COMMERCIAL ELECTROLYTIC CELL FOR PERIODIC ACID PRODUCTION C H A R L E S L. M A N T E L L Department of Chemical Engineering, .\'ewark
College of Engineering, Newark 2, X. J .
Commercial cells were needed for production of dialdehyde starch. The design of such cells for the Northern Utilization Research and Development Division, Agricultural Research Service, U. S. Department of Agriculture, is given in sufficient detail, with bills of material, for reproduction, along with operating results.
N THE PRODUCTION PROCESS
as described by Pfeifer et al.
I (7-7) starch is reacted with periodic acid, under controlled
conditions, times, and temperatures. The starch is converted to a dialdehyde form, and the periodic acid is reduced to iodic acid, with the formation of small amounts of organic side products. After concentration, necessary because of dilution by wash waters, the iodic acid solution is cycled back to the electrolytic cell for regeneration to periodic acid, and then is available for treatment of another batch of starch. About 1% of the iodic acid is lost from the system, primarily in discarded wash waters which are too dilute for economic concentration. This loss is most economically replaced not by addition of purchased iodic acid, but by the electrolytic formation of the needed iodic acid from elemental iodine (4,5). The commercial cell should be adaptable to the formation of iodic acid from iodine as well as the conversion of iodic acid to periodic acid.
General Form of Cell Periodic acid can be formed from iodic acid in a two-electrolyte cell, where the acidic iodic acid solution is separated by a diaphragm from an alkaline catholyte containing no iodine compound. The iodic acid is the anolyte in the cell-that isj the electrolyte adjacent to the anode. The anode found best is composed of a 1y0nominal silver content lead on which an adherent film of lead peroxide has been formed. A satisfactory cathode material is carbon steel. I t is essential that the anolyte be kept as free as possible from all heavy metals, inasmuch as a number of these, such as nickel and chromium, are catalytic for the decomposition of periodic acid or alter the course of the reaction with starch. Therefore, in the circulation system of the anolyte through the cell, it must not come in contact with any metallic part such as pump rotors and casings, pipe, and the like. I n this cell all of these mechanisms are nonmetallic, being constructed either of rigid nonplasticized plastics or of glass.
Diaphragms Efficient diaphragms are a n essential component of every two-fluid electrolytic cell. I n the present instance many diaphragm materials were tried in small test units. Some of these were reasonably satisfactory in small-scale cells whose diaphragms did not exceed 1 foot by 1 foot. With larger areas there were problems of support but also problems of the mechanical failure of the diaphragms. 144
l&EC PROCESS DESIGN A N D DEVELOPMENT
Papers of the parchment type, made by either the zinc chloride or the sulfuric acid process, were disintegrated after a few hours and failed. Cotton, which had been useful in the early development of electrolytic metal cells, was not sufficiently resistant to the anolyte and catholyte. Its porosity, no matter what variants were employed in reference to yarn sizes, yarn types, weave construction, and tightness of such weave, was too high and allowed exchange of liquid volumes between the anolyte and catholyte. IVhen the situation was improved by the application of synthetic textiles whose chemical resistance was satisfactory, such as poly(viny1 chloride), dynel, Orlon, and polyethylene, again no matter what yarn size, type, construction, and tightness of weaving were employed, the porosities were too high as measured in volume exchange under pressure differences of 1 inch in the height of the anolyte or catholyte as against the other electrolyte. Calendering and partial fusion decreased the porosity but insufficiently for the needs of the cell operation. Ceramics, such as alumina, were satisfactory to a degree, but with increasing time increased in porosity and became unsatisfactory. I n addition, the construction of the ceramic and the mechanical properties in sizes larger than 1 square foot introduced so many manufacturing problems that to all intents and purposes these were unavailable in the sizes required for the final cell. Some interesting results were achieved with asbestos diaphragms which, by themselves, were too porous, but when impregnated, laminated, or impregnated and calendered after treatment with poly(viny1 chloride), poly(viny1 chloride) and phenol-formaldehyde resins, or combinations with polyethylene, showed interesting characteristics. However, the mechanical development of such diaphragms appeared to necessitate an inordinate amount of research time for the determination of all the variables of manufacture. Cooperative development programs with Pfaudler-Permutit in connection with the development of ion-exchange diaphragms led to materials which could be obtained in large sizes in sheet form and whose mechanical characteristics could be supplemented by supporting mechanisms. Another development program with the American Felt Co. on calendered diaphragms made of felted synthetic fibers, particularly poly(vinyl chloride) and polyethylene, was carried forward to the point where satisfactory chemical resistance was achieved, but more work was needed to satisfy the minimum porosity requirements. The diaphragm finally developed and used in the cell design \vas a specific ion-exchange unit diaphragm of Pfaudler-Permutit which showed satisfactory chemical resistance and minimum porosity. Intensification of the desirable
1. Anode
1 7G iig-Pb
Anolyte chamber 3. Support sheet 4. Diaphragm
RiKicl PVC
Table I. Component Specifications Current Density, Length X Width X Thickness, Electrical Amp./Sq. Inches Connection Cm . On busbar bolted 0.02 38 X 34 X l / g to end None None 36 X 36 X 2
Unplasticized PVC
36 X 36 X
Cationic permselective membrane Carbon steel
Composition
2.
