CA1287732C - Process and apparatus for low temperature amine removal of acid gases - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A hydrocarbon stream containing acid gases is contacted with an aqueous alkanolamine for acid gas removal. The rich amine is flash-atomized and absorbed acid gases are desorbed from a disperse liquid phase to a continuous gaseous phase.

Description

~6~0 Goddin, Ho, Reed 7~7~s~

PROCESS AND APPARATUS FOR LOW TEMPERATURE
AMINE REMOVAL OF ACID GASES
FIELD OF THE INVENTION
The invention relates to a process for acid gas removal and more particularly to a process for removal of 15 carbon dioxide and/or hydrogen sulfide from gases. In a further aspect the invention relates to a process for carbon dioxide removal from hydrocarbon containing gaseous streams utilizing aqueous alkanolamines.
BACKGROUND OF THE INVENTION
The removal of acid gases from gaseous streams by absorption in aqueous alkanolamines has long been a part of industrial technology. In the past, such pro-cesses have been used in connection with the production of natural gas to remove, for example, carbon dioxide (CO2) 25 present at relatively low levels typically not exceeding 20-30 mol % CO2 and hydrogen sulfide (H2S) from the pro-duced gases.
In recent years, the development of CO2-miscible flooding has created a need for processes capable oE
30 treating gases which contain large quantities of acid gases including CO2, in the range for example of 15 to about 90 mol ~ CO2 or even higher from gaseous streams produced in connection with such CO2 miscible floods. The gas produced during CO2 miscible flooding can have in 35 addition to CO2, H2S, and significant amounts of methane, and especially, of ethane and higher hydrocarbons. It is geneLally desirable to recover the CO2, for example, for reinjection into the reservoir undergoing CO2 miscible ~ ~7~3~

flooding or for other uses, and the hydrocarbons for fur-ther processing.
In addition, low quality natural gas reservoirs occur ln nature which contain high levels of CO2. Such 5 reservoirs have sometimes not been produced for lack of processes which can recover salable gases efficiently and economicallyr while meeting environmental requirements.
A number of processes using aqueous amine solu-tions have been proposed for the removal of acid gases 10 from gaseous streams containinq high levels of CO2. Some of these processes have accomplished the removal of CO2 in two or more stages for efficiency, utilizing, for example~
at least a first stage for bulk removal of CO2, and at least a second stage for final removal of CO2. One such 15 process is described in U.S. Patent 4,466,946.
A major object of such processes is to accom-plish removal of CO2 and other acid gases such as hydrogen sulfide from gas streams containing hydrocarbons with a low consumption of energy. Significant factors in deter-20 mining energy consumption are the operating temperaturesmaintained in the CO2 absorption and regeneration stages.
In conventional operation, a rich amine liquid containing C2 and other acid gases such as H2S is regenerated by passing the liquid downwardly in generally continuous 25 fashion through a column having a plurality of vapor liquid contact devices therein. Adjacent to the bottom of the column, stripping vapor, predominantly water vapor or steam is generated by heat input typically by a reboiler.
The rising vapor passes upward countercurrently to the 30 descending liquid stripping CO2 and other acid gases from the descending liquid. Such a system can be considered to form a disperse system in which the liquid amine forms the generally continuous phase and the rising bubbles of vapor comprising steam and stripped gases form the discontinuous 35 or disperse phase. Desorption of the acid gases is from the generally continuous phase to the dispersed phase.

The heat requirement Eor generating the stripping vapors in such systems can be considerable. The heat requirement for regeneration of the rich amine solution, by such steam stripping, for example, can be reduced greatly when the 5 stripping temperature is reduced below the boiling point of the amine solution. The heat requirement continues to decrease as the temperature of stripper operation decreases, so it is advantageous to operate at the lowest reasible temperature. Regeneration at lower temperature 10 also facilitates major investment cost savings by elimina-tion of the amine cooler and lean/rich amine exchanger which are an integral part of many such acid gas removal stages operated at higher temperatures.
SUMMARY OF THE INVENTION
During absorption of acid gases from fluid streams using aqueous alkanolamine solutions a lean alka-nolamine stream is contacted with the stream undergoing treating, acid gases such as carbon dioxide and hydrogen sulfide are absorbed, and a rich alkanolamine stream com-20 prising absorbed acid gases and a product fluid stream reduced in acid gas content are produced. During regener-ation, the acid gases are desorbed from the rich alkanola-mine solution and a lean alkanolamine solution is pro-duced. The lean alkanolamine solution is not completely 25 free of absorbed acid gases but contains a residual acid gas loading which at equilibrium is a function of tempera-ture, acid gas partial pressure, and alkanolamine concen-tration in the base of the stripper. It is the difference between the residual acid gas loading oE the lean alkanc-30 lamine stream and the acid gas loading of the rich alkano-lamine stream which determines the acid gas "carrying capacity" or "net loading capacity" of the alkanolamine solution. It is therefore highly advantageous that the residual acid gas loading of the lean alkanolamine stream 35 be substantially the equilibrium acid gas loading. How-ever, it has been found that equilibrium acid gas loading values become more diffi_ult to attain at low stripping temperatures.

