Constraint control of distillation processes

Abstract

Abstra+ThereIS a growmg mterest to design and operate chemical processes for reduced energy consumphon As an example a compmson IS made between the dlstdlation of bmary rmxtures m a coaventmnal dxxtillation column, a vapour recompresslon system and a two column heat mtegrated system For ali three con@rahons constramt control schemes are proposed UtWy costs can be reduced with about a factor of two by usrng a heat u&grated system lNTRODUCTlONIn the way it IS commonly apphed m the chemxal mdustry, dMllation 1s an energy consummg process heat IS added 111 order to evaporate and dlstlll a mixture and coohng has to be Introduced to urlthdraw the added energy (Fig 1) It should come as no surpnse that ideas have been proposed to decrease energy consumption m dlsaation [l-4] One system wluch was designed to conserve energy was the use of a heat pump by usmg the heat of condensation of overhead vapour for reborn (Fw 2) The attractiveness of tfus opt1011 e&rely depends on the thermodynamic etlicrency of the heat P-P Another way to conserve energy is to replace a smgle column by two heat mtegrated columns m parallel, as shown m FGg 3 The overhead vapour from the first column 1s used as a heatmg medium for the second column reboller The first column must be operated at a higher pressure than the second column because the bottom of the second column will be ncher m higher bolllng components than the top of the first column, and a temperature dtierence 1s required for heat transfer In this paper optunal operation of the different configurations for dlstrllatlon wti be studied startmg from a more or less optunal design ST pwhere V IS the vapour flow from the reboder, SAH/GT IS the vmatlon of the heat of vapormtion w&h temperature and c,, IS the spectic heat of the feed In our particular study the first term m eqn (3) was about five tunes higher than the second term Therefore the costs of feed preheatmg were ignored and for the thud term m eqn (2) the rebodmg costs were taken For the conventional smgle column and the heat mtegrated two column system electnclty costs were ignored. resultmg m an objective function J=c,,D+cBB-c&-cwW (4) There are five degrees of freedom for a gven feed, which enters the column under botimng pomt conditions (see Fig 1) the pressure P, the bottom product flowrate B, the dlstlllate flowrate D, the top reflux ratio RT and the bottom reflux ratio Rg The bottom reflux ratio can be associated with the steam flow to the reboller As D and B have to be used for level control, there are three remauung degrees of freedom In this study the top product quality was specdied for which RT was chosen somewhat arbltrardy as manipulated vanable Hence for the conventtonal dlstdlatlon process the two degrees of freedom were the pressure P and the bottom reflux ratto Ra or vapour flow from the reboder V (11) The vupour recompressron system (see Fig 2) Compared to the conventional dlstdlatlon process there 1s one extra degree of freedom the pressure after compression However, the reboller and condenser are not mdepende...

Implementation of Optimal Operation for Heat Integrated Distillation Columns

“…The work by Roffel and Fontein (1979) is most similar to our work. They discuss some aspects related to constrained control.…”

Heat pump assisted distillation. VI: Classified references for heat pump assisted distillation systems from 1945 to February 1986

“…al., 1982 Brousse er al., 1985Carta et al, 1982 Chiyoda Engineering and Construction Co. Ltd., 1984Danziger, 1979 Danziger and Baurner, 1978 DeLair, 1981 Duriez, 1982Ellis, 1982 Erickson, 1982a Fahmi and Mostafa, 1983 Fair, 1977Farbenfabriken Bayer A. G., 1960 Ferre e? al., 1985Flores et al, 1984Franz and Edward, 1965Glavic et al, 1982Heimbach et al, 1983 Hernandez, 198 1 Hodgett, 1984Howard, 1980Huhta-Koivisto, 1984 Joseph and John, 1966Katz and Fogel, 1981Latham, 1945Leitner, 1983Menzies and Johnson, 1971 Messing, 1958Morikawa, 1979 Mostafa, 1981OBrien, 1976ONeill et al, 1985Paculea and Schitea, 1983 Robert, 1959Roffel and Fontein, 1978Roffel and Fontein, 1979 Schorre er al., 1982Slegers, 1984Sobron and Fernandez, 1984 Spangler, 198 1 Sterlini, 1982Swain, 1982Swain, 1985Tsuruda, 1984 Tsuruta, 1982Vochet, 1983Walter and Kenneth, 1960 Zenty, 1981 4. REFERENCES FOR OVERALL ASSESSMENTS OF HEAT PUMP ASSISTED DISTILLATION Anthony, 1981 Baasch, 1982 Badische Anilin-und-Soda-Fabrik Akt.…”

Studies in the synthesis of control structures for chemical processes. Part V: Design of steady‐state optimizing control structures for integrated chemical plants

“…Current industrial practice dictates that the optimal operating point for a chemical plant will move from the intersection of one set of constraints to another as disturbances change. The importance of constraints in plant economics and operation has been demonstrated on distillation columns (Duyfjes et al, 1973;Rijnsdorp, 1967;Maarleveld and Rijnsdorp, 1970;Kenney, 1979;Roffel and Fontein, 1979), fluid catalytic crackers (Lee and Weekman, 1976;Davis et al, 1974;Webb et al, 1977;Prett and Gillette, 1979) and ethylene plants (Kaiser et al, 1966;Ishida, 1975;Rijckaert et al, 1978;Duncanson and Youle, 1970) with a growing interest to operate chemical plants under "constraint control. "…”