Gas Velocity, ft/sec~: 0.15-0~30 CO Gonce~tration in Fe~d Gas; ppm: 1-110 O Co~centration in D~scharge Gas, ppm: 1-1000 O Concentration in F~d &as, ppm: 0-50 H S Concentration in F~ed Gas, ppm: 0-20 For good ga~-liquid contacting and stable operat-ion) the gas velocity should not exceed the column flooding velocityO Flooding velocities were calculated using a typical correlation for sieve towers for 5 percent *
Reactant weight percent based on the weight of the total solutio~
~ 3.
~5~3~ ll,g52-2 NaOH/TEG solutions as set forth in Table I:
T LE
Solution: 5 percent NaOH~TEG
Gas: C~H, Colu~n ~i~meter: 28 mm Liquid-~o- Column Column ~looding Gas Mass Pressure,Temperature9 Velocity*
Flow Ratio __2~8__ ~C.
:LOO, 1 300 20 1 o 0~
O d 2 300 20 0 5 97 O~ 1 400 20 l o 13 O ~ 2 400 20 iL ~ 05 Ool 15 75 5 0~2 15 ~5 ~5 Ool 115 40 1~8 ____ Based on total cross-sectional area of the tower.
For column operatîon above 250 psig~ the calcu lated flooding veloci~y is about 1.0 ~t/sec~ The data base for this correlation consista of mostly industrial operations with aqeous solu~ions, and thus, liquid v~scosity is not included in ~he correlation~ Thereor , its applicability to ~ viscous medium such as caustic/
glycol solutions is uncertain. The pilot uni~ was operated at gas veloci~ies 0.15 ~o 0.30 ft/sec. When the g8S
~eloci~y exceeded 0~7 ft~sec. column operations beeame un~
stable. This value corresponds to roughly 60 percent of the calculated flooded velocity, suggesting hat the flood-ing velocity correlations should be adjustet for viscosity.
14.
~.~LS~391~ ll,9S2 ~
With bo~h ethylene g:lycol and triethylene ~lycol/
caustic solutions, tray action was excellent ln the indi-cated operating range.
The results of the C02 remo~7al experiments are set forth irl Table IIo 15 .
11, 952-2 ~L ~ ~ O
o u e~ ~ _ ~ o o s:i ~J O O
i~ b _ ~ _ ~
a ~1 ~ ~ ~ ~ ~3 ~ o ~ o o g a ~ ol o ~3 o o c~ e~ o ~ Q ~ O O e~ e~ O C~ O
~ ~
0~ CCC~
,~ r 8 .~ ~: 8 ~ 8 ~ ~ ~ ~ ~ ~
i O ~ ~ ~ N ~ ~ C N ~t C ~ o o o ~ o ~ o o o o o .
~5 ~ o ~ 2; 8 ~ o ~ o ~ 9 8 o o o o ~ o ~> ~ ~ ~
_ 1~ O N N ~ ~'9 0 ~ O O F~ t-- ~ IC 11 oN o O ~0 ~ q? b3 ~ ~ ~7 0 ~n ~ ~ ~ b~ o ~
~c~gOgOOOgOOOOOO '~ O
a ~ _ Qf e D~ O 1~1 C
4" ~5~
~ ~ 8 ~ $ 5~ d ~ ~
~ ~ p., æ
~6 .
11,952-2 Based on these results, the following was concluded:
1. Both ethylene glycol ~nd triethylene glycol/
caustic mixtures are very effec:tive for trace CO2 removal.
Triethylene glycol, however~ is preferred due to its lower vapor pre~sureq 2. It is possi~le to reduce the CO2 concentra-tion in ethylene to less than 0.5 ppm wi~h a ten tray column, provided that good gas~liquid contact is ~aintained.
There is described in U.S~ Patent NoO 2,~85,332, patented January 2, 1940, a process for dehydrating and acid seques~ering of refrigeration systems by supplying to the refrigeration systems of an alkali alcoholate~ parti-cularly sodium methylola~e. The alcoholate is supylied to the sys~em to remove water by a methathysis reaction which yields the alkali hydroxide, such as sodium hydroxide.
Sodium hydroxide, or caustic, is left in ~he systemO The r .-. 1 '`'~, ~ 3 ~ 11,952-2 consequences are obvious. Caustic is a corrosive age~t and its presence in the refrigerant gas cannot be bene-icial in the long run. In add:ition, the pa~en~ speaks of removing acids as well. For example, hydrogen chloride, presumably as hydrochloric acid, is converted to sodium chloride. It may be that HCl ls more corrosive than NaClg bu~ certainly NaCl is not ad~antageous to leave in the refrigerant gas. Thus, this patent utilizes known chemis~
try to solve one problem, but the patent's process creates other problemsO As bad a problem that water may be, caustic formation cannot be perceived as significa~t improvement.
U.S. Patent Mo. 2,177,068, patented October 24, 1939, is concerned with the treatment of natural gas to remove wa~er and acid gases. The patent utilizes poly hydric alcohols in ~ombination wi~h amines, e,g " alkanol amines, or this purpose. The paten ack~owledges that it was old~ even the~, to use such amines to remove acid gases, and polyols have ~ong bee~ know~ as humectants.
In the above two processes 3 the gases beIng treated are sa~urated hydrocarbons whose inertness have been well established. The impurities being removed do not adversely affect the ~unc~ion of these gases, they only adversely affect the environment in which the gases are used.
In any event, the process of this patent suff~rs from the rellance upon amines to extract the acid gases.
These amine systems ar~ known to be corrosiveO
The evolution of industrial gas purification has followed the improvements in impurity detection~ With ~ ~ 5~ ~ 3~ 952-2 improved analytical procedures that can be applied to evaluation of industrial gas streams has come the need for superior processes or the removal of newly detectable impurities.
As employed herein, the terms '~on~reacting"
and "inert gases'l refer to those industrial gases which are capable of treatment in accordance with the process of the invention without reacting with the seLected alkali/
polyhydric alcohol solutions defined below.
It is generally understood that olefins~ due to their manner of production, storage and/or handling, may typically contain in trace amounts in the range of parts per million of some or all of the followlng impurities:
water vapor, hydrogen sulide, carbon dioxide~ carbonyl sulide, sul~ur dioxide, and mercaptans. Many commercial syntheses require an olefin feed having below 1, and preferably below 0.5 ppm o each of the impurities~
For example, in order to make suitable grade ethylene for the production of certain grades of poly-ethylene, small concentrations of CO2 (of approximately10-25 ppm) in the olefin feed ha~e to be reduced to less than 1 ppm. One present practice uses a caustic pellet absorber bed~ Two major problems associated with this procedure are: ~i) only about 3 percent of the NaOH
presPnt in ~he bed is converted to Na2CO3 (via 2NaOH +
C2 = Na2CO3 ~ %2); and (ii) after the outer coat of the pellet is conver~ed to ~he carbonate, ~he w~ter generated can cause particl~ agglomexation and bridging which in tur~ causes cha~neling and finally complete solidification o the bed. The latter is a particularly difficult problem ~ 3 ~ 11,95~-2 because the bed conten~s at times have had to be removed manually. An aqueous caustic solution has been proposed as a~ alternatlve. One problem with this approach is that the wa~er vapor from the solution is introduced into the ethylene, thus necessitating 7eolite water-adsorption beds which: (i) would be large and costly to install; a~d tii) would be e~pensive to regenerateO
The process of this invention employs a liquid, essentially non-aqueous treating medium, that is, the medium typically does not contribute more water to ~he gas undergoing treatment. The liquid treating medium exhibits (i) a high capacity (i.e. 9 solubility~ for the alkali, (ii) a very low vapor pressureg (iii) when combined with the alkali, the system is chemically stable, and (iv~ a sufficien~ly low viscosity to permit proper wetting of the gas being treatedO
The process of this invention is the use of an alkali (or alkaline ear h) metal/liquid polyhydric alcohol solution to r~move the aforedefined trace impurities, and, in particular, to si~ultaneously remove substantially all of the deleterious trace impurities when they are present, rather tha~ ~ust a single impuri~y, as has been prlor art approach.
Thus, in accordance with the present invention, a process is provided for the purification of an industrial gas stream of hydrogen, earbon monoxide, air, nitrogen, oxygen, helium, argon9 mono-olefins having from two to five carbon a~oms per molecule, diolefins having four or ive carbon atoms per molecule, paraffins, or acetylenes~
by the removal of trace amounts of reacting impurities, ~ 3 ~ 11,952-2 i~troduced therein prior to or cluring a process for the production or treatment of said gas, and including, for example, hydrogen sulfidej carbon dioxide, carbonyl sulfide, sulfur dioxide, hydroge~ chloxide, hydrogen cyanide, ~itric a id, and mercaptans. This is accomplish-ed by contacting said gas, at a temperature between about lS and 100C" with an essentially non-aqueous liquid solution comprising from about 0,5 to 15 weight percent of alkali or alkaline earth metal, measured as its hydroxide or weak inorganic acid salts, in a liquid aliphatic poly-hydric alcohol havi~g a ca~bon to oxygen ratio of one (1) to five (5), and at least two (2) oxygens thereof being separated by not more than ~wo (2) sequential carbon atoms.
In addition, the process of this invention com-p~ises continuously feeding an industrial gas streamg as described, to a body of liquid alkali/polyhydric alcohol solution, as described herein, maintained at a temperature between about 15~ and lOO~C., in which the gas s~ream is removed from contac~ wi~h the body into an atmosphere in which the partial pressure of water ther~in is insufficient to significantl~ increase the water concentration in the stream. In the preferred embodiment, the water concen-tration of the s~ream removed from the liquid body and the atmosphere is ~ess than was contained in the stream fed to the body.
