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Teaching of industrial inorganic chemistry.

Teaching of Industrial Inorganic Chemistry

One important factor for the existence of the chemical industry is the profit motive, even to some extent in controlled economies. It costs about $2-million to build a 400-tonne per day sulfuric acid plant.[1] If it were not feasible that by so doing one could earn at least $200,000 per year, or an annual 10% of the capital invested from the sale of the output from the plant, then the incentive for doing this is negligible. With less trouble and risk, the same $2-million could earn 10% or better in term investments with financial institutions. Profit from its operations is not only in the company's best interests, but also in that of the government and the public, due to the employment provided and the revenue resulting from taxes on profits and employee income.

Beyond the profit motive there are many other corporate interests and responsibilities involved in the operation of the chemical plant. These include obligations to the country and the local community in which they operate met by the payment of local taxes etc., as well as to proper management of raw materials, products, and wastes, emission control, safety, and the provision of good working conditions. With multinational corporations, the relative priorities of their local and international obligations occasionally become blurred to the detriment of operations in one or other country of operations; sometimes requiring guiding legislation.

Many factors need to be considered in estimating costs of operation: the amount of raw materials and products, utilities and labour required, and capital costs to construct the processing unit needed to carry out the chemical conversions. Raw material and product estimation is difficult, if not impossible with the usual research (academic) yields specified in research publications. Research yields usually do not take into account starting material that is not reacted (converted), as this is generally not of primary importance in the research result being reported. For process consideration, however, knowing the fraction of starting material that is recoverable unchanged can be vital to the commercial viability of a process. For process value, therefore, it is usual to specify a conversion, meaning the fraction (or percent) of starting material chemically changed in the reaction, as well as a fractional yield, or the selectivity with which the converted starting material has been changed to the desired product. Thus the fractional conversion times the fractional selectivity (industrial yield) equates to the commonly used research definition of yield.[2] Very often a reaction taken to higher conversions is less selective for the desired product. This is the reason why process considerations require separation of these two components of research yields.

Patent and Trademarks

Few students have seen a patent, or know much in detail about the privileges that this system of public documents confers. Show them one or two as part of the teaching process; a brief process patent may consist of only one or two pages. Some are considerably longer.[4]

Explanation of the patent application process, and the value of composition of matter, process, and machine patents, with examples can be of great interest. Not all new advances in the chemical industry are the subject of patents; much is retained as in house `know-how', company proprietary information that may not even be patentable. This information does not enter public documentation and, therefore, the company hopes that this will remain proprietary for even longer than the normal 20 years from date of filing in Canada, of 17-year useful life of a North American patent. Trademarks and tradenames of companies are publicly recognized company symbols which also form a part of Patent Office business. These symbols are registered with the Patent Office and remain in force for as long as annual registration fees are paid. They form a significant part of company promotion. As such they are as important and valuable to the marketing side of chemical companies as the process or composition of matter patent is to the production side.

Scale and Price Information

What inorganic chemicals are produced commercially on the largest scale? Current information on this is available from Canadian Chemical Processing and Statistics Canada for Canadian data, and from special issues of Chemical and Engineering News [5] for both the US and Canada, with further details available from recent editions of the Statistical Abstract of the United States[6] and Minerals Yearbooks[7,8] (Table 1). Less current data for these and other parts of the world, for comparison purposes, is available from editions of the UN Statistical Yearbooks.[9] [Tabular data omitted]

Chemical price information is occasionally given in issues of Canadian Chemical Processing, and for the US is accessible on a weekly basis from the Chemical Marketing Reporter.[10] Combining data from these sources with the timing of the major oil price increases provides material for valuable discussion regarding the significance of energy costs in chemicals production. Thus, the costs of ammonia and sodium hydroxide production, which have substantial energy components, are significantly affected by petroleum pricing (see Figure 1). Pricing of sulfuric acid, with a lower energy component in its production cost is less affected by oil prices.

The volume of chemicals produced is affected in much the same way as business is generally by recessionary cycles. Thus, while the general trend for the production volume of any commodity chemical is upward, the recession of 1982 clearly shows up in these plots of volume of production versus time (see Figure 2). The dominant position of pulp and paper production in Canada provides a substantial fraction of the market for chlorine and caustic soda (sodium hydroxide). These commodity chemicals, as well as hydrogen, are mostly produced via the electrolysis of a saturated solution of sodium chloride in water.

Chlorine and Sodium Hydroxide

Prototypes of the two principal kinds of electrolytic units used, the diaphragm and the mercury cell, were proved in Germany in 1891 for the former, and simultaneously but independently by H. Castner in the US and K. Kellner in Austria for the latter, both in 1892.[11] These represent the independent development of parallel technologies to solve a single commodity need at the same time. It was only a year after the development of the initial batch electrochemical diaphragm process that E.A. LaSueur of Ottawa introduced refinements to allow continuous and more efficient chloralkali production by this cell type.[12] The electrochemistry of commercial chloralkali production lends credence to electrochemical theory in that the relationship of volume of production to current flow is quite close to that predictable from Faraday's Law. If desired, the instructor can pursue this further by demonstration of the validity of the Gibbs-Helmholtz equation for estimation of the theoretical operating voltage, and the relationship of this to the actual operating voltage via consideration of overvoltages, concentration gradients, cell resistance etc. LaSueur's contributions are one example of Canadian contributions to chemical technology, and still form the basis of operation of all percolating diaphragm cells operated today.

