t here are two basic approaches to minimizing &stillation column energy

Transcription

1 8 Miniizing Energy Requireents 8.1 INTRODUCTION t here are two basic approaches to iniizing &stillation colun energy requireents: 1. Conseraatiandesigning and operating a colun so that it akes the specified separation with the least aount of energy per pound of feed. 2. Energy recovqrecovering and reusing the heat in the colun product streas, whether they be liquid or vapor. The ain ephasis of this chapter is on the latter approach, but it is pointless to uy to recover energy unless we also try to conserve it. Consequently we will discuss conservation first. 8.2 CONSERVATION For distillation, conservation eans designing and operating a colun so that it akes the specified separation with the least aount of energy per pound of feed. We have a nuber of techniques to accoplish this: 1. Autoatic control of coposition of product streas. Operators coonly overreflux conventional coluns with a single top product and a single botto product. Extra heat is used to ensure the eeting or exceeding of specified product purities. Geyer and Kline' give, as an exaple, a 70tray colun separating a ixture with a relative volatility of 1.4 and with specifications of 98 percent low boilers overhead and 99.6 percent high boilers in the base. If the operator adds enough boilup and reflux to increase overhead purity to 99 percent and base purity to 99.7 percent, an increase of 8 percent in energy consuption results. 2. Feed provided at the proper feed tray. It can be shown that this results in a lower energy requireent per pound of feed than would feedg on any 181

2 182 Minitzzw E M Requireents other tray. As feed coposition or enthalpy deviates &o design values, the optiu feedtray location also changes. 3. Colun operation at iniu pressure.2 Lower pressure usually eans higher relative voia&ty. Therefore, the necessary separations can be accoplished with lower boilup/feed and reflux/feed ratios. Condenser capacity ay be liited, however, and the colun ay flood at lower boilup rates than it would when operating at higher pressures. 4. Use of lowest pressure stea available.' In any plants excess lowpressure stea is available that otherwise would be vented to the atosphere. This stea is usually cheaper than highpressure stea. Where reboiler AT ight be too sall if the stea were throttled, one ay use a partially flooded reboiler (see Chapters 4 and 15) and throttle condensate. Since lowpressure stea is seldo available at constant pressure or stea quality, pressure and teperature copensation of flow easureents is highly desirable if stea is throttled instead of condensate. 5. Use of stea condensate receivers. In any plants stea traps require considerable aintenance and have sigficant leakage. The use of stea condensate receivers instead of traps reduces aintenance and stea losses. 6. Possible use of echanical vacuu pups. For vacuu coluns there is soe opinion' that echanical vacuu pups offer energy savings over stea jets. The difference, however, is usually sall. 7. Dry distillation. For coluns now using live stea, it is soeties econoical to switch to steaheated reboilers. 8. Insulation. Older coluns, designed before the energy crunch, can often benefit fro new, increased insulation. 8.3 DESIGN CONSIDERATIONS IN HEATRECOVERY SCHEMES Energy recovery in a distillation colun eans, practically spealung, recovering or reusing heat contained in the colun product streas, whether they are liquid or vapor. A nuber of schees have appeared in the literature. The two chief ones involve (1) "ultiple effect" distillation, analogous to ultiple effect evaporation, and (2) vapor recopression. But, regardless of the schee, there are five design factors that ust be considered: 1. Reserve capacities that ay be required: Extra heating capacity Extra cooling capacity Extra distillation capacity These are iportant for startups and shutdowns, changes in production rate, changes in feed coposition, and changes in product specifications. Generally speaking, however, auxiliary" reboilers and condensers should be avoided, if * "Auxiliary" condensers and reboilers are those installed in parallel with "noral" condensers and reboilers for startup or peak load purposes.

