Pinch-Point Analysis The Role, Approach and Results of Systematic Energy Integration
With the increase in the cost of energy, the ability to optimize the use of resources, and in particular of energy, is becoming an extremely important skill for chemical engineers. Optimal integration of energy has a major role for every process unit.
Systematic methods for optimal energy integration are well established in the framework of pinch - point analysis.
It is useful to remember that pinch designates the location among process streams where the heat transfer is the largest constraint. The pinch can be identified in an enthalpy – temperature plot as the nearest distance between the hot and cold composite curves. Accordingly, the energy management problem is split into two parts: above and below the pinch. In principle, only heat exchange between streams belonging to the same region is energetically efficient. Moreover, heat should be supplied only above and removed only below the pinch. When the pinch principle is violated energy penalties are incurred. The designer should be aware of it and try to find measures that limit the transfer of energy across the pinch.
The essential merit of pinch point analysis is that makes possible the identification of key targets for energy saving with minimum information about the performance of heat exchange equipment.
The key results of are:
- Computation of minimum energy requirements.
- Generation of an optimal heat exchangers network.
- Identification of opportunities for combined heat and power production.
- Optimal design of the refrigeration system.
The Overall Approach
Figure illustrates the overall approach by pinch point analysis.
The first step is extraction of stream data from the process synthesis. This step involves the simulation of the material balance by using appropriate models for the accurate computation of enthalpy. On this basis, composite curves are obtained by plotting the temperature T against the cumulative enthalpy H of streams selected for analysis, hot and cold, respectively. Two aspects should be taken into account:
- Proper selection of streams with potential for energy integration.
- Adequate linearization of T – H relation by segmentation.
The next step is the selection of utilities. Additional information regards the partial heat transfer coefficients of streams and utilities, as well as the price of utilities and the cost laws of heat exchangers.
After completing the input of data, one can proceed with the assignment of tasks for heat recovery by targeting optimization procedure. In the first place, the minimum difference temperature ΔT min is determined as a trade-off between energy and capital costs. If the economic data are not reliable, selecting a practical ΔT min is safer. Next, initial design targets are determined as:
- minimum energy requirements for hot and cold utilities,
- overall heat exchange area, and
- number of units of the heat - exchanger network.
The approach continues by design evolution. This time, the design of units is examined in more detail versus optimal energy management. Thus, the “appropriate placement” of unit operations against pinch is checked. This may suggest design modifications by applying the “plus/minus principle”. The options for utility are revisited. Capital costs are the trade-off again against energy costs. The procedure may imply several iterations between targeting and design evolution. Significant modifications could require revisiting the flowsheet simulation.
The iterative procedure is ended when no further improvement can be achieved.
Note that during different steps of the above procedure the individual heat exchangers are never sized in detail, although information about the heat transfer coefficients of streams is required. Only after completing the overall design targets can the detailed sizing of units take place.
Optimization methods can be used to refine the design. Then, the final solution is checked by rigorous simulation.
The value of pinch analysis
An important feature of the methodology is determining the appropriate placement of unit operations with respect to pinch. The analysis can find which changes in the design of units are necessary and perform a quantitative evaluation of these changes.
The strongest impact has the design of the chemical reactor, namely the pressure and temperature. It is useful to know that higher reaction temperatures give better opportunities for heat integration.
Another important source of energy saving is the integration of distillation columns by thermal coupling or by integrated devices, such as the divided wall column.
However, very tight energy integration might be detrimental for controllability and operability, by removing some degrees of freedom. Thus, the analysis of heat integration should investigate the consequences on process control.
Summing up, pinch point analysis consists of a systematic screening of the maximum energy saving that can be obtained in a plant by internal process/process exchange, as well as by the optimal use of the available utilities. The method is capable of assessing optimal design targets for the heat exchanger network well ahead of detailed sizing of the equipment.
Furthermore, the method may suggest design improvements capable of significantly enhancing the energetic performance of the whole process.
Tools to use for pinch-point analysis
Most of the known process simulation packages are suitable for the development of pinch point analysis, and their list can be viewed in the article Complete List of Process Simulators.
There are also free and simple tools available that are specialized only for simple pinch point analysis, their names are PRO_PI1 and Hint.