INTEGRATION OF TVC DESALINATION SYSTEM WITH COGENERATION PLANT: PARAMETRIC STUDY

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1 Fourteenth International Water echnology Conerence, IWC 4 2, Cairo, Egypt 7 INEGRAION OF VC DESALINAION SYSEM WIH COGENERAION PLAN: PARAMERIC SUDY Fuad N. Alasour * and Hadyan F. Alajmi ** * Kuwait University, College o Engineering and Petroleum, Mechanical Engineering Department P.O. Box 5969 Saat 36, Kuwait alasour@kuc.kuniv.edu.kw ** Kuwait Oil Company, Major Project Group II P.O. Box 9758 Alahmadi 68, Kuwait Halajmi@kockw.com ABSRAC he aim o this research is to perorm a parametric analysis on VC desalination system. he study was based on irst and second law o thermodynamics. Four units o thermal vapor compression (VC) desalting system, rom Sidem, with a capacity o MIGD and a gain ratio o 8 were utilized. his system was considered to be integrated with the Azzour South cogeneration plant. Several parameters were investigated in this study; compression ratio, temperature dierence across the stages and number o stages. Results showed that the gain ratio, exergy destruction were sensitive to the variations o those parameters. Results also showed that the evaporator and steam ejector are the main sources o exergy destruction occurred in VC system. he study recommends that eorts should be directed to achieve the best setting or the desalination process to minimize the exergy destruction and increase the gain ratio by improving design o such components. Keywords: Desalination, Cogeneration, SE-VC, ME-VC, Exergy, Gain ratio, Heat consumption INRODUCION Researchers have continued working or achieving additional improvements to enhance desalination system perormance; they are trying to reach a reduction in speciic energy consumption, speciic heat transer area, speciic cooling water low rate and speciic exergy destruction. On the other side researchers are seeking to ind ways to increase desalination gain ratio and perormance ratio. Several investigators studied the eect o operational and design parameters using energy and exergy analysis such as: Darwish and El-Dessouky [6] compared the perormance between thermal vapor compression process, conventional multi eect (ME) system and conventional

2 8 Fourteenth International Water echnology Conerence, IWC 4 2, Cairo, Egypt multistage lash (MSF) desalting system. heir results showed that gain ratio o 4 eects VC is very close to that o ME eects and 24 stages MSF desalting systems. Nevertheless, the MSF and ME are supplied with a steam at low pressure, while the VC requires a high pressure steam. Al-Najem et al. [] conducted a parametric analysis or VC system components; steam ejector, evaporator, and condenser, as well as the system as a whole. he study presented the energy and exergy analysis o individual components or a single and a multi eect thermo vapor compression systems. Results showed that the steam ejector and the evaporator are the main source o exergy destruction in the VC system. hey recommended minimizing theses losses by improving the design o such component. El-Dessouky and Ettouney [8] evaluated the variations o thermal perormance ratio, speciic heat transer area, and speciic low rate o cooling water as a unction o brine boiling temperature, vapor compression ratio and motive steam pressure, the study aimed to analyze a single eect thermal vapor compression (SE-VC) desalination process. hey recommended that the single eect vapor compression desalination unit can be operated at intermediate values o evaporation temperature (7 to 8 C) and low compression ratios (values close to 2). his is necessary to have perormance ratios close to or higher than.5. hey concluded that when the VC unit operates at intermediate values, a large reduction is observed in the speciic heat transer area and speciic cooling water low rate. his reduction will lower the construction cost o evaporator, condenser, and seawater pump. In addition, they concluded that operating cost would be lower as a result o reduction in the energy required to operate the seawater pumping unit. El-Dessouky et al. [9] proposed a novel system to increase the perormance o a multistage lash desalting process (MSF) by a combination o MSF with thermal vapor compression (VC). wo schemes, which included vapor entrainment and compression rom the heat recovery or rejection sections, were evaluated. hey evaluated the perormance o the proposed schemes as a unction o top brine temperature and location o vapor entrainment. hey concluded that thermal vapor compression enhances the perormance o MSF system as a result o the increase in the perormance ratio and the reduction in the speciic low rate o cooling water and the speciic heat transer area. Kamali et al. [] developed simulation model o ME-VC system or parametric optimization techniques. his model predicts the eect o all parameters on total capacity, perormance ratio o the system, temperature dierence between eects and pressure on each eect under design and operating conditions. he results showed that by means o parametric study, the computer simulation tool developed will help designers to achieve the best settings or desalination process to increase GOR value and minimize the energy consumption. Bin Amer [3] developed a Matlab algorithm and used it to solve a mathematical model optimization problem, where dierent numbers o eects were tested to maximize the

