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" ENERGY ANALYSIS OF A THERMAL DESALINATION PLANT IN SAUDI ARABIA"

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TABLE OF CONTENTS

Acknowledgement 3 List of Tables 4 List of Figures 5 Summary (Arabic) 6 Summary 7 Introduction 8

Literature Survey 10

Research Methodology 10

The Studied processes 10

Single stage flashing unit 11

Multi-Flash Distillation 12

Multi-Effect Distillation 12

Multi-Effect Distillation with vapor pre-heating 13

Multi-Effect Distillation with vapor/liquid pre-heating 15

Results and Discussion 16

References 25

Appendices 26

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ACKNOWLEDGEMENT

The principal investigator and his team wish to acknowledge the financial assistance

from the Deanship of Scientific Research and the Research Center of the College of

Engineering for all its contribution to make this project a success.

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LIST OF TABLES

List of Tables

TABLE 1. Data used on the simulation of the flash units

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LIST OF FIGURES

Figure 1 HYSYS process flow diagram for a single evaporator

Figure 2 HYSYS process flow diagram for MSF distillation

Figure 3 HYSYS process flow diagram for Multi-Effect-Distillation

Figure 4 HYSYS process flow diagram for Multi-Effect-Distillation

with vapor pre-heating.

Figure 5 HYSYS process flow diagram for Multi-Effect-Distillation

with liquid/vapor pre-heating.

Figure 6 Composite Diagram of the Single Stage Flash distillation

Figure 7a Pinch analysis of the SSF showing energy savings

Figure 7b Pinch analysis of the SSF showing energy savings

Figure 8 Grid diagram of the existing network

Figure 9 Composite Diagram of the Multi-Stage Flash distillation

Figure 10a Pinch analysis of the MSF showing energy savings

Figure 10b Pinch analysis of the MSF showing energy savings

Figure 11 Grid diagram of the existing network for MSF

Figure 12 Composite Diagram of the Multi-Effect Distillation without pre-heating

Figure 13 Composite Diagram of the Multi-Effect distillation with liquid pre-heating

Figure 14 Composite Diagram of the Multi-Effect distillation

with vapor/liquid preheating.

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SUMMARY For an arid country like Saudi Arabia, the quest for more fresh water never ends. The

country has adopted the multi stage flash distillation (MSF) process as the prominent

process for the production of fresh water. Nevertheless the process is complex, costly

and energy intensive. Consequently, energy analysis has evolved to be a useful tool

for the optimization of the thermal process. Benefits are not limited to the reduction in

energy consumption. Several energy optimization projects resulted in a decrease in

unit product cost, an elongation of the service life of the plant through the

conservation of its material of construction and an increase in profit margin generated

by the process.

The simulation of five plant configuration types (namely, Single Stage Flash

distillation, Multi-Stage Flash distillation and three types of Multi-Effect Distillation)

was run using HYSYS. The various runs established the energy utilization of the

various processes.

The results of the various HYSYS runs were then fed to Super Target software

program. The program was developed by Linnhoff-March [9], which contains a suite

of pinch-technology programs that provide fundamental insights to heat recovery

options in a process, which then results in lower operating costs or reduced capital

cost by auditing energy consumption or emission. The program was used to find and

calculate the maximum possible heat recovery and to compare the existing design

with targets.

It was clear from the analysis that there was a lot of wastage in the SSF plants and that

increasing the number of stages improved the energy utilization. In the SSF, the

energy that could be saved was as high as 82.4 % while it was only 63.5 % when an

MSF plant is used. When a conventional MED plant was used, the savings of 80.5 %

that was achieved was comparable to that of SSF. However, when the feed was pre-

heated by the vapor and liquid/vapor of the columns savings of –10.6 % and –11.5 %

were achieved respectively. For the latter, the minimum delta Ts indicated the need

for heat exchangers with small areas.

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1.0 INTRODUCTION

For an arid country like Saudi Arabia, the quest for more fresh water never ends. The

country has adopted the multi stage flash distillation (MSF) process as the prominent

process for the production of fresh water. The implementation of the MSF process has

placed the country among the top 16 % of fresh water producers. At present, MSF

accounts for about 66 % of the world’s production of desalinated water [5].

