Process and plant improvement using extended exergy analysis, a case study

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1 Process and plant improvement using extended exergy analysis, a case study ALHASSAN STIJANI 1, NAVEED RAMZAN 1, WERNER WITT 1 Lehrstuhl Anlagen und Sicherheitstechnik Brandenburgische Technische Universität Burger Chaussee 2, Cottbus, Germany In this paper, energy and exergy analyses of a distillation unit is conducted to study thermodynamic efficiency of the unit, performance evaluation and total annualized cost (TAC) optimization A systematic procedure for analysis as well as optimization have been proposed and demonstrated by two case studies Side stream withdrawal and operating conditions have been selected as variables for the optimization Compared with the base case, alternative case with side stream (SS) achieved a higher thermodynamic efficiency (2681 %) Keywords: Optimization, Total annualized cost, Distillation column unit Distillation is the most used separation operation in chemical and petrochemical industries and its evergrowing application is accompanied by a large increase in consumption of energy [1] According to some recent estimates about 40% of energy involved in refinery and other continuous chemical processes is consumed in distillation [2] For this reason the energy efficient design and operation of distillation processes is an important issue discussed in various literatures [3, 4] These analyses have focused only on energy and ignored the quality of energy Thus going a step further, to evaluate the quality of energy lost via Exergy analysis is an efficient technique for reducing inefficiencies A careful evaluation of process and plant design using Exergy analysis enables the identification and quantification of the sources of inefficiencies or process irreversibility-related losses Exergy analysis includes conservation of mass and energy balance together with the second law of thermodynamics Al-Muslim et al [5] published a thermodynamic analysis of crude oil distillation systems which includes aspects like energy and exergy analysis, performance evaluation and system optimization Ishada et al [6] has shown that large savings could be obtained because of reduction of high quality energy The use of irreversible thermodynamics principles in the field of distillation is still under development [7] In this study, thermodynamic analysis of a distillation unit is presented Different distillation unit configurations related to minimum entropy production, energy consumption and economics (TAC) are analysed Methodology Figure 1 represents the proposed methodology s structure showing the inter-linking of the software tools used The process is modeled using Aspen Plus TM simulator Mass and energy data from the Aspen Plus TM model are transferred to MS-Excel to compute the exergy of the streams and thermodynamic efficiency of the distillation unit under study The successive quadratic programming algorithm (SQP) of Lang and Biegler [8], which is integrated in Aspen Plus TM has been adopted to the model requirements and is used for economic optimization The base case is improved by generating structural alternatives such as variation of feed stage and side stream withdrawal Basic aspects considered The exergy of a stream of matter is the maximum work (useful energy) that can be obtained from it in taking it to complete equilibrium (of temperature, pressure and composition) with the environment (Rivero 1993) [9] Exergy loss is a measure of irreversibility in the column due to momentum loss (pressure driving force), thermal loss (temperature driving force/mixing), and chemical (1) Fig 1 Methodology * alhatj@yahoocom Tel: Fax

2 potential loss (mass transfer driving force/mixing) In the column there is entropy production due to heat and mass transfer between the fluid streams In the condenser there is entropy production due to heat transfer only [10] Column exergy balance Figure 2 shows the balance regions of the distillation unit under study For a steady state process, in terms of the first and second law of thermodynamics, the energy and entropy balance (inner balance region) equations are: Minimum work and thermodynamic efficiency The minimum amount of work required for separation can be calculated as follows: Thus thermodynamic efficiency of the column can be express as: (6) (8) Entropy balance: (2) Calculation procedure Figure 4 shows the procedure used for calculating thermodynamic efficiency of each alternative generated where S irr is the entropy production in the distillation unit Exergy loss and entropy production in distillation are related to each other [7] The exergy balance for the distillation unit (outer balance region) is: (3) (4) (5) Fig 4 Calculation procedure of thermodynamic efficiency Economic model The cost effectiveness of a process can be evaluated by applying attributes like cost profitability, return on investment and total annualized cost (TAC) [11,12] TAC (Operational cost + Annualized fixed capital investment) is considered in this paper as the economic objective function Capital investment calculation is based on the sum of the costs of column, trays and condenser Operating cost is estimated in terms of utility cost Operating cost Fig 2 Exergy balance of distillation unit (9) Capital investment cost The bare-module which is appropriate for optimization studies [12] is applied in the estimation of installation cost of column, condenser and tray Installed cost of condenser (10) Installed cost of column Fig 3 Component balance on tray Stage exergy loss The exergy loss on tray n is calculated with an exergy balance over the tray (fig 3) as follows: (6) The right side of equation 6 represents the exergy of streams into and out of the tray Heat loss and heat transfer via tray outer balance is not considered Installed cost of trays Thus total annualized cost is (11) (12) (13) 393

