040344 Increase Capacity

040344 Increase Capacity - Reactions and Separations...

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Unformatted text preview: Reactions and Separations Increase Capacity and Decrease Energy for Existing Refinery Distillation Columns Mamdouh Gadalla, Megan Jobson and Robin Smith, UMIST This method optimizes the existing distillation system and its heat-exchanger network simultaneously, lowering energy consumption and freeing up capacity at a minimum capital investment. D ISTILLATION COLUMNS ARE AMONG the biggest energy consumers in the chemical industries, particularly in oil refineries. Retrofit projects in refineries mostly aim at reducing energy consumption and increasing throughput to increase profits and meet market demands. Usually, plants aim to achieve their retrofit objectives by reusing the existing equipment efficiently, rather than installing new towers and heat exchangers, which requires a substantial capital investment. Retrofitting schemes A number of modifications have been suggested to the distillation column and the heat exchanger network to meet these two goals. Sittig suggests changes to improve the efficiency of distillation systems, including the installation of new internals with higher efficiency, the use of intermediate reboilers, etc. (1). Bannon and Marple recommend making other column modifications, such as installing pump-arounds at suitable locations in the unit and adjusting the cooling duty for each pump-around (2). Harbert’s idea is to install preflash units or prefractionators before the crude oil distillation unit. This would save energy and increase the throughput of the column (3). Rivero and Anaya call for installing additional trays, as well as adding reboilers to the stripping columns (4). Fraser and Sloley propose increasing the capacity of crude-oil units by adding pump-arounds, reducing the operating pressure and increasing the preflash overhead vapor (5). 44 www.cepmagazine.org April 2003 CEP In refinery distillation systems, the energy-efficiency of the process strongly depends on the heat-exchanger network design. For example, the duty and temperature drop of each pump-around affects how much heat can be recovered, and the connections between the heat exchangers (also known as matches) and heat-exchanger areas determine how much heat is actually recovered. Within the last decade or so pinch analysis has been applied to identify modifications to the column and the heat exchanger network. Linnhoff and Dhole’s idea is to use the column’s grand composite curve (CGCC) to identify suitable modifications that would save energy (6). Dhole and Buckingham extended this method for energy saving and debottlenecking of refinery distillation systems. Their method has three stages (7). First, column modifications are made using the CGCC, then the heat-exchanger network design is changed to save energy by adding more heat-transfer area, and, finally, design changes are instituted to debottleneck the arrangement. Liebmann’s approach is a two-step method for retrofitting (8). The distillation column is first modified to reduce its energy demand, and the CGCC provides guidelines for improving the heat-recovery potential. Afterwards, the column is remodified, followed by a reanalysis of the CGCC to further increase the heat recovery. Overall, two levels of modifications are proposed, those that are relatively inexpensive and those that require larger investments. Examples of inexpen- sive modifications include piping changes to avoid mixing unlike streams together, and adjusting the stripping-steam flowrates. Capital-intensive options include replacing internals and relocating feeds and draws. Briones et al. (9) successfully applied Liebmann’s retrofit approach to discover modifications for reducing the energy consumption of the crude-oil distillation unit. Bagajewicz et al. adapted Liebmann’s approach by linking the pinch analysis with rigorous simulation and optimizing the column’s operating parameters (10). Improved method As helpful as they are, none of these retrofit methods assesses the existing heat-exchanger network together with the crude distillation column. Further, they do not Nomenclature A = a, b, c = Aexist = Aret = B = Csb = D = DC = DT = Eret = FLV = L = m, c = N = Nmin = NR = NS = R = RHK = RLK = Rmin = xfHK = xfLK = Udes = Umax = V = VV = total area required for heat exchangers, m2 flooding parameters (based on stage spacing) existing