Document o9YBVx5nDg0VQjpJb2XmN5pKr

Ur r; Monomer Storage and Protection \ 1 i | < l I I 4 r < / Styrene and vinyl chloride are two major petrochemicals; both require care in handling to pre vent explosions and loss of quality. P. 6. 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 m large volume by the Dow Chemical Co and others. These are styrene and vinyl chloride. Styrene production in the United States in 196S 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 exception, 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 maj'or 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 lbs./yr. 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 ing 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 is fairly remote. Eye pro tection should be worn where the possibility of eye contact exists, as m sampling, transferring and main tenance activities Washing contaminated eyes im mediately with water for 15 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. f Table 1. Selected properties of styrene monomer. Chemical Formula Density, Ibs./gal @ 77'F. Vapor Pressure, mm Hg @ 77" F @ 104F. Boiling Point *F. @ 760 mm flash Point, Tag dosed Cup, " F. Fire Point Tag Open Cup, "F. Explosive Limits in Air at Room Temperature % Viscosity, Centipoise @ 77"F. 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 Purity (by freezing point) % Minimum . . Color APHA, Maximum Saybolt Minimum Aldehydes (as benzaldehyde). % Maximum by wt. Peroxide (as HjOi) % Maximum by wt. Sulfur (as S), % Maximum by wt Chlorides (as Cl) % Maximum by wt Polymer Content Maximum--ppm. Sales Spec. 1 99.5 Typical Anal. 99.73 10 27 . <5 o.u^j 0.010 .004 .0007 0.0025 .0001 0.01 .0001 10 0 CHEMICAL ENGINEERING PROGRESS (Vol <i, No 4) I I April 1959 29 STG 3400739 RSV0033496 - MOWtifldoioitAMeE COVHtAOEtnKATiON. 'Sr, r*1- ^"* Oi Figure 1. Disappearance of TBC from styrene mono mer in storage. Vspor 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 material 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 12tol5ppmTBC 30 to75 ppm TBC Saturated with Less Than Temp. Oxygen 3 ppm Oxygen Saturated with Oxygen 60*F. 5 to 6 mo. 10 to 15 days More than 1 year S5*F. 1 to 2 mo. 4to5days 3to4months 110`F. 8 to 12 days Less than 24 hrs. Lessthan 30 days Table 4. Selected properties of vinyl chloride monomer. Chemical Formula Boiling Point (760 mms.), *F. Melting Point, *F. Vapor Pressure at 77*F. Rash Point (Cleveland Open Cup), *F Explosive Limits Vol % in Air Liquid Density grams/ml. 77* F. --4'F. Liquid Viscosity cps @ 77* F. -CH,:CHC1 7.2 -244.4 -- 56 pals -108.4 3.6-26.4 0.9013 0.9834 Q.18S in Table 2. For c< jsraison, 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 tatter 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 some 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 oxygep. Table S 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 m storage can be drawn from it: 1. The lower the temperature the better. 2. An ade quate concentration of TBC must be maintained. 8. 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 70- 75*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 ou 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 he 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 he 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 fiom buildup of aldehydes and 30 April 1969 CHEMICAL ENGINEERING PROGRESS [Vol , No <| sTG 3400140 RSV0033497 .l.ifT- SC=" analysis as man-] i yisM Sr difficult qual4, "*y,,potymer._con 1 except as a poly-' ^ ners. It is in the, sed as a co-mono-Figure 2. Icicle grovrth in an air at- * resins, that smallmosphere on the unlined roof and ely affect finishedsupporting structure of a 70 ft. by re considerations, 24 ft. high tank after 125 days reaction, which is1 service. i ing. This can re-j 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 3f/2 years showed no change. jation, involving0t}ier oxygenated compounds. Shelf life as shown in ' opTM* in a co*i-Tabie 3 applies, jin short, styrene Je commodify and Air storage problems . I Storing under air can, however, create problems as i1.11 containers weu ag sojve them. Since TBC is a high boiling com- jutnough various pound, monomer vapors above the liquid level in the j.ectiveness, para-tanh are uninhibited. Normal temperature swings re- j one almost uni- suvt Jn a continuous cycle of vaporization and conden- ^ncentration for nation on any structure in the vapor space, including ppm. TBC also the roof and sidewaQs of the tank. Droplets will adhere enough, is effec- to any rough or porous surface. In the presence of air, in the presence these droplets polymerize readily into a highly dis- alcation of the colored and oxidized