Water Supply from Seawater Desalination: Considerations for State-of-the-art Technologies

S. V. CABIBBO and R. M. SZOSTAK Kuljian Corporation, 3700 Science Center, Philadelphia, PA 19104, USA

1. INTRODUCTION

This note reviews the current situation in the desalination industry wherein two primary tech- nologies are competing for the most cost effective solution to the production of potable water from seawater for capacities greater than one million gallons per day. The competing technologies can be classified as thermal and membrane processes represented by multistage flash distillation and reverse osmosis, respectively.

On a stand-alone basis, the comparison between both technologies, membrane versus thermal, is straightforward and tends to favor the membrane process where fuel costs are expensive. However, when electricity production is also desirable and a dual purpose plant is considered, then the comparison favors the thermal process for any fuel cost.

Municipal and industrial water recovery from seawater desalination has assumed considerable importance in many parts of the world. The application of this technology is sensitive to many social and economic constraints. However, as the need for potable and high quality industrial water has increased in recent years, similarly, the application of desalting technology has increased. The growth in land based desalination units greater than 25 000 gallons per day capacity from 1950 to 1977 is shown in Fig. 1. A corresponding distribution of capacity according to international region is shown in Fig. 2. The dominant technology through 1977 has been the distillation process represented by the multi-stage flash (MSF) technology.

In recent years, the membrane process rep- resented by the reverse osmosis (RO) technology has been commercially developed and applied. A unique quality of the RO process is the ability to recover water with minimum energy consumption. As a result of the RO process energy efficiency, this technology has the potential of becoming a dominant technology in the future.

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Fig. 1. New seawater desalination facilities from 1950-1977. Based on J 977 Office of Water Research and Technology inventory of land-based desalination plants (El-Ramly and Congdon, 1977).

Natural Resources Forum @ United Nations, New York, 1984

179

180 S. V. CABIBBO & R. M. SZOSTAK NRF VOL. 8 NO. 2 , 1984

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Fig. 2. Worldwide distribution of seawater desalination capacity, 1977.

In general terms the MSF technology consists of multi-stage distillation process wherein seawater brine is heated to an elevated temperature by steam. Subsequent flashdown of the brine in each stage yields a quantity of water vapor free of salt which is recovered by condensing. The cumulative water collected for each stage is combined to yield the total product flow from the evaporator. The flashdown available for production is determined by the upper brine temperature, limited by scale formation, and the lower brine temperature, limited by the cooling seawater temperature.

The RO technology consists of semi-permeable membranes and high pressure feed pumps. The osmotic tendency of water to flow through the membrane to the salt laden seawater is reversed by the pressure gradient created by the feed pumps. The reverse flow through the membrane produces desalted water while concentrating the soluble salts in the brine stream. Production from the RO process is limited by the scale formation of the soluble salts and the specific membrane productiv- ity based on feed water pressure, salinity, temperature and salt rejection.

Several factors need to be considered in evaluating both the RO (membrane) and MSF

(thermal) processes at their current state of development:

(a) form of energy required; electric power or low pressure steam;

(b) equipment replacement during the plant life;

(c) equipment cost; (d) product water quality; (e) potential for cogeneration (dual purpose

( f ) land requirement; (g) seawater requirement.

plant) or utilization of waste heat;

2. THERMAL DESALINATION TECHNOLOGY

The MSF process has provided the bulk of worldwide installed capacity to date. There are a large number of alternative thermal processes which have the potential for future development; however, none have been demonstrated to the extent of MSF technology. Characteristic of the alternative thermal processes is multiple effect (ME) distillation and is discussed later in this section.

Table I shows the performance parameters of MSF. All distillation processes yield a high purity

TABLE I Performance Characteristics of a Multi-stage Flash

Desalination Facility (MSF)

Product water salinity Performance ratio Electrical energy 700 kW/MGD

25 ppm TDS 8 Ib product/1000 BTU

(7.4 X lo6 BTU/HR/MGD)

44.2 X lo6___ BTU/HR

(BTU basis at 10 500 BTU/KWHR)

Thermal energy

Capital cost $5.90/GPD Seawater required 10 MGD/MGD

MGD

water of less than 25 parts per million (PPM) total dissolved solids (TDS). For the MSF process, a thermal efficiency performance ratio of 8 pounds of water produced per 1000 BTU of heat input is typical. Performance ratios may vary from 6 to 12 for a particular application. The electrical energy required is 700 kW per million gallons per day (MGD) of product water. Converting this to a thermal (BTU) basis and using a typical power

