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A water desalination course
Desalination, 94 (1994) 331-337 Elsevier Science B.V., Amsterdam
331
A water desalination course Hisham T. El-Dessouky ChemicalEngineering Department,College of Engineering and Petroleum,Kuwait University,P.O.Box 5969, Safat (Kuwait).Tel: 965 4817662: Fax: 965 4811772 (Received January 15.1993) SUMMARY
Water desalination is a required course in the Chemical Engineering Curriculum at Kuwait University. The importance of the course and curriculum placement are described. A detailed course syllabus and examples of project assignments are presented. INTRODUCTION
like the other Arabian Gulf countries, suffers from severe weather conditions, scarcity of rain water and absence of rivers and lakes and has very limited underground resources of fresh water. The average rainfall is 100 mm per year and the average evaporation rate is 17 mm per Kuwait,
&Y
[Il.
Desalination of seawater is the main source for supplying the required fresh water at steady state with reasonable reliability in many Middle East countries. By 31st December 1991, a worldwide total of 8,886 desalination units with a total capacity of 15,582,OOOm3/d had been installed or contracted [2]. The major proportion of desalination capacity is installed in Saudi Arabia (24.4%), United States (15.2%), Kuwait (9.1%) and the United Arab Emirates (10.6%) [2]. The majority of the desalination plants operating in the Gulf countries are based on the Multi Stage Flash (MSF) process. Most of the Reverse Osmosis (RO) plants are installed in the USA, followed by Saudi Arabia and Bahrain. The Multiple Effect Evaporation (MEE) and the Vapor Compression (VC) desalination processes seem to be very promising, especially for small and medium-sized plants.
OOll-9164/94/$07.00
0 1994 Elsevier Science B.V. All rights reserved.
~~I)~OO11-9164(93)E0139-Z
332
Many graduates of chemical engineering from Kuwait University are working in the field of water desalination. There are about 15 institutions and companies in Kuwait concerned with desalination [3]. Their work covers these areas: scientific engineers and operations, research and development, plant management, operation and maintenance, water resources planning, sharing in design, construction and commission of desalination units. Furthermore, desalination technology is increasingly used for the treatment of waters other than sea and brackish waters. The current areas of application include the treatment of effluent waters for reuse, river water to obtain ultrapure water for boiler feed water and electronic industries and groundwater. Therefore, a water desalination course is compulsory at the Chemical Engineering Department at Kuwait University. The main purposes of offering the water desalination course are to equip the students with critical and analytical tools to work on different desalination processes (which are extremely difficult problems), to make the students more interested in, and knowledgeable about real problems affecting their economic and social life, and to bridge an unfortunate gap that too often exists between industry and universities. There are very few publications dealing with water desalination eduction. Glasgow University is the pioneer school with a Master of Engineering described by Silver and Hanbury [4]. The development of the course and the discussion of some of the lessons gained during teaching this course were illustrated by Hanbury et al. [5]. Bakish and Arthur [6] gave a detailed description of the baccalaureate program in water desalination offered by the College of Science and Engineering at Fairleigh Dickinson University. This paper presents the undergraduate course in water desalination offered by the Chemical Engineering Department at Kuwait University. COURSE DESCRIPTION
The ChE 461 water desalination course is a three-hour lecture. The prerequisite is ChE 343 heat transfer which is a standard one-semester first course in engineering heat transfer. On the other hand, the prerequisites for ChE 461 by topics are: Principles of energy and material balance, Equilibrium thermodynamics, Physical chemistry, Elements of plant economics, Heat exchangers calculations, Fluid mechanics and pressure drop calculations for the fluid flow inside pipes.
333
In order to design a desalination course within the short time of one semester, it was necessary to concentrate the detailed analysis on the most widely used desalination processes. We have tried, however, to introduce the basic principles and the concepts for the other desalination processes. Throughout the course the emphasis is on applications of the fundamentals of basic sciences and general concepts to develop a process design for the desalination processes most widely used in industry in the Gulf countries. Due to lack of time, detailed mechanical design and cost estimation, are not dealt with. The course simply had to stop somewhere. The range of topics covered in the materials course are summarised below: Introduction [I h]
Importance and the objective of the desalination course -prerequisites - course organization and the way it is. Needs for Water Desalination 12 h] Water sources in the Middle East - Water consumptions in different countries - quality of water required in drinking, domestic, agricultural and important industries in the Gulf area - Alternative solutions of the water shortage problem. Desalination Processes [3 h] Main components of desalination plants - Classification of desalination processes - General description and flow charts for the desalination processes. Seawater Properties [3 h] Chemical composition of seawater - Estimation of the following seawater properties: salinity, alkalinity, hardness, pH, density, thermal conductivity, viscosity, specific heat, and boiling point elevation. Multiple EfSect Evaporation [8 h] Principle of evaporation-Steam economy and specific heat transfer for a single evaporator - Thermal analysis of Multiple Effect Evaporators - Effect of number of Effects on the specific heat transfer area and the performance ratio - Overall heat transfer coefficients in horizontal and vertical falling film evaporators Process design of multiple effect evaporators desalination plants - Multiple effect evaporation desalination plants in practice.