The "alkali and alkaline ea~th metals" referred to herein are tho~e from groups IA, and II~ of the Period Table of Elements; specifically they are: lithiumg sodium, potassium, rubidium, ce~ium, magnesium, calci~m, strontium, and barium. As discussed hereinbelow, the preferred 6~
11,~5~-2 ~ ~ 5 ~ ~ 3 alkali metals a~e sodium and po~assium~ wi~h po~assi~
constituting the most preferresl metal ~or the pract~ce of the process o~ the present invention. For purposes of discussion only, the term "alkali" shall mean alkali metal and alkaline earth metal as hereinabove defined.
For brevity in description and for convenience, the process of the invention is primarily discussed here-in~elow ~n connection with the removal of trace amounts of impurities of the group set forth hereinabove from e~hylene gas.
The alkali value pxovided in the polyhydric alco-hol solution may be obtained from the hydroxides and weak inorganic acids of the alkali or alkaline earth metals.
The polyhydric alcohol includes glycerol and alkylene glycols, for e~Emple, those having up to twelve carbon atoms ~such as: ethylene glycol, diethylene glycol, trieth~lene glycol, tetrae~hylene glycol~ propylene glycol, dipropylene glycol, l~-butylene glycol, di-l/2~butylene glycol, and the like), and alkyl ne oxide adductsi Such alkylene oxide adduc~s are made by reacting ~h4 alkylene oxide with a polyol such as the aforementioned glycols, glycerol and other riols (for example, 1,2,6-hexane triol7 monomethyl ether o erythritol~ and the monomethyl ether of pentaerythritol). Ethylene oxide and propylene oxide adducts of the polyols are preferred. Most preferred of the adducts are the ethylene oxide adduct, alone or reacted with propylene oxide~
The process of the present invention is for the purification of a gas stream which contains as its major component an inert gas which does not reac~ chemically ~ 3 3~ ll,952-2 (non-reacting) with the liquid ~lkali/polyhydric alcohol solution selected for the proce'ss and which stream contains, in trace amounts, a minor co~ponent which chemically reacts with ~he same liquid alkali/polyhydrie al~ohol solution~
Examples of the non-reacting ma,jor gas components are:
hydrogen; carbon monoxide; air; oxygen; nitrogen; helium;
~rgon; lower mono-ole~ins, lower diolefins and derivatives ~hereof; parafins', such as methane, ethane and the like;
and acetylenes. "olefins" referred to herein are under stood to be those selected from the group consisting of mono-olefins having from two to five carbon atoms per molecule, diolefins having four or five carbon ato~s per molecule and derivatives ~hereof~ Typical of such olefins are ethylene, propylene, the butylenes, the pent~lenes, 1,3-butadiene,lS4-pentadiene and derivatives thereof, . The reacting components are primarily those which form acids in aqueous solutions. These materials react ~i h the alkali of the solvent to form inorganic or organic salts therewith, thereby proYiding an effective means o their removal from the gas. Examples of acidic or reacting components include hydrogen sulfide, carbon dioxide, carbo~yl sulfide, sulur dioxide, mer~aptans9 hydroge~ chloride, hydrogen cyanide, nitric oxide, and the like~
It has been found in practicing this process that water vapor may be simultaneously removed from t~e feed gas m2terial even though it is usually present in quantities much higher than trace quantities~ The removal of water vapor with the alkali1polyhydric alcohol solution is believed to follow a completely different mechanism ~ 3 9 11,952-2 from that followed by the removal of the ~race quantities of the reacting components of the feed material~ The mechanism for wa~er removal may be based on ~he humectant properties of ~he polyhydric alcohols. In addition, i~ is believed that fuxther solution capaci~y can be effected by such metathesis reactions as:
RONa ~ H2O = ROH ~ NaOH
and Na2GO3 + ~ O = ~NaOH ~ ~2 It has additionally been found that water removal by the alkali/polyhydric alcohol solution can ~e effectet with respect to contained water in the industrial gas stream whîch is present up to the limi s of saturation in the stream (viz. dew point). The hygroscopicity q~lity of the polyhydric alcohol which induces water absorption (via hydrogen bonding mechanism) has bePn found not to diminish capacity of the alkali/polyol solution for reaction of the solution wi~h other reacting components in trace æmounts listed above~ Thus, such reactions which involve the reacting component in which water can become an equi-librium component seem not to be adversely affected by the presence of such large amounts of water in ~he solution.
Accordingly, it is understood that any relatively small amount of water vapor contained in ~he non-reacting gas feed (introduced therein prior to or dur~ng a process for the production or treatment of ~he non-reacting gas) is scavenged by the polyhydric alcohol and alkali of the absorbent solution. Similarly, any water of reaction formed in ~itu is also scavenged by the alkali solution components as it is ound. Thus, the absorbent solu~ion is characterized herein as an "essentially non-aqueous" solution during ~ 3~ 11,952-2 the period in the process of the inven~ion when it i5 act-ing to effec~ puri~ication by removal of trace amounts o reac~ing impurities from the non-rP cting gas stream.
It should be noted in passing that the partial pressure of water maintained in the vapor phase above the absorbe~t solution is lower thc~ that in the non-reacting ga~ stream (mole basls), so that water is not reintroduced into ~he non~reacting gas stream.
The typical, non-aqueous absorbent solutions of the inve~tion may initially be prepared by the addition of alkali or alkaline earth metal values to a solution of the selected liquid polyhydric alcohol ~glycerol and glycols of he stated group). This addition to the poly-hydric alcohol solution may be by the direct addition of alkali metal (such as by ~he addition of sodium turnings~, by the ad~ition of ~he alkali or alkaline earth metal ~n the form of i~s hydroxide, or by the addition of the alkali or alkaline ear~h metal in the fonm of its carbonate or bicarbonate, or other similarly weak inorganic acid salts. The addition in the metal or hydroxide or salt form ~ may reac ~n a desired manner to form the poly-hydric alcoholate of the alkali or alkaline earth metal.
It may be that a carbonate form is first hydrolyzed by water to ~he hydroxide before alcoholate formationO
The alkali/polyhydric alcohol solutions are formed by simply mixing the alkali component wi~h the poly~
hydric alcohol component until a solution of the two compo-nents is formedO This can be done at room temperature or at elevated temperatures, viz, temperatures as high as the boiling point of the alcohol, Normally, mixing be ween 10.
~'~ 5 ~ 9 3 9 Ll,952-~
the two components is effe~t~d at temperatures o~ from abou~ 30C. to about 250C7, preferably at temperatures of from about 40C. to about 2~QC.
The alkali value supplied to the polyhydric alcohol may be in ~he orm of the hydroxide, and as ~alts of weak inorganic acids 7 ViZ. carbonio acid~ Preferably, the alkali is supplied as the hydroxide, thP carbonate and/or the bicarbonateO A3 pointed out below, the greatest efficiency of impurity removal ~ypically whe~ ~he alkali is employed as the hydroxide~ but that the most stable system is achieved with the carbonate. Illustrative of suitable alkali are, by way of example only, lithium hydroxide, lithi~m carbonate, lith1um blcarbonate~ sodium hydroxide, sodium carbonate~ sodium bicarbonate, potassium hydroxide, potassium carbonate, potassium bicarboDa~e9 rubidium hydroxide, rubidium carbonate, rubidium bicar-bona~e, cesium hydroxide, cesium carbonate, cesium bicar-bonate~ magnesium hydroxideg magnesium carbonate, magne-sium bicarbonate, calcium hydroxide~ calcium carbonate, c~lcium biearbonate~ strontium hydroxide, strontium carbo-nat~, barium hydroxide, barium carbonate, and barium bi-carbonate.
It has been found that addition of from about 0~5 to 15 weight percent of contained alkali or alkaline earth metal values may be employed to form an operable non-aqueous absorben~ solution h~ving a satisactory contained salt concentration3 after reaction with a minor portion of the liquid polyhydric alcohol (glycerol or selected glycol)3 presuming salt formation. It has additionally been found that elevation to a ~emperature o the order of about 200C.
~ 3 ~ 1~,952-2 is desirable to effect an activation of the carbonate to the hydroxide f ormO The alkali, in the hydroxide form, may enter in to aD equilibrium reactiorl which generate the alcoholate and a small amo~t of water~ Whe~her ~uch alcoholates contribute to the process of this invention is hard to determine, but it is believed alcoholate formation plays a significan~ role.
TG develop process design data and to study the charac~eristics of the column operation, a high-pressure pilot unit was constructed, An Oldershaw type glass column, 28 mm ID, containing 20 trays was employed. The glass column was mounted in a pressure gauge (Jergusen Co.) for high-pressure operations. The feed gas (ethylene3 was preheated to the desired column temperature in a shell-and-tube heat exchangerO Another heat exchanger between the d rculating pump and the column was used to ~ontrol the temperature of ~he liquid entering the top of the columnu Caus ic/glycol solutions were prepared by feed-ing the desired amoun~q of sodium hydroxide into the glycol in a mix tank. The solution was stirred at 50-75C. under nitrogen purge, Three hours of stirring was sufficient to dissolve the raustic pellets completely. The solution was then transferred ~o ~he reservoir section of the Oldershaw column for water s~ripping. The solution was circulated at S0CO~ 50 psig, ~th nitrogen gas flowing eountercurrent to the liquid flow. The s~ripping was continued until the water concentration in the purge nitrogen decreased below 10 ppm.