3 Fourteenth International Water echnology Conerence, IWC 4 2, Cairo, Egypt 9 gain ratio. he results showed the maximum gain ratio varied between 8.5 and 8.5 or 4 and 2 eects with an optimal top brine temperature ranging between 55.8 to 67.5 o C and reasonable speciic heat transer area. - Single and Multi Eect hermal vapor compression desalting system he main components o the VC unit are the steam jet ejector, the condenser and the evaporator. he SE-VC consists o only one evaporator whereas the ME-VC consists o multi number o evaporators (eects). Multi eect thermal vapor compression desalting systems is shown in igure ; the motive steam at a medium pressure is used to compress some o the vapor generated in the last evaporator (the lowest temperature eect) by the steam ejector. he recompressed vapor discharged rom the ejector is used (along with the expanded motive steam) as a heating source. he expanded motive steam and recompressed vapor leaving the steam ejector are directed to and condensed in the irst and highest temperature eect. he vapor generated in the irst eect ( D ) at ( P ) is directed to the second eect where it condenses. his vapor ( D ) acts as a heat source or this second eect and heats the eed ( F 2 ) to that eect rom its eed temperature ( ) to its evaporation temperature ( 2 ) and generates vapor by evaporation and lashing at a rate equal to ( D 2 = D e 2 + D l2 ) at (P 2 ). Similarly, the vapor ( D 2 ) generated in the second eect is used as a heat source or the next eect and so on to the last eect. he vapor generated in the last eect ( D n ) is divided to ( D ev ) and ( D c ). he stream ( D ev ) is directed to the steam ejector where it is recompressed and returned to the irst eect, along with the motive steam ( S ) as a heating medium. he stream ( D c ) is directed to the end o the condenser where it condenses and heats the seawater eed rom seawater temperature ( cw ) to the eed temperature ( ). he brine leaving the irst eect ( B ) is directed to the second eect where its temperature is spontaneously decreased rom ( ) to ( 2 ) where ( 2 = ) by lashing a part o this brine equal to ( B C / L ) into a vapor in the second eect. he vapor generated in the second eect ( D 2 ) consists o the part obtained by evaporation ( D e2 ) and the part obtained by lashing ( D l 2 ). Similarly, the brine leaving the second eect ( B 2 ) is directed to the third eect and so on to the last eect. he brine leaving the last eect ( B n ) is the brine blow down. Part o the condensate returns to the boiler, and the other part join the product water. he energy and exergy analysis or each components o multi eect system will be presented in this section. In the ollowing analysis, some assumptions will be made, whenever adequate, to simpliy the analysis such as: the speciic heats C, C d and C br are averaged as C, the latent heats L, L2,... Ln are assumed to be equal to L and temperature drop across evaporators, i i+ are assumed to be equal to e. Steam ejector he energy balance and exergy destruction (irreversibility) in the steam

4 2 Fourteenth International Water echnology Conerence, IWC 4 2, Cairo, Egypt ejector are expresses, as the ollowing: Sh + D h = + I ej s ev = Ψ ej gn = S ( S D ev ) h dc [ ] [( h h ) ( s s )] D ( h h ) ( s s ) s dc o s dc ev dc gn o dc gn () (2) First eect he energy balance and exergy destruction (irreversibility) in the irst eect are expresses, as the ollowing: ( S + Dev )( hdc hc ) = FC ( ) + D L ( )[( ) ( )] ( ) I = Ψ = S + Dev hdc hc sdc sc FC ln D L v (3) (4) Second eect he energy balance and exergy destruction (irreversibility) in the second eect are expresses, as the ollowing: ( F D ) C( ) = F C( ) D L D L (5) = Ψ = ( ) ( ) 2 I D L F D C 2 ln D2 L F2C( 2 ) ln 2 2 n-eect (6) he energy balance and exergy destruction (irreversibility) in the n-eect are expresses, as the ollowing: D [( F D ) + ( F2 D2 ) + ( Fi Di )] C = Fi C( i ) D i Li L (7) I i i i ( ( Fi Di )) C o i = Ψ = o i Di Li + ln v( i) o i D i L Fi C( i ) ln vi (8) End condenser he energy balance and exergy destruction (irreversibility) in the n-eect are expresses, as the ollowing: D c L n = ( M + F ) C( ) cw t cw (9)

