Document 2jm0w6Q55Vykvp3zVymmN6dzN

Monomer X>=K ywerVn)PY _po n c ______ yc Protection Styrene and vinyl chloride are two major petrochemicals; both require care in handling to pre vent explosions and loss of quality. P. G. Shelley, and E. J. Sills, The Dow Chemical Co., Texas Div., Freeport, Tex. This article will discuss two monomers which are manufactured, stored and handled in large volume by the Dow Chemical Co. and others. These are styrene and vinyl chloride. Styrene production in the United States in 1968 was about 3.5 billion lbs. and is expected to rise to 4.3 billion lbs. by 1972. Individual locations producing from 500 million to one billion lbs. of styrene monomer annually are not uncommon in the industry. Distribution from some of these plants is worldwide. Almost without ex ception, locations of major use are far removed from the producing points. This adds up to a lot of storage and handling. For example, Dow stores styrene at seven major locations in the United States and in per haps ten foreign countries. Vinyl chloride production in the United States in 1968 was about 2.8 billion lbs. and worldwide in excess of 10.0 billion lbs. Production is expected to rise in the United States to 4.5 billion lbsVyr. and worldwide to 13.0 billion by 1970. As with styrene, the location of VCM users is in most cases remote from the producing site, which makes large bulk storage and handling systems a nec essary adjunct of the VCM production business. In many instances, some storage facilities at the user end are required as well. Unlike styrene, vinyl chloride is a gas at N.T.P., and therefore requires pressure storage under its own vapor pressure, or as an alternate, refrigerated vapor recompression for low pressure storage systems. Despite these storage requirements, the growing trend is to distribute vinyl chloride on a worldwide basis. Styrene characteristics Styrene is a colorless non-corrosive, aromatic hydro carbon liquid. Table 1 gives its more important prop erties as related to storage and handling. In devel oping techniques and equipment for styrene storage and handling, three major objectives, and a few minor ones which will be developed, must be considered. These are personnel protection, protection of the quality and monomeric status of the material under normal cir cumstances and protection against fire. Personnel protection is accomplished primarily by training and utilizing common sense. Styrene is low in single dose oral toxicity, but accidental ingestion cases should be referred to a physician immediately. Vomit<ng should not be induced because of possible aspira tion of the monomer into the lungs. Styrene in the eyes can be very painful, but the like lihood of permanent damage ia fairly remote. Eye pro tection should be worn where the possibility of eye contact exists, as in sampling, transferring and main tenance activities. Washing contaminated eyes im mediately with water for IS minutes is the recom mended treatment. Again, medical attention should be obtained as soon as possible. Casual skin contact normally causes little if any irritation. On the other hand, prolonged contact, as would result from continually wearing clothing soaked with styrene, or frequently repeated contact is capable of causing blistering and possible skin swelling. Wash ing with soap and water is the remedy. No case of styrene absorption through the skin with resultant systemic effects is on record. Table 1. Selected properties of styrene monomer. Chemical Formula Density, Ibs./gal. @ 77*F. Vapor Pressure, mm Hg <g 77*F. @ 104* F. Boiling Point *F. @ 760 mm Flash Point Tag Closed Cup, *F. Fire Point, Tag Open Cup, *F. Explosive Limits in Air at Room Temperature % Viscosity, Centipoise @ 77F. C*H,CH:CH, 7.5 6.5 16.0 293 94 99 1.1 to 6.1 .71 Table 2. Specification for styrene monomer. Property Sales Spec. Typical Anal. Purity (by freezing point) % Minimum.......................... Color APHA, Maximum ................. Saybolt, Minimum .............. Aldehydes (as benzaldehyde), % Maximum by wt............... Peroxide (as HiOj) % Maximum by wt............... Sulfur (as S), % Maximum by wt. ......... Chlorides (as Ci) % Maximum by wt............... Polymer Content Maximum~ppm................... 99.5 10 27 0.020 0.010 0.0025 0.01 10 99.73 <5 ,004 .0007 .0001 .0001 0 Chemical engineering progress (Voi. as. no. 4) ucc 032200 April 1969 29 Vapor concentrations below about 100 ppm in air are neither likely to be harmful nor particularly objec tionable to most personnel. Concentrations above this level become increasingly more irritating to the nose and eyes. In cases of excessive exposures, an individual could be overcome, as by any other volatile hydrocarbon. Fresh air and rest should bring him back with little likelihood of permanent effects. As protection against possible leaks and spills, any styrene handling installation should be designed to minimize human exposure and provide adequate venti lation