Thermodynamics and Energetics
Anirban RoyAssistant Professor, Department of Chemical
Engineering, BITS Pilani Goa

Shubham LanjewarHigher Degree (M.E.) Student, Department of
Chemical Engineering, BITS Pilani Goa

Ridhish KumarFirst Degree (B.E.) Student, Department of
Chemical Engineering, BITS Pilani Goa

Anupam MukherjeePh.D student, Department of Chemical
Engineering, BITS Pilani Goa

Most of the water available on the earth's surface is saline in nature, as the need for the development of new freshwater supplies increases, global seawater desalination capacity will continue to expand. Desalination, in general, is the removal of salt from seawater and Brackish water which makes desalinated water healthier in comparison to rivers and ground waters. Evaporation of water over the oceans in the water cycle is a natural desalination process. Due to the requirement for clean water to check pollution, the most signifi cant one is water reuse. In this perspective, desalination of water is a good solution at the present situation. Most of the modern interest in desalination is focused on the cost-effective provision of fresh water for human use. Along with recycled wastewater, it is one of the few rain-fall independent water resources. Due to its high energy consumption, desalinated water is generally costlier than available freshwater resources. The potential for developing technologies that can help in minimizing the reverse osmosis power consumption has been recently increased due to the global energy crisis and the global warming. Currently, approximately 1% of the world's population is dependent on desalinated water to meet daily needs, but the UN expects that 14% of the world's population will encounter water scarcity by 2025. Therefore, there is considerable worldwide interest in the assessment of the potential environmental impacts of all aspects of desalination processes including both thermal and reverse osmosis. The least energy- intensive, and as a result often the most economical, seawater desalination process is reverse osmosis (SWRO).

Energy Issues in Desalination
There are some energy intensive processes like Multi-Stage Flash and Multiple Effect Distillation which requires thermal as well as electrical energy to separate fresh water from sea water or Desalination, although these processes are energy intensive but were used. When membrane separation achieved a milestone of reducing the energy demand of desalination these processes are now less used. Reverse osmosis is one of the membrane separation processes, now everywhere RO water filter is installed for the domestic purpose. This process is not much energy intensive because it requires only electrical energy for pumping the feed water, but if we discuss the energy consumption for Desalination for the same process since from the early 1970s energy requirement is gradually decreased from 12 KWh /m3 to 2 KWh/m3 till year 2008 but the question is how it has been achieved, We can say it as development in the technology, the energy has been recovered by using Energy recovery devices such as pressure exchangers ( Pressure exchanger transfers pressure energy from high-pressure fluid stream to low-pressure fluid stream) shown in fig.1.

Figure 1: Schematic of Energy Recycling in SWRO

Thermodynamically, the theoretical minimum energy required to separate one drop of fresh water through desalination is 0.76 KWh/m3 and for 50 % recovery, it is 1.56 KWh/m3. The extra energy is required for pre-treatment and post-treatment of the feed stream and permeates stream respectively. Hence, the reduction of energy consumed to produce potable water is one of the most active research areas in the Desalination Plant. Now, from 2 KWh/m3 can novel materials or membranes will reduce the energy demand? This is also an active research area to develop a high permeability membrane so that less hydraulic pressure will be required and will reduce the energy consumption for SWRO. But, Energy is governed by the osmotic pressure of the concentrate , high permeability membranes will have a negligible effect.

Future of Seawater Desalination
FO-RO Hybrid Process - With increased interest in the application of Forward osmosis (FO) phenomenon in recent years has led researchers to carry out a proper integration of FO technology in current seawater desalination techniques. Forward osmosis (FO) is the movement of water across the semi -permeable membrane due to the osmotic pressure gradient between two sides of membranes. There is no requirement of hydraulic pressure in FO system which makes it different from Reverse Osmosis (RO) system. The advantage of FO is that it has a high rejection of a wide range of contaminants, lower membrane fouling and theoretically, higher water fluxes and recoveries than RO. Keeping the above features of FO, researchers proposed a hybrid model of FO/RO desalination system. Generally, FO desalination process is comprised of two steps. The first step is an extraction of fresh water from raw water source using a suitable draw solution (Osmotic agent). The second step leads to separation of the osmotic agent from the fresh water. But to reduce the cost of desalination FO is suggested as membrane pretreatment step for seawater prior to RO desalination. Cost reduction leads to ultimately lower energy requirement for pretreatment and RO process, fewer pretreatment processes, the lower maintenance cost of RO modules, decrease/elimination of chemical cost, and the decrease in RO membrane replacement cost. The only pressure associated with FO process is due to flow resistance in the membrane module and equipment requirement is also less in FO. The FO process for energy requirement is economically feasible up to 80% recovery of the impaired water source, significantly decreasing the cost of desalinated water. The best improvement in FO/RO hybrid model is related to process energy requirement. FO process has a capability of recovering potential energy of RO brine and renewable potential energy of naturally available osmotic solution such as seawater. Recovery of potential energy of RO brine for utilization in the pretreatment process ultimately leads to the reduction of overall energy consumption for desalination. This aspect plays a vital role for desalination units installed in remote areas where energy cost represent a greater share of total desalination system.


Figure 2: Schematic diagram of hybrid FO-RO desalination system.

An integrated FO-RO system for desalination of seawater or brackish water consists of one open-end loop for feed water and one-closed loop for the draw solution as shown in Fig. 2. The FO stage serves as a pretreatment step prior to RO desalination. The draw solution used is RO brine. This process plays an important role in RO desalination as it eliminates the need for the chemical used for conventional pretreatment steps. Although, apart from having advantages of energy reduction by FO/RO hybrid system it has few limitations also like low water flux due to currently available FO membranes and complexity of understanding membrane chemistry.

References.
1. K. H. Mistry and J. H. Lienhard, "Effect of Non Ideal Solution Behaviour on Desalination of a Sodium Chloride Solution and Comparison to seawater," Journal of Energy Resources Technology, 2013.
2. M. H. Sharqawy, J. H. Lienhard and S.M. Zubair, "Thermophysical properties of sea water:A Review of Existing Correlations and Data," Desalination and water treatment, pp. 354-380, 2010.