Michael T. Ligayo - NU12 9/12 new process - seawater to freshwater conver

Prof. Jorge,

Here is my NU12 - 9/12.

Regards,
Michael Ligayo

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FRESH WATER FROM SEA WATER?!

The Water Resources of the World

Over 70% of our Earth's surface is covered by water ( we should really call our planet "Ocean" instead of "Earth"). Although water is seemingly abundant, the real issue is the amount of fresh water available. 

•       97.5% of all water on Earth is salt water, leaving only 2.5% as fresh water 

•       Nearly 70% of that fresh water is frozen in the icecaps of Antarctica and Greenland; most of the remainder is present as soil moisture, or lies in deep underground aquifers as groundwater not accessible to human use. 

•       < 1% of the world's fresh water (~0.007% of all water on earth) is accessible for direct human uses. This is the water found in lakes, rivers, reservoirs and those underground sources that are shallow enough to be tapped at an affordable cost. Only this amount is regularly renewed by rain and snowfall, and is therefore available on a sustainable basis.

Since antiquity, irrigation, drainage, and impoundment have been the three types of water control having a major impact on landscapes and water flows. Since the dawn of irrigated agriculture at least 5000 years ago, controlling water to grow crops has been the primary motivation for human alteration of freshwater supplies. Today, principal demands for fresh water are for irrigation, household and municipal water use, and industrial uses. Most supplies come from surface runoff, although mining of "fossil water" from underground aquifers is an important source in some areas. The pattern of water withdrawal over the past 300 years shows the dramatic increases in this century.

A timeline of human water use:

•       12,000 yrs. ago: hunter-gatherers continually return to fertile river valleys 

•       7,000 yrs. ago: water shortages spur humans to invent irrigation 

•       1,100 yrs ago: collapse of Mayan civilization due to drought 

•       Mid 1800's: fecal contamination of surface water causes severe health problems (typhoid, cholera) in some major North American cities, notably Chicago

•       1858: "Year of the Great Stink" in London, due to sewage and wastes in Thames 

•       Late 1800s-early 1900: Dams became popular as a water management tool 

•       1900s: The green revolution strengthens human dependency on irrigation for agriculture 

•       World War II: water quality impacted by industrial and agricultural chemicals 

•       1972: Clean Water Act passed; humans recognize need to protect water

The Alternative Process to Obtain Freshwater from Seawater – The Chloralkali Process

The production of caustic soda and chlorine is one of the most important heavy chemical industries. Hydrogen gas is also produced, but in lesser amounts. The two major chemicals that is, chlorine and caustic soda, among many chlorine compounds produced as by-products, are formed entirely from the electrolysis of brine. Caustic soda and its co-product chlorine are used in large quantities as raw materials in the manufacture of organic chemicals, plastics, pulp and paper, aluminum, and in waste treatment. Gaseous hydrogen can be shipped to ammonia plants or used as fuel.

There are three ways of manufacturing caustic soda and chlorine from brine depending of the type of cell used. The first process uses diaphragm cells, made from asbestos. This type produces at most 12% caustic soda, still contaminated with sodium chloride salts. Further evaporation and purification steps are required, which makes the process rather inefficient economically because of the large amount of energy expended (Faith, 1957). Also, the caustic produced may still be too contaminated for use, especially in the synthetic fiber industry, which requires high purity caustic (Black, 1994).

The mercury cell or amalgam cell uses a flowing pool of mercury as an electrode in the conversion of brine to caustic soda, and hydrogen and chlorine gases. The process produces at least 50% caustic soda, essentially free from contaminant salts. The process requires minimum processing and expends less energy than do diaphragm cells. Inherent in the process is the production of a mercury-sodium alloy (amalgam) that is not decomposed by the brine present. This amalgam is decomposed in a separate vessel and leads to a small loss of mercury in the environment. This is mainly discharged into nearby waters. Various studies in Japan have shown that consumption of this mercury-contaminated water produces very serious health complications. This led to efforts to control the effluent mercury concentration or entirely replace the manufacturing process.

The latest electrolytic cells, membrane cells make use of specially fabricated membranes that facilitate electrolysis. These cells have the main advantage of the least energy requirement among the three with a caustic product that has a purity intermediate of the diaphragm and mercury cells. However, the cells are very expensive (Austin, 1984). Further, membrane cells do not withstand extreme pH values (Kuhn, 1971). The presence of the divalent cations Ca²⁺ and Mg²⁺ also damages the cells. Although the current trend is shifting towards the use of membrane cells, many companies have found it more economical to continue manufacturing caustic and chlorine from the old mercury cell units (Black, 1994).

On the other hand, with the discovery of large mining sites of soda ash (sodium carbonate), efforts were put forth to manufacture caustic from soda ash by reacting it with slaked lime or calcium hydroxide. Caustic soda was successfully manufactured by plants operating within the vicinity of the mining sites. Although this method, the lime-soda process, does not release mercury effluents, it unavoidably releases huge amounts of excess calcium chloride in the environment, which pollutes as well (Black, 1994).

