Heat integration and HEN Design

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HEAT INTEGRATION HEAT INTEGRATION: Department of Chemical Engineering University of Engineering And Technology, Lahore A.N.Tabish 2009-MS-Chem-25 HEAT EXCHANGER NETWORK DESIGN

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INTRODUCTION: A cold process stream can be heated by using steam or any hot utility, available at a temperature higher than the target temperature of process stream and a hot process stream can be cold by using cooling water or any cold utility, available at a temperature lower than the target temperature of process stream. Situation refers the maximum use of utility that is merely an operating cost, and maximum annualized cost that demands a larger rate of return to make the process profitable. A cold process stream can be used to lower the temperature of hot process stream, which heats up the cold stream as well. Hence minimizing the use of utility and ultimately operating cost and total annualized cost. HEAT INTEGRATION tabish288@yahoo.com

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“Heat Integration”, enables the maximum heat exchange between process streams using “Pinch Technology”, that revealed various methods to maximize process-to-process heat exchange and minimized the use of utilities through an integrated network of heat exchangers. Application of Pinch analysis minimizes the energy consumption of chemical processes by calculating thermodynamically feasible energy targets and achieving them by optimizing heat recovery systems, energy supply methods and process operating conditions. It is also known as process integration, heat integration, energy integration or pinch technology. tabish288@yahoo.com

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Improvement in the energy consumption obtained after successive designs of given product.

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PROFITABILITY OPTIMIZATION: Design for “Minimum Capital Cost” Design for “Minimum Energy Cost” Optimum Design for “Minimum Energy Cost”

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Simple process flow sheet Heat integrated flow sheet tabish288@yahoo.com

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PINCH TECHNOLOGY: Data Extraction: To formulate the process flow sheet and perform heat and mass balance. Minimum Approach Temperature: To select the minimum temperature difference that can be allowed across any heat exchanger in the network. Composite Curves: To draw the enthalpy vs. temperature graph for cold streams as well as for hot streams Minimum utility targets: To calculate the minimum heating and cooling requirements that must be supplied by utility system. Heat Exchanger Network (HEN) Design: To design a network of heat exchangers to exchange the energy between process and utility streams. Network Relaxation and Optimization: To modify the network to eliminate the small exchangers which are not cost effective. Process Change: To alter the process conditions of unit operations and other streams to maximize heat integration. tabish288@yahoo.com

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DATA EXTRACTION: Process Flowsheet Operating Conditions for process streams & unit operations Heat and mass balance Hot and cold stream allocation tabish288@yahoo.com

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MINIMUM APPROACH TEMPERATURE (ΔTmin):

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Area and cost variation with ΔTLM

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COMPOSITE CURVES: tabish288@yahoo.com

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The hot streams plotted separately The composite hot stream

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The cold streams plotted separately The composite cold stream

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MINIMUM UTILITY TARGET: For an efficient network synthesis it is required to evaluate MAXIMUM ENERGY RECOVERY (MER) so that MINIMUM UTILITY TARGET can be selected and network may be designed so as to satisfy the energy requirements for each process stream that is the determination of minimum hot and cold utility requirements in the process. Three methods are widely used for the estimation of minimum utility requirement. Graphical method Algebraic method Computer based methods tabish288@yahoo.com

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MINIMUM UTILITY TARGET: (Graphical Method) Production of Maleic Anhydride from Benzene

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HEAT EXCHANGE STREAM DATA tabish288@yahoo.com

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INITIAL TEMPERATURE INTERVAL TABLE

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Initial composite diagram with ΔTmin = 28 oC Hot stream pinch Temperature = 358 oC Cold stream pinch Temperature = 330 oC. So, ΔTmin at pinch point is 28 oC. While allowable ΔTmin for chemical processes is 3 - 10 oC.

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Revision of temperature interval table Slop of the curve at pinch point = 0.0475 Intercept of the curve = - 440 For y = mx + c Modified enthalpy change rate at 358 oC is 16800 kJ/sec. This value of 16800 must be increased to 17000 to make the temperature approach at the pinch point to 10oC. Therefore 200 kJ/hr must be added to every enthalpy change rate value associated with the streams to be heated. Hence, Difference of baseline enthalpy change rates = 200 kJ/sec So, Revised baseline enthalpy change rate for streams to be cooled = 17200 kJ/sec. tabish288@yahoo.com

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Revised temperature interval table

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Revised composite diagram with ΔTmin = 10 oC Minimum heating utility required = 17232 – 17200 = 32 kJ/sec Minimum cooling utility required = 11000 – 1200 = 9800 kJ/sec Revised composite diagram with ΔTmin = 10 oC tabish288@yahoo.com

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MINIMUM UTILITY TARGET: (Algebraic Method) Algebraic method is also termed as “Problem Table Method”. Although composite curves can be used to set energy targets, yet they are inconvenient because they are based on a graphical construction. The problem table is the name given by Linnhoff and Flower to a numerical method for determining the pinch temperatures and the minimum utility requirements; Linnhoff and Flower (1978) also termed as “Temperature Interval Method”. Once understood, it is the preferred method, avoiding the need to draw the composite curves. tabish288@yahoo.com

