Chlor-Alkali Technology S. Venkatesh and B. V. Tilak' Occidental Chemical Corporation, Hooker Research Center, Long Road, Grand Island, NY 14072 Chlor-alkali technology is one of the largest electrochemical industries in the world, the main products being chlorine and caustic soda (sodium hydroxide) generated simultaneously bv the electrolvsis of sodium chloride. which is enerev intentries. Chlorke and caustic soda are indkpensabie intermediates for the chemical industry. Chlorine is a strong oxidizer, and the largest volume use for chlorine is in the production of vinyl chloride monomer, which, in turn, is polymerized to the major plastic, polyvinylchloride (PVC). Chlorine is also widely used as a bleaching agent, especially in the pulp and paper industry, and as a disinfectant. Chlorinated organic compounds, such as chlorinated ethanes and fluorocarbons, are used as intermediates in the manufacture of polymers like polyesters and urethanes. Caustic soda, on the other hand, has wide industrial applications in mineral processing, the pulp and paper industry, and textile and glass manufacturing operations. uf&turing processes. The reader may refer to (1-9) for details related to the technology. Electrochemical Principles Basic Electrochemistry Chlorine and caustic soda are the co-products of the electrolysis of sodium chloride (commonly termed "brine") following the overall reaction 2NaCI + 2HzO 2NaOH + Clzt + Hz1 (1) AF2yc = 100.9 Kcal Since eqn. (1)has a positive free energy, AF, energy has to be supplied in the form of electricity for the reaction to occur. The amount of electrical energy . . needed for the reaction de11v111l. U C . t.Iv~.tr.-lw< p,ir,,mt tvrs w I1 ,a: t urrt III d v ~ ~ - i t v , vt,.t:i~:t.,. t o . d ~J I I J, I I I I I I L I C i n ~ t v ~ i : iill, l d cell dr,i;~t. he comnonent electrochemical reactions involved in the overall reaction (1) are
-
+ 2e- (at the anode; E o s s y = 1.36V)
(2)
Hp + 2 0 H (at the cathode; E02s-C= 0.84 V)
(3)
2 C1- - * C12 2 Hz0
+ 2e-
-
The minimum voltage required for electrolysis to hegin for a given set of cell conditions, such as an operational temperature of 95"C, is the sum of cathodic and anodic reversible potentials ( E n ) ,and is known as the "thermodynamic decomposition voltage." This can be calculated to be 2.23 V a t 9 5 T for a caustic concentration of 3.5 M. However, chlor-alkali cells operate around 3.5 V a t a current density of approximately 230 mA ~ m and - not ~ a t 2.23 V, since in order to drive eqn. (1) a t an acceptable rate, additional voltage (driving force) is required to overcome cell resistance and electrode overvoltages
276
Journal of Chemical Education
(10). The total cell voltage for a diaphragm type chlor-alkali cell comprises the following components = 2.23 V = 0.06 V = 0.38 V
Thermodynamic decomposition voltage Anode overvoltage Cathode overvoltage Voltage drop across electrolyte due to gap, separator and hardware Total (for a current density of 230 mA cm+)
==V 3.50 V
For further details related to cell voltage components, see (5-9). Efficiency and Energy Consumption Faraday's Law states that 96,487 Coulombs (amp. see) are required to produce one gram equivalent weight of the electrochemical reaction product. This relationship determines the minimum electrical requirement for chlorine, caustic production in terms of kiloampere hours (KAh) as KAh --96,847 X 1000 metric tone of Clz 60 X 60 X 35.45 = 756 KAh - 96,487 X 1000 metric ton of NaOH 60 X 60 X 40 = 670.1 The current efficiency of an electrolytic process (q,,,,,,,) is the ratio of the amount of material produced to the theoretically expected quantities. The inefficiencies arise from the secondary reactions occurring a t the cathode, anode and in the hulk, and these are discussed below, under Diaphragm Cells. The voltage efficiency is the ratio of the thermodynamic decomposition voltage t o the actual cell voltage (V,,II). T h e energy efficiency (q,,,,,,), which is a product of the current and voltage efficiencies, can therefore he expressed as (for a diaphragm cell) 2.23vcurrent Vd However, the industry's popular terminology is the energy consumption expressed in terms of kilowatt hours (KWhr) per ton Cla or NaOH, and an estimate of this value requires a knowledge of cell voltage, current efficiency, and also the efficiency of the rectifier to convert AC power to DC (vrectifier). Thus, the energy consumption for producing C ~ ~ ( E CisL J 756 V,,a ACKWhr ECI,= 0 ~ ~ ~ ~ ~ ~metric t ? ~ ton ~ .C t12i 5 ~ ~ venerzy
=
(
)
Manufacturing Processes A distinguishing feature of chlor-alkali cells lies in the manner by which the products, viz., Cla and NaOH, are pre-
' Person to whom correspondence should be sent.
I
1
I
I
Figure 2. Schematic of an ion-exchange membrane cell.
Figure 1. Schematic of a diaphragm type chlor-alkali cell.
vented from mixing with each other to ensure generation of the desired products. Three types of chlor-alkali cells in commercial use are( 1) diaphragm cells, (2) mercury cells, and (3) membrane cells. These cells are schematically illustrated in Figures 1-3. Diaphragm Cells (Fig. 1 )
During electrolysis, chlorine is generated a t the anode following eqn. (2). Commercial anodes are typically ruthenium-based or platinumliridium-based coatings on titanium substrates. The chlorine evolved a t the anode first dissolves in the electrolyte until it is saturated and then evolves as bubbles of chlorine gas. Since the soluhility of chlorine is a strong function of temoerature, hieh temoeratures (-95-
.