334 Vapor Compression [6 h] Principles of vapor compression-Thermal analysis of mechanical and thermal vapor compression processes - Process design of vapor compression processes - Comparison between mechanical and thermal vapor compression processes Vapor compression desalination plants in practice. Multi-Stage Flash [8 h] Principles of multi stage flash desalination process - Thermal analysis of once through MSF process - Thermal analysis of brine recirculation MSF desalination process - Flashing chambers configuration - Estimation of different thennodynamic losses - Process design of brine recirculation MSF desalination plants - Detailed design of flashing chambers (length, height, width, liquid level, dimensions of the submerged orifice, demister depth and cross sectional area, and condenser layout) - multi stage flash desalination plants in practice - Comparison of the performance ratio and the specific heat transfer surface area of the MSF plants with that of MEE and VC plants. Reverse Osmosis 16 h] Principles of reverse osmosis desalination process - Energy requirements Membrane polarization and pressure losses - Hollow fibre and Spiral Wound membranes - Factors influencing the capacity of RO members - Procedure for estimating permeator capacity - Determination of salt passage and product concentration - Energy recovery in RO Plants - Projection of RO plants - RO plants in practice. Scale Formation and Control 14 h] Effect of scale formation on the desalination plants’ performance - Principles of scale formation reactions in seawater - Kinetics of scale formation Removal of deposited scale - Minimizing the tendency for scaling - Removal of scale-forming constituents - Threshold additives - Additions to provide ahernative sites for deposition - Scaling potential in RO membrane - Prediction of required pretreatment processes for RO membranes - Cleaning of contaminated membranes - Product water post-treatment. Cogeneration Plants [3 h] Single purpose power and desalination plants - Dual purpose plants - Cogeneration steam turbine - Gas turbine cogeneration - Water-to-power ratio Advantages and disadvantages of dual purpose plants.
335
Material of Construction [4 h]
Composition and properties of the construction materials widely used in destination plants - Selection of the most suitable materials of construction for: flashing chambers, evaporators calandrias, membranes shell, heat transfer tubes, steam jet ejectors, pumps, pipeworks, and pretreatment equipment. ASSIGNMENTS
Some of the homework assignments are presented in this section. This illustrates the nature of the course. The class is divided into several subgroups of 2 or 3 students each. The required assignments are the same for all groups. However, values of the operating and design parameters are different for each group. We have considered the following factors in choosing these assignments: l The assignments deal mainly with the most widely used and promising desalination processes. l
The solution of these assignments requires thoughtful implementation of the tools of engineering idealizations and approximations, and the order of magnitude of numbers to be expected in practical situations.
l
The assignments should serve a motivational role by relating the theoretical basis to real engineering situations.
l
The numerous possible number of rules, steps and principles and their applications, in either designing and sizing of equipment or the evaluation of performance of a particular process, are usually left for the students to discover them by themselves while solving the assignments.
l
Students will gain the ability to function effectively as a team member, integration of a deep understating of basic principles with practical technology, hands-on orientation, and avoiding the abstractions that go with usual discussions of innovation.
l
Students will search for where to find appropriate formulae and how to recognise and apply them sensibly.