3 Fig 5 Base case Fig6 Design alternative (with side stream) Table 1 SIMULATION RESULTS FOR OPTIMAL OPERATING CONDITIONS Case study The methodology is illustrated with the help of case study of a distillation column unit (see figure 5) This unit is part of the Hydrocarbon recovery (HCR) plant, which removes hydrocarbons and other components from the offgass The feed stream to the stripping column enters normally on plate 16 The column operates with live steam injection into the base on tray 35 at temperature 140 o C and 375 bars From the physical limitations, some more constraints are usually enforced For example, acetone recovery must be kept within a certain range of specification as shown below (mass %) Distillate: x Water < 10 %, x Acetone > 50 %, Base: x Acidity < 3 %, x Acetone < 022 %, x Methanol < 2 % The base case was modified by introducing a side stream at tray 30 (fig 5) This modification contributed to energy saving and reducing the TAC Comparison of base case and design alternative at optimal operating conditions The steady state simulation model is configured in Aspen Plus TM using built in RADFRAC method Detailed degree of freedom analysis is performed for determining the number of decision variables (independent variables Table 2 RESULTS OF THE CASE STUDY we can affect) needed to execute the simulation With given values for feed composition, feed flow rate, operating pressure, total number of trays, feed tray location, the distillation unit has two degrees of freedom (steam flow rate and reflux ratio) Thus, these decision variables are used for optimization and satisfaction of purity constraints which are product key component composition Table 1 shows optimum operating conditions and purity achieved for base case and side stream case There is a slight change in the product purity of the base case compared with side stream case (table 1) The side stream case favors energy reduction (table 2) The other advantage of the side stream case is, that it removes the middle boiling components Cost and exergy analysis Main results of the simulation are summarized in table 2 Savings in TAC is evident in side stream case Table 3 gives the basis used for cost calculation for each alternative As shown in table 2 the base case design gives higher irreversibility loss (exergy loss) compared with the side stream case This results to higher thermodynamic efficiency The reason is that the side stream design has less separation work because preferred middle boiling components are removed via the side stream Exergy loss in the column is a measure of irreversibilities in the column due to momentum loss (pressure driving Table 3 VALUES USED FOR COST CALCULATION 394

