Plant design electrolytic processes

In the heavy-water plants constmcted at Savannah River and at Dana, these considerations led to designs in which the relatively economical GS process was used to concentrate the deuterium content of natural water to about 15 mol %. Vacuum distillation of water was selected (because there is Httle likelihood of product loss) for the additional concentration of the GS product from 15 to 90% D2O, and an electrolytic process was used to produce the final reactor-grade concentrate of 99.75% D2O. 

At present about 77% of the industrial hydrogen produced is from petrochemicals, 18% from coal, 4% by electrolysis of aqueous solutions and at most 1% from other sources. Thus, hydrogen is produced as a byproduct of the brine electrolysis process for the manufacture of chlorine and sodium hydroxide (p. 798). The ratio of H2 Cl2 NaOH is, of course, fixed by stoichiometry and this is an economic determinant since bulk transport of the byproduct hydrogen is expensive. To illustrate the scde of the problem the total world chlorine production capacity is about 38 million tonnes per year which corresponds to 105000 toimes of hydrogen (1.3 x I0 m ). Plants designed specifically for the electrolytic manufacture of hydrogen as the main product, use steel cells and aqueous potassium hydroxide as electrolyte. The cells may be operated at atmospheric pressure (Knowles cells) or at 30 atm (Lonza cells). 

Investment protection Superior. The need to keep electrolytic cells at temperature even during reactor shutdown is more easily met if cells are heated by electrical heaters or hydrogen combustion products as is proposed for plant design that uses low temperature process heat loop. 

Finally, there are sometimes aspects of plant electrical engineering in which the electrical design will interface directly to the process, most commonly in electrolytic processes. Here there is no mechanical equipment intervening between the process and the electrical supply. The flow of information within the project team is different. This must not be allowed to be the cause of error or omission, such as failure to completely specify the electrical characteristics of the cells, the back e.m.f. and its significance under shutdown conditions, and the insulation requirements for busbars and potentially energized and accessible equipment. 

Since every plant is constructed for a specific process often using specific raw materials, the design of cells, nature of electrode materials and the processes which accompany the electrolysis step vary considerably. However, there are some features which are common to most electrolytic processes These will be discussed first and later several processes will be briefly described. 

Dead Sea Works Process. The Dead Sea Works, a subsidiary of Israel Chemicals Ltd., aimounced plans ia 1992 to constmct a 25,000 t/yr magnesium plant at Beer-Sheva, Israel. The plant, to be based on Russian camaHite technology, is designed to use an existing potash plant as the source of camaHte. The chlorine by-product can be either Hquefted and sold, or used ia an existing bromine plant. Waste streams from the camaHite process, as well as spent electrolyte from the electrolytic cells, can be returned to the potash plant. 

The values of plant process variables for steady-state hydrogen production rates between 75 and 100% of full power are given by the load schedule reported here. The objective in designing this schedule was to achieve near constant hot side temperatures in both the nuclear and chemical plants. Briefly, mass flow rates are maintained proportional to power throughout, inventory control is used in the PCU, and electrolytic cell area and current are maintained proportional to hydrogen production rate. 

A discussion of electrochemical reactors is available in books by Prentice (Electrochemical Engineering Principles, Prentice-Hall, 1991), Hine (Electrode Processes and Electrochemical Engineering, Plenum Press, 1985), Oloman (Electrochemical Processing for the Pulp and Paper Industry, The Electrochemical Consultancy 1996), and Goodridge and Scott (Electrochemical Process Engineering A Guide to the Design of Electrolytic Plant, Plenum, 1995). 

Stone Webster/lonics A flue-gas desulfurization process in which the sulfur dioxide is absorbed in aqueous sodium hydroxide, forming sodium sulfite and bisulfite, the sulfur dioxide is liberated by the addition of sulfuric acid, and the reagents are regenerated electrolytically. Designed by Stone Webster Engineering Corporation and Ionics Inc. Operated in a demonstration plant in Milwaukee, WI, in 1974, but not commercialized. 

F. Goodridge and K. Scott, Electrochemical Process Engineering A Guide to the Design of Electrolytic Plant, Plenum Press, New York (1995). 

The first production plant in the world for electrolytic aluminum coating from organoaluminum complexes was put into operation by SEDEC (Berlin) in 1983. In order to shorten the processing time required by the HGA plant, this unit was designed as a rectangular cell. The production cell has an electrolyte volume of 15,000 L. The capacity of this automatic aluminum plating unit amounts to 32 m /h, with a layer thickness of 10 pm. Articles mounted on 32 frames, each 500 X 1000 mm in size, can be simultaneously coated. Fig. 19 shows the electrolysis... 

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