Assignment I
Two thousand tons of fresh water are to be produced in a forward-feed seven effects evaporator. The saturated steam flowing to the first effect is at 122” C and the saturation temperature of the formed vapor at the last effect is 45” C. The thermodynamic losses other than the boiling point elevation
336
are constant in all effects and equals 0.75’ C. The feed water salinity and temperature are 42000 ppm and 35 ’ C, respectively. Overall heat transfer coefficient U is related to the effect of saturation temperature. All effects are to have the same heating surface. What is (a) Plant performance ratio, (b) Specific heat transfer area, (c) Cooling water flow rate, (d) Mass of vapor formed by flashing in the third effect and in the flashing box of the fifth effect. Assignment 2 A brine recirculation MSF desalination plant has the following data. Plant capacity, Md = 5000 m3/d, feed water temperature and salinity are 30” C and 44 g/kg, respectively, number of flashing stages n = 24, number of stages in heat rejection section J = 3, temperature of the brine in the last stage Z’, = 32” C, top brine temperature TTb, = 112” C, heating steam temperature T, = 116.75 ’ C and terminal temperature differences in both the brine heater and preheaters are 3” C. Calculate the following: (a) thermodynamic losses, (b) plant performance ratio, (c) cooling water flow rate, (d) brine heater surface area, (e) flow rate of make-up water, (f) the following parameters for stage number 5: boiling point elevation, gate height, liquid level, demister temperature loss, stage height, stage length, preheater surface area. Assignment 3 A reverse osmosis seawater desalination plant of three-stage reverse osmosis; the plant uses DuPont 8” diameter B- 10 Permeators. The plant capacity is 1000 m3/h, the feed water salinity is 37500 ppm and the maximum and minimum temperatures of the seawater are 15 and 33 ’ C, respectively. Find: l the number of permeators in each stage; 0 salt concentration in product water; 0 specific energy consumption. CONCLUSION
The demand for steady and economical supply of water with reasonable reliability is constantly increasing all over the world. The Gulf countries have invested heavily in building large desalination plants. There is a very
337
need for a substantial number of professionally qualified men and women to manage and supervise the planning, construction, modification and operation of the existing and foreseen large number of desalination plants. The water desalination course at Kuwait University exposes the students to the theory and practice of the different desalination processes. It prepares the students to understand fully the desalination processes parameters and the behaviour of plants under different circumstances. strong
REFERENCES 1 A.M.R. Al-Marafie and M.A. Danvish, Desalination, 71(1989) 45-55. 2 K. Wangnick, IDA Worldwide Desalination Plants Inventory Reports No. 12, Intemational Desalination Association (IDA), 1992. 3 M. Balaban, Desalination Directory, Hannover, 1992. 4 R.S. Silver and W.T. Hanbury, Postgraduate Course in Desalination, Proceedings of the Fourth Int. Symp. on Fresh Water from the Sea, Vol. 2, 1973, pp. 413-425. 5 W.T. Hanbury, W.S. McCartney, W. Cunningham and T. Hodgkiess, Desalination, 30 (1979) 449459. 6 R. Bakish and W. Arthur, An Undergraduate Program in Desalination, Proceedings of the Int. Congress on Desalination and Water Reuse (Tokyo), IDEA, 1977, pp. 291-298.
331
A water desalination course Hisham T. El-Dessouky ChemicalEngineering Department,College of Engineering and Petroleum,Kuwait University,P.O.Box 5969, Safat (Kuwait).Tel: 965 4817662: Fax: 965 4811772 (Received January 15.1993) SUMMARY
Water desalination is a required course in the Chemical Engineering Curriculum at Kuwait University. The importance of the course and curriculum placement are described. A detailed course syllabus and examples of project assignments are presented. INTRODUCTION
like the other Arabian Gulf countries, suffers from severe weather conditions, scarcity of rain water and absence of rivers and lakes and has very limited underground resources of fresh water. The average rainfall is 100 mm per year and the average evaporation rate is 17 mm per Kuwait,
&Y
[Il.
Desalination of seawater is the main source for supplying the required fresh water at steady state with reasonable reliability in many Middle East countries. By 31st December 1991, a worldwide total of 8,886 desalination units with a total capacity of 15,582,OOOm3/d had been installed or contracted [2]. The major proportion of desalination capacity is installed in Saudi Arabia (24.4%), United States (15.2%), Kuwait (9.1%) and the United Arab Emirates (10.6%) [2]. The majority of the desalination plants operating in the Gulf countries are based on the Multi Stage Flash (MSF) process. Most of the Reverse Osmosis (RO) plants are installed in the USA, followed by Saudi Arabia and Bahrain. The Multiple Effect Evaporation (MEE) and the Vapor Compression (VC) desalination processes seem to be very promising, especially for small and medium-sized plants.
OOll-9164/94/$07.00
0 1994 Elsevier Science B.V. All rights reserved.
