acid. The glycol yield is 95 mol %, Brownstein says. This is 20 percentage points above the best yield from the ethylene oxide/glycol process. The substantially increased yield of the acetoxylation process looks especially attractive as the price of ethylene shoots up. The proportionate share of ethylene in the total production cost of ethylene oxide/glycol in a 500 million lb-per-year glycol unit built with 1974 dollars has increased from 49% to 65% in the past 18 months, Brownstein notes. Further increases in the price of ethylene, perhaps to 16 cents to 19 cents per lb in 1980, will increase the advantage of acetoxylation, Brownstein calculates. A year and a half ago, when ethylene cost just 3 cents per lb, the ethylene glycol sales price by conventional technology at 71% yield was 8 cents per lb for a new plant. In comparison, acetoxylation-produced ethylene glycol would have required a sales price of 8.6 cents for the same profitability (figured at 15% discounted cash flow). Now, with ethylene costing 8.5 cents per lb, conventional technology requires a 12.7-cent price for ethylene glycol. For acetoxylation technology, the corresponding price would be 11.0 cents. By 1980, if ethylene costs 17 cents per lb, Brownstein figures that acetoxylation-derived ethylene glycol would have a price about 4 cents cheaper than the 19 cents of glycol from conventional production. Capital cost is about the same for both acetoxylation and ethylene oxide/ glycol plants. This means the principal cost advantage in acetoxylation comes from the greatly reduced cost of ethylene per lb of ethylene glycol—4.55 cents compared with 8.15 cents in the ethylene oxide/glycol route. For the long-term future, even this cost advantage in acetoxylation may be eclipsed. Using basic data from Union Carbide, Brownstein figures that synthesis gas-derived ethylene glycol is essentially competitive right now with both conventional and acetoxylationbased ethylene glycol. This calculation assumes 8.5 cents per lb for the cost of ethylene, $1.25 per million Btu as the cost of synthesis gas, 400 atm pressure in the synthesis gas process (way under published developmental process pressures of 3400 atm), and a 20 mol % conversion of synthesis gas with 50 mol % selectivity for ethylene glycol among the various reaction products. These coproducts bring up another potential plus for the synthesis gas route. Brownstein remarks that if credits are assigned to even half of the byproducts of the synthesis gas route (propylene glycol, glycerol, and methanol), synthesis gas technology becomes far more attractive than acetoxylation by 1980. However, Brownstein concludes that even without by-product credits, synthesis gas technology would be quite attractive by that date. D
Caprolactam plant at Flixborough, U.K., was destroyed by flashing cyclohexane
More safety urged in process designs With its first anniversary still two months away, the great Nypro caprolactam plant disaster at Flixborough, U.K., last June 1 is having ever-widening repercussions. The political consequences, such as increased governmental concern worldwide with chemical plant siting and safety, are the most obvious. But on a more specialized level, there is a mass of considerations for plant managers and engineers. Addressing the latter audience, Trevor A. Kletz of Imperial Chemical Industries' petrochemicals division in Wilton, U.K., detailed a number of plant safety lessons from the Flixborough explosion at the American Institute of Chemical Engineers meeting in Houston earlier this month. His points range from plant design and operation to R&D practices. The top consideration on Kletz's list is the extraordinary danger of flashing liquids in plant operations. Kletz says that flashing liquids are much more dangerous than gases or liquids kept below their normal boiling point. It was just such a flashing liquid that led to the Flixborough blast. Collapse of a temporary 20-inch pipe between reactors at the plant, Kletz explains, allowed about 50 tons of raw material cyclohexane to escape. The cyclohexane had been kept in inventory at about 120 psig and 145° C, a temperature well above cyclohexane's normal boiling point. Although there is nothing special about cyclohexane that makes it particularly likely to leak or explode, its escape at Flixborough as a flashing liquid resulted in devastation. The explosion caused 28 deaths and more than $100 million in damage. Preventing flashing liquid leaks can start all the way back in research, Kletz points out. He asks whether it is
not possible to develop more gaseous phase processes, in which any leak would be in the form of less dangerous gas. At the same time, Trevor brings up the question whether processes that use tubular reactors could not be increased. In these reactors, the maximum leak is limited by the size of the tube. Plant inventory also might be more limited, Kletz suggests. He says that in the past, research workers have tended to accept whatever inventory the process required and have not made low inventory one of their objectives. Kletz does not suggest an absolute bar on high-inventory processes. Rather, he wants more consideration of ways to curb inventories. "So often we keep a lion and build a strong cage to keep it in. Our cages are usually very strong, and only rarely, as at Flixborough, does the lion break loose. I am sure that by good design and operation one can make the chance of it breaking loose acceptably low. But before we keep a lion, we should perhaps ask if a lamb will do instead." Cutting inventories also can come from changes in plant engineering, Kletz says. He zeros in on distillation columns, refrigeration systems, heat transfer systems, and reactors. The reason is that boiling or flashing liquids occur mainly in these stages. In one example of many, Kletz asks whether more use could not be made of packed columns or film trays in distillation columns. The cost per theoretical tray is lower for a packed column. Film trays are more expensive per tray, but this cost is wholly or partially offset . by reduced column height. Kletz also mentions possible design of distillation columns with tall, thin bases to reduce base-level inventory. D March 31, 1975 C&EN
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