EQC-IPMEKT FOR EXPERIMEVTAL STUDYOF ACETIC ACID SYSTHESIS USDER PRESSURE
Synthesis of Acetic Acid from Methanol and Carbon Monoxide
aT
HE oucces:2ful (winniercia1 u r o t l u c tion of t:iethanol t'roni hydrogeii ani1 oxide- of cartion and t!w re-iilting price r e d u c t i o n ha- iiisiie nvaili!ile a new ranniuteris! f 1 . r cheniical synthesis. I t i - t h e r e f o r e desirable to invediyatr tile possil)ilitie~ of ti > i 11g I I I e t I: ii n I ) 1 in n e x reaction?. The pre-ent paper deals with t!ie sj-nthe-i.. of acetic acid f r o n ~ilie t h a n o 1 a n t l carhon n!I) II IJ si i!e in the .sense of the IolliwiIig rwctic i n :
The catalytic, vapor-phase synthesis of acetic acid from methanol and carbon monoxide under pressure has been studied. The effect of factors such as temperature, pressure, space velocity, composition of reactants, and recirculation of by-products is shown. Active carbon impregnated w-ith phosphoric acid is catalytically active in this reaction, but its life is limited. The commercial applicability of this process depends o n the development of a stable, active catalyst and the elimination of side reactions to form products not reconvertible to methanol or acetic acid.
R e a c t i o n s 3 and 4 have been considered bj- Christianaeii ( 2 ) whose c a l c u l a t i o n s indicate that significant a m o u n t s of niet'hanol could be converted to other p r o d u c t s tby t h e b e reactions. An early observation hy Frolich (4) reported the formation of acetates w h e n soditini methylate was treated with carCH.C)M CC) +CHSCOOH b o n m o n o x i d e a t 190" C . (1) Stahler, howeyer, could not duplicate this result, ( 6 ) ; he fount! ILkTIJIT D. SIAGH 4 Y D \ORTIAU W.h R I S E that alkyl forniates r e s u l t e t l acciwate t ~,erriiuiI~iiaiiiic calculaUniversity of Illinois, Urbana. Ill. when using carbon monoside t i o w tin thi- e q i i i l i tiriuni heeven a t high pressures. Fixher cau-e the theriiial Iiroriert . ies of (3)used reaction 1 to explain the format& bf acids in his ' acetic acicl nl:~! it? mixtures are not knon-n with accuracy. hi,prijsiiii3tii?:i~, however, indicate that the reaction is exothol" made from carbon monoxide and hydrogen. S u m pat'ents, especially in recent years, have described a variety t!ieriiiic ti! t!ie extent of 20,000 to 30,000 calories and that the equi1il)rixn a t temperatures between 300" and 500" C. and of catalysts and conditions for the production of acetic acid eyen a t pre-zres helow 300 atmospheres is very favorable to by pressure synthesis. Hardy ( 5 ) reports laboratory resultz acetic acicl fc:,riiiation. In this case the pmsibility of side reobtained by bubbling mixed carbon monoxide and methanol actil~ri-d - i ] enters. In order t,o account for by-products vapor through pho;phoric acid containing copper phosphate. foiitic! expsimental , it is posiible to formulate reactions a t 330" to 340" C. Under his optimum conditions, using a -olrLewk.:lt a\ follonrecirculation method, 44.9 per cent of the alcohol was converted to acetic acid, 9.5 per cent to dimethyl ether, and 4.8 'LCHIOH CHsOCHa H10 (2) per cent to carbon, and 5.0 per cent mas loss. The remainder methyl ether was unconverted alcohol. I n the present work attention is primarily given to yaporCHjOH CO +H,COOCH, (3) phase, catalytic reactions carried on with solid catdy3ts. methyl formate
+
+
+
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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Experiments in which liquid methanol, containing dissolved or suspended materials, was subjected to high-pressure carbon monoxide gave no evidence of acetic acid formation. I n order to use higher temperatures than were possible with liquid methanol, compressed carbon monoxide was bubbled through the alcohol, and the mixed vapor and gas passed
but i t seems desirable to describe certain phases of the work because of frequent speculation in the literature regarding the merits of this process. Considerable data collected over a period of several years have been omitted because of their qualitative nature. These relate to catalyst searches and to tests bearing on reaction mechanism. The experimental work involved two t'ypes of apparatus and operation. The first was the "once-through" method in which the reactants passed through t'he catalyst, were cooled to recover the products, and expanded to atmospheric pressure. The second mas similar to the usual pressure synthesis cycle for methanol or ammonia in that, after recovery of products, the noncondensable gas and vapor were recirculated without e x p n n s i o n , p a ss i n g through the catalyst repeatedly toget'her with "makeup" reactants. Obviously, the results in the two cases might differ markedly since in the second case certain products of the reaction accumulate in the recirculated gas and the composition of reaction mixture changes. This point will be discussed later. The first apparatus used was the expansion or oncethrough type, shown in Figure 1:
j .THERIlOCOURL
Approximately 300 cc. of methanol (or a solution containing alcohol) were introduced above the upper piston of the intensifier, 3, and pressure applied under the lower (large) piston by means of a nitrogen cylinder, 1, operating through the oil reservoir, 2. By a suitable unbalance of pressures in the oil reservoir and in the system beyond the intensifier, controlled by the valve, 4, liquid alcohol was introduced at a slow, measured rate into the vaporizing furnace, 5. T h i s f u r n a c e was normally held at 300" C. Carbon monoxide from pressure storage, 6, was mixed with the alcohol vapor and entered the loner end of the catalyst furnace, 7. In most cases 30 cc. of catalyst were used, the temperature being measured by a chromel-alumel couple inside a well in the catalyst bed. The reaction products passed through a watercooled condenser, 8, and into a separating trap, 9. The liquid
c
bubbled through phosphoric acid a t 410" to 480" C., a definite test for methyl acetate was obtained but large quantities of carbon were deposited. Subsequent work indicated that a fairly satisfactory catalyst could be made by impregnating activated carbon granules with phosphoric acid. Catalysts of this type were used in most of the experiments reported here. Catalyst development is a matter that requires multiplicity of testing equipment and considerable man power; i t did not seem desirable to undertake such a problem. The goal was rather to develop a catalyst sufficiently active to permit a study of the chemical characteristics of the reaction and to outline the possible operating conditions. The yield of product and the extent of side reactians depend, of course, on the catalyst properties, and the results presented are limited by this fact. While phosphoric acid exhibits sufficient activity for the immediate purpose, it also possesses properties that are undesirable and often lead to experimental diffi-
Experimental Methods The experimental work on the synthesis of acetic acid was planned t o yield data from which a selection of satisfactory experimental conditions could be made and then to operate a small pilot plant for the purpose of obtaining further process data from which an appraisal of the economic value of the development could be made. Both of these aims have to some extent been realized. Obvious gaps in the data still exist,
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CIRCULATION METHOD
i
product was periodically withdrawn into the glass receiver, 10. Soncondensable gas passed through an expansion valve, 11, and through scrubbers containing sulfuric acid, 12. The volume and rate of flow were measured by meters 13 and 14. Expanded gas was collected in a holder. In this type of operation, considerable dimethyl ether was formed. Weighing the absorbers, 12, gave data on this quantity. The liquid product from 10 usually contained acetic acid, methanol, water, dimethyl ether, and methyl acetate. The composition of this liquid was determined by careful fractionation and titration. The second method of operation-namely, without expansion-is shown in Figure 2:
recirculation
Two sizes of catalyst chamber were used during different stages of the work. One had a capacity of 30 cc., the other contained
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INDUSTRIAL AND ENGINEERING CHEMISTRY
911
1-olume gas a t 1 atm. = 3, \Volume catalyst X time in hours
Experimental Data EXPAXSION METHOD. The data froin typical expansion experiments are shown in Figures 3, 4, and 5 . I n order to place all measurements on the same basis the total production, T. P., is calculated as the number of grams of acetic acid, free and combined, per 100 cc. of catalyst. The rate, R, is the number of grams of acetic acid per 100 cc. catalyst per hour. Per cent conrersion is calculated on the basis of the total alcohol put through the catalyst. I n Figure 3, T. P., R, and per cent conversion are shown as a function of temperature. Two curves at 29,000 space veIocity (S. V.) and for periods of 1.5 and 3 hours indicate that the total production increases with temperature u p to about 450' C. and then begins to decrease as the temperature rises. One curve a t a lox-er space velocity, 14,500, shows a steady increase with temperature up to 475' C. For several reasons to be discussed later it was not feasible to carry the temperature higher; doubtless this curve would also coritain a maximum if it could be investigated over a wider range. These curves illustrate the marked effect of temperature on the rate of reaction and the importance of time of contact and temd ' . ! ! ! ! ! ! ! ' ~I ]L i I ,1 ! I perature in determining production of acetic acid. The rate of production, R, in Figure 3 is shon-n as a function of temperature and space velocity. Percentage conversion of alcohol t o acetic acid and t o dimethyl ether, in Figure 3, depends on the temperature. The fraction of alcohol converted to ether i j many times that converted