The Oldershaw colu~n contained three sample taps to analyze the gas entering and leaving the column. The 12~
~ 3 ~ 11,952 2 gas samples were continuously analyzed for C02 and ~O.
Also periodic liquid samples were taken for viscosity de~ermina~ions.
The column was tested under a varlety of operat ing conditions. These tests can be summari7ed as follows:
Solvents: Ethylene Glycol Triethylene Glycol (TEG) Reactan~s:* 2% (wt) NaOH
or 5% (wt~ NaOH
270 Pyrogallol 2% Mn~12 Feed Gas: N2 or C2H4 Column Pres ure, psig: 100-365 Column Temperature, G.: 38-60Co Liquid-to-Gas M~ss Flow Ratio: 0.023-0.167 Gas Velocity, ft/sec~: 0.15-0~30 CO Gonce~tration in Fe~d Gas; ppm: 1-110 O Co~centration in D~scharge Gas, ppm: 1-1000 O Concentration in F~d &as, ppm: 0-50 H S Concentration in F~ed Gas, ppm: 0-20 For good ga~-liquid contacting and stable operat-ion) the gas velocity should not exceed the column flooding velocityO Flooding velocities were calculated using a typical correlation for sieve towers for 5 percent *
Reactant weight percent based on the weight of the total solutio~
~ 3.
~5~3~ ll,g52-2 NaOH/TEG solutions as set forth in Table I:
T LE
Solution: 5 percent NaOH~TEG
Gas: C~H, Colu~n ~i~meter: 28 mm Liquid-~o- Column Column ~looding Gas Mass Pressure,Temperature9 Velocity*
Flow Ratio __2~8__ ~C.
:LOO, 1 300 20 1 o 0~
O d 2 300 20 0 5 97 O~ 1 400 20 l o 13 O ~ 2 400 20 iL ~ 05 Ool 15 75 5 0~2 15 ~5 ~5 Ool 115 40 1~8 ____ Based on total cross-sectional area of the tower.
For column operatîon above 250 psig~ the calcu lated flooding veloci~y is about 1.0 ~t/sec~ The data base for this correlation consista of mostly industrial operations with aqeous solu~ions, and thus, liquid v~scosity is not included in ~he correlation~ Thereor , its applicability to ~ viscous medium such as caustic/
glycol solutions is uncertain. The pilot uni~ was operated at gas veloci~ies 0.15 ~o 0.30 ft/sec. When the g8S
~eloci~y exceeded 0~7 ft~sec. column operations beeame un~
stable. This value corresponds to roughly 60 percent of the calculated flooded velocity, suggesting hat the flood-ing velocity correlations should be adjustet for viscosity.
14.
~.~LS~391~ ll,9S2 ~
With bo~h ethylene g:lycol and triethylene ~lycol/
caustic solutions, tray action was excellent ln the indi-cated operating range.
The results of the C02 remo~7al experiments are set forth irl Table IIo 15 .
11, 952-2 ~L ~ ~ O
o u e~ ~ _ ~ o o s:i ~J O O
i~ b _ ~ _ ~
a ~1 ~ ~ ~ ~ ~3 ~ o ~ o o g a ~ ol o ~3 o o c~ e~ o ~ Q ~ O O e~ e~ O C~ O
~ ~
0~ CCC~
,~ r 8 .~ ~: 8 ~ 8 ~ ~ ~ ~ ~ ~
i O ~ ~ ~ N ~ ~ C N ~t C ~ o o o ~ o ~ o o o o o .
~5 ~ o ~ 2; 8 ~ o ~ o ~ 9 8 o o o o ~ o ~> ~ ~ ~
_ 1~ O N N ~ ~'9 0 ~ O O F~ t-- ~ IC 11 oN o O ~0 ~ q? b3 ~ ~ ~7 0 ~n ~ ~ ~ b~ o ~
~c~gOgOOOgOOOOOO '~ O
a ~ _ Qf e D~ O 1~1 C
4" ~5~
~ ~ 8 ~ $ 5~ d ~ ~
~ ~ p., æ
~6 .
11,952-2 Based on these results, the following was concluded:
1. Both ethylene glycol ~nd triethylene glycol/
caustic mixtures are very effec:tive for trace CO2 removal.
Triethylene glycol, however~ is preferred due to its lower vapor pre~sureq 2. It is possi~le to reduce the CO2 concentra-tion in ethylene to less than 0.5 ppm wi~h a ten tray column, provided that good gas~liquid contact is ~aintained.
3. Liquid-to~gas flow rate ratios as low as 0.05 are sufficient for efficien~ C02 removal.
4. Five percent NaOH/TE& solution~ flow esily at 40-50C. with good tray ac~ion. The viscosity of thls solution at 40Co is 110 cps~
5. After three weeks of continuous operation with one batch of 5 percent NaOH/TEG solution, no viscos~
ity increase was observed, even though ~he solution contain-ed the reaction by-product Na2GO3.
ity increase was observed, even though ~he solution contain-ed the reaction by-product Na2GO3.
6. No Na2CO3 sol~d settlîng was apparent in the reservoirO However, some solids were tr~pped in the liquid filters. It appears that the settling velocity is very slow.
7. C2 removal was equally efficient before and after the wa~er content of the solution was stripped off.
One batch of 5 percent NaOH/TEG solution was operated continuously for three weeks until the C02 concen-tration in the column discharge ~tarted increasing due to decreased NaOH content relative to the ~otal solution. To test whether the decrease in CO~ removal efficiency is due to lack of free water in the system or due to decreased 3~ 11,g52-2 ~aOH content, a pint of TEG containing 0.1 weight percent H~O was added to the liquid reservoir and the column was restarted, No improvement in CO2 removal was observed, indicating that the decreased NaOH content was responsible for the decreased column effici.ency.
D~ring the lengthy period of continuous operation, ~hree liquid samples were taken or viscosity measurements.
No change in viscosity was observed, suggesting the dimeri-zation and oligomerization reactions are insignifican~.
The same solution was filtered to remove the Na2C03 solids9 because no noticeable set~ling occurred in the liquid reservoir. This observation is consistent with settling velocity calculations using the Stokes equation. Thes~
calcula~ions show, for example, for a Na2C03 particle one micron in di$meter, the settling velocity is 0O005 ft/day at 40C. in 5 percent NaOH/TEG solution. From seanning electron microscopy measurements, Na2CO3 paxticles ran~e in size from 1-10 microns, mostly in the lower size range.
These results suggest that a filtration step may be necessary for solids removal.
In addition to the employment of sodiu~ and lithium as alkali metals for the in situ reaction with the selected glycol to form the non-aqueous absorben~ solu~ion9 tests were conducted to determine the effectiveness of loading various glycolates with various alkaline earth metal hydroxides. The following Tables III and IY show, respectively., the loading of equivalent weight percentages of strontium, barium and calcium hydroxides to diethylene glycol, and the resultant carbon dioxide lo~s and percent-age of theoretical carbsn dioxide displaced for nine (9?
18.
~5~93~ 11,952-2 cycles of regeneration employing these alkaline earth metal hydroxides.
Fig. 3 of the drawings set forth a graphical representation of comparative non-aqueous reversible carbon dioxide scavenging efficiency for barium, calcium, potassium and sodium-based systems through the nine cycles of regeneration. It is to be noted tha~ the sodium~based system presents a consistently high efficiency o~ operatio~
through all ni~e cycles.
T~BLE III
INITIAL CO LOADINGS IN VARIOUS GLYCOLATES
~~~
Solvent CO~ Uptake, Solvent W
5 wto % Sr(OH) in diethylene ~l~col 125 1O95 5 wto % Ba~OH) in diethylene gl~col 125 2~80 5 wt. % Ca(OH) in diethylene gl~col 125 4O50 Conditions:
Room temperature C2 pressure ~ atmO
19 .
~l~5~93'~ ll,9S~ 2 TABLE IV
=~_ Regenleration C02 Loss, g of Theoretical Solvent Cycle _ gm~
5 wt % Sr(OH)2 in diethylene glycol 1 0.8 44 2 0.3 17 3 0.5 28 ~ 0.9 50 0.4 22 6 0.4 22 7 0.4 ~2 ~ 0.3 17 9 0.35 19 5 wt % BatOH)2 in diethylene glycol 1 0.5 39 2 0.5 39 3 0.35 27 . 4 0.45 35 D.4 31 6 0.5 39 ? 0.5 39
One batch of 5 percent NaOH/TEG solution was operated continuously for three weeks until the C02 concen-tration in the column discharge ~tarted increasing due to decreased NaOH content relative to the ~otal solution. To test whether the decrease in CO~ removal efficiency is due to lack of free water in the system or due to decreased 3~ 11,g52-2 ~aOH content, a pint of TEG containing 0.1 weight percent H~O was added to the liquid reservoir and the column was restarted, No improvement in CO2 removal was observed, indicating that the decreased NaOH content was responsible for the decreased column effici.ency.
D~ring the lengthy period of continuous operation, ~hree liquid samples were taken or viscosity measurements.
No change in viscosity was observed, suggesting the dimeri-zation and oligomerization reactions are insignifican~.
The same solution was filtered to remove the Na2C03 solids9 because no noticeable set~ling occurred in the liquid reservoir. This observation is consistent with settling velocity calculations using the Stokes equation. Thes~
calcula~ions show, for example, for a Na2C03 particle one micron in di$meter, the settling velocity is 0O005 ft/day at 40C. in 5 percent NaOH/TEG solution. From seanning electron microscopy measurements, Na2CO3 paxticles ran~e in size from 1-10 microns, mostly in the lower size range.