5.
Cathode
I
ri
/O
Perforation None
Effective Area 30 X 30 inches both sides (2 X 6.25 sq. ft.) 6.25 sq. ft.
None
None
30 X 30 inches open 75 min., 90 max.
34 X 34 X
None
None
None
1.5 sq. ft. on 751. open basis 6.25 sq. ft.
38 X 34 X
Cu busbar bolted at end Kone
0.09
.4pprox. 75%
1.5 sq. ft. (one side)
l/8
None 30 X 30 inches 6.25 sq. ft. Catholyte Rigid PVC 36 X 36 X 2 open chamber Circulation rate, gallons per minute per cell chamber. Anolyte 4.4. Catholyte 2.0. Cell voltage 5-7. Initial compositions, grams/100 ml. Starting from elemental iodine A. Anolyte: 7-9 iodine, 3 sodium hydroxide. Catholyte: 5 sodium hydroxide. Starting from iodic acid B. Anolyte: 6-8 iomdic acid. Catholyte: 5 sodium hydroxide. p H : During conversion of iodine to iodate, pH should be maintained above 7.0 to prevent formation of free iodine in solution; after all iodide is converted, pH can be permitted to drop to 1 . 5 to 2.0. During conversion of iodic acid to periodic acid, pH should be controlled between 1.5 and 2.0, Temperature, 25-40” C. 6.
properties of these diaphragms, with further development of mechanical strength, is still going on. Microporous rubber was tried out, but was deficient in mechanical properties.
Table II. Bill of Materials for 2000-Ampere Cell No. Material Size Required Item
Cell chambers
1YC Ag lead alloy Rigid PVC
Support sheet
Rigid PVC
Diaphragm
Cathode
Cationic permselective membrane Carbon steel
Ifanifolds
Rigid PVC
Launders
Rigid PVC
Gaskets
Saran
End insulating plate Pressure plate
Rigid PVC
38 X 34 X l / 8 inch with 30 inchsquare area perforated equal to 75VG of area 5 inch diam. X 8 feet 2 ea. with 16-‘/2inch 2 ea. with 8‘/?-inch outlets, all with l-’/n-inch inlet 4 inch wide X 7 inch deep open trough, 8 feet long with 1‘/$-inch outlet 36 X 36 X 1 / 8 inch with 30 - inchsquare central opening 36 X 36 X l / 8 inch
Cast iron
As per drawing, with
2 4
24 24
Anode Anolyte and Catholyte Compartments
I t is advantageous, although not absolutely necessary, to have the catholyte-that is, the liquor adjacent to the iron cathodefree of all metals except traces of iron. The catholyte is 5 grams of sodium hydroxide per liter. Steel or cast iron pumps may be employed for 1iquor circulation, although for minimum contamination plastic pipe is preferred and used in the circulation systems. T h e cell is a system of anodes in parallel and cathodes in parallel, with anolyte frames and catholyte frames of the same size and construction in a gasketed arrangement so that both sides of each anode are employed. I n the cell described here, only one side of each cathode is used, the cathode chamber having a perforated cathode on each side, with a diaphragm adjacent to each cathode, and a perforated poly(viny1 chloride) sheet serving as a “billow plate” to hold the diaphragm in place. The anolyte and catholyte frames are 3 feet square by 2 inches thick with a 30-inch-square central opening. They are fabricated of rigid poly(viny1 chloride) unplasticized, type I, and have two 45’ corner entrances and two 45’ corner exits.
Billow Plates Even with every care in design and operation to avoid pressure differences or hydraulic heads between the anolyte and catholyte. in the larger sizes all of the diaphragms needed support against billowing-that is, bulging of the diaphragm because of uneven mechanical properties, particularly tensile strength. These properties were not anisotropic, being better in the direction of fabrication than a t right angles to this direction. even when differences in strength as a result of method of forming were eliminai.ed or carefully controlled during fabrication of the diaphragm. I t was therefore necessary to support the diaphragm mechanically by perforated polyvinyl sheets which are termed “billow plates.”
