Process unit flowsheet

This section on flowsheets basically aims to provide some illustrative examples of the use of the various mineral processing unit operations that have been described. A general flowsheet involving almost all the unit operations pertinent to mineral processing is shown in Figure 2.32. The others refer specifically to beach sands, lead-zinc concentration, molybdenum, and the rare earths. 

Figure 3.2 shows the flowsheet for the illustrative example. The data for the example is given in Table 3.1, where i, fc, and is are consecutive processing units, while d2,3 is a dedicated intermediate storage vessel between processing units and is. The time horizon of interest in this example is 9 h. 

Ql Which process units should be used in the process flowsheet ... 

The answers to questions Ql and Q2 provide information on the topology/structure of the process flowsheet since they correspond to the selection of process units and their interconnections, while the answer to question Q3 provides information on the optimal values of the design parameters and operating conditions. 

By assigning, for instance, binary variables to represent the existence or not of process units in a total process system, the potential matches of hot and cold streams so as to reduce the utility consumption or the existence of trays in a distillation- based separation system, the resulting total number of binary variables can grow to 1000 or more which implies that we have to deal with a large combinatorial problem (/. ., 21000 process flowsheets). 

Step 1 Representation of Alternatives A superstructure is postulated in which all process alternative structures of interest are embedded and hence are candidates for feasible or optimal process flowsheet(s). The superstructure features a number of different process units and their interconnections. 

Conceptually, the representation of alternative process flowsheet(s) is based on elementary graph theory ideas. By representing each unit of the superstructure as a node, each input and output as a node, the interconnections among the process units as two-way arcs, the interconnections between the inputs and the process units as one-way arcs, the interconnections between the process units and the outputs as one-way arcs, and the interconnections between the inputs and the outputs as one-way arcs, then we have a bipartite planar graph that represents all options of the superstructure. 

As an illustration let us consider the representation of alternative flowsheets that have one input, two outputs, and three process units. The complete superstructure is shown in Figure 7.5. 

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. 

Process design for continuous processes is carried out mostly using steady-state simulators. In steady-state process simulation, individual process units or entire floivsheets are calculated, such that there are no time deviations of variables and parameters. Most of the steady-state floivsheet simulators use a sequential modular approach in which the flowsheet is broken into small units. Since each unit is solved separately, the flowsheet is worked through sequentially and iteration is continued until the entire flowsheet is converged. Another way to solve the flowsheet is to use the equation oriented approach, where the flowsheet is handled as a large set of equations, which are solved simultaneously. 

Process synthesis tries to find the flowsheet and equipment for specified feed and product streams. We define process synthesis as the activity allowing one to assume which process units should be used, how those units will be interconnected and what temperatures, pressures and flow rates will be required [2.15, 2.16[. 

The combination of unit processes 25 Flowsheet for firework manufacture... 

The economic construction and efficient operation of a process unit will depend on how well the plant and equipment specified on the process flowsheet is laid out. 

Whatever the code used to solve material and energy balance problems, you must provide certain input information to the code in an acceptable format. All flowsheeting codes require that you convert the information in the flowsheet (see Fig. 5.4) to an information flowsheet as illustrated in Fig. 5.5, or something equivalent. In the information flowsheet, you use the name of the mathematical model (subroutine for modular-based flowsheeting) that will be used for the calculations instead of the name of the process unit. 

The sequential modular method of flowsheeting, as mentioned previously, is the one most commonly encountered in computer packages. A module exists for each process unit in the information flowsheet. Given the values of each input stream composition, flow rate, temperature, pressure, enthalpy, and the equipment parameters, the module calculates the properties of its outlet streams. The output stream for a module can become the input stream for another module for which the calculations proceed until the material and energy balances are resolved for the entire process. 

The phenomena-driven method for the process synthesis analyzes not the processing units, the so-called building blocks, but the phenomena that occur in those blocks. This method is based on opportunistic task identification and integration. It was applied to separation process synthesis, based on thermodynamic phenomena. It explored the relationship between the physicochemical properties, separation techniques, and conditions of operation. The number of alternatives for each separation task is reduced by systematically analyzing these relationships. Then, possible flowsheets are produced with a list of alternatives for the separation tasks. 

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