3~ 3~

We have found that acid gases can be clesorbed Erom a rich aq-leous tertiary alkanolamine solution com-prising absorbed acid gases, by atomizing and flashing the rich aqueous tertiary alkanolamine solution under condi-5 tions of droplet size and settling time effective for sub-stantially attaining equilibrium acid gas loading values while in the resulting disperse droplet phase, the temper-ature of the rich tertiary alkanolamine solution being below about 200F. The resulting lean tertiary alkanola-10 mine solution droplets can coalesce to form a lean ter-tiary alkanolamine solution having substantially equili-brium acid gas loading values which can be returned, without further acid gas desorption, for further absorbing acid gases.
In accordance with an aspect of the invention, an acid gas-rich feed gas can be contacted with an aqueous tertiary alkanolamine under pressure in a first contacting zone at a temperature below about 200F. The rich aqueous tertiary alkanolamine can then be removed from the first 20 contacting zone and then flashed with atomization or dis-persal into finely divided droplets into one or more regeneration zones and carbon dioxide and other acid gases desorbed while in the droplet phase to substantially equi-librium loading values in the alkanolamine solution.
25 Regenerated lean alkanolamine from the regeneration zones can be recycled to the first contacting zone. According to a further aspect of the invention, the acid gas com-prises carbon dioxide, H2S, and a minor proportion oE
hydrocarbon components.
As indicated, the first contacting zone com-prises a tertiary alkanolamine absorber operated at a tem-perature well below the normal boiling point of the amine solution, ~or example, below about 200F, preferably in the range of about 190F, to about 140F or even lower.
35 The operating temperatures of the lean alkanolamine stream leaving regeneration and entering the Eirst contactinq zone can be about the same, so neither heat exchange between lean and rich amine streams, nor cooling of the 77~2 lean amine prior to entering the Eirst contacting zone need be used. Consequently, heat input to the stripper can be limited primarily to heat leakage from the system and vaporization of water into the stripper overheacl 5 vapor.
In a further embodiment of the invention, the gaseous stream from which a first major quantity of CO2 has been removed in the first contacting zone can be introduced into a second contacting zone in which residual 10 CO2 and other acid gases can be removed. Preferably, according to this embodiment~ the first contacting zone uses an aqueous tertiary alkanolamine and the second con-tacting zone uses aqueous secondary or primary alkanola-mines to accomplish final cleanup.
The invention will be further understood and other features, embodiments, and applications of the invention will be apparent from the following detailed description of the invention and from the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGURE 1 represents schematically a first embod-iment of the invention.
FIGURE 2 represents schematically a second embodiment of the invention.
D ILED DESCRIPTION OF THE INVENTION
The present invention is based on the discovery that carbon dioxide and other acid gases can be effec-tively removed from a rich tertiary aqueous alkanolamine solution by flashing accompanied by atomization or dis-persal of the rich amine into small droplets. This method 30 of desorbing carbon dioxide is particularly advantageous at low temperatures such as in the range of about 1~0F to about 200F where conventional desorbing techniques using a plurality of vapor liquid contact devices were found incapable of acnieving equilibrium values of acid gas 35 residual loading. As will be described hereafter the present invention contemplates subjecting the rich alkano-lamine solution to flashing and atomization in one or more regeneration stages. The flashing and atomization is ~ ~.3~ 3~

