Introduction to Desalination - Louis Theodore

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levels is increasing at a faster rate than originally realized.

      2 Water demand at both the national and international levels is increasing at a faster rate than population growth.

      3 There is indeed growing water scarcity.

      4 One approach to solving this water scarcity problem is to develop and/or find new sources of water.

      5 Nonconventional water sources will become more attractive in the future.

      6 Water conservation measures should be expanded in the future.

      7 Seawater and brackish groundwater are nonconventional water sources serving as feedstock for desalination processes.

      8 Poor seawater quality can limit RO applications but generally does not have as great an effect on evaporation and crystallization processes.

      9 Desalination processes featured evaporation in earlier times, while RO is generally the process of choice today.

      10 The increasing demand for potable water will ensure that improvements in current technologies and development of new approaches for potable water production will continue to develop in the future.

      1.7 Illustrative Examples

      Five illustrative examples complement the material presented in this chapter.

      1.7.1 Illustrative Example 1

      Solution. Process industries rank above all others as users of water. No process plant could operate without water as a cooling medium and as a direct raw material in certain phases of a process. The local water supply, therefore, must be studied before an area can even be considered as a possible site for a processing facility. A detailed estimate of present and future water requirements must precede a plant siting study. Then the availability of water in the region being considered should be carefully investigated. If well water is to be used, a complete study of the history of the groundwater table is necessary. If groundwater supplies are adequate, they are preferred because of their lower temperature and generally higher water quality as compared to surface water sources.

      Surface waters from streams or lakes also require careful consideration since both their quantity and quality can vary greatly on a seasonal basis. Streams discharging into the ocean during times of low runoff can become saline as a result of seawater intrusion. Under such conditions, plant design may have to include large storage facilities for fresh water to be collected during periods of high runoff and used during dry seasons when the stream becomes saline.

      Companies moving into relatively nonindustrialized areas often fail to consider the possibility of other plants following suit. The availability of the water supply should be adequate not only for the future needs of the proposed plant but also for the anticipated needs of other industries that might move into the area. It is also desirable to consider alternate sources of supply that may be required if the preferred water source becomes depleted. Water quality must be studied as well as water quantity. Chemical and bacteriological examination will indicate the extent of treatment required and can aid in the development of water cost figures for comparison with other locations. The possible contamination of the water source by other industries in the area should be anticipated. Note that “contamination” may consist of raising the temperature of the water to a level that renders its use as a cooling medium impossible.

      1.7.2 Illustrative Example 2

      A range of water needs typically exist within a facility, all of which may have different water quality requirements for specific uses. Describe five of these common needs.

      Solution. Five common industrial facility water uses include:

      1 Cooling Water. Natural and forced-draft cooling towers are generally used to provide the cooling water required at a site unless water can be drawn from a convenient river or lake in sufficient quantity to meet a facility’s cooling demands. Seawater or brackish water can be used at coastal sites but, if used directly, necessitates more expensive materials of construction for heat exchangers because of potential corrosion problems resulting from the high dissolved solids content of this water. Often, this cooling water does not have to be of high purity and can be taken from blowdown streams from boilers or process lines that require much more stringent water quality conditions.

      2 Potable and General Use Water. The water required for general purposes on a site is usually taken from a local municipal or private supplier, unless a cheaper source of suitable quality water (e.g. a river, lake, or well) is available. If the cost of this supply is low, incentives for water use reduction will also be low. It should be remembered, however, that the cost of the supply may be a small portion of the overall cost of managing this water, particularly if it is used in process rinsing, general facility cleaning and wash-down, etc. More water use requires more energy to convey it from the source and within a plant. When the water becomes contaminated, costs for its handling increase exponentially. In addition, waste treatment becomes progressively less efficient and more costly as a waste stream is diluted, and load penalties may be incurred based on the volume of waste discharged to publicly owned treatment works (POTWs). It should be evident, then, that numerous benefits can be associated with increased water use efficiency within a plant.

      3 Demineralized Water. Water from which all the minerals have been removed by ion exchange must be used where ultrapure water is needed to meet process demands and strict boiler feedwater requirements. Mixed and multiple-bed ion exchange units are used for this purpose, with resins exchanging multivalent cations for hydrogen. Boiler water as condensed steam and process water must be removed on a routine basis (blow down) to prevent the build-up of unwanted constituents within these systems, and this ultrapure water may be effectively reused within a plant for other unit operations that demand much less stringent water quality characteristics, i.e., cooling water, process rinse water, and so on.

      4 Refrigeration. Refrigeration is needed for processes that require temperatures below those that can be economically obtained with cooling water. Chilled water can be used to lower process temperatures down to approximately 10°C. For lower temperatures, to -30°C, salt brines (NaCl and CaCl2) are used to distribute the “refrigeration” around the site from a central refrigeration unit. Vapor compression machines are normally used for this purpose. As with boilers, evaluation of the operating conditions of a chiller can lead to significant pollution prevention opportunities. Consider, for example, that a 1% improvement in chiller efficiency can be expected for each 1°F increase in the chiller set point.

      5 Steam. The steam for process heating is generated in either fire - or water-tube boilers, using the most economical fuel available. The process temperatures required can usually be obtained with low-pressure steam (typically 25 psig), with higher steam pressures needed only for high process temperature requirements. A significant pollution prevention, energy conservation, and cost reduction opportunity may exist in a facility’s steam generation system, as many facilities operate at higher than necessary steam pressures, increasing their energy demands for steam production and the cost of producing this steam. In addition, repair and replacement of leaking steam and condenser lines will prevent the wasting of steam and the associated energy and cost needed to produce this wasted steam.

      Briefly describe the wastewater streams typically generated in the petroleum industry.

      Solution. Throughout the life cycle of crude oil, through extraction, transportation, and refining, roughly 5 to 7 gal of water are used for every 1 gal of crude oil processed. Of this total, roughly l to 2.5 gal of water are used for every 1 gal of crude oil stock. In the United States, 1 to 2 billion gal of water are used per day in the refining industry. Water consumption varies for different regions and different qualities of crude oil. These water estimates do not include unconventional shale formations, where water usage is generally much higher. Water consumption in refining is a result of evaporation and drilling, blowdown, process discharge,

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