NRF VOL. 8 NO. 2, 1984 SHORT NOTES 181

plant heat rate of 10 500 BTU/kWh results in a BTU equivalent of 7.4 x lo6 BTU/HR/MGD. The thermal efficiency derived from the perfor- mance ratio is 44.2 x lo6 BTU/HR/MGD. A typical installed cost for an MSF plant is US$5.90/GPD for large capacity units. The installed cost includes the desalination plant as well as the seawater intake and discharge. Installed cost for thermal processes is sensitive to plant size and increases rapidly for small capacity units. The seawater required, including the cooling demand, is 10 MGD per 1 MGD of production. Feedwater to the MSF process is obtained from the cooling water discharge. Only 3 MGD is actually fed to the MSF distillation equipment as makeup, producing 1 MGD of product and 2 MGD of brine for discharge normally with the remaining seawater coolant.

The outstanding economic advantage of cogen- eration is shown in Table 11. Although the data shown are representative of a large dual purpose plant, the comparison has been reduced to a 1 MGD unit size for ease of comparison. The electric

exhaust steam can be utilized by a thermal process rather than rejected to the condenser, the overall cycle efficiency is significantly improved. The proper proportioning of energy and capital cost between power and water is always subject to interpretation in a dual purpose plant; however, the benefit derived is clear no matter the method of allocation used.

Table I11 illustrates the performance charac- teristics of the multiple effect distillation process. The product water TDS is 25 PPM; similar to the MSF technology. However, the typical thermal

TABLE I11 Performance Characteristics of a Multiple Effect

Desalination Facility

Product water salinity Performance ratio 12 Electrical energy 300 kW/MGD Thermal energy 29.5 x lo6

25 ppm TDS

BTU/h

Capital cost $4.60/GPD Seawater required 6 MGD/MGD

TABLE I1 Comparison of Dual Purpose and Single Purpose

MSF (1 MGD unit size)

Dual Single

Electrical demand (kW) - 700 - 700

Net electric output (kW) t4,500 - 700 Thermal equivalent (BTU/h)a +47.3 X lo6 -7.4 x lo6 Thermal energy (BTU/h) -44.2 X lo8 -44.2 X lo6 Net thermal equivalent(BTU/h) t3.1 x 10' -51.6 x lo6

Electric generation (kW) +5,200 0

aBased on 10 500 BTUlkW h.

power generation is based on a typical dual purpose plant operating at a turbine throttle condition of 1500 PSIG/95O0F. The positive net thermal equivalent required by the dual purpose plant is a result of a bookkeeping anomaly caused by the effective heat rate in cogeneration plants being reduced to approximately 5000 BTU/kWh. Total thermal equivalent requirement for a single purpose MSF plant is 51.6 x lo6 BTU/HR for a 1 MGD unit size.

When the energy contained in the turbine

performance ratio of 12 pounds of product/lOOO BTU of heat input is significantly better than MSF. The range of thermal efficiency is a performance ratio of 10 to 15 for larger units. Electrical energy required is also reduced to 300 KW/MGD of product. The thermal energy requirement derived from a performance ratio of 12 is 29.5 x lo6 BTU/MGD. The capital cost at $4.60/installed GPD is significantly less than the MSF plant cost. However, the cost for the ME plant is based on estimates since large ME plants have not been built comparable to the large MSF plants operating throughout the world. The seawater requirement for the ME plant is 6 MGD seawater/l MGD product due primarily to the higher performance ratio.

3. MEMBRANE DESALINATION TECHNOLOGY

Seawater reverse osmosis (RO) membrane technology is a relatively new development and dominates all other membrane processes. Elec- trodialysis and ion exchange techniques, which are competitive with RO for brackish water applica- tion, are not currently economic for seawater

7 82 S. V. CABIBBO & R. M. SZOSTAK NRF VOL. 8 NO. 2, 1984

desalting. The RO process utilizes hydraulic pressure to force product water through a semi-permeable membrane which rejects the soluble salts, leaving them in the brine stream to be discharged. Since the brine stream is still under considerable pressure after leaving the RO module, various energy recovery techniques may be used.

