Introduction

A complex system consists of many subsystems, and each subsystem consists of many pieces of equipment (e.g., pumps, compressors, turbines, and motors) consuming energy (steam, power, fuel, etc.) at various levels and capacities. A typical block diagram of a total system and its subsystems is shown in figure 4-1. Each subsystem has its own energy consumption pattern, and when the energy efficiency of each subsystem is optimized, energy consumption of the total system will also be optimized if the subsystems are discrete and have no energy dependency on the other subsystems.

Figure 4-1: Typical discrete system/subsystem layout

By contrast, when there exists a process integration among subsystems, it is necessary to consider the total integrated system energy efficiency in the optimization process. Figure 4-2 shows a subsystem in which some process units are integrated to achieve high energy efficiency. Even within the subsystem (process unit), there exists a process integration between energy-intensive equipment to minimize energy consumption. In this configuration, hot streams from the subsystem (unit I) transfer energy to the feed to another subsystem (unit II), and hot products from unit II transfer energy to the feed to unit III. This is one of the methods of energy integration of subsystems of a main system. By this energy integration, overall specific energy consumption will be minimized. There are some associated problems in this configuration, especially when either of the subsystems (unit II or III) is shut down owing to operational/maintenance problems.

Figure 4-2: Typical energy-integrated system

In the...