heat exchanger area, m2 area added for retrofitting, m2 bottom product flow, kmol/h capacity factor, dimensionless top product flow kmol/h ratio of downcomer area to total area, dimensionless stage diameter, m total energy consumed after retrofit, MW flow parameter, dimensionless liquid mass flow, kmol/h retrofit area model parameters in Eq. 12 total number of stages in column minimum number of stages at total reflux number of stages in rectifying section number of stages in stripping section reflux ratio fractional recovery of heavy key to bottom product fractional recovery of light key to top product minimum reflux ratio, dimensionless mole fraction of heavy key in feed mole fraction of light key in feed design vapor velocity, m/s flooding velocity, m/s vapor mass flow, kmol/h vapor volumetric flow, m3/s Subscripts: Fensk = Fenske Gill = Gilliland Kirk = Kirkbride Greek letters: αLK = relative volatility of light key αHK = relative volatility of heavy key ρL = liquid mass density, kg/m3 ρV = vapor mass density, kg/m3 ξ = ratio defined in Eq. 3 φ = factor defined in Eqs. 4 and 5 ΨGill = factor in Gilliland correlation, see Eq. 2 offer a systematic approach to retrofitting. Rather, they propose various modifications. These methods suffer from two more drawbacks: Some of the proposed column modifications would require a substantial capital investment, while others sometimes violate such constraints such as the maximum tray capacity. This approach aims to identify the set of operating conditions in an existing distillation column that will allow the existing heat-exchanger network (modified by adding heattransfer area or changing the piping arrangements between exchangers, for example) to best recover heat. The hydraulic limits of the column constrain which design solutions can be considered. Thus, we offer a systematic approach for retrofitting existing refinery distillation columns. This method simultaneously considers the existing heat-exchanger network along with reducing energy consumption and increasing the throughput of the existing distillation unit. Framework used The authors’s optimization framework uses models for the column and the heat-exchanger network, as well as pinch analysis. Optimization is carried out using a successive quadratic programming (SQP) solver. The solver uses an algorithm that is aimed at large, sparse nonlinear programs. In essence, a quadratic approximation of the highly nonlinear problem is solved during each iteration (11). The system is represented using a column retrofit model and a heat-exchanger network retrofit model. The column model captures the relationships between operating conditions, product quality and column design. The heat-exchanger model presents the results of a detailed study of options for improving heat recovery in the network by making minor changes to the heat-exchange hardware and configuration. For example, the sequence of heat exchangers may be changed or a new exchanger may be installed. The column design is optimized using these two models. That is, its operating conditions are selected for a column with a fixed design (the given number of stages, diameter, feed and draw locations, etc.) to minimize the sum of the utility costs and investment in the heat-exchanger network. The network retrofit model is used to calculate th cost of additional heat-exchange area during the optimization. Thus, the optimization couples the two independent models and accounts for interactions between the column design and the heat-exchangernetwork performance. The column retrofit model uses the existing parameters, such as the number of stages and their distribution, locations of condenser, reboiler and pump-arounds, and product specifications. The heat-exchanger network retrofit model takes into account the network’s details, such as the heat-transfer areas and duties for each exchanger, the ex- CEP April 2003 www.cepmagazine.org 45 Reactions and Separations isting matches, and the existing energy consumption. During optimization, the user can vary the column’s operating conditions, such as changing the feed preheating temperature, steam flows to each section, reflux ratio, and temperature drop and flow of liquid recycled by pumparounds for minimum energy consumption. The hydraulic constraints and capacity limitations of the existing column are taken into account during this process. Optimization of an existing refinery distillation unit identifies the optimum