Prefractionation units

Most of the benzene in the gasoline pool comes from the reformer unit (reformate). To reduce the reformate s benzene, one must modify the feedstock quality and/or operating conditions. Benzene s precursors in the reformer feed (C, and C ) can be prefractionated and sent to an isomerization unit. The reformer operating pressure can be reduced... 

From the above discussion it can be observed that the initial diluted feed can be sent to the water-column and not to the ethanol recovery column (azeotropic distillation). In consequence, this alternative has only two columns, and has been occasionally presented as innovative. Doherty Malone (2001) discusses in detail all the alternatives discovered along the time using benzene as entrainer. The twO-column sequence is not the most economical because of large entrainer recycle and expensive columns. It seems that the two-column sequence with prefractionator, in total three units, offers the best compromise between investment and operation (solvent recycle) costs. 

A simulation of a DWC based on the equivalent Petlyuk column has two recycle loops, which requires two recycle blocks (R1 and R2), as shown in Figure 9.6b. The prefractionator column (Pref) has neither reboiler nor condenser, which can be simulated using an absorber column. This column has two recycle streams, which includes a recycled vapor stream to a prefractionator (Vapor pre in) at the bottom, and the recycled liquid stream to the prefractionator (Liquid pre in) at the top. These two streams need the initial estimates to solve the Pref column. A distillation unit for the main column, the material streams. Feed, Vapor pre out. Liquid pre out. Liquid Main out. Vapor Main out. Distillate, Side, and Bottom for Main column are added to the flow sheet. Two recycle blocks need to be added to the flow sheet one recycle block (Rl) for the Liquid Main out and another (R2) for Vapor Main out to initialize the recycle stream. Once the Pref is solved, the main column is simulated with an estimated number of stages and stage locations from the shortcut calculation. 

Large polymers of the type found in plant cell walls are difficult to analyze directly. Therefore, most studies use prefractionation by size-exclusion chromatography (SEC) or reduction to smaller units by partial acidic hydrolysis or enzymolysis. For mixtures of large polymers such as dextrans ionized by MALDI (Figure 19.5), there is a fall in signal intensity with increasing mass that has been attributed to signal suppression by the smaller. 

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