• Introduction • Techniques available for process
integration• Pinch technology• Features and Benefits of Pinch• Where pinch technology is used?• Concept of pinch technology• Phases of pinch technology• A Retrofit Project
Contents
PROBLEMS OF PROCESS INDUSTRY RELATED TO ENERGY
ENERGY MANAGEMENT THROUGH PROCESS INTEGRATION - A REALITY
• Problems of Indian industries can be solved by using techniques that minimize energy consumption with minimum investment.
• Process integration is one such technique.
• Borrowed, often obsolete technology
• Energy consumption per unit production much higher than in western industries
• No concept of process integration
PROCESS INTEGRATION
The Process Integration is defined as “Systematic and general methods for designing integrated production systems, ranging from individual processes to total sites, with special emphasis on the efficient use of energy and reducing environmental effects”.
Process Integration is a part of Process Intensification (PI).
Ramshaw, 1995, defined PI as: “A strategy for making dramatic reductions in the size of a chemical plant so as to reach a given production objective”.
Techniques available for process Integration
Pinch Technology Approach
MILP/MINLP Approach
State-Space Approach
Genetic Algorithm Approach
Process Graph Theory Approach
Supertargeting Approach
Methods in Process Integration
The three major features of Process Integration methods are:
• The use of heuristics (insight)• The use of thermodynamics • The use of optimization techniques.
Pinch Analysis is a method with a particular focus on Thermodynamics. Hierarchical Analysis and Knowledge Based Systems are rule-based approaches with the ability to handle qualitative (or fuzzy) knowledge. Finally, Optimization techniques can be divided into deterministic (Mathematical Programming) and non-deterministic methods (stochastic search methods such as Simulated Annealing and Genetic Algorithms).
HierarchicalAnalysis
HeuristicRules
KnowledgeBased Systems
Thermodynamic Methods
Optimization Methods
QUALITATIVE
QUANTITATIVE
INTERACTIVEAUTOMATIC
Fig. 1 One possible Classification of Process Integration Methods
One possible classification of Process Integration methods is to use the two-dimensional (automatic vs. interactive and quantitative vs. qualitative) representation in figure 1.
Pinch technology reveals all the possible savings and their corresponding financial benefits.
• It defines the maximum possible savings.• It looks at the overall site.• It does not bench-mark but takes into account all specific mill
factors, age, location, process equipment, operating preferences, product, etc.
• It reveals the maximum cogeneration potential
PINCH TECHNOLOGY
Pinch Technology was introduced by Linnhoff in 1978 to solve heat exchange problems as an energy saving tool. Pinch Technology forms the essence of optimization of processes by energy and resource analysis (OPERA).
Features and Benefits of Pinch…
• Targets for minimum heating & cooling.
• Quantifies scope for heat recovery.
• Analysis Includes the process unit or the whole site, as appropriate:
• Design tools define appropriate project.
• Shows what to do with low-grade waste heat.
• Combined Heat and Power (CHP).
• Practical application brings real benefits.
WHERE PINCH TECHNOLOGY IS USED?
Heat integration Distillation column targeting Cogeneration & total site targeting Batch process targeting Emission targeting Mass exchange network ( Water & waste water
management & recovery of valuable materials) Hydrogen management in refineries
CONCEPT OF PINCH TECHNOLOGY
ONION DIAGRAMFig. 2. The process design hierarchy can be represented by “onion diagram” as shown below.
Reactor
SeparatorHeat exchange network
Utilities
The heat and material balance is at this boundary
Site-Wide Utilities
Fig. 2 Onion Diagram
1
2
34
Main points from onion diagram
Design of a process starts with the reactors.
Separator can be designed for known feeds, products, recycle concentrations and flow rates.
For heat and material balance, heat exchange network (HEN) can be designed.
For remaining heating and cooling duties, the utility system is designed.
PROBLEM ADDRESSED Generally two types of problem are addressed:
• Creating New Designs
This is related to the design of HEN for a new plant, which is in design stage.
• Retrofit – Revamping Existing Designs
This is related to the retrofitting of an already existing HEN in a plant to improve its exchange efficiency.
PHASES OF PINCH TECHNOLOGY
There are four phases of pinch analysis in the design of heat recovery systems for both new and existing processes:
DATA EXTRACTION
It relates to the extraction of information required for pinch technology from a given process heat and material balance. PERFORMANCE TARGETSTargeting provides a fundamental insight into heat recovery options in a process. It does this by giving a system-wide view of the heating and cooling requirements at different temperature levels.
NETWORK DESIGNINGIn design the user will typically work with an incomplete network and try to follow the pinch design rules.
NETWORK OPTIMIZATIONHeat exchange network for maximum energy recovery established by pinch design method, should only be regarded as initial designs and some final optimization is required.
Graphical Representation
Composite curve
Fig. 6. The HCC and CCC show the heat availability and heat requirement for the overall process
0
50
100
150
200
0 1000 2000 3000 4000 5000
Heat Content Q (kW)
T (
C)
HCC
CCC
Region of heat recovery by process to process exchange
QHmin
QCmin
Tmin
Above pinch
Below pinch
Analytical ProcedureProblem Table Algorithm (PTA)
For the calculation of energy targets, only the inlet temperatures, outlet temperature and heat capacity flow rates are required.
The steps involved in PTA are:
• Determination of temperature intervals
• Calculation of net MCP in each interval
MCp,int = MCp,c – MCp.h for each interval
• Calculation of net enthalpy in each interval
• Calculation of cascaded heat
• Revision of cascaded heat
• Determination of energy targets
Implementation of Problem Table Algorithm
Interval i.