to acid, and t'he effect of temperature is opposite in the two cases. It is .surprising to find the extent of conversion of alcohol to acid and t o et'her greater a t 29,000 S. I-.than a t 14,500 S.I-. .ibore 450" C. the values for acid are in the usual relationl;liip--namely, higher conversion a t lower d. V. The data indicate the possibility of regarding ether as a necessary interniediate in acid 300 Y O 400 450 500 formation since both products increase v i t h S.T-. and show FICCRE3. D i ~ 4F R O V TIPICIL E X P ~ R S I OEXPERIMEATS \ oppoHite trends with temperahre. Figure 4 s1io11-s the favorable effect of iiicreasing S. V. on 400 cc of catalyst hlcohol mas introduced into the ->-tern by R and percentage conrersion a t se\-eral temperatures. It n \mall HilI~-McCannapropoitioning pump, 1, draning from a is significant that at the highest temperature! 475" C., the calibrated ie>eivoir, 2. hfter pacsing through the vaporizing fraction of alcohol going to acetic acid decreases with S. V., furnace, 3, the alcohol vapor mived vith carbon monoxide in the whereas a t lower t'emperatures slight inp r c h e a t e r , 4. T h e floir through catalyst chamber 6. creases are noticed. Figure 5 sho& the condenser 7 , and trap 8, was decreasing activity of the catalyst with s i mi 1a r t o that described time. The curves a t 300" and 350" C. above. When product was withdrawn and collected in refer to anhydrous alcohol, and the t\To 9, the dissolved gases evolved a t h i g h e r temperatures were obtained were absorbed in scrubbers containing sulfuric acid, 10. Q Soncondensable g a s f r o m the separator, S, was picked up by the circulating booster p u m p , 11, a n d f o r c e d through the pressure flowmeter, 12, to return to the preheater, 4. The pressure on the system was controlled by carbon monoxide f r o m storage, 5. Thc p r c s s u r c flowmeter wag calibrated bjR e x p a n d i n g gas through an accurate volume meter b e fore and after cxperiments. T h i s device has been described ( I ) . I n all experiments space velocity hai been calculatefl on the basis of the exit gas and TT aq as follows.
FIGL'RE
1.
TYPICALEKP.~NSIOS EKPERIMESTS
D.kTA FROM
FIGVRE5
D i ~ FROM i TIPIC~L Expi\EXPERI\IE\TS
SIO\
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INDUSTRIAL AND ENGINEERING CHEMISTRY
FIGIJRE 6 . D 4 ~ 4FOR TYPICAL CIRCUL.4TIO\ EXPERIXEXTS
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u s i n g 85 p e r c e n t aqueous alcohol. The decrease with time is approxiiiiately linear. Thii: will be tiiacuwetl later. C I R c IT L A T I o s METHOD. Noit, of the data obtained (luring c i r c 11 I a t i o ri experimente refer to a total p r e s s u r e o f 2000 p o i i n d ~p e r s q u a r e inch, d i i l e all experin i e n t s u s i n g the expansion iriethod were a t 4000 pounds pressure. This change was one of c o n v en i e II c e since, within the range, t h e results a r e n o t significantly af i'e c t e (1 by pressure. All exI I I I periment)? in Figure 6 PRESSURKZOOO LBS TEMP. 3 5 0 D t Q . C . are a t 10,000 S.1 7 . and BO PERCENT MEOH. CIRUILATIOY 2000 pounds presmre, and with 80 per cent a 1c o h 01. Total proSPACE VCLOCITY duction, rate of produc0 5 000 10000 I5000 20 D tion, and percentage FIGURE 7 . D.4~.4FOR TYPICAL CIRCULlTlOX c o n v e r si o n increase EXPERIMENTS with temperature. In contrast with e x p a n sion experiments no maximum appears in thece curveq. Figure 7 illustrates the effect of S.J'. on these 3ame quantitieb. The behavior here is entirely normal. Figure 8 include^ (lata for t v o pressures, three spaces velocitieq, and three temperatures on rate of production as a function of time The results a t 4000 pounds pressure also include wrrie indication of the effect of water on the life of the catalyit This latter factor is more exten51vely +hewn in Figure 0, TI hich also gives the distribution of acetic acid and methyl acetate in the product a3 it depend. o n the concentration of alcohol in the feed. A direct comparison of the t n o ex p e r in1 e n t a 1 method$ is shown in Figure 10. rntier the came conditions the rate of production of acid with circulation is approximately double that oi.ltainet1 with once-through operation. RESCLTS.The experimental re-ult. *ho\i that acetic acid can he synthebizetl from riiethaiiol and carbon monoxide by vapor-1iha.e catalytic methodi. The reaction proceeds under n hat are not u~iially conqidered extreme conditions for -uch reaction-namely, 300" to 500" C. and 2000 to -1000 pcrundi per square inch pressure. Kithout attempting to develop and refine a satiqfactory, cvnimercial cataly-t, it has been shown that phosphoric acid depo\itetl on active carbon catalyzes the main reaction miti givea product containing acetic acid, methanol, methyl acetate, water, and dissolved dimethyl ether An important point to be empha-ized iq that no waterinsoluble oils need be formed and that the organic by-products are capable of ultimate and complete conversion to acetic acid.
