Operation and regeneration of permselective ion-exchange membranes in brine electrolysis cells

Methods of membrane regeneration of permselective ion-exchange membranes of a brine electrolysis cell are greatly improved when the cells are fed brine which contains little or no carbon dioxide, carbonate anions or bicarbonate anions during normal electrolysis and when the methods of membrane regeneration are those wherein at least one liquid solution contacts the membrane and the pH of that solution is below that of the pH of the electrolyte in contact with the membrane during normal electrolysis.

BACKGROUND OF THE INVENTION 
1. Field of Invention 
This invention relates to rejuvenating permselective ion-exchange membranes 
employed as selective barriers between the anolyte and catholyte of brine 
electrolysis cells. 
2. Definitions Used Herein 
"Carbon oxide" is used herein to mean carbon dioxide, or carbonic acid, or 
a carbonate or bicarbonate of an alkali metal or an alkaline earth metal 
(including magnesium), or a combination of any of these. 
"Cathodic protection voltage" is defined herein to mean a cell voltage 
drop, as measured between the anode to the cathode of a cell, which is 
just large enough to cause reduction of water to hydrogen and hydroxyl 
ions at the cathode. Such a cell voltage is, therefore, capable of 
providing cathodic protection for the cathodes to prevent them from 
corroding. 
3. Discussion of Prior Art 
The electrolysis of chlorides of monovalent cations (including lithium, 
sodium, potassium, rubidium, cesium, thallium and tetra methyl ammonium) 
with cation selective membranes is well known for the production of 
chlorine and the hydroxides of such cations, particularly with respect to 
the conversion of sodium chloride to chlorine and caustic. Representative 
of such permselective cation exchange membranes are the perfluorosulfonic 
acid membranes made and sold by the E. I. duPont de Nemours & Co., Inc., 
under the tradename, Nafion, and the perfluorocarboxylic acid membranes of 
the Asahi Glass Industry Co., Ltd. of Tokyo, Japan. See U.S. Pat. No. 
4,065,366 to Oda et al for a description of the latter carboxylic acid 
type membranes. 
In the process of electrolyzing sodium chloride into chlorine and caustic 
wherein such membranes are used, the membrane divides the cell into anode 
and cathode compartments. Brine is fed to the anode compartment and water 
is fed to the cathode compartment. A voltage impressed across the cell 
electrodes causes the migration of sodium ions through the membrane into 
the cathode compartment where they combine with hydroxide ions (created by 
the splitting of water at the cathode) to form an aqueous sodium hydroxide 
solution (caustic). Hydrogen gas is formed at the cathode and chlorine gas 
at the anode unless a depolarized cathode is used. (When a depolarized 
cathode is used, H.sub.2 gas is not generated.) The caustic, hydrogen and 
chlorine may subsequently be converted to other products such as sodium 
hypochlorite or hydrochloric acid. 
It is known that over a long period (&gt;100 days) of use of such 
membrane-type cells, there occurs an undesirable increase in the cell 
voltage and electrical energy consumed per unit (e.g. ton) of product 
made. The prior art in general has attributed this undesirable increase to 
the fouling of the membrane by hardness and other multivalent cation 
impurities contained in the brine feed..sup.1 The calcium cation in 
particular has been singled out as the most damaging impurity. 
FNT .sup.1 See U.S. Pat. No. 3,793,163 to R. S. Dotson (1974); The Asahi 
Chemical Membrane Chlor-Alkali Process, page 5 of a paper presented by 
Maorni Seko of Asahi Chemical Industry Co., Ltd., of Tokyo, Japan, at The 
Chlorine Institute, Inc., 20th Chlorine Managers Seminar, at New Orleans, 
Louisiana on Feb. 3, 1977; Effect of Brine Purity on Chlor-Alkali Membrane 
Cell Performance, a paper originally presented by Charles J. Molnar of E. 
I. duPont de Nemours & Co., Inc., and Martin M. Dorio of Diamond Shamrock 
Corporation at The Electrochemical Society Fall Meeting held October, 
1977, at Atlanta, Georgia; The Commercial Use of Membrane Cells in 
Chlorine/Caustic Plants, pages 6-9 of a paper presented by Dale R. Pulver 
of Diamond Shamrock Corporation at The Chlorine Institute's 21st Plant 
Manager's Seminar, at Houston, Texas, on Feb. 15, 1978; Nafion.RTM. 
Membranes Structured for High Efficiency Chlor-Alkali Cells, a paper 
presented by Charles J. Hora of Diamond Shamrock Corporation and Daniel E. 