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Hot stream Temperature: Cold stream Temperature: Shifted temperatures for data: tabish288@yahoo.com

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Shifted temperatures for data: The temperature interval heat balance:

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HEAT EXCHANGER NETWORK DESIGN HEAT EXCHANGER NETWORK DESIGN: tabish288@yahoo.com

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PROBLEM STATEMENT: Given a number NH of process hot streams (to be cooled) and a number NC of process cold streams (to be heated), it is desired to synthesize a cost-effective network of heat exchangers that can transfer heat from the hot streams to the cold streams. Given also are the heat capacity of each process hot streams, FCP,u; its supply (inlet) temperature, Ts,u; and its target (outlet) temperature, Tt,u, where u = 1, 2 ,………., NH. Given also are the heat capacity of each process cold streams, FCP,v; its supply (inlet) temperature, Ts,v; and its target (outlet) temperature, Tt,v, where v = 1, 2 ,………., Nc. Available for service are NHU heating utilities and NCU cooling utilities whose supply and target temperatures (but not flowrates) are known.

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CAPITAL AND ENERGY COSTS: Heat exchanger network that would seem appropriate to most when energy is cheap and capital expensive. Heat exchanger network that would seem appropriate to most when energy is expensive and capital cheap.

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CAPITAL AND ENERGY COSTS Cont’d…..: Capital Cost = f(Thermodynamic effects) = f(Driving forces, Heat loads) Evidently, as we go to tighter designs (i.e. to reduce driving forces) we need less utility and the overall heat load decreases. Capital cost then increases with reduced driving forces (we all know that) but decreases with reduced heat load (we rarely consider this point).

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CAPITAL AND ENERGY COSTS Cont’d…..:

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HEAT EXCHANGER NETWORK DESIGN: The process streams are drawn as horizontal lines, with the stream numbers shown in square boxes. The Hot streams are drawn at the top of the grid, and flow from left to right. The cold streams are drawn at the bottom, and flow from right to left. The stream heat capacities CP are shown in a column at the end of the stream lines. Heat exchangers are drawn as two circles connected by a vertical line. The circles connect the two streams between which heat is being exchanged; that is, the streams that would flow through the actual exchanger. Heater and coolers are drawn as a single circle, connected to the appropriate utility.

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THE PINCH DESIGN METHOD: If the energy target set by the composite curves is to be achieved, the design must not transfer heat across the pinch by: Process-to-process heat transfer Inappropriate use of utilities QHmin QCmin tabish288@yahoo.com

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THE PINCH DESIGN METHOD Cont’d…..: Stream matching is started at the pinch point to avoid the violation of the assumption of ∆Tmin. If the design is started away from the pinch at the hot end or cold end of the problem, then initial matches are likely to need follow-up matches that violate the pinch or the ∆Tmin criterion as the pinch is approached. If the design is started at the pinch, then initial decisions are made in the most constrained part of the problem. This is much less likely to lead to difficulties later. If such matches are not made, the result will be either use of temperature differences smaller than ∆Tmin or excessive use of utilities resulting from heat transfer across the pinch. tabish288@yahoo.com

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THE PINCH DESIGN METHOD Cont’d…..: tabish288@yahoo.com

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THE PINCH DESIGN METHOD Cont’d…..:

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THE PINCH DESIGN METHOD Cont’d…..:

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THE PINCH DESIGN METHOD Cont’d…..: tabish288@yahoo.com

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THE PINCH DESIGN METHOD Cont’d…..: At the pinch, the match starts with a temperature difference equal to ∆Tmin. The relative slopes of the temperature–enthalpy profiles of the two streams mean that the temperature differences become smaller moving away from the pinch, which is infeasible. On the other hand, in second Figure match involving the same hot stream but with a cold stream that has a larger CP. The relative slopes of the temperature–enthalpy profiles now cause the temperature differences to become larger moving away from the pinch, which is feasible. Thus, starting with ∆Tmin at the pinch, for temperature differences to increase moving away from the pinch. tabish288@yahoo.com

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THE PINCH DESIGN METHOD Cont’d…..: tabish288@yahoo.com

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THE PINCH DESIGN METHOD Cont’d…..: If a cold stream is matched with a hot stream with smaller CP, (i.e. a steeper slope), then the temperature differences become smaller (which is infeasible). If the same cold stream is matched with a hot stream with a larger CP (i.e. a less steep slope), then temperature differences become larger, which is feasible. Thus, starting with ∆Tmin at the pinch, for temperature differences to increase moving away from the pinch. tabish288@yahoo.com

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THE PINCH DESIGN METHOD Cont’d…..: tabish288@yahoo.com

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THE PINCH DESIGN METHOD Cont’d…..:

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THE PINCH DESIGN METHOD Cont’d…..: tabish288@yahoo.com

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THE PINCH DESIGN METHOD Cont’d…..: tabish288@yahoo.com

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THE PINCH DESIGN METHOD Cont’d…..: tabish288@yahoo.com

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HEAT EXCHANGER NETWORK DESIGN: