Document gE92E195QKVwQR187wG5y3G3

NEWS FEATURES PVC makers o'i N f \ tfMnWrtIcks move cleaning devices from one reactor to the next at GGee.orgia-Pacific Plaquemme, La., PVC plant. move to mop up monomer emissions Production improvements lead the drive by U.S. polyvinyl chloride producers to meet strict federal limits on suspected carcinogen vinyl chloride. Photo: Gorgta<PaciAc Corp, Q U.S. manufacturers of polyvinyl chloride are girding themselves for stringent strictures on vinyl chloride monomer (VC) levels. For instance, when a U.S. Occu pational Safety and Health Admin istration (OSHA) standard for VC comes into full force on April 1, the ceiling for worker exposure to the monomer will harden, eliminating today's option that allows plants to run at up to 25 ppm without using respirators. A link--which became a cause c4lebre in early 1974--between VC and a rare liver cancer called angio sarcoma is spurring the dampdown from the pre-April 1974 limit of 500 ppm. And U.S. plants are not alone in facing stiff VC curbs: exposure regulations also are tightening in Western Europe and Japan. Sweden, like the U.S., has adopted a 1-ppm limit. Most other nations are head ing for monoma ceilings in the 5-25-ppm range, though. SOME SUCCESS-Through new or improved manufacturing methods, many operators of larger and newer PVC plants already claim to be un der 1 ppm--at least for most normal operations, if not during upsets or maintenance or in every single part of the plant. They've been buttoning sourdfc. Already, Uniroyal Chemical, up their facilities to hold in fugitive a division of Uniroyal Inc. (New emissions (such as leakage through York) has revealed plans to close its flanges), changing both polymer decades-old Painesville, Ohio, PVC ization formulations (recipes) and plant (Ckem, Eng., Oct. 13, p. 67) be vessel-cleaning procedures to cut cause of the "exceedingly large capi down the frequency of opening up tal expenditures" needed to satisfy reactors, and adopting improved OSHA and EPA standards. At least techniques for stripping residual VC some industry spokesmen expect one from product PVC resin. or two other older and smaller PVC However, a few smaller and older plants to shut down, too. PVC plants apparently are limping CAPTURING FUGITIVE VC-Before along without making much head the VC health hazard was recog way toward OSHA's limit. New en nized, fugitive emissions were the gineering controls for VC either largest source of monomer losses, ac can't mesh very well into these in cording to an EPA analysis. But stallations' equipment setups, or companies are clamping down. can't be economically justified. "One of the first things we did was Adding to their dilemma, the U.S. to tighten all the flanges," says a Environmental Protection Agency Continental Oil Co. (Stamford, (EPA), at presstime, was on the Conn.) spokesman. And throughout verge of proposing standards to the industry, flanged pipe couplings regulate VC emissions to neighbor are ~ replacing leak-prone threaded hoods around plants. Many of the, couplings, and welded joints some- steps needed to satisfy -the -OSHA -tunei are eliminating both. ceiling undoubtedly will go hand in ' "We are continuing to look for hand with efforts required to meet better gasket materials and pump EPA's standards. Hovieverl^Scom- i iieal^iadds'John Nelson,-vice presi pliance with the latter may demand dent of manufacturing for B. F. sizable additional outlays. Goodrich Chemical Co. (Cleveland, "If an operator was staggered by Ohio), the largest U.S. PVC pro the OSHA rules, the coupe-de-grace ducer. EPA, for its part, plans to in will be EPA's," predicts one industry sist on double-sealed or canned 25 _____________ CHEMICAL ENGINEERING NOVEMBER 24, 1975 ucc 006728 NEWS FEATURES V \ pumps (or an equivalent): the agency also will call for rupture disks under relief valves, to avoid leaks if a valve reseats improperly. The trend to larger-size polymer ization reactors, aimed primarily at boosting productivitv, also helps stem fugitive emissions, because one large vessel has only half the possible leakage points of two smaller units of the same total capacity. Industry had relied on less-than-7,500-gal re actors for 85% of its output as late as 1972, but now much bigger vessels are favored. For instance, GeorgiaPacific Corp. (Portland, Ore.) uses 20,000-gal vessels at its new Plaquemine, La., plant, and Shintech, Inc. (Freeport, Tex.) features 35,000-gal units at its Freeport installation. Massive, 55,000-gal reactors already are employed by Germany's Chemische YVerke Hiils AG (Marl) and BASF AG (Ludwigshafen). COPING WITH CLEANING-The opening of reactors for manual cleaning can also allow monomer to escape. Pre-venting vessels and flush ing them with an inert gas prior to opening is limiting VC losses, espe cially at the handful of smaller and older plants where workers wearing respirators still do the necessary chipping and scraping by hand. However, companies are striving to reduce the frequency of the manual cleaning. "Balancing the reactant recipe" can thwart the formation of