Process simulation—batch operations

Example 14.1 Consider again the chlorination reaction in Example 7.3. This was examined as a continuous process. Now assume it is carried out in batch or semibatch mode. The same reactor model will be used as in Example 7.3. The liquid feed of butanoic acid is 13.3 kmol. The butanoic acid and chlorine addition rates and the temperature profile need to be optimized simultaneously through the batch, and the batch time optimized. The reaction takes place isobarically at 10 bar. The upper and lower temperature bounds are 50°C and 150°C respectively. Assume the reactor vessel to be perfectly mixed and assume that the batch operation can be modeled as a series of mixed-flow reactors. The objective is to maximize the fractional yield of a-monochlorobutanoic acid with respect to butanoic acid. Specialized software is required to perform the calculations, in this case using simulated annealing3. 

The counterparts of dissolving particles are the processes of precipitation and crystallization the description and simulation of which involve several additional aspects however. First of all, the interest in commercial operations often relates to the average particle size and the particle size distribution at the completion of the (batch) operation. In precipitation reactors, particle sizes strongly depend on the (variations in the) local concentrations of the reactants, this dependence being quite complicated because of the nonlinear interactions of fluctuations in velocities, reactant concentrations, and temperature. 

Dynamic simulations of a batch-operated scrubber with pump-around will be presented showing the effect of process configuration on destruction demands, where the method of hypochlorite destruction is a fixed-bed catalytic reactor. 

Batch processing is used primarily when the reaction time is long or the required daily production is small. The same batch equipment often is used to make a variety of products at different times. Otherwise, it is not possible to generalize as to the economical transition point from batch to continuous operation. One or more batch reactors together with appropriate surge tanks may be used to simulate continuous operation on a daily or longer basis. 

Industrial-scale adsorption processes can be classified as batch or continuous. In a batch process, die adsorbent bed is saturated and regenerated in a cyclic, operation. In a continuous process, a countercurrent staged contact between lire adsorbent and die feed and desorbent is established by cidier a true or a simulated recirculation of die adsorbent. The efficiency of an adsorption process is significantly higher in a eoiuinuous mode of operation than in a cyclic batch mode. For difficult separations, batch operation may require 25 times more adsorbent inventory and twice die desorbent circulation rate than does a continuous operation. In addition, in a batch mode, the four functions of adsorption, purification, desorption, and displacement of the desorbent from the adsorbent are inflexibly linked, wtiereas a continuous mode allows mure degrees of freedom with respect to these functions, and thus a better overall operation. 

Several simulations have been carried out under process parameter uncertainties e.g. in pre-exponential rate constant (ko) and activation energy (Ea). In all case studies we considered 10 time intervals when reactor temperature and switching time are optimized while minimizing the final batch operation time. Results, reported in the value of minimum batch time to obtain the desired product C and the amount of the desired product C at the end of batch operation, from on-line dynamic optimization strategy are also compared with those from the off-line strategy. 

We study the separation of 77-hexane-ethyl acetate mixture by using acetonitrile as a heavy heterogeneous entrainer. The simulation of the process is performed with the batch process simulator ProSimBatch [10]. It enables to evaluate operational parameters like the entrainer amount that are not provided by the feasibility and synthesis analysis The column model consists of usual plate by plate Material balance, Equilibrium, Summation of fractions and Heat balance... 

The principle of integral process development [26] covers much more than just the optimization of a process. This approach begins with computer-aided decision procedures in the conception phase. Tools are available in which the process structure is suggested, for example should the process be a batch or a continuous operation The software tool for process synthesis PROSYN uses databases which include knowledge of experts, material data and calculation models for unit operations. Interfaces to process simulation tools such as ASPENPLUS and material databases are also supplied. PROSYN also delivers an economic evaluation of the future production process. 

Three simulated tests were run using only nitric and formic acids. In each case, the reaction began promptly and proceeded smoothly. After the second test, the denitrated material was evaporated and additional nitric acid added to simulate the tandem semi-batch operation to be used with actual process solution. At the end of formic acid feed for Test 3, the material was refluxed for 2 hours and evaporated to 2500 L to simulate the final canyon product batch. A final formic acid denitration reduced the acidity of the simulated concentrate to <1M HNO3. 

Unsteady-state or dynamic simulation accounts for process transients, from an initial state to a final state. Dynamic models for complex chemical processes typically consist of large systems of ordinary differential equations and algebraic equations. Therefore, dynamic process simulation is computationally intensive. Dynamic simulators typically contain three units (i) thermodynamic and physical properties packages, (ii) unit operation models, (hi) numerical solvers. Dynamic simulation is used for batch process design and development, control strategy development, control system check-out, the optimization of plant operations, process reliability/availability/safety studies, process improvement, process start-up and shutdown. There are countless dynamic process simulators available on the market. One of them has the commercial name Hysis [2.3]. 

To overcome the need for large amounts of material and the associated handling problems, agglomerates are sometimes crushed and reused as fresh feed which introduces a completely new set of problems. If, on the other hand, a continuous process is simulated in a batch operation, the influence of recycle and the question of process equilibrium remain unsolved problems. 

Be able to use the batch process simulators to cany out material and energy balances, and to prepare an operating schedule in the form of a Gantt chart for the process. 

In BATCH PLUS, every simulation is for an overall batch process, with stream values always reported on a per batch basis. Continuous operations, however, can be inserted. For these units, a feed is loaded, the vessel is filled to its surge volume, and an effluent stream immediately begins to transfer the product downstream. This differs from normal batch operation, which involves loading all of the feed and completing the processing steps before unloading. Specific units in BATCH PLUS, such as the Fermenter, can also operate as fed-batch. In such operations, a feed is added continuously to the batch while an operation is taking place. 

Figure 4.27 BATCH PLUS simulation for Example 45. (a) Operations recipe for the process-BATCH PLUS output, (b) Last three columns of stream table (for the complete table, see ASPEN —> TutoM — Batch Process Simulation — tPA Manufacture on the CD-ROM. (c) Mixing Tank report (fa Fermenter 1 and 2 reports, see the CD-ROM), (d) 3-batch Gantt chart.

Similarly, the reports for Fermenters 1 and 2 are shown in ASPEN — Tutorials —> Batch Process Simulation tPA Manufacture on the CD-ROM, indicate maximum volumes of 22.8 L and 322 L after operations 1.7 and 1.8. On this basis, 40-L and 400-L vessels are selected for Fermenters 1 and 2. 

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