process changes for minimum energy consumption. Since the retrofit design does not change the dimensions or internals of the column, these modifications do not require a major capital investment. Rather, they are simply changes to the operating conditions. Reducing the energy consumption of the existing crude distillation unit allows the column throughput to increase, due to the resulting reduced vapor flows. In determining the maximum increase in throughput, the model identifies any column bottlenecks that would limit this increase, and evaluates proposed modifications for debottlenecking. To run the model, a FORTRAN code was developed that allows modeling via short-cut models, and an interface was created between the code and a rigorous simulation package. This allowed the users to obtain thermodynamic and physical data for the components and pseudocomponents. A rigorous simulation was first performed on the existing arrangement (the base case) to initialize the shortcut calculations, e.g., specifying the key components and their recoveries, plus matching the product flowrates to those in the existing operation. This rigorous simulation should yield a reasonably accurate picture of the product flows and compositions, steam flows, pump-around duties, flowrates and temperature drops, among others. The simulation results are useful in checking the validity of the column retrofit model, as well as for initializing this model. The user’s manual of the simulation software that the reader employs will provide guidance on how to rigorously model refinery columns. Further instructions on setting up the problem are found in Ref. 12. Retrofit model for the column The short-cut column model is based on the Underwood equation for the calculation of the minimum vapor flow in a column. The basic model equations are those of Fenske, Gilliland and Kirkbride together with consecutive flash calculations (13), and key-component material balances. The retrofit equations for distillation columns with reboilers are: N min = N (1 − ψ Gill ) − ψ Gill 46 www.cepmagazine.org April 2003 (1) CEP where: 1 + 54.4ξ ξ − 1 ψ Gill = 1 − exp 0.5 11 + 117.2ξ ξ (2) and: R − Rmin R +1 The following two terms are defined as: ξ= φ Fenske φ Kirk α = LK α HK N (3) min B x fHK = D x fLK (4) 1/ 2 NR NS 2.427 (5) The recovery of the heavy key in the top product is given by: 1/ 2 RHK 2 φ φ Fensk φ Fensk − 1 + 1 Kirk = 1− +4 2(φ Fensk – 1) φ Fensk (φ Fensk ) 2 φ Kirk (φ Kirk + 1) + (6) 2φ Kirk (φ Fensk − 1) While the recovery of light product in the bottoms is: RLK 2 (φ φ Kirk Kirk + 1) = 1 − +φ 4(φ Fenske − 1) ( Fenske − 1) + (φ Kirk + 1) 2(φ Fenske − 1) 1/ 2 (7) A similar retrofit model can be written for distillation columns that employ steam stripping. In this case, consecutive flash calculations are used to model the stripping section (12). The two retrofit models relate the product compositions to the existing number of stages, the distribution of the stages, and the existing operating conditions. The models treat the distillation column as fixed and calculate the product flows, temperatures, compositions and duties. This provides the basis for optimizing the existing column. Hydraulic analysis To evaluate the column hydraulics, the diameter required for vapor flow is calculated for those stages where there is a significant change in the vapor and liquid flows. Such stages include the top and bottom trays, pump-around stages, and the feed stage. For sieve plates, the diameter is calculated from the flooding limits via Water 8 Water 8 LN PA3 Existing Heat-Exchanger Network Retrofit Models Retrofit Shortcut Models 7 HN 6 8 7 7 LD PA3 PA1 Steam 2 PA2 HD 3 PA1 2 1 Steam 1 6 5 4 3 2 Feed 1 LN HN LD Steam 2 PA2 Heat Exchanger Area 5 4 Heat-Exchanger Network HN Steam 1 LD 4 PA1 Heat-Exchanger Network Retrofit Energy Demand HD 5 3 Retrofit Area Steam 2 HD 2 Feed 1 Steam 1 RES RES Existing Distillation Column Existing Heat Exchanger Network 7 6 Water PA2 Feed PA3 LN RES Existing System with Maximum Energy Recovery and Minimum Additional Exchanger Areas Column Decomposition and Simulation Optimizer (SQP) Existing Stages (Fixed) Existing Diameter (Fixed) s Figure 1. Optimization requires decomposing the column and coupling the heat-exchanger network using the retrofit models. Key: PA = pump-around; LN = light naphtha; HN = heavy naphtha; LD = light distillate; HD = heavy distillate; and Res = residue. Eqs. 8–11 (14): −c Csb = a − b × exp FLV (8) ρV ρL (9) FLV = L V U max = Csb DT = ρ L − ρV ρV 4VV πUdes (1 − DC ) (10) (11) Fair (14) provides values for the parameters a, b and c found in Eq. 8. Kister (15) lists flooding correlations for a range of internals and operating conditions. The correlation and the parameters used should be suited to the existing internals, and yield a reasonable prediction of the entrainment flooding characteristics. In Eq. 11, the design velocity Udes is less than the flooding velocity Umax by some safety factor. Typically, Udes is 70–80% of Umax. Calculation of diameters of the distillation column allows the analysis of the hydraulic performance of the column. The diameter profile along the column is obtained by plotting the diameters for various stages vs. the stage number. This profile allows identifying the column bottlenecks that limit throughput enhancement. Column bottlenecks occur on those stages in which the required diameter is larger than the existing one. The diameter calculations allow the existing hydraulic limitations of the tion column to be considered in the optimization framework. Therefore, during this process, the diameter is calculated for the key stages. Then, the calculated diameters CEP April 2003 www.cepmagazine.org 47 Reactions and Separations Water 2 14 13 1 4 6 8 10 12 3 5 7 9 11 LN 9 11 12 13 6 21 PA3 8 HN 1 22 18 15 7 2 8 5 7 PA2 10 6 15 19 LD 18 25 3 26 5 PA1 16 Steam 4 22 27 9 24 HD Crude 23 5 17 Steam 9 10 Res 28 s Figure 2. The atmospheric crude unit and its heat-exchanger network before optimization was carried out. Key: PA = pump-around; LN = light naphtha; HN = heavy naphtha; LD = light distillate; HD = heavy distillate; and Res = residue. are compared with the existing ones to guarantee that the existing diameters are not exceeded; otherwise a penalty is imposed. Hence, the optimum distillation column will not have bottlenecks. Retrofit model for the heat-exchanger network This model calculates the required area of the retrofitted heat-exchanger network, while considering the fixed parameters of the network (e.g., heat-transfer areas, duties, matches, stream splits). The retrofit model and the associated parameters, m and c, are obtained from an extensive retrofit study on the existing heat exchanger network. The model, although simple, incorporates the details of the existing Heat-exchanger network in the process optimization framework. The model allows the benefits of energy savings to be weighed against the capital investment required to modify the heat-exchanger network. Details on the model are found in Ref. 12, and these should aid the reader in performing his or her own analysis.he optimization considers the details of the existing heat exchanger network simultaneously with the existing crude distillation column and accounts for the hydraulic 48 www.cepmagazine.org April 2003 CEP constraints of the column. Figure 1 illustrates the approach. The retrofit curve is obtained from an extensive retrofit study on the heat exchanger network. The model relates the exchange area required for reducing energy consumption, Aret, to the reduced energy consumption, Eret. The model may take different forms; the power law form has been found suitable for a number of case studies investigated. A = m( Eret )c (12) The additional area requirement for the retrofitted network is related to the existing area of the network, Aexist, and the total area requirement, A: Aret = A − Aexist (13) The heat-exchanger retrofit model mathematically describes a retrofit curve of an existing heat exchanger network (e.g., Figure 1). The retrofit curve is a graphical representation of the capital-energy trade-offs in an existing heat-exchanger network; it consists of a plot of retrofit area Liquid Liquid Vapor Steam Vapor 1. Increase Temperature Drop Across Pump-around 2. Reduce Steam Flow Vapor Cooling Vapor Liquid 3. Increase Liquid Flow Through Pump-around 4. Adjust Feed Preheating s Figure 3. Column modifications that can overcome bottlenecks. vs. energy demand. The model is based on the network pinch developed by Asante and Zhu (16). The amount of heat recovery that can be achieved is limited by the network pinch. Network pinch analysis Table. Base case vs. optimum case for constant feed flowrate. Parameter Feed preheat temperature, °C Liquid Base case Optimum case 360 363 PA1 liquid flow, kmol/h 1,228 1,233 PA2 liquid flow, kmol/h 2,396 3,989 PA3 liquid flow, kmol/h 5,868 3,953 PA1 temperature difference, °C 40 50 28.1 PA3 temperature difference, °C 20 58.9 1,200 1,088 The optimization framework The optimization considers the details of the existing heat-exchanger network simultaneously with the existing crude distillation column and accounts for the hydraulic constraints of the column. Figure 1 illustrates the approach. Note that the column configurations before and after optimization are the same. The overall strategy consists of: 1. Decomposing the crude distillation column into an equivalent sequence of simple columns, which is simulated using the retrofit model. 