Wednesday, April 9, 2008

Pinch Technology : Part 01

Introduction.
While oil prices continue to climb, energy conservation remains the prime concern for many process industries. The challenge every process engineer is faced with is to seek answers to questions related to their process energy patterns. A few of the frequently asked questions are:
  1. Are the existing processes as energy efficient as they should be?
  2. How can new projects be evaluated with respect to their energy requirements?
  3. What changes can be made to increase the energy efficiency without incurring any cost?
  4. What investments can be made to improve energy efficiency?
  5. What is the most appropriate utility mix for the process?
  6. How to put energy efficiency and other targets like reducing emissions, increasing plant capacities, improve product qualities etc, into a one coherent strategic plan for the overall site?
All of these questions and more can be answered with a full understanding of Pinch Technology and an awareness of the available tools for applying it in a practical way. This article aims to provide the basic knowledge of the concepts in pinch technology and how they have been be applied across a wide range of process industries. The articles comprises of following six sections: 1. What is Pinch Technology?
  • Meaning of the Term Pinch Technology
  • Basis of Pinch Technology
  • Objectives of Pinch Analysis
  • A Simple Example of Process Integration by Pinch Analysis
  • Development of Pinch Technology Approach
  • Areas of Applications of Pinch Technology
2. Basic Concepts of Pinch Analysis
  • Composite Curves
  • DTmin and Pinch Point
  • Grand Composite Curve
  • Energy Cost, Capital Cost, and Total Cost Targeting
  • Energy Cost and Capital Cost Trade-Off
  • Plus/Minus Principle of Process Modification
  • Appropriate Placement Principles for Key Process Equipments
  • Total Site Analysis
3. Steps of Pinch Analysis
  • Identification of Hot, Cold, and Utility Streams in the Process
  • Thermal Data Extraction for Process and Utility Streams
  • Selection of Initial DTmin Value
  • Construction of Composite Curves and Grand Composite Curve
  • Estimation of Minimum Energy Cost Targets
  • Estimation of Heat Exchanger Network Capital Cost Targets
  • Estimation of Optimum DTmin Value
  • Estimation of Practical Targets for HEN Design
  • Design of Heat Exchanger Network (HEN)
4. Benefits and Applications of Pinch Technology
  • General Process Improvements
  • Industrial Applications
5. The Future Outlook Of Pinch Technology
  • Regional Energy Analysis
  • Total Site Analysis
  • Network Pinch
  • Top Level Analysis
  • Combined Heat and Power Optimisation
  • Water Pinch
  • Hydrogen Pinch
What is Pinch Technology? Meaning of the term "Pinch Technology" The term "Pinch Technology" was introduced by Linnhoff and Vredeveld to represent a new set of thermodynamically based methods that guarantee minimum energy levels in design of heat exchanger networks. Over the last two decades it has emerged as an unconventional development in process design and energy conservation. The term ‘Pinch Analysis’ is often used to represent the application of the tools and algorithms of Pinch Technology for studying industrial processes. Developments of rigorous software programs like PinchExpressTM, SuperTargetTM, Aspen PinchTM have proved to be very useful in pinch analysis of complex industrial processes with speed and efficiency. Basis of Pinch Analysis Pinch technology presents a simple methodology for systematically analysing chemical processes and the surrounding utility systems with the help of the First and Second Laws of Thermodynamics. The First Law of Thermodynamics provides the energy equation for calculating the enthalpy changes (dH) in the streams passing through a heat exchanger. The Second Law determines the direction of heat flow. That is, heat energy may only flow in the direction of hot to cold. This prohibits ‘temperature crossovers’ of the hot and cold stream profiles through the exchanger unit. In a heat exchanger unit neither a hot stream can be cooled below cold stream supply temperature nor a cold stream can be heated to a temperature more than the supply temperature of hot stream. In practice the hot stream can only be cooled to a temperature defined by the ‘temperature approach’ of the heat exchanger. The temperature approach is the minimum allowable temperature difference (DTmin) in the stream temperature profiles, for the heat exchanger unit. The temperature level at which DTmin is observed in the process is referred to as "pinch point" or "pinch condition". The pinch defines the minimum driving force allowed in the exchanger unit. The basic concepts of Pinch Analysis are discussed in the next section, Basic Concepts of Pinch Analysis. Objectives of Pinch Analysis Pinch Analysis is used to identify energy cost and heat exchanger network (HEN) capital cost targets for a process and recognizing the pinch point. The procedure first predicts, ahead of design, the minimum requirements of external energy, network area, and the number of units for a given process at the pinch point. Next a heat exchanger network design that satisfies these targets is synthesized. Finally the network is optimized by comparing energy cost and the capital cost of the network so that the total annual cost is minimized. Thus, the prime objective of pinch analysis is to achieve financial savings by better process heat integration (maximizing process-to-process heat recovery and reducing the external utility loads). The concept of process heat integration is illustrated in the example discussed below. A Simple Example of Process Integration by Pinch Analysis Consider the following simple process [Figure 1(a)] where feed stream to a reactor is heated before inlet to a reactor and the product stream is to be cooled. The heating and cooling are done by use of steam (Heat Exchanger -1) and cooling water (Heat Exchanger-2), respectively. The Temperature (T) vs. Enthalpy (H) plot for the feed and product streams depicts the hot (Steam) and cold (CW) utility loads when there is no vertical overlap of the hot and cold stream profiles.

Figure 1(a): A Simple Flow Scheme with T-H profile

An alternative, improved scheme is shown in Figure 1(b) where the addition of a new ‘Heat Exchanger–3’ recovers product heat (X) to preheat the feed. The steam and cooling water requirements also get reduced by the same amount (X). The amount of heat recovered (X) depends on the ‘minimum approach temperature’ allowed for the new exchanger. The minimum temperature approach between the two curves on the vertical axis is DTmin and the point where this occurs is defined as the "pinch". From the T-H plot, the X amount corresponds to a DTmin value of 20 oC. Increasing the DTmin value leads to higher utility requirements and lower area requirements. The estimation of optimum economic value of DTmin is discussed later.

Figure 1(b): Improved Flow Scheme with T-H profile

Source : http://www.cheresources.com

3 Comments:

Obakasan said...

Menarik, tapi adakah yg mengetahui tentang software yang digunakan di bidang pinch technology? terutama water and energy

yantze said...

google prospect research group, utm, malaysia kebetulan saya baru ikutan this research group. Group ini ada yang sudah buat komersil softwarenya.

Obakasan said...

Ya, saya juga mendengar tentang UTM yg membanggakan software pinch untuk water and energy. Apakah saya bisa mendapatkan softwarenya?

 




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