Rissler's presentation provoked an intriguing exchange later during the session. Florence M. Wambugu, a plant pathologist with the Kenyan Agricultural Research Institute who is now working at Monsanto in an effort to develop virus-resistant sweet potatoes through genetic engineering, pointed out that in Africa, women are responsible for the majority of agricultural labor. African women working in the fields spend as much as 60% of their time pulling weeds, Wambugu added, suggesting that it was disingenuous for Rissler to state that feminists should oppose herbicide-resistant crops. "Agricultural problems in Africa are due to a lack of intensive agricultural development, a lack of inputs, and a lack of knowledge," Wambugu says. "We need to transfer agricultural technology from the U.S. and Europe to improve our food production." Wambugu's work at Monsanto on sweet potatoes, which is being funded by a grant by the U.S. Agency for International Development and Monsanto, will help improve the sturdiness of one of the major Third World food crops. It will also, Wambugu says, provide her with knowledge she can transfer to other African scientists upon her return to Kenya. USDA's Duke puts the issue of herbicide-resistant crops into some perspective. Herbicide-resistant crops will have to compete with other options a farmer has for weed control and will not eliminate a farmer's options, as activists like Rissler suggest, he says. And herbicidetolerant crops face obstacles other than the antipathy of environmental activists, Duke points out. There are questions about whether the public will accept genetically engineered food, about the rise of herbicide tolerance in weed species, and about pleiotropic effects such as reduced yield accompanying herbicide tolerance. Herbicide-resistant crops are unlikely to have much effect on the total acreage treated with herbicides, Duke says, because most agricultural land already is treated with herbicides. They are, however, likely to reduce the total volume of herbicides used. Although such crops will improve the efficiency of reduced-tillage agriculture, they are also likely to accelerate the evolution of herbicide-resistant weeds. Ironically, herbicide-resistant crops are likely to be a tool in managing such resistance, Duke concludes. •
Catalysis facilitates oil refinery changes The "oil refinery of the future" is here now. Environmental regulation has made it necessary, technical and economic conditions notwithstanding. As regulations become more restrictive and prescriptive, the oil refiners of the past, the so-called oil boilers, are converting their refineries into the largest chemical plants in the world. The Netherlands has been in the forefront of this changeover, and some insights from the Dutch perspective come from Jeop E. Naber, director of process research and Ian E. Maxwell, catalytic research and development manager for Shell Research B.V. Both are industry consultants to the Netherlands new Institute for Catalysis Research (C&EN, March 1, page 27). Catalysis is a key discipline in the modification of processes and the development of new ones to make environ-
Looking
mentally acceptable fuels. Conversion of crude oil is expected to remain the principal source of motor fuels for many years, but production of fuel additives in large quantities along with conversion of natural gas will become significant. The various oil crises arising from political problems in the Middle East during the past 20 years provided the push to convert more crude oil to motor fuels. This usually has been accomplished, at least in Dutch refineries, through catalytic cracking and flexicoking, a process for gasifying high-sulfur coke. Although bottom-of-the-barrel upgrading has helped to minimize the effects of higher crude prices, it has also given birth to a new petrochemical processing industry. In particular, the upgrading has meant increased attention to top-of-the-barrel technology. The lighter, C, to C4 components have become important feedstocks. Using these feedstocks, though, means increasing their molecular weights through such processes as oligomerization and paraffin alkylation.