3 8.4 Multiple Loads Supplied /y a Sin5h Source 183 at all possible. Their use increases investent, as well as instruentation and control coplexity. Soe users have had probles with turning auxiliary condensers and reboilers on and off, and they prefer not to do it. Instead they always aintain at least a sall load on these heat exchangers. This obviously wastes energy. 2. Priorities. If recovered energy is to be distributed to several loads, what is the order of priorities? 3. Interactions. Elaborate heatrecovery schees are often highly interactive; how is this to be dealt with? 4. Overall heat balance. How is this aintained? 5. Inerts (low boiler) balance. With elaborate heatrecovery schees, this is soeties a proble. Too high a concentration of inerts or low boilers will blanket processtoprocess heat exchangers; too low a concentration will result in product losses through the vents. In view of the above, it is apparent that control of coluns with heatrecovery schees is ore difficult than control of conventional coluns. Let us now look at three types of dtipleeffect distillation. 8.4 MULTIPLE LOADS SUPPLIED BY A SINGLE SOURCE Soeties, as shown in Figure 8.1, a colun that is a very large energy user becoes the energy source for a nuber of loads, each of which acts as a condenser. Two ethods have been used to allocate the energy to be recovered (1) throttling the vaporheating ediu to each condenser, as shown in Figure 8.1, and (2) operating each condenser partly flooded by throttling the condensate. Soe priority schee ust be established for startups and for any other occasion when vapor supply is teporarily short. One ethod of handling the priority probles is to use overrides and to splitrange the various valves involved, as shown in Figure 8.2. The schee shown illustrates the use of pneuatic devices, but the concepts readily ay be ipleented with soe digital or analog electronic controls. For the six loads in Figure 8.2, we eploy six gain 6 relays. For load 1, which has the highest priority, the gain 6 relay is calibrated to have an output span of 315 psig for an input span of 35 psig. Load 2 has the next highest priority, so its gain 6 relay is calibrated for 57 psig input, 315 psig output. This continues until the load with lowest priority, load 6, has a gain 6 relay calibrated for 1315 psig input, 315 psig output. At the design stage of such a syste, considerable care ust be exerted to obtain suitable value of controller gain and also proper valve sizing. Processtoprocess heat exchangers are coonly designed for very sall teperature differences, say 810 C. If vapor throttling is used, it should be recognized that the vaporsupply valves will tend to have sall pressure drops. Accordingly, it is advisable to have vapor flow control to each load, with a set

4 184 Miniizing Energy Requireents 3 0 a ta c, i 1 3 s a a d C I 2 a 5 i? :8 *s 32 s5 E l

5 8.4 Multiple Lo& Supphd ly a SinJle Source 185 cn Q + s & c UI E s 3 5 NL ode 2g 3s gg

6 186 Miniiziv Energy Requireoats point fio a priary controller. In the absence of vapor flow control, interactions ay be severe, and very close control of supply and load pressures ay be required. 8.5 SINGLE SOURCE, SINGLE LOAD When there is only one source and one load (see Figure 8.3), control ay be both sipler and ore flexible. The colun that is the source does not need to be operated at a constant pressurein the schee shown, it finds its own pressure. For the illustrative exaple, the overhead coposition of the supply colun is controlled via reflux; the base coposition of the load colun is controlled by boilup in the supply colun. The schee of Figure 8.3 has an interesting dynaics proble. The controls ust be so designed that changes in vapor flow fro the supply colun ust reach the condenserreboiler at about the sae tie as feed flow changes fro the supply colun. If there is a serious discrepancy, particularly if the secondcolun bottoproduct flow is sd, base level in the second colun ay experience serious upsets. Another proble associated with this schee is the selection and sizing of the feed valve to the first colun. This colun will run at a low pressure at low feed rates and at a higher pressure at high feed rates. Assuing that the feed coes fro a centrifugal pup, one can see that valve pressure drop will be very high at low flow, and low at high flow. The variation in valve pressure drop with flow will be uch greater than that norally encountered in a puped syste. In another version a following colun in the train supplies heat to a preceding colun as shown in Figure 8.4. In this particular case, the first colun gets only part of its heat fro the second colun; the reainder coes fro an auxiliary reboiler. Interactions between the two coluns ay be severe. Again, for the cases studied, we have found it advantageous to let pressure find its own level in the second colun, that is, the one supplying heat. An interesting practical proble here is how to adjust the auxiliary reboiler on the first colun. After exaining soe coplex heatbalance schees, we decided that the siplest approach was to use colun AI'. Vapor flow to the first colun fro the condenserreboiler will not be constant, but the AI' control will provide a rapid ethod of ensuring aonstant boilup. The AP control, in turn, ay have its set point adjusted by a coposition controller for the lower section of the first colun. It should be noted that for the schees of both Figure 8.3 and Figure 8.4, axiu colun pressure occurs at axiu feed rate and boilup rate. For coluns we have studied to date, there has been no proble with flooding at lower rates and pressures.