product. Polymer "icicles" develop ia cleariy shows [ftnd continuing refluxing action dissolves the material (and carries it hack into th e bulk of theliquid. If allowed " e** |to proceed indefinitely, these icicles will grow to tre- `f "f 13 .mendous size and eventually fall into the tank, gen- Mats backed by jerally throwing the entire contents off specification, jbeveral condu- j Figures 2 and 3 illustrate the condition, j1 atrage be The situation described occurs to a harmful de 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, aie used or produced, a tank breathing to the atmospheie creates an untenable situation with respect to ail pol lution. Styrene concentrations in air as low as a few parts/biDion 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 aie vented through an independently fueled flare. Such precaution 15 necessary regardless of the storage at mosphere. Color development ft o a j I gree only in uncoated steel tanks having internal roof fjtter. z. An ade- , supporting structures as shown. Rusty steel and corn- maintained. (plicated structure seems to be the preferred environjn must be ,ment for polymer buSilddupn. A non-nporous smooth sur- 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 jface provides no sites for retaining the uninhibited - considers it lmonomer and, even in the presence of air, little poly i styrene storaperature exJefrigeration is mer buildup can be expected. All modern styrene stor age is designed to provide such a surface within the bounds of reason. Self supporting dome roofs are em i required to ployed. API 650 tanks having a dome radius equal to "around 70- 0.8 the dia. of the tank are satisfactory, and can be Ided advantage constructed in Bizes up to atleast 70 ft. in dia. The roof stores in the and sidewalls down to within about 2 ft. of the bottom }mer condensa- are coated with one of several suitable coatings. For va the liquid large tanks, a coating which will cure at atmospheric previously, is the extraction of color bodies from oxi dized polymer attached to structures within the tank. Another is a compound which sometimes is Ruined 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 snitable.) temperature is the most practical. Several epoxy types oe In the fall within this category. The bottom and lower 2 ft of ! 1 shows wall is coated with a rust resisting inorganic zinc sili }l to fall below totned simply means of cir- 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. 'brtant. TBC is it 15-20%, in tsse in overall jihecked every i i I 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 T~ wb^a sty* i latter is usually cheaper and j u st as satisfactory. In the ijcozLcentratlon j absence of oxygen, polymer formation is retarded to at room temlest and gen- Isne where the accumulation b to eliminate (ldehydes and 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. 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 `Amoved and must be replaced immediately. Particulate matter can be extremely detrimental m 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 Eire commonly used Figure 5 illustrates loading pumps and filters in a Dow plant. Note especially the canopy for shading. 5, No. 4) CHEMICAL ENGINEERING PROGRESS (Vol A5. No 4) April 1969 31 STG 3400741 RSV0033498 the liquid surface ./also near the center of the tank provides a particularly desirable situation. t - The test tank was approximately 11 ft. in dia. and contained G ft 3 in. (about 3,400 gal.) of styrene. The foam used was National Foam Systems, Inc. Aero-O- Foam XL3 liquid, a foam containing fluonnated com ' ( t pounds. The foam was applied in 4 % concentration and admitted to the center of the tank bottom, having Ii passed through an aeration device or "foam maker." Foam was started 5 min, and 16 sec. after light-off at a rate of approximately 0.1 gal./min./sq. ft. of liquid i Figure 5. Styrene loading pumps and filters. surface. Results of this test are tabulated below: TIME DESCRIPTION t 0'0" . Light off I5'15" . Foam injection started (valve opened) Although styrene is classified as a reactive flam 6'32* Sound of foam on hot tank walls 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 6'0' 6'20" . 