Maloney of E. I. duPont de Nemours & Co., Inc., at The Electrochemical 
Society Fall Meeting, October, 1977, Atlanta, Georgia; U.S. Pat. No. 
4,115,218 to Michael Krumpeit (1978); U.S. Pat. No. 4,073,706 to Zoltan 
Nagy (1978); U.S. Pat. No. 3,988,223 to S. T. Hirozawa (1976); U.S. Pat. 
No. 4,204,921 to W. E. Britton et al (1980); U.S. Pat. No. 4,202,743 to 
Oda et al (1980); and U.S. Pat. No. 4,108,742 to Seko et al (1978). 
To prolong the useful life of these membranes many techniques have been 
developed to reduce the amount of contaminants in the brine which foul the 
membrane. Many of the references cited above give methods for further 
reducing the multivalent cation impurities contained in the cell's feed 
brine. A very recent technique discovered for reducing membrane fouling 
centers around using brine which contains very little carbonate anions or 
carbon dioxide (carbon oxides) in the feed brine. This technique is 
disclosed in a recently filed patent application entitled, "Membrane Cell 
Brine Feed", having Ser. No. 248,670 and a filing date of Mar. 30, 1981, 
and having as inventors Bobby Ray Ezzell and Harry Stevens Burney, Jr. The 
latter named inventor is a co-inventor of the instant invention, and this 
prior patent application is incorporated by reference herein as is set 
forth at length for purposes of prior art teachings and for the new 
techniques taught therein pertaining to obtaining improved brine for 
electrolysis in an electrolytic cell which employs a permselective 
membrane disposed between the anode and cathode. 
To further prolong the life of these permselective membranes, several 
techniques for regenerating them in place have been developed. For 
example, U.S. Pat. No. 4,115,218, by Michael Krumpelt (issued Sept. 19, 
1978) teaches that such membranes can be rejuvenated by merely reducing or 
interrupting the cell current or voltage alone or in combination with a 
concomitant flushing of the catholyte portion of the cell. This process is 
limited to the instance where the brine fed to the cell during its normal 
operation contains a calcium content which is less "than is ordinarily 
used". 
Another example of membrane regeneration is found in U.S. Pat. No. 
3,988,223, by Stanley T. Hirozawa (issued Oct. 26, 1977). This patent 
teaches unplugging the membrane by a process which comprises maximizing 
the brine head, adding a chelate or chelate forming agent to the anolyte, 
shunting the electrical current to the cell, flushing the cell, and 
removing the shunt. 
A third example of membrane regenerating is found in U.S. Pat. No. 
4,040,919, by Jeffrey D. Eng (issued Aug. 9, 1977). This patent teaches 
these membranes can be regenerated by increasing the acidity of the 
anolyte, diluting the electrolyte located immediately adjacent to the 
anolyte and separated from the anolyte by a membrane, reducing the current 
density, and maintaining such conditions during electrolysis for a period 
sufficiently long to rejuvenate the membrane. Note, usually the 
electrolyte referred to in this patent can be the catholyte, but it does 
not have to be. It can be an electrolyte located between two spaced 
membranes which are both located between an anode and a cathode. 
These membrane regenerating techniques are an improvement over the 
alternative of replacing the membranes, but only marginally so in many 
instances. Generally these techniques produce only a short term 
improvement, particularly short term improvements insofar as are concerned 
the cell voltage and cell energy requirement (unit of energy used to make 
a unit of cell product). 
It is not certain why these membrane regenerating techniques usually 
produce only short term improvements, but it seems in accordance with the 
discovery of the present invention that these techniques can readily 
remove some salts from the membrane, but can remove substantial amounts of 
impregnated calcium carbonate only at the expense of doing considerable 
damage to the membrane. It would be advantageous to overcome these 
deficiencies, and the method of the present invention at least partially 
does. 