7 8.5 Siwk Swr, Load a3 tn C vl ai u 5 8 a3 z.i i? eg "Os 22 ss ZX

8 188 Miniizing Ew Requireents N E W 5 i z 0 W C VI i s 2l 8 c i F?g os e l

9 8.8 Energy R e7 iy Vapor Recopresswn SPLIT FEED COLUMNS A third arrangeent, which is used in soe sy~tes,~.~ involves splitting feed between two coluns that ake the sae separation (Figure 8.5). The supply colun, however, runs at a higher pressure than the load colun. The feed split is controlled to aintain a heat balance. 8.7 COMBINED SENSIBLE AND LATENT HEAT RECOVERY In addition to the recovery of the latent heat of vapor streas, in any cases it is practical to recover part of the sensible heat in the colun botto product and stea condensate by exchange with colun feed. Such schees have been used in the cheical and petroleu industries for years. Since feed flow is typically set by level controllers or flowratio controllers, its flow rate will not be constant. The feed enthalpy or teperature, therefore, is apt to be variable. This ay ake coluncoposition control difficult unless one eploys either feedforward copensation or a tri heater with control for constant teperature or enthalpy. (See Chapters 5 and 11.) 8.8 ENERGY RECOVERY BY VAPOR RECOMPRESSION In the past vapor recopression ( heat pups ) has often been considered for distillation of aterials boiling at low teperatures. The incentive in any instances was to be able to use watercooled condensers, thus avoiding the expense of refrigeration. Another factor favoring vapor recopression is a sall teperature difference between the top and botto of the colun. Today the ain interest is in getting the colun vapor copressed to the point where its teperature is high enough to perit using the vapor as a heat source for the reb~iler.~.~ An auxiliary, steaheated reboiler and/or auxiliary watercooled condenser ay be necessary for startup (see Figure 8.6). A review of copression equipent and ethods of estiating operating costs has been presented by Beesley and Rhine~ith.~ Mosler7 discusses the control of a nuber of vapor recopression schees. Nd9 presents investent equations and data. Fahi and Mostafa indicate that the optiu location at which to use the copressed vapor ay not be in a reboiler at the colun base, but rather at an interediate site. Other papers on energy integration for distillation coluns include those by OBrien and by Rathore et al.12

10 190 Miniizing Enevgy Requirenents U 0 w c I % e + 7 E 2g ZE s5 dl

11 8.8 EWJJ~ Recovery Czy Vapor Repession 191 rnuuuli FIGURE 8.6 Heat recovery via vapor recopression

12 192 Miniking Energy Requireents 1. Geyer, G. R., and P. E. Kline, CEP, 4951 (May 1976). 2. Shinskey, F. G., Dtitillation ControL, McGrawHill, New York, Tyreus, B. D., and W. L. Luyben, CEP, 5966 (Sept. 1976). 4. Rush, F. E., CEP, 4449 (July 1980). 5. Shaner, R. L., CEP, 4752 (May 1978). 6. Bley, A. H., and R. D. Rhinaith, CEP, 3741, (Aug. 1980). 7. Mosler, H. A., Coneol of Sidestrea and Energy Conservation Distdlation Towers, Proceedangs, AIChE Workshop on Industrial Process control, Tapa, Fl., Chiang, T., and W. L. Luyben, Heat REFERENCES Integrated Distillation Configurations, paper subitted to IOEC Proc. Des. Dev. (1983). 9. Null, H. R., Heat Pups in Distillation, CEP, 5864 (July 1976). 10. Fahi, M. F., and H. A. Mostafa, Distillation with Optiu Vapor Recopression, Cbe. Eng., Res, Des., (Nov. 1983). 11. O Brien, N. G., Reducing Colun Stea Consuption, CEP, 6567 (July 1976). 12. Rathore, R. N. S., K. A. Vanworer, and G. J. Powers, Synthesis of Distillation Systes with Energy Integration, AICbE J., 20(5): (Sept. 1974).