OPTIMIZATION OF THE TUBE SIZE AND THE ARRANGEMENT OF EVAPORATOR TUBE BUNDLE TO IMPROVE THE PERFORMANCE OF MED-TVC SYSTEMS

Transcription

1 Eleventh International Water Technology Conference, IWTC Sharm El-Sheikh, Egypt 359 OPTIMIZATION OF THE TUBE SIZE AND THE ARRANGEMENT OF EVAPORATOR TUBE BUNDLE TO IMPROVE THE PERFORMANCE OF MED-TVC SYSTEMS Ramin K. Kamali and S. Mohebbinia Fan Niroo Company, Tehran, Iran s: ABSTRACT The present work summarizes parametric optimization technique for the tube size and arrangement of tube bundle inside MED-TVC system (Multi Effect Desalination with Thermal Vapor Compression) to increase GOR (Gain Output Ratio) value. For this purpose, a simulation model of MED-TVC system is presented. This program provides engineers a cost effective tools for thermodynamics and thermo hydraulic designing, developing and optimizing the thermal desalination plants. It is the objectives of this article to develop a mathematical model which predicts the influence of all factors on total capacity, performance ratio, temperature difference between effects and pressure on each effect to select the optimum size and arrangement of the tubes for various capacities of the MED units. The pressure drop across the system should be minimized in order to decrease the overall energy consumption of the system and therefore to improve the gain output ratio of the plant. The simulation package is applied for a specified capacity in order to achieve the best size and arrangement for tube bundles of the system. A comparison of GOR values between two actual systems with different size and arrangement in their evaporators shows that a system which has designed according to results of simulation package, has higher amount of GOR value. The comparison between the simulation results, with experimental data well proves optimization method s validity. INTRODUCTION The need for high quality water significantly increased during the second half of the last century. It has been a complex task to develop an effective thermo hydraulic design without actual testing which usually requires costly test procedures. The desalination industry is the life live for several countries and zones around the world, especially the countries around the Persian Gulf such as IRAN. Expansion in desalination industry is associated with reduction in the power consumption and increase GOR value. For this reason, a general computer code for MED type of desalination has been developed and is currently used by the only MED-TVC Manufacturer Company in IRAN (Fan-Niroo Co.)[1]. Uche [2], Kafi [3], Hisham [4,5], Jernqvist [6] and Hisham [7] developed a simulation code for MED system with

2 360 Eleventh International Water Technology Conference, IWTC Sharm El-Sheikh, Egypt shell and tube evaporators. Vertical channels instead of horizontal tubes were considered by Mitrovic and Raach [8] in 2005 but there is no corrugation on the plates in their investigation. The present work deals with both shell and tube and plate type evaporators with horizontal corrugation and in addition, thermo-compressor and ejectors are designed as well. TVC systems are affecting on increasing GOR value in MED systems [9].In this paper, it is shown that parametric study as one of the optimization method on thermo hydraulic data, strongly help to increase GOR value inside MED-TVC systems. Some of the energy losses in the system take place due to pressure drop in the evaporators. By decreasing the overall pressure drop of the system, the energy losses decreased and the performance of the system will increase consequently. Steam pressure drop in the system is a function of tube length, tube internal diameter and the arrangement of the tube bundle. The purpose of this work is the investigation of the effect of tube length and tube arrangement in the evaporators. THERMO HYDRAULIC MODELING A schematic of MED-TVC system is shown in Figure (1). The system is consisting of some evaporators, a condenser, and thermo-compressor. In each effect, two phase flow inside the evaporators is modeled by conservation equations to account pressure drop and flow specifications [10], [11], [12], [13]. Figure 1. Schematic of MED-TVC system. In mathematical modeling, mass and energy balance equations have been developed for the system and then heat exchangers, thermo-compressor and ejectors are designed based on the results of mass and energy balance.

3 Eleventh International Water Technology Conference, IWTC Sharm El-Sheikh, Egypt 361 Evaporators and Condenser Design Evaporators are the main part of desalination units. Therefore, it is very important to design them more cost effective and more efficient. Two types of configuration would be applied to desalination plants are conventional shell and tube and plate type evaporators. Plate type evaporators are a new technology to have a compact and portable system. Shell and Tube Evaporators To design a shell and tube evaporators, there are some parameters should be calculated such as tube size (diameter and length), number of pass and number of tubes. Figure (2) illustrates the design algorithm which was used to determine these parameters and finalize evaporators design. Figure 2. Shell and tube evaporators design algorithm.