5 Fourteenth International Water echnology Conerence, IWC 4 2, Cairo, Egypt 2 I c = Ψ = o ( + ) ( ) c Dc Ln M cw Ft C cw o ln () n cw he total product distillate output is expressed by D = D = D + D D t i 2 n () 2- hermal vapor compression desalting system perormance parameter In this section, some o important parameters that have signiicant eect on the perormance o VC system will be presented. Speciic energy consumption he speciic energy consumption per unit distillate is expressed by: Q D ( ) = S h s h c (2) Gain ratio he amount o resh water product per unit mass o motive steam or the gain ratio is expressed by: D GR = (3) S Speciic exergy destruction he speciic exergy destruction per unit distillate is expressed by: Ψ ψ = (4) D Speciic heat transer surace area he speciic heat transer surace area o whole system is expressed by: A D Ae Ac = + (5) D he heat transer area o evaporator (irst eect) is expressed by: A e ( ) ( S + Dev )( hdc hc ) U ( ) Qe = = (6) U e dc e dc he heat transer area o the second eect to the last eect is expressed by:

6 22 Fourteenth International Water echnology Conerence, IWC 4 2, Cairo, Egypt D( i) L A i = (7) U i ( ) i i+ he heat transer area o condenser is expressed by: A c = U c D L c ( LMD) c (8) he log mean temperature dierence is expressed by: LMD = cw vn ln cw (9) vn AZZOUR DESALINAION PLAN One o Kuwait cogeneration power desalting plants is the Azzour South cogeneration plant, which consists o 8 steam turbines with a 3 MW nominal capacity each. Each turbine is designed to supply extracted steam rom cross tube connecting the IP and LP turbines to two MSF units o 7.2 MIGD capacities each. able in Appendix -A presents a comparison based on speciic energy consumption by three analytical methods or our dierent desalting systems (MEE, VC, MSF and RO). Based on results showed in able in Appendix- A, VC was selected to be integrated with Azzour south cogeneration plant. A parametric study using irst and second law o thermodynamics is applied to an existing and operating VC designed and manuactured by Sidem o France (able 2 in Appendix- A). his system is considered to be integrated to an existing Azzour South power desalting plant. he considered system is similar to the our units o MIGD capacity each, built by Sidem in the western remote area o Abu-Dhabi in the UAE. Each unit has our eects and an end condenser. It is directly operated by a boiler generating saturated steam at 25 bar pressure. he top brine temperature ( ) at about 58.8 C in the irst eect; ( P ) =.859 bar and last eect temperatures ( 4 ) at about 46.8 C, ( P 4 ) =.859 bar. he data shown in able 2 in Appendix-A is designed or units directly operated by the boiler, where in Azzour south power desalting plant, the steam is extracted rom cross over tube connecting the intermediate and low pressures turbines. So to add the Sidem system to Azzour plant, the steam supply condition must be changed to the conditions o extracted steam: P = 3 bar, = C (actual condition o Azzour plant at 75% load).

