Bulatov, I

Pan, M

Smith, R

English

The University of Manchester - Institutional Repository

 CHEMICAL ENGINEERING TRANSACTIONS   VOL. 29, 2012 A publication of  The Italian Association of Chemical Engineering Online at: www.aidic.it/cet Guest Editors: Petar Sabev Varbanov, Hon Loong Lam, Jiří Jaromír Klemeš Copyright © 2012, AIDIC Servizi S.r.l., ISBN 978-88-95608-20-4; ISSN 1974-9791 DOI: 10.3303/CET1229238  Please cite this article as: Pan M., Bulatov I. and Smith R., (2012), Retrofit procedure for intensifying heat transfer in heat exchanger networks prone to fouling deposition, Chemical Engineering Transactions, 29, 1423-1428 1423 Retrofit Procedure for Intensifying Heat Transfer in Heat Exchanger Networks Prone to Fouling Deposition Ming Pan*, Igor Bulatov, Robin Smith Centre for Process Integration, The University of Manchester, Sackville street, Manchester, M13 9PL, UK  ming.pan@manchester.ac.uk This paper presents a novel optimization method of addressing fouling effects in heat exchanger network (HEN) retrofit with intensified heat transfer. In the new approach, the fouling deposition in each heat exchanger is taken into account when heat transfer intensification is regarded as a retrofit option in HENs, and the optimization procedure proposed by Pan et al. (2012b) is developed to solve the addressed problems efficiently. A practical industrial process is carried out to show that with the consideration of fouling influence. The benefit that can be derived from implementing intensified technologies is not only increasing heat recovery but also extending exchanger operating times with the mitigation of fouling deposition. 1. Introduction A number of retrofit approaches have been reported for retrofitting heat exchanger network (HEN) in the last few decades. The existing approaches include implementing intensified heat transfer techniques, adding more exchanger area, installing new exchangers, moving exchangers and repiping streams (Ponce-Ortega et al., 2008; Nguyen et al., 2010; Pan et al., 2012a, 2012b). However, these conventional approaches are proposed based on the assumption of constant fouling resistances in exchangers during the whole operating period, which might suffer from the chronic operational problem caused by fouling, and thus compromises energy recovery and environmental welfare in the retrofitted HENs.  Fouling concerns the formation of unwanted material on heat transfer surfaces. Fouling problems manifest themselves as loss or reduction in production, increased energy consumption, increased pressure losses, antifouling chemical costs, cleaning costs, and so on. To reduce fouling effects in HENs, most researchers tried to increase exchanger area or modify operating conditions in the existing HENs. Brodowicz and Markowski (2003) proposed several control methods to increase exchanger area or increase the capacities of heaters and coolers. Rodriguez and Smith (2007) optimized the operating conditions (such as wall temperature and flow velocity) to mitigate fouling in HENs. Coletti et al. (2010) used a dynamic and distributed mathematical model for shell-and-tube heat exchangers undergoing crude oil fouling to simulate the existing HENs. Recently, Yang et al. (2011) found that, fouling of the heat exchanger depends on two major operational parameters, namely the wall shear stress and the surface temperature. The use of tube inserts (one type of enhancement techniques) has been shown to be effective in mitigating crude oil fouling whilst enhancing heat transfer at the same time, as there will be axial and radial distributions of local shear stress in tube side with the introduction of inserts (Ritchie and Droegemueller, 2008; Krueger and Pouponnot, 2009). Thus, in this paper, the fouling model of tube inserts proposed by Yang et al. (2009) is adopted, and afterward combined in the optimization approach of HEN retrofit proposed by Pan et al. (2012b). Finally, an industrial study is  1424 presented to demonstrate the validity and efficiency of the enhancement techniques in HEN retrofit scenarios. 