These results suggest that a filtration step may be necessary for solids removal.
In addition to the employment of sodiu~ and lithium as alkali metals for the in situ reaction with the selected glycol to form the non-aqueous absorben~ solu~ion9 tests were conducted to determine the effectiveness of loading various glycolates with various alkaline earth metal hydroxides. The following Tables III and IY show, respectively., the loading of equivalent weight percentages of strontium, barium and calcium hydroxides to diethylene glycol, and the resultant carbon dioxide lo~s and percent-age of theoretical carbsn dioxide displaced for nine (9?
18.
~5~93~ 11,952-2 cycles of regeneration employing these alkaline earth metal hydroxides.
Fig. 3 of the drawings set forth a graphical representation of comparative non-aqueous reversible carbon dioxide scavenging efficiency for barium, calcium, potassium and sodium-based systems through the nine cycles of regeneration. It is to be noted tha~ the sodium~based system presents a consistently high efficiency o~ operatio~
through all ni~e cycles.
T~BLE III
INITIAL CO LOADINGS IN VARIOUS GLYCOLATES
~~~
Solvent CO~ Uptake, Solvent W
5 wto % Sr(OH) in diethylene ~l~col 125 1O95 5 wto % Ba~OH) in diethylene gl~col 125 2~80 5 wt. % Ca(OH) in diethylene gl~col 125 4O50 Conditions:
Room temperature C2 pressure ~ atmO
19 .
~l~5~93'~ ll,9S~ 2 TABLE IV
=~_ Regenleration C02 Loss, g of Theoretical Solvent Cycle _ gm~
5 wt % Sr(OH)2 in diethylene glycol 1 0.8 44 2 0.3 17 3 0.5 28 ~ 0.9 50 0.4 22 6 0.4 22 7 0.4 ~2 ~ 0.3 17 9 0.35 19 5 wt % BatOH)2 in diethylene glycol 1 0.5 39 2 0.5 39 3 0.35 27 . 4 0.45 35 D.4 31 6 0.5 39 ? 0.5 39
8 s.45 35 ~ 0.5 39 5 wt % Ca(OH)2 in diethylene glycol 1 1.~ 51 2 1.1 37 3 1.2 40 4 1.~5 35 1.0 34 6 1.2 40 7 1.0 34 8 0.9 30
9 0.95 32 Conditions: C0 addition at 1 aOm and room temperature Re~eneration dt 200 C
Solvent weight - 125 gm 20.
~ 11,952-2 In the drawings:
Figo 1 is a ~implified schematic flow sheet o~
an apparatus arrangement suitable for practicing the proces~
of the invention for gas~puæification;
Figo 2 is a schematic flow sheet of an apparatus arrangement similar to that of Fig. 1 and addit~onally having means for the regeneration of spent non-aqueous absorbent solution and recycling the regenerated ~olution to the liquid-gas co~tactor for further use;
Fig. 3 is a curve showing the carbon dioxide displacement efficiency for a number of cycles of thermal regeneration of four differen alkali or 21kaline ear~h metal based scavenging systems;
Fig. 4 i~ a simplified schematic flow sheet for a pilot plan~ having con~inuous purification of gas streams with absoxbent regeneratlon wherein no metal carbonate prec~pita~e is formed during operation~ and Fig~ 5 is a simplified schemati flow sheet for a pilot plant having continuous purification and cyclic regeneration, wherein insoluble metal carbonate precipitate is formed and remov~d during operationO
Apparatus suitable or commercial operation of the process of the invention is shown schematically in Fig. 1 of the drawlngs. As ~here showng the contacting device may alternatively be a vessel illed with the caus~ic (NaOH) extractant solution through which the gas stream ~o be treated is passedO
In the process of the invention, feed gas contain-ing impurities such as C02, H20, S02, H2S, COS aad 21, ~ ~,5 ~ ~ 3 ~ 11,9~2~2 mercaptans, is mixed with a no~-aqueous solution in a gas-liquid contactor containing one or more contac ing zones.
The $mpurities are removed from the gas by rea~tion with or dissolutio~ into the solutionO I~ necessary for purity considerations, the purifled gaLs leaving the gas-liquid con~actor may be further treated~ eOg., by passing through a 6elective adsorbent bed, to remove traces of the ~on-aqueous solution.
Depending on the concentration of impurities in the feed a~d the amount of feed processed, the solution may have ~o he renewedO This can be done intermittently or con~inuously by (1) removing solid products of reaction, (2) purging spent solution, and/or (3) separating spent solution and absorbed impurities from active solution. To compensate for any removal of solution by cleanup or by entrainme~t or vaporization into the pu~ified gas, provision is made for introductions of mak up non-aqueous solution into the process.
A demister may be provided in the top of the column to remove large size spray particles. Ethylene gas leaving the demister passes ~hrough a filter which removes sub-micron sized glycol mis~ Depending on the size, the liquid can be returned to the strlpping stlll or for small flows it can be collected in a drum and be disposed of periodically~ D~wnstream o~ the filter, an activated carbon trap ~ay be provided to remove last traces of TEG
from ethylene. At 50C., 450 psig, TEG concentration in ethylene will be less than 0.1 ppm, In the event that TEG
removal is desired below that level, it is easily accomplish-ed by adsorption with activated carbon. The TEG loading on 22~
3~ 11,9~2-2 activated carbon i9 expected to bP greater than 20 percent by weight. At this loading, 800 pounds of activated carbon is sufficient to complel:ely remove all traces of TEG from a column treating 40,000 lbs/hr ethylene in one year (8000 hrs) of the operation. Therefore~ activated carbon can practically be used on a throw-away basis, and no regeneration facilities are needed to allow reason2bly economical operation.
A double filter arrangement may b~ provided to remove the carbonate solids from the solutio~. The caustic/
glycol solution leaving the filters is separated into two streams9 Approximately 20 percent of the flow i5 de-pressurized to 20 psig and rou~ed ts the stripping st~ll, and the remaining 80 percent is circulated to the top of the absorber via the circulating pump. For ~ colu~n treat-ing 40,000 lbs/hr ethylene, liquid flow should be ?t least lO gallon/min (L/& ~ 0.125)~
The Rtripping still operates at 10 20 psig, 100C~, and it has two primary functions:
l. To decrease the moisture content of the fresh caustic/glycol solu ions to less than 10 ppm in the gas phase. ~his is accomplished by nitrogen purging at 100Co intermitten~ly whe~ makeup solid caustic is added to the system.
2. To remove the moisture adsorbed by the circulating solution with continuous nitrogen purging at 100C. Wi~h this mode of opera~ion, the ~austic/glycol solution will remove C02 by reaction and H20 by physical adsorption~ Since the moisture content of the incoming ethylene is small ( ~ 5 ppm), only a small portion of the ~ 3 3 ~ 11,952-2 circulating solution needs to 'be desorbed. The still should be sized ~o hold 120 gallons of lO percent NaOH/TEG
solution, The stripping still is equipped with a turbine-type agitator to dissolve the solid caustic in glycol solutions~ As NaOH is removed rom the system by reaction with CO29 resh sodium hydroxide mus~ be added. To add solid caustic, the still is depressurized and purged with ni~rogen, the tank is opened and flake caustic is added to bring the solution to, for example, 15 percent NaOHO
After the moisture content in the purge nitrogen is below
Solvent weight - 125 gm 20.
~ 11,952-2 In the drawings:
Figo 1 is a ~implified schematic flow sheet o~
an apparatus arrangement suitable for practicing the proces~
of the invention for gas~puæification;
Figo 2 is a schematic flow sheet of an apparatus arrangement similar to that of Fig. 1 and addit~onally having means for the regeneration of spent non-aqueous absorbent solution and recycling the regenerated ~olution to the liquid-gas co~tactor for further use;
Fig. 3 is a curve showing the carbon dioxide displacement efficiency for a number of cycles of thermal regeneration of four differen alkali or 21kaline ear~h metal based scavenging systems;
Fig. 4 i~ a simplified schematic flow sheet for a pilot plan~ having con~inuous purification of gas streams with absoxbent regeneratlon wherein no metal carbonate prec~pita~e is formed during operation~ and Fig~ 5 is a simplified schemati flow sheet for a pilot plant having continuous purification and cyclic regeneration, wherein insoluble metal carbonate precipitate is formed and remov~d during operationO
Apparatus suitable or commercial operation of the process of the invention is shown schematically in Fig. 1 of the drawlngs. As ~here showng the contacting device may alternatively be a vessel illed with the caus~ic (NaOH) extractant solution through which the gas stream ~o be treated is passedO
In the process of the invention, feed gas contain-ing impurities such as C02, H20, S02, H2S, COS aad 21, ~ ~,5 ~ ~ 3 ~ 11,9~2~2 mercaptans, is mixed with a no~-aqueous solution in a gas-liquid contactor containing one or more contac ing zones.
The $mpurities are removed from the gas by rea~tion with or dissolutio~ into the solutionO I~ necessary for purity considerations, the purifled gaLs leaving the gas-liquid con~actor may be further treated~ eOg., by passing through a 6elective adsorbent bed, to remove traces of the ~on-aqueous solution.
Depending on the concentration of impurities in the feed a~d the amount of feed processed, the solution may have ~o he renewedO This can be done intermittently or con~inuously by (1) removing solid products of reaction, (2) purging spent solution, and/or (3) separating spent solution and absorbed impurities from active solution. To compensate for any removal of solution by cleanup or by entrainme~t or vaporization into the pu~ified gas, provision is made for introductions of mak up non-aqueous solution into the process.