SUMMARY OF THE INVENTION 
This invention relates to a method of operating and regenerating an 
electrolysis cell which electrolyzes an aqueous alkali metal halide 
solution (a brine) to a halogen at the anode of the cell and to an alkali 
metal hydroxide at the cathode of the cell. The particular cells for which 
this method is particularly useful are those which contain a permselective 
ion-exchange membrane so disposed between the anode and cathode as to form 
a selective barrier between the anolyte and catholyte, thereby separating 
the space around the anode into an anolyte compartment and the space 
around the cathode into a catholyte compartment. This method comprises the 
combination of steps of: 
A. feeding to and electrolyzing in such a cell a brine which contains no 
more than about 5 ppm hardness (expressed as ppm calcium) and no more than 
about 70 ppm "carbon oxide" (expressed as ppm CO.sub.2) during at least 
50% of the cell's normal electrolysis operation, with said maximum 
concentrations of "carbon oxide" and hardness occurring prior to, or at 
least immediately prior to, the brine's becoming part of the cell's 
anolyte; and 
B. regenerating the membrane after it has eventually become at least 
partially fouled with compounds of multivalent cations from the brine fed 
to the cell during the cell's normal electrolysis step (Step A above) by 
contacting the membrane on at least one of its sides with a solution 
capable of dissolving the multivalent cation compounds fouling the 
membrane for a time sufficient to dissolve a substantial amount of said 
compounds fouling said membrane. Preferably both sides of said membranes 
are contacted. In any event the pH of the solution is maintained below the 
pH of the electrolyte which was in contact with that side of the membrane 
during the normal electrolysis step (Step A above) for a time sufficient 
to dissolve most of the compounds of polyvalent cations plugging and/or 
fouling the membrane. 
Halides are taken to mean their ordinary meaning herein, i.e. primary 
compounds of the halogens. Examples are sodium chloride, potassium 
chloride, sodium bromide and the like. 
Membranes have been found to be much better regenerated with less damage 
done to the membrane using the above method of cell operation and 
rejuvenation. 
Preferably the membrane is regenerated in place in the cell. In this case 
reducing the pH in Step (B) above can be achieved by a number of methods. 
The current density and/or cell voltage can be significantly reduced or 
completely cut off. Increasing the flow rate of water to the catholyte 
compartment over that rate used during normal cell electrolysis (Step A) 
will reduce the catholyte pH. Adding more acid to the anolyte compartment 
or brine being fed to the anolyte compartment will reduce the pH in the 
anolyte compartment. Other methods of achieving the lowering of pH 
required by Step (B) above will readily occur to those skilled in the art 
if it is kept in mind that the object of reducing the pH is to reduce the 
pH inside the membrane to dissolve the foreign salts impregnated therein 
by maintaining a liquid solution in contact with the membrane on one or 
both sides to receive these salts when dissolved. 
A further feature of this invention is the protection of the cathodes from 
corrosion during the membrane regenerating step (Step B above). This can 
be achieved by the addition of corrosion inhibitors to the catholyte 
compartment and/or reducing the cell voltage to the "cell cathodic 
protection voltage" defined above. 
A yet further feature of this invention is that if the membrane is dried 
after the contaminating salts have been dissolved from it in Step (B) 
above, the membrane regeneration is further enhanced.

DETAILED DESCRIPTION OF THE INVENTION 
This invention is the discovery that better membrane regenerations can be 
obtained by operating the cell with certain newly discovered brine 
conditions. These newly discovered brine feed conditions are that the 
brine fed to the cell's anolyte compartment have no more than about 70 ppm 
"carbon oxide" (as defined above and expressed as ppm CO.sub.2) prior to 
the brine feed becoming part of the anolyte. In the anolyte virtually all 
of the "carbon oxide" is or becomes carbon dioxide, and is swept from the 
cell without harming the membrane. It is a theory of the inventor of this 
co-pending application that a residual of the carbon dioxide close to the 
membrane in the cell's anolyte chamber is in the form of carbonate anions. 
It is a further theory that a very small, but significant, part of these 
residual carbonate anions react with calcium and are deposited on and in 
the membrane. That invention, of course, is not limited to those theories, 
but it is the only explanation of which the inventors can think which 
explains the significant improvements they discovered when feeding such 
low "carbon oxide" containing brine to membrane-type brine electrolysis 
cells. The improvements discovered for use of such low "carbon oxide" 
containing brine are that there is a substantially less increase in cell 
voltage and electrical energy consumed per unit of product made over a 
long term use of a membrane-type brine electrolysis cell, without having 
to resort to such drastic and expensive brine preparation techniques such 
as the one taught wherein the calcium concentration must be reduced to 
less than the extremely low concentration of 0.08 ppm., U.S. Pat. No. 
4,202,743 to Yoshio Oda et al (issued May 13, 1980). 
As is taught in the above cited co-pending aplication incorporated by 
reference herein, there are more desirable parameters for the brine feed 
than the upper limit of 70 ppm "carbon oxide" (expressed as ppm CO.sub.2). 
For example the less "carbon oxide" present, the better the cell performs. 
