Batch process flowsheet

To illustrate the difference between SSN and STN, consider a simple batch process flowsheet shown in Fig. 1.9 (Ierapetritou and Floudas, 1998). The process involves 3 consecutive tasks, i.e. reaction, mixing and purification, and 4 consecutive states. The corresponding STN for this flowsheet is shown in Fig. 1.10a, whilst the SSN is shown in Fig. 1.10b. The existence of the arc between state 1 and state 2 signifies the presence of a task, which corresponds to mixing in this particular case. 

Figure 4.16 Final flowsheet for the production of butadiene sulfone in a batch process.

Process Flowsheet Batch vs. Continuous operation Detailed unit operations selection Control and operation philosophy Information above plus process engineering design principles and experience... 

The goal of a conceptual design for a continuous process is to select the process units and the interconnections between these units, identify the dominant design variables and estimate the optimum design conditions, and identify the process alternatives and find the best four or so alternatives. For batch processes, we must also decide which units should be batch and which should be continuous, whether or not some process operations should be carried out in the same process unit or separate units, whether or not parallel units should be used, and how much intermediate storage is required. Thus, batch processes require more decisions to fix the structure of the flowsheet (there are more alternatives to consider). Since there are many situations in which it must be decided whether to develop a batch or a continuous process, both procedures should be present in a general conceptual design code, whereas the current trend is to develop separate codes for batch processes. 

As was described in the review of previous work, over the last ten years MINLP optimization models have been reported for the synthesis of process flowsheets, heat-exchanger networks, separation sequences, reactor networks, utility plants, and design of batch processes. Rather than describing in detail each of these works, we will briefly highlight several examples from our research group at Carnegie Mellon to illustrate the capabilities and the current limitations of the MINLP approach. 

Flowsheets drawn up for batch processes normally show the quantities required to produce one batch. If a batch process forms part of an otherwise continuous process, it can be shown on the same flowsheet, providing a clear break is made when tabulating the data between the continuous and batch sections, i.e., the change from kg/h to kg/batch. 

Many of the high-value API compounds are formed in multistep syntheses starting from compounds that are themselves specialty chemicals. The patents that are cited give the preparation in the form of a laboratory recipe rather than a process flowsheet, and hence the chemist s recipe must be scaled up to the production recipe. A decision on whether to use batch or continuous production must also be made. These are therefore difficult design problems. 

The overall model of an SMB process is developed by linking the models of individual chromatographic columns (Section 6.2). As with the chromatographic batch process, the plant set-up of Fig. 6.35 is converted into a simulation flowsheet. Figure 6.36 shows the SMB column model. 

We elected to wash the oxide using a batch process. This allows for greater process flexibility and enables us to segregate material that might contain impurities that need to be fed at a reduced rate to the plant. The batch system is also easy to automate and does not require constant operator intervention. The overall washing process is best described by the flowsheet shown in Figure 1. 

In this chapter, the principles behind the use of several widely used flowsheet simulators are introduced. For processes in the steady state, these include ASPEN PLUS, HYSYS.Plant, CHEMCAD, and PRO/n. For batch processes, these include BATCH PLUS and SUPER-PRO DESIGNER. 

As in the steady-state simulation of continuous processes, it is convenient to convert from a process flowsheet to a simulation flowsheet. To accomplish this, it is helpful to be familiar with the library of models (or procedures) and operations provided by the simulator. For example, when using SUPERPRO DESIGNER to simulate two fermentation reactors in series, the process flowsheet in Figure 4.25a is replaced by the simulation flowsheet in Figure 4.25b. In BATCH PLUS, however, this conversion is accomplished without drawing the simulation flowsheet, since the latter is generated automatically on the basis of the recipe specifications for each equipment item. 

When a SUPERPRO DESIGNER flowsheet is defined to be in continuous mode, streams are reported on a per hour basis. Scheduling information is not required, and no overall batch time is calculated. Individual batch processes can be inserted into the flowsheet, with their batch and turnaround times specified. 

Naot, I., and D.R. Lewin, Analysis of Process Dynamics in Recycle Systems Using Steady State Flowsheeting Tools, Proc. 4th IFAC Symposium on Dynamics and Control of Chemical Reactors, Distillation Columns and Batch Processes (DYCORD 95), Helsingor, Danish Automation Society, Copenhagen (1995). 

Monomer conversion can be adjusted by manipulating the feed rate of initiator or catalyst. If on-line M WD is available, initiator flow rate or reactor temperature can be used to adjust MW [38]. In emulsion polymerization, initiator feed rate can be used to control monomer conversion, while bypassing part of the water and monomer around the first reactor in a train can be used to control PSD [39,40]. Direct control of surfactant feed rate, based on surface tension measurements also can be used. Polymer quality and end-use property control are hampered, as in batch polymerization, by infrequent, off-line measurements. In addition, on-line measurements may be severely delayed due to the constraints of the process flowsheet. For example, even if on-line viscometry (via melt index) is available every 1 to 5 minutes, the viscometer may be situated at the outlet of an extruder downstream of the polymerization reactor. The transportation delay between the reactor where the MW develops, and the viscometer where the MW is measured (or inferred) may be several hours. Thus, even with frequent sampling, the data is old. There are two approaches possible in this case. One is to do open-loop, steady-state control. In this approach, the measurement is compared to the desired output when the system is believed to be at steady state. A manual correction to the process is then made, based on the error. The corrected inputs are maintained until the process reaches a new steady state, at which time the process is repeated. This approach is especially valid if the dominant dynamics of the process are substantially faster than the sampling interval. Another approach is to connect the output to the appropriate process input(s) in a closed-loop scheme. In this case, the loop must be substantially detuned to compensate for the large measurement delay. The addition of a dead time compensator can... 

Recall that solids do not flow well. Solids are difficult to convey between units, and solids do not flow downward through cylinders as well as liquids. In practice the ZnO adsorber and oxidizer are batch processes rather than continuous processes as implied by the flowsheet above. The adsorber is initially packed with ZnO. After all the ZnO has converted to ZnS, the solids are dumped into the oxidizer and the adsorber is recharged with ZnO. 

Batch process equipment must be properly configured in a plant, process, or unit operation in order to be operated and maintained in a reasonable manner. A flowsheet for a general batch plant is shown in Fig. 22.18. The Instrument Society of America (ISA) SP-88 standard deals with the terminology involved in batch control (Parshall and Lamb, 2000 Strothman, 1995). 

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