and drainage in critical operating sites. Person nel should be well instructed as to the nature and haz ards of the materia] and provided with safety goggles and ready access to a safety shower and eye bath. Styrene is marketed under the specification shown Table 3. Shelf life of styrene monomer. Effect of Inhibitor and Oxygen at Various Temperatures 12 tol5 ppm TBC 50 to 75 ppm TBC Saturated with Less Than Temp. Oxygen 3 ppm Oxygen Saturated with Oxygen 60*F. 5 to mo. 10 to 15 days More than 1 year 85* F. 1 to 2 mo. 4 to 5 days 3 to 4 months 110'F. 8 to 12 days Less than 24 hrs. Less than 30 days Table 4. Selected properties of vinyl chloride monomer. Chemical Formula Boiling Point (760 trims.). *F. Melting Point, 'F. Vapor Pressure at 77*F. Flash Point (Cleveland Open Cup), *F.' Explosive Limits Vol. % in Air Liquid Density grams/ml. 77*F. --4*F. Liquid Viscosity cps @ 77 *F, CHjXHCl 7.2 -244.4 ~ 56 psia -108.4 3.6*26.4 0.9013 0.9834 0.185 in Table 2. For comparison, a typical analysis as man ufactured is included. Storage problems Polymer content and color are the most difficult qual ities to maintain in storage, especially polymer con tent. Styrene has no significant value except as a poly mer or copolymer with other monomers. It is in the latter field of use, especially when used as a co-mono mer in the preparation of polyester resins, that small quantities of the homopolymer adversely affect finished product quality. In addition to end use considerations, it is possible for the polymerization reaction, which is exothermic, to become self accelerating. This can re sult in an extremely^ dangerous situation, involving high temperature and pressure development in a con tainer, as well as loss of the product. In short, styrene monomer is a heat sensitive perishable commodity and must be recognized and treated as such. Polymerization in storage and shipping containers is controlled by adding an inhibitor. Although various compounds exhibit tome degree of effectiveness, paratertiarybutyl catechol (TBC) is the one almost uni versally used. Minimum effective concentration for prolonged storage is approximately 10 ppm. TBC also acts as an antitioxidant and, strangely enough, is effec tive as a polymerization inhibitor only in the presence of oxygen. Table 3 gives a rough indication of the TBC-oxygen system's effectiveness and clearly shows the effect of temperature on the system. Shelf life means the time the monomer can be ex pected to remain within specification. The table is based on a combination of laboratory tests backed by many years of practical experience. Several conclu sions pertinent to quality protection in storage can be drawn from it: 1. The lower the temperature the better. 2. An ade quate concentration of TBC must be maintained. 3. An adequate concentration of oxygen must be maintained. Although not absolutely necessary, Dow considers it good practice to insulate and refrigerate styrene stor age in areas where average ambient temperature ex ceeds 80F for a substantial period. Refrigeration is external and the monomer is circulated as required to maintain the body of liquid in the tank around 7075* F. Such an installation has the added advantage of tending to equalize day to night temperatures in the vapor space and thereby minimize monomer condensa tion on the tank's roof and sidewalls above the liquid level. TBC is gradually depleted from styrene in the course of doing its job as an inhibitor. Figure 1 shows this effect quantitatively. Concentration should never to allowed to fall below 10 ppm. System effectiveness can be maintained simply by adding TBC as required. Here again, means of cir culating the tank contents becomes important. TBC is added as a concentrated solution, about 15-20%, in monomer to achieve any desired increase in overall concentration. TBC content should be checked every few days in dead storage. Oxygen content' is adequately maintained when sty rene is stored under air. The equilibrium concentration of oxygen in the liquid is around 50 ppm at room tem perature. Under air is always the simplest and gen erally most satisfactory way to store styrene where the tank is designed to minimize polymer accumulation on the roof; turnover is frequent enough to eliminate any problems arising from buildup of aldehydes and 30 April 196? UCC 03221 CHEMICAL ENGINEERING PROGRESS [Vol. 65. No. ) Figure 2. Icicle growth in an air at mosphere on the unlined roof and supporting structure of a 70 ft. by 24 ft. high tank after 125 days service. Figure 3. Horizontal steel tank measuring 10 ft. by 36 ft. in proc ess of being cleaned after four years in styrene service. Figure 4. Top of epoxy coated 10 ft. by 36 ft. tank after six months in styrene service. Inspection after 3% years showed no change. other oxygenated compounds. Shelf life as shown in Table 3 applies. Air storage problems Storing under air can, however, create problems as well as solve them. Since TBC is a high boiling