A demister may be provided in the top of the column to remove large size spray particles. Ethylene gas leaving the demister passes ~hrough a filter which removes sub-micron sized glycol mis~ Depending on the size, the liquid can be returned to the strlpping stlll or for small flows it can be collected in a drum and be disposed of periodically~ D~wnstream o~ the filter, an activated carbon trap ~ay be provided to remove last traces of TEG
from ethylene. At 50C., 450 psig, TEG concentration in ethylene will be less than 0.1 ppm, In the event that TEG
removal is desired below that level, it is easily accomplish-ed by adsorption with activated carbon. The TEG loading on 22~
3~ 11,9~2-2 activated carbon i9 expected to bP greater than 20 percent by weight. At this loading, 800 pounds of activated carbon is sufficient to complel:ely remove all traces of TEG from a column treating 40,000 lbs/hr ethylene in one year (8000 hrs) of the operation. Therefore~ activated carbon can practically be used on a throw-away basis, and no regeneration facilities are needed to allow reason2bly economical operation.
A double filter arrangement may b~ provided to remove the carbonate solids from the solutio~. The caustic/
glycol solution leaving the filters is separated into two streams9 Approximately 20 percent of the flow i5 de-pressurized to 20 psig and rou~ed ts the stripping st~ll, and the remaining 80 percent is circulated to the top of the absorber via the circulating pump. For ~ colu~n treat-ing 40,000 lbs/hr ethylene, liquid flow should be ?t least lO gallon/min (L/& ~ 0.125)~
The Rtripping still operates at 10 20 psig, 100C~, and it has two primary functions:
l. To decrease the moisture content of the fresh caustic/glycol solu ions to less than 10 ppm in the gas phase. ~his is accomplished by nitrogen purging at 100Co intermitten~ly whe~ makeup solid caustic is added to the system.
2. To remove the moisture adsorbed by the circulating solution with continuous nitrogen purging at 100C. Wi~h this mode of opera~ion, the ~austic/glycol solution will remove C02 by reaction and H20 by physical adsorption~ Since the moisture content of the incoming ethylene is small ( ~ 5 ppm), only a small portion of the ~ 3 3 ~ 11,952-2 circulating solution needs to 'be desorbed. The still should be sized ~o hold 120 gallons of lO percent NaOH/TEG
solution, The stripping still is equipped with a turbine-type agitator to dissolve the solid caustic in glycol solutions~ As NaOH is removed rom the system by reaction with CO29 resh sodium hydroxide mus~ be added. To add solid caustic, the still is depressurized and purged with ni~rogen, the tank is opened and flake caustic is added to bring the solution to, for example, 15 percent NaOHO
After the moisture content in the purge nitrogen is below
10 ppm this concentrated stre~m i~ gradually mixed with the circulatlng solution. During the solid caustic addi~ion and moisture removal stepl all of the liquid lea~ing ~he filters is circulated to the column with no side flow to the stripperO
The heat source for the still can be low-pressure steam on the jac~etsO It should be capable of heating the solution to 150C" although a maximum of 100co is suffirient for efficient moisture removal.
The following Table V sets ~orth typical examples of the practice of the process of th~ present invention employing varying feed gases, sodium and potassium-based absorbent systems, flow rates and operat-ing temperatures employed.
~rom the examples set forth herein, it may be seen that, in the process o the present invention, bulk quantities (major proportions of the absorbent) of the glycol/glycolate solution are employed to effect gas purification (of nitrogen or ethylene) by reduction of 24.
~ ~ 5 ~ ~ 3 ~ 11,952-2 impurities such as c~rbon dioxide, carbonyl sulfide, and hydrogen sulfide from a relatively high concentration level ~o a low parts per million level.
The heat source for the still can be low-pressure steam on the jac~etsO It should be capable of heating the solution to 150C" although a maximum of 100co is suffirient for efficient moisture removal.
The following Table V sets ~orth typical examples of the practice of the process of th~ present invention employing varying feed gases, sodium and potassium-based absorbent systems, flow rates and operat-ing temperatures employed.
~rom the examples set forth herein, it may be seen that, in the process o the present invention, bulk quantities (major proportions of the absorbent) of the glycol/glycolate solution are employed to effect gas purification (of nitrogen or ethylene) by reduction of 24.
~ ~ 5 ~ ~ 3 ~ 11,952-2 impurities such as c~rbon dioxide, carbonyl sulfide, and hydrogen sulfide from a relatively high concentration level ~o a low parts per million level.
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11,952~2 In a pilot plant of t:he type shown in ~che flow sheec of Fig. 5 of the drawings, an on-stream example of the purification process of the invention was carried out.
A preliminarily purified ethylene stream from ~ commercial e~hylene production facillty was passed for a period of 56 days through a non-aqueous absorbent solution containing 6% sodium hgdroxide (94% triethylene glycol) ln a non-aqu~ous scrubber. The scrubber temperature was 30C. and the ethylene feed pressure 450 psig. The inlet carbon dioxide range over the period was 1-6 ppm and the outl~t carbon dioxide range was 0.05-0.15 ppm. The ethylene feed was 30 pounds per hour and the gas velocity in the scrubber was 0.6 feet per minute. The scrubber in ~he final days of operation was found to reduce an inlet carbon dioxide impurity in the gas strea~n of 5.35 ppm to an outlet content of Oo lO ppm.
In the initial e~perimeTltal practice of the process of the i~vention, a sodlum-based system was investigated wherein the absorben~ solution was prepared by addition of sodium hydroxide or carbonate, or metallic sodium to the excess glycol or glyceride solution to provide the desired sodium glycol or glyceride salt in unreacted glycol or glycerol sol.venf~
The process of such sodium-based system can present a problem in stream processing. When employing absorbent solutions based on NaOH and a liquid polyhydric alcohol, there may arise the need for filtratlon of the spent solution to effect removal of very fine (1-40 microns) Na2C03 solids produced whe~ any moisture is present. The process of the sodium~based system also presents the inherent 27~
~ 3 ~ 11,952-2 limitation (when ~ot further treated) of being a one-~hot, non-continuous process in terms of the caustic employed~, It has been found that a potassium-based absorbent system, as described hereinbelow, pro~ides advantages over the sodium-based absorbent systemO The non-aqueous absorbent solution is a potassium glycolate or glycerate formed by the reaction of either elemental potassiu~, potassiwm hydroxlde or potassium carbonate wlth the polyol, e,g., glycol and/or glycerol. When the preferred potassium carbonate is employed as the source of po~cassium, a thermal activation step at about 200C.
is desired for optimization of alcoholate formationO
The continuous system 1B typified by the thermal regeneratioa of a potassium-based glycolate or glycerate containing solution which are used to scavenge C02 from a varie~y of feed streams. It has been found that K2C03 dissolved in glycol solutions is quite soluble up to concentra~ions of about 10 weight percent at ambient tempe~
ratures or slightly above (viz. 20-25C.). It also has been found that, when ~hese K2C03-continuous solutions are hea~ed to ~ 1~0-2~0~C., the K~C03 decomposes to release C02 with the simultaneous formation of a potassiun glycolate salt.
The reaction is believed to be:
(180-200C~) 2 2 (A) wherein ROH represents any glycol. The potassium glycol~
ate salt, ROK, is therefore of the s~me species as if it were initially produced from KOH, as follows:
28.
~ ~ 5 ~ ~ 3 ~ 11,952-2 ROH + KO~ ROK + ~2 (B) (125C ) When ROK is formed in he presence o water9 due to the use of KO~ as the alkali metal source material (se Equation B) the H~O is scavenged from ~he system at the regenerator by displacement with N2 gasO
Upon C02 scavenging in he absP~ce of water, the reaction is believed to be as follows:
o ROK ~ C02~ ?RO-C-OK (C) (25-100C,) o The product, RO-~-OKl is also soluble ln the excess glycol solvent and will also decompose at 180-200C. to split out C02~ ~hus regenerating the ROK
salt ~or re use. If trace quantities of water enter the ~crubber with the eed it is beliPved that ~he inorganic carbonate ~2C03 will be formed, but unlike t~e Na2~03 which would be present as a solid~ K2 3 re~ains in solu~ion and can again be ~hermally regen ratedO ~ence, the advantage of ~his aspect of the invention is that under the indicated conditlons no precipitate will form and all carbonate species formed can be thermally de-composed, thus providing a reactivated C02 scavenging solution after proper cooling before it is returned to the scrubber section~
This aspect of the invention, therefore, provides a thermally regenerable CO~ scavenging system withou~ the need for filtra~ion e~uipment~
The KOH-K2C03/glycol process for C02 removal 29.
3~ 11,952-~
~s based on the use of ~table non-aqueous solvents, such as triethylene glycol, wi~h low ~por pressure and with high dissolving capacity.
The chemistry o KO~-K2C03/glycol 601utions is not entirely understood at present. It is not believed that potassium carbonate is 6imp1y physleally dissolved in glycols. According ~o our best understandlng, ~ome re-actions between the alkali and, e~g., triethylene ~lycol (TEG) is achieved; probably a partial ~lcoholation ls effected.
Commercially a~ailable triethylene glycol (TEG) contains ~bout 0.1 percent (weight) water. In addi~lon, XOH/TEG reactions form ~ater~ Therefore, ~hen a glycol-ate or glycerate is prepared, the mixture will contain some water after preparation. This water is typically s~ripped away, prior to use of the solution for impuri~
t;es removal. During the stripping operation, the equilib-riu~ will con~inuously shift to the righ~ as the ~ater is removed from the system avoring alcoholate formation. I
the major portion of the water is removed from the system, the sol.ution will contain potassium glycolate, unreacted glycol and small amounts of free unreacted KOH and/or K2G03 (if used as a starting material). The KOH/glycol solutions are extremely hygroscopic and there will be some chemically condensed wa~er complexed with the excess glycol and remain in the solution even following the dry~
ing step. This water is removed from t,h~ system by heat~
ing and purging with an inert gas or by an equivalent means and is not permit~ed to accumulateO However, sucn water removal is not critical to the invention~ Thus, the - ~ 30~
,. :. .. .
