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Process Integration for optimum energy efficiency

The process industry sector uses a large amount of energy for heating, cooling and motive power.

The traditional approach to reducing energy consumption, and thereby CO2 emissions, has tended to concentrate on specific equipment, eg pumps, motors, boilers, etc. It is also well known that further savings may be found by examining energy systems rather than individual items of equipment in isolation. A typical example is investigating a site compressed air system as a whole rather than looking only at the compressors.

On many process plants this can be taken a stage further by adopting a systematic and holistic Process Integration approach to heating, cooling, and power generation across the site as a whole. This often identifies sizeable additional savings and provides the company with important guidance for longer-term efficiency improvements. Where water consumption is significant, including water systems in the analysis may increase the savings even more.

Key aspects of the Process Integration approach

It is constructive to include not only Pinch technology but a portfolio of more or less conventional process/chemical engineering analysis techniques under the general heading of Process Integration. This ranges from simple process flowsheets to sophisticated heat and material balance simulation plus a number of less obviously energy related tools such as Hazop analysis. Collectively they provide a powerful toolkit for optimising the use of natural resources in process plants, individual components being applied as appropriate on a case by case basis.

Outside the group of specialists practicing it, the scope of Process Integration is widely misunderstood. The terms Process Integration (PI) and Pinch Technology are sometimes used synonymously but this can be misleading. Process Integration is better seen as a suite of techniques of which Pinch is just one element, albeit an important one. Several other aspects of PI methodology (eg heat and material balances) are prerequisites to carrying out a formal Pinch analysis. In many cases Pinch analysis is essential to realising the full scope for savings. On the other hand, it is not infrequent for a large percentage of the benefit to be obtained from the preliminary stages without needing to carry out a formal Pinch analysis.

Inconsistent uptake of PI across the process sector

Much of this wider PI shopping basket has been available for a long time as standard good practice Chemical Engineering methodology. At the hi-tech end of the industry it is sometimes applied as standard practice together with increasingly sophisticated software, often based on PI principles for process optimisation.

At the other end of the spectrum, many process plants still don’t have a decent process flowsheet, let alone a heat and material balance or a pinch analysis. This has generally got worse over the last 25 years as relentless pressure to minimise costs has decimated technical manpower not directly related to day to day production. For many sites in this category, even an unsophisticated approach to PI can lead to significant long term energy savings.

Understanding the flowsheet – material and energy flows

On many sites, there are many material and energy flows, including a significant number of heating and cooling duties. Although the overall energy consumption may (or may not) be reasonably well documented, individual flows are often unknown. In order to obtain an optimum solution, it is important to understand the principal energy and material flows – ie to have a reliable and consistent heat and material balance (H&M balance), to know what those flows, temperatures, pressures etc are and why they are necessary.

This is often confused with simply taking more measurements. Clearly an H&M balance requires some measurements to be made. However, because the balance is constrained by conservation of mass and energy, it can be used both to validate (or discredit) individual measurements and to interpolate values with a minimum of additional (expensive) measurement devices.

H&M balances vary from simple hand calculations to sophisticated modular simulations. Different tools are applicable depending on the context. Unfortunately, many operating process sites are unaware of the right tools and even fewer have access to them or practical experience of their application.

At a more basic level still, many process sites don’t even have appropriate flowsheets on which to define and record a conceptual understanding of the process under review.

Energy and water

Water is used for a wide range of purposes on many process sites. Some of these are directly related to energy, eg cooling water, evaporative cooling towers, boiler feed water, steam condensate, hot wash water etc. In many processes water is part of the process flow stream, eg paper pulp, food and drink manufacture etc. There are often opportunities to reuse water as well as to recover heat. Sometimes saving water automatically saves heat by retaining it in the system but the interactions are more complicated. For example, reducing water throughput can lead to temperature build up in the process and the need for additional cooling. It is therefore important on many sites to tackle energy saving and water minimisation in an integrated manner.

Pinch Technology

Pinch Technology is a set of techniques for systematic analysis of process thermal requirements (and water use/reuse). The methodology results in a rigorous understanding of both the quantity and the temperature of all the individual process heating and cooling duties in the context of the site as a whole. This allows the maximum possible heat recovery and therefore the minimum overall heating and cooling utility demands (targets) to be defined. It also identifies the effect of changes to individual process operations on the minimum utility demands, thus showing which changes really are beneficial. Pinch methodology also includes efficient methods of designing the optimum heat recovery configuration and it identifies which existing heat transfer duties are incompatible with maximum energy recovery. Thermal Pinch is now a relatively mature technology with practical applications on a large number of process plants and a successful track record over almost 20 years.

Water Pinch is an analogous approach based on quantity of aqueous flows and concentrations of contaminants. Being multi-dimensional, it is rather more difficult to apply. In many cases, however, thermal Pinch analysis also results in large water savings because temperature, and therefore heat, are often the key water quality parameters.

Pinch Technology provides a clear and systematic way for process plant sites to understand how and how much they can improve the efficiency with which they use heat, and sometimes also water. Implementing the findings leads to optimum reduction in attributable carbon emissions.

Because Pinch is concerned with optimising the use and reuse of heat and doing so at the most appropriate temperature, it is an important tool for optimising combined heat and power (CHP).

There are two key aspects:

Optimising energy consumption for the site as a whole

It may not be immediately obvious that the optimum energy solution for a site as a whole is not necessarily achieved by minimising the amount of energy used in each of the individual items of equipment. The thermodynamic value of energy, especially heat, depends on its quality as well as its quantity. For example, high quality (ie high temperature) heat rejected from one operation can be reused in another less demanding application. Thus a project which minimises the quantity of energy used by the first operation may actually increase the total energy demand if it results in the heat rejected being too cold to be reused in the second operation.

Optimising thermal energy consumption therefore requires that the best opportunities for heat recovery are identified and also any changes to individual operations themselves which would further increase the scope for heat recovery.

Process Integration in practice

Whereas PI is applied fairly regularly in some high-tech process industries, on the majority of other process sites it is still seldom used. Moreover, relatively few engineers are aware of what it can and cannot achieve in a given context. In the early days of Pinch Technology, some aspects were oversold, leading to a number of case studies which did not always meet expectations. Subsequently, practical experience has developed but there is still a residual school of thought that “Pinch is not relevant to my industry”. This is unfortunate because it is not necessarily true and a number of good carbon saving opportunities are failing to be identified.

PI crosses the boundaries of several disciplines and because relatively few engineers have a working knowledge of what it entails, PI frequently fails to appear as a category in registers of expertise, technology lists etc – to the extent that readers not already familiar with PI and Pinch would be unlikely to appreciate its relevance to their own sites.

The consequence of this situation is a self-perpetuating under-application of PI and Pinch techniques with the result that many opportunities for cost effective reduction of energy and water consumption are failing to be identified. At a time when there is growing political will to improve energy efficiency, more extensive application of PI should be pro-actively encouraged.

Copyright © HRC Consultants Ltd 2014

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