4 362 Eleventh International Water Technology Conference, IWTC Sharm El-Sheikh, Egypt Figure (3) illustrates a schematic view of the MED-TVC design cod. Plate Type Evaporators For being compact, easy to clean, efficient and more flexible, the gasket plate type heat exchangers are employed in desalination process. To design a plate type heat exchanger, there are some parameters which have to be calculated such as number of plates, plate size, chevron type, the length of gap between plates and so on. The design algorithm for plate type evaporator is illustrated in Figure (4).

5 Eleventh International Water Technology Conference, IWTC Sharm El-Sheikh, Egypt 363 Figure 4. Plate type evaporators design algorithm Shell and Tube Condenser A part of vapor generated in the last effect is passed through condenser and condensed by seawater intake to the system. Condenser is usually shell and tube type in MED system which has the same design algorithm as shell and tube evaporators. Thermo-Compressor and Ejectors Thermal desalination systems operate at pressures lower than atmosphere pressure. Therefore, using vacuum devices in these systems are unavoidable. Ejectors and thermo compressors are common thermal devices can provide vacuum requirement for these systems. It should be noted that thermo compressor is one kind of an ejector. The ejector is a pumping device which uses jet action of a high pressure and temperature primary

6 364 Eleventh International Water Technology Conference, IWTC Sharm El-Sheikh, Egypt motive steam to entrain and accelerate a slower secondary steam (load). Due to the simplicity of design and absence of motive parts, ejectors are very reliable; require practically no maintenance and have a relatively low installation cost [14]. The ejectors are powered by heat, which is low-grade energy and it is obviously less expensive to run than electrical or mechanical-related power. The steam required for the jet ejector is commonly drawn from boilers. These devices used in vapor compression desalination systems as a heat pump. A thermo vapor compression desalination unit mainly comprises a steam jet ejector, a single or multi effect evaporator, and a condenser. The thermo compressor is used to compress the vapor from pressure P s [which is the vapor pressure leaving the last effect or condenser depends on the system design] to P 1 [which is the vapor pressure entering the first effect] by using an external source of steam at a pressure P e greater than the vapor pressure. Two types of ejectors are usually used in the systems. They are hogging ejector and NCG ejector. First one provides the initial vacuum of the system and the other one discharge non-condensable gases (NCG) from system. The ejector design can be classified into two categories which is known "Constantarea mixing ejector" and "Constant-pressure mixing ejector". In this case, the ejectors and thermo compressor have been designed based on "Constant-pressure mixing ejector" [14], [15]. Thermo compressor and ejectors design flowchart is shown in Figure (5). Simulation Algorithm Finally, the simulation algorithm of thermo hydraulic design of MED-TVC system can be considered as Figure (6). The program is modular in structure and includes a number of modules for evaporators, condensers, thermo-compressor, steam jet ejectors and etc. Each module has its own mathematical model. The program also includes a comprehensive database for the physical properties of seawater. There is a library containing correlations for heat transfer coefficient of different heat transfer surfaces and flow regime. [16], [17], [18].

7 Eleventh International Water Technology Conference, IWTC Sharm El-Sheikh, Egypt 365 Inputs: m, P P p, η, T p, P s, T s, m s P Calculate nozzle throat and nozzle outlet diameters. Determine number of required nozzles. Set Mach number of the nozzle outlet equal to 3.5 and calculate the nozzle outlet diameters, then determine the number of required nozzle (usually 3) Set the Mach number of secondary flow at the inlet of mixing section equal to 1 and calculate the pressure and temperature of both of streams, and determine the diameter of constant area section. By assuming "constant pressure mixes ", calculate the flow condition before shock (Pressure, Mach number...) Calculate flow condition after Mach number, and its condition at the outlet of ejector, By means of diameter of constant area, determine the lengths of mixing section, constant area section and diffuser section Figure 5. Thermo-compressor and ejectors design algorithm. Figure 6. Thermo hydraulic design algorithm.