~ ~ ~4~ 3~ 11,952-2 KOH/glycol solutions also can function as water scavengers.
Table V hereinabove sets forth typical examples o the practice of the process of the invention employing varying ~eed gases, NaOH and ]KOH/solvent systems, flow rates and operating temperatures employedO
From the examples set forth~ it may be seen tha~, in the process of the invention, bulk quanti~ies (major proportions of the absorbent) o the glycol/
glycolate solution are employed to effect gas purification (of nitrogen or ethylene) by reduction of impurities &uGh as carbon dioxide, carbonyl sulfide~ and hydrogen sulfide from a relatively high concentration level to a low par~s per million level.
The use of the potassium based system adds improved water scavenging capabilities simultaneously with its application as CO2 soavenger, sueh being accomplished without solids forming. In case o the ethylene feed which also contains trace amou~ts of water, this sys~em eliminates the need for water-removing ad-sorbents or at least reduce~ their size considerably.
One ideal application is for the removal of atmospheric C2 and H2O from air feed in the production of N~ and 2 by cryogenic air separation.
As discussed hereinabove, the spent non-aqueous absorbent solution is regenera~ed in the cyolic process as is graphically shown in the flow sheet of Figo 2 of ~he drawings. As ~here shown, a nitrogen gas purge ls employed in th~ solution regeneration s~ep in com~ination with heatin8 to a ~emperature of about 180 200Co to de~
compose (to carbon dioxide) the carbonate formed in the 31.
3 ~
11,952~2 solution and remove the carbon dioxide along with water vapor and other vapor imp~ri ies which are released and purgedO
As indicated herein~bove, the flow sheet ~et forth in Figo 4 of ~he drawings shows cycle embodying the process of the invention for the potassium-based system or any other system in which insoluble, solid carbonate precipitate is not formed in the a~sorbent solutio~. ~s there shown, valves Vl and V2 would be open only for a frac~ion of daily, continuous operation. The cycle will permit a batch or continuous regeneration operationO If con~inuous, a cooler is required in the line between the makeup and regeneration vessel and the storage vessel.
It is to be noted that the solution is cooled in the re-generation vessel prior to transfer to the storage vessel.
The embodiment of this figure o~ ehe drawings is designed to opera~e for the purification of ethyle~e~ employing 6%
by weight K2C03 feed in triethylene glycoL solution~ The use of nitrogen purge gas for evol~ed C02 and o~her vapor impurities is optional wi~h ~hls potassium based systemO
The use of a sodium-based non-aqueous absorbent solution for extended time periods can result in the ~ormation of fine sodium carbonate precipitate particles (of the order of 1-40 mlcro~s in size), making the removal of the precipitate merely by employment of a filtration step more difficulto It has been determined that such fine precipitate may b~ agglomera~ed into particles of larger size ~of the order of 200-400 microns) by a number of alte~native processing steps. Such larger precipitate particles can then be removed from the solution 32.
~5~3~ 11, 952-2 by the use of a s~mple direct filtration step~ as indi-cated in the embodiment of apparatus shown in ~lg~ 5 of ~he drawings.
This agglomeration can be e~fected by any one of a number of procedures such as: the addi~ion of up to 15% by weigh~ of water prior to heat ~reatment at about 750C.; a~d ~he addition of up to 2% by weight of common ion addition (e,g., sodium chloride for chloride ion)~ with or without up to abou~ 5% by weight water.
Such additions to the cycl~ to effect agglomerat~on are not ~hown in the simplified flow sheet of Figo 5 but may be made along with the use o a related heater in the line i~mediately upstream of the filter pressJ
The NaOH-based non-aqueous scrubber has the disadvantage that a precipitate, ~a2CO3, tends to accumulate under certain operating conditionsO However, the NaOH-based non-aqueous ~crubber is still more attracti~e when compared to the NaOH solid pellet absorb-er system. AS indicated hereinabove3 it was found that the use of the K2CO3-based non-aqueous scrubber r~s111ted in a thermally regenerable sys~em. The K2CO3 system has two distinct advantages over the NaOH system in that it is free o precipitation of solids upon CO2 cycling and it is more thermally stable. However, the K2C03 has only about half ~he chemical efficiency as the NaOH
system for cyclic CO2 scavenging. Earlier studies indicated that NaOH-based systems can displace approxi-mately 42% of the theoretical amount of CO2 absorbed while K2C03-based systems displace approxLmately 20% C0 Table VI summarizes the results of cyclic C02 33.
~ 3 g 1~,952~2 scrubbing using solutions consisting of 3 weight percen~
LiOH dissol~ed în diethylene~ triethylene, and dipropyl-ene glycol. Theoretlcally, 5.5 grams of C02 can be absorbed by 100 grams o~ 3 welght p~rcent LiOH in these solvents, based on the organic carbonate s oichiometry.
Using 5.5 grams ~s a igure for total C02 absorption, one can calculate theoretically the percent C02 displaced for each C02 absorption/desorption cycle~ Through ive cycles of C02 absorption/desorption, 3 weight percent LiOH/diethylene glycol theoretically displaced an average 14.7% of the theoretical amount of C02 per oycle while the LiOH-based solu~ions of dipropylene and triethyle~e glycol respectively3 displaced an average 15.7 and 14.3%
of the ~heoretical amoun~ of C02 per cycle. From a standpo;nt of efficient CO~ displacemcnt, this s udy shows there is lit~le if any significance in selec ing o~e LiOH-based glycol solution in preference to another.
Furthermore, from Table VI, it is seen that C02 dis~
placement (i.e., chem~cal) efficiency decli~ed drastioally with each succeeding cycle o C02 absorption/desorption, regardless of glycol selectionO For comparative purposes~
Table VXI shows the results for NaOH, K2C03 and LiOH-based non-aqueous solutions o diethylene glycol. From Table VII
the NaOH-based, non-aqueous scrubber appears to have superior chemical efficiency compared to K2C03 and LiOH-based non-aqueous scrubbers. This conclusion ls supported by the fart that after five cycles of C02 cycling, NaOH
theoretically displaces a mean value of 42~0% of the theoretical amount of C02 per cycle while K~C03 and LioH-based non-aqueous scrubbers displace 19,9 and 17~4%, respec~ively, per cycle.
34.
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11,952-2-C
~ 3~
SUPPLEMENTARY DISCLOSURE
EX~MXLES
Solutions of potassi~m hydroxide in diethylene glycol were made up and activated at 170C. while being purged with nitrogen. Subsequently the activated solutions were contacted at 25~C. and one psig with a car-bon dioxide feed stream until the solutions were equili-brated with the feed. The solutions were then regenerated at 155C. or 170~C. in one of two ways. The first way consisted of purging the solution for three to 16 hours with dry nitrogen. The second way consisted of subjecting the solution to vacuum tapproximately 150 mm mercury absolute) for three to 16 hours. Following reactivation the solutions were contacted as before with a carbon dioxide feed stream, and the capacities to absorb carbon dioxide were determined. Results of these tests demon-strate the ability of the potassi~n-containing solutions to undergo absorption of carbon dioxide and subsequent desorptic~n .
The methods of desorption include two fea~ures:
temperature increase and a means to further facilitate removal of the absorbed carbon dioxide. Temperature increase serves to weaken the bonds by which carbon dioxide is held in the solution. The use of a sparging gas or the use of vacu~n serves the same purpose: by both processes the partial pressure of carbon dioxide in the vapor phase is reduced, which enhances the transfer of carbon dioxide from the li~uid to the vapor phase. It will be readily apparent to one skilled in the art that a combination of `` 36a.
,, 11,952-2-C
~ S~ 9 3~
these two processes could also be used to facilitate removal of carbon dioxide from the solution.
Many of the compounds listed hereinabove, though suitable as absorbents for the variety of gases listed above, do not possess the capability of being regenerated. Furthermore, with some gases such as hydro-gen chloride, essentially none of the alkali and alkaline ear~h containing solutions can be regenerated. However, with carbon dioxide, regeneration of solutions containing lithium, sodium or potassium can be effected, as is shown in the examples contained herein. (This refers to the following Table VIII as well as Tables VI andVII above.) Thus, the instant regeneration process is limited to gas feeds in which carbon dioxide is the only component absorbing and solutions of the three alkali metals mentioned above are employed.
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11,952~2 In a pilot plant of t:he type shown in ~che flow sheec of Fig. 5 of the drawings, an on-stream example of the purification process of the invention was carried out.
A preliminarily purified ethylene stream from ~ commercial e~hylene production facillty was passed for a period of 56 days through a non-aqueous absorbent solution containing 6% sodium hgdroxide (94% triethylene glycol) ln a non-aqu~ous scrubber. The scrubber temperature was 30C. and the ethylene feed pressure 450 psig. The inlet carbon dioxide range over the period was 1-6 ppm and the outl~t carbon dioxide range was 0.05-0.15 ppm. The ethylene feed was 30 pounds per hour and the gas velocity in the scrubber was 0.6 feet per minute. The scrubber in ~he final days of operation was found to reduce an inlet carbon dioxide impurity in the gas strea~n of 5.35 ppm to an outlet content of Oo lO ppm.
