The Chlor-Alkali Industry James J. Leddy Chlor-Alkali Technology Center Oyster Creek Division DOWChemical Company Freeport. TX 77541 edited by: W. Conard Fernelius, Harold Wittcoff, and Robert E. Varnerin
The chlor-alkali industry (chlorine and sodium hydroxide) is an extremely important part of the total chemical industry. An earlier paper ( 1 )entitled " S a l t A Pillar of the Chemical Industry" presented an overview of the products obtained by means of chlorine and caustic. The nresent Daner -. . . deals snecifically with the electrolytic conversion of salt and water (NaC1 brine) into chlorine, sodium hydroxide (caustic) and hydrogen. This energy intensive industry bas an annual U.S. capacity of about 60 billion pounds of chlorine and caustic (2). The annual growth rate is about 5%. requiring almost a quarter of a billion dollars of new ~ a p i t ainvestment l annually in the U.S. alone. About one-third the total world chlor-alkali c a ~ a c i t v exists in tht! 1l.S.in about 70planrs. by far the largerohhirh are concentrated in rhi: Gulf Conit area of Texas and Louisiana. This concentration derives from the combined availability in that part of the U.S. of salt, hydrocarbons (fuel and organic products based on chlorine and caustic) and deep water ports for transportation. Such world-scale plants are measured in capacities of thousands of tons of products per day. While the basic chemistry of this conversion of salt and water to chlorine and caustic is simnle. the ~bvsical chemistrv . . ilnd ~mgineeringare more complex. Discussion I I procws ~ fundamentals nnd alternatives is found in hlantell (31, Kuhn ( 4 ) , and Sconce (5).As in many areas of old process chemistry, much remains to he learned ahout Drocess fundamentals, particularly reaction mechanisms. Also, technologips such as this which are Dra~tiredun a huae scale often hem suhsuntid economic gains for apparently small changes in the basic physics or chemistry of the process. For example, a reduction of 1%in the energy consumed in this process in this country is equivalent to the energy required to keep a half million 100 W lieht bulbs hurnine" constantlv. To understand this i m ~ a c t on our economy let's take a look at some fundamentals. The conversion of common salt and water into chlorine. calistic and hydrogen is represented by the stoichiometry ~
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2NaCI + 2Hz0
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Clz + Hz + 2NaOH(,,
The standard free energychange at 25OC for this conversion is +I00 kcal. Ohviouslv t o make this reaction eo reauires a sizeable energy input. since both Hz and N a ~ ~ > e a c & e a d i l y with Cl2, it is necessary that the cell used to perform this reaction provide physical separation of the H2 and NaOH from the chlorine. The earliest means of achieving this conversion commercially used direct current electricity as the input energy and a porous separator called a diaphragm to minimize mixine of the chlorine and caustic. Hence. the name diaphra& cell. Purified brine (saturated NaCl solution) is fed to the anode compartment of the cell. Chlorine is discharged a t the anode (formerly graphite, now largely titanium metal coated with a thin laver of an electroactive coatine - 16. . . 7)). .. The level of brine in the anode compartment of the cell is maintained higher than in the cathode compartment. This arrangement keeps brine slowly percolating through the diaphragm, which has the good effect of discouraging backmigration of hydroxyl ions into the anode compartment, but also has the nuisance effect of "contaminating" the caustic produced in the cathode compartment with salt. Hydroxide ion 640 1 Journal of Chemical Education
is produced along with hydrogen by the electrolytic reduction of water at the mild steel cathode of the cell. The anode and cathode half-cell reactions and their associated thermodynamic electrode potentials (8)are as follows: Anode: 2C1- 2e- + Cls E" = - 1 3 3 2Hz0 + 2e- --Hz + 20HEe = -0.83V Cathode:
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Net Reaction:
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+
2C1- + 2H20 Clp Hp + 20H- E o = -2.19V The Eo value for the net reaction represents the cell voltage below which it is imoossible to make the reaction oroceed as written. In realit);, Gigher cell voltages are necessary t o comnensate for kinetic effects such as those determined bv composition of the elertrolyte and natured the e l ~ c r r d smfares. e These effects are referred to as ovarpotenlials. They increase the minimum cell voltage at whir11 it is possible to make any uroduct at all. In addition to these twoessential inaredients bf cell voltage we must contend with all of the elec&cal resistances, or IR drops, which exist in the cellcircuit. These IR drops, as well as the overpotentials, are also dependent on current density, which is the total current through the cell divided hv the nominal surface area of either the anode or rathurle. The greater the current d ~ n s i t ythe more product is ma& for a m e n electrode area. as orcdictahle from Faradav's Law. But a'iso, the higher the &kt density the greater the contrihutions of overnotentials and IR drons to overall cell voltage. In a practical sense this translates to a compromise between cell cost and current density in order to minimize the cost of production. Tvnical nroduction diaphraam cells range in size from amp, designed to operate at &rent densities 30,060 to 15b,0~ from 1500 to 3000 amp/m2. Faraday's Law with allowance for modest current inefficiencies indicates that approximately 30,000 amp must be driven through a single cell for 24 hr to produce 2000 lh of chlorine, 2100 lb of NaOH and 57 lh of hydrogen. Thus, a production plant with a capacity of 500 short tons of chlorine per day requires, for example, five hundred 30,000-amp