com pound, monomer vapors above the liquid level in the tank are uninhibited. Normal temperature swings re sult in a continuous cycle of vaporization and conden sation on any structure in the vapor space, including the roof and sidewalls of the tank. Droplets will adhere to any rough or porous surface. In the presence of air, these droplets polymerize readily into a highly dis colored and oxidized product. Polymer "icicles'' develop and continuing refluxing action dissolves the material and carries it back into the bulk of the liquid. If allowed to proceed indefinitely, these icicles will grow to tre mendous size and eventually fall into the tank, gen erally throwing the entire contents off specification. Figures 2 and 3 illustrate the condition. The situation described occurs to a harmful de gree only in uncoated steel tanks having internal roof supporting structures as shown. Rusty steel and com plicated structure seems to be the preferred environ ment for polymer buildup. A non-porous smooth sur face provides no sites for retaining the uninhibited monomer and, even in the presence of Bir, little poly mer buildup can be expected. All modem styrene stor age is designed to provide such a surface within the bounds of reason. Self supporting dome roofs are em ployed. API 650 tanks having a dome radius equal to 0.8 the dia. of the tank are satisfactory, and can be constructed in sizes up to at least 70 ft. in dia. The roof and sidewalls down to within about 2 ft. of the bottom are coated with one of several suitable coatings. For large tanks, a coating which will cure at atmospheric temperature is the most practical. Several epoxy types fall within this category. The bottom and lower 2 ft. of wall is coated with a rust resisting inorganic zinc sili cate material such as the Dimetcote protective coat ings. This permits static charges in the liquid to drain off through the tank ground. Figure 4 demonstrates the coating effectiveness. In situations where it is necessary to continue using uncoated tanks for styrene storage, particularly those containing internal structures, the best solution to polymer formation in the vapor space is to pad the tank with an inert gas such as nitrogen or natural gas. The latter is usually cheaper and just as satisfactory. In the absence of oxygen, polymer formation is retarded to the extent that a tank may remain in service from five to ten years without cleaning. However, under this con dition, the oxygen in the liquid is rapidly depleted and polymerization will proceed, as indicated in Table 3. A serious quality problem is created unless the tank is subject to rapid turnover as, for example, a day tank. The solution is straightforward, but requires careful control if a flammable pad gas is used. Enough air is pumped into the liquid periodically or continuously to maintain a minimum of 10 ppm oxygen in the liquid phase. If styrene is to be stored within or near a chemical complex wherein halogens, particularly bromine, are used or produced, a tank breathing to the atmosphere creates an untenable situation with respect to air pol lution. Styrene concentrations in air as low as a few parts/billion will form an extremely potent lachrymator when contacted by bromine in sunlight. Only slightly higher concentrations will do the same thing with chlorine. Where this situation exists as, for ex ample, in Dow's Texas Div., all styrene tanks within range of a probable halogen containing atmosphere are vented through an independently fueled flare. Such precaution is necessary regardless of the storage at mosphere. Color development Color development in storage can be one of the most exasperating quality problems with styrene. The reason is usually not immediately apparent and is sometimes never determined. One cause, discussed previously, is the extraction of color bodies from oxi dized polymer attached to structures within the tank. Another is a compound which sometimes is formed by the reaction of TBC, moisture and iron oxide (rust). Still another is prolonged contact with copper-bearing alloys such as brass and bronze. Copper reacts with styrene to impart a characteristic blue-green color to the monomer, and should not be used in styrene han dling equipment. (All other common metals such as aluminum, stainless steel, galvanized steel, etc. are suitable.) Assuming the system is adequately designed to main tain overall quality as previously discussed, the best de fense against color problems is to avoid contamination by strict adherence to high cleanliness standards. For example, styrene which has been laying in a line for a week or so, especially an exposed line, should never be flushed into a storage tank without prior inspection. If a batch of styrene turns up off-color it can usually be restored by contact with activated alumina in a fixed-bed filter. TBC will also be removed and must be replaced immediately. Particulate matter can be extremely detrimental in some styrene