Column distillation simulation problems

This is by no means an exhaustive list of the reasons that computer-based simulations fail. Indeed, in many cases it is a combination of more than one of the above factors that leads to difficulty. In those cases it may be necessary to combine several of the strategies outlined above to solve the simulation problem. Often, however, there is no substitute for trial and error. Haas [chap. 4 in Kister (op. cit.), 1992] offers some additional insight on using simulators to solve distillation column models. 

For example, Figures 4.12 to 4.17 show column profiles for the distillation problem introduced in Example 1.1, which is described in more detail in Examples 4.3 and 4.4. The column was simulated in UniSim Design. 

A real Process Flow Diagram (PFD) must be translated in a scheme compatible with the software capabilities and with the simulation goals. The flowsheet scheme built up for simulation purposes will be called in this book Process Simulation Diagram (PSD). PSD is in general different from PFD. For example, some simple units, as for pressure or temperature change, may be lumped in more complex units (from simulation viewpoint). Contrary, complex units, as distillation columns or chemical reactors, may need to be simulated as small flowsheets. Hence, a preliminary problem analysis is necessary. The steps in defining a simulation problem are ... 

In the design or operation of a distillation column, a number of variables must be specified. For both design and simulation problems we usually specify column pressure (which sets the equilibrium data) feed conposition, flow rate and feed tenperature or feed enthalpy or feed quality and tenperature or enthalpy of the reflux liquid. The usual reflux condition set is a saturated liquid reflux. These variables are listed in Table 3-1. The other variables set depend upon the type of problem. 

For the distillation performance problem in Section 19.2. assume that scale-down occurs while maintaining constant boil-up rate. Determine the conditions in the reboiler, column, and condenser for these operating conditions. A process simulator should be used for the distillation column. 

One might think that this problem can be very easily overcome by simply ratioing the flowrates of the two fiesh reactant feeds. This strategy works in computer simulations, but it does not work in a real plant environment. The reasons why ratio schemes are not effective are inaccuracies in flow measurements, which are always present, and/or changes in the compositions of the feedstreams. Either cause will result in an imbalance of the stoichiometry. Therefore, it is necessary to have some way to determine the amount of at least one of the reactants inside the column so that feedback control can be used to adjust a fresh feed flowrate. Sometimes temperatures or liquid levels can be used. Sometimes a direct composition measurement on a tray in the column is required. This issue is the heart of the reactive distillation control problem and will be quantitatively smdied in detail in subsequent chapters. 

Design and Operation of Azeotropie Distillation Columns Simulation and design of azeotropic distiUation columns is a difficult computational problem, but one tnat is readily handled, in most cases, by widely available commercial computer process simulation packages [Glasscock and Hale, Chem. Eng., 101(11), 82 (1994)]. Most simida-... 

The principle of the perfectly-mixed stirred tank has been discussed previously in Sec. 1.2.2, and this provides essential building block for modelling applications. In this section, the concept is applied to tank type reactor systems and stagewise mass transfer applications, such that the resulting model equations often appear in the form of linked sets of first-order difference differential equations. Solution by digital simulation works well for small problems, in which the number of equations are relatively small and where the problem is not compounded by stiffness or by the need for iterative procedures. For these reasons, the dynamic modelling of the continuous distillation columns in this section is intended only as a demonstration of method, rather than as a realistic attempt at solution. For the solution of complex distillation problems, the reader is referred to commercial dynamic simulation packages. 

Hydraulic analysis of the Aspen Plus simulator produces thermodynamic ideal minimum flow and actual flow curves for rigorous distillation column simulations. These types of calculations are performed for RADFRAC columns. Using the input summary given in problem 4.48 construct the stage-flow curves. Assess the thermodynamic performance of the column. 

The results from the rigorous model with the inputs specified as above show a flow rate of 1084.5 kg/h of n-hexane in the distillate product. This exceeds the requirements calculated from the problem statement (1065.5 kg/h). The simplest way to get back to the required specification is to use it directly as a specification for the column. From the Design tab on the column window, we can select Monitor and then Add spec to add a specification on the distillate flow rate of n-hexane, as shown in Figure 4.50. This specification can then be made active, and the bottoms flow rate specification can be relaxed. When the simulation is reconverged, the bottoms flow rate increases to 19,350 kg/h, and the n-hexane in the distillate meets the specification flow rate of 1065.5 kg/h. 

Simulate the vinyl chloride process (Problem 5.4) using Aspen Plus. Take the feed at room temperature and 20 psia. Operate the direct chlorination reactor at 65°C and 560 kPa. A distillation column removes the trichloroethane and the rest of the stream is sent to the furnace. Heat the stream to 1500 F so pyrolysis takes place. Cool the effluent from the furnace, and recycle the vapor (mostly HCl). Send the hquid (vinyl chloride and ethylenedichloride) to a distillation column for separation. 

Simulate the ethanol process (Problem 5.6) using Aspen Plus. The feed streams are at 1 atm and room temperature, but the reactor operates at 960 psia and 570°F. Thus, you must heat the reactor feed, and after the reaction occurs you must cool the product. The first splitter is a vapor-liquid separator (you choose the temperature that will separate ethanol from ethylene), and the remaining splitters are distillation columns. You will have to decide on the number of stages and reflux ratio using DSTWU first might be useful. 

---