Pinch Analysis: For the Efficient Use
WATER PINCH TECHNOLOGY
Introduction
Process integration (PI) is an efficient approach that allows industries to increase their
profitability through a bunch of methods, including reductions in energy, water and raw
materials consumption, reductions in green house gas (GHG) emissions, and in waste
generation. Therefore, process integration is always defined as all improvements made to
process systems, their constituent unit operations, and their interactions to maximize the
effective use of energy, water, and raw materials. [1] Among PI methodologies, pinch analysis is
certainly the most widely used.[2] This is due to the simplicity of its underlying concepts and,
especially, to the spectacular results it has obtained in numerous projects worldwide.
Pinch analysis techniques for integrated network design were originally developed from the
1970s onwards at the ETH Zurich and Leeds University (Linnhoff and Flower 1978; Linnhoff
1979).[1] At first, it had been used to design the heat exchange network. Later, with the
development of techniques, pinch analysis is no longer limited to solve the energetic
problem, it has been extended to the optimization of water and raw material using systems.
In general, pinch analysis is a unique process design expertise dedicated to maximize your
process energy efficiency at minimum capital expenses.[3] Different from the specific methods
focus on the improvement of single unit or partial region of consuming system, pinch
analysis treats the flow system as an organic whole, considers the distribution quality and
quantity of water, energy or raw materials between different unit and finally reaches the
optimization of the whole system, including: (1) maximize the match between supplies and
demands of individual commodity (principally energy, hydrogen or water); (2) minimize the
import of purchased utilities; (3) minimize the discharge of relevant waste.
Overview of Pinch Analysis Method
In order to find out the minimum water requirement in the plant, pinch analysis method is
often used in two steps [5]:

Step 1: Determine the process minimum energy usage

Step 2: Find the design that meets this minimum value
In practice, it is a complex problem involved amount of problems, like construction,
operation and profits. An effective way to approach the complexities of energy, water and
raw material systems is to divide the problem into different phases. It is not the
fundamentals of pinch methodology in detail, which we will discuss later, but an overview of
the phases that comprise a pinch analysis.
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Figure 1 shows a breakdown of tasks normally performed during a typical pinch study. The
dark blue (heavily shaded) boxes are main activities; the light blue (lightly shaded) boxes are
activities that can be used in separate or combination.
Figure1. Structured Approach of Pinch Analysis
DATA COLLECTION
Design Data
Simulation
Measurements
MODELING
PINCH ANALYSIS
ITERATIVE SOLUTION
PROJECT IDEAS
Measurements
Site Expertise
Simulation
PI Experience
SITE IMPROVEMENT
In the data collection phase, all three types of data are valuable for performing a pinch
analysis:

Measurements are the most appropriate basis for evaluation, which define the process
currently in operation.

Simulation is the best data source for a study, which includes the previous category and
a consistent usage and consumption balance.

