Styrene homopolymerizes albeit slowly but steadily, even at ambient temperatures to a clear glassy solid.
To avoid polymerization, a minimum 10 ppm of the inhibitor para Tertiary Butyl Cathecol (TBC) is added when styrene is stored or shipped.
Since polymerization is promoted by higher temperatures, storage tanks are usually insulated.
Lyondell gives interesting data on sampling, tests and testing frequency for polymer and TBC while in storage, and the maximum recommended storage periods as function of temperature.
More info about Styrene.
Sorry for this long thread
FCMaua
Storage Tanks
o Materials - Stainless steel is preferred although its cost can be excessive. Clean mild steel or aluminum are acceptable but must be properly coated before use with styrene. Contamination with rust imparts colour and accelerates polymerization. The use of copper or copper alloys (e.g. brass, bronze and some aluminum) for fittings etc. is not acceptable. Styrene will corrode copper, and the copper will inhibit polymerization and impart a blue/green colour to the styrene.
o Structure - A self-supporting or exterior-supported domed roof is preferred. Styrene vapour will condense and collect upon internal structures or surface irregularities. This condensate does not contain polymerization inhibitor and so will polymerize. High molecular weight polystyrene will collect on any internal structures, forming "stalactites" which can eventually break off and contaminate product or buckle the roof.
o To make the interior surface as smooth as possible, the number of nozzles etc. should be minimized, where possible, duplicating functions at a single point. Where possible, nozzles should be placed below the normal liquid level.
o The tank design must include a frangible roof to shell joint.
o Interior coating - For a mild steel tank, an inorganic zinc silicate coating is required for all interior surfaces of the tank (floor, walls and ceiling). This acts as a rust inhibitor, and to dissipate static charge. Other coatings may be acceptable, provided that corrosion resistance and avoidance of static charge buildup are achieved. A coating is not required for a stainless steel tank.
o Tank gauging - A gauge hatch is necessary, and the dip stick must be properly calibrated. Manual tank gauging is preferred. Internal structures should be eliminated. A Differential Pressure (DP) cell may be installed at the bottom of the tank to give approximate values by measurement of the pressure difference to ambient. This can also serve as a high pressure alarm.
o Venting - Goose-neck vents are adequate for tank breathing unless environmental considerations do not allow. Dryers are not recommended, since they provide a surface where polymerization can occur which may block a vent. In an area where styrene vapours can come into contact with halogens (chlorine and especially bromine), the tanks should be independently vented to flare. Styrene will react with halogens to form a powerful lachrymator (tear gas), even at low concentrations. In areas where legislation or environmental concerns do not allow venting to atmosphere, venting to flare is necessary.
o Gas blanketing - Oxygen in air will react with styrene to form unwanted oxidation products. This not only contaminates the product, but some can deplete the inhibitor and lead to polymerization. However, without some oxygen (approx. 10 ppm in solution), the Tertiary Butyl Catechol (TBC) is not effective as inhibitor. The most effective gas mixture for blanketing is nitrogen with 6-10% v/v oxygen. Other gases such as natural gas have also been used. However, gas blanketing is complicated and costly, and so is not normally recommended.
o Temperature - Depletion of TBC, and formation of polymer and oxidation products are temperature dependent reactions. The lower the storage temperature, the better. Higher temperatures also increase styrene vaporization and subsequent build up of polymer. Normally, the preferred storage temperature is l0-18°C (50-65°F). If the temperature approaches 20°C (70°F), the tank must be cooled. Under no circumstances should the temperature exceed 25°C (77°F). To maintain a proper storage temperature, the following features are required:
In cool or moderate climates, insulation is not required. For an uninsulated storage tank, the exterior surface must be painted with a white, reflective coating. If the tank is mild steel, the coating should also be formulated to prevent rust. There must be some way to cool the tank and its contents. A recirculation pump with refrigeration capability may be used. Alternatively, there must be a facility to spray the exterior of the tank with cool water. A permanent spray fixture on the roof of the tank is preferred.
In warm or hot climates, insulation is required. For an insulated storage tank, a white, reflective exterior coating will increase the effectiveness of the insulation. Exterior cooling is not effective, so a refrigerated recirculation facility is recommended.
o Without recirculation, polymerization inhibitor (TBC) added has a tendency to sink to the bottom. A recirculation pump is not essential but highly recommended. Recirculation helps to minimize build up of polymer in lines. Localized heating is minimized. Recirculation decreases the effect of temperature cycling, so reduces styrene vaporization and condensation and subsequent build up of polymer in the head space.
o The facility to add TBC at the recirculation pump is recommended. This makes it unnecessary for personnel to climb to the top of the tank under hazardous conditions.
o A permanent temperature indicator with high temperature alarm is useful to indicate the start of a polymerization problem. Probes should be mounted below the normal liquid level. In the absence of recirculation facility, several probes at various locations are recommended.
o Storage tanks should be separately diked with pumps placed outside the dike. Dike drains should be routinely closed, but with the ability to be opened if necessary. Spacing and construction of dikes is usually covered by Government safety, fire and/or environmental legislation. This must be followed.
o Foam facilities to blanket the interior of the tank are recommended.
B. Pumps and Lines
o Wherever possible, all tanks, hoses, pumps and lines should be dedicated to styrene service. Any facilities used for other materials must be thoroughly cleaned and, dried before use for styrene.
o Styrene should not be allowed to sit in lines, hoses, pumps etc., since this greatly increases the possibility of localized heating and polymerization. Regular recirculation is preferred. The frequency and duration of recirculation depends upon local conditions. In hot weather the lines should be recirculated daily for 1 hour. In cooler weather, the frequency may be reduced to once or twice per week, depending on results of testing done on site which show this is adequate. If recirculation is not available, then equipment must be emptied, blown out with air or inert gas (preferred) or pigged, and kept dry.
o Equipment should he chosen or designed such that there is no place where product can collect when not in use. Thus, lines should be sloped to drain, and all interior surfaces (especially at connections) should be as smooth as possible.
o Valves and bleed lines between on-spec. product lines and off-spec. or other product lines must be double blocked. All material left in the dead volume between blocks must be bled out.
o All equipment must be made from materials totally inert to styrene monomer. Copper and copper alloys are not allowed. Hoses made from stainless steel or polypropylene have been found suitable.
o The build up of static charge must be avoided. When filling tanks, tank cars, tank trucks or drums, the dip pipe should be submerged in the liquid to avoid splashing. Filling rate must be restricted to 1 m/sec or less until the dip pipe is submerged.
o Because of the danger of static charge build up, filling hoses etc. should be checked for electrical continuity. If any doubt exists, an external bonding wire must be fitted. When filling drums, a bonding wire should join fitting pipe to drum.
o If there is any suspicion or the presence of water in the line, or if the line contains a microfilter, then the pumping rate should not exceed 1 m/sec. The pumping rate must not exceed 3 m/sec under any conditions.
o When loading or unloading a ship, an insulated flange in the ship-shore line is recommended. The lines on both sides of the flange should be continuous, but use of a ship-shore continuous bonding line is not recommended (Ref. ISGOTT).
o For jointing, compressed asbestos fiber has been commonly used. Health hazards associated with asbestos make it undesirable. Care and proper protective equipment must be used when handling asbestos. Styrene will swell many different types or rubber. Teflon, cork and leather gaskets and seals have been used.
o Pumps - All types of pumps have been used with styrene monomer, but centrifugal pumps with mechanical seals and PTFE packing are preferred. Provision for a 0.8 mm clearance between the stuffing box follower and impeller shaft provides an exit for any polystyrene which has formed which might otherwise solidify and score the shaft. The pump should not contain any rubber or copper containing parts which might come in contact with styrene.
o Pumping against closed valves can cause overheating and subsequent polymer formation, and so must be avoided.
o Use of an in-line microfilter is recommended, since the presence of particulate matter is unacceptable to most customers. When a filter is used, additional attention to static charge and blockage with polymer and particulates is required. If there is any evidence of particulate contamination, the product must be filtered. A filter over the end of the filling line is satisfactory. Constant attention to cleanliness on-site, especially during transfer operations, helps to minimize contamination problems.
Table II
Shelf Life of Styrene Monomer
General Effects of Inhibitor and Oxygen at Various Temperatures
12 to 15 ppm TBC 50 to 75 ppm TBC
Temp. O2 Sat'd. <3 ppm O2 O2 Sat d.
°F °C
60 15.6 5-6 mo. 10-15 days more than 1 year
85 29.4 1-2 mo. 4-5 days 3-4 mo.
110 43.3 8-12 days <24 hours less than 30 days
*Ref. Chem. Eng. Prog. Vol. 65, No. 4, April 1969.
o TBC is such an effective polymerization inhibitor that no styrene polymerization will occur until the TBC is almost completely exhausted.
o In the absence of TBC, when a molecule of styrene is activated by an initiator, it will react with another styrene molecule, and a chemical bond is formed between them. The activity is transferred to the second molecule, which then reacts with another molecule. This process, called polymerization propagation, continues as more styrene molecules are added one at a time, creating a linear chain. Eventually, the chain can grow to form a very large polystyrene molecule consisting of hundreds or thousands of styrene monomer "links".
o This polymerization is the cause of the potential danger. Each time a bond is formed with a new styrene molecule in the growing chain, energy is released (i.e. an exothermic reaction). Also, the rate of polymerization increases with increasing temperature. These two factors create an autocatalytic or "snowballing" situation.
o The problem is compounded as the concentration and molecular weight of polymer increases. The viscosity of the solution increases, reducing heat transfer. Polystyrene is a poor heat conductor, and the high viscosity reduces convection. Heat produced by the reaction is not dissipated. The high viscosity also makes it difficult to pump the solution or effectively mix in a polymerization inhibitor.
o The actual course of the reaction in a storage tank will depend upon many factors, some of which include the amount of styrene present, volume to surface ratio, head space volume, ambient temperature, heat transfer characteristics, facility for pressure release, etc.
o An unattended, runaway polymerization of a tank car would likely proceed as follows:
- After total TBC exhaustion, the reaction would start quite slowly, taking 10 days or more to reach a temperature of 65°C. At this point, about 15% of styrene would be converted to polymer.
- During the next 10-36 hours, the reaction rate would increase exponentially. As the remaining 85 % of the monomer polymerizes, the temperature would rise very rapidly to 200-250°C.
o If the storage tank is not vented, or the vents have become plugged with polystyrene, pressure may increase to the point that the tank will rupture explosively.
o If the tank is fully vented, the temperature will rise to the boiling point of styrene (145°C). The heat used to convert the styrene liquid to vapour will prevent the temperature from reaching the high levels attained in a closed system. however, large amounts of styrene vapour will be released to the air, and quantities of boiling styrene/ polystyrene solution will escape through the opening in the tanks.
o The reaction will stop when all of the styrene has polymerized. The mass will cool gradually. If allowed to cool completely, the polystyrene will solidify, creating a substantial cleanup problem.
Prevention
o A runaway polymerization reaction will never occur if the following precautions and procedures are followed:
i) Maintain the storage tank at the proper temperature (see page 6)
ii) Regularly monitor the TBC level, and dose as necessary to maintain 10-15 ppm at all times (see sections III and VI X)
iii) Prevent contact between styrene monomer and polymerization initiators, e.g. oxidizing agents, metallic hydrides, iron chlorides
iv) Regularly monitor the polymer level
v) Regularly inspect relief valves and vents for polymer buildup. Use only atmospheric vents. Avoid PV vents (see section IV)
vi) Plan for fast stock turnover
vii) In a situation where a shipment may be in transit for a long period of time, particularly in a hot environment, overexposing with TBC may be required.
How can I calculate the proper flow rate for styrene monomer recirculating in order to avoide its polymerization resulted by heat transferred into the pipe fluid? which consideration should be taken into account?
Thanks
I have not read the details of FCMaua's recommendations in detail, but believe that they are similar to my research following investigation of a fire/explosion in a barrel of styrene monomer used at a fibreglass facility.
I cannot say for sure whether the lack of TBC due aging and degradation was the cause, but there was a strong arguement for it.
I can only stress the importance of following the guidelines due to my own second hand experience.
Please review the Lyondell website for information on styrene monomer it is pretty informative.