end-uses, and is another reason for clean liness. It is standard practice to filter the monomer between bulk storage and shipping containers. Many users filter again immediately prior to use. Cartridge type filters, capable of removing 10 micron particles are commonly used. Figure 5 illustrates loading pumps and filters in a Dow plant. Note especially the canopy for shading. ucc * 032202 i CHEMICAL ENGINEERING PROGRESS (Yol. 45. No. 4) April 1969 31 Figure 5. Styrene loading pumps and filters. Although styrene is classified as a reactive flam mable liquid, it is a relatively low hazard material with respect to fire protection. Figure 6 is a plot showing the range of temperature and pressure at which an explosive mixture with air will occur in a confined vessel. Ignition sources Sources of ignition should be eliminated from sty rene storage areas, no smoking rules adhered to, and totally enclosed electrical equipment used. Explosion proof equipment is recommended only in confined areas. Static electricity is probably one of the more important ignition hazards. All tanks, pumps, etc. should be adequately grounded and submerged filling practiced. Styrene tanks should, in general, conform to the Flammable Liquids Code NFPA-30. Tanks of over 10,000 bbls. capacity should be individually diked, the dike having 100% of tank capacity. Dike drains are kept normally closed but with valve accessible under emergency conditions. Pumps should be located outside the dike. Distance between tanks should be a minimum of one half the sum of their diameters. From the standpoint of polymer buildup, the number of roof nozzles on a tank should be the minimum. A flame arrestor and vacuum-pressure relief are usually mounted on the same nozzle. Emergency relief is com bined with a manhole (''A A gauge hatch is the only other necessary opening, but a float operated tape level indicator is frequently provided. Flow line and drain valves adjacent to the tank must be steel (preferred) or ductile iron as per API 604. Fixed foam protection is the recommended provision for extinguishing styrene tank fires. The foam cham ber is separated from the tank by a frangible such as aluminum foil or glass to prevent polymer accumula tion. Inspection is at least annually. Provision should be made to supply liquid foam solution at a rate of 0.1 gal./min./sq. ft. of surface. In September 1967, Dow, in conjunction with Na tional Foam Systems Inc., conducted a test to deter mine the feasibility of subsurface foam injection for extinguishing fires in tanks containing styrene. Essen tially, the method consists of pumping a foam solution into the tank through a product line. As the foam en ters the tank, it rises to the liquid surface, providing a fire smothering blanket. It has generally been ac knowledged that such a system, if effective, would offer cost, maintenance and reliability advantages over the commonly used foam chamber systems. These require a separate, external piping system arranged to dis charge foam on the liquid surface inside the tank. Ad mitting the foam through a swing line, placing it near the liquid surface and also near the center of the tank provides a particularly desirable situation. The test tank was approximately 11 ft. in dia. and contained 6 ft. 3 in. (about 3,400 gal.) of styrene. The foam used was National Foam Systems, Inc. Aero-OFoam XL3 liquid, a foam containing fluorinated com pounds. The foam was applied in 4% concentration and admitted to the center of the tank bottom, having passed through an aeration device or "foam maker." Foam was started 6 min. and 15 sec. after light-off at a rate of approximately 0.1 gal./min./sq. ft. of liquid surface. Results of this test are tabulated below; TIME DESCRIPTION . O'O' ........... Light off 5'15*........... Foam injection started (valve opened) 5'32".......... Sound of foam on hot tank walls 6'0' .......... Fire very k>w-mostly around edges 6'20' ..........Fire around edge only 7'15* .......... Fire controlled-small flickers only 7'45"...........Fire nearly out-two small flickers 8'25*............Eire extinguished 8'56"............Foam stopped The test was considered successful on all counts. Other than the extinguishment of the fire, original areas of concern included the possibility of undesirable reac tions or degradation of the monomeer from the foam and the extent of entrainment of monomer in the foam. Neither of these proved to be of consequence. The sty rene was very simply reprocessed to a salable product and entrainment proved to be substantially less than that experienced with gasoline and hexane. Figure 7 illustrates a schematic of a typical storage tank used by The Dow Chemical Co. Vinyl chloride Vinyl chloride monomer (VCM) at normal ambient temperature and pressure is a colorless gas with a faint sweet odor. VCM is stored and shipped as a liquid un der pressure and the greatest hazards in the handling of this material are the dangers of fire and explosion. Table IV gives the more important properties of VCM as related to storage and handling. The same three major objectives which apply for styrene mono mer also apply for developing technique and equipment for safe handling and storage of vinyl chloride mono mer; plus some techniques peculiar to VCM due to its high vapor pressure and extreme flammability. The objectives are; 1. Personnel protection. 