The Production and Recovery of C 2 4 4 Olefins from Syngas C. 6. Murchison, R. L. Weiss, and R. A. Stowe Dow Chemical Co., 1776 Bldg., Midland, MI 48640
The petrochemical industry in the U.S. owes its historic erowth to inexoensive hvdrocarbon raw materials obtained From beneath t h e earth;s surface. Low price has been so fundamental to the chemical industry's profitability that it is no wonder that the seemingly uncontrolled price escalation of crude oiland LPG's in the 1970's led many companies to look for alternative sources of petrochemical feedstocks. Although there are a number of alternative natural carhon-containing materials, coal, lignite, peat, biomass, and garbage, none are as convenient and easy to transport as oil and gas. Solids handling is much more difficult and capital intensive than processing gases or liquids. Nevertheless. there comes a uoint where the cost of fluid hydrocarbons is greater than the cost of solid hydrocarbons olus the cost of the extra cauital to handle them. This point Is generally predicted to- he when gas and oil reach $&12/mmBTU. They are currently about $3.50/mmBTU. Coal, even with its large ash content (10-20%), is generally considered to he the leading alternative to oil and gas because of its relatively high carbon content and concentrated abundance. In this paper we will discuss reacting hydrogen and carbon monoxide (syngas) a t relatively high selectivity to ethylene, propylene, and butenes over novel catalysts. The syngas would come from the partial oxidation of coal with steam and oxygen. In addition we will give data illustrating a unique ethylene removal step which is compatible with operating the olefin synthesis a t low conversion. Synthesis of Olefins Ideally, it would be nice to have 100%selectivity to ethylene or a t least ethylene and propylene. Thus, instead of forming the paraffin from syngas and then cracking i t to the olefin we could conserve energy by direct synthesis of the olefin. This reasoning would he fine if it were not for the chemistry of the process. Syngas to hydrocarbons or Fischer-Tropsch (FT) chemistry, as it is called, is a polymerization of carbon monoxide or its surface derivatives and obeys the statistics of a stepwise addition mechanism (1,2).The elementary statistical expression correlating weight fraction of a certain-carbonnumber polymer with the probability of adding one more unit to the growing chain is given by C, = n(1 - P)2P"-1. Figure l!shows a plot of this function versus probability of adding one more unit to the growing chain (chain growth probability, CGP). The polymer chemist usually concerns himself or herself with polymers of >0.9 CGP. In this case we are interested in short polymers, those of CGP = 0.4. In Figure 1 one sees the C1 "monomer" decrease as the CGP increases. The C2-C4 fraction goes through a maximum and then a t the higher CGP the C5+ material begins to form in appreciable amounts. So far, this analysis only relates to carhon number and says nothing about the olefinic character of the product. With a few exceptions essentially all steady state F T catalysts operated at reasonable conversion give a product distribution described by these statistics. One unfortunate exception is a negative deviation for the C2 species. Where the statistics suggest that 30%C2 is possible, most catalysts with the exception of molybdenum give about 20% C2 at best. Molybdenum catalysts can give the CGP maximum of 30%
routinely and in combination with cobalt the paraffin and alcohol synthesis can give up to 45-50s C2 (3-5). The first step in maximizing the C2-C4 olefins is to maximize the total C2-C4 product as suggested by the polymer statistics (50-60s). As is customarv in F T chemistrv the ieleutiv~t~es are on a CO>-freecarbon mole 11asili. The was unr m o w bark and f m h on the vrohabilitv axis of the chahgrowth plot is to vary the acid/dase character of the catalyst. Adding basic compounds to the catalyst increases the heat of chemisorption of CO relative to Hz (6.77. On an elementary level one can think of the surface concentration of carbon species available for polymerization to be increased relative to the concentration of hydrogen. Hydrogen is generally considered involved in the chain-termination step. For catalysts based on elements such as molyhdenum, for which the reduced oxide has a low CGP, adding a basic compound, such as KzC03, will increase the CGP. On the other hand, elements like iron have a relatively high chain-growth probability lying to the right of the C2-C4 maximum in Figure 1. In this case to increase the amount of material in the desired C2-C4 region one adds an acid component such as a chlorinated compound or an acidic support. In our work with iron catalysts, which usually are a mixture of iron carbides, reduced oxides of iron, and spinels formed with other promoters, we have often chosen to add methyl chloride to the feed a t ppm levels. In the presence of Hz and the catalyst, CH4and HC1 are formed, thereby acidifying the Figure 2 illustrates this for a sintered FesOa catalyst (8,9). catalyst. I t should he mentioned that process parameters such as temperatures and pressure can also be used to move up and down the probability axis. Increasing the reactor temperature lowers the average molecular weight (lowers GCP) whereas increasing pressure raises the average molecular weight (higher CGP). For moly catalysts temperatures of 380-410 "C and pressures of 300-500 psig are appropriate. For iron catalysts, temperatures of 280-310 OC and pressures of 150-300 psig are used.
PROBABILITY OF CHRIN G W T H
Figure 1. Selectivity versus probability of chain growth.