The high demand for fresh water as well as the abundance of thermal energy in the

form Natural Gas and Petroleum derivatives has made the process rather attractive for

Saudi Arabia and the neighboring countries. Nevertheless the process is complex,

costly and energy intensive. Consequently, energy analysis has evolved to be a useful

tool for the optimization of the thermal process. Benefits are not limited to the

reduction in energy consumption. Several energy optimization projects resulted in a

decrease in unit product cost, an elongation of the service life of the plant through the

conservation of its material of construction and an increase in profit margin generated

by the process.

Seawater is an aqueous solution of salts, basically sodium chloride. The salt

concentration varies depending on the location of the sea being used. However the salt

concentration in the Arabian Gulf region has been documented to range between

0.036 wt percent to 0.042 wt % salts. This concentration is typical to most sources of

desalination plants in the area.

The removal of such salts from such aqueous solutions is done in two ways. The first

is by thermal process whereby pure water is evaporated and the salt or a more

concentrated salt solution is left behind. The other method is by membrane separation,

where pure water passes through the pervious membrane without allowing the salt to

go through. Though this method is the new trend and is undergoing a lot of research

work, the former is more pervasive in the Middle East at large and in the Kingdom in

particular.

Distillation is one of the oldest known methods of separating fresh water from

a solution of salt water. When the latter is boiled, freshwater evaporates leaving the

salt behind. The water vapor is cooled down, condensing the steam back to fresh

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water. It is thermodynamically known that raising water to its boiling point is not

enough to vaporize it, rather more heat is required (heat of vaporization) to change the

liquid to vapor or steam. Thus the heat of vaporization is of major importance in the

desalination industry.

The solution of salt water has its own saturation temperature. This temperature drops

at low pressures as the altitude of the location increases or if vacuum is created in the

evaporator. Saturated liquid water contains maximum amount of energy and will

vaporize if additional heat is provided or pressure is reduced.

In MSF plants, concentrated brine is heated to just below the brine recycle

pump delivery pressure and at the top brine temperature. The brine then flows into the

evaporator or flash chamber, where it finds a pressure lower than its saturation

pressure. The brine instantaneously evaporates until water vapor pressure decreases to

the first stage pressure and the corresponding temperature also decreases to the first

stage vapor temperature. The single stage plant can be extended to any number of

stages. The pressure of each stage is successively reduced, until the interaction

between vapor volume, equilibrium and heat rejection help to fix the minimum

temperature of the last stage. For most purposes, the increasing specific volume of the

flashed vapor limits the bottom brine temperature. On the other hand proscription of

scale formation dictates the value of the top brine temperature. It is in this region that

energy is expended. Studies based on energy and exergy considerations have been

conducted [2]. These were done to obtain optimum design parameters for the plant. It

would be useful to implement a degree of heat exchanger network that will help

reduce losses.

In the light of current world energy crunch, the use of pinch analysis (Linnhoff) could

expose areas of savings. It is interesting to note that, for the first time, the research

work addresses the use of PINCH analysis in desalination.

In this research work, it is intended to evaluate the thermal performance and carry out

energy savings analysis of conventional commercial MSF plants under standard

conditions using PINCH analysis.

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2. THE STUDIED PROCESSES

The processes studied are given in Figs. 1-5. The plants are either single flash unit or

a series of flash units. In these units seawater is evaporated to obtain distilled water.

To reduce corrosion problems, process feed must be properly treated and heater

temperatures must not exceed 120 ° C. [4]. Where recycle is used, the recycle flow is

limited by the salt maximum admissible concentration, which depends on material

and the equipment useful life. In all flow sheets, the data used is as shown in Table 1

and is typical of the conditions in the desalination plants in the Kingdom.

TABLE 1. Data used on the simulation of the flash units

Top Brine Temperature 90 ° C.

Temperature of reject brine 60 ° C.

Motive steam temperature 100 ° C.

Intake seawater temperature 30 ° C.

Thermodynamic loss

Condenser terminal temp. Diff.

Salinity of intake seawater 42000ppm

Salinity of reject seawater 70000ppm

For the single stage flash unit the actual plant consists of a pre-heater, an evaporator

equipped with a demister and a condenser. The simulation substitutes the evaporator

with a flash column. The flash unit is effective in representing the evaporator and the

demister in that the demister is used to remove excessive vapor and reduce pressure

build up, while in a flash column the separation of vapor/liquid is total and the

pressure drop in the condenser removes any pressure build up.

Appendix A2 shows the actual flow sheet for an MSF plant. The MSF plant is a

combination of several SSF units. Appendix A1 shows an actual multi-effect

distillation unit and is similar to the MSF except that in Appendix A1, the flows are

subdivided into n-equal streams.