2. Protection of quality and monomeric status. 3. Protection against fire and explosion. Adequate training in the potential hazards of the material and the proper personnel protection equip ment and techniques for it are essential to the safety of all personnel handling VCM. The vapors should be controlled to no more than 500 ppm for repeated 7-8 hr. daily exposures. Concentration above this figure can cause dizziness, disorientation and "drunkenness." Concentrations ap proaching the lower explosive limit (3.6 vol. % in air) can cause helplessness and unconsciousness from a very short exposure (anesthesia). Thus, operating areas must be designed with ade quate drainage and ventilation and equipment should be selected to minimize the possibility of leaks and spills. Operating'personnel should be equipped with and trained in the proper use of respiratory equip ment Organic vapor canisters have been shown to be un- 32 April 1969 UCC 032203 CHEMICAl ENGINEERING PROGRESS (Vol. sJ, No- ^ dependable and ineffective against vinyl chloride va pors. Self-contained breathing apparatus supplied with air or oxygen with full face piece and approved by the U. S. Bureau of Mines for this purpose should be used for protection against vinyl chloride. Liquid VCM spilled on the skin can cause frostbite due to rapid evaporation. Moderate chemical burns can occur to the skin due to liquid exposure, particularly with inhibited VCM, since the most commonly used in hibitor is phenol. Skin areas splashed with VChf should be immedi ately washed with soap and water; eye exposures must be immediately washed with water for 15 min. mini mum, and medical attention obtained immediately. Contaminated clothing, gloves, boots, etc. should be removed immediately to avoid prolonged skin contact. Trained operating personnel must be fully aware of the flammable and explosive characteristics of VCM to preclude the possibility of human error causing a ma jor spill, or providing an ignition source for minor leaks or spills. Safety considerations Summing up VCM storage systems should be designed to minimize leaks and spills; have adequate ventilation and drainage to conduct any leaks or spills away from critical operating areas; and the storage and process areas treated strictly as hazardous, both from an engineering and operating procedural stand point. All personnel involved should be: well trained in the nature and hazards of the monomer and in the proper handling procedures; equipped with suitable eye pro tection such as safety glasses with side shields ot their equivalent; have adequate respiratory equipment for emergency use; and provided with ready access to an eye wash and safety shower. Vinyl chloride is marketed under the specification shown in Table 5. For comparison, a typical analysis as manufactured is included. Until 1956, essentially all VCM was inhibited as pro duced with 50-100 ppm of phenol to preclude the pos sibility of spontaneous polymerization during handling and storage. During the late 1950's it became apparent to most producers that unihibited VCM could be stored and handled safely under the proper conditions. Since deleting the inhibitor makes the inhibitor re moval system at the users plant site unneessary, a considerable capital and operating expense incentive exists to do so. Thus, a major proportion of VCM is now manufactured, stored and shipped in the uninhib ited state, although some users still specify phenol inhibted monomer. Since large volumes of VCM are now stored and han dled in the uninhibited state, the proper design, main tenance and operating procedures for storage systems to protect the quality and monomer status of the stored VCM are absolutely essential to safety and loss pre vention. Coincidentally these same steps also optimize the conversion to and quality of the end use product. Like styrene, the only significant end use for VCM is as a poJymer (PVC) or co-polymer with other mono mers. The impurities or monomer characteristics which affect the polymer Quality adversely are mainly color, oxygen, honopolymer in the monomer iron content, and unsaturated hydrocarbon impurities which produce CHEMICAL ENGINEERING PROGRESS (Vol. 4S, No 4) Figure . Conditions for explosive mixture with sty rene under air. variable polymerization rates and polymer quality. High water content is bIso undesirable. These characteristics can all be controlled within specification limits by the proper engineering and operating control of the final stages of the VCM finish ing train and storage facilities. Construction materials