7 Fourteenth International Water echnology Conerence, IWC 4 2, Cairo, Egypt 23 Steam Jet Ejector S + D ev Motive steam Feed Water F, D ev M cw D c F, F 2, F 3, F 4, D D 2 D n- D n v v2 V (n-) Vn 2 n- n Seawater B, B 2, 2 B n-, n- B n, n Figure Schematic diagram o ME-VC Rejected Brine D t, d Fresh Water

8 24 Fourteenth International Water echnology Conerence, IWC 4 2, Cairo, Egypt RESULS AND DISCUSSION Results are presented in terms o variations in the system design parameters as a unction o temperature dierence across the evaporator ( e ), compression ratio (Cr), top brine temperatures ( ) and number o eects (N). he system parameters include variations in speciic heat transer area per unit distillate (A/D), speciic heat consumption per unit distillate (Q/D), gain ratio (D/S), Speciic exergy destruction per unit distillate ( Ψ / D ) in evaporator, condenser, ejector and the whole system he results are examined both the single eect and multi eect thermo-vapor compression desalting systems Figure 2 showed the variation o the speciic heat transer area per unit distillate (A/D) as a unction o temperature dierence across the evaporator ( e ) in the evaporator, condenser and the whole system. Results are obtained at a temperature dierence ( e ) ranging rom 2 to 6 C. As ( e ) decreases, the heat transer area o the evaporator (and that o the whole system) is increased and consequently the unit cost. he decrease o ( e ) is insensitive in the condenser whereas it is sensitive in the case o the evaporator. he drop in the speciic heat transer area o the evaporator rom, ( e ) = 2 to 8 is about 83% while rom, ( e ) = 8 to 6, the drop is small, only 9%. 7 condenser 6 evaporator total Speciic heat transer area per unit distillate A/D,m 2 (kg/s) emperature dierence across the evaporator e, o C Figure 2 Eect o temperature dierence across the evaporator on the speciic heat transer area o the evaporator and condenser (SE-VC)

9 Fourteenth International Water echnology Conerence, IWC 4 2, Cairo, Egypt 25 Figure 3 showed the variation o speciic heat transer area per unit distillate (A/D) as a unction o compression ratio (Cr) under our dierent top brine temperatures. he results are obtained at motive steam pressure o 3 bar and top brine temperature range o 46 to 7 C. Results showed that the speciic heat transer area decreases drastically as the compression ratio is increased and the top brine temperature is increased. At constant top brine temperatures and at higher compression ratios, the pressure o the discharge vapor is greater. his is because the pressure o the entrained vapor does not change at constant top brine temperatures. Simultaneously, the temperature o the discharge vapor is also increased as the compression ratio is elevated. he increase in the temperature o the discharge vapor enhances the rates o heat transer. his is caused by the increase o the driving orce or the heat transer across the evaporator ( e ). As a result, the evaporator heat transer area is reduced at higher compression ratios. Regardless, the heat transer area increases in the condenser. his is because o the increase in the condenser load, which is caused by the reduction in the amount o entrained vapor at higher compression ratios. However, the decrease in the evaporator area is more pronounced than the increase in the condenser area. he net result o the above is the decrease in the speciic heat transer area upon the increase o the compression ratio. 4 Speciic heat transer area per unit distillate A/D,m 2 (kg/s) = 46 o C = 58 o C = 7 o C Compression ratio Cr Figure 3 Eect o compression ratio on the speciic heat transer area at dierent top brine temperatures

10 26 Fourteenth International Water echnology Conerence, IWC 4 2, Cairo, Egypt Figure 4 shows the speciic heat consumption per unit distillate (Q/D) and gain ratio (GR) as a unction o temperature dierence across the evaporator ( e ). he results are obtained at temperature dierences ranging rom 2 to 6 C. he results showed that the value o the speciic heat consumption per unit distillate is sensitive to variations in temperature dierence, where it is increased rom [kj/(kg/s) o distillate] or ( e ). = 2 C to 443 [kj/(kg/s) o distillate] or ( e ) = 6 C. he drop in gain ratio is 65% or ( e ) ranges rom 2 to 8 C, whereas rom ( e ) = 8 to 6 C, the drop is relatively less it is about % only. he drop that occurred is due to the increase o compression ratio as ( e ), increases which yield to an increase the amount o motive steam consumed to compress the entrained vapor. hereore, the system gain ratio is reduced. 2 2 D/S 6 Q/D Speciic heat consumption per unit distillate Q/D,kJ/kg Gain ratio,kg/kg D/S emperature dierence across the evaporator e, o C Figure 4 Eect o temperature drop across heat transer suraces in the evaporator on the speciic heat transer consumption per unit distillate and gain ratio Variations in the gain ratio (D/S) as a unction o number o eects (N) under dierent top brine temperatures, ( ) are shown in Figure 5. he results were obtained or the same temperature dierence between the top brine temperature ( ), (evaporating temperature o the irst eect) and ( n ), (o the evaporating temperature in the last eect), whereas the evaporation temperature in the last eect assumed to be ( n ) = 46 C. he increase in the number o eects reduces the temperature across each evaporator (eect), and consequently decrease compression ratio. At low compression ratios, the amount o motive steam consumed to compress the entrained vapor is small. hereore, the system gain ratio increases.