38 X 34 X
inch
8
36 X 36 X 2 inch with 30 inchsquare central opening, fitted with inlet and outlet ports 36 X 36 X 1/8 inch punched, central 30 inches square, to equal 75Y0 open 36 X 36 X inch
25
l/8
-
-
Insulators
Porcelain
Electrode bar connectors Electrode frame Bus bar
Copper
pressure bars 5 X 18 inches with end fittings 1 X l / g X 34 inches
Rigid PVC
As per drawing
Copper coil
l/16
Valves Tees Tubing Pipe, misc.
Rigid PVC Rigid PVC Tygon Rigid PVC
VOL.
4
2
66
2
inch X 6 inches X 150 feet
‘/L
inch diaphragm inch inch I.D. inch
l / ~
”4 ’/?
1
NO. 2
1 50 25 225 feet 70 feet
APRIL 1 9 6 2
145
T h e perforations of the billow plate were not critical as long as the holes were less than '/2 inch in diameter and the perforations were such that a minimum of 7570 and a maximum of 90% of the diaphragm area was exposed. This was achieved by staggering drilling holes in alternate lines of holes. I n the final cell construction, the diaphragm was supported by the perforated cathode on one side and the perforated unplasticized PVC billow support plate on the other.
This unit could be expanded both forward and backward from the original-for example, 1234565432123456543212345654321 or to whatever extent desirable. The anolyte and catholyte chambers are the same structurally and dimensionally and are interchangeable, so that 2 and 6 are the same. However, the chamber arrangement is such that there are two anolyte chambers for each catholyte chamber and the anolyte volume is twice that of the catholyte.
Cell Arrangement
The complete cell consists of multiples of this basic unit operated in parallel. For convenience they are held together in a Screw press arrangement. The basic cell arrangement is:
Component Specifications
The sizes, capacities, and operating values are tabulated in Table I. The bill of materials for a 2000-ampere cell for iodate (or iodic acid) formation from iodine and caustic, and periodic acid from iodic acid, is given in Table I1 along with frame and cell volumes, as well as manifold dimensions. The bill of materials is for the arrangement:
1. Solid silver-lead anode plate with gasket 2 . Anolyte chamber with gasket 3. Support PVC sheet perforated 4. Diaphragm with gasket 5. Perforated steel cathode with gasket 6 . Catholyte chamber 5. Perforated steel cathode with gasket 4 . Diaphragm with gasket 3. Support PVC sheet,. perforated 2. Anolyte chamber with gasket 1. Solid silver-lead anode with gasket 1-2-3-4-5-6-5-4-3-2-1
6543212345654321---1---1---1---1--1--123456:
but a bill of materials for 123456543212345654321 - - 1 - - 1 - -1 is obvious and can be readily calculated. As required to malntain pH
Iodate solution for reoxidation
I
I_
J
H~ vented Cooling
-4
4
+ Coollnq
t Anolyte Circulating System
-
I
!
I
'
-
Reclrculate
-
Recirculate
(Periodic Acid)
I
Catholyte Circulation System
4
A
I
Purification
'I
ELECTROLYSIS CELLS
,
Tanks Wash Water
Evaporator Cenwlfuges
Iodate rich wash
Dilute wash water
.1
Dialdehyde Starch Cake
Sewer I
Figure
1.
I
Schematic flowsheet, showing cell used in production of dialdehyde starch
146 l & E C P R O C E S S D E S I G N A N D D E V E L O P M E N T
-- 1-- 1
k G
- GASKET
PP- PResaunE PLATE
9P- SUPPORT PLATE
Figure 2.
Flowsheet
A4simple flowsheet indicating circulation systems and external reaction of cell liquors with starch is shown in Figure 1. By-product Removal
By-products build-up in used oxidant after cyclic use, particularly formic acid and its derivatives. These, if recycled, reduce the oxidation efficiency. An effective removal method consists of treating liquors with 0.1% of their weight of an
Cell assembly
activated adsorption carbon of the glycerol decolorizing type, granular in form, in a column a t room temperature. T h e minimum contact time would be 3 minutes. Equivalent treatment with powdered activated carbon with agitation can be substituted, followed by filtering. Purification treatment is necessary every sixth cycle as a minimum, or better continuously. An expanded assembly is shown in Figure 2 . The detail parts of the cell assembly are shown in Figure 3, the electrolyte frames in Figure 4, and the cell support plates in Figure 5. Performance data for iodine to iodate and for iodate to periodate are given in Table 111.