2. Fouling deposition in tube side Based on the work presented by Yang et al. (2009), fouling on a heat exchanger surface can be described as two steps. In the first step (induction period), the active fouling species adhere to the heat transfer surface and gradually cover it from a fractional coverage of θ = 0 to total coverage at θmax. This pre-conditioning layer is very thin, where the increase in fouling resistance Rf can be negligible, thus, changes in surface roughness are ignored. In the second step (fouling period), the fouling layer may start to grow immediately on the covered/pre-conditioned surface when it may be assumed that the growth rate is proportional to θ. The overall rate of fouling resistance growth can therefore be expressed as: ffRdtdR     (1) where, R’f is the form of established fouling rate expression, and the simple threshold model (Polley et al., 2007) is utilized in this paper: 8.0w33.08.0f ReTREexpPrReR       (2) 3945.0Re8526.1Ree     (3) In Equations (2) and (3), α and γ are constants, Re and Ree are Reynolds numbers before and after the implementation of tube inserts, Pr is Prandlt number, E is activation energy, R is universal gas constant, and Tw is tube wall temperature. Yang et al. (2009) also proposed second order growth rate (θg) and one order removal rate (θr) on fractional surface coverage (θ). Equation (4) describes that in the early stage of surface pre-conditioning, active species can be captured and adhered to the surface; meanwhile, the particles that stick to the surface act as seeds, attracting more foulant around them, such that fouling proceeds in a micro-growth manner.   11kdtd g   (4) Based on the concept of removal or release from the surface as in adsorption science, the removal rate (θr) of the surface coverage is assumed to be proportional to the surface coverage, as shown in Equation (5). 2kdtd r     (5) Thus, the net growth rate of θ is obtained with combining Equations (4) and (5):    21 1 kkdtd    (6) Integrating Equation (7) gives the fractional surface coverage (θ): t)kk(eckkk2111121    (7) where c is constant, t presents time, k1 and k2 are lumped growth rate and removal rate constants.  1425 wiTREexpAk1    (8) 8.02 urk     (9) In Equations (8) and (9), Ai and r are constants, Tw is tube wall temperature, R is universal gas constant, E is activation energy, and u is flow velocity. Compared with the induction time (when the growth of θ reaches its maximum at 0.5θmax), the fouling time is much longer (normally several months vs. several days). Thus, the induction time may be negligible in the whole operational period, and the overall rate of fouling resistance growth in Equation (1) can be simplified as: fmaxfRdtdR   (10) According to Equation (7), θmax is given by: 121maxkkk   (11) The fouling resistance at t operational time can be estimated based on the integration of Equation (10): tRR fmaxf   (12) The procedure of HEN retrofit addressed in this paper is to change the heat transfer coefficients of heat exchangers in tube side by implementing tube inserts under constant heat transfer area, which provides energy saving without any topology modifications. In addition, heat transfer area, stream flowrates and heat capacities in each exchanger maintain constant during HEN retrofit. Therefore, the fouling resistance formulations can be combined in the optimization approach proposed by Pan et al. (2012b) for HEN retrofit, and only two types of nonlinear terms are generated in the proposed model, namely, logarithmic mean temperature difference (LMTD) and fouling resistance (Rf). Most importantly, this paper considers more details for calculating overall heat transfer coefficient (U) rather than simply calculation of U in the previous work (Pan et al., 2012b):  exexextubeexexexexexexexexex SRFTRFDSUDkIDODlnODDTUDIDODDUDAAD2 (13) where DTUDex, DSUDex and DUDex are the reciprocal values of tube-side, shell-side and overall designed heat transfer coefficients for exchanger ex, ADex and Aex are designed and required area of exchanger ex, ODex and IDex are outer and inner tube diameters of exchanger ex, ktube is tube conductivity, TRFex and SRFex are tube side and shell side fouling resistances. To resolve the computational difficulties related to those nonlinear terms, the iterative procedures developed by Pan et al. (2012b) are utilized to find the optimal solutions of implementing tube inserts in HEN retrofit with fouling consideration. 3.  