of the system can all be car bon steel, indeed all equipment, instruments, guages etc. must be scrutinized to exclude the use of copper and copper bearing alloys due to the possibility of trace acetylene reacting with copper to form copper acetylides. Aluminum and aluminum alloys must also be ex cluded due to its reactivity with VCM. All valves, direct connecting instrument cases, pumps, casings, etc. must be carbon steel or equivalent, and not cast iron or ductile iron, to preclude the possi bility of major spills from equipment fracture or breakage. Stainless steels are acceptable but unneessarily cost ly for this service due to the size of most systems and in view of the suitability of carbon steels. It is men tioned only because some minor specialty items such as instruments may be more available in stainless steel. Storage tank pressure For ambient condition storage systems, design pres sure of the storage tanks should be 100 psig, with 150 psig flanges, fittings, etc. used throughout the system. Horizontal cylindrical tanks are generally used for relatively small installations, 50,000 gal. or less. For larger installations, up to multimillion lbs., spheres are preferred. In the case of vapor recompression and refrigerated systems, lower design pressures can be used for the storage tanks, but pumping and piping systems should still adhere to the 150 lb. design. For safety reasons as well as monomer quality pres ervation, the oxygen content of stored vinyl chloride vapor phase must be maintained below 1000 ppm by volume. Thus storage tanks must be cleaned, dried and inerted to remove oxygen prior to introducing VCM into the system. Experience has indicated that the best way to remove atmospheric oxygen from a system is ucc 032204 April 1969 33 NOTE1 ROOF AND SIDE WALLS COATED WITH AIR CURED EPOXY. PRODUCT OUT VACUUM PRESSURE RELIEF W/FLAME ARRESTOR GAUGE HATCH THERMO- ''*1 WELL--*,4 MANHOLE AND EMERGENCY ELIEF -TAPE-LEVEL INDICATOR Figure 7. Rec ommended de sign for styrene storage tanks. TO SLUDGE SEWER In TANK DIAMETER 1/2 TANK DIAMETER to pad and depad the system, (including tanks, lines, pumps, etc. as a unit) with.nitrogen until the oxygen content reaches the desired level. For example, padding a system originally containing one atmosphere of air to 30 psig with nitrogen, depadding to atmosphere and repeating until the pad-depad procedure has been done five times will reduce the oxygen content of the system to 0.082 volume %. Experience has indicated strongly that vinyl chlor ide which is produced on the alkaline side, 0.2-0.5 ppm alkalinity as NaOH, has a more stable storage life. It is recommended that VCM which is to be stored or handled for any period more than a few hrs. be proc essed on the alkaline side. This can be achieved by pumping to storage via flake NaOH traps. It is also recommended that flake NaOH traps be in stalled in the tank farm with the necessary piping systems to load builk VCM shipments via these NaOH beds. It is essential to protect VCM against contami nation by air, water, or any oxidizing chemicals, (per oxides or peroxide precursors in particular), to pre clude degrading the monomer quality. Steps to achieve this are obvious, i.e., do not permit tie-ins of any of these deleterious compounds to VCM systems. VCM should not be exposed to Bunlight, and storage temperatures should be maintained as low as possible. Sight glasses if used, should be the suitably rated re flex type with actinic proof shields. However, Dow recommends the use of non-glass devices for level in dication. A DP cell located at the bottom of the tank with steam traced vapor leg connected to the tank top is quite satisfactory. Tanks should be painted white or any other heat reflective paint system used to protect the monomer quality. In tropical and subtropical lo cations it is recommended that vapor recompression systems be used to maintain storage tank temperatures low enough to protect the monomer quality. Fire protection Vinyl chloride storage systems should be located in a segregated area well separated from the major proc ess unit and engineered as a Class! Group D Division I area and treated procedurally as such. Any sections of the process, or any adjacent process which has a higher than average fire or explosion haz ard, such as reactors or furnaces should be located as far away from a VCM storage facility as practical. The VCM storage system should be designed to elim inate and/or minimize the possibility of leaks and spills by paying close attention to design pressures, safety valve settings, selection of valves, instruments and design temperatures. VCM should not be allowed to "free fall" from top entries into tanks. Inlets should be into the bottom of tanks or, if top inlets are used, grounded dip pipes pro vided. In addition, all tanks, pipelines and auxiliary equipment such as pumps and compressors must be grounded to preclude ignition of leeks due to static electricity. A major