~~I)~OO11-9164(93)E0139-Z
332
Many graduates of chemical engineering from Kuwait University are working in the field of water desalination. There are about 15 institutions and companies in Kuwait concerned with desalination [3]. Their work covers these areas: scientific engineers and operations, research and development, plant management, operation and maintenance, water resources planning, sharing in design, construction and commission of desalination units. Furthermore, desalination technology is increasingly used for the treatment of waters other than sea and brackish waters. The current areas of application include the treatment of effluent waters for reuse, river water to obtain ultrapure water for boiler feed water and electronic industries and groundwater. Therefore, a water desalination course is compulsory at the Chemical Engineering Department at Kuwait University. The main purposes of offering the water desalination course are to equip the students with critical and analytical tools to work on different desalination processes (which are extremely difficult problems), to make the students more interested in, and knowledgeable about real problems affecting their economic and social life, and to bridge an unfortunate gap that too often exists between industry and universities. There are very few publications dealing with water desalination eduction. Glasgow University is the pioneer school with a Master of Engineering described by Silver and Hanbury [4]. The development of the course and the discussion of some of the lessons gained during teaching this course were illustrated by Hanbury et al. [5]. Bakish and Arthur [6] gave a detailed description of the baccalaureate program in water desalination offered by the College of Science and Engineering at Fairleigh Dickinson University. This paper presents the undergraduate course in water desalination offered by the Chemical Engineering Department at Kuwait University. COURSE DESCRIPTION
The ChE 461 water desalination course is a three-hour lecture. The prerequisite is ChE 343 heat transfer which is a standard one-semester first course in engineering heat transfer. On the other hand, the prerequisites for ChE 461 by topics are: Principles of energy and material balance, Equilibrium thermodynamics, Physical chemistry, Elements of plant economics, Heat exchangers calculations, Fluid mechanics and pressure drop calculations for the fluid flow inside pipes.
333
In order to design a desalination course within the short time of one semester, it was necessary to concentrate the detailed analysis on the most widely used desalination processes. We have tried, however, to introduce the basic principles and the concepts for the other desalination processes. Throughout the course the emphasis is on applications of the fundamentals of basic sciences and general concepts to develop a process design for the desalination processes most widely used in industry in the Gulf countries. Due to lack of time, detailed mechanical design and cost estimation, are not dealt with. The course simply had to stop somewhere. The range of topics covered in the materials course are summarised below: Introduction [I h]
Importance and the objective of the desalination course -prerequisites - course organization and the way it is. Needs for Water Desalination 12 h] Water sources in the Middle East - Water consumptions in different countries - quality of water required in drinking, domestic, agricultural and important industries in the Gulf area - Alternative solutions of the water shortage problem. Desalination Processes [3 h] Main components of desalination plants - Classification of desalination processes - General description and flow charts for the desalination processes. Seawater Properties [3 h] Chemical composition of seawater - Estimation of the following seawater properties: salinity, alkalinity, hardness, pH, density, thermal conductivity, viscosity, specific heat, and boiling point elevation. Multiple EfSect Evaporation [8 h] Principle of evaporation-Steam economy and specific heat transfer for a single evaporator - Thermal analysis of Multiple Effect Evaporators - Effect of number of Effects on the specific heat transfer area and the performance ratio - Overall heat transfer coefficients in horizontal and vertical falling film evaporators Process design of multiple effect evaporators desalination plants - Multiple effect evaporation desalination plants in practice.
334 Vapor Compression [6 h] Principles of vapor compression-Thermal analysis of mechanical and thermal vapor compression processes - Process design of vapor compression processes - Comparison between mechanical and thermal vapor compression processes Vapor compression desalination plants in practice. Multi-Stage Flash [8 h] Principles of multi stage flash desalination process - Thermal analysis of once through MSF process - Thermal analysis of brine recirculation MSF desalination process - Flashing chambers configuration - Estimation of different thennodynamic losses - Process design of brine recirculation MSF desalination plants - Detailed design of flashing chambers (length, height, width, liquid level, dimensions of the submerged orifice, demister depth and cross sectional area, and condenser layout) - multi stage flash desalination plants in practice - Comparison of the performance ratio and the specific heat transfer surface area of the MSF plants with that of MEE and VC plants. Reverse Osmosis 16 h] Principles of reverse osmosis desalination process - Energy requirements Membrane polarization and pressure losses - Hollow fibre and Spiral Wound membranes - Factors influencing the capacity of RO members - Procedure for estimating permeator capacity - Determination of salt passage and product concentration - Energy recovery in RO Plants - Projection of RO plants - RO plants in practice. Scale Formation and Control 14 h] Effect of scale formation on the desalination plants’ performance - Principles of scale formation reactions in seawater - Kinetics of scale formation Removal of deposited scale - Minimizing the tendency for scaling - Removal of scale-forming constituents - Threshold additives - Additions to provide ahernative sites for deposition - Scaling potential in RO membrane - Prediction of required pretreatment processes for RO membranes - Cleaning of contaminated membranes - Product water post-treatment. Cogeneration Plants [3 h] Single purpose power and desalination plants - Dual purpose plants - Cogeneration steam turbine - Gas turbine cogeneration - Water-to-power ratio Advantages and disadvantages of dual purpose plants.