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213
Table 1. Olefln Content versus Dlsperslon and Wt% Alkell
After the molecular weight distribution has been more or less maximized in the C2-C4 reaion - one concentrates on maximizing the olefinlparaffin ratio. Fortunately with moIvhdenum the addition of alkali not only moves the molecular weight distribution in the desired direction but also increases the olefinlparaffin ratio. Again this has to do with the suppression of hydrogen chemisorption relative to CO. For iron-based catalysts we usually start with an alkalized catalyst that already has a relatively high olefinlparaffin ratio, hut the molecular weight distribution is shifted toward the heavy end. In this case we can essentially back-titrate with CI, decreasing the average molecular weight. Although at first the OIP ratio decreases, additional chloride will reduce the intrinsic activity of the catalyst such that a somewhat higher OIP is obtained. Figure 2 illustrates this effect. We have found that an important way of significantly increasing the C2-C4 olefin content of an F T catalyst is to reduce the dispersion of the active element or create large crystallites rather than small. We have found this t o be particularly effective for reduced molybdenum catalysts. Table 1 illustrates the differences in olefin selectivity between highly dispersed reduced moly catalysts and poorly dispersed examples. High dispersion, small crystallite size (20 A) is created by impregnating high surface area supports with low loadings of (NH&Mw02~4H20. In some cases where appreciable free OH groups exist on the support, high dispersion can be achieved by reacting those OH groups with MoOa. In contrast, by impregnating very low surface-area supports (1-10 mzlg) or high surface-area supports with high loadings one can obtain catalysts with low dispersion and crystallites >I000 A (10). In addition, commercial MOOJ powders that have crystallite sizes of about 1000 A, can be fabricated into catalysts that will give relatively high olefin yields upon partial reduction (11,lZ). Calcining catalysts a t temperatures u p to 700 "C is also useful in lowering dispersion. Inkll cases alkali is still necessary for high olefin selectivity, as shown inTable 1.We take this crystallite size effect as further evidence for the structure-sensitive nature of the F T reaction in contrast to the structure-insensitive reaction of olefin hydrogenation. Finally, an important consideration for industrial catalyst research is that the catalyst behavior must be stable over an extended period of time. Figures 3 and 4 show the CZ-C4 olefin and ethylene selectivity versus hours on stream for the catalysts being discussed here. For the moly catalyst the final conversion was arbitrarily lowered to illustrate high ethylene selectivity a t low conversion. One of the important criteria for an olefins plant is that the product mix should match the market needs. In the US., the ethylene industry evolved based on cracking ethanelpropane feedstocks. The resulting market demand is for an ethylenelpropylene ratio of about 211. On the other hand, in Europe, which historically did not have large ethane supplies, the market demand is about 111. Because Dow is a worldwide company, i t was important in our long range research to he able t o vary the olefin ratios from any F T 214
Journal of Chemical Education
q CI/g CRTRLYST
Figure 2. Chaln grown?and O/Pratio versus CI,
4 8 8 C. S 8 8 p.80.
H1ICO-.8
COW
Flgure 3. Pertormame MOO?olelin catalyst.
Figure 4. FT pilot plm-FesOdTi02/K/CI
catalyst
plant. Table 2 shows the results for catalysts discussed here. The first column gives the as-synthesized ratios. The second column results from cracking the coproduced light paraffins and oil and then comhining the C2-C4 olefina with the directly produced olefins. In our experience all F T catalysts in which the hydrogenation has been suppressed enough to
Table 2.
Olelln Ratlos (C2H41C3HdCaH.) Aner
Moly low S A support Reduced Mooa F e promote Reduced Mooa e x n u d a t e Sintered FesO,
cracking ~ a r a f f i n as n d oil
28
0.91110.4 0.6/1/0.6 1.6/1/0.4 0.41110.7
1.81110.4 1.21110.6 2.51110.6 1.0/1/0.6
16
12
