Recovery of Benzene from the Aqueous Wastes of a Styrene Manufacturing Process (adapted from El-Halwagi, 1997). Styrene is manufactured by dehydrogenating ethylbenzene over an oxide catalyst at 600 to 650=C. Steam is generally co-injected with the ethylbenzene into the reactor, shown in Problem 3, Figure 1. Styrene and hydrogen gas are the primary reaction products. Byproducts include benzene, ethane, toluene and methane. The products and byproducts from the reactor are cooled to approximately ambient temperature. Light products such as hydrogen, methane and
ethane which do not condense when cooled to ambient temperatures are vented. The condensed materials are sent to a decanter where an organic layer and an aqueous layer form. The organic layer containing benzene, toluene, and unreacted ethylbenzene is recycled. The aqueous portion of the wastewater stream (R2) generated in the decanter is saturated with benzene and must be treated. Another wastewater stream (R1) is generated earlier in the process, during the manufacture of ethylbenzene. The ethylbenzene fed to the styrene manufacturing operation is generally produced on site from ethylene and benzene and produces a condensate wastewater stream that is saturated with benzene. Thus, there are two sources of benzene saturated wastewater (approximately 1770 ppm or 0.00177 kg benzene/kg water) in the process: R1 (1000 kg/hr) and R2 (69,500 kg/hr). The concentrations of benzene must be reduced to at most 57 parts per billion in the wastewater. Design a recycle and reuse system for the benzene using steam stripping and adsorption onto activated carbon. The system is shown in Problem 3, Figure 2. In the design:
• A steam stripping unit is used which removes benzene from the wastewater and recycles it. This unit produces a wastewater stream saturated with benzene (70 kg/h; see Problem 3, Figure 2) that must be sent to the recovery system.
• Spent activated carbon is regenerated with steam. The steam is recovered by condensation , producing a wastewater stream saturated with benzene (37 kg/h; see Problem 3, Figure 2). This wastewater is sent to the benzene recovery system. a) Draw a composition interval diagram for the benzene rich streams.
a) Draw a composition interval diagram for the benzene rich streams.
b) Data on the lean stream compositions are given below. The target compositions are based on a variety of factors. For example, the supply composition of benzene in activated carbon corresponds to the residual benzene remaining on the carbon after regeneration. Using these data, draw the composition interval diagram for the lean streams.
c) Using the equilibrium relationships shown below, draw a pinch diagram for this system.
where yi and xi are the mass ratios (kg benzene/kg benzene free stream) in the rich and lean streams. Assume mi for steam is 0.001 and mi for activated carbon is 0.00071.
d) Determine the location of the pinch point assuming no minimum driving force for mass transfer and calculate the amount of benzene recovered. What is the value of the recovered benzene if it can be sold for $0.20/kg? Repeat your calculation for a minimum mass transfer driving force of 0.001.
e) Compare your results to the flow rates shown in Problem 3, Figure 2 and discuss any differences.