335
Material of Construction [4 h]
Composition and properties of the construction materials widely used in destination plants - Selection of the most suitable materials of construction for: flashing chambers, evaporators calandrias, membranes shell, heat transfer tubes, steam jet ejectors, pumps, pipeworks, and pretreatment equipment. ASSIGNMENTS
Some of the homework assignments are presented in this section. This illustrates the nature of the course. The class is divided into several subgroups of 2 or 3 students each. The required assignments are the same for all groups. However, values of the operating and design parameters are different for each group. We have considered the following factors in choosing these assignments: l The assignments deal mainly with the most widely used and promising desalination processes. l
The solution of these assignments requires thoughtful implementation of the tools of engineering idealizations and approximations, and the order of magnitude of numbers to be expected in practical situations.
l
The assignments should serve a motivational role by relating the theoretical basis to real engineering situations.
l
The numerous possible number of rules, steps and principles and their applications, in either designing and sizing of equipment or the evaluation of performance of a particular process, are usually left for the students to discover them by themselves while solving the assignments.
l
Students will gain the ability to function effectively as a team member, integration of a deep understating of basic principles with practical technology, hands-on orientation, and avoiding the abstractions that go with usual discussions of innovation.
l
Students will search for where to find appropriate formulae and how to recognise and apply them sensibly.
Assignment I
Two thousand tons of fresh water are to be produced in a forward-feed seven effects evaporator. The saturated steam flowing to the first effect is at 122” C and the saturation temperature of the formed vapor at the last effect is 45” C. The thermodynamic losses other than the boiling point elevation
336
are constant in all effects and equals 0.75’ C. The feed water salinity and temperature are 42000 ppm and 35 ’ C, respectively. Overall heat transfer coefficient U is related to the effect of saturation temperature. All effects are to have the same heating surface. What is (a) Plant performance ratio, (b) Specific heat transfer area, (c) Cooling water flow rate, (d) Mass of vapor formed by flashing in the third effect and in the flashing box of the fifth effect. Assignment 2 A brine recirculation MSF desalination plant has the following data. Plant capacity, Md = 5000 m3/d, feed water temperature and salinity are 30” C and 44 g/kg, respectively, number of flashing stages n = 24, number of stages in heat rejection section J = 3, temperature of the brine in the last stage Z’, = 32” C, top brine temperature TTb, = 112” C, heating steam temperature T, = 116.75 ’ C and terminal temperature differences in both the brine heater and preheaters are 3” C. Calculate the following: (a) thermodynamic losses, (b) plant performance ratio, (c) cooling water flow rate, (d) brine heater surface area, (e) flow rate of make-up water, (f) the following parameters for stage number 5: boiling point elevation, gate height, liquid level, demister temperature loss, stage height, stage length, preheater surface area. Assignment 3 A reverse osmosis seawater desalination plant of three-stage reverse osmosis; the plant uses DuPont 8” diameter B- 10 Permeators. The plant capacity is 1000 m3/h, the feed water salinity is 37500 ppm and the maximum and minimum temperatures of the seawater are 15 and 33 ’ C, respectively. Find: l the number of permeators in each stage; 0 salt concentration in product water; 0 specific energy consumption. CONCLUSION
The demand for steady and economical supply of water with reasonable reliability is constantly increasing all over the world. The Gulf countries have invested heavily in building large desalination plants. There is a very
337
need for a substantial number of professionally qualified men and women to manage and supervise the planning, construction, modification and operation of the existing and foreseen large number of desalination plants. The water desalination course at Kuwait University exposes the students to the theory and practice of the different desalination processes. It prepares the students to understand fully the desalination processes parameters and the behaviour of plants under different circumstances. strong
REFERENCES 1 A.M.R. Al-Marafie and M.A. Danvish, Desalination, 71(1989) 45-55. 2 K. Wangnick, IDA Worldwide Desalination Plants Inventory Reports No. 12, Intemational Desalination Association (IDA), 1992. 3 M. Balaban, Desalination Directory, Hannover, 1992. 4 R.S. Silver and W.T. Hanbury, Postgraduate Course in Desalination, Proceedings of the Fourth Int. Symp. on Fresh Water from the Sea, Vol. 2, 1973, pp. 413-425. 5 W.T. Hanbury, W.S. McCartney, W. Cunningham and T. Hodgkiess, Desalination, 30 (1979) 449459. 6 R. Bakish and W. Arthur, An Undergraduate Program in Desalination, Proceedings of the Int. Congress on Desalination and Water Reuse (Tokyo), IDEA, 1977, pp. 291-298.