effective for removing carbon dioxide, and other acid gases if present, substantially to equilibrium, under the conditions of temperature, and pressure, and alkanolamine solution employed, during the time that the rich absorp-5 tion solution is in the atomized dispersed state. A sub-stantial approach to equilibrium as used in the descrip-tion of this invention shall mean a residual acid gas loading in the lean alkanolamine solution within 10% of equilibrium acid gas loading values, more preferably 10 within 5% of equilibrium acid gas loading values.
By substantially attaining equilibrium acid gas loading values while in the disperse phase, retention of liquid for further acid gas desorption is unnecessary;
consequently retention time, vessel size, total volume of 15 alkanolamine solution and the like can be reduced. Fur-ther, at the temperatures of interest herein, it has been found that desorption of acid gases from pooled liquid even at retention times as high as 30 minutes was ineffec-tive to achieve equilibrium values of acid gas loading.
It will be appreciated that a wide range of dro-plet sizes can be employed, the range selected being that which will accomplish mass transfer of carbon dioxide and other acid gases from the liquid droplet phase to the vapor phase so that desorption oE the acid gases can occur 25 substantially to equilibrium. The mean droplet size is determined by the average droplet settling time which in turn is dependent on the average droplet velocity and the vessel geometry.
Generally, the lower limit of droplet size can 30 be determined so as to prevent entrainment and loss of absorption solution droplets in the gaseous phase removed from the regenerator; the mean-droplet size will be that necessary to provide the necessary droplet surface; and the upper limit of droplet size will depend upon the dro-35 plet size distribution characteristic of the nozzle. Inany specific application, selection of proper droplet size, _ettlincl time, and equipment configuration can be readily determined by a skilled person in accordance with ~3~3~

the invention as hereinafter described and set forth. In the illustrated embodiment described below, mean droplet diameter of the finely divided droplets can be gerlerally in the range of about 100 mlcrons to about 5,000 microns, s preferably in the range of abou~ 1000 microns to about 5000 microns; settling time can be, ~or example, 0.050 seconds or longer, preferably in the range of 0.10 to 0.50 seconds.
According to a preferred embodiment of the 10 invention a gaseous stream comprising a significant quan-tity of carbon dioxide can be introduced into the base of a first absorber (first contacting zone) for removal of at least a major portion of the carbon dioxide from the gaseous stream. Carbon dioxide can be present in the 15 gaseous stream from about 15 mol % CO2 to 90 mol ~ or higher. For effective absorption and regeneration according to the invention, the CO2 should generally have a partial pressure in the gaseous stream of about 50 psi to about 500 psi or higher.
The gaseous stream can be derived, for example, from gases produced during a CO2 miscible flood operation.
Such produced gases from CO2 miscible flood operations, after CO2 breakthrough, can contain as much as 90 mol % or even higher levels of CO2, as well as methane, and espe-25 cially, ethane and higher hydrocarbons. Hydrogen sulfide will also generally be present~
Such gaseous streams can contain significant levels of higher hydrocarbons which can cause foaming problems during acid gas absorption if the gas is not pre-30 treated to remove at least a portion of the hydrocar-bons. One method for such hydrocarbon removal is described in U. S. Patent No. 4,466,946. Briefly, the hydrocarbon removal can be achieved so that the hydro-carbon dew point profile as CO2 is removed in the first 35 and subsequent contactors is preferably at least 10F
below the operating temperature profile of the absorber. One convenient method of such hydrocarbon 3~

removal is by chilling the feed to condense and remove heavier hydrocarbons which can cause Eoaming and other operatlng problems in the contactor.
The gaseous stream containing hydrocarbons and 5 significant amounts of carbon dioxi~e can also be derived from natural gas reservoirs or from other sources; and in such instances, of course, the instant invention will also be useful.
As indicated above, we contemplate that in most lO cases, the instant invention will find greatest applica-bility when the CO2 content is above about 15 mol % of the gaseous feedstream since below about this level conven-tional absorber systems may be preferred. A typical range f C2 content can be in the range of about 30 mol % to 15 about 90 mol % CO2, although as indicated, more or less C2 can be present.
In a preferred embodiment, the first contactor can be any suitable contactor for contacting the gaseous feedstream with a lean aqueous tertiary alkanolamine solu-20 tion introduced in the top of the first absorber and pro-duced in accordance with the invention for removal of carbon dioxide and other acid gases present to substan-tially equilibrium values of acid gas loading. Suitable contactors can include, for example, towers filled with 25 packing material, the alkanolamine flowing down through the packing and gas flowing upward in countercur.ent fashion; spray towers; tray towers containing bubble cap, sieve or valve trays; stirred vessels, and the like.
Preferably, packed towers or tray towers can be used.
The aqueous tertiary alkanolamine is preferably selected from the group consisting of triethanolamine (TEA), methyldiethanolamine (MDEA), and the like. The alkanolamine can be present in the aqueous solution in the range of about 20 mol % to about 90 mol %, preferably in 35 the range of about ~0 mol % to about 60 mol %. Activators can also be present, such as, for example, piperazine and the like.