7T5" Fire very low--mostly around edges .. Fire around edge only . Fire controlled--small flickers only i explosive mixture with air will occur in a confined 7'45* .. Fire nearly out-two small flickers vessel, B^S" .. Fire extinguished I .Ignition sources 4 8'66* . . Foam stopped The test was considered successful on all counts. Other Sources of ignition should be eliminated from sty than the extinguishment of the fire, original areas of rene storage areas, no smoking rules adhered to, and concern included the possibility of undesirable reac s totally enclosed electrical equipment used. Explosion tions or degradation of the monomeer from the foam proof equipment is recommended only in confined and the extent of entrainment of monomer in the foam. areas. Static electricity is probably one of the more Neither of these proved to be of consequence. The sty Important ignition hazards. All tanks, pumps, etc. rene was very simply reprocessed to a salable product should be adequately grounded and submerged filling and entrainment proved to be substantially less than 1 practiced that experienced with gasoline and hexane < Styrene tanks should, in general, conform to the Figure 7 illustrates a schematic of a typical storage Flammable Liquids Code NFPA-30, Tanks of over 10,- tank used by The Dow Chemical Co. 000 bbls. capacity should be individually diked, the t. dike having 100% of tank capacity. Dike drains are kept normally closed but with valve accessible under Vinyl chloride Vinyl chloride monomer (VCM) at normal ambient emergency conditions.Pumps should be located outside temperature and pressure is a colorless gas with a faint the dike. Distance between tanks should be a minimum sweet odor. VCM is stored and shipped as a liquid un of one half the sum of their diameters. der pressure and the greatest hazards in the handling Prom the standpoints polymer buildup, the number of this material are the dangers of fire and explosion. of roof nozzles on a tank should be the minimum A Table IV gives the more important properties of flame arrestor and vacuum-pressure relief are usually VCM as related to storage and handling. The same mounted on the same nozzle. Emergency relief is com three major objectives which apply for styrene mono bined with a manholef1). A gauge hatch is the only mer also apply for developing technique and equipment other necessary opening, but a float operated tape level for safe handling and storage of vinyl chloride mono indicator is frequently provided. Flow line and drain mer; plus same techniques peculiar to VCM due to its valves adjacent to the tank must be steel (preferred) high vapor pressure and extreme flammability. The or ductile iron as per API 604. objectives are: Fixed foam protection is the recommended provision 1. Personnel protection. for extinguishing styrene tank fires. The foam cham 2. Protection of quality and monomeric status. ber is separated from the tank by a frangible such as 3. Protection against fire and explosion. aluminum foil or glass to prevent polymer accumula Adequate training in the potential hazards of the tion. Inspection is at least annually. Provision should, material and the proper personnel protection equip be made to supply liquid foam solution at a rate of 0.1 ment and techniques for it are essential to the safety gal./min./sq, ft. of surface. In September 1967, Dow, in conjunction with Na tional Foam Systems Inc., conducted a test to deter of all personnel handling VCM. The vapors should be controlled to no more than 600 ppm for repeated 7-8 hr. daily exposures. 1 mine the feasibility of subsurface foam injection for Concentration above this figure can cause dizziness, extinguishing fires in tanks containing styrene. Essen disorientation and "drunkenness," Concentrations ap tially, the method consists of pumping a foam solution proaching the lower explosive limit (3.6 vol. % in air) I into the tank through a product line. As the foam en can cause helplessness and unconsciousness from a ters the tank, it rises to the liquid surface, providing very short exposure (anesthesia). a lire smothering blanket. It has generally been ac- Thus, operating areas must be designed with ade * -knowledged that such a system, if effective, would offer quate drainage and ventilation and equipment should cost, maintenance and reliability advantages over the be selected to minimize the possibility of leaks and commonly used foam chamber systems. These require spills. Operating personnel should be equipped with a separate, external piping system arranged to dis and trained in the proper use of respiratory equip charge foam on the liquid surface inside the tank Ad ment. mitting the foam through a swing line, placing it near Organic vapor canisters have been shown to be un- 32 April 1969 CHEMICAL ENGINEERING PROGRESS (Vol 4i No 4) STG 3400742 RSV0033499 "1er of the tynk '?. , irt. dia, and ' styrene. The | l ; 1 dependable and ineffective ''Vunst vinyl chloride vapors. Self-contained breathy .'