Dimethyl Ether Formation CIRCULATION EXPERIMENTS FIGURE8. DATAFOR TYPICAL
One of the main by-products of the reaction appears to be dimethyl ether, and under a once-through
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AUGUST, 1933
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be formed by dehydration. In order to estab1i.h conditions tending to repress dehydration of the catalyqt, aqueous alcohol solutions were used. Much more sati>factory operation of the apparatus resulted. The comparimi of various alcohol concentrationq (Figure 5 ) indicate aloo an improved behavior of the catalyst both as regards life and activity. Despite the addition of nater to the reaction iniuture, the activity diminished n i t h time a t practically a liiieai rate as hhonn in Figure 8. Such a result itrengtheni the helief that volatilization 1- the cause of the decrease. Sample. of catalyst taken before and after a 20-hour teqt at 300' c'. u-ing feed alcohol containing 20 per cent n ater gave the idlowing analysis :
1 b
2
2
Grams &PO; per 50 Cc Catalyst
2
Original Cpetream Downstream
R
24,97 12.59 14.10
I
The results shoTT not only a large 103s from the cataly-t but also "creep" of catalyst in the direction of gas flow. Concentration of Product P z50
82 f
FIG 9 RECIRCL~LATION I CRLSbURL4000L15. S.V.29000 TEMP. 400DCG.C.
5 a Y
The concentration of acetic acid in the product of thip reaction varied with many factors. Two Sets of (lata taken at 400" C., 4000 pounds pressure, and 29,000 S. V. compare absolute and 85 per cent alcohol.
Y
L
70
FIGURE 9.
80
D.4T.4
90
100
TABLE
I.
FOR TYPIC.4L CIRCULlTION
EXPERIMEXTS
type of operat,ioii most of the alcohol is converted to ether (Figure 3). Fort'unately this product is formed as the result of a definitely reversible reaction so that recirculation of the noiicondensable products stops further formation of ether as soon as sufficient alcohol has been processed to build the ether concentration up to its equilibrium value. Analyses of gas taken from the circulat,ing system after 4 to 6 hours of operation averaged 2.4 per cent et,her by volume 17-hen the catalyst temperature was 400" C. The liquid product expanded from the collectiiig trap is saturated with ether which it evolves n-hen the pressure is releaied. Since the molal ratio of carbon nionoxide to methanol R used in these e x p e r i 111 e n t s n a - usually 40 to 1, no s t a t e ment regarding t h e e f f e c t of alcohol concen1.5 45 6 tration in the HOURS FIGLRE 10. COMPIRISO> OF THE T \ \ O gas stream can ETPERIVEST.4L 3IETHODS he made. Concentration of -4lcohol in Feed Early experiments using anhydrous methanol resulted in considerable difficulty due to volatilization of the catalyst and condensation in cooler parts of the apparatus. From the dissociation pressure data of orthophosphoric acid, it appeared probable that the more volatile metaphosphoric acid might
period, Hours
vARIAT1ON O F -%CETIC A C I D
CONCENTRATION B Y
EXPANSION METHOD Acid Concn., Absolute
yo by K e i g h t S570
Grame d c i d per 1 3 Hours Absolute 85%
OF -ICETIC .IUD CONCENTRATION KITH TABLE11. 1-ARIATIOS TEMPERATURE BY EXPANSION ~IETHCID
Catalyst t e m p . . O C. Concn. of acid, 7*by weight
330 14 6
400
42.5
475
23,l
IS 3
15 3
TABLE111. Y.IRIATIOSOF ACETIC -IUD BY RECIRCC-L.XTIOX METHOD 2000 pounds; temperature = 353' C . ; $. V , == 10,000: alcvhol = 80 per cent) Acid Concn., .Xcid C'cinrn., Hourly Samples yo b y K e i g h t Hourly Samples "; by Weight First hour 16.6 Fifth hour 13 0 Second hour 20.4 Sixth hour
