Production of Styrene
Comparison of some processes that produce Styrene...
Background Styrene is one of the essential reactant that we require for the creation of products we use day to day. The reason why styrene is so widely used is because it is strong, durable; and can be used in many things such as foam, cases, appliances, containers and carpets. In 2011, the demand for styrene was 26,111,000 metric tons and since this compound is so versatile the demand for it will only continue to rise as the population rises as well. Styrene began its large scale production in 1930s because of new chemical discoveries allowing it to become readily available. Today there is no chemical alternative that can compete effective crosslinking and that is also cheap as well as styrene does. But styrene is a dangerous chemical; it is highly flammable and it can readily link itself to other styrene molecules when exposed to light and heat. Polystyrene however, has a major impact to global warming. Because polystyrene takes 500 years to decompose and is resistant to photolysis meaning it is very difficult to discard. Burning polystyrene will release Carbon Monoxide and Styrene Monomers into the environment which is dangerous to our health. Exposure to styrene includes aspiration into the lung which can cause chemical pneumonitis which can be fatal, it is irritating to skin and eyes and dangerous to the central nervous system therefore the best solution to control exposure is incorporate safety practice in the workforce. Styrene is a dangerous molecule but there are two feasible ways to produce styrene. One of them is responsible for 90% of today’s styrene production and that is dehydrogenation of ethylbenzene in the presence of a catalyst. The other process involves the oxidation of ethylbenzene to its peroxide that is reacted with propylene to produce methylphenyl carbinol which is then dehydrated to form styrene. But there is also a new process that has emerged such as Toluene Alkynation with Methanol to from Styrene, because this process has eliminated the ethylbenzene dehydrogenation unit operating cost would be reduced.
Figure 1: Styrene Demand 2011
Alternative Process Dehydrogenation of Ethylbenzene The dehydrogenation reaction of ethylebenzene is usually operated at temperatures above 600oC and is an endothermic reaction which is reversible. High temperatures and a low benzene partial pressure are used in the reaction to shift the equilibrium reaction to favour the production of styrene because the conversion of ethylbenzene increases as the reaction temperature increases.
Figure 2: Reaction to Form Styrene from Ethylbenzene
Main side reactions include: Ethylbenzene -> Benzene + C2H4
Ethylbenzene + H2 -> Toluene + CH4
The side reaction which involves toluene is determined by the temperature of the reactor that is currently operating. If there is toluene in the reaction, this can be separated by a distillation column whilst leaving the ethylbenzene and styrene to be recycled and mixed with fresh ethylbenzene before the reactor. Ethylbenzene exists as a colourless liquid but is mostly found as a vapour in the environment, it can move through water and soil because it doesn’t easily bind to soil. But it can move towards a water supply where it breaks down with the chemicals naturally found in water. Residential areas close to an underground storage tank of ethylbenzene of landfills may have some contamination with ethylbenzene due to the leakage from the site. Luckily ethylbenzene isn’t classified as a hazard to the environment One important aspect of the dehydrogenation of ethylbenzene is its reactor. The major by-product of the reaction from ethylbenzene to form styrene is complex mixtures of aromatics and well as the formation of coke therefore in order to minimize these formations the reactors had to be adiabatic. These reactors generally had one circular flow path that entered and Figure 3: Axial Reactor Figure 4: Radial Reactor then exited the vessel. This circular design required a low inlet volume to obtain an appropriate distribution of the feed through
to the catalyst bed compared to an axial flow reactor. These radial reactors also had the advantages to provide a low pressure drop since its path through the catalyst bed is shorter than an axial reactor. Because the dehydrogenation of ethylbenzene is an energy demanding process, many technologies have aimed to decrease this high energy consumption. Most styrene plants that involve the dehydrogenation of ethylbenzene to form styrene are either based on the Lummus Crest/UOP technology or the Fina/Badger process technology that have adiabatic reactors. Although these processes are very alike there are some key differences within these processes such as the temperature, the reaction pressure and the amount of unit operations required to obtain styrene. However Lummus Crest undergoes a Zeolite-base liquid phase dehydrogenation of ethylbenzene whilst the Fina/Badger undergoes a vapour-phase dehydrogenation of  ethylbenzene . Both processes have super-heated steam around 750oC-850oC that is fed into the first reactor to prepare the system for a forward reaction. The steam and ethylbenzene mixture has a ratio of 1.8 and is fed into the first reactor at 610oC-650oC which differs depending on the quality of the catalyst. The reaction mixture cools during conversion and the then leaves the first reactor at 560oC-570oC. The conversion of ethylbenzene is about 35%. The process is repeated into a second reactor which ethylbenzene is then evaporated by numerous heat exhangers and mixed with dilution steam to prevent coke formation to maximise the conversion to 65%-70%.