_9_ The first contactor can be operated under condi-tions of temperature and pressure effective for removal of at least a major portion of the carbon dioxide from the feed stream. Preferably, the operating temperatures are 5 well below the boiling point of the amine solution employed. For example, in a preferred embodiment, the temperature of the lean amine fed to the first contactor can be broadly in the range of about 1~0F to about 200F, or even lower. Most preferably, the operating tempera-10 tures are in the range of about 160F to about 190F.Typically the contactor operating pressures can be in the range of about 200 psi to about 800 psi, although lower or much higher pressures can be used depending on the pres-sure of the inlet gas source. The CO2 preferably has a 15 partial pressure of about 50 psi to about 500 psi.
The rich aqueous tertiary alkanolamine solution can be removed from the base of the first contactor and introduced with pressure reduction (flashed) into a flash regeneration vessel such as a tank or tower. Whereas a 20 regeneration vessel is typically equipped with trays or packing or other contact devices, we have found that due to the very low volume of stripping vapor ascending within the vessel at the low temperatures of interest, the oper-ating efficiency of the trays or packing is much impaired.
25 Accordingly, trays or packing are not required or desired when regenerating alkanolamine solutions in accordance with the present invention. In fact, the presence of trays or packing can interfere with regeneration. Accord-ingly, an empty vessel can be used and is preferred.
30 Demisting packing can, of course, be used in such gener-ally empty vessels to minimize entrainment of liquids overhead.
The flashing can be accompanied, preceded, or followed by mechanical dispersal of the rich aqueous ter-35 tiary alkanolamine solution into droplets, for example, byspraying, atomizing, misting, or nebulizing. Preferably, the mechanical dispersal is accomplished by spraying the rich amine solution by atomiæing nozzles into the flash 7~

vessel. Preferably, the mechanical dispersal ls accom-plished by upwardly directed atomizing nozzles since such an arrangement has been found to result in slightly better stripping than downwardly directed spray nozzles. Other 5 arrangements can, of course, also be used as can other mechanical devices effective Eor dispersal of the rich amine into finely divided droplets.
Any suitable atomizing nozzles can be used in accordance with the invention. Preferably, pressure 10 nozzles such as hollow cones, solid cones, and fan nozzles can be used. Hollow cone nozzles are preferred for their high atomization efficiency; most preferably wide angle, low capacity nozzles operating with moderately high pres-sure drops are used because such are effective for gener-15 ating small droplets and increasing the rate of masstransfer.
The nozzles can be oriented within the regenera-tion vessels to maximize contact between the droplets and vapor phase. Opposing sprays may be advantageous since 20 collision between finely divided droplets can result in shattering and further fragmentation of the droplets.
Either vertical or horizontal regeneration vessels can be used. Use of a plurality of nozzles at low liquid rates per nozzle facilitates generation of smaller droplets.
25 There also is a turndown advantage since sorne of the nozzles can be taken out of service, thus maintaining suf-ficient pressure drop across the remaining in-service nozæles to give adequate atomization.
The flashing of the rich amine can be accom-30 plished at an inlet temperature about the same as that atwhich the rich amine is removed from the first contactor, that is, in the range generally of about 140F to about 200F; and the lean amine after the flashing and mechan-ical dispersal step(s~ can be returned directly to the 35 first contactor. When operating according to the inven-tion the lean/rich amine heat exchanger and the lean amine cooler can be eliminated reducing equipment require-ments.

--11~
The ~lashing can be accomplished in one stage, or in more than one stage. Typically, if the CO2 product must be recompressed prior to, for example, reinjection during CO2 miscible flooding, it ~ill be desirabLe to use 5 more than one stage of flashing, for example, two or more, to reduce recompression requirements. Thus regeneration can be achieved by stages of flashing. Optionally a small amount of heat can be introduced prior to the first ~lash to maintain the lean amine temperature in the desired 10 range. When two or more stages are used for flashing to conserve recompression energy, the flash pressures can be selected to maintain a ratio between stage outlet pres-sures of about 2.5 to 3.5, i.e., equivalent to the com-pression ratio for a single stage of compression. Thus, 15 for example, for a two-stage flash with rich amine leaving the contactor at 300 to 400 psia, and final Elash at about 20 to 25 psia, the first stage Elash can be accomplished at about 50 to 90 psia.
According to a preferred embodirnent of the 20 invention, the process stream from the first contactor from which at least a major portion of CO2 has been removed can be provided to a second stage of carbon dioxide removal. Preferably, the CO2 content will be below about 20 mol %, more preferably in the range of 25 about S to about 15 mol % CO2 since these ranges can be more efficiently handled by primary alkanolamines such as, for example, monoethanolamine and the like or secondary alkanolamines such as, for example, diethano]amine (DEA) and the like than by tertiary alkanolamines~ A primary or 30 secondary amine is preferred for final cleanup according to the invention because stringent sales gas specifica-tions are more readily obtained with these more reactive amines than with tertiary amines. The use of such primary and/or secondary alkanolamines can be conventional and 35 need not be described here in detail. See also U. S.
Patent 4,466,946. In any event regeneration can be accom-plished under conventional conditions of high temper-3773~