cells. If each cell requires 3.4 V to drive 30,000 amp through it, rectification equipment is required equivalent ot at least 1700 V. Production logistics demand this circuit be divided into a t least two smaller circuits. In anv ewnr. the elertriral energy consumed is spproximately 2460 kC\' hrlton of rhlorine ~roduced.The Dercentaaes of the energy used for the thermodynamic, kinetic, a n i IR requirements in this cell are, respectively, 65% lo%, and 25%. Optimization is paramount. As Dotson (9)pointed out recently, rapidly rising energy costs have made power consumption the primary criterion in evaluating cell performance. However, energy is also consumed in this process in deriving product caustic for sale from the heavily salt-laden caustic effluent which is produced by the diaphragm cell. This effluent contains 10-12% NaOH and 14-16'3 NaC1. Since the form of raustir in the market plilce for most usera is an aqueous solution nmtainina 50% NaOH. the diaohraem cell effluent must be evaporatk to remove both watkr a& salt. The resulting 50% caustic contains about 1% NaCl and smaller amounts of carhonate, sulfate and chlorate. The evaporation is performed in multi-effect evaporators using steam as the heat source. ~~
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There is a chlor-alkali process which consumes only electrical energy which has some advantages over the diaphragm process, and that is the mercury cell process. This cell uses a moving film of mercury over a bed of steel as the cathode. Anodes are titanium metal with an electroactive coating. There is no diaohraem used because there is essentiallv no . hydrogen or caustic produced in the cell itself. The anodk reaction is the same as for the diaohranm cell, but the cathode reaction is now the deposition oisodium metal into the mercurv to oroduce a 0.5% sodium amalgam. This requires a higher thermodynamic potential t h a n k the cathode in the diaohraem cell. Water is not decomposed on the mercury surface L a u s e of the high hydrogen ov&mtential. The sodium amalgam oroduced at the mercury cathode is removed from the cell and allowed tu react with water in a vessel called a decomposer. By adjusting the water flow to the decomposer, M o o caustic can be made directly with no evaporation. Mercury cell caustic also contains very little salt compared to 50% caustic made from diaohrarm cell effluent. One might con. clude that, because ho steam is required in the m e r c b cell, less enerw totallv is reauired. This is not true, however, since the volta& requ&ed to drive the mercury cell is much greater than for the diaohraem cell. Further, the mercury cell costs . more, requiring much higher currentdensities which in turn increases the IR drops, increasing the voltage even more. Production model mercury cells in the 300,000 a'mpere class are not uncommon. Cell voltages are typically 4-5 V. In areas of cheap electricity (e.g., hydroelectric) where fuel costs are high (with consequently high steam costs) the mercury cell can be a real advantage. It is also advantageous compared to the more cumbersome diaphragm cell-evaporator plants where smaller tonnages are r&&d along with a highergrade of caustic. All in all, the economic choice between these technologies is often difficult to make. Environmental pressures (10, 11) on mercury cell operations to reduce mercury discharges to essentially zero, the need for a cell technology to produce high quality caustic without the need for the additional processing steps required with diaphragm cell effluent, and a technology which uses less energy has placed much emphasis lately on the use of cation-exchange membranes (12, 13) to replace the present
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separators in diaphragm cells. The development of membranes to achieve this new cell technolow has consumed the concerted efforts of many researchers f& 20 years. Consider the difficultv in achieving desirable ion-exchange properties and life with a film for$ng polymeric materia which is a t once resistant to attack by both aqueous chlorine and concentrated solutions of NaOH a t 90-100°C. One way of achievina this is described by Grot (14, 15) using cation-exchange membranes made from sulfonated perfluorinated vinyl ethers. While the membrane cell is presently in its infancy. there is speculation in the industry that its higher capital cost will he offset by overall process energy savings and the produrtivn of 50°0 caustic of better than mercurv cell oualitv. Much more is yet to he heard from this exciting new area, even thouah areat strides have been made in mercury cell ooerations-toeliminate mercury discharges by improved process control and the use of ion-exchange resins (I I ) to recover and recycle the mercury which was previously lost to the environment. In summary, the chlor.alkali industry plays a vital role in all industralized economies. With a third of total world cauaritv in the 1J.S. at arnund 30 hillion ~ o u n d of s chlorine and i 2 biiion pounds of NaOH annually k s important part of our overall economic well being continues to evolve toward increasingly sophisticated technology particularly in the direction of increased efficiency of energy utilization. A
Literature Cited
1971. (5) Smnce, J.
S.,A.C.S.MonogrgrphNo.l 5 4 , Publishins Corn..Near York. 1962.
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l
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Uam!'Rehhold
(10) Chemical Engineering, p. 84, July 27, (1970). (11) Him, F., et al.,lnt'l. Cham. Eng., 11.1(1977). (12) Seko, M., Near York Meeting, April 4-9 (1976).
A.C.S.
(13) Cmk,E. H..etal..U.S.Pa&nt,3,%8,737(1976). ( 1 4 Gmt.E.G.,U.S.Pafenl.3.718.627 (1973). (15) Gmt. E. G.. Chom-lng. Tech., 44,161 (1972).
Volume 57, Number 9, September 1980 I 641
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