Case study A HEN of existing preheat train for a crude oil distillation column in a refinery plant is carried out to evaluate the proposed optimization approach. As shown in Figure 1, the retrofit objective is to reduce the hot utility (HU) consumption, namely, reduce the heat duty of heat exchanger 30 (target exchanger). The stream data and initial exchanger data can be found in Tables 1 and 2. Moreover, the heat-flow capacities of hot utility and cold utility are 93 kW/ ºC and 9652.5 kW/ ºC, the inlet temperatures of hot utility and cold utility are 1500 ºC and 12.45 ºC, the minimum temperature  1426 difference approaches (∆Tmin) before and after heat transfer intensification are 19 ºC and 5 ºC, and the minimum operating time of exchanger in the original HEN is 3.1 months (exchanger 28).   Figure 1: A HEN for the case study Table 1:  Stream details in case study Stream C1 C2 C3 H1 H2 H3 H4 H5 H6 H7 H8 H9 H10 H11 FCP  (kW/ ºC) 323 358.5 474 14.2 181.5 113 100 22.2 39.5 28.0 176 24.5 25.0 69.6 Tin (ºC) 33.5 91.34 151 335 253.2 294 212 213 174 364 290 284 240 179 Tout (ºC) 95.6 157.3 352 69.4 116.1 130 156 61.7 43.3 65.6 211 65.6 57.8 69.3 Table 2:  Exchanger details in original HEN EXs hi (kW/m2· ºC) h0 (kW/m2· ºC) U (kW/m2· ºC) Area (m2) Operating Time (month) Designed Required Designed Required Designed Required Designed Required 1 0.263 0.263 0.375 0.375 0.125 0.125 192.5 175.0 9.4 2 0.208 0.208 0.304 0.304 0.101 0.101 12.8 11.7 7.1 3 0.521 0.521 0.671 0.671 0.224 0.224 256.7 233.3 11.9 4 0.703 0.703 0.847 0.847 0.282 0.282 192.5 175.0 3.5 5 0.453 0.453 0.898 0.898 0.224 0.224 220.0 200.0 5.4 6 0.479 0.479 0.627 0.627 0.209 0.209 715.0 650.0 5.2 7 0.211 0.211 0.307 0.307 0.102 0.102 22.0 20.0 8.8 8 0.893 0.893 1.008 1.008 0.336 0.336 33.0 30.0 5.7 9 0.903 0.903 1.017 1.017 0.339 0.339 61.1 55.6 4.5 10 0.235 0.235 0.339 0.339 0.113 0.113 305.6 277.8 15.3 11 0.832 0.832 0.959 0.959 0.320 0.320 61.1 55.6 9.6 12 0.204 0.204 0.298 0.298 0.099 0.099 244.5 222.2 7.2 13 0.411 0.411 0.552 0.552 0.184 0.184 141.4 128.6 12.7 14 0.786 0.786 0.920 0.920 0.307 0.307 94.3 85.7 6.0 15 0.392 0.392 0.530 0.530 0.177 0.177 47.1 42.9 8.0 16 0.927 0.927 1.035 1.035 0.345 0.345 471.5 428.6 7.9 17 0.422 0.422 0.564 0.564 0.188 0.188 228.5 207.7 9.9 18 0.275 0.275 0.390 0.390 0.130 0.130 228.5 207.7 12.1 C1 C2 C3 CU H1 H2 H3 H4 H5 H6 H7 H8 H 9 H10 H11 HU 30 30 29 29 21 21 28 28 27 27 26 26 24 24 20 20  4  4 18 18 17 17 16 16 23 23 13 13 22 22 12 12  6  6  5  5  3  3  1  1  2  2 25 25 31 31 19 19 15 15 14 14  7  7 11 11 10 10  8  8  9  9  H Hot stream:  HU Hot utility:  C Cold stream:  CU Cold utility:  1427 Table 2: Exchanger details in original HEN (Continued) 19 0.340 0.340 0.469 0.469 0.156 0.156 152.3 138.5 13.2 20 0.663 0.663 0.810 0.810 0.270 0.270 1523.1 1384.6 5.4 21 0.504 0.504 0.653 0.653 0.218 0.218 76.2 69.2 4.9 22 0.658 0.658 0.805 0.805 0.268 0.268 114.2 103.8 3.6 23 0.736 0.736 0.876 0.876 0.292 0.292 152.3 138.5 4.7 24 0.308 0.308 0.431 0.431 0.144 0.144 1142.3 1038.5 3.4 25 0.335 0.335 0.463 0.463 0.154 0.154 73.3 66.7 10.0 26 0.984 0.984 1.079 1.079 0.360 0.360 733.3 666.6 4.3 27 0.239 0.239 0.344 0.344 0.115 0.115 256.6 233.3 4.3 28 0.881 0.881 0.998 0.998 0.333 0.333 1122.0 1020.0 3.1 29 0.556 0.556 0.706 0.706 0.235 0.235 264.0 240.0 4.2 30 0.910 0.910 1.022 1.022 0.341 0.341 198.0 180.0 15.7 31 0.290 0.290 0.408 0.408 0.136 0.136 330.0 300.0 5.3 Table 3:  Exchanger details in retrofitted HEN EXs hi (kW/m2· ºC) h0 (kW/m2· ºC) U (kW/m2· ºC) Area (m2) Operating Time (month) Designed Required Designed Required Designed Required Designed Required 1 0.263 0.253 0.375 0.375 0.125 0.122 192.5 175.0 12.0 2 0.208 0.124 0.304 0.304 0.101 0.071 12.8 11.7 40.1 3 0.521 0.521 0.671 0.671 0.224 0.224 256.7 233.3 11.9 4 0.703 0.561 0.847 0.847 0.282 0.251 192.5 175.0 8.0 5  0.455 0.452 0.898 0.898 0.225 0.224 220.0 200.0 9.2 6 0.588 0.555 0.627 0.627 0.232 0.226 715.0 650.0 9.3 7 0.211 0.214 0.307 0.307 0.102 0.103 22.0 20.0 8.0 8 1.227 1.155 1.008 1.008 0.386 0.376 33.0 30.0 8.2 9 0.903 0.789 1.017 1.017 0.339 0.317 61.1 55.6 8.0 10 0.236 0.219 0.339 0.339 0.113 0.108 305.6 277.8 25.3 11 0.832 0.634 0.959 0.959 0.320 0.278 61.1 55.6 26.3 12 0.204 0.194 0.298 0.298 0.099 0.096 244.5 222.2 10.4 13 0.452 0.445 0.552 0.552 0.194 0.192 141.4 128.6 21.5 14 0.786 0.733 0.920 0.920 0.307 