ature and high energy input for the fina:L stage o~ regen-eration regardless of the amine choice.
The invention will be further understood and appreciated from the following Examples in which EXAMPI,E I
5 indicates that conventional stripping of tertiary alkano-lamines is inefficient at the low temperatures of interest and EXAMPLE II indicates the effectiveness of mechanical dispersal of rich amine into small droplets during flashing.
EXAMPLE I
A 60 gpm (gallons per minute) skid mounted amine unit having a contactor and a stripper was modified for testing and data retrieval. The contactor and stripper were each 2.5 feet in diameter with 20 Nutter valve trays.
15 The range of operating conditions is given in Table IA.
TABLE IA
TEA TEST CONDITIONS

AMINE TEA
Rate - GPM 30-100 Net Loading - SCF/gal1.5-4.0 FEED GAS
Rate - MSCFD 400-900 C2 in Feed - mol % 38-68 CONTACTOR
Top Press. - psia 220-570 C2 in Offgas - mol % 2-30 STRIPPER REBOILER
Temp - F 160-235 Press. - psia 21-24 Data collected using the skid mounted amine unit demonstr~ted operability of the tertiary amine process for bulk CO2 removal from a natural gas containing C3 and heavier hydrocarbons. With stripping temperatures above 35 about 190F, performance of the TEA unit conformed to that predicted, but at lower temperatures, the net CO2 solution loading fell below predicted values.

3~

The liquid and vapor rates in the contactor were sufficiently close to the normal operating rates that it was considered unlikely that the average tray efficiency would differ greatly from the 15~-30% anticipated for 5 tra~s in such service. On the other hand, vapor rates in the stripper column were far below the rates desired for effective tray action in the stripper. Accordingly, the burden was placed on the stripper as the principal source of deviation from predicted TEA performance at lower tem-10 perature levels.
In an attempt to discern whether mass transfer or reaction kinetics was controlling in the stripper, reaction rate constants were calculated based on the first order stripping equation:
kr = ~ ln A0 here: ~ = residence time Ao = CO2 concentration in rich amine A = CO2 concentration in lean amine Since the residence time in the stripper system is not well defined, a pseudo kr was calculated in which 25 the amine flow rate in GPM was substituted for 1/~.
Logarithms of the pseudo-reaction rate constants were plotted versus the reciprocal of the reboiler temper-ature in degrees Kelvin. The straight line representing a least squares fit to the data had a slope of -3500 which 30 corresponds to E/R in the Arrhenius equation:

kr = Ae Johnstone (see Johnstone and Thring, Pilot 35 Plants, Models and Scale Up Methods, Ch. 6, McGraw-Hill, 1957) has defined a "temperature coefficient of reaction rate" which represents the ratio of reaction rates for a temperature rise of 10C above a base temperature of 15C.

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In this instance the coefficient amounts to 1.50.
According to Johnstone, the temperature coef~icient for most chemical reactions lies between 2 and ~. A coeffi-cient below 1.5 characterizes a dynamic (mass transfer-5 controlled) regime. The coefficient was determined to beabout 1.5 which indLcated that mass transfer was probably the controllin~ factor.
Another useful criterion is that if the system were chemical reaction rate controlled, a change in fluid 10 velocity would not affect the reaction rate constant at a given temperature. The effect of changing amine flow rate through the stripper system on the pseudo-reaction rate constant at constant reboiler temperature was investi-gated. Results indicated that decreasing the amine rate 15 at constant temperature decreased the reaction rate, which supported the concept that mass transfer, which is sensi-tive to velocity, was the controlling mechanism in the stripper.
It was concluded that decreasing temperatures in 20 the stripper impaired the mass transfer due to decreasing diffusivity, increasing liquid viscosity, and increasing partial pressure of CO2 in the stripping vapor; and that valve trays in the unit stripper were ineEfective due to very low vapor velocity.
_AMPLE II
As indicated, tests conducted on the skid-mounted amine unit indicated deterioration in bulk CO2 removal performance when bottoms temperature of the stripper was decreased below about 190F. Since process 30 energy and investment cost savings can be realized by operation at lower amine temperature, the decision was made to conduct additional tests. The primary objective of these tests was to define the minimum practical oper-ating temperature with a modified stripper column design.
35 The 20 Nutter valve trays in the stripper column were removed and a 10-foot Flexipac* high efficiency packing was installed in the upper section of the stripper column.
Spray nozzles located at four different levels were * Trademark 3~ 2 installed, three directed downwards and one upwarcls. A
top nozzle lnstalled above the packing served to dls- tri-bute liquid over the packing. The remaining three nozzles were installed at different levels in the empty column 5 below the packing to accomplish spray atomization.
The amine unit was restarted, and ten test runs were completed. The schedule of run conditions is given in Table IIA. All tests were conducted with two amine pumps on line, corresponding to about 65 gpm. TEA concen-10 tration was 50-53 weight %.
TABLE II