The product water salinity is shown in Table IV to be 300ppm TDS, which is greater than the distillation processes but within World Health Organization standards (500 ppm TDS max- imum). A typical product water recovery rate of

TABLE IV Performance Characteristics of a Reverse Osmosis

Desalination Facility

Product water salinity Product water recovery

Electrical energya (BTU basis at 10 500BTU/kWh)

Thermal energy Capital cost Typical membrane life Seawater required

(overall)

300 ppm TDS

27% 11 00 kW/MGD (11.6 x 10sBTU/h)

0 $5.50/GPD 2-5 years 4 MGD/MGD

MGD

aWith cnergy recovery.

27% is comparable with the thermal processes. The electric power requirement of 1100 KW/MGD of production includes a credit for energy recovery. On an equivalent thermal energy basis, 11.6 x 1 O6 BTU/HR/MGD are required by the RO process. However, there is no requirement for direct thermal energy. A typical cost of RO technology is $5.50/installed GPD including

seawater intake. A recurrent capital expenditure for membrane replacement is not shown and can be a significant factor dependent upon the actual membrane life. A typical RO membrane life of 2 to 5 years is expected in seawater application. Several factors which influence membrane life are: membrane composition, membrane configuration, seawater salinity, operating temperature, pre- treatment and product water recovery. Since the RO process requires no seawater cooling, the seawater required is only for desalination or 4 MGD seawater/l MGD product water.

4. TECHNOLOGY COMPARISON

A comparison of thermal (MSF) and membrane (RO) desalination technologies can be made on the basis of process requirements. The primary considerations are product quality and energy requirements. Table V shows a direct comparison between thermal processes represented by Dual Purpose MSF and Single Purpose MSF and membrane processes represented by RO with Energy Recovery and without. Both technologies yield potable quality water; however, the low TDS product characteristic of distillation processes carries the advantage of greater total water production through blending with brackish water, if available. In addition, the water quality characteristic of distillation processes can be applied directly for industrial uses.

The net energy required on a thermal energy equivalent basis favours RO with or without energy recovery compared with a Single Purpose MSF. However, the Dual Purpose MSF is favored over RO with or without energy recovery for application wherein the cogeneration of both

TABLE v MSF vs RO (1 MGD unit size)

MSF MSF RO RO dual purpose single purpose energy recovery

Product water salinity (ppm) Seawater required (MGD) Electricity consumed (BTU/h basis) Electrical energy generated

(BTU/h basis) Thermal energy consumed (BTU/h) Net energy (BTU/h)

25 25 300 300 I 0 10 4 4

-7.4 x 10' -7.4 x 10' -16.5 x 10' -16.5 X 10'

c54.6 x lo6 0 +4.9 x 10' 0 -44.2 X 10' - 4 4 . 2 ~ 10' 0 0 +3.1 x 10' -51.6 x 10' -11.6 X 10' -16.5 X 10'

NRF VOL. 8 NO. 2, 1984 SHORT NOTES 183

water and power is desirable. As noted earlier, the positive net energy for the Dual Purpose MSF is an anomaly which results from a cogeneration power heat rate of approximately 5000 BTU/kWh while the comparison table has consistantly applied a heat rate of 10 500 BTU/kWh which is charac- teristic of a stand-alone power generating station.

From this comparison, it is determined that when water and export electricity are desirable, the Dual Purpose MSF technology is particularly advantageous. The RO technology with or without energy recovery has a large potential energy saving compared to the Single Purpose MSF.

The availability of waste heat, low fuel cost, or application of the ME distillation technology have certain advantages for thermal processes. In such cases, a more detailed analysis is appropriate.

High fuel costs should accelerate the develop- ment of thermal processes with improved performance ratios. However, poor experiences with early high efficiency thermal desalting plants has instilled caution and conservatism in consider- ing other thermal processes. The practical limit for MSF technology is about 12 pounds of pro- duct/1000 BTU of heat input. Multiple Effect (ME) distillation plants have the potential for performance ratios greater than 12. Unlike MSF,

the performance ratio increases directly with the number of effects.

Similarly, the RQ membrane technology has the potential for reduced membrane cost as fabrica- tion techniques continue to develop and improved membranes and more energy efficient systems are brought to the marketplace.

5. CONCLUSION

Developments in the field of desalination technology have brought forth highly energy efficient systems in both membrane and thermal processes. The RO membrane systems require the least amount of energy on an equivalent thermal basis and are appropriate for stand-alone plants when fuel costs are high. The advantages derived from cogeneration ensure the continued applica- tion of thermal desalination whenever water and power production are planned.

The selection of the appropriate technology for a single purpose plant, where fuel costs are low or waste heat is available, requires a detailed analysis of the particular site techno-economic factors and careful coordination with government/industrial planners.