Flowsheet program

Many aspects of a process can be evaluated with a chemical process flowsheet program such as Aspen. These programs handle mass balances and heat loads on each component with great accuracy. Separation components can also be handled accurately as long as they are rather straightforward. Cost databases also exist on these programs that allow rapid costing of many components. 

However, the chemical reactor still requires considerable thinking and hand calculations because there are few situations where databases exist on reaction kinetics. Especially for catalytic reactions, data either do not exist or are uniehable far from the conditions under which the data were obtained. The number of possibUities of catalyst chemical and geometrical properties are so large that no flowsheet programs can handle anything beyond extremely simple situations. 

Several companies (D.B.R. Oilphase/Schlumberger, Infochem Computer Services, Ltd., Calsep) have commercially available computer programs (DBR hydrate, Multiflash, PVTSim) for the prediction of hydrate properties, and such methods are incorporated into process flowsheeting programs such as ASPEN , HYPERCHEM , and SIMCI . Researchers in the CSM laboratory (Sloan and Parrish, 1983 Sloan et al., 1987 Mehta and Sloan, 1996) generated new parameters for the prediction of si, sll, and sH hydrates, which were incorporated into the program, CSMHyd. 

To gain the maximum benefit from the use of a flowsheet program, the operator/designer must be adequately trained. A suitable program will have 20-30 standard units available, numerous equation-solving procedures, control facilities and probably optimization facilities. The unit-equipment subroutine must adequately represent the process equipment, recycle streams need to be specified, and suitable solution convergence is required. For the effective use of CAD packages, it... 

Since pure equation-based models involving as many as a few hundred thousand equations arise and require solution, ways are needed to ensure that this activity can occur with some degree of success. Perhaps the oldest of such approaches comes with flowsheeting programs. 

We cannot go into all of the details of flowsheeting program construction and application here. Refer to Chapter 5 and the to supplementary references at the end of this chapter for further information. However, let us look at an example of a very simple process with recycle to compare the simultaneous equation and modular solution techniques just to illustrate the concepts. 

System is shown in Fig. 15. A flowsheeting program, HYSIS or other comparable one, is used to develop the process flow diagram. The flowsheeting program determines the operating conditions and the utilities required—steam and cooling water. Then, a value-added economic analysis is performed to estimate the profitability of the plant. If the profitability is acceptable. 

In the period 1970-80 s the major engineering bureaus, as well as some large manufacturing companies in refining and petrochemical industries developed in-house flowsheeting programs. Mostly adopted the modular sequential architecture. However, some were based on the equations oriented approach, as SPEEDUP at Imperial College in London (UK) and TISFLO at DSM in The Netherlands. 

Aylott, M.R., Ponton, J.W., Lott, D.H. (1985) Development of a dynamic flowsheeting program. Proceedings of Process Systems Engineering, IChemE Symposium Series 92,... 

The development and the design of new processes are today extensively based on mathematical modelling and simulation. The entire process is modelled through combining individual unit operation models. Several flowsheeting programs (Aspen+ , HySim , ChemCAD Pro II etc.) have already found their place in engineering... 

The parameters for these phenomena models are obtained from literature, estimation methods or experiments with special equipment designed for these purposes. Parameter estimation routines are sometimes available in flowsheeting programs mentioned above, but more often specific programs are used. 

At the laboratory of Chemical Engineering at HUT there are facilities for VLE measurements, which have also been used to retrieve parameters that are of interest in the dimerization process. The Laboratory of Industrial Chemistry at HUT has studied reaction kinetics and several kinetic models have been developed. The process models have been developed using the flowsheeting program Flowbat. 

It is important to remember that a simulation results are only as good as the mathematical models on which those are based. However, there are still many items that are neglected or treated insufficiently in mathematical models, such as ageing and deactivation of the catalyst and the stability of the products under actual production conditions. Many commercial flowsheeting programs still rely totally on idealistic behaviour. Many programs have only very limited number of reactor types like tube and CSTR. Common multiphase reactors where mass transfer phenomena also plays important role are missing. Also idealized separation models are common. 

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