Table 111.
l-+-l Et.ccraocc
Figure 2;.
FRAME ( R \ G # P~V C )
Performance Data
Anode Effective anode Anolyte, g. j1. Iodine Iodic acid Sodium hydroxide Sodium sulfate PH Temp., O C. Current density, amp./sq. cm.
Cathode Catholyte, g./l. Sodium hydroxide Sodium sulfate Current density, amp./sq. cm. Total current, amp. Cell voltage Current efficiency,
Iodine to Iodate, 7 % 'Q Lead, Solid PbOn
PeAcid, 77~- 4 Lead, ~ Soizd
Iodic riodic
IO
80-90
60-80
30-50
0.2-0.3
50-75 2.5-3.5 25-40 0.02-0.03
Perforated carbon steel 50-60
0.9-0.14 2000 5-6 75-90
20-30 0.9-0.14 2000 5.5-7 80-90
Electrode assembly VOL. 1
NO. 2
APRIL 1 9 6 2
147
I Figure 4.
Electrolyte frame
I 39'
NGTE: ALL
RIBEIIN~ IS I * T H ! C & M ~ T E R II S~ L C a s r IRON
Acknowledgment
Figure 5.
The development group consisted of Robert Halfon, chemical engineer; Allyn Heit, chemist; Daniel Bauer, junior mechanical engineer; and Walter Dzingala, chemical engineer, with assistance on test runs by Robert Casciano, chemical engineer. literature Cited
(1) Conway, H. F., Sohns, V. E., IND.ENG.CHEM.51, 637 (1959). (2) Dvorch, W., Mehltretter, C. L. (to L.S. A., Secretary of Agriculture), U. S. Patent 2,648,628 (Aug. 11, 1933). (3) Mehltretter, C . L. (to U. S. A., Secretary of Agriculture), Zbid., 2,713,553 (July 19, 1955).
Cell support plate
(4) Zbid., 2,770,589 (Nov. 13, 1956). (5) Zbid., 2,830,941 (April 15, 1958). (6) Mehltretter, C. L., Rankin, J. C., Watson, P. R., IND.ENG. CHEM.49, 350 (1957). (7) Pfeifer, V. F., Sohns, V. E., Conway, H. F., Lancaster, E. B.,Dabic, S., Griffin, E. L., Jr., Zbid., 52, 201 (1960). RECEIVED for review February 17, 1961 ACCEPTEDOctober 18, 1961 Work done under contract with the U. S. Department of .4griculture and authorized by the Research and Marketing Act. Contract supervised by the Northern Utilization Research and Development Division, Agricultural Research Service.
CONTINUOUS AUTOMATIC MANUFACTURE OF HYPOCHLORITE SOLUTIONS FROM SODA ASH ROBERT L. M C B R A Y E R AND N E W L I N S. N I C H O L S Chemical Engineering Research, Wyandotte Chemicals Cor)., Wyandotte, Mich.
The success of continuous automatic systems for the manufacture of hypochlorite solutions from caustic soda and milk of lime led to experimental work on the manufacture of hypochlorite solutions from soda ash. Potential applications for soda ash-based solutions exist in the pulp, textile, and metallurgical industries. Experimental data demonstrated the applicability of oxidation potential as a means of control in a continuous system Reproducible data gave a correlation between oxidation potential and available chlorine concentration. Stability of the solutions as a function of temperature and concentration was studied.
HE USE OF OXIDATIOS POTESTIAL to control the continuous Tautomatic manufacture of sodium hypochlorite solutions was proposed by Pye in 1950 (70). Since that time, information has appeared on experimental work and commercial installations of such systems to produce sodium and calcium hypochlorite (7-3, 8, 72). The sodium hypochlorite solutions have been made from caustic soda. Continuous automatic operation offers two primary advantages over batch operation-reduction of manpower and reduction of required production space. Less evident, but economically important advantages are-a more stable and uniform product and the reduction of the danger of overchlorination.
148
l & E C PROCESS D E S I G N AND DEVELOPMENT
The success of continuous automatic units producing hypochlorites from caustic soda and milk of lime led to experimental work using soda ash. Potential applications for soda ash-based hypochlorite exist in the pulp, textile, and metallurgical industries. In the pulp and textile industries, it may be desirable to use a bleach with a comparatively low p H (between 8 and 9) (73). Soda ash-bleach solutions are buffered to a p H of' 8.5 to 9. In addition, the caustic removal problem is eliminated. For certain metallurgical applications, such as the separation of cobalt and nickel ( 7 7 ) , soda ash-based hypochlorite could supply the chlorination agent and would neutralize the liberated acid.