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In all the simulation runs the flow sheets were simplified to depict the main unit

operation activities pertaining to the processes, and in our view, enables the process

engineer to make quick and fast judgment on the thermal performance of the plant

under study.

2.1. SINGLE-STAGE FLASHING UNIT (SSF)

In the first part of the work, a single-stage flash distillation column was used. The

flow sheet is shown in Fig. 1. In this figure, a feed, at the conditions specified in

Table 1, enters a pre-heater. The pre-heater (which is presumed to have steam with a

flow rate and temperature) is used to raise the temperature of the inlet cold flow to

that of the conventional top brine temperature (TBT). In the heat exchanger unit of a

plant, the steam used releases its latent heat. The energy is used to heat the entering

seawater from ambient temperature to the stated temperature. The hot brine then

enters the flash-chamber, which is operated at a pressure lower than the saturation

pressure at the top brine temperature conditions. The concentration of the rejected

brine was kept at 7.4 wt %. The distillate was cooled back to 60 ° C using

conventional heat exchanger. The latent heat of condensation of the produced vapor is

first removed before removing the sensible heat of the resulting liquid.

Fig. 1. HYSYS process flow diagram for a single evaporator

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2.2 MULTI FLASH DISTILLATION (MSF)

Multi-stage flash distillation is the most widely used in industry. A schematic diagram

for the system is shown in Fig. 2. In the original flow sheet of Dessouky et al [5], the

system consisted of 14 flash units. The simulation here is done with ten flash units

because the last unit had exceeded the recommended brine concentration. The system

consists of 10 flash units and the brine heaters. Three sections are clear in this flow

sheet namely, the brine heater, the flash section and the heat rejection section. The

brine heater drives the flashing process by heating the feed to the flash conditions

required to achieve separation. Flashing then occurs in the columns where vapor

formation occurs in equilibrium with brine liquid. The flashed off vapors are all

accumulated and sent to condenser for cooling at the designated temperature of 40 ° C.

Fig. 2. HYSYS process flow diagram for MSF distillation

2.3 MULTI EFFECT DISTILLATION (MED)

Fig. 3 below shows the use of multi-effect distillation to achieve the same degree of

separation. The flow sheet was developed by Garcia et al [3] but was used in solar

energy applications. The feed is initially divided into 10 equal flows to suite the

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number of flash columns. There is also the same number of pre heaters. However in

the flow sheet the bottom brine liquid of the previous column is used to pre-heat the

second feed to a pre-designed temperature before heating again to attain the top brine

temperature value. This way the load on the pre-heaters is reduced. This arrangement

is repeated for all columns as shown in the flow sheet. The vapor flows from all the

flash units are collected and sent to a condenser for cooling to the required

temperature.

2.4 MULTI EFFECT DISTILLATION (MED) WITH VAPOR PRE-HEATING

Fig. 4 is a flow sheet showing a modification of that in Fig. 3. In this, the vapor

product rather than the liquid product is utilized as the pre-heater. Initially, the feed

was similarly divided into 10 equal flows. The first feed is pre-heated to the top brine

temperature before being flashed. At that condition, the liquid mixture is separated

into vapor and liquid. The vapor goes through the top of the column and the liquid

through the bottom. The top vapor is used to preheat the next feed to as near the TBT

as possible. It is then properly conditioned in a separate pre-heater. This seriously

reduces the load on the pre-heater. The conditioned feed is then flashed. This is

repeated for other columns as shown in the flow sheet. The resultant vapor lines are

all mixed and sent to a condenser where the temperature is cooled to 40 C.

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Fig. 3. HYSYS process flow diagram for Multi-Effect-Distillation

Fig. 4. HYSYS process flow diagram for Multi-Effect-Distillation with vapor pre-

heating

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2.5 MULTI EFFECT DISTILLATION (MED) WITH VAPOR/LIQUID

PRE- HEATING

Fig. 5, shows another combination of the MED scheme. Here, the same strategy of 10

equal flows is maintained. However, the feed is first preheated by the hot liquid

product to 45 °C. Beyond this temperature, there is a temperature cross and therefore

a limit is established beyond which the hot liquid cannot preheat the cold one. The hot

vapor stream then does the rest of the preheating. This stream heats the cold liquid

from 45 °C to the TBT. The last heat exchanger is then used to properly condition the

feed before flashing. Flashing occurs in the column which then separates the stream

into its liquid and vapor components. The process is repeated for all units. The vapor

lines are then collected and cooled to the nominal temperature of 45 °C.