In the initial e~perimeTltal practice of the process of the i~vention, a sodlum-based system was investigated wherein the absorben~ solution was prepared by addition of sodium hydroxide or carbonate, or metallic sodium to the excess glycol or glyceride solution to provide the desired sodium glycol or glyceride salt in unreacted glycol or glycerol sol.venf~
The process of such sodium-based system can present a problem in stream processing. When employing absorbent solutions based on NaOH and a liquid polyhydric alcohol, there may arise the need for filtratlon of the spent solution to effect removal of very fine (1-40 microns) Na2C03 solids produced whe~ any moisture is present. The process of the sodium~based system also presents the inherent 27~
~ 3 ~ 11,952-2 limitation (when ~ot further treated) of being a one-~hot, non-continuous process in terms of the caustic employed~, It has been found that a potassium-based absorbent system, as described hereinbelow, pro~ides advantages over the sodium-based absorbent systemO The non-aqueous absorbent solution is a potassium glycolate or glycerate formed by the reaction of either elemental potassiu~, potassiwm hydroxlde or potassium carbonate wlth the polyol, e,g., glycol and/or glycerol. When the preferred potassium carbonate is employed as the source of po~cassium, a thermal activation step at about 200C.
is desired for optimization of alcoholate formationO
The continuous system 1B typified by the thermal regeneratioa of a potassium-based glycolate or glycerate containing solution which are used to scavenge C02 from a varie~y of feed streams. It has been found that K2C03 dissolved in glycol solutions is quite soluble up to concentra~ions of about 10 weight percent at ambient tempe~
ratures or slightly above (viz. 20-25C.). It also has been found that, when ~hese K2C03-continuous solutions are hea~ed to ~ 1~0-2~0~C., the K~C03 decomposes to release C02 with the simultaneous formation of a potassiun glycolate salt.
The reaction is believed to be:
(180-200C~) 2 2 (A) wherein ROH represents any glycol. The potassium glycol~
ate salt, ROK, is therefore of the s~me species as if it were initially produced from KOH, as follows:
28.
~ ~ 5 ~ ~ 3 ~ 11,952-2 ROH + KO~ ROK + ~2 (B) (125C ) When ROK is formed in he presence o water9 due to the use of KO~ as the alkali metal source material (se Equation B) the H~O is scavenged from ~he system at the regenerator by displacement with N2 gasO
Upon C02 scavenging in he absP~ce of water, the reaction is believed to be as follows:
o ROK ~ C02~ ?RO-C-OK (C) (25-100C,) o The product, RO-~-OKl is also soluble ln the excess glycol solvent and will also decompose at 180-200C. to split out C02~ ~hus regenerating the ROK
salt ~or re use. If trace quantities of water enter the ~crubber with the eed it is beliPved that ~he inorganic carbonate ~2C03 will be formed, but unlike t~e Na2~03 which would be present as a solid~ K2 3 re~ains in solu~ion and can again be ~hermally regen ratedO ~ence, the advantage of ~his aspect of the invention is that under the indicated conditlons no precipitate will form and all carbonate species formed can be thermally de-composed, thus providing a reactivated C02 scavenging solution after proper cooling before it is returned to the scrubber section~
This aspect of the invention, therefore, provides a thermally regenerable CO~ scavenging system withou~ the need for filtra~ion e~uipment~
The KOH-K2C03/glycol process for C02 removal 29.
3~ 11,952-~
~s based on the use of ~table non-aqueous solvents, such as triethylene glycol, wi~h low ~por pressure and with high dissolving capacity.
The chemistry o KO~-K2C03/glycol 601utions is not entirely understood at present. It is not believed that potassium carbonate is 6imp1y physleally dissolved in glycols. According ~o our best understandlng, ~ome re-actions between the alkali and, e~g., triethylene ~lycol (TEG) is achieved; probably a partial ~lcoholation ls effected.
Commercially a~ailable triethylene glycol (TEG) contains ~bout 0.1 percent (weight) water. In addi~lon, XOH/TEG reactions form ~ater~ Therefore, ~hen a glycol-ate or glycerate is prepared, the mixture will contain some water after preparation. This water is typically s~ripped away, prior to use of the solution for impuri~
t;es removal. During the stripping operation, the equilib-riu~ will con~inuously shift to the righ~ as the ~ater is removed from the system avoring alcoholate formation. I
the major portion of the water is removed from the system, the sol.ution will contain potassium glycolate, unreacted glycol and small amounts of free unreacted KOH and/or K2G03 (if used as a starting material). The KOH/glycol solutions are extremely hygroscopic and there will be some chemically condensed wa~er complexed with the excess glycol and remain in the solution even following the dry~
ing step. This water is removed from t,h~ system by heat~
ing and purging with an inert gas or by an equivalent means and is not permit~ed to accumulateO However, sucn water removal is not critical to the invention~ Thus, the - ~ 30~
,. :. .. .
~ ~ ~4~ 3~ 11,952-2 KOH/glycol solutions also can function as water scavengers.
Table V hereinabove sets forth typical examples o the practice of the process of the invention employing varying ~eed gases, NaOH and ]KOH/solvent systems, flow rates and operating temperatures employedO
From the examples set forth~ it may be seen tha~, in the process of the invention, bulk quanti~ies (major proportions of the absorbent) o the glycol/
glycolate solution are employed to effect gas purification (of nitrogen or ethylene) by reduction of impurities &uGh as carbon dioxide, carbonyl sulfide~ and hydrogen sulfide from a relatively high concentration level to a low par~s per million level.
The use of the potassium based system adds improved water scavenging capabilities simultaneously with its application as CO2 soavenger, sueh being accomplished without solids forming. In case o the ethylene feed which also contains trace amou~ts of water, this sys~em eliminates the need for water-removing ad-sorbents or at least reduce~ their size considerably.
One ideal application is for the removal of atmospheric C2 and H2O from air feed in the production of N~ and 2 by cryogenic air separation.
As discussed hereinabove, the spent non-aqueous absorbent solution is regenera~ed in the cyolic process as is graphically shown in the flow sheet of Figo 2 of ~he drawings. As ~here shown, a nitrogen gas purge ls employed in th~ solution regeneration s~ep in com~ination with heatin8 to a ~emperature of about 180 200Co to de~
compose (to carbon dioxide) the carbonate formed in the 31.
3 ~
11,952~2 solution and remove the carbon dioxide along with water vapor and other vapor imp~ri ies which are released and purgedO
As indicated herein~bove, the flow sheet ~et forth in Figo 4 of ~he drawings shows cycle embodying the process of the invention for the potassium-based system or any other system in which insoluble, solid carbonate precipitate is not formed in the a~sorbent solutio~. ~s there shown, valves Vl and V2 would be open only for a frac~ion of daily, continuous operation. The cycle will permit a batch or continuous regeneration operationO If con~inuous, a cooler is required in the line between the makeup and regeneration vessel and the storage vessel.
It is to be noted that the solution is cooled in the re-generation vessel prior to transfer to the storage vessel.
The embodiment of this figure o~ ehe drawings is designed to opera~e for the purification of ethyle~e~ employing 6%
by weight K2C03 feed in triethylene glycoL solution~ The use of nitrogen purge gas for evol~ed C02 and o~her vapor impurities is optional wi~h ~hls potassium based systemO
The use of a sodium-based non-aqueous absorbent solution for extended time periods can result in the ~ormation of fine sodium carbonate precipitate particles (of the order of 1-40 mlcro~s in size), making the removal of the precipitate merely by employment of a filtration step more difficulto It has been determined that such fine precipitate may b~ agglomera~ed into particles of larger size ~of the order of 200-400 microns) by a number of alte~native processing steps. Such larger precipitate particles can then be removed from the solution 32.
~5~3~ 11, 952-2 by the use of a s~mple direct filtration step~ as indi-cated in the embodiment of apparatus shown in ~lg~ 5 of ~he drawings.
This agglomeration can be e~fected by any one of a number of procedures such as: the addi~ion of up to 15% by weigh~ of water prior to heat ~reatment at about 750C.; a~d ~he addition of up to 2% by weight of common ion addition (e,g., sodium chloride for chloride ion)~ with or without up to abou~ 5% by weight water.
Such additions to the cycl~ to effect agglomerat~on are not ~hown in the simplified flow sheet of Figo 5 but may be made along with the use o a related heater in the line i~mediately upstream of the filter pressJ
The NaOH-based non-aqueous scrubber has the disadvantage that a precipitate, ~a2CO3, tends to accumulate under certain operating conditionsO However, the NaOH-based non-aqueous ~crubber is still more attracti~e when compared to the NaOH solid pellet absorb-er system. AS indicated hereinabove3 it was found that the use of the K2CO3-based non-aqueous scrubber r~s111ted in a thermally regenerable sys~em. The K2CO3 system has two distinct advantages over the NaOH system in that it is free o precipitation of solids upon CO2 cycling and it is more thermally stable. However, the K2C03 has only about half ~he chemical efficiency as the NaOH
system for cyclic CO2 scavenging. Earlier studies indicated that NaOH-based systems can displace approxi-mately 42% of the theoretical amount of CO2 absorbed while K2C03-based systems displace approxLmately 20% C0 Table VI summarizes the results of cyclic C02 33.
~ 3 g 1~,952~2 scrubbing using solutions consisting of 3 weight percen~
LiOH dissol~ed în diethylene~ triethylene, and dipropyl-ene glycol. Theoretlcally, 5.5 grams of C02 can be absorbed by 100 grams o~ 3 welght p~rcent LiOH in these solvents, based on the organic carbonate s oichiometry.