2. Modeling the existing heat-exchanger network using the retrofit area model. 3. Simultaneously optimizing the operating conditions of the existing distillation operation to minimize the sum of the utility costs and additional exchanger area costs. 4. Taking as fixed, the existing number of stages, the column configuration and diameters. 44.1 PA2 temperature difference, °C suggests that topology changes (e.g., resequencing of exchangers, installation of new exchangers) are necessary to overcome the limits caused by the pinch. For no topology changes, the network is optimized by adding area to the existing exchanger units. The white bar under the curve in Figure 1 indicates the additional heat-exchanger capacity needed for the retrofit. The heat-exchanger network retrofit model simply considers the total energy demand and total area requirement, which greatly simplifies the characterization of the heat-exchanger network during process optimization. The model allows the benefits of energy savings to be weighed against the capital investment required to modify the heat-exchanger network. Details on the model are found in Ref. 12 and these should aid the reader in performing his or her own analysis. Note that the column configurations before and after optimization are the same. Main steam flow, kmol/h HD-stripper steam flow, kmol/h 260 247 R/Rmin 1.2 1.11 Example The approach was tried, using data from an actual unit (Figure 2). The numbers inside of the column represent the number of stages in each section; note that this notation differs from that used in Figure 1. The grid in Figure 2 represents the process streams, the heat exchangers connecting them, and any other heaters or coolers, including the CEP April 2003 www.cepmagazine.org 49 Reactions and Separations furnace and steam heaters. The hot streams (i.e., those that require cooling) run from left to right, while the cold streams run from right to left. The numbers in the circles of the heat-exchanger grid stand for individual exchangers in the network. The vertical lines represent the heat exchange between process streams. This figure shows the base case, that is, before optimization. Two aims of the retrofit design are considered: (1) to improve the energy-efficiency of the process; and (2) to increase the throughput by 20% over the current capacity. The atmospheric tower is fed 100,000 bbl/d of crude oil. Before the retrofit, the scheme was consuming power at 99.5 MW, with a total operating cost of $28.4 million/yr. The heat-exchanger network retrofit model was found to be: A = 6.75 × 10 6 Eret −1.61 (14) Literature Cited 1. Sittig, M., “Petroleum Refining Industry Energy Saving and Environmental Control,” Noyes Data Corp., Park Ridge, NJ (1978). 2. Bannon, R. P., and S. Marple, “Heat Recovery in Hydrocarbon Distillation,” Chem. Eng. Progress, 74 (7), pp. 41–45 (July 1978). 3. Harbert, W. D., “Preflash Saves Energy in Crude Unit,” Hydrocarb. Proc., 57 (7), pp. 23–125 (1978). 4. Rivero, R., and A. Anaya, “Exergy Analysis of a Distillation Tower for Crude Oil Fractionation,” and “Computer Aided Energy Systems Analysis,” Proc. of Winter Annual Meeting of ASME, 1 (11), pp. 25–30 and 55–62, Dallas, TX (1990). 5. Fraser, A. C., and A. W. Sloley, “Consider Modeling Tools to Revamp Existing Process Units,” Hydrocarb. Proc., 79 (6), pp. 57–63 (2000). 6. Dhole, V. R., and B. Linnhoff, “Distillation Column Targets,” Computers Chem. Eng., 17 (5/6), pp. 549–560 (1993). 7. Dhole, V., and P. Buckingham, “Refinery Column Integration for De-bottlenecking and Energy Saving,” paper presented at ESCAPE IV Conf., Dublin, Ireland, sponsored by IChemE, Rugby, U.K. (Mar. 1994). 8. Liebmann, K., “Integrated Crude Oil Distillation Design,” PhD thesis, UMIST, Manchester, U.K. (1996). 9. Briones, V., et al., “Pinch Analysis Used in Retrofit Design of Distillation Units,” Oil & Gas J., No. 6, pp. 41–46 (June 1999). 