11 Fourteenth International Water echnology Conerence, IWC 4 2, Cairo, Egypt 27 2 = 54 o C = 58 o C 6 = 62 o C Gain ratio, kg/kg D/S Number o Eects N Figure 5 Eect o increasing the number o eects or the same n on the gain ratio at dierent top brine temperatures he variations in the speciic heat transer area per unit distillate, (A/D), as a unction o number o eects (N) at dierent top brine temperatures, ( ) is shown in Figure 6. he evaporation temperature in the last eect assumed to be n = 46 C. he results showed that the heat transer area increases by adding more eects. he heat transer area or the our eect units, or example, is more than our times than the area o tehg single eect unit producing the same distillate and working between same ( ) and ( n ). he variations in the exergy destruction per unit distillate; in the condenser, evaporator, steam ejector, and the system as whole, as a unction o top brine temperature is shown in Figure 7. Results are obtained at motive steam pressure o 3 bar and with our eects. he results showed that the increase o the top brine temperature, while keeping the same ( n ), increases the exergy destruction per unit distillate. he results indicated that the main exergy destruction occurs in the evaporator and steam ejector.

12 28 Fourteenth International Water echnology Conerence, IWC 4 2, Cairo, Egypt 2 Speciic heat transer area per unit distillate A/D m 2,(kg/s) = 54 o C = 58 o C = 62 o C Number o Eects N Figure 6 Eect o the number o eects on the speciic heat transer area at dierent top brine temperatures 2 condenser Speciic exergy destruction per unit distillate Ψ/D,kJ/kg evaporator ejector total op brine temperature, o C Figure 7 Eect o the top brine temperature on the speciic exergy destruction in the condenser, evaporator and ejector (ME-VC)

13 Fourteenth International Water echnology Conerence, IWC 4 2, Cairo, Egypt 29 CONCLUSION A parametric study based on irst and second law analysis was applied on a suggested VC system, which was considered to be integrated with an existing Azzour South cogeneration plant. Results showed that the speciic energy consumption per unit distillate and the gain ratio are sensitive to variations in temperature dierence across evaporator. he gain ratio is also sensitive to variations o number o eects. Moreover, results showed that the speciic exergy destruction per unit distillate is sensitive to the variations in temperature dierence across evaporator, and top brine temperature. he parametric analysis o VC desalting system combined with Azzour South plant showed that the steam ejector and the evaporator are the main sources o exergy destructions in VC system. hereore eorts should be directed to minimize these losses by improving design o such components. he exergy analysis enables us to develop a systematic approach that can be used to identiy sites or real losses o valuable energy in thermal devices. NOMENCLAURE A Heat transer area, m 2 B Brine low rate, kg s - C Liquid (distillate, eed and brine) speciic heat kj kg - K - C r Compression ratio D Distillate low rate, kg s - E r Expansion ratio F Feed low rate, kg s - GR Gain ratio h Speciic enthalpy, kj kg - I Irreversibility rate, MW L Speciic latent heat, kj kg - LMD Low pressure, bar m Mass low, kg M Mass low rate, kg s - MIGD Million imperial gallons per day N Number o eects P Pressure, bar Q Motive steam supply low rate, kg s - s Saturation temperature, C S emperature, C S op brine temperature, C Overall heat transer coeicient, kw m -2 C B op brine temperature, C U Overall heat transer coeicient, kw m -2 C