0.296 94.3 85.7 8.8 15 0.392 0.304 0.530 0.530 0.177 0.152 47.1 42.9 24.4 16 1.335 0.858 1.035 1.035 0.402 0.333 471.5 428.6 33.4 17 0.698 0.593 0.564 0.564 0.241 0.224 228.5 207.7 15.0 18 1.667 1.376 0.390 0.390 0.257 0.246 228.5 207.7 9.7 19 0.340 0.282 0.469 0.469 0.156 0.140 152.3 138.5 34.4 20 3.311 2.356 0.810 0.810 0.456 0.426 1523.1 1384.6 8.0 21 0.504 0.454 0.653 0.653 0.218 0.205 76.2 69.2 8.0 22 3.289 1.509 0.805 0.805 0.453 0.377 114.2 103.8 8.0 23 3.676 1.901 0.876 0.876 0.484 0.420 152.3 138.5 8.0 24 1.541 0.636 0.431 0.431 0.269 0.205 1142.3 1038.5 8.0 25 0.335 0.274 0.463 0.463 0.154 0.137 73.3 66.7 24.5 26 4.926 2.283 1.079 1.079 0.567 0.486 733.3 666.6 8.1 27 1.193 0.705 0.344 0.344 0.220 0.190 256.6 233.3 8.0 28 4.405 1.725 0.998 0.998 0.535 0.433 1122.0 1020.0 8.0 29 2.762 1.263 0.706 0.706 0.408 0.334 264.0 240.0 8.0 30 0.910 0.785 1.022 1.022 0.341 0.317 198.0 180.0 26.7 31 0.290 0.149 0.408 0.408 0.136 0.088 330.0 300.0 41.5 Bold exchanger number presents intensified exchanger.   In case study, tube side fouling deposition and intensification are considered simultaneously to improve energy saving in the whole network. In addition, shell side heat transfer coefficient and fouling resistance are assumed constant in exchangers during HEN retrofit, as only tube-side intensified techniques are considered. From Table 3, it can be noted that to increase the operational time of the whole process, the proposed method is utilized to obtain the optimal solutions under the constraint of operating times ≥ 8 months. In the retrofit solution, the implementation of tube inserts not only  1428 increases the heat transfer coefficients in enhanced exchangers, but also prolongs their operating times significantly, which might save substantial capital investment because of potential production losses during cleaning process. 4. Conclusions Implementation of intensified heat transfer is an efficient way to increase energy recovery and overcome fouling deposition in retrofitting HEN. In order to solve large scale HEN retrofit problems efficiently, a new optimization method has been proposed. The proposed design approach is able to give realistic and practical solutions for the debottlenecking of HEN as detailed intensified techniques are systematically applied. This leads substantial capital saving as no structural modification in heat recovery system configuration is considered. The case study shows that, based on the new approach, more energy saving is achieved (5.35%) under long operating times (3.1 months to 8 months). Acknowledgement The financial support is gratefully acknowledged from the EC FP7 project “Intensified Heat Transfer Technologies for Enhanced Heat Recovery – INTHEAT”, Grant Agreement No. 262205. 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2012b, Optimisation for the retrofit of large scale heat exchanger networks with comprising different intensified heat transfer techniques, Applied Thermal Engineering,

All-at-once and step-wise detailed retrofit of heat exchanger networks using an MILP model,

Calculation of heat exchanger networks for limiting fouling effects in the petrochemical industry,

Determination of crude oil fouling thresholds,

Effects of fouling on performance of retrofitted heat exchanger networks: a thermo-hydraulic based analysis,

Extraction of crude oil fouling model parameters from plant exchanger monitoring,

Heat Exchanger Design Handbook, section 3.21.2: Application of Tube Inserts in Heat Exchangers: Benefits of Tube Inserts, Eds.

Heat exchanger performance enhancement through the use of tube inserts in refineries and chemical plants – successful application examples: Spirelf, Turbotal and Fixotal systems,

Modelling of fouling induction period.

Novel optimization method for improving energy recovery in heat exchanger networks with intensified heat transfer,

Optimization of operating conditions for mitigating fouling in heat exchanger networks,

Simultaneous retrofit and heat integration of chemical processes,

Retrofit Procedure for Intensifying Heat Transfer in Heat Exchanger Networks prone to Fouling Deposition

Retrofit Procedure for Intensifying Heat Transfer in Heat Exchanger Networks prone to Fouling Deposition

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