Flash distillation simulating

G6. Use a process simulator to solve the following flash distillation problem Feed is 2 mol%... 

Chapter 2 Appendix A. Computer Simulation of Flash Distillation... 

Multicomponent flash distillation is a good place to start learning how to use a process simulator. The problems can easily become so conplicated that you don t want to do them by hand, but are not so complicated that the working of the simulator is a mystery. In addition, the simulator is unlikely to have convergence problems. Although the directions in this appendix are specific to Aspen Plus, the procedures and problems are adaptable to any process simulator. The directions were written for Aspen Plus V 7.2, 2010 but will probably apply with little change to newer versions when they are released. Additional details on operation of process simulators are available in the book by Seider et al. (2009) and in the manual and help for your process simulator. 

The purposes of this lab are to become familiar with your simulator and to explore flash distillation. This includes drawing and specifying flowsheets and choosing the appropriate physical properties packages. 

In Section 2.7 we looked at solution methods for multiconponent flash distillation. The questions asked in that section are again pertinent for multiconponent distillation. First, what trial variables should we use As noted, because N and Np are required to set up the matrices, in design problems we choose these and solve a number of simulation problems to find the best design. We select the tenperature on every stage Tj because tenperature is needed to calculate K values and enthalpies. We also estimate the overall liquid Lj and vapor Vj flow rates on every stage because these flow rates are needed to solve the conponent mass balances. 

To demonstrate the earlier procedure, the problem of flash distillation is considered. Flash calculations are very common, perhaps one of the most common chemical engineering calculations. They are a key component of simulation packages like Hysys and Aspen. 

Simple distillation is not the same as flashing because the vapor is removed out of contact with the liquid as soon as it forms, but the process can be simulated by a succession of small flashes of residual liquid, say 1% of the original amount each time. After n intervals,... 

In addition to handling the conventional vapor/liquid process operations, the ASPEN library of process models includes solids handling and separation units, a set of generalized reactors, improved flash and distillation unit models and process models from the FLOWTRAN simulator. The user can also include his or her own model or key elements of a model, such as the reaction kinetics, in FORTRAN code. 

Example 5.15 Retrofits of distillation columns by thermodynamic analysis The synthesis of methanol takes place in a tube reactor in section 3 in the methanol plant shown in Figure 5.7. The reactor outlet is flashed at 45°C and 75 bar, and the liquid product (stream 407) containing 73.45 mol% of methanol is fed into the separation section (see Figure 5.8), where the methanol is purified. Stream 407 and the makeup water are the feed streams to the section. Table 5.2 shows the properties and compositions of the streams in section 3. The converged simulations are obtained from the Redlich-Kwong-Soave method to estimate the vapor properties, while the activity coefficient... 

Thermodynamic calculations are used to evaluate vapor-liquid equilibrium constants, enthalpy values, dew points, bubble points, and flashes. Established techniques simulate the heat exchangers and distillation columns, and handle convergence and optimization. 

It is often necessary to add user components to complete a simulation model. The design engineer should always be cautious when interpreting simulation results for models that include user components. Phase equilibrium predictions for flashes, decanters, extraction, distillation, and crystallization operations should be carefully checked against laboratory data to ensure that the model is correctly predicting the component distribution between the phases. If the fit is poor, the binary interaction parameters in the phase equilibrium model can be tuned to improve the prediction. 

Decomposition in elementary simulation blocks. Example an azeotropic distillation column may be decomposed in reboiled stripping column, heat exchanger, three-phase flash separator and reflux splitter. 

Specialised units are used to simulate complex fractionation processes in petroleum refining. Typical configuration consists of a main column with pump-around and side strippers (Fig. 3.14). Among applications, we may cite pre-flash tower, crude atmospheric distillation, or Fluid Catalytic Cracking (FCC) main fractionator. 

After developing a robust steady-state simulation, the next step is the sizing of units whose dynamics is considered. Typically these are units with a significant material inventory, as flash vessels, distillation columns, and liquid-phase reactors. Heat exchangers, pumps and compressors may be considered in many situations as reaching fast steady state. 

Parametric column simulations for the I POAc system were performed with different Damkohler numbers, reflux ratios, reboil ratios as well as total number of stages, (N-I-) and feed tray location, (/). The distillate and bottoms compositions obtained were recorded in transformed composition space. Fig. 6.9 compares the products obtained from column simulations with 30 stages and using different values of r and s at D = 0.25 and D = 0.75. The column feed specification is the same as that to the co-current flash cascade. The flash trajectories provide a good estimate of the product compositions from a continuous column. We also compared the product compositions from column simulations with the flash trajectories in mole fraction space. We found that product compositions from column simulations surrounded the flash trajectories, in agreement with the hypothesis that the flash trajectories lie in the feasible product regions for continuous RD. 

As mentioned during the discussion of the synthesis steps, process simulators are very use fill. They are used to calculate heats of reaction, heat added to or removed from a stream power requirements for pumps and compressors, performance of a flash separator at variou temperatures and pressures, and bubble- and dew-point temperatures associated with distillates and bottoms products, among many other quantities. 

Return to the design of the toluene hydrodealkylation process, as it is presented in Section 4.3. In the reactor section, after heuristics are utilized to set (1) the large excess of H2 in the hydrodealkylation reactor, (2) the temperature level of the quenched gases that enter the feed-product heat exchanger, and (3) the temperature in the flash vessel, the simulator is used to complete the material and energy balances and to examine the effects of these heuristics on the performance of the reactor section. In the distillation section, after heuristics are used to set (1) the quahty of the feed, (2) the use of partial or total condensers, (3) the use of cool-... 

A depropanizer example is provided to illustrate the use of Aspen IPE. The depropanizer is a distillation tower to recover propane and lighter species from a normal-paraffins stream, as shown in Figure 1. The simulation flowsheet and selected results are shown in Appendix I and in the multimedia tutorial on the CD-ROM that contains these course notes ASPEN Tutorials —> Separation Principles -> Flash and Distillation). Also, a copy of the file, RADFRAC.bkp, is provided on the CD-ROM. 

This example involves the single distillation column shown in Figure 1, with its simulation flowsheet and selected results shown in Appendix I and on the multimedia tutorial on the CD-ROM that contains these course notes (ASPEN Tutorials Separation Principles Flash and Distillation). 

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