4 Table 4 RESULTS OF PRODUCT QUALITY FOR STEAM AT 110 o C force), thermal loss (temperature driving force/mixing), and mass transfer driving force/mixing Exergy loss is greatest at the base of the column for both cases studied This situation is illustrated in figure 7 In terms of thermodynamic efficiency, energy consumption and cost, the side stream solution should be preferred The main contribution to the exergy loss within the distillation unit is therefore due to steam condition (too high temperature) Figure 9 shows the exergy content of the outlet streams, it can be observed, that the bottom stream has high exergy content In the original plant, a heat exchanger is used to preheat the feed stream via the bottom stream The effect is shown in figure 9 Because of the exergy level of bottom stream behind the heat exchanger, there is some potential for optimization The distillate stream is send to the acetone recovery plant Because of this, it is not considered in the further analysis of this study Conclusion In this work the exergy analysis is used for improving thermodynamic efficiency of a distillation unit The exergy loss profiles in the distillation column indicated that much exergy supplied by the steam is wasted In future work exergy related aspect like extension of bottom product will be considered, also heat exchanger and heat pump will be analyzed before other objectives like safety and operability will be integrated Fig 7 Exergy loss profiles in columns In order to reduce the exergy loss at the bottom of the column, the steam condition was changed The temperature of entering steam was reduced from C to C Figure 8 shows the resulted exergy profile The exergy loss at steam inlet decreased by approximately a factor of 3 while product quality remains the same ( table 4) Fig 8 Exergy loss profiles in column (steam at 110 o C) Nomenclature A m 2 area c $/kg specific cost c* cor $/kg cooling water cost C $/yr cost per year C* el $/MJ electricity cost D D - m diameter d 1/yr depreciation factor E MJ/ h exergy rate e MJ/kmol specific exergy e* MJ/kg specific exergy F c - fixed capital cost factor F cw kg/h cooling water flowrate M kg/h mass flow rate N kmol/h mole flow rate N act - Number of trays P bar pressure P kw electrical power S kj/k entropy s kj/(kg*k) specific entropy T K temperature w mj/h separation work Q kw heat duty ~ xi kmol/kmol liquid mole friction xi kg/kg liquid mass fraction ~ yi kmol/kmol vapour mole fraction yi kg/kg vapour mass fraction Subscript Fig9 Distribution exergy of streams for base case design B strm - exetergy of bottom stream;b strm, Hx, Hx - exergy of bottom stream after integration it with heat exchanger to preheat feed stream, D strm - exergy of distillation stream 395

5 B Bottom col Column cw Cooling water D distillate el electrical F feed irr irreversible R reflux S steam Superscript L liquid V vapour References 1 FARIA, S H B, ZEMP, R J, Using exergy loss profiles and enthalpytemperature profiles for the evaluation of thermodynamic efficiency in distillation columns, Thermal Engineering, Vol 4, No 1, 2005, p 76 2 OLUJIC, Z, FAKHRI, F, DE RIJKE, A, DE GRAAUW J, JANSENS, PJ, Internal heat integration the key to an energy-conserving distillation column, Journal of Chemical Technology and Biotechnology, 78, 2003, p HSUAN, C, JR-WIE, L, A new exergy method for process analysis and optimization, Chemical Engineering Science, 60, 2005, p FLORES, O A, CARDENAS, C, HERNANDEZ, S, RICO-RAMÝREZ, V, Thermodynamic Analysis of Thermally Coupled Distillation Sequences, Ind Eng Chem Res, 42, 2003, p AL-MUSLIM H, DINCER I, Thermodynamic analysis of crude oil distillation systems International Journal of Energy Research, 29, 2005, p TAPRAP, R, ISHIDA, M, Graphic Exergy Analysis of Processes in Distillation Column by Energy-Utilization Diagrams, AIChE Journal,Vol 42, no 6, 1996, p RIVERO, R, KOEIJER, G, Entropy production and exergy loss in experimental distillation columns, Chemical Engineering Science, 58, 2003, p YD LANG, L T BIEGLER, Comp Chem Eng, 1987, 11, RIVERO, R, RENDON, C, MONRY L, The exergy of crude oil mixtures and petroleum fractions: calculation and application, IntJ Applied thermodynamics, Vol2, No3, 1999, p ROSJORDE A, KJELSTRUP S, The second law optimal state of diabatic tray distillation column, Chemical Engineering Science, 60, 2005, p DIRK BRUSIS, (2003), Synthesis and optimization of Distillation processes with MINLP Techniques, Fortschritt-Berichte VDI Reihe 3 Verfharenstchnik 12 SEIDER, W D, SEADER, JD, LEWIN, DR, Process Design principles, Synthesis, Analysis and Evaluation, John Willey & Sons, Inc, New York, 1998 Manuscript received: 396