Figure 5: Fina/Badger Dehydrogenation
Figure 6: Lummus Crest/UOP Dehydrogenation
Figure 7: Selected Process
The Picked Process PRODUCTION OF STYRENE BY ADIABATIC DEHYDROGENATION Fina/Badger Styrene Process
The fresh ethylbenzene is mixed with the recycled ethylbenzene [P-21] then it is mixed with steam [P-4] and is fed to the primary and secondary dehydrogenation reactors. When steam exits the re-heater as a cool product, it is then reheated to superheated steam [P3] which then enters the primary dehydrogenation reactor [P-4]. Superheated Figure 8: Styrene Molecule steam is used for heating the mixture for the reaction in the secondary dehydrogenation reactor as well [P-6]. These dehydrogenation reactors [E-2 and E-3] are designed to have low pressure and an even flow distribution. The dehydrogenation reactors liquid waste is then cooled through a number of heat exchangers [E-5, E-6 and E-7] which heat the recycled and fresh ethylbenzene[P-9]. The reactors liquid waste is divided and condensed [P-20] which then enters a vent gas compressor which further minimized pressure drop.This process has three distillation towers that operate to reduce polymer formation, to have low temperatures and to operate under vacuum conditions. The first column [E-13] separates the toluene and benzene by-products from unconverted ethylbenzene and styrene [P-22], the benzene and toluene is then sent to a different ethylbenzene plant where it is further fractionated[P-19]. The second distillation tower [E-15] separates the unconverted ethylbenzene [P-21] that is then recycled for the feed for the dehydrogenation reactors. The temperature of this tower is below 100oC to reduce the polymerisation of styrene which occurs at temperatures higher than 100oC. This recovery stream [P-23] is then Figure 9: Selected Process fed to the third and final column [E16] where styrene itself [P-24] is purified by removing any heavy residue [P-25]. 4-tertbutylcatechol is injected at the top of the third column to prevent polymerisation during
storage. Preventing polymerisation during storage is maintained by lowering the temperatures below 20oC and by frequently adding 4-tert-butylcatechol. Adiabatic dehydrogenation of ethylbenzene processes leads to superheated steam above 600oC which may have dangerous outcomes if not handled properly. However if we based our selection with an isothermal dehydrogenation of ethylbenzene which was established by BASF, their reactor is built as a reactor-heat exchanger where ethylbenzene and steam flow through tubes that are packed with catalysts, the heat however is supplied with hot flue gas in the outer portion of the reactor, therefore the temperature of the steam is lower than the adiabatic process. But a disadvantage to this is that the size of the isothermal reactor that constrains the size limitations of a single-train chemical plant, therefore there will be an increase of investment costs for chemical plants. But heat recovery from there flue gas where it will still enter the atmosphere will have impact on global warming. Because when fuels are burned (besides ash and certain gas molecules) their remains are still there. But it depends on the composition of the fuel that really has an impact to global warming. Typically it consists of CO2 which is the main greenhouse gas responsible for global warming which might conclude to nearby societies protesting against the chemical plant. This is why an adiabatic process is more sustainable than the isothermal process. Figure 9: Flue Gas
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