Brown & Root, Phillips Collaborate on a More Selective Catalyst for Ethylene Processing
Looked at one way, the announcement from the two companies, who have collaborated on a number of ethylene plants over the years, represents a marginal improvement over B&R's ethylene technology, which is an also-ran in worldwide ethylene plant design. But looked at another way, the new development could be a significant factor in making B&R's technology the preferred method of producing ethylene, one of the largest and most basic petrochemical feedstocks. At the same time, it could be a nifty business for Phillips, which will be supplying the catalyst material.
What the companies have announced is a new grade of catalyst, the E-Series, that is used in the acetylene-to-ethylene reactor of B&R's and similarly designed units. The catalyst offers improved selectivity for reacting only acetylene to only ethylene (as opposed to producing ethane, or reacting ethylene to ethane), while providing better overall reactor stability. B&R looks on it as a key selling point of its "Low Capital Ethylene Reactor" (See figure 1).
A little background puts the issue into perspective. Acetylene is a critical impurity in polymer-grade ethylene; it is an unwanted byproduct of the ethane or naphtha cracking furnace that is the basic first step in ethylene production. Over the years, the spec for acetylene has declined from 10 ppm(v) to 1[Ref. 1]. Acetylene is usually present at a level of 0.2-0.4 mol% in the cracked gas leaving the pyrolysis furnace. At that level, it is usually uneconomical to recover the acetylene as a salable byproduct, so it must be removed. The conventional way to do this is the so-called back-end catalytic hydrogenation reactor, which takes the C2 fraction (from which hydrogen, CO and other light components have been removed) and reacts it with hydrogen over a palladium catalyst to form ethane, which is recycled back to the pyrolysis furnace.
B&R, through its acquisition of C.F. Braun several years ago, has been the primary supplier of what it calls "front end" processing, which takes cracked gas directly from the deethanizer (or depropanizer in other configurations)-still containing hydrogen and light components. (See figure 2). Here, the Phillips catalyst technology enters. Phillips has been marketing a C-Series ethylene, based on palladium on alumina substrate, that selectively reacts acetylene to ethylene. The C-Series depends to some extent on CO and propadiene to moderate the hydrogenation reaction, says Bob Rhoades, a Phillips marketing manager, "but today's modern pyrolysis-furnace technology has changed the composition of cracked gas entering the front end." CO production is better controlled, so that it is low or nil at steady-state, yet it will still spike to higher levels on startup or unsteady-state conditions. Varying CO content, in turn, makes it harder to control the acetylene reactor, and-most importantly-to prevent runaways.
Rhoades says that the E-Series, also a palladium-based catalyst with proprietary "modifiers," has been developed to take account of the new processing conditions. The catalyst is more stable in conditions of varying CO content, and has a cycle life before regeneration that is measured in years, rather than months for back-end catalysts. Most significantly, the catalyst is more than 70% selective for acetylene, meaning that the majority of acetylene is successfully converted to product, and little or no product is converted to ethane. "In effect, you've debottlenecked your plant, and added to capacity," says Rhoades.
Wil Tam, VP for technology at Brown & Root (and one of the originators of front-end processing), seconds this opinion, adding that the overall process provides a lower-capital-cost alternative to back-end processing, is more cost-effective to operate, and is more stable than competing front-end designs. "There are a selected number of ethylene projects going forward at this time around the world, and we feel that we can offer a tremendous advantage over conventional designs," he says.
Phillips has used the E-Series catalyst in two large ethylene plants in Sweeny, TX for three years, demonstrating the selectivity and stability. It will load the catalyst in the acetylene reactor of a third ethylene plant during an August '98 turnaround. Phillips has four ethylene plants at Sweeny, with a total capacity of 4.5 billion lb/yr. Three of the plants employ Brown & Root technology.
References
For more information:
Phillips Chemical Co., 9685 W. 105th Way, Broomfield, CO 80021. Tel: 303-464-0204; Fax: 303-464-0205.
Brown & Root Engineering and Construction, Process Technology, 1000 S. Fremont Ave., Alhambra, CA 91803. Tel: 628-300-2020; Fax: 626-300-2633.