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Elements of Pinch Analysis

The majority of process plants contain a number of streams which need either to be heated or cooled (known respectively as cold streams and hot streams). It is often immediately apparent that some of the heating and cooling can be carried out by heat recovery between one or more of the hot streams and one or more of the cold streams. On most of these plants, it is also apparent that some heating and some cooling are actually being carried out by thermal utilities, eg process steam, cooling water etc which results in a net energy cost to the site.

However, on all but the simplest plants it is far from immediately apparent:

  1. What is the maximum amount of heat which can be recovered, ie what is the minimum amount of (paid for) heating and/or cooling required?
  2. What configuration of heat exchangers between hot streams and cold streams is required in order to achieve the maximum amount of heat recovery?

As an introduction to the concepts of Pinch analysis, it is instructive to consider a simple heat exchanger. Heat exchangers work on the principle that heat flows from high temperature to low temperature. If two process streams are separated by a thermally conducting surface, heat will flow from the hot stream to the cold stream. The following diagram illustrates this principle and shows the energy temperature profile in diagramatic form.

Composite curves

Pinch analysis is a systematic examination of thermally intensive processes in which all heating and cooling duties (actual or potential) are extracted as temperature/energy flow (T/H) profiles and combined into composite curves for the whole process and/or site. The following diagrams illustrate the principle of composite curves and show how by combining the hot and cold composites the total potential for heat recovery on the plant can be quantified.

This defines the thermal energy targets (minimum heating and cooling (utility) duties necessary to operate the process assuming perfect heat recovery). By comparison with the existing consumption, scope for improved heat recovery is quantified.

In classical Pinch procedure, the analysis is extended to consider alternative process (as opposed to heat recovery) configurations and further opportunities to improve thermal efficiency identified. In practice, on more complex plants (eg multiple effect evaporators) it is often impossible to implement a heat recovery project without altering the temperature and/or flowrate of one or more process streams. In these situations the distinction between heat recovery and process modification projects becomes somewhat academic.

The Pinch

The Pinch is a key concept in the analysis. This is the temperature region in a process where the hot and cold composites are closest together. It is therefore the region where heat recovery is most constrained and it is most important to ensure that heat transfer duties are correctly configured.

Above the pinch temperature there is a shortfall of heat and below it there is a surplus. The majority of processes exhibit a pinch at some intermediate temperature between that of the hottest and the coldest process stream. This is visible as the temperature at which the composite curves are closest together. A minority of processes, known as threshold problems, do not exhibit a pinch. Threshold problems only need a single thermal utility (either hot or cold but not both). A feature of all conventionally pinched processes is that all process heating below the pinch and all process cooling above the pinch can be carried out by heat recovery. In order to minimise thermal utility consumption it is therefore essential all heat available above the pinch must be used above the pinch and all process heating below the pinch must be carried out by heat recovery from below pinch hot streams. Likewise hot utilities should only be used above the pinch and cold utilities should only be used below the pinch.

Heat transfer configuration and the grid diagram

The heat transfer grid diagram is a powerful method of representing process heat transfer. Thermal utilities and process heat recovery duties can be clearly visualised. By dividing the diagram into below and above pinch regions, inappropriate heat transfer is immediately obvious.

Significance of temperature and the grand composite

Pinch analysis is based on the significance of temperature in process thermal duties. Thermal utilites are increasingly expensive as their temperature is increasingly far away from ambient (both for heating and for cooling). The concept of utility pinches can be used to analyse which utilities should be applied to specific thermal duties. In order to minimise utility costs it is important that heat transfer does not cross either a process or a utility pinch.

The grand composite curve (GCC) is another important construction. This is effectively the net process heating or cooling requirement as a function of temperature based on the assumption that all feasible heat recovery will be implemented. Thus the GCC can be used to examine the most efficient thermal utility temperatures for a given process.

It can therefore also be used to guide changes to the process itself which will result in lower thermal utility costs. One significant aspect of this form of analysis is that thermal utility targets can be determined before the heat recovery configuration is defined.

Some processes exhibit multiple near pinches. It is important to understand this in order to achieve energy efficient heat recovery designs. In some cases multiple process pinches exhibit some of the characteristics of utility pinches. Eg multiple effect evaporation plants are divided into temperature regions by pinches between each effect and the next one hotter or colder. Because of the multiple effect mechanism, heat is progressively more valuable above each successive pinch. Projects to optimise ME evaporators must take into account the implications of these multiple pinches and that almost any change in heat transfer duty will alter several process streams and thereby modify the composite curves. It is often appropriate to carry out the pinch analysis iteratively by resimulating the process balances between iterations.

Pinch analysis is essential when integrating unit operations with simultaneous heating and cooling requirements, eg evaporators or distillation units, into a process as a whole. If both heating and cooling can be configured to take place the same side of the pinch, the unit can be operated with zero net thermal utility cost. If the unit operates across a pinch any marginal increase in unit duty inevitably increases the plant-wide thermal utility demand.

Energy targeting and heat exchanger cost

If a fixed minimum approach temperature (ΔTmin) is assumed to be applicable for all heat transfer duties, pinch analysis can be used to target and design for minimum thermal utility (energy) cost. In practice, different process stream matches incur different specific heat exchanger costs. For a given exchanger type and metallurgy the exchanger surface area is a function of the thermal duty and the temperature driving force (LMΔT). In some cases, by factoring in heat exchanger cost correlations and applying a discount factor to relate capital and operating costs, the targeting concept can be extended to include capital as well as energy costs.

For a more extensive treatise on Pinch analysis, please refer to specialist literature on the subject.

Copyright © Harry Cripps 2018

Harry Cripps MA MSc DMS CEng FIChemE MEI hrc at hrcconsultants dot co dot uk