The 10 to 12% sodium hydroxide product from a conventional percolating diaphragm cell is contaminated by similar concentrations of non-electrolyzed sodium chloride. The sodium chloride can be largely removed by partial evaporation and crystallization of the salt, but newer ionselective (and non-percolating) membrane cells permit preparation of up to 28% sodium hydroxide virtually free of sodium chloride.[13] The operating principles behind these improvements are intriguing,[14] and worth discussion. They will be employed in the largest chloralkali facility in the world now under construction in Texas.[15] Prototype experiments using a [Beta]-alumina diaphragm to act as a selective, sodium-permeable membrane, which separates a redhot sodium chloride melt anolyte from the near 100% sodium hydroxide catholyte, promise to further change the fundamental operating characteristics of future commercial chloralkali cells.[16]

Balancing Supply and Demand

Electrochemical production of chlorine and sodium hydroxide from sodium chloride brine necessarily locks the ratio of the relative masses of the two products to 1.000: 1.128 (1.000:(40.00/35.45)), which seldom parallels demands. Price adjustments can help. Reducing the price of the commodity produced in excess can enlarge existing markets and to stimulate new markets for that commodity. But there are limits. Currently, the price of sodium hydroxide is about 2 1/2 times the price of chlorine; 20 years ago it was only about 20% higher. Producers who have the means to produce chlorine without sodium hydroxide, or sodium hydroxide without chlorine to flexibly cope with market demands which stray from the 1.00 to 1.13 direct electrochemical production ratio, will have a market advantage. Chlorine may be made by chemical oxidation of sodium chloride with nitric acid (Salt Process). Or it may be produced by the oxidation of anhydrous hydrogen chloride,

6NaCl + 12 [HNO.sub.3] + 2[Na.sub.2][CO.sub.3] [right arrow]

3[Cl.sub.2] + 10[NaNO.sub.3] + 2[CO.sub.2] + 2NO + 6[H.sub.2]O

itself a large scale by product of the production of chlorinated solvents. This can be accomplished with manganese dioxide (Weldon Process)

[CH.sub.4] + 4[Cl.sub.2] [right arrow] [CCl.sub.4] + 4HCl

which only returns half of the element contained in the starting hydrogen chloride, or with air (Deacon Process) which in theory returns all of it.[17] Sulfuric acid is sometimes employed for water removal

4HCl + [MnO.sub.2] [right arrow] [MnCl.sub.2] + [Cl.sub.2] + 2[H.sub.2]O

[Mathematical Expressions Omitted]

sometimes employed for water removal in the Deacon Process to decrease potential chlorine losses from the reverse reaction on cooling for product recovery (Kel Chlor Process(17)). Hydrogen chloride can also be oxidized to chlorine electrochemically. Alternatively, a sodium chloride melt may be electrolyzed in a

2HCl electrol. [H.sub.2] + [Cl.sub.2]

firebrick-lined Down's cell to produce chlorine, and liquid metallic sodium. This is the method used to produce about 3% of the chlorine

2NaCl electrol. 2Na + [Cl.sub.2]

in North America, and as much as 10% in the UK(11). Now molten sodium is an interesting commodity to move through pipes, and ship; any leak, or spill, and a fire is likely, requiring less than common firefighting techniques to handle!

The hydrogen co-product of chloralkali production or hydrogen chloride electrolysis is used to provide part of the hydrogen for ammonia or methanol synthesis, when the scale of the chloralkali facility is large enough to make this worthwhile. Smaller or isolated facilities usually burn the hydrogen to provide part of their operating energy requirements. This has to be one of the most environmentally benign fuels, with zero contribution to atmospheric carbon dioxide. Heat and water vapour are the only products of combustion.

Potassium Chloride (Potash)

One last example of particular interest in the Canadian context is the production of potash, which can be used to illustrate the application of innovative methods to handle the mining and recovery of commodity chemicals. Potash is so named because of the original commercial recovery of potassium carbonate from wood ash by extraction with water. In industry the term usually refers to potassium chloride (more specifically, `muriate of potash'), a key fertilizer ingredient. However, the dominant minerals containing potassium chloride in Canada, as in the other major world deposits, consist of potassium chloride co-crystallized with either magnesium chloride (carnallite, KCl*[MgCl.sub.2]*6[H.sub.2]O) or sodium chloride (sylvinite, 2NaCl*KCl). Ingenuity is required to recover the potassium chloride from the large beds located in Saskatchewan, where it is estimated that reserves of some 10(11) metric tonnes resides.