Using 5.5 grams ~s a igure for total C02 absorption, one can calculate theoretically the percent C02 displaced for each C02 absorption/desorption cycle~ Through ive cycles of C02 absorption/desorption, 3 weight percent LiOH/diethylene glycol theoretically displaced an average 14.7% of the theoretical amount of C02 per oycle while the LiOH-based solu~ions of dipropylene and triethyle~e glycol respectively3 displaced an average 15.7 and 14.3%
of the ~heoretical amoun~ of C02 per cycle. From a standpo;nt of efficient CO~ displacemcnt, this s udy shows there is lit~le if any significance in selec ing o~e LiOH-based glycol solution in preference to another.
Furthermore, from Table VI, it is seen that C02 dis~
placement (i.e., chem~cal) efficiency decli~ed drastioally with each succeeding cycle o C02 absorption/desorption, regardless of glycol selectionO For comparative purposes~
Table VXI shows the results for NaOH, K2C03 and LiOH-based non-aqueous solutions o diethylene glycol. From Table VII
the NaOH-based, non-aqueous scrubber appears to have superior chemical efficiency compared to K2C03 and LiOH-based non-aqueous scrubbers. This conclusion ls supported by the fart that after five cycles of C02 cycling, NaOH
theoretically displaces a mean value of 42~0% of the theoretical amount of C02 per cycle while K~C03 and LioH-based non-aqueous scrubbers displace 19,9 and 17~4%, respec~ively, per cycle.
34.
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11,952-2-C
~ 3~
SUPPLEMENTARY DISCLOSURE
EX~MXLES
Solutions of potassi~m hydroxide in diethylene glycol were made up and activated at 170C. while being purged with nitrogen. Subsequently the activated solutions were contacted at 25~C. and one psig with a car-bon dioxide feed stream until the solutions were equili-brated with the feed. The solutions were then regenerated at 155C. or 170~C. in one of two ways. The first way consisted of purging the solution for three to 16 hours with dry nitrogen. The second way consisted of subjecting the solution to vacuum tapproximately 150 mm mercury absolute) for three to 16 hours. Following reactivation the solutions were contacted as before with a carbon dioxide feed stream, and the capacities to absorb carbon dioxide were determined. Results of these tests demon-strate the ability of the potassi~n-containing solutions to undergo absorption of carbon dioxide and subsequent desorptic~n .
The methods of desorption include two fea~ures:
temperature increase and a means to further facilitate removal of the absorbed carbon dioxide. Temperature increase serves to weaken the bonds by which carbon dioxide is held in the solution. The use of a sparging gas or the use of vacu~n serves the same purpose: by both processes the partial pressure of carbon dioxide in the vapor phase is reduced, which enhances the transfer of carbon dioxide from the li~uid to the vapor phase. It will be readily apparent to one skilled in the art that a combination of `` 36a.
,, 11,952-2-C
~ S~ 9 3~
these two processes could also be used to facilitate removal of carbon dioxide from the solution.
Many of the compounds listed hereinabove, though suitable as absorbents for the variety of gases listed above, do not possess the capability of being regenerated. Furthermore, with some gases such as hydro-gen chloride, essentially none of the alkali and alkaline ear~h containing solutions can be regenerated. However, with carbon dioxide, regeneration of solutions containing lithium, sodium or potassium can be effected, as is shown in the examples contained herein. (This refers to the following Table VIII as well as Tables VI andVII above.) Thus, the instant regeneration process is limited to gas feeds in which carbon dioxide is the only component absorbing and solutions of the three alkali metals mentioned above are employed.
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Claims (19)
1. The process for the purification of an industrial gas stream of one or more of hydrogen, carbon monoxide, air, nitrogen, oxygen, helium, argon, mono-olefins having from two to five carbon atoms per molecule, diolefins having four or five carbon atoms per molecule, paraffins and acetylenes, by the removal of trace amounts of impurities therein, of one or more of water, hydrogen sulfide, carbon dioxide, carbonyl sulfide, sulfur dioxide, hydrogen chloride, hydrogen cyanide, nitric acid, and mer-captans which process comprises the steps of: providing a liquid essentially non-aqueous solution comprising from about 0.5 to about 15 weight percent of at least one member selected from the group consisting of alkali and alkaline earth metal, present as a hydroxide or a carbonate, in a liquid aliphatic polyhydric alcohol having a carbon to oxygen ratio of one to about five, and at least two oxygens thereof being separated by not more than two sequential carbon atoms; heat treating said solution to remove water originally in the solution as well as water formed by natural hygroscopic reaction with said alcohols and by the reaction:
selected group metal hydroxide + polyhydric alcohol H2O + metal alcoholate, as well as the following reaction to activate said solu-tion (if a carbonate is selected):
selected group metal carbonate + H2O metal hydroxide + CO2;
stripping the resultant solution to remove the water, 37.
11,952-2 effecting purification by contacting said industrial gas, at a temperature between about 15 and 100°C., with the stripped solution; and separating the purified industrial gas from the reaction products of said contacting step.
selected group metal hydroxide + polyhydric alcohol H2O + metal alcoholate, as well as the following reaction to activate said solu-tion (if a carbonate is selected):
selected group metal carbonate + H2O metal hydroxide + CO2;
stripping the resultant solution to remove the water, 37.
11,952-2 effecting purification by contacting said industrial gas, at a temperature between about 15 and 100°C., with the stripped solution; and separating the purified industrial gas from the reaction products of said contacting step.
2. The process in accordance with claim 1, wherein said gas stream contains dissolved water vapor in an amount up to its limit of saturation therein, said water vapor being concurrently removed from said gas stream with other of such impurities.
3. The process in accordance with claim 1, wherein the concentration of said alkali metal is from 1 to 10 weight percent.
4. The process in accordance with claim 1, wherein the alkali or alkaline earth metal is sodium or potassium provided as the hydroxide.
5. The process in accordance with claim 4, wherein said alkali metal hydroxide is sodium hydroxide.
6. The process in accordance with claim 1, wherein said alkali metal or alkaline earth metal is potassium provided as the hydroxide or carbonate.
7. The process in accordance with claim 1, wherein the selected gas is ethylene.
8. The process in accordance with claim 1, wherein the selected polyhydric alcohol is triethylene glycol.
9. The process in accordance with claim 1, wherein said stream is continuously fed to a liquid body 38.
11,952-2 of said solution, the gas stream is removed from contact with said liquid body solution and is transferred into an atmosphere contiguous with said body in which the partial pressure of water vapor therein is insufficient to signifi-cantly increase the water concentration in said stream.
11,952-2 of said solution, the gas stream is removed from contact with said liquid body solution and is transferred into an atmosphere contiguous with said body in which the partial pressure of water vapor therein is insufficient to signifi-cantly increase the water concentration in said stream.
10. The process in accordance with claim 9, wherein the water concentration in said stream leaving said body and said atmosphere is less than that contained in the stream fed to the body.
11. The process in accordance with claim 10, wherein the alkali metal is potassium provided as a carbonate.
12. The process in accordance with claim 1, wherein the solution is regenerated periodically by thermal treatment.
13. The process in accordance with claim 12, wherein the thermal treatment is heating to a temperature to about 180° to about 200°C.
14. The process in accordance with claim 1 wherein said contacting involves countercurrently passing said stream through said solution.
39.
CLAIMS SUPPORTED BY THE SUPPLEMENTARY DISCLOSURE
39.
CLAIMS SUPPORTED BY THE SUPPLEMENTARY DISCLOSURE
15. In a regenerative process for the purification of an industrial gas stream by the removal of trace amounts of impurities therefrom in accordance with claim 1, the further steps comprising increasing the temperature of said reaction products to between 150 and 200°C.; reducing the partial pressure of carbon dioxide in the vapor phase above said reaction products; separating carbon dioxide from said reaction products; and recycling the treated reaction products to said liquid solution for contacting a further quantity of said industrial gas stream.
16. The process in accordance with claim 15, wherein the temperature of said reaction products is increased to between 155 and 170°C.
17. The process in accordance with claim 15, wherein said separation of carbon dioxide is effected by displacement with an inert gas from said reaction products.
18. The process in accordance with claim 15, wherein said separation of carbon dioxide is effected by displace-ment with a vacuum applied to said reaction products.
19. The process in accordance with claim 18, wherein the temperature of said reaction products is increased to between 155 and about 170°C.
40.
40.
Applications Claiming Priority (4)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US3873679A | 1979-05-14 | 1979-05-14 | |
| US038,736 | 1979-05-14 | ||
| US8931679A | 1979-10-30 | 1979-10-30 | |
| US089,316 | 1979-10-30 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1154939A true CA1154939A (en) | 1983-10-11 |
Family
ID=26715492
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000351897A Expired CA1154939A (en) | 1979-05-14 | 1980-05-14 | Process for the purification of nonreacting gases |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1154939A (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN114917742A (en) * | 2022-05-14 | 2022-08-19 | 浙江大学 | Low-temperature CO realization by using metal oxide and vicinal diol 2 Method for capturing and recycling |
-
1980
- 1980-05-14 CA CA000351897A patent/CA1154939A/en not_active Expired
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN114917742A (en) * | 2022-05-14 | 2022-08-19 | 浙江大学 | Low-temperature CO realization by using metal oxide and vicinal diol 2 Method for capturing and recycling |
| CN114917742B (en) * | 2022-05-14 | 2023-02-03 | 浙江大学 | Low-temperature CO realization by using metal oxide and vicinal diol 2 Method for capturing and recycling |
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