Aspen Plus extractive distillation

Solvent-based separation through extractive distillation consists of two distillations. The first is an extraction column with two feed (Aspen Distill was used designing this column), while the second is a simple distillation column (the driving force concept was used for designing this column). The design was then verified by rigorous simulation using Aspen Plus . The residue curve map (see Fig. 3) was used... 

This unit can simulate any type of separation processes, as distillation, absorption, stripping, or extraction columns, modelled as cascade of counter-current equilibrium stages. The model Radfrac in Aspen Plus is particular powerful. It is first built on the inside-out algorithm that increased dramatically the robustness in simulating distillation-based operations (Boston, 1980). Columns with multiple feeds, side streams products, stage heaters or coolers, can be treated, as illustrated in Fig. 3.13. The following capabilities are generally available ... 

Figure 8-14. Residue curves for water-ethanol-ethylene glycol for extractive distillation to break ethanol-water azeotrope. Curves generated by Aspen Plus 2004 using NRTL for VLE. Mass...

Figure 8-A3. Aspen Plus screen shot of completed extractive distillation system...

The vapor-liquid equilibrium (VLB) and liquid-liquid extraction (LLE) correlations in Aspen Plus are not always as accurate as possible. This can cause significant errors, particularly near pinch points in distillation columns. If data is available, Aspen Plus will find values of the parameters for any of the VLB or LLE correlations by doing a regression against the data you input. This is illustrated to obtain an improved fit for the non-random two-liquid (NRTL) VLB correlation for the binary system water and isopropanol (IPA). VLB data for water and isopropanol is listed in Table B-1. This system has a minimum boiling azeotrope at 80.46°C. The Aspen Plus fit to the data with NRTL is not terrible, but can be improved. 

Sidestream Column. The RadFrac block in Aspen Plus can be used for the ordinary distillation column with one feed stream and two outlet streams (distillate and bottoms). This block can also be used for multiple feed streams or more than two ordinary outlet streams. For example, the extractive distillation column in Section 3.1 demonstrated that two feed streams can be connected to the RadFrac block. In a similar way, more outlet streams can be drawn out of the RadFrac block as a sidestream column. The sidestream draw-off can be either a liquid stream or a vapor stream. AU these varieties of different columns can easily be configured by adding material streams to the flowsheet with the RadFrac block (see previous Fig. 3.5). 

The Aspen Plus file of this extractive distillation system is exported to Aspen Dynamics after dynamic parameters are specified (equipment sizes). Figure 11.8 shows the control stmcture developed for this system, which is based on the extractive distillation control structure proposed by Grassi. Relay-feedback testing and Tyreus-Luyben tuning of the temperature loops give the controller parameters given in Table 11.2. The temperature controllers have 1 min deadtimes in the loops. Reflux ratios are held constant in each column (3.44 in the extractive column and 1.61 in the methanol column). 

The Aspen Plus files for each of the three extractive distillation systems are exported to Aspen Dynamics after dynamic parameters are specified. Aspen Plus tray sizing is used to fix column diameters. Table 11.4 gives column sizes. Pump heads and valve pressure drops (typically 3 atm) are selected to give reasonable rangeability so that 20% increases... 

The separation in the extractive column depends on the amount of solvent circulating around the system. Figure 12.4 shows that high solvent flowrates reduce the impiuity of chloroform in the distillate acetone product. For each solvent flowrate, there is a nonmonotonic effect of reflux ratio. To achieve the desired distillate purity of 99.5 mol% acetone, the minimum solvent flowrate is 145kmol/h (solvent-to-feed ratio of 1.45). These results are obtained with the impurity of acetone in the bottoms held at 0.1 mol% acetone using the design spec/vary feature of Aspen Plus and manipulating distillate flowrate. 

CHP production The solid residue from the beer column and me concentrated syrup from the evaporation plant can be used to generate heat to the process and electricity in a CHP plant. If necessary, bark from the debarking unit can also be used. The CHP plant (back-pressure steam turbine unit with steam extraction at appropriate pressme levels to supply utility and process steam) was simulated in Aspen Plus (for input dam, see Figure 4.5). Utility steam is used for process heating in, for example, distillation column reboilers and can be merefore replaced by another hot utility at me same temperature. Process steam is used in the process directly as, for example, in the pretreatment step where steam is used for steam explosion to separate to wood components. As steam in this case is an essential part of the process, it cannot be replaced by another utility. 

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