Fig. 5. HYSYS process flow diagram for Multi-Effect-Distillation with liquid/vapor

pre-heating

2.6 SUPER TARGET

SuperTarget is the world's leading energy pinch tool. It is a modular product with

options for process, site or column analysis. SuperTarget [9] has led from the front for

almost 10 years and whilst it has imitators, it has no serious rival. Still the preferred

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tool of energy pinch experts world-wide SuperTarget is also ideal for less experienced

users and makes pinch analysis a routine part of process design.

3.0 RESULTS AND DISCUSSION

The results of the simulation runs using HYSYS are shown in Appendix A-1. For Fig.

1, the total energy requirements are given by Q_101 and Q_102. Q_101 is the heating

requirement and is given by 45 kJ. This is equivalent to 1000 tons of low-pressure

steam required for this heating. On the other hand Q_102 requires –60 kJ of energy to

cool the vapor stream. Using cooling water at 20°C, this gives an equivalent of 2 tons

of cold water. This establishes the energy utilization of the process.

The results were then fed to Super Target software program. The program was

developed by Linnhoff-March [9], which contains a suite of pinch-technology

programs that provide fundamental insights to heat recovery options in a process. This

results in lower operating costs or reduced capital cost by auditing energy

consumption or emission. In using this program, it is expected to find and calculate

the maximum possible heat recovery and to compare the existing design with targets.

Fig. 6, shows the composite diagram for the SSF distillation unit. The red curve

shows the heating curves while the blue shows the cooling composite curve. In Fig. 6,

the temperature, T, is plotted against enthalpy or heat transferred with the slope being

the inverse of heat capacity. For hot streams, the curves are cooling composites that

begin at highest temperature and end at the lowest temperature after energy has been

removed. The Q required for the cooling composite is given as 23728.3 kW. For the

cold stream, the curve is a heating composite beginning from 30°C and ends at 99.6

°C after 23243.6 kW of energy has been added. Since the slopes of the two curves are

constant, the composite curves have no segments but it is clear that utility steam is

required at portions where heat flow from cooling composite is not covered for on the

heating composite.

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Fig. 6. Composite Diagram of the Single Stage Flash distillation

A cross pinch analysis from the program shows an energy savings of 82 %. From the

figure, it is clear that the lower end of the cooling composite is not fully covered and

will require the use of utility.

Figs. 7a and 7b show both the energy due to the total energy savings that can be

attained. Savings of 195227.2 kW can be made on the hot utility if the generated

vapor is used as a pre-heater to the incoming feed.

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Fig. 7a. Pinch analysis of the SSF showing energy savings

Fig. 7b. Pinch analysis of the SSF showing energy savings

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Fig. 8, shows the grid diagram of the existing network, including the split mix

structure as well as the cross pinch heat exchangers. The hot stream is red and is

depicted to flow from left to right. The temperature starts from 99.6 °C and is cooled

to 26.4 °C. The heat extracted in this process is 23243.9 kW.

On the other hand, the cold stream is blue and is depicted to flow from right to left. It

was seen to begin at 30°C up to the Top Brine Temperature.

Fig. 8. Grid diagram of the existing network

The composite diagram for MSF unit with 10 flash columns is shown in Fig. 9. Here

again, the heating composite as well as the cooling composite curve both have

constant slopes with the closest point of approach giving a min 12.7T C°∆ = . As usual

the upper and lower points that are not covered by heating or cooling curves require

the use of utilities. This shows in the upper and lower portions of the heating

composite.

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Figs. 10a and 10b, show potential savings of 63.5 %. Of this savings 51% comes from

the use of hot utility. Thus it is important to seriously consider the use of hot utility.

Fig. 11, shows the grid diagram for the MSF distillation unit. The cooling composite

shows the use of two condensers to achieve this. The heating composite shows the use

of several pre-heaters.

It is interesting to note that the use of several flash units has rather reduced the

amount energy savings to be made. This means that more energy savings are made

with the use of more flash columns at the expense of capital cost. The composite

diagrams of Figs. 12, 13 and 14 add to the fact that not only are energy saved from the

addition of more flash units, but energy can and is also conserved from the use of the

vapor from each unit as a pre-heating medium.

When the feed was divided into seven equal streams and fed into different flash units,

the energy savings that could be attained was found to be 80 % as shown in Fig. 12.

