Method for flowsheeting

C.L. Chen, Ph.D. thesis, A class of successive quadratic programming methods for flowsheet optimization, University of London, 1988. 

Chen, C.L., A Class of Successive Quadratic Programming Methods for Flowsheet Optimisation. PhD Thesis, (Imperial College, London, 1988). 

Equation (8) is of the form of the Newton-Raphson method. The A(X) matrix, however, is not necessarily the Jacobian, J(X). Just how the A(X) is set up depends on the application. Bending and Hutchison (88) developed the method for pipe flow networks. Hutchison and Shewchuk (89) applied the method to multiple distillation towers. Gorczynski and Hutchison (90) detail the method for flowsheeting systems. Quasilin (91) is a flowsheeting system based on this approach. MULTICOL (92) appears to solve interconnected columns by means of this approach as well. 

Ihese difficulties vanish if the system equations are simply collected and solved for all unknown variables. Several powerful equation-solving algorithms are available in commercial programs like Maple , Mathematica , Matlab , Mathcad , and E-Z Solve that make the equation-based approach competitive with the sequential modular approach. Many researchers in the field believe that as this trend continues, the former approach will replace the latter one as the standard method for flowsheet simulation. (Engineers are also working on simultaneous modular methods, which combine features of both sequential modular and equation-based approaches. We will not deal with these refinements here, however.)... 

Selection of a flotation technique for gold preconcentration depends very much on the ore mineralogy, gangue composition and gold particle size. There is no universal method for flotation of the gold-bearing minerals, and the process is tailored to the ore characteristics. A specific reagent scheme and flowsheet are required for each ore. 

The next section will reflect on common conceptual process design practices for overall flowsheet development. The remainder of this chapter reviews recent developments in process synthesis methods for reaction and separation systems that systematically guide the design decision-making process toward novel and improved designs. 

Methods for making higher strength P205 acid have been known for a long time. The basic hemihydrate-dihydrate process shown in the Hydro Fertilizer flowsheet, Fig. 23.12, is similar to the initial process attempted in 1931 at the Cominco plant at Trail, BC. The hemihydrate-dihydrate process failed there, mostly because of inadequate filters, but the Dorr dihydrate process did emerge successfully. 

A better method for the presentation of data on flowsheets is shown in Figure 4.2. In this method each stream line is numbered and the data are tabulated at the bottom of the sheet. Alterations and additions can be easily made. This is the method generally used by professional design offices. A typical commercial flowsheet is shown in Figure 4.3. Guide rules for the layout of this type of flowsheet presentation are given in Section 4.2.5. 

Frequently process plants contain recycle streams and control loops, and the solution for the stream properties requires iterative calculations. Thus efficient numerical methods for convergence must be used. In addition, appropriate physical properties and thermodynamic data have to be retrieved fi om a data base. Finally, a master program must exist that links all the building blocks, physical property data, thermodynamic calculations, subroutines, and numerical subroutines, and that also supervises the information flow. You will find that optimization and economic anafy-sis are really the ultimate goal in the use of flowsheet codes. 

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 most characteristic task of CAPE and historically the most popular is process synthesis. The development of process synthesis is confirmed by the use of a very broad spectrum of methods for building a process flowsheet. 

A different approach to process synthesis is offered by means-ends method. It is based on the observation that the purpose of material processing is to apply various operations in such a sequence that the differences in properties between the raw materials and the products are systematically eliminated. As a result, the raw materials are transformed into the desired products. The means-ends method starts with an initial state and successively applies transformation operators to produce intermediate states with fewer differences until the goal state is reached. The hierarchy for the reduction of property differences is as follows identity, amount, concentration, temperature, pressure, and finally form. This property changing method has its limitations, as it ignores the influences and the impacts on other properties. Moreover, the search method takes an opportrmistic approach, which cannot guarantee the generation of a feasible flowsheet. The means-ends analysis approach has been used as a systematic process synthesis method for overall process flowsheet synthesis, as well as for the more detailed case of a separation system to resolve the concentration differences in nonideal systems that include azeotropes. 

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. 

Evolutionary Methods. The objective of all evolutionary methods for selecting and sequencing separation processes is a systemelic approach to an optimal system. These methods all mek to improve the existing flowsheet. Stephanopoulos describes the elements of an evolutionary strategy as follows ... 

Improvements in methods for developing optimal flowsheets for the overall process, including a more sophisticated approach to incorporating separations into the flowsheet. 

In addition to attempting to provide heuristic roles for choosing Ihe base case, process synthesis research attempts also (0 develop systematic methods for the evolution of the flowsheet from the base case to the... 

The remainder of the book deals with scrubbing processes, the most publicized of the control methods. Process flowsheets for several absorbents are proposed and their applications in emission control are discussed. 

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