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 ' * Inc, Aero-Oiuorinated comncentration and fcottom, -having Pfoam maker." ter light-off at iq. ft. of liquid Jed below: : 1 i i ; 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 VCM 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. valve opened) ' Contaminated clothing, gloves, boots, etc. should be v ik walls I removed immediately to avoid prolonged skin contact. >und edges ilkera only II flickers I Trained operating personnel must be fully aware of j the flammable and explosive characteristics of VCM to I preclude the possibility of human error causing a ma jor spill, or providing an ignition source for minor h counts. Other leaks or spills. Safety considerations ,inal areas of ( Summing up VCM storage systems should be .pirable reac- designed to minimize leaks and spills; have adequate jm the foam ventilation and drainage to conduct any leaks or spills . I in the foam, away from critical operating areas; and the storage 1 ipce. The sta and process areas treated strictly as hazardous, both ble product from an engineering and operating procedural stand- ; }ly less than point. ical storage 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 laal ambient tection such as safety glasses with side shields or their . equivalent; have adequate respiratory equipment for h a faint iquid un ite handling Explosion, hperties of I The same jene mono- quipment !Lde monodue to its ility. The emergency use; and provided with ready access to an eye wash and safety shower. Vinyl chloride is marketed under the specification shown in Table 6. 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 1 producers that unihibited VCM could be stored and handled safely under the proper conditions. Since deleting the inhibitor makes the inhibitor re Jatus. moval system at the users plant site unncessary, a considerable capital and operating expense incentive Ids of the i equip- te safety lould be fated 7-8 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 in- 1 hibted monomer. Since large volumes of VCM are now Btored and han dled In the uninhibited state, the proper design, main _ izziness, 1 ions api > in air) | from a tenance and operating procedures for storage systems to protect thequality and monomer status of the stored VCM are absolutely essential to safety and loss pre vention. Coinciderudly these same steps also optimize the iith ade- t should aks and <pd with r equip- conversion to and quality of the end use product. Like styrene, the only significant end use for VCM is as a polymer (PVC) or co-polymer with other mono mers The impurities or monomer characteristics which affect the polymer quality adversely are mainly color, be on- oxygen, honopolymer m the monomer iron content, and unsaturated hydrocarbon impurities which produce "IN Figure 6. Conditions for explosive mixture with sty rene under air. variable polymerization rates and polymer quality. High water content is also 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 unnceasarily 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 No. 4 CHEMICAL ENGINEERING PROGRESS (Vol iS. No 0 ! I STG 3400743 Apn! 1969 33 RSV0033500 1 NOTE* ROOF AND SIDE W*___5 COATED WITH AIR CURED EPOXY. ') f- L. <* "A* ---* >5-A.11bN&nU**a-*n!. PRODUCT OUT VACUUM PRESSURE RELIEF W/FLAME ARRESTOR GAUGE HATCH THERMO- V*T> WELL-^ MANHOLE AND EMERGENCY ELIEF TAPE-LEVEL INDICATOR PRODUCT IN Figure 7. Rec ommended de sign for sfyrene storage tanks. PACKAGE REFRIGERATION TO _ SLUDGE SEWER 1/2 TANK DIAMETER 1/2 TANK DIAMETER to pad and depad the system, (including tanks, lines, far away from a VCM storage facility as practical. 1 i pumps, etc. as a unit) with nitrogen until the oxygen The VCM storage system should be designed to elim i* content reaches the desired level. For example, padding inate and/or minimize the possibility of leaks and a system originally containing one atmosphere of air spills by paying close attention to design pressures, to 30 psig with nitrogen, depadding to atmosphere and safety valve settings, selection of valves, instruments repeating until the pad-depad procedure has been done and design temperatures. five times will reduce the oxygen content of the system VCM should not be allowed to "free fall" from top to 0.082 volume %. entries into tanks. Inlets should be into the bottom of Experience has indicated strongly that vinyl chlor tanks or, if top inlets are used, grounded dip pipes pro ide which is produced on the alkaline side, 0.2-0 6 ppm vided. In addition, all tanks, pipelines and auxiliary alkalinity as NaOH, has a more stable storage life It equipment such as pumps and compressors must be is recommended that VCM which is to be stored or grounded to preclude ignition of leaks due to static handled for any period more than a few hrs be proc electricity. A