No Heat Integration

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. 

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These heuristics are based on observations made in many practical applications. In addition to being restricted to simple columns, the observations are based on no heat integration (i.e., all reboilers and condensers are serviced by utilities). Difficulties can arise when the heuristics are in conflict with each other, as the following example illustrates. 

From Tables 21.1 and 21.2, when both columns operate at 1 bar, no heat integration is possible. Thus,... 

Constraints (11.3) stipulates the quantity of heat transferred to storage from a hot unit at the beginning of the time horizon. Constraints (11.4) and (11.5) quantify the amount of heat transferred and received from storage unit, respectively. They ensure that if there is no heat integration between a processing unit and storage, then the amount of heat related to storage is not disturbed. 

Constraints (11.6) ensures that only one unit is heat integrated with storage at any given point in time. Constraints (11.7) and (11.8) ensure that the temperature of the storage unit is not changed if there is no heat integration with any unit. These... 

No heat integration Direct heat integration Heat storage... 

No heat integration takes place, and hence hot and cold utilities provide the required loads in the reboilers and condensers, respectively. Also, it is assumed that all columns operate at fixed pressures (i.e., the pressure or temperature of each column is fixed). 

The batch process allows high flexibility with respect to the composition of the feedstock. In turn, the economic indices are on the lower side because of lower equipment productivity and higher operation costs, such as manpower and automation. The use of a large excess of methanol is reflected in higher energy consumption if no heat-integration measures are taken. Large amounts of wastewater formed by acid-base neutralization need costly treatment. 

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. 

Step 3. The open-loop instability of the reactor acts somewhat like a constraint, since closed-loop control of reactor temperature is required. By design, the exothermic reactor heat is removed via cooling water in the reactor and product condenser. We choose to control reactor temperature with reactor cooling water flow because of its direct effect. There are no process-to-process heat exchangers and no heat integration in this process. Disturbances can then be rejected to the plant utility system via cooling water or steam. 

A prefractionator arrangement with no heat integration saves about 30% energy compared to the best of the direct or indirect sequence. A prefractionator with further heat integration where the columns are run at different pressures, can have savings of around 50% compared to the best of the direct or indirect sequence (Ding Luyben, 1990). 

For the designed plant with no heat integration, there are 38 control degrees of freedom in this process. These degrees of freedom represents the available manipulated variables in the process and can be characterised as follow four feed valves, direct reaction and oxy-reaction coolers valves, direct reaction and oxy-reaction product valves, oxy quench cooler valve, three decanter product valves, pyrolysis preheater and heater valves, pyrolysis product valve, pyrolysis quench cooler valve, HCl heater valve, eight valves for the heating and cooling systems of the four distillation columns, thirteen valves for the base, top and reflux streams of the four distillation columns. 

We begin with the THF-water system. A comparison of systems with no heat integration, with partial heat integration and with complete heat integration will be presented. The phase equilibria for this system and a nonheat-integrated system have been discussed in Chapter 5. 

With complete heat-integration, the heat removal in the condenser of the high-pressure column Qc2 must be equal to the heat input to the reboiler of the low-pressure column. Therefore, a degree of freedom is lost, and we can only set the reflux ratio on one column, not both as is the case with partial heat integration or no heat integration. 

Figure 6.8 Flowsheet conditions with no heat integration.

TABLE 6.2 Controller Tuning Parameters No Heat Integration and Partial Heat Integration THF-Water. 

In making these comparisons, we developed the core model, a level/flow/composition model that neglects the effect of thermal (temperature) and pressure dynamics. For this plant, with only one recycle stream and no heat integration, the assumption is that the temperature and pressure control loops are largely isolated and noninteracting. This assumption has to be tested for accuracy via simulation. 

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