.
"
of the solution. There are two ~ a r a s i t i creactions off-settina the anode ef-
Figure 3. Schematic of a mercury cell
oxidation of 0 ~ 1 - i o nto chlorite, eqn. (5).
+ 4e-
2H20 -* O2 t 4Ht 60CI-
+ 3HzO
-
+ 4CI- + 6H+ +
2CIO3-
(4) 3 O n + fie2
(5)
The oxygen contribution form these reactions is dependent on the nature of "anode material" and the pH of the medium-the current efficiency for oxygen being generally 1-3% using commercial anodes. If graphite anodes are used, another reaction leading to inefficiency is the oxidation of C to C02 as C
+ 2Hz0
-
COP
+ 2H2
(6)
The electrolyte from the anode compartment flows through the diawhraem to the cathode compartment (see Fig. 1).The
.
compartment hack-migrates to the anode compartment and reacts with dissolved chlorine (C12(S)) to form chlorate as described by equs. (71, (81,and (9).
+ OHHOCl + OH-
CMS)
2HOCl+ OCl-
-
+ C15 HzO + OCIHOCl
* CIO1-
+ 2Ht + 2CI-
(7)
(8) (9)
There are two reactions that influence the cathodic efficiency, namely the reduction of OC1- and ClOn- as given by
While these reactions are thermodynamically favorable, they are not kineticallv sienificant under normal oneratine conditions. Hence, the cathodic efficiency is usually high (395%)-the catholvte tvnicallv contains aonroximatelv 12% 9
-
Membrane Cells (Fig. 2) In these tvoes of cells, instead of using a diauhrazm, an plex copolymer of tetrafluoroethylene (TFE), perfluoro vinyl sulfonyl fluoride and perfluoro vinyl ether made by DuPont-and perfluorocarhoxylic acid membranes, made hy Asahi Chemical Company. Pure hrine, containing less than 0.1 ppm in Ca2+ and Mg2+ and prepared hy ion-exchange methods is fed into the anode compartment, and pure deionized water is fed into the cathode compartment. The electrochemical reactions a t the cathode and anode are the same as in the diaphragm cell. However, in ion-exchange membrane cells, the Na+ ions from the anode compartment are exchanged by the cation exchange membrane to the cathode comparment to form NaOH with the OH- ions produced a t the cathode. Since chloride ion migration is selectively hindered by an ideal cation exchanee membrane, it is uossihle to produce pure caustic, a t c o n c e n t h m s of up to 50%, containing negligible amounts of Cl- and Cl0:~-.However, in practice, the caustic concentrations obtained without significant loss in current efficiency are in the range of 10-35% depending on the nature of membrane used and operating conditions. The ultimate goal of membrane cell technology is to produce caustic of sufficient strength that the caustic evaporation stage (see the Volume 60
Number 4
April 1983
277
section helow on Unit Operations) can he completely eliminated. Mercury Cells (Fig. 3 )
While the anolyte-catholyte separation is accomplished in the diaphgram and memhrane cells using separators and ion-exchange memhranes, respectively, the mercury cell contains no diaphragm. Separation is achieved using the mercury cathode itself. Chlorine is generated a t the anode and Nai is discharged a t the cathode to form sodium amalgam which is passed into a second cell and reacted with water to form NaOH, Hz, and Hg as expressed by 2NaHg + 2HzO
-
2NaOH
+ H2 + Hg
(12)
The regenerated Hg is cycled back to the electrolyzer. Due to the damaging environmental effects of mercury, these cells are being phased out gradually Unit Operations in a Chlor-Alkali Plant The unit operations in a commercial chlor-alkali plant using diaphragm cells can he classified as (1)brine purification, (2) electrolytic cells, ( 3 )Hz and Clz collection, (4) caustic purification and salt removal. Brine Purification
The feed material for the manufacture of chlorine is saturated hrine, usually pumped from salt depos~ts,which typically contains 23-25% NaC1,1500 ppm Caz+, 40 ppm Mg2+, 4000 ppm SO$, and 40 ppm Fez+ and Fe". The presence Ca, Mg, and Fe is harmful to the performance of ashestosbased diaphraums . and ion-exchange memhranes since these metals would precipitate (as hydioxides) in the separator (where the pH is >7), leading to an increase in the ohmic dron across t h e diaphragm and, hence, an increase in ceil voltage. These impurities are normally removed by treating the impure hrine with catholyte containing -12% NaOH 15% NaC1, and saturated with flue gases (i.e., COz, etc.). This solution is again mixed with the catholyte to precipitate Mg(0H)a and Fe(OHI3. After settling these solids, which is facilitated by adding starch or a flocculating agent, the solution containing suspended material is nassed through nreu coated filters. he ciear solution, analyzing less than 10 ppm Ca2+, 1ppm Mg2+,4000 ppm S04z-, and 1ppm Fe3+, is fed into the anode compartment of the electrolytic cells.
+
.~
Electrolytic cells
The C12 gas from the anode compartment is first cooled by direct or indirect contact with water before drying wlth 98% Hi304 in a countercurrent series of operations. It is then compressed and liauified in pressure storage tanks made of special carbon s t e e f ~ h C12 e gas typically contains 97% C12and 1.5%Oz. Cell liquor contains approximately 12% NaOH 15% NaCl and is processed to produce 50 to 70% NaOH, free from Fe, C103-, O C 1 which are objectionable for certain end uses (e.g.,