10.Bagajewicz, M., et al., “Energy Savings Horizons for Crude Fractionation,” Computers Chem. Eng., 23 (1), pp. 1–9 (1998). 11. Biegler, L. T., et al., “Systematic Methods of Chemical Process Design,” pp. 761–763, Prentice Hall, Englewood Cliffs, NJ (1997). 12.Gadalla, M., “Retrofit Design of Heat-Integrated Crude Oil Distillation Systems,” PhD thesis, UMIST, Manchester, U.K. (2003). 13.Suphanit, B., “Design of Complex Distillation Systems,” PhD thesis, UMIST, Manchester, U.K. (1999). 14.Fair, J. R., “How to Predict Sieve Tray Entrainment and Flooding,” Petro/chem. Engr., 33 (10), pp. 45–62 (1961). 15.Kister, H., “Distillation Design,” McGraw-Hill, New York, Chap. 6 (1992). 16.Asante, N. D. K., and X. X. Zhu,“An Automated and Interactive Approach for Heat Exchanger Network Retrofit,” Trans. IChemE, 75, Part A, pp. 349–360 (Mar. 1997). 50 www.cepmagazine.org April 2003 CEP Table 1 compares the base case with the optimized arrangement when the feed flowrate is unchanged. The optimum energy consumption of the crude unit is 77.4 MW, a reduction of 22% and a savings of $6.3 million/yr. Some investment is needed to improve the performance of the heat-exchanger network; the energy savings arise from changing process operating conditions (e.g., furnace inlet temperature, pump-around duties, feed temperature) such that more heat recovery is possible. The payback for these modifications is 4 mo. However, to increase the throughput would require using larger diameters than exist in some sections inside the column. Therefore, to increase its capacity, the column had to be debottlenecked. The column model and hydraulic analysis identified the bottlenecked sections. Proposed modifications that can eliminate the bottleneck are shown in Figure 3. Parts 1 and 3 in the figure show increasing the amount of heat removed during a pumparound; Part 2 represents reducing the flow of stripping steam and, hence, the total vapor flow; and Part 4 illustrates reducing the temperature and, therefore, the vapor fraction of the column feed. The unshaded arrows that point toward each other mean that the required diameter would be decreased. The optimized distillation unit has a 20% increase in throughput and requires 94.8 MW of heating. The operating cost saving is $1.9 million/yr, relative to the base CEP case, with a payback of less than 1 yr. MAMDOUH GADALLA recently completed his PhD at UMIST, Dept. of Process Integration (P .O. Box 88, Manchester M60 1QD, U.K. He has four years of research and teaching experience with the Atomic Energy Authority of Egypt and the Chemical and Petroleum Engineering Dept. of the United Arab Emirates Univ. Gadalla designs equipment for the retrofitting of crude-oil distillation units. He holds a bachelor’s and master’s in chemical engineering from Cairo Univ. MEGAN JOBSON is a lecturer in the Dept. of Process Integration at UMIST (Phone: 44 161 200 4381; Fax: 44 161 236 7439; E-mail: m.jobson@umist.ac.uk). She carries out research, teaches and undertakes industrial studies on the synthesis and design of distillation, absorption and reactive separations. Previously, she worked as a process engineer in the food industry. She did her undergraduate work in chemical engineering at the Univ. of Cape Town, South Africa and holds a PhD in the same field from the Univ. of the Witwatersrand, Johannesburg, South Africa. ROBIN SMITH is a professor and head of the Dept. of Process Integration at UMIST (Phone: 44 161 200 4382; Fax: 44 161 236 7439; E-mail: r.smith@umist.ac.uk). He has extensive industrial experience with Rohm & Haas and ICI. Smith has consulted extensively for process integration projects. He is widely published in process integration and is the author of “Chemical Process Design,” (McGraw-Hill). He is a Fellow of the Royal Academy of Engineering and of the Institution of Chemical Engineers in the U.K., as well as being a chartered engineer. In 1992, he was awarded the Hanson Medal of the Institution of Chemical Engineers for his work on waste minimization. His main research activities include the design of reaction and separation systems, site utility systems, waste minimization and water-system design. Smith holds BSc, MSc and PhD degrees in chemical engineering from the Univ. of Bradford, U.K. ...
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This note was uploaded on 12/29/2011 for the course CHE 128 taught by Professor Scott,s during the Fall '08 term at UCSB.

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