Sep-2013

Using pinch technology in operations?

Pinch Technology is a well established concept, and a tool used to optimise waste heat recovery and design efficient heat integration schemes in a wide range of applications, throughout the process industries.

Zoran Milosevic, Allan Rudman and Richard Brown
KBC Process Technology, UK

Article Summary

What is less obvious is that Pinch Technology can assist an operating engineer in his daily work. The paper discusses the basic pinch principles and how the knowledge of them helps in understanding the operation and behaviour of heat exchanger networks, finding the operational improvements, calculating the effects of exchanger fouling, benchmarking the energy performance of existing process units, and identifying their improvement potential.

Introduction
Pinch Technology is arguably the most rigorous, systematic and best documented methodology in all process design.^1,2,3 Virtually, no design involving the optimisation of process heat recovery is carried out without applying some form of pinch analysis. Pinch Technology is particularly useful when designing very complex processes, such as refineries and petrochemical plants, aiming at and achieving high energy efficiency of the individual processes, as well as of the whole site. It deserves its special place in the hierarchy of design methodologies because of the exactness of the fundamental principles that it uses, its simplicity, the magnitude of design improvements/benefits that it brings about, and its wide applicability.

However, the fact that Pinch Technology provides a “final” methodology, in the point in time when not many new processing facilities are being built, or are expected to be built in the near future, led to occasional remark that “pinch technology may be obsolete”.⁴ Proponents of this view feel that since any recently built petrochemical plants have been designed close to optimum, while new ones are unlikely to be constructed in the current global energy climate, practically all major pinch work had already been “done”.

While the underlying logic of such viewpoint can be understood, it is nevertheless incorrect for two main reasons, apart from the obvious statement that thermodynamic principles can hardly be described as outdated.

Firstly, the optimum in design is a moving target. Process plants that have been optimised today may not operate in an optimal fashion in the future. Many “pinch” revamps in existing refineries and petrochemicals have been carried out not because the original design was suboptimal at the time, but because the optimum has moved since the plant was commissioned. What was not economical to install 30 years ago may be economical now. Even the pinch and/or energy projects that have been carried out only 10 years ago should be reviewed against the changing economics, because the cost of energy grows faster than the plant construction cost (Figure 1). This renders viable those projects that have previously been considered uneconomical. Pinch Technology is a design tool that guides optimum retrofits. In fact, its retrofit theory is more complex than the theory of designing new units and it is still developing.

Secondly, Pinch Technology helps the operating engineer to understand and manage a range of process issues that are related to day-to-day operation of their process units.

The present paper focuses on this last area of applicability of Pinch Technology, i.e. its use in the daily work of the operating engineer, but also aims to provide useful revamp guidelines.

Pinch Technology – A Brief Description
The Pinch principles have been described extensively in chemical engineering literature.^1,2,3 Essentially, Pinch Technology is a technique that is used to analyse heat availability in process’ hot streams and to match it against the heat demand of suitable cold streams, in an optimum fashion. This optimises the preheating of the cold streams by using hot streams’ waste heat, and saves fuel in furnaces and other heaters. The technique owes its name to the discovery and the conceptual importance of the thermodynamic “pinch point” – the point of the closest temperature approach between the combined hot and cold heat availability curves. This thermodynamic bottleneck limits the recoverability of the hot stream’s energy.

Pinch Technology has four principal functions:
• Energy versus capital targeting and optimisation.
• Design of optimum heat exchange networks.
• Optimisation of the use of utilities.
• Revamping of existing networks.
These functions are briefly described below.

Energy and capital targeting and optimisation
The energy targets for the optimum use of energy are determined ahead of designing a unit. The methodology is based on the use of heat availability curves (the Composite Curves – Figure 2), and the optimisation of capital cost (exchanger area) versus energy cost (fuel), to calculate energy “targets” – the optimum achievable heat recovery, and hence the optimum energy consumption of a process. Composite curves represent heat availability and heat demand profiles. When superimposed, they show the recoverable energy (where curves overlap), and the external heating and cooling requirements (the uncovered parts of the curves). Moving the curves apart illustrates the effect of increasing the temperature approach between the composites: this reduces the required exchanger area, but also reduces heat recovery between hot and cold composite, thus increasing the consumption of both heating and cooling energy (case B in Figure 2).

Heat Exchanger Network Design
Pinch Technology further provides the design methodology which ensures that the “pinch” targets are met in the actual design. An introduction and the explanation of the design methodology can be found in.² Today, this is to a large extent a computer-led process.

Optimisation of the use of utilities (utility placement)
The utility-placement function is based on the use of Grand Composite Curves (GCC), whereby the cost of the targeted energy is minimised by utilising a cheaper utility – for example by using low pressure steam instead of high pressure steam or fuel, where possible. While Composite Curves show the total demand of the heating and cooling utilities, the GCC shows the distribution of this demand in various temperature intervals of the heat transfer region, and are used to determine how much of the lower temperature utility can be optimally used. The Grand Composite curve in Figure 3 shows how the heating target can be met by using HP (high pressure) steam (left), but also illustrates the option of partly using LP (low pressure) steam and reduce the use of HP steam (right).