Sylvinite recovery from the northern shallower deposits of 1000 m or so is by conventional mining, but how would you economically separate the valuable potassium chloride from the 60% or so sodium chloride co-crystallized with this? Froth flotation in water, essentially the same technology that concentrates several sulfide minerals, is used. Froth flotations, to separate water-soluble minerals!? To prevent losses of the water-soluble minerals during separation, saturated brine is used as the flotation medium. Proper prior surface treatment of the finely ground sylvinite with guargum plus a tallow amine produces a stable surface froth of ca. 96% KCl, and an underflow of sodium chloride, with KCl recoveries in the 90 - 95% range.[18] This is truly amazing technology, which can provide material on surface chemistry, contact angles and the like, at the instructor's discretion. An example: substitution of the tallow amine by a fatty acid salt allows collection of the sodium chloride as a froth and produces a potassium chloride underflow, the reciprocal of the result obtained with tallow amine!

The deeper sylvinite deposits some 1,600 m below the surface are too expensive to mine by conventional means. Solution mining appears to be possible, but on first glance would seem to require the handling of almost twice as much sodium chloride as potassium chloride at the salts recovery stage. How to economically accomplish this? Here too, innovation is used to decrease the economic penalty of this factor by employing the much steeper hot and cold differential solubility of potassium chloride over sodium chloride (see Figure 3). By using hot water already nearly saturated in sodium chloride to contact the sylvinite, far more potassium chloride than sodium chloride is dissolved.[19] After filtration of the insolubles from the returned, still hot brine, staged evaporations and crystallizations allow separate recovery of sodium chloride and 99+% potassium chloride. This is more than adequate purity for fertilizer formulations and is suitable pure for many chemical applications such as the preparation of potassium hydroxide or the sulfate or nitrate salts, used for fertilization of chloride sensitive crops.

References

[1.] F.A. Lowenheim and M.K. Moran, Faith, Keyes, and

Clark's Industrial Chemicals, 4th ed., John Wiley and

Sons, New York, 1975, p.795. [2.] M.B. Hocking, Modern Chemical Technology and

Emission Control, Springer-Verlag, New York, 1985,

p.15. [3.] M.B. Hocking, Production of Isocrotonic Acid, Can. Pat.

881,556, to the Dow Chemical Company, 1971; U.S.

equivalent Pat. 3,539,548. [4.] D.M. Findlay and R.A.N. McLean, Treatment of Mercury

Contaminated Aqueous Media, Can. Pat.

1,083,272 to Domtar Inc., 1979; U.S. equiv. Pat.

4,147,626. [5.] Chemical and Engineering News, weekly, put out by

the American Chemical Society, 1155-16th St. N.W.,

Washington, DC 20036. [6.] U.S. Bureau of the Census, Statistical Abstract of the

United States, 109th edition, Washington, DC, 1989,

and earlier annual editions. [7.] 1988 Canadian Minerals Yearbook, Energy, Mines and

Resources Canada, Ottawa, 1989, and earlier annual

editions. [8.] 1987 Minerals Yearbook, Vol. I. Minerals, Metals, and

Fuels, US Dept. of the Interior, Washington, 1989. [9.] 1985/86 Statistical Yearbook, United Nations, New

York, 1988, and earlier editions, biennial. [10.] Chemical Marketing Reporter, weekly, two volumes per

year, 100 Church Street, New York, 10007. (Formerly

called the Oil, Paint and Drug Reporter). [11.] Reference 2, p. 135. [12.] E.A. LeSueur, Improvements in or Connected with

Means or Apparatus Employed in Electrolysis, British

Pat. 5983, April 7, 1891. [13.] R.L. Dotson, "Modern Electrochemical Technology",

Chem. Engin. July 17, 1978, p. 106. [14.] M.B. Hocking, and M. Gellender, "New Chemical

Processes Avert Industrial Mercury Pollution",

Chemistry International 1981(2), p.7. [15.] E.J. Rudd and R.F. Savinell, Report of the Electrolytic

Industries for the Year 1988, J. Electrochem. Soc., 136(9), 449C, Sept. 1989. [16.] Y. Ito, S. Yoshizawa, and S. Nakamatsu, "A New Method for the Electrolysis of Sodium Chloride Using a [beta]-alumina-Molten Salt System", J. Appl. Electrochem., 6, 361 (1976). [17.] L.E. Bostwick, "Recovering Chlorine from HCI", Chem. Engin., 83(21), 46, Oct. 11, 1979. [18.] Reference 2, p. 113. [19.] J.B. Dahms and B.P. Edwards, Solution Mining Method of Soluble Minerals or Mineral Compounds from Defined Subterranean Multiple Deposits, Can. Pat. 672,308, to Pittsburgh Plate Glass Co., Oct. 15, 1963.
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Author:Hocking, Martin B.
Publication:Canadian Chemical News
Date:Mar 1, 1990
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