VCM storage facility should be equipped with its own lightning rod. All operating equipment in a process unit and/or storage system must be inerted to preclude oxygen contamination prior to commissioning, and the obvious steps taken to preclude the introduction of oxygen slugs during service must be routinely followed. Examples are in loading tank cars, tank trucks, barges, etc., the receiving container vents back to the storage tank. Therefore, these receiving vessels must be inerted to remove oxygen, preferably by padding up and down as already outlined. It is also important that loading containers be elec trically grounded during the loading operation. It is desirable where feasible to have shipping con tainers returned from customers with a "heel" of about 5 psig vinyl chloride vapor to preclude the possi bility of atmospheric air leaking into the container in transit. It is a good precaution to analyze the heel of VCM vapor in returned shipping containers for oxygen content prior to reloading. Storage tanks of greater than 10,000 USG capacity should be diked inside a con crete dike with a capacity to contain the entire con tents of the storage tank, or tanks, if more than one tank is involved. .Also, the diked area must have an underground drain equipped with block valves. This drain should not lead into the main plant sewer system as the danger exists of flooding the entire plant, in cluding ignition source areas with flammable VCM-air mixtures. Since vinyl chloride is lighter than, and es sentially immiscible in water, it will float on top of a flowing sewer. The VCM storage dike drain should discharge to a 34 April 1969 ucc 032205 CHEMICAL ENGINEERING PROGRESS (Vol. AS, Na. 4) LIQUID IN Figure 8. Sche matic diagram of a typical vinyl chloride storage system. separate underground sewer preferably leading to a re mote disposal or recovery sump. The drain block valve should be accessible from outside the dike wall, and left normally in the closed position; checked each day and opened when necessary to drain casual water from the diked area. Pumps and other auxiliary equipment, level gauges, etc. associated with the storage tanks should be located outside the dike wall. Major storage installations, say greater than 50,000 USG, should be equipped with an open head water del uge invoked by heat actuated devices and/or manually invoked remotely to provide emergency flushing in the event of major spills or fires. Water alone will not effectively extinguish or control large VCM fires due to its lighter than water character. Water may even spread the fire by floatation, but will serve to protect the tanks and contents from overheat ing while the fire is being brought under control by chemical means. In the event of a VCM fire, a point to remember is that one of the products of combustion is HC1 gas. It is recommended that in areas where mobile fire fighting rigs equipped with foam or powder capa- Table 5. Specification for vinyl chloride monomer. Property Sales Specification Typical Anal. Acetylene, ppm Acidity as HC1, ppm Aldehyde, ppm Iron, ppm Non Volatile Matter (% by weight) Phenol, ppm Inhibited Uninhibited Sulfur (as S), ppnt Water (% by weight) 2 maximum 5 maximum 5 maximum 0.5 maximum 0.05 maximum 25 minimum 100 maximum 2 maximum 5 maximum 0.03 maximum <1 <0.5 nil < 0.5 < 75 ppm 25-75 nil <5 < 100 CHEMICAL ENGINEERING PROGRESS (Vol ii, No 4) bilities are not readily accessible, fixed foam facilities be installed adjacent to the storage system and piped permanently inside the dike at required points to con trol and extinguish fires at any point within the dike area. Figure 8 is a schematic of a VCM storage facil ity. Summing up The best way to provide safety in VCM storage and handling facilities is to engineer the installation to minimize spills and leaks; preclude any ignition source for minor leaks and spills that might occur; provide a drained, deluged, dike area; guard against contami nating the system with air or oxidizing chemicals and train people thoroughly in the proper safe procedures for handling the product. # Literature Cited 1. Diu, E., H. end C. Jona. "Practical Way to Size Safety Diaki," CArm. Eng, (September IB, 1M1). \z P. G. Shelley graduated from the Univer sity of Oklahoma in petroleum engineer ing and received an M.S. degree from the same institution. Following two years as an instructor at Pennsylvania State University he spent five years in research with Phillips Petroleum before joining Dow. He has been associated with styrene manufacturing and tech nology for more than 25 years and is presently production manager for sty rene at Dow's Texas Div. r '\i E. J. Sills is a graduate in chemical en gineering from the University of Toronto and has worked in the Production De partment of Dow Chemical of Canada in the Chlorinated Hydrocarbon production area since 1951. He is currently super intendent of chlorinated hydrocarbon process development. UC^ April 1969 032206 35