give appreciable amounts of olefins, will produce a significant amount of aliphatic oil. This oil makes a very suitable cracking stock (13) for additional olefins. From these catalysts ethylenelpropylene ratios of 0.411 to 2.511 can be obtained. One of the dilemmas of the synthesis of ethylene from syngas is that the selectivity of this olefin is a strong inverse function of conversion. Thus. as shown in Fieure 5. the highest selectivity to ethylene is obtained a t a b o k 20%'conversion. At this low conversion. considerable auantities of unreacted CO (bp -192 "C) andHz (hp -253 O C ~are present along with product CH4 (bp -184 O C ) , making the concentration of ethylene (bp -104 OC) in this low-boiling fraction of exit gas very low. Recycling to increase the net conversion effectively is obviated by the high reactivity of ethylene toward hvdroeenation. Cryogenic distillation to remove such low conckntritions of ethylene is not economically feasible. Thus, it appears difficult to take full advantage of the high ethylene selectivity (20%) possible with our new moly catalysts because of the relatively low conversion necessary for that selectivitv. Ethylene is a large-volume commodity chemical with worldwide production capacity in 1980 of about 100 billion Iblyr. Styrene (made by dehydrogenating ethyl benzene, which in turn is ~ r o d u c e dfrom ethvlene and benzene) is also a large-volume >ommodity chemical with about 24 billion Iblyr capacity. With this background in mind we have proposed a novel F T process that incorporates a member of the new generation of high-olefin F T catalysts plus an alkylation section such that the resulting plant is capahle of being world scale in both ethylene and styrene in an exceptionally cost-effective manner (12). In this process the effluent from a highselectivitv olefin catalvst would have the CO9. -. HvO. - .and C3+ hydrocarbons separated from it by conventional means (scruhbine. drvine. and crvozenic distillation) to remove C3+. ~ h & e m a i n &gases ~ wo&d be co-fed with benzene (or other aromatic hvdrocarhon) to an alkvlatiou reactor cont a i n i n ~an acid catalyst. We have carried out experiments with a sample of HZSM-5 zeolite prepared in our laboratory benzene plus the (14,15). able 3 gives the results of F T product stream with the C3+ removed over HZSM-5 (benzenelethylene = 17.511).The 98% ethylene removal and 97% selectivity to ethylbenzenes demonstrate the effectiveness of the concept. The aromatics stream, which is easily separated from the light gases, would he recycled to the inlet of the alkylation reaction for further reaction. This zeolite catalyst showed no activity loss or visible sign of coking after 40 h on stream. Conclusion
We have seen some of the criteria, practical constraints and planning that go into a goal-oriented industrial research project. We have shown that relatively high C2-C4 olefin yields approaching 50% of the C02-freeproduct can be realized. By varying the catalyst, ethylenelpropylene ratios of 0.4 to 2.5 can be achieved meeting market demand and maximizine ROI. For a comDanv forward-integrated into styrene the unique problem bf separating the iery dilute, directly produced ethylene from the low-boiling, essentially
;\
-
Directly svnthesized
8
4
-
\
l
8
I8
,
l
28
,
l
,
l
~
l
,
l
,
48 58 68 78 I: SYNGRS CONVERSION
l
88
38
,
l
98
Figure 5. Olefin selectivity versus conversion
Table 3.
Alkylation wlth Dllute Ethylenew
H2 C2H1 C~HB CIHB
mole % feed
mole % product
36.8 2.1
36.9 0.05 1.51 0.02
1.4
...
N"
1.9
1.9
ethylbenzene diethylbenzene xylene toluene
... ... ... ...
2.3 0.052 0.032 0.047
noncondensable hydrogen and carbon monoxide is solved by alkylating benzene. We feel this research contributed significantly to the practical substitution of synthetic feedstocks for natural hydrocarbons in the event of uncontrolled price escalation or nonavailahility of conventional feedstocks. Literature Cited (11 Billmeyer, F. W ' 7 e r t b m k ofPolymer Science": Wiles: New York,1972:p 245. (21 Jacobs. P. A,: Wan Wouwr,D.J. J. Mol. Cat. 1982.17.145. (3) Murchisan, C. B. In "4th lnt. C o d on the Chem. Uses of Moly.': Barry, H. F.; Mi~hcll,P.C.H.,Eds.;ClimaxMoly.Co.:AnnArbor,MI, 1932: p197. (41 Pedenen. K.;Jorgensen, I. 0 . H.; Roetrap-Nielsen, J. R., GB 2065491A. "Process & Cat. for the Prep. of a Gas Mixture Having s High Content af C2 Hydraearbans". (51 Murchison. C. B.;etal. 1984Id.Chem Cons. of PaciiieBssinSoe., Honolulu,HI.De~. 19W. paper 03G35 "Syngsa to Higher Alcohols over Sulfur Tolerant Catalysts". (61 D w , M E.;Shinglm,T.: Boshoff. L. J.;Oosthuiren,G.J. J. Cotolysis 1969,15,190. (7) Mras, W. 0. Cot. Rev. 198%,25(41,591. (8i Da"i8, H. G.: Wilson. T. P. U S Patent 2,711,259: "Hydrocarbon Synthmk Employmg An Iron Catalyat In The Prp4once of A Halogen Containing Regulator". (91 Davis, W. D.; Wilson, T. P.; Kunz, A. N. U.S. Patent 2,717.260; "Hydmearban Synthssi~". (LO1 Murchison. C. B.: Murdiek. D. A. U.S. Patent 4.L99.522; '"Pracms far Pmdueing Olefins From Carban Manoxide and Hydrogen". Murdick. D. A. US. Patent 4,380,589; "Novel Fiseher~Tmpneh ( I l l Murehiron. C. 8.; "o*o,.-.~-
.
112) Murchi~on.C. 8.; Stove, R. A.; Weim R. A. US. Patent 4,447,664 "Integrated Fircher-Tropneh and Ammatic Alkylation Process". ( l a ) stove.R.A.: M U T C ~ ~ S OC.~ B . .,~ y d r m ~ r h o ~locesring n 1~84,63(6),95. 1141 Chang, C. D.:Siluestti.A. J. J. Cat. 1977.47.249, (15) Stowe, R. A,; Murehiron, C. B. Hydrocarbon Pwcesaing 1982 (Jsnuaryl, 61,147.
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