14 3 Fourteenth International Water echnology Conerence, IWC 4 2, Cairo, Egypt Greek symbols ψ Speciic exergy, kj kg - Ψ Exergy rate, kj s - e emperature dierence across the evaporator, C i emperature drop o the lashing stream per stage, C Subscripts First eect br Brine c Condensate or condenser d Distillate dc Steam ejector discharge condition e Evaporator or evaporation ej Ejector ev Entrained vapor Feed l Flashing g Gain gn Saturated vapor generated in the evaporator i Stages numbers l Loss n Last eect o Surroundings s Steam t otal v Vapor Desalting System Appendix-A able Speciic energy consumption by our dierent desalting systems at design and actual data [5] hermal Energy kj/kg Equivalent Mechanical Due to hermal kj/kg Pumping Energy kj/kg otal Equivalent Mechanical kj/kg (kwh/m 3 ) Speciic Fuel Energy kj/kg Design Actual Design Actual Design Actual Design Actual Design Actual MSF VC MEE RO (9.85) 44.8 (2.44) (8.2) 23. (6.4) (9.38) 43.5 (2.9) 28.6 (7.82) 23. (6.4)

15 Fourteenth International Water echnology Conerence, IWC 4 2, Cairo, Egypt 3 able 2 Data o Sidem system built in UAE [] Parameter Delivery Eect Eect 2 Eect 3 Eect 4 (saturation), C P, bars Distillate output, MIGD (kg/s) (52.6) Motive steam Saturated at 25 bar op brine temperature, C 58.8 Last eect brine temperature, C 46.8 Average boiling point elevation, C BPE = Number o eects 4 emperature drop/eect (4 C) REFERENCES [] Al-Najem, N.M., Darwish, M.A., and Yousse, F.A., hermovapor Compression Desalters: Energy and Availability Analysis o Single and Multi Eect Systems, Desalination, Vol., pp , 997. [2] Al-Shuaib, A., Al-Bahu, M., El-Dessoukey, H., and Ettouney, H., Progress o the Desalination Industry in Kuwait, Desalination, Vol. IV, No. 3, pp. 9-22, 999. [3] Bin Amer, A.O., Development and optimization o ME-VC desalination system, Desalination, Vol. 249, pp , 29. [4] Cengel, Y.A., and Boles, M.A., hermodynamics: An Engineering Approach, McGraw-Hill, 994. [5] Darwish, M.A., Alasour, F.N., and Al-Ajmi, H.F., Energy and Exergy Analysis o Azzour South Cogeneration Power Desalting Plant in Kuwait, Second Workshop on Desalination echnologies: Future trends and Economics, ADS, Alexandria, pp. 4-5, 2. [6] Darwish, M.A., and El-Dessouky, H., he Heat Recovery hermal Vapour Compression Desalination with Other hermal Desalination Processes, Applied hermal Engineering, Vol. 6, No. 6, pp , 996. [7] El-Dessouky, H., and Ettouney, H., Fundamentals o Salts Water Desalination, Department o Chemical Engineering, College o Engineering and Petroleum, Kuwait University. [8] El-Dessouky, H., and Ettouney, H., Single Eect hermal Vapor Compression Desalination Process: hermal Analysis, Heat ranser Engineering, Vol. 2, No. 2, pp , 999. [9] El-Dessouky, H., Ettouney, H., Al-Fulaij, H., and Mandani, F., Multistage Flash Desalination Combined with hermal Vapor Compression, Chemical Engineering and Processing, Vol. 39, pp , 2. [] El-Nashar, A.M., Cost Allocation in a Cogeneration Plant or the Production o Power and Desalted Water Comparison o the Exergy Cost Accounting Method with the WEA Method, Desalination, Vol. 22, pp. 5-34, 999.

16 32 Fourteenth International Water echnology Conerence, IWC 4 2, Cairo, Egypt [] Kamali, R.K., Abbassi, A., Sadough Vanini, S.A., Saar Avval, M., hermodynamic design and parametric study o MED-VC, Desalination, Vol. 235, pp , 29. [2] Kotas,.J., he Exergy Method o hermal Plant Analysis, Butterworths, 985. [3] Li, Kam W., Applied hermodynamics: Availability Method and Energy Conservation, aylor & Francis, 996. [4] Utgikar, P.S., Dubey, S.P., and Prasada Rao, P.J., hermoeconomic Analysis o Gas urbine Co-generation Plant a Case Study, Journal o Power and Energy (I Mech. E), Vol. 29, pp , 995.