ChE 455 Styrene Production
ChE 455 Styrene Production
Styrene is the monomer used to make polystyrene, which has a multitude of uses, the most common of which are in packaging and insulated, styrofoam beverage cups. Styrene is produced by the dehydrogenation of ethylbenzene. Ethylbenzene is formed by reacting ethylene and benzene, and one of the ways benzene is made is by the hydrodealkylation or transalkylation of toluene, which is obtained as a byproduct of gasoline manufacture. There is very little ethylbenzene sold commercially. Most ethylbenzene manufacturers convert it directly into styrene in the same plant.
The plant at which you are employed currently manufactures ethylbenzene and styrene. This plant was recently acquired by your company in a takeover, and a team of engineers has been assigned to solve the problems observed in the process over the last few years. The unit to which you are assigned, Unit 400, converts the ethylbenzene into styrene, producing around 100,000 metric tons per year of 99.8 wt % styrene.
Process Description
The process flow diagram is shown in Figure 1. The reactions, the kinetics, and the
equilibrium equations are detailed in Appendix 1. Ethylbenzene feed is mixed with recycled ethylbenzene, heated, and then mixed with high-temperature, superheated steam. Steam is an inert in the reaction, which drives the equilibrium (shown in Equation 1 in the Appendix 1) to the right by reducing the concentrations of all components. Since styrene formation is highly endothermic, the superheated steam also provides energy to drive the reaction to the right. The reactants then enter two adiabatic packed beds with interheating. The products are cooled, producing steam from the high-temperature reactor effluent. The cooled product stream is sent
to a three-phase separator, in which light gases (hydrogen, methane, ethylene), organic liquid, and water each exit in separate streams. The hydrogen stream is further purified as a source of hydrogen elsewhere in the plant. The small amount of benzene and toluene is distilled and either incinerated for its fuel value or returned to the ethylbenzene process (since the benzene raw material always has some toluene impurity). The ethylbenzene and styrene stream is distilled to separate unreacted ethylbenzene for recycle from the styrene product.
The styrene product can spontaneously polymerize at higher temperatures. Since our product styrene is sent directly to the polymerization unit, experience suggests that as long its temperature is maintained below 125°C, there is no spontaneous polymerization problem. Since this is below styrene’s normal boiling point, and since low pressure pushes the equilibrium to the right, much of this process is run at low pressures, with much of the separation section at vacuum.
Tables 1 and 2 show the design conditions for Unit 400. Table 3 contains an equipment list.
Other pertinent information and calculations are contained in Appendix 2.
DETAILED ASSIGNMENT