Global Assessment of Caustic and Chlorine

           

Electrolytic production of caustic soda and chlorine occurs in fixed, stoichiometric amounts. However, the demand for the products varies individually, and according the National Statistics Office (1996-2001), worldwide demand has frequently vacillated. Further, international companies are looking for ways to replace entirely pure chlorine compounds, citing the environmental hazards connected with organic compounds made with chlorine (organochlorine compounds). This led to price cutoffs that prompted less production and marketing of the products to avoid financial losses. However, caustic soda can be converted to soda ash, which has a number of applications, while chlorine can be converted to sodium chlorate, whose demand has increased for the last decade (Thompson, 1995). At any rate, both caustic soda and chlorine can still be used as basis for new products.

Local Assessment of the Mercury Cell Process

Mabuhay Vinyl Corporation, established since 1963, is the leading local producer of caustic soda and chlorine, among other products, using the mercury cell as the electrode. When stricter standards were imposed, the plant shifted to using diaphragm cells instead. A process engineer was quoted saying that economically speaking the mercury/amalgam process is very viable, but the local culture of avoiding anything connected with using mercury was so prevalent that it drove off possible local investors. The plant shifted to using diaphragm cells in 1979, when there was yet no technology that could lessen the effluent concentration to safety limits.

Removal of Mercury from Amalgam Wastewater

Previous ways of disposing mercury (II), which include ion exchange and reaction with sulfhydryl groups, proved to be very expensive. Interestingly, German researchers from the National Research Centre for Biotechnology discovered that when treated in a bioreactor inoculated with Pseudomonas putida, about 97% of ionic mercury could be reduced to liquid mercury (Von Canstein, et al., 1999). Another technical-scale study was made, this time using genus Pseudomonas sp. The results in the removal of mercury were similar (Dobler, et al., 2000).  These studies present a novel way to treat wastewater from the amalgam process and eventually revive the method that has been losing to more environmentally friendly cells.

Description of the Process

The conventional chlor-alkali process utilizes either rock salt or concentrated brine as raw material. We propose a method based on the former. Rock salt is dissolved in a mixer, called a resaturator, together with the depleted brine, which is a recycle stream from the mercury cell unit. Next, the clear brine is freed of the ionic contaminants [viz., Iron (II), Ca (II), Mg (II), and SO₄^2-] by treatment with barium carbonate in the first reactor, and subsequently, with soda ash in the second reactor. The ionic reactions are as follows:

Ba²⁺ + SO₄^2-                                             BaSO_4(s)

Fe²⁺ + CO₃^2-                                             FeCO_3(s)

Ca²⁺ + CO₃^2-                                             CaCO_3(s)

Mg²⁺+ CO₃^2-                                             MgCO_3(s)

These ionic contaminants, with the exception of potassium ions (which occur in negligible amounts) are precipitated out of the solution as carbonate salts and as barium sulfate. Heavy metals, like cadmium, mercury, and lead, if present, are precipitated with sodium hydroxide. The treated brine is subjected to gravity settling in two sedimentation tanks operating in series. Slurry, consisting of ionic salts, is withdrawn from the mixture.

The clear brine is pumped to the holding tank and brought to pH 5.5 by addition of aqueous hydrochloric acid and solid NaOH. Water formed from the neutralization reaction is vaporized in the evaporator. The acidified brine passes into the mercury cell where sodium ions are reduced according to the electrolytic reactions:

                        Na⁺ + Cl^-                      Na + Cl₂

Metallic sodium dissolves in mercury, forming amalgam.

                        Na + 4Hg                     NaHg₄

The amalgam passes to the mercury denuder, where the following reactions occur.

                        Na + H₂O                      NaOH + 0.5 H₂

In the electrolytic production of caustic soda, hydrogen and chlorine gases are evolved. Hydrogen can be used for a number of purposes, one of which is the production of hydrochloric acid.

The chlorine gas from the mercury cell is entrained with water vapor.  The entrained water vapor of the hot gas is condensed in a heat exchanger. Water that exists in equilibrium with chlorine gas is further stripped from the mixture in the scrubbing tower using sulfuric acid. It is then compressed to liquid chlorine and packed.

The depleted brine from the electrolyzer is freed of entrained chlorine by air stripping. One reason of stripping chlorine from the depleted brine is for microorganisms to thrive, as chlorine has a disinfecting effect. Chlorine gas and air are liberated as waste gas. The stripped brine, prior to bioremediation is neutralized through the addition of sodium hydroxide and cooled to the optimum temperature for microbial growth. The biologically treated brine is the recycled to the resaturator.

The primary product, 50% by weight NaOH, is essentially free of contaminants. It can further be processed to produce a more concentrated liquid and/or rolled into flakes. 4

Main Products of the Process: Freshwater and Sodium Hydroxide