Run CO2-Mol ~ CO2 in Feed Lean Aminel Rich Amine2 Number Feed Offgas (psi) Temp-FNozzle No.
15 1 53 14 17~3 204 53 12 181 162 1 & 4 25 NOTES:
1. Lean Amine (a) The stripper reflux drum was at 24-26 psia.
(b) The temperature of the lean amine to the con-tractor was held equal to the lean amine leaving the stripper reboiler (no cooling).
2. Spray Nozzle Arrangement (a) Nozzle 1 was in the top of the column, directed downwards; Nozzles 2 and 3 were intermediate in the column directed downwards; Nozzle 2 was located below the packing about 25 feet above liquid in the base; Nozzle 3 about 14 Eeet above the liquid; No zle 4 was in the base of the column and directed upwards.

Regeneration column performance data collected in these runs showed excellent conformance to predlcted values, indicating that the nozzles maintairled essentially single-stage equllibrium flash performance independent of 5 the temperature level, representing a significant improve~
ment over valve tray operation at temperatures below about 190F. It is considered that the high transfer area created by atomization is much more effective for mass transfer than the valve tray action at very low vapor 10 rates. The use of Nozzle No. 4 located in the lower por-tion of the column with the spray directed upwards appeared to have a slight edge in performance.
The improvement in operation resulting from the use of nozzles confirms mass transfer by diffusion to have 15 been limiting in the valve tray stripper. The close cor-relation between the modified stripper and predicted values based on one equilibrium flash indicated that the mass transfer rate throughout the temperature range explored (140 F-200F) was adequate to attain a close 20 approach to e~uilibrium.
From this study it was concluded that perform-ance of the stripper at lower amine temperatures was much improved by the use of spray nozzles compared to valve trays; that high efficiency pac~ing in combination with a 25 distribution spray was comparable to downward directed atomizing spray nozzles in an open column; and that slightly better stripping was observed by the use of an upward-spray nozzle located in the base of the column.
The improved regeneration column performance facilitated a 30 closer evaluation of absorber performance. It was con-cluded that a decline in absorber efficiency occurred with decreasing amine temperatures which could be attributed to reaction kinetics becoming the dominant mechanism at lower absorption solution temperatures.
DETAILED DESCRIPTION OF THE DRAWINGS
The invention will be further understood and appreciated from the following description of the draw-ings.

~ %~3t;~73~

Referring now to the drawings, FIGIJRE 1 repre-sents schematically a first embodiment of the invention in which an acid gas feed havlng 20 mol ~ or more CO2 aCl well as methane, ethane, and higher hydrocarbons is processed 5 in a feed pretreatment zone A to remove hydrocarbons which might otherwise condense during operation of the amine towers, a bulk CO2 removal zone B in which CO2 is reduced to less than about 20 mol ~, preferably in the range of about 5 to about 15 mol % CO2, a second feed pretreatment 10 zone C and a cleanup CO2 removal zone D in which remainin~
C2 is removed from the process stream.
Thus a gaseous feed stream can be introduced by line 101 into feed pretreatment zone A where the feed is chilled and hydrocarbons condensed which can otherwise 15 condense out during CO2 removal. This can be accomplished by any suitable arrangement of chilling and condensing equipment. In the illustrated embodiment of FIGURE 1, the feed can be chilled in air cooler 102, then removed via line 104, exchanger 106, line 107, chiller 108, and line 20 109 to separation vessel 110 where liquid hydrocarbon can be removed by line 111. The pretreated vapor can be removed from vessel 110 by line 112, heated in indirect exchange with the feedstream in exchanger 106, and then provided by line 114, heater 116, and line 118 to tertiary 25 amine contactor 120 in bulk CO2 removal zone B.
In the bulk CO2 removal zone B, the stream in line 118 can introduced at the base of absorber 120 and contacted therein with lean tertiary amine, such as, for example, aqueous MDEA or TEA provided to the top of 30 absorber 120 by line 124 and a substantial amount of CO2 removed to produce a first contactor overhead stream in line 122 having preferably about 5 to about 15 mol ~ CO2.
A rich amine stream can be removed from contactor 120 by line 125, exchanger 126, line 127, throttle valve 128, and 35 line 129 to flash atornization vessel 130.
In flash atomization vessel 130, the rich amine can be atomized, as llustrated, by upwardly directed atomizing nozzles 133, and flashed to regenerate the amine 77~:

in accordance with the invention. The releasecl CO2 can be removed by line 13S after removal oE entrained liquids by demisting packing 131. The lean regenerated amine can be removed from the base of vessel 130 by line 132 and pump 5 134 and lean amine returned by line 124 to contactor 120.
The process gas stream in line 122 from which a bulk removal of CO2 has been accomplished can be provided to a second pretreatment zone C in which the feed can be chilled and hydrocarbons condensed and removed which can 10 otherwise condense out during CO2 removal in final CO2 removal zone D. Thus, as shown in the illustrated embodi-ment, the stream in line 122 can be provided via air cooler 136, line 137, exchanger 138, and line 139 to vessel 140 from the base of which liquid hydrocarbons can 15 be removed. The treated process stream can be provided by line 141 to, for example, DEA contactor 142 which can be operated conventionally and need not be described here in detail since such operation will be familiar to those skilled in the separation arts.
In contactor 142, the process stream from which bulk CO2 has been removed can be contacted with, for example, lean DEA solution provided by line 158 to remove remaining CO2 and to provide a sweet hydrocarbon product stream 143 from which substantially all CO2 and H2S has 25 been removed. The rich amine can be provided to regener-ator 150 by line 144, exchanger 145, line 146, throttle valve 147, and line 148. CO2 can be stripped from the rich amine in regenerator 150 and an acid gas stream can be removed via an overhead cooler/condenser indicated gen-30 erally by 151 and by line 160. Lean amine can be removedfrom the base of regenerator 150 and returned by line 153, exchanger 145, line 154, pump 155, line 156, air cooler 157, and line 158 to contactor 142.
~eferring now to FIGURE 2, FIGURE 2 represents 35 an embodiment of the invention in which regeneration of rich amine in contact zone B can be accomplished in two stages. Except for zone 3 described in detail bel~w, FIGURE 2 iS the same with the same reference numerals as FIGURE 1 described above.

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Referring now to FIGURE 2, the Eeed stream after pretreatment in zone ~ is provided to bullc CO2 removal zone B by line 118. In zone B the pretreated feed stream is contacted in contactor 220 with a tertiary amine pro-5 vided by line 224, producing a product stream in line 222having preferably 5-15 mol % CO2 therein which can be pro-vided to zone C as described above for line 122 for FIGURE 1. Rich amine from contactor 220 can be withdrawn by line 22S and flash-atomized in two stages in strippers 10 230 and 240, operated respectively, for example, at 75 psia and at 20-25 psia. Thus, rich amine can be with-drawn by line 225 and provided by exchanger 226, line 227, throttle valve 22~, line 229, and atomized by nozzles 233 into flash vessel 230 operated at the intermediate pres-15 sure. Released CO2 after demisting by packing 231 can bewithdrawn by line 235. Semirich amine can be withdrawn from flash vessel 230 by line 232 and provided by throttle valve 235, line 236, and by atomizing nozzles 237 to flash vessel 240. Released CO2 can be removed by line 241 20 after demisting by packing 238; and lean amine can be returned by line 242, pump 243, and line 224 to contactor 220.
It will be appreciated that the invention can provide an economical and efficient method for removing 25 acid gases such as CO2 and H2S from gaseous streams at low temperatures below about 200~F and regenerating the loaded absorbent solution substantially to equilibrium values.
Other advantages and applications will be apparent to those skilled in the art from the description herein; how-30 ever, the invention is not limited thereto by the claimsappended hereto.

Claims (17)

1. Process comprising:
desorbing acid gases from a rich aqueous tertiary alkanolamine solution comprising absorbed acid gases by atomizing and flashing the rich aqueous tertiary alkanolamine solution under conditions of droplet size and settling time effective for substan-tially attaining equilibrium acid gas loading values while in the resulting disperse phase, the tempera-ture of the rich tertiary alkanolamine solution being below about 200°F; and coalescing the resulting lean tertiary alkanolamine solution droplets and forming a lean tertiary alkanolamine solution having substantially equilibrium acid gas loading values.