8 366 Eleventh International Water Technology Conference, IWTC Sharm El-Sheikh, Egypt PARAMETRIC STUDY As GOR value is depend on temperature difference between effects, so it is important to investigate the influence of temperature difference between effects on GOR value. For a four effects system with high pressure steam inlet, this effect is observed on Figure (7). It can be seen that, if the temperature difference between effects decrease, the amount of GOR value will increase. In order to decrease the temperature difference between effects, the pressure drop between effects should be decreased. Therefore it is very important to optimize evaporator s thermo hydraulic design inside MED-TVC systems to minimize pressure drop between effects GO R Temperature Difference between Effects [ o C] Figure 7. Variation of GOR value with temperature difference between effects. In MED-TVC systems with shell and tube evaporators, thermo hydraulic design depends on definition of some parameters such as tube lengths, pitches and diameters. In addition, tube bundle arrangement between tube sheets can affect on heat transfer coefficient and required heat transfer surface area. To fix Thermohydraulic design condition, it is necessary to define the best values for tube lengths, pitches and diameters in order to minimize pressure drop between effects. As the vapor generated in each effect is passed through tube bundles and demisters and enter inside tubes in the next effect, so the pressure drop between effects is approximately equal to summation of pressure drop around the tubes and pressure drop inside the tubes in the next effect. So, it is necessary to find optimum points for thermo hydraulic data because some of them affect against each other. For example, to minimize the pressure drop around the tube bundles, it is better to select tubes with higher length but to minimize the pressure drop inside the tube bundles, it is better to select shorter tubes. Therefore, parametric study should be done to find optimum points and minimize pressure drop between effects.

9 Eleventh International Water Technology Conference, IWTC Sharm El-Sheikh, Egypt 367 EXPERIMENTAL SETUP A sample system with 2400 cubic meter fresh water per day capacity is selected and an experimental test was performed on this system to prove the validity of parametric study. Figure (8), shows a schematic of the experimental unit. Figure 8. A schematic of experimental Unit. The desalination package is controlled by a Unit Control Panel (UCP) as shown in Figure (9). Table (1) shows the list of controllable parameters and their tag numbers in the package. The desalination unit is automatically controlled from the Unit Control Panel but is interfaced for monitoring purpose. Figure (10) shows a schematic of monitoring instruments in the control room. These monitoring instruments can show the general faults, status of the operation and the entire amount of temperature and pressure gauges in the system. Figure 9. A schematic of unit control panel.

10 368 Eleventh International Water Technology Conference, IWTC Sharm El-Sheikh, Egypt Table 1. List of control valves and their nominal value in the package Main Number Code Description 1 PT01 Inlet Steam Pressure 2 PT02 Pressure in Effect#1 3 PT03 Pressure in Condenser 4 LT01 Product Level in Condenser 5 LT02 Brine Level in Effect#5 6 CT01 Product Conductivity 7 CT02 Condensate conductivity 8 FT14 Condense water Flow 9 FT01 Product Flow 10 FT02 Total Feed Flow 11 FT03 Effect #1 Flow Water 12 FT04 Sea Water Flow 13 TE01 Sea Water Temperature 14 TE02 Feed Water Temperature 15 TE03 Effect #1 Temperature 16 TE04 Condensate Water Temperature 17 TE05 Condenser Temperature Figure 10: A schematic of monitoring instruments in the control room. To investigate the effect of optimized thermo hydraulic data, a new system was installed according to new thermo hydraulic data in order to compare the results. All of the mentioned parameters in Table (1) are extracted in order to do parametric study on the sensitive parameters. It should be noted that, there was no difference between

11 Eleventh International Water Technology Conference, IWTC Sharm El-Sheikh, Egypt 369 process condition, mechanical parameters and materials between optimized and nonoptimized system. In addition, all of the experimental results were obtained according to summer condition. RESULT AND DISCUSSION Steam produced from seawater evaporation in each effect, while passes across the tube bundle and condenses through the tubes of the next effect has pressure drop. Decreasing the pressure drop of the MED system signifies lowering the load of the thermo-compressor which has to endure. It means that by decreasing the P, the amount of thermo-compressor steam consumption will decrease. In other words, by decreasing the overall pressure drop of the system, the energy losses decreased and the performance of the system will increase consequently. Steam pressure drop in the system is a function of tube length, tube internal diameter and the arrangement of the tube bundle. Increasing the tube length leads to increase in pressure drop; whereas increasing the tube length itself leads to decreased number of tubes and consequently increases pressure drop. Pressure drop in these two zones of the system changes differently, so that in a specific interval of the tube length its amount will be minimum. By comparison of the obtained results from actual system and simulation data, it will be understood that the systems in which the length of the tubes are comparable with the length obtained from simulation, result higher efficiencies than the others. Increasing the tube pitch decreases the pressure drop in tube side and as a result leads to higher efficiency of the unit; but on the other hand it increases the shell diameter and extra capital costs. Finding the optimum value for this parameter depends on the importance of operating costs versus capital costs. As mentioned, thermo hydraulic performance of a sample system with 2400 cubic meter per day capacity with shell and tube evaporators has been evaluated. Figure (11) shows that the optimum length for the evaporator s tubes with 1 inch diameter to achieve the minimum pressure drop between effects is 3.8 meters. Pressure drop (bar) Length of Tubes [m] Figure 11. Variation of pressure drop with length of tubes.