Col. ATint
Col. BMCp,int
Col. CQint
Col. DQcas
Col. ERcas
0123456
streamMCp
16512211555503530
01025-15251020
0430175-900125150100
0-430-60529517020-80
605175
0900775625525
H1 H2 C3 C410 40 20 15
The composite curve gives the information as:
Minimum hot utility (QHmin) = 605 kW
Minimum cold utility (QCmin) = 525 kW
Hot pinch temperature = 125 ˚C
Cold pinch temperature = 105 ˚C
Tmin is known as the “pinch” and once the pinch is recognized it is possible to consider the process as two separate systems: one above the pinch and one below the pinch. The system above the pinch requires a heat input and is therefore a net heat sink. Below the pinch, the system rejects heat and so is a net heat source.
Concept of Pinch
Concept Of Multiple Utility
The energy requirement for a process is supplied via several utility levels e.g. steam levels, refrigeration levels, hot oil, furnace flue gas etc.
The general objective is to maximize the use of the cheaper utility levels and minimize the use of the expensive utility levels.
The composite curve provide overall energy targets but do not clearly indicate how much energy needs to be supplied by different utility levels. For this purpose, the grand composite curve is used.
Grand Composite Curve
Fig. 7. GCC shows the multiple utilities
0
20
40
60
80
100
120
140
160
180
0 100 200 300 400 500 600 700 800 900 1000
Heat flow Q (kW)
T (
*C
)
Pinch
High temperature process sink profile
Low temperature process source profile
HU1
CU1
CU2
HU2
Process to process heat exchange
The balanced composite curve is generated to estimate the targeted area.
Fig.8. The balenced composite curves
020406080
100120140160180200
0 1000 2000 3000 4000 5000Heat Content Q (kW)
T (C
) BHCC
BCCC
Interval i. Th,i.-1
Tc,i.-1
Th,i.
Tc,i.
BALANCED COMPOSITE CURVES
Fig. 4: Potential energy savings in some major industrial sectors
Fig. 5: Potential water consumption savings in some major industrial sectors
A RETROFIT PROJECT
Even in a simple process made up of two unit operations (Fig. 6 & 7), a reactor and separator, with a recycle stream, pinch technology has something to offer. In this case, a pinched design (right) reduces steam consumption by 38%, eliminating the need for external water cooling, cutting the number of heat exchangers needed from six to four, and reducing heat transfer surface area requirements from 629 to 533 m2.
Reactors
Unpinched
Steam
Steam
Recycle
Separator
Feed
Cooling mater
Product
Reactors
Pinched process
Steam
Recycle
Separator
Feed Product
Figure.6 Figure.7
RETROFIT PROJECT
The pinch technology principle is applied on various projects. One such project, Fig. 8, consists of a complex refining system. The process is already highly integrated, with various streams being heat-exchanged to reduce overall energy requirements. Application of pinch technology in this project results a minimum energy target of about 31 MW for the hot utility, which is steam. The original process consumed nearly 39 MW of steam. Thus, the “scope” for improvement is 8 MW or 20% of original demand. A brief analysis of the existing flow sheet uncovered the specific reasons for current utility requirements being greater than the target:1. One instance of utility heating below the pinch.2. One instance of utility cooling above the pinch.These violations totaled 8MW. If eliminated, they would bring the utility requirements to target. The problem was, it would take six new exchangers and a substantial new investment to accomplish this.
Steam
Reactor
Recycle
3 1 2 4 5
17,000 lb/hr Steam
70psi
Feed and recycle Recycle
Flash Stripper
Preheater
20 psi
Fig. 8 Process prior to retrofit.
At this stage, one should always look for “process modifications” that included the following:
1. Decrease the pressure of column No. 2 by 5 psi (34 kPa).2. Decrease the pressure of column No. 5 by 10 psi (69 kPa).
The effect of these modifications resulted the energy target for the process so modified was 27 MW representing another 15 % potential savings. The problem was, it would still take six new exchangers and a substantial new investment to accomplish this.At this stage, one should look for “second order” process modification aimed at simplifying the necessary hardware changes. Modify the process slightly so that its enthalpy changes fit as many of the existing exchangers as possible? These major adjustments led to the sacrifice of 1.3 MW but helped to save four exchangers reducing the number of new exchangers from six to two.The flow sheet for the process finally recommended is shown in Fig. 9. Process modifications and two new exchangers combined give 28 % energy savings at six months payback.
Steam
Reactor
3 1 2 4 5
11,000 lb/hr Steam
60psi
Feed and recycles
Recycle
Flash
Stripper Preheater
15 psi
Fig. 9 The process after retrofit.
New exchangers
CONCLUSIONS
With all of the tools that pinch analysis provides, one of the most important challenges before is to properly integrate pinch tools into the conceptual process design phase. Decisions made in the phase of planning affect the entire life cycle of a process facility. Using pinch technology tools & understanding the process doesn’t ensure the desired results. These tools must be applied at the right point in the process design phase. Just as it be incorrect to conduct a pinch analysis after completion of the process design phase, wherein critical process parameters have been fixed, it is just as incorrect to conduct a pinch analysis without a direct interaction with the process specialists & downstream engineering disciplines. It is Pinch Technology’s role to identify “what might be”. However, input from other engineering disciplines ultimately determines “what can be”.
REFERENCES
1. Plant Design And Economics For Chemical EngineersMax S Peter,Ronal E West 5th Edition,McGraw Hill
2. Richardson & Colson Vol.6
3. Genaral Process Improvement Through Pinch Technology B.linnoff , G.T pollen
Top Related