Design data may be used where Measurements cannot be preformed.
Project ideas need to be evaluated with one or more methods and then adjust these ideas
though modeling process. This may be an iterative solution, because all these processes may
be repeated several times.
Water Pinch Analysis
Water Pinch Analysis (WPA) is a systematic technique for reducing water consumption and
wastewater generation through integration of water-using activities or processes. [2] Key target
of water pinch analysis is maximizing water reuse and minimizing the amount of wastewater.
Techniques for setting targets for maximum water recovery capable of handling any type of
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water-using operation including mass-transfer-based and non-mass-transfer based systems
include the source and sink composite curves and water cascade analysis (WCA).[3]
The source and sink composite curves is a graphical tool for setting water recovery targets as
well as for design of water recovery networks.[4] It considers each water-using operation as
being described by the mass transfer of a certain contaminant(s) from the process itself to
the water stream (Figure.2a). First of all, in order to achieve a better understanding of the
composite curves, a single water using unit is analyzed in this section.
Figure.2 (a) Model of Mass Transfer in Water Using Unit
(b) Loading and Concentration Figure
(a)
Limiting Water Supply Line
Water Supply Line
(b)
This process can also be described by the C-M figure (Figure.2b). The horizontal ordinate M
represents the contaminant mass loading, while vertical ordinate C is the contaminant
concentration. The highest concentration, which is also the highest line in C-M figure, is the
contaminant line; the other lower lines are water supply lines. The larger the slope of water
supply line is, the smaller the flowrate of the supplied water will be. In order to determine
whether the recovered water can be used in the specific unit, maximum inlet contaminant
concentration (C In, Max) should be provided, which is named as limiting inlet concentration.
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And, in order to certify the minimum inlet flowrate, the maximum outlet flowrate (C Out, Max)
is needed. With these two values, the limiting water supply line is certified (the first solid
line under the contaminant line in Figure.2b). From Figure 2b, we can see that any water
supply line under limiting water supply line can meet the requirement of the water using
unit. It is the explanation of single unit analysis, which is also a component of a whole water
pinch analysis.
In order to optimize the whole system, we need to consider the water using condition
entirely. Therefore, it requires us to recombine all the composite curves as one curve and
analyzed this integrative curve. The detailed process is show in Figure.3.
Figure.3 Composite Curve and Limiting Composite Curve
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3
□
1
□
2
□
1
2
4
3
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By specifying the maximum allowable inlet and outlet contaminant concentrations for each
operation unit, a composite curve of all the units is built (Figure.3.1). Then, on the basis of
the composite curve, a limiting profile can be constructed. These are combined to form a
limiting composite curve (Figure.3.3, 3.4), against which a water supply line can be matched.
As we mentioned before, the entire water supply curve which are underneath the limiting
composite curve can meet the requirements of water using in the units. We assume the inlet
concentration of the fresh water is zero. In order to minimize the consumption of fresh water,
the outlet concentration should be as large as possible. When the slope of the water supply
curve increases to touch the limiting composite curve, the mass transfer driving force is
minimum, outlet concentration is maximum, and consumption of the fresh water is
minimum. This touching point is the water pinch point, as shown in Figure.3.4. Water pinch
point has a significant meaning for the water system design. Above the water pinch point,
limiting inlet concentration of the unit is larger than the pinch point concentration, there’s
no need for the unit to use fresh water. Blow the water pinch point, limiting outlet
concentration is smaller than the pinch point concentration, the wastewater in this unit
should not be discharged, since it can be reused as the inlet water of the other unit after
some treatment or directly.
Application of Water Pinch Point
There are several types of water reuse solutions, with or without water treatment. One of a
simple solution is: a water pinch study begins with the assumption that existing inlet
concentrations (concentration of water sinks) are at their maximum acceptable limits for all
site processes/equipments. This indicates the minimum water usage under currently
imposed constraints on inlet concentrations. Projects identified at this stage will be low-cost
opportunities involving only pipework modifications, but generally allowing only small
water reuse opportunities.
Figure.4 One Design of Water Reuse
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Based on this water pinch analysis, continue to use the example above, the total saving
percentage of the fresh water and the reduction of the wastewater can be figured out, which
is nearly 23% (Figure.4).
Reference
[1] Linnhoff, B., 1993. Pinch analysis: a state-of-the-art overview. Trans. I Chem E (Part A) .71,
503_522.
[2] Polley, G.T., Polley, H.L., 2000. Design better water networks. Chem. Eng. Prog 96 (2)., 47_52.
[3] Savelski, M.J., Bagajewicz, M.J., 1999. WaterSave: a New Approach to the Design of Water
Utilisation Systems in Refineries and Process Plants. AIChE Spring Meeting, Houston.
[4] Kim K J，Smith R．Automated design of diacontinuous water system[J]．Process Safety and
Environmental Protection，2004．82(3)：238—248．
[5] Wang, Y.P., Smith, R., 1994. Wastewater minimisation. Chem. Eng. Sci. 49, 981_1006.
Ran Zhao Factsheet 2
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