12 370 Eleventh International Water Technology Conference, IWTC Sharm El-Sheikh, Egypt Figure (12) shows that the optimum length for the evaporator s tubes to achieve the minimum pressure drop between effects is varied by amount of tube pitch. For a tube bundles with a higher tube pitch, the amount of pressure drop and the optimum length for tube bundle decreases Pressure Drop [bar] Length of tube bundle [m] Tube Pitch=1.36 in Tube Pitch=1.31 in Figure 12. Variation of pressure drop with length of evaporators tubes. Figure (13) shows that the pressure drop inside condenser tube bundles decreases if the length of tube bundle increases to a higher value Pressure Drop [bar] Length of tube bundle [m] Figure 13. Variation of pressure drop with length of condenser tubes. For mentioned system with shell and tube heat exchangers, parametric study on thermo hydraulic data has been done. Table (2) shows the comparison between optimized and non-optimized system. Unfortunately, there was not any available

13 Eleventh International Water Technology Conference, IWTC Sharm El-Sheikh, Egypt 371 system in order to verify the results of optimized plate type evaporators, but it is interesting to note here that a new system is going to be built based on optimized plate type MED-TVC design data. According to this Table, it is obviously clear that, a system with optimized thermo hydraulic data has lower required heat transfer surface area. Therefore, GOR value in optimized system for a same heat transfer surface area is more than GOR value for a non-optimized system. Table 1. Optimized and non-optimized design data for MED-TVC system with shell and tube evaporators Parameter Unit Design Optimized Deviation Data Data (%) Total distillated product Ton/day Seawater temperature o C Motive steam Ton/day First Effect steam temperature o C Feed water temperature o C Number of effects Number of tubes in each effect Effects Tube length m Effects Tube diameter cm Number of condenser tubes Condenser Tube length m Condenser Tube outer diameter cm Steam Pressure barg GOR Shell depth m Shell length m CONCLUSIONS The method of parametric study is an appropriate tool to estimate the optimum thermo hydraulic parameters. By comparison of the obtained results from actual system and simulation data, it will be understood that the systems in which the length of the tubes are comparable with the length obtained from simulation, result higher efficiencies than the others. Decreasing the pressure drop of the MED system signifies lowering the load of the thermo-compressor which has to endure. It means that by decreasing the P, the amount of thermo-compressor steam consumption will decrease. In other words, by decreasing the overall pressure drop of the system, the energy losses decreased and the performance of the system will increase consequently. Another conclusion is that the optimum length of tubes inside the evaporators is strongly depending on the capacity of the system. Parametric study on thermo hydraulic data is one of the best methods to arise amount of GOR value of the system.

14 372 Eleventh International Water Technology Conference, IWTC Sharm El-Sheikh, Egypt NOMENCLATURE B D D r R F M c T S m P Brine blow down mass flow rate, ton/hr Distillate product mass flow rate, ton/hr Entrained steam mass flow rate, ton/hr Rejected water mass flow rate, ton/hr Seawater feed to effects flow rate, ton/hr Seawater mass flow rate, ton/hr Temperature of effect, o C Motive steam mass flow rate, ton/hr Mass flow rate Steam pressure SUBSCRIPTS i Index B Brine F Feed f Condenser product n Number of effects p Primary steam s Motive steam sw Seawater η Nozzle Efficiency P REFERENCES 1. R.K. Kamali, A. Abbassi, and S.A. Sadough, A simulation model and parametric study of MED-TVC process, EDS international conference, EuroMed (2006). 2. Javier Uche, Javier Artal, and Luis Serra, Comparison of heat transfer coefficient correlations for thermal desalination units, Journal of Desalination 152 (2002) F. Kafi, V. Renaudin, D. Alonso, and J.M. Hornut, New MED plate desalination process: Thermal performances, Journal of Desalination 166 (2004) Hisham M. Ettouney, and Hisham El-Dessouky, A simulator for thermal desalination process, Desalination 125 (1999) Hisham T. El-Dessouky, H.M. Ettouney, Multiple effect evaporation desalination systems: thermal analysis, Desalination 125 (1999) Jernqvist, M. Jernqvist, and G. Aly, Simulation of thermal desalination process, Desalination 134 (2001) Hisham Ettouney, Visual basic computer package for thermal and membrane desalination processes, Desalination 165 (2004)

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