Molten Salt Reactors and Desalination

 

WATER: A NATIONAL SECURITY ISSUE

There is a finite amount of fresh water readily available for human consumption and use. This supply is already strained due to competing demands for farming, industry, commercial and domestic uses. With world population expected to exceed ten billion by 2040, the draw on these resources will become even less sustainable. Water problems will hinder the ability of key countries and domestic states to produce food and generate energy, posing a risk to global food markets and hobbling economic growth.

During the next 10 years, many countries important to the United States will likely experience water shortages and poor water quality. North Africa, the Middle East, and South Asia will face major challenges coping with water problems. These water shortages will risk instability and state failure, increase regional tensions, and distract foreign countries from working with the United States on important US policy objectives. Domestically, the situation is equally serious. Water shortages are already impacting the quality of life and economic prospects of US residents living in the southwestern states.

Without more effective water resource management, fresh water availability will not keep up with demand. Molten Salt Reactor (MSR) technology offers a powerful potential solution to the impending global freshwater crisis. MSRs produce economical, high temperature process heat that can be used for a variety of valuable applications. A co-generation/manufacturing synergy of a MSR and coal with access to seawater or brackish groundwater can provide electricity, industrial heat, gaseous and liquid transportation fuels, fertilizer, and most importantly, clean potable drinking water for agriculture, industrial, and domestic use.

 

DESALINATION: OPPORTUNITIES AND CHALLENGES

Over 97 percent of the world’s water supply is held either as saline ocean water or brackish groundwater reserves. If an effective, economical desalination process could be developed, it would provide a virtually unlimited supply of fresh water, solving the world’s water issues.

Currently, most of the word’s desalination capacity is based on thermal technologies, primarily multi- stage flash (MSF) distillation and multi-effect distillation (MED). MSF and MED are popular in parts of the world where thermal energy is readily available and inexpensive. These technologies require large thermal energy inputs to vaporize water, produce problematic brine discharges, and result in relatively low water recoveries. Significantly, these technologies have electrical requirements for ancillary operations that often approach those of Reverse Osmosis.

Reverse Osmosis (RO) continues to gain popularity as a successful desalination technology; nearly all desalination plants built today are RO plants. Its benefits include the capability of achieving higher recoveries than those typical of its thermal counterparts, while using less overall energy. However, electricity use is still high, and brine discharge problems remain.

Brine production is a significant drawback for these technologies. Plants must be located near an ocean, where discarded brine has less environmental impact. In brackish water applications, the quantity of brine produced precludes the use of economic solar pond evaporation, and ground well injection increases source water salinity over time. Since there is no sustainable, environmentally sound way to dispose of the brine on site, MSF, MED and RO technologies cannot be used to desalinate inland brackish groundwater sources.

 

NEXT GENERATION DESALINATION: FORWARD OSMOSIS

Next generation desalination technologies address outstanding issues of energy use, brine management and water recovery. Forward Osmosis (FO) is a membrane-based separation process, like Reverse Osmosis, which relies on a semipermeable membrane to remove salt. However, the driving force for separation in Forward Osmosis is osmotic pressure, not the hydraulic pressure used in RO.

By using a concentrated solution of high osmotic pressure called a draw solution, water can be induced to flow across the membrane, rejecting the salt. The draw solution must then be removed from the draw solution-laden water, yielding potable water. Recycling the draw solution is key to economic viability of Forward Osmosis.

 

THE IDEAL DRAW SOLUTION: AMMONIA-CARBON DIOXIDE

A draw solution for use in a continuous FO desalination process, in which the draw solution is recovered, must have certain characteristics to make the process economically viable. For this FO process the draw solution must have high solubility, a low molecular weight, and easy removal.

The Forward Osmosis next generation desalination process currently uses a recyclable solution composed of ammonium salts. These salts (a mixture of ammonium bicarbonate, ammonium carbonate and ammonium carbamate) are formed when ammonia and carbon dioxide gases are mixed in an aqueous solution. The salts are highly rejected by the semipermeable membrane used in FO and are highly soluble, leading to the reliable generation of high osmotic pressures for the FO process.

Once the concentrated draw solution is used to effect separation of water from the saline feed source, the subsequently diluted draw solution may be heated to remove its ammonium salt solution, producing fresh water as the primary product of the FO process. This thermal separation of draw solution is based on the useful characteristic of these salts to decompose into ammonia and carbon dioxide gases when the solution is heated. The temperature at which this occurs is dependent on the pressure of the solution. If a vacuum distillation column is used for this separation, the temperature of heat required can be quite low, in the range of 35-40°C (95-104°F) given an ambient temperature of 15-20°C (59-68°F).

The use of an ammonia-carbon dioxide draw solution thereby allows for effective desalination of saline feedwater sources using little more than low-grade heat (very little electricity is required for unpressurized process pumping). Furthermore, the high osmotic pressures that solutions of this type may generate allow for very high feedwater recoveries. This has the benefit of reducing brine discharge volumes, electrical requirements for feedwater pumping and process capital costs.

 

BENEFITS OF SYNERGY: FORWARD OSMOSIS, MSR AND SNG

The use of an ammonia and carbon dioxide based salt to create osmotic pressure to effect the separation of fresh water from saline sources allows for higher feedwater recoveries, lower brine discharge volumes, lower (and less expensive) energy use and a lower total water cost. Using a vacuum distillation column for solution recovery, it is possible to use very low-grade heat as the primary energy source for FO and hence radically reduce the cost of desalination.

The high recoveries and subsequent low brine discharge volumes make it possible to reduce the negative environmental impact of desalination of all types, opening up the possibility of effectively desalting inland saline water sources. With high recovery FO, it is possible to obtain fresh water economically from brackish groundwater without producing a liquid brine stream. This could be of great benefit to arid regions with such resources, such as the southwestern US.

Cost-effective Forward Osmosis will rely on ample supplies of high purity carbon dioxide and ammonia. Both can be produced from the Synthetic Natural Gas (SNG) process driven by an Molten Salt Reactor (MSR), offering the promise of a synergy, especially for developing nations that are in dire need of electricity, transportation fuel, and water. Transforming various carbon-based feed stocks such as coal into SNG creates carbon dioxide and ammonia. MSR power makes this an economically attractive proposition.

MSRs offer other benefits to the FO process. Residual heat captured from a MSR used in an SNG production process could be used in the desalination process. The low cost electricity generated could power pumps, making it economically viable to transport water long distances. This is preferable to drawing water from environmentally sensitive streams and fresh water habitats, positively impacting the natural environment, while enabling continued, sustainable economic growth.

Forward Osmosis is a compelling potential solution to the world’s impending water crisis. Molten Salt Reactor technology, powering Synthetic Natural Gas production, can create the raw materials for affordable FO desalination and the low-cost energy needed to maximize the benefits of these technologies, offering communities, states, and nations a means to produce and secure their own prosperity.

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