Synthesis network

In order to demonstrate the applicability of the fmegoing feasibility criteria, we now revisit the dephenolization case study. As has been drscussed die synthesis ought to start at die pinch and proceed in two directions die rich and the lean ends. Let s begin widi die rich end of the problem. Alxive the pinch, we have two waste streams and two MSAs. Hence, minimum number of exchangers above the pinch can be calculated according to Eq. (5.7b) as 

Immediately above the pinch, the number of rich streams is equal to the numbi of the MSAs, thus d feasibility critmon given by Eq. (5.8a) is satisfied. Tt second feasibility crifmion (Eq. 5.12a) can be checked through Fig. 5.7. By criirpming the valu of with G,- for each pot tial pinch match, one can readily deduce that it is feasible to match Si with either Ri or R2 immediately above tte pinch. Nonetheless, while it is possible to match S2 with R2, it is infeasible to pair S2 with Ri immediately above the pinch. Therefore, one can match Sj with Ri and S2 with R2 as rich-end pinch exchangers. 

CHAPTER FIVE Synthesis of mass-exchange networks 

M)kg/s C2sl.00kg/s L /m,=250i[gfs L2 =lJ6kg/s Fiflure 5.7 Feasibility ciileria above the pinch for the dephenolization example. 

Fll rc 5.9 Feasibility criteria below the pinch for the dephenolization example. 

When two streams are paired, the exchangeable mass is die lower of the two loads of the streams. For instance, the mass-exchange loads of Ri and S are 0.0664 and 0.0380 kg/s, respectively. Hence, the mass exchanged from Ri to Si is 0.0380 kg/s. Owing to this match, the capacity of Si above the pinch has been completely exhausted and S] may now be eliminated from any further consid tion in the rich-end subproblem. Similarly, 0.0132 kg/s of phenol will be transferred from R2 to S2 thereby fulfilling the required mass-exchange duty for R2 above the pinch. We are now left with two streams only above the pinch 

Increased heat utilization does not always mean a trade-off, and many studies have shown a reduction in energy consumption as well as capital cost which in itself is a remarkable recommendation for a systematic approach to network design. 

Kirk-Othmer Encyclopedia of Chemical Technology (4th Edition) 

Simulation tools are available for sizing and analyzing plants. However, these tools do not replace the designer as the architect of the plant because selection of process and the sequenciag of units are the designers choices. The same is tme for heat-exchanger networks. Most of the commercial process simulator companies market computer modules that perform some of the tedious steps ia the process but none is able to remove the designer from the process. 

Alternative representations of stream temperature and energy have been proposed. Perhaps the best known is the heat-content diagram, which represents each stream as an area on a graph (3) where the vertical scale is temperature, and the horizontal is heat capacity times flow rate. Sometimes this latter quantity is called capacity rate. The stream area, ie, capacity rate times temperature change, represents the enthalpy change of the stream. 

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Cerda, J., Westerberg, A. W., Mason, D., and Linnhoff, B., Minimum Utility Usage in Heat Exchanger Network Synthesis—A Transportation Problem, Chem. Eng. ScL, 38 373 1983. 

Saboo, A. K., Morari, M., and Colberg, R. D., RESHEX An Interactive Software Package for the Synthesis and Analysis of Resilient Heat Exchanger Networks II. Discussion of Area Targeting and Network Synthesis Algorithms, Computers Chem. Eng., 10 591, 1986. 

Wood, R. M., Wilcox, R. J., Euid Grossmann, I. E., A Note on the Minimum Number of Units for Heat Exchanger Network Synthesis, Chem. Eng. Commun., 39 371, 1985. 

For the purpose of network synthesis, the overall heat-transfer coefficient is usually idealized as a constant value. This iadependence of the heat-transfer coefficient makes possible the iterations necessary to solve the network problem. Usually, the overall heat-transfer coefficient for each exchanger (match) is defined as... 

Combinatorial. Combinatorial methods express the synthesis problem as a traditional optimization problem which can only be solved using powerful techniques that have been known for some time. These may use total network cost direcdy as an objective function but do not exploit the special characteristics of heat-exchange networks in obtaining a solution. Much of the early work in heat-exchange network synthesis was based on exhaustive search or combinatorial development of networks. This work has not proven useful because for only a typical ten-process-stream example problem the alternative sets of feasible matches are cal.55 x 10 without stream spHtting. 

The eadiest use of heat-exchange network synthesis was in the analysis of cmde distillation (qv) units (1). The cmde stream entering a distillation unit is a convenient single stream to heat while the various side draws from the column are candidate streams to be cooled in a network. So-called pumparounds present additional opportunities for heating the cmde. The successful synthesis of cmde distillation units was accompHshed long before the development of modem network-synthesis techniques. However, the techniques now available ensure rapid and accurate development of good cmde unit heat-exchange networks. 

Heat Exchange Technology, Network Synthesis" in ECT3rd ed., Suppl. Vol., pp. 521—545, by E. Hohmann, California State Polytechnic University. 

R. A. Greenkorn, L. B. Koppel, and S. Raghavan, "Heat Exchanger Network Synthesis—A Thermodynamic Approach," 71 stAlChE Meeting, Miami, Fla., 1978. 

K. C. Hohmann and F. J. Lockhart, "Optimum Heat Exchanger Network Synthesis," AICHE 82nd National Meeting, Adantic City, N.J., 1976. 

J. J. Siirola, "Status of Heat Exchanger Network Synthesis," AIChE National Meeting, Tulsa, OHa., 1974. 

Shenoy, U. V. (1995). Heat Exchange Network Synthesis Process Optimization by Energy and Resource Analysis. Gulf Phb. Co., Houston, TX. 

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