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MARCH 8, 1993 C&EN
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In addition to adjusting the molecular weights of the various refinery fractions, operators have had to limit certain types of compounds in fuels because of envi ronmental considerations. Among these compounds are aromatic fractions, the more volatile components, and photoreactive constituents. The virtual stipulation of oxygenated compounds as octane-acceptable addi tives with low volatility has produced an entirely new petrochemical industry al lied to refining. By adding up to 3% ox ygenates to fuels, tailpipe emissions may be lowered. Methyl ferf-butyl ether (MTBE), ethyl terf-butyl ether (ETBE), and tert-amyl methyl ether (TAME) are the chief con tenders in the oxygenate derby. They are intrinsically high-octane components and contain no environmentally objec tionable heteroatoms. Although ETBE and TAME have the advantage of lower volatility, they cost more to make than does MTBE, and research is under way to find better processes for making these two oxygenates. One possible direction for development work is improving ionexchange catalysts. Better resins for this purpose would increase the yields of TAME and ETBE. In the near term, growth of MTBE de mand is expected to cause a shortage of isobutylene, which is now mainly a by product of steam- and catalytic-cracking. This, in turn, prompts investigation of new routes to isobutylene. One possibility, mentioned by the Shell researchers, is adding ZSM-5 zeo lites to the fluid catalytic cracker. This addition would increase production of C3 and C4 hydrocarbons, including iso butylene, from the cracker, but would be done at the expense of gasoline pro duction. Nonetheless, the overall eco nomic advantages remain. Isoolefins can also be made by catalytic isomerization and dehydrogenation of paraf fins. Several commercial processes for this purpose are now in use. Paraffin isomerization can yield isoparaffins that are desirable for their higher octane numbers. However, the isomerization is equilibrium limited and, for thermodynamic reasons, is best done at low temperatures. But it is difficult to carry out catalytic isomerization at low temperatures. Some catalysts have been developed that function at temperatures as low as 150 °C, but these are easily poi soned. Naber and Maxwell say that zeolites
Top-of-the-barrel technology is key to modern oil refinery Shell has developed can function at up to 250 °C, are relatively insensitive to impurities, and do not suffer much from attrition in the reactor. Such cata lysts have also been developed by Union Carbide and are now produced by Allied-Signal's UOP. One of the lines of research that Shell would like to pursue is to extend paraffin isomerization beyond the C5 and C6 com pounds to increase the amount of total isomerate produced in the refinery. Isobutylene is alkylated with C3 and C4 hydrocarbons to produce C7 and C8 paraffins. These products have high octane numbers but are hard to make by this method because of the specific acid catalysts that are required. So far, the only acids that can be used profit ably are sulfuric and hydrofluoric acids. But these acids are environmen tally unacceptable and are highly cor rosive to equipment. Hence, develop ment of solid acids for alkylation is un der way at Shell, as elsewhere, with the expectation that sulfuric and hydroflu oric acid catalysts can be eliminated in the near future. Although crude oil conversion is ex pected to remain the principal source of fuels and petrochemicals in the future, natural gas reserves are emerging as a major hydrocarbon resource. This trend, say Naber and Maxwell, is likely to re sult in a shift toward use of natural gas (methane) as a significant feedstock for chemicals and, possibly, for fuels as well. Deployment of technology for direct and indirect conversion of methane would probably displace much of the current production of liquefied natural gas. At present, natural gas from remote resources is the most viable, in terms of cheap feedstocks. However, domestic development in developing countries, as well as sale of high-value products on international markets, would seem to argue in favor of natural gas conver sion rather than liquefaction. Some of the natural gas conversion processes already in use include indi rect production of sy: .thesis gas (a mix ture of hydrogen and carbon monox ide), which is subsequently converted to methanol and then to gasoline by the Mobil MTG process. Alternatively, syn thesis gas may be converted to a vari ety of olefins, alcohols, and paraffinic waxes via Fischer-Tropsch chemistry. Shell has developed a process that produces mainly diesel and gas oil from synthesis gas derived from natural gas.
Crude oil fractions
Products Uas
Î treatment
Gas
Π^ς
Alkyiation
Light ends
Isomerization Reforming J
Naphtha
Kerosine
Middle distillates
1
Hydrotreatment
I Fluid catalytic I cracking
Flashed | distillates
Gasoline
Gas oil
Hydrocracking Residue
Residue upgrading
| •
•
I
huei on Source: Shell Research B.V.
A major plant that will use the process is now under construction in Malaysia. The process overcomes some kinetics limitations by conducting the chemistry in two stages. In the first stage, conven
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tional Fischer-Tropsch paraffin waxes are produced. In the second stage, the waxes are hydrocracked to a middledistillate boiling ranee. Joseph Ha^in
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