deposits in the first place, says A. B. Steele, operations manager for Union Car bide Corp.'s Performance Plastics Dept. He adds that firms are regu larly improving their understanding of fouling and how to cope with it through changes in recipe and oper ating conditions. But the recipe work remains highly proprietary. Conti nental Oil, for example, declines fur ther comment on a "clean-wall for mulation'' that it says it developed last year--but it has told EPA that the frequency of reactor openings might be cut to once every 200 batches. However, in Italy, ANIC SpA (Milan) does identify its pro prietary additive as an "inorganic colloid,'' which : minimizes deposits and makes those that do form softer and easier to remove. Special reactor-wall surfaces can also help. For instance, Shintech uses a chemical coating, developed by its half-owner, Shin-Etsu Chemical In dustry Co. (Tokvo). Before each pohmerization batch, walls are "thinly coated with a chemical, so thin you can see the basic structural material, stainless steel," reveals Teruhiko Aihara, assistant general manager of Shin-Etsu's PVC div. and a member of the Japanese firm's board. "Scale just does not stay on the walls." A simple, low-pressure water spray after each batch is enough to wash away any "specks" on the reactor. The technique al ready has proven itself in five years' commercial use by Shin-Etsu. RELYING ON WATER-PVC makers in the U.S. (and even more so in Western Europe and Japan) use water sprays themselves as the main means to keep reactors clean. A number of proven techniques are available, including the HRC system from Goodrich Chemical, which fea tures a 6,000-8,000-psi spray, and a rival method from Pfaudler Co. (Rochester, N.Y.), which works at much lower pressure (several hun dred psi) but consumes more water. Overall, water cleaning can halve or even further reduce the frequency of opening reactors for manual cleaning, depending on vessel design and the PVC-resin grades being made, notes Harry Connors, vice president and general manager, Plastics Div., Diamond Shamrock Corp. (Cleveland, Ohio). But on a 0-100 scale, he adds, it's "closer to being 50% effective than it is to being 100% effective" in reducing cleaning frequency. A patent-pending system invented and manufactured by Georgia-Pa cific promises to raise the score. Computer positioning of a waterspray nozzle is the key. The specially designed nozzle is programmed to come within an inch or so of reactor walls, heads and internals, a spokes man says, in contrast to conven tional systems in which nozzles oscil late within the vessel sometimes a few feet away from surfaces. The firm's Plaquemine plant fea tures two such units, handling four and five reactors, respectively. Two tall denicks,^hpwnjn the; photo on p. 25, move the system's cleaning heads into and out of the reactors, and from one vessel to the next. A 200-gal/min flow of 5,000-6,000-psi water ensures cleaning. SOLVENTS GAINING?-Nonaqueous cleaning fluids had been proposed even a decade ago as a way of doing a more thorough job. Ac tual use remains limited, though, be cause of obstacles such as high capi tal cost, complexity of solventrecovery equipment, solvent traces left in the reactor, and solvent safety and environmental hazards. How ever, with better reactor cleaning now a priority, producers of PVC seem more anxious to tackle the problems. A number of solvents rate as possi bilities: ethylene dichloride, featured in a system from Stauffer Chemical Go. (Westport, Conn.); methyl-ethyl ketone, used by Union Carbide at its Texas City, Tex., PVC plant; M-Pyrol, GAF Corp.'s (New York) nmethyl-2-pyrrolidone solvent; tetrahydrofuran, favored by Monsanto; and dimethyl formaldehyde. The Union Carbide solvent-wash ing system has been running for about ten years. But Carbide's Steele points out that the firm has the sol vent available at Texas City for a myriad of other tasks there, so exten sive special facilities did not have to be built for the PVC-reactor clean ing chore. Carbide washes vessels with the solvent after every 5-15 batches. GAF, another pioneer in solvent cleaning, is working to bring to mar ket a complete process/equipment package using M-Pyrol. An al ready piloted solvent-recovery step rates as the key to GAF's new push. In it, water is added to spent solvent to precipitate polymer, which is then removed by centrifugation; the sol vent/water mixture is distilled to re coup the M-Pyrol. PURIFYING POLYMER-The PVC product in the reactor also needs cleaning to remove VC. Stripping by post-polymerization venting of head gases from the reactor eliminates some monomer, which then passes through a refrigerated-condenser system and is recycled. However, sig nificant quantities of VC remain, and they give emissions problems in subsequent resin processing and han dling operations. , ^ ... To avoid specifying diverse point controls on all the various post-strip ping operations, EPA plans to set a limit on VC content in resin leaving the stripping step: 2,000 ppm for dis- 26 CHEMICAL ENGINEERING NOVEMBER 24. 1975 ucc 006729