major VCM storage facility should be essed on the alkaline side This can be achieved by equipped with its own lightning rod. pumping to storage via flake NaOH traps. All operating equipment in a process unit and/or It is also recommended that flake NaOH traps be in storage system must be inerted to preclude oxygen tt stalled in the tank farm with the necessary piping contamination prior to commissioning, and the obvious 5 systems to load builk VCM shipments via these NaOH beds. It is essential to protect VCM against contami steps taken to preclude the introduction of oxygen slugs during service must be routinely followed. nation by air, water, or any oxidising chemicals, (per Examples are in loading tank cars, tank trucks, oxides or peroxide precursors in particular), to pre barges, etc, the receiving container vents back to the clude degrading the monomer quality. Steps to achieve storage tank. Therefore, these receiving vessels must s. this are obvious, i.e, do not permit tie-ins of any of be inerted to remove oxygen, preferably by padding up A these deleterious compounds to VCM systems. and down as already outlined VCM should not be exposed to sunlight, and storage It is also important that loading containers be elec temperatures should be maintained as Eow as possible. trically grounded during the loading operation. Sight glasses if used, should be the suitably rated re It is desirable where feasible to have shipping con- flex type with actinic proof shields. However, Dow ` tamers returned from customers with a "heel" of recommends the use of non-glass devices for level In about 5 pslg vinyl chloride vapor to preclude the possi dication. A DP cell located at the bottom of the tank bility of atmospheric air leaking into the container with steam traced vapor leg connected to the tank top m transit. is quite satisfactory. Tanks should be painted white or It is a good precaution to analyze the heel of VCM any other heat reflective paint system used to protect vapor in returned shipping containers for oxygen - the monomer quality. In tropical and subtropical lo content prior to reloading. Storage tanks of greater cations it is recommended that vapor recompression than 10,000 USG capacity should be diked inside a con system.'. De used to maintain storage tank temperatures crete dike with a capacity to contain the entire con low enough to protect the monomer quality. tents of the storage tank, or tanks, if more than one i i Fire protection tank is involved. Also, the diked area must have an underground drain equipped with block valves. This i i Vinyl chloride storage systems should be located in a segregated area welt separated from the major proc ess unit and engineered as a Class I Group D Division drain should not lead into the mam plant sewer system as the danger exists of flooding the entire plant, in cluding ignition source areas with flammable VCM-air I area and treated proceduratly as such. mixtures. Since vinyl chloride is lighter than, and es Any sections of the process, or any adjacent process sentially immiscible m water. It will float on top of a which has a higher than, average fire or explosion ha2- flowing sewer. ard, such as reactors or furnaces should be located as The VCM storage dike drain should discharge to a * 34 Apr.! 19dP CHEMICAt ENGINEERING PROGRESS {Vo! . No l| ST6 3400744 RSV0033501 >*Cr i--y U ' 1 ' Figure 8. Sche matic diagram of a typical viny! chloride I storage system. j I siid jtic for jen ons t)?en 'cks, the oust STOP elec- conr of tKKSSiainer vcm: sygen reater acone conlh one 1 iye an i. This system mt, injCM-air and es* top of a' 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 V CM 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 ECl 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 Acetylene, ppm Acidity as HC1, ppm Aldehyde, ppm Iron, ppm Non Volatile Matter <% by weight) Phenol, ppm Inhibited Uninhibited Sulfur (as S), ppm Water (% by weight) Sales Specification 2 maximum 5 maximum 5 maximum 0.5 maximum 0.05 maximum ' 25 minimum 100 maximum 2 maximum 5 maximum 0.03 maximum Typical Anal. <1 <05 ml < 0.5 < 75 ppm 25-75 nil <5 < 100 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 leakg; 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 Dim, E* H. Karim, and C. Jonaa, -Frmetieal Way to Sue Safaty Dialer. Cham Snf (Saptambar It. 1861). 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 Sow 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. 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 mtendent of chlorinated hydrocarbon process development. '&S, No- ) , ! CHEMICAL ENGINEERING PROGRESS (Vol . No 4) STG 3400745 April 1959 35 } RSV0033502