Heat-Integrated System

In this section we compare the steady-state design and the dynamic control of heat-integrated extractive and pressure-swing processes. The same numbers of trays used in the base-case designs are used in both systems. The systems have not been reoptimized for heat integration. Only partial heat integration is considered in which an auxiliary reboiler is used. 

The higher pressure in the methanol column requires a higher reflux ratio and more heat input than the low-pressure operation because the higher pressure makes the separation more difficult. The reflux ratio increases from 1.61 to 2.91, and the reboiler heat input increases from 7.1 to 11.4 MW. 

The condenser duty in the methanol column is now 9.31 MW, which can be used to produce vapor in the base of the extractive column. The total energy needed in the extractive column is 11.4 MW, so an auxiliary steam-driven reboiler is required with a duty of 2.1 MW. The extractive column itself is unchanged except it has two reboilers. 

The reboiler/condenser is sized by using an overall heat transfer coefficient of 0.00306 GJh m K The heat transfer rate is 33.53 GJ/h (9.31 MW) and the temperature difference is 21.2 K, giving an area of 514 m.  

The TAG of the heat-integrated extractive system is 2,980,000/y. This should be compared with the base-case TAG of 3,750,000/y. Total capital investment is essentially the same in the base and heat-integrated designs ( 3,000,000), while energy cost is reduced from 2,720,000 to 1,980,000. 

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T. Umeda, T. Harada, and K. Shiroko, "A Thermodynamic Approach to the Synthesis of Heat Integration Systems in Chemical Processes," Proceedings of the 12th Symposium on Computer Applications in Chemical Engineering, Montreaux, Swit2edand, 1979, p. 487. 

Umeda, T., Itoh, J., and Shiroko, K. (1979). A thermodynamic approach to the synthesis of heat integration systems in chemical processes. Comp. Chem. Eng. 3, 273-282. 

Itoh J, Shiroko K and Umeda T (1982) Extensive Application of the T-Q Diagram to Heat Integrated System Synthesis, International Conference on Proceedings Systems Engineering (PSE-82), Kyoto, 92. 

Umeda T, Harada T and Shiroko K (1979) A Thermodynamic Approach to Synthesis of Heat Integration Systems in Chemical Processes, Comp Chem Eng, 3 273. 

Schilling, G., Pantehdes, C.C., 1996. A simple continuous-time process scheduling formulation and a novel solution algorithm. Comput. Chem. Eng., 20(Suppl.) S1221-1226 Umeda, T., Harada, T., Shiroko, K., 1979. A thermodynamic approach to the synthesis of heat integration systems in chemical processes. Comput. Chem. Eng., 3 273-282 Wang, Y.P., Smith, R., 1994. Wastewater minimization. Chem. Eng. Sci., 49(7) 981-1002... 

High system efficiency levels can be achieved only with intensive heat integration within the fuel cell micro CHP systems. Hence, heat integration system studies are of utmost importance along with the development of novel reforming catalysts, cleanup systems, and PEM fuel cell components if on-site hydrogen production is desired for micro CHP applications. 

Fig. 1.23. Flowsheet for a heat-integrated system of an autothermal gasoline reformer (ATR) and a high-temperature shift stage (HTS) interconnected with a heat exchanger [37].

To illustrate the very large energy savings that are possible with this complex/heat-integrated system, consider the separation of a benzene, toluene, and xylene mixture. A conventional two-column light-out-first separation flowsheet with no heat integration uses twice the energy7 that the prefractionator-reverse flowsheet uses. 

A heat-integration system is attached. The combination of the MTA quality and the unique crystallization system generates less waste. In a following step, the PTA crystals are separated from the mother liquor and finally dried for further processing in the polyester production system. 

These heat-integrated systems were economically advantageous because of the large feed flow rate. The last configuration was built. 

Table 11.4 Characteristics of prefractionator/side stream column heat integrated system...

The heat integration system consists of a catalytic burner reactor (BUR) and three heat exchangers (HXl, HX2, HX3). In the catalytic burner reactor, the unconverted hydrogen is combusted at 370 °C to deliver the heat necessary for the vaporizer and the reformer. The whole FP-FC system operates autothermally. 

Evaporator systems are major pieces of process equipment and are often purchased on a total responsibility basis. This is especially true of vapor compression and highly heat integrated systems. Specific design information and fabrication are often proprietary to vendors. Evaporator manufacturers generally are rather specialized. Few offer a complete range of evaporator types some specialize in one type only. 

These results clearly demonstrate that heat integration can result in very significant reductions in energy consumption. However, as we demonstrate in the next section, the dynamic controllability of the completely heat-integrated system is not as good as that of the nonheat-integrated system. 

These results demonstrate that the fully heat-integrated system can only handle fairly small disturbances without having purity specifications violated. 

In the following sections the fuUy heat-integrated system is compared with a completely nonheated-integrated system and with a partially heat-integrated system. 

Figure 6.20 gives results for changes in feed composition from 6 to 7 mol% THF (solid lines) and from 6 to 5 mol% THF (dashed lines). The transient deviation in x kthf) is about 70 ppm, which is much smaller than that seen in the heat-integrated system for a small feed composition disturbance (6-6.5 mol% THF). 

These results demonstrate that there is a significant dynamic controllability penalty associated with the fully heat-integrated system. 

Notice in the faceplates shown at the bottom of Figure 6.23 that the setpoints of the two temperature controllers are different in the partially heat-integrated case fiom those used in the fully heat-integrated system because the temperature profiles are slightly different. Of course the TCI temperature controller must be retuned since its output signal is a ratio. 

TABLE 6.3 Design Results for Heat-Integrated Systems Acetone-Methanol. 

These results show that there is no control penalty for going to the partial heat-integrated systems when auxiliary reboilers or condensers are added. This result is to be expected because there is no loss of control degrees of freedom. 

These conditions correspond to the first and second laws of thermodynamics. Figure 11.12 shows the flowsheet equations for the extractive heat-integrated system. The two conditions listed above are specified in the top two equations in the Text Editor window. 

The third equation is used to provide a pressure-compensated temperature measurement in the methanol column. This is needed because, in the heat-integrated system, the pressure in the methanol column is not controlled. It floats with operating conditions. If more heat transfer is required in the reboiler/condenser, a larger temperature difference is required, and this is achieved by the pressure in the methanol column increasing, which raises the bubblepoint temperature in the reflux drum. 

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