2. Process as in Claim 1 wherein the lean ter-tiary alkanolamine solution is within about 10% of equili-brium acid gas loading.

3. Process as in Claim 1 wherein the lean ter-tiary alkanolamine solution is within about 5% of equili-brium acid gas loading.

4. Process as in Claim 1 wherein:
the rich aqueous tertiary alkanolamine solution comprises a tertiary alkanolamine selected from the group consisting of triethanolamine and methyldiethanolamine.

5. Process as in Claim 4 wherein:
the droplets have a mean diameter in the range of about 100 to about 5,000 microns; and a set-tling time of greater than about 0.050 seconds.

6. Process for the recovery of carbon dioxide from a gaseous stream comprising carbon dioxide and hydro-carbon components, the process comprising:
introducing the gaseous stream into the base of a first contacting zone and contacting the gaseous stream with an effective lean aqueous ter-tiary alkanolamine solution introduced at the top of the contacting zone, absorbing at a temperature below about 200°F at least a major portion of the carbon dioxide and producing a CO2-lean gaseous stream and a rich aqueous tertiary alkanolamlne solution;
desorbing acid gases from the rich aqueous tertiary alkanolamine solution comprising absorbed acid gases by atomizing and flashing the rich aqueous tertiary alkanolamine solution into a first regenera-tion zone under conditions of droplet size and set-tling time effective for substantially attaining equilibrium acid gas loading values while in the resulting disperse phase, the temperature of the rich tertiary alkanolamine solution being below about 200°F;
coalescing the resulting lean tertiary alkanolamine solution droplets and producing the lean tertiary alkanolamine solution having substantially equilibrium acid gas loading values; and returning the thus regenerated lean tertiary alkanolamine solu-tion to the first contacting zone.

7. Process as in Claim 6 wherein:
the lean aqueous tertiary alkanolamine solution comprises a tertiary alkanolamine selected from the group consisting of triethanolamine and methyldiethanolamine; and comprising:
introducing the CO2-lean gaseous stream into a second contacting zone and contacting the CO2 lean gaseous stream with a second effective aqueous alkanolamine solution and absorbing substantially all of the remaining CO2.

8. Process in as Claim 6 comprising:
flashing and atomizing the rich aqueous tertiary alkanolamine solution by dispersing the rich aqueous tertiary alkanolamine solution into droplets having a mean diameter in the range of about 100 microns to about 5,000 microns a settling time of greater than about 0.050 seconds during introduction into the first regeneration zone.

9. Process as in Claim 8 comprising:
introducing the rich aqueous tertiary alka-nolamine solution into the first regeneration zone by atomizing nozzles.

10. Process as in Claim 9 comprising:
introducing the rich aqueous tertiary alka-nolamine solution into the first regeneration vessel by more than one upwardly directed atomizing nozzles.

11. Process as in Claim 6 comprising:
flashing and atomizing the rich aqueous tertiary alkanolamine stream by dispersing the rich amine stream into droplets in the first regeneration zone in at least a first regeneration stage and a second regeneration stage, the first regeneration stage being operated at a higher pressure than the second regeneration stage.

12. Process as in Claim 6 comprising:
flashing and atomizing the rich aqueous tertiary alkanolamine stream in the first regenera-tion zone at an effective temperature below the boiling point of the rich amine.

13. Process as in Claim 6 comprising:
flashing and atomizing the rich amine stream and desorbing absorbed carbon dioxide at a temperature below about 200°F.

14. Process as in Claim 6 comprising:
flashing and atomizing the rich amine stream and desorbing carbon dioxide at a temperature in the range of about 140°F to about 190°F.

15. Process as in Claim 7 wherein:
the lean aqueous alkanolamine solution com-prises methyldiethanolamine; and the second effective aqueous alkanolamine solution comprises diethanolamine.

16. Process as in Claim 6 comprising:
introducing the gaseous stream into a dew-point control zone and removing hydrocarbons there-from such that the hydrocarbon dewpoint profile during CO2 removal is at least 10°F below the operating temperature profile during CO2 removal and then introducing the thus treated stream into the first contacting zone.

17. Process as in Claim 16 further comprising:
introducing the CO2-lean gaseous stream into a second contacting zone and contacting the CO2 lean gaseous stream with a second effective aqueous alkanolamine solution and absorbing substantially all of the remaining CO2.