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ch08 - CHAPTER 8 SOLUTIONS Exercise 8.1 Cumene process with...

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8-1 CHAPTER 8 SOLUTIONS Exercise 8.1 – Cumene process with drag (purge) streams Page 1 The flowsheet for the process is a modification to that shown in Figure 8.1 as follows: 1. Add a distillation column, C4, whose feed is the bottoms from C3. The distillate from C4 is sent to the Trans-alkylation Reactor and the bottoms is a alkylation heavies and trans-alkylation heavies. 2. Add a purge stream, Drag 1, from the distillate of the benzene recovery column, C2. 3. Add a purge stream, Drag 2, from the bottoms of the cumene recovery column, C3. The process has two feed streams, a benzene-rich feed, and a propylene-rich feed, which provide the main reactants. The modified process has five exiting streams: 1. Propane-rich product 2. Cumene-rich product 3. Alkylation heavies-rich product 4. Drag 1 5. Drag 2 The two feeds contain a total of six inert species: Water, Ethane, Propane, Isobutane, MCP, and MCH The reactions produce the following species, which are not products: n-Propylbenzene, t-Butylbenzene, and p-Cymene. In addition, all of the toluene in the benzene-rich feed is not reacted. All of the propylene, and 1-butene in the two feeds are consumed in the Alkylation Reactor. All of the m-DIPB and p-DIPB produced in the Alkylation Reactor are consumed in the Trans-alkylation Reactor. Thus, provisions for exiting the process must be provided for: Water, Ethane, Isobutane, MCP, MCH, Toluene, n-Propylbenzene, t- Butylbenzene, and p-Cymene. These exits are primarily as follows: Propane-rich product: Water, Ethane, and Isobutane. Drag 1: MCP, MCH, and unreacted Toluene. Drag 2: n-Propylbenzene, t-Butylbenzene, and p-Cymene. Some losses of Benzene, Cumene, m-DIPB, and p-DIPB occur in the products. One approach to the solution of this exercise is as follows: 1. Using a spreadsheet with the given feeds and the product distribution of the Alkylation Reactor, compute the reactor effluent. As a first approximation, assume a benzene recycle stream of pure benzene at a flow rate sufficient to provide a molar ratio of benzene to propylene in the combined feed of 4. 2. Calculate the distillate and bottoms from the depropanizer using the shortcut FUG method for a column pressure that will give a vapor distillate at 120 o F. 3. Make preliminary calculations for the Trans-alkylation Reactor.
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8-2 Exercise 8.1 – Cumene process with drag (purge) streams Page 2 4. Estimate the benzene recycle to the Trans-alkylation Reactor. 5. Perform a complete process calculation using a simulator. Material balance for Alkylation Reactor in lbmol/hr computed from given feeds and product distribution for the reactor. Using the given molecular weights, the given product distribution in “change in pounds per 100 pounds of propylene” is first converted with a spreadsheet to “change in lbmoles per 1,209.2075 lbmoles of propylene. The benzene recycle stream is a preliminary estimate, assuming pure benzene and a molar ratio of 4 for benzene to propylene. The reactor effluent is then computed. Other components in the benzene recycle will pass through the reactor and add to the reactor effluent.
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