DETALLES DE PROCESOS
Major improvements have recently been realized in several of these processes, resulting in improved products and reduced operating costs.
Each process offers unique advantages in producing certain of the large number of PVC products currently marketed.
In addition, a vapor-phase polymerization technique has recently received considerable attention and may shortlybe commercialized.
POLIMERIZACION POR SUSPENSION
Suspension-polymerization processes since the 950s have been the major method used for polymerization of vinyl chloride. Although attempts have been made to develop continuous-flow variations of suspension polymerization, only one type of commercial flow process has apparently been developed. Various modified batch processes will be described later;Fig 6-3 is the flowsheet used by one major producer.
ALMACENAMIENTO DEL MONOMERO
Vinyl chloride at ambient temperatures and pressures is a colorless gas with a taint, sweet odor. Mixed with air, it is explosive; a large plant was severely damaged by an explosion that occurred after a sight glass on a storage tank broke
High concentrations of vinyl chloride in the air are toxic to man. In storingvinyl chloride precautions are necessary to protect plant personnel, to prevent fires and explosion, and to maintain high-quality monomer.
In the past vinyl chloride to be shipped contained a small amount of phenol inhibitor to minimize polymerization. This inhibitor was removed before polymerizations by washing the vinyl chloride with caustic and then with water.
Inhibitors, however, are notnecessary in highly purified vinyl chloride and are generally not used now. The vinyl chloride is frequently stored in a wet state, as is also the recycle vinyl chloride. The temperature of the vinyl chloride in the insulated storage tanks is often maintained at 60°F or less. Vapor recompression or external cooling is preferred in these tanks to internal coils. At such temperatures, hydrolysisreactions or peroxide formation is effectively suppressed.
Formerly, both glass-lined and stainless-steel storage were used. In one plant, 36 stainless steel was preferred to 304 stainless steel(50). Trace quantities of water and HCl , led ti corrosion and iron pickup. With the use of lower temperatures for storage and for high-purity vinyl chloride, low-carbon steel is suitable (89). Care should betaken to exclude copper or brass from the system because trace amounts of acetylene may react to form copper acetylides.
One company uses several storage tanks of 15000 gal capacity, and each tank can hold up to 10000 lb of liquid vinyl chloride. At 60°F, the pressure in the tank is approximately 27 lb/in gage. The number of storage vessels depends of course on the plant size, size of vessels, andthe availability of vinyl chloride. When vinyl chloride is purchased, more storage capacity is generally needed than vinyl chloride is produced at the plant.
Shelley and sills(89) describe in considerable detail the equipment and techniques to be used in storing vinyl chloride.
Batch reactors provided with one or more agitators are used for the polymerizations step of the suspensionprocess. Figure 6-4a shows the tops of several reactors in which the agitator shaft enters through the top. Figure 6-4b is a sketch showing the main featurs of a reactor.
Temperature control of the exothermic polymerizations is a key factor in designing and operating the jacketed reactors. Water at the desired temperature is rapidly pumped through the jacked. Part of this water is recirculated,and part is discarded. Normal cooling water, refrigerated water, steam, or blends of the three provide the necessary makeup for the jacket(52). The reactors are jacket generally both on the side walls and on the bottom. Jacketing the top head is also employed on occasion. Water and specially vinyl chloride vaporized during polymerizations, condense on the cold top, and then reflux back to the...