Then when the feed was preheated with either the liquid product from the flash unit or

the vapor product, the savings targets were fully achieved.

Fig. 9. Composite Diagram of the Multi-Stage Flash distillation

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Fig. 10a. Pinch analysis of the MSF showing energy savings

Fig. 10b. Pinch analysis of the MSF showing energy savings

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Fig. 11. Grid diagram of the existing network for MSF

Fig. 12. Composite Diagram of the Multi-Effect Distillation without pre-heating

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Fig. 13. Composite Diagram of the Multi-Effect distillation with liquid pre-heating

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Fig. 14. Composite Diagram of the Multi-Effect distillation with vapor/liquid

preheating

It is interesting to note that most savings were achieved through the utilization of the

latent heat of the vapor. As the savings target is achieved, the DT is also improved.

Recall that the smaller the DT, the larger the exchanger area required achieving the

cooling or heating. The DT for MSF was 10.5, that for MED was 12.2 but with liquid

cooling and vapor/liquid DTs of 69 and 69.56 were observed respectively. This

indicates the need for smaller heat exchanger areas.

4.0 CONCLUSION In this project, comparative analysis was carried out on multi-effect desalination

plants. Five plant types were used. The first was the theoretical SSF, followed by a

conventional MSF plant used in the Kingdom and then three types of MED plants

used in the field. It was clear that there was a lot of wastage in the SSF plants and that

increasing the number of stages improved the energy utilization. In the SSF, the

energy that could be saved was as high as 82.4 % while it was only 63.5 % when an

MSF plant is used. When a conventional MED plant was used, the savings of 80.5 %

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that was achieved was comparable to that of SSF. However, when the feed was pre-

heated by the vapor and liquid/vapor of the columns savings of –10.6 % and –11.5 %

were achieved respectively. For the latter, the minimum delta Ts indicated the need

for heat exchangers with small areas.

5.0 REFERENCES

[1] O. A., Hamed, M. A. K., Al-Sofi, M., Imam, G. M., Muatafa, K., Bamardouf,

H., Al-Washmi., ‘Simulation of multistage flash desalination process’,

Desalination, 134, pp195-203, 2001.

[2] O. A., Hamed, M. A. K., Al-Sofi, M., Imam, G. M., Muatafa, K., Bamardouf,

H., Al-Washmi.,., ‘Thermal performance of multi-stage flash distillation plants in

Saudi Arabia’, Desalination, 128, pp281-292, 2000.

[3] L.,Garcia-Rodriguez, C., Gomez-Camacho, ‘Thermo-economic analysis of a

solar multi-effect distillation plant installed at the Plataforma Solar Almeria

(Spain)’, Desalination, 122, pp205-214, 1999.

[4] E. E., Tarifa, S. F., Dominguez, D., Humana, D. L., Martinez, A. F., Nunez,

N. J. Scenna, ‘Fault analysis for MSF plants’, Desalination, 182, pp131-142,

2005.

[5] H., El-Desouky, I., Al-Atiqi and H. Ettouney, ‘Process synthesis: the multi-

stage flash desalination system’, Desalination, 115, pp155-179, 1998.

[6] A., Jernqvist, M., Jernqvist and G., Aly, ‘Simulation of thermal desalination

processes’, Desalination, 126, pp147-152, 1999.

[7] H. T., El-Dessouky and H. M. Ettouney, ‘Multi-effect evaporation desalination

systems: thermal analysis’, Desalination, 125, pp 259-276, 1999.

[8] A., Jernqvist, M., Jernqvist and G., Aly, ‘Simulation of thermal desalination

processes’, Desalination, 124, pp187-193, 2001.

[9] Linnhoff-March, Super Target software program, V. 6.

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6.0 APPENDICES

APPENDIX A1: MULTI-EFFECT DISTILLATION (MED)

Source: Garcia-Rodriguez et al[3]

Fig. A-1. Typical flowsheet of Multi-Effect Distillation (MED)

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APPENDIX A2: MULTI-STAGE FLASH DISTILLATION (MSF)

Source: El-Dessouky et al[5]

Fig. A-2. Typical flowsheet of Multi-Stage Flash Distillation (MSF)

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APPENDIX A3: SINGLE STAGE FLASH DISTILLATION (SSF)

Source: El-Dessouky et al

Fig. A-3. Typical flowsheet of Single-Stage Flash Distillation (SSF)