Thus brine feed containing less than about 50 ppm "carbon oxide" is better 
than that containing 70 ppm; brine containing less than about 30 ppm is 
better than that containing 50 ppm; brine containing less than 10 ppm is 
better than that containing 30 ppm; and brine containing less than 2 ppm 
is very much to be preferred. Also brine which has a low hardness content 
(expressed as ppm calcium) in addition to having a low "carbon oxide" 
content was discovered to produce even better results. Brine containing 
less than about 5 ppm hardness is acceptable; brine containing less than 
about 3 ppm hardness is preferred; and brine containing less than about 
1-2 ppm hardness is even more preferred. The pH of the brine after it 
becomes anolyte was also found to have a significant effect on cell 
performance. A pH of less than about 4 is acceptable; a pH of less than 
3.0 is preferred; and a pH of about 2.0 is most preferred. 
The low "carbon oxide" content of this brine can be achieved by several 
methods. One is not to place it there in the first instance, but the most 
practical method is to remove it after using a conventional brine 
treatment wherein: (a) sodium carbonate (in molar excess with respect to 
the calcium present in the brine) is added to the brine to form insoluble 
forms of calcium carbonate and sodium hydroxide (in molar excess with 
respect to the magnesium present in the brine) is added to the brine to 
form insoluble forms of magnesium; and (b) these insoluble forms of 
calcium and magnesium are substantially all then separated from the brine 
leaving a brine containing the excess amounts of carbonate and hydoxide 
anions. This conventionally treated brine can then be treated with a 
sufficient amount of mineral acid, preferably hydrochloric acid, to 
convert the carbonate anions to carbon dioxide. This carbon dioxide can be 
removed by allowing it to set for a few days much like an opened bottle of 
a carbonated soft drink; or it can be removed more rapidly by agitation 
such as shaking or stirring; or more rapidly by a gas purge with an 
innocuous gas such as chlorine gas, air, nitrogen, or the like; or even 
more rapidly by a combination of agitation and gas purge. 
The hardness can also be reduced by methods such as contacting the brine 
with chelating ion exchange beds, or solvent extraction techniques. 
The anolyte pH can be lowered and controlled by methods such as adding 
hydrochloric aid and/or flow controlling the brine to the cell. 
Better appreciation of the present invention can be obtained by those 
skilled in the art from a study of the following six examples. The first 
two examples are examples of prior art while the latter four are examples 
of the present invention. The two prior art examples both show the 
inferior regenerative effect obtained by regenerating membranes after they 
had been fed brine containing relatively normal concentrations of "carbon 
oxide" during the normal cell electrolysis step preceding the membrane 
regeneration step. In the first of these prior art examples, the "carbon 
oxide" was predominately in the form of carbonate anions 
(CO.sub.3.sup.--), whereas in the second prior art example, the "carbon 
oxide" was predominately in the form of entrained carbon dioxide gas. The 
pH of the brine feed determines what forms the "carbon oxide" will take. 
Before presenting these examples, however, it is useful to present a set of 
definitions of cell performance and a description of the type of cell used 
in all six examples. 
DEFINITION OF CELL PERFORMANCE 
One parameter which is important in considering a cell's energy performance 
is the strength of the caustic produced, for the more concentrated the 
caustic produced, the less energy is later required in evaporating water 
from the caustic after it has left the cell and is being concentrated. The 
purity of the caustic soda product is also important to over-all process 
economics. Preferably sodium chloride and sodium chlorate in the caustic 
are maintained as low as possible. The actual level of these impurities is 
a function of cell operating parameters and the characteristics of the 
membrane. Over the life of a membrane cell these impurities are preferably 
maintained at the same level as when the cell was new. 
The two other parameters required for a complete energy view of the overall 
process, particularly over a long period of time, are current efficiency 
and cell voltage. Cell voltage is defined to be the electrical potential 
as measured at the cell's anode connection to the power supply and the 
cathode connection to the power supply. Cell voltage includes the chemical 
decomposition voltages and the IR associated with current flowing through 
electrodes, membrane and electrolytes. 
Current efficiency is a measure of the ability of the membrane to prevent 
migration into the anode compartment of the caustic produced at the 
cathode. Herein it is also referred to as caustic efficiency and NaOH 
efficiency. Caustic efficiency is defined as the actual amount of caustic 
produced divided by the theoretical amount of caustic that could have been 
produced at a given current. The most common method of comparing the 
performance of an electrolytic process combines both current efficiency 
and voltage into a single energy term. This energy term is referred to as 
the cell's "energy requirement", and is defined to be the amount of 
electrical energy consumed per unit of NaOH produced. It is usually 
expressed in killowatt hours (KWH) of electricity consumed per metric ton 
(mt) of NaOH produced. The method of determining this energy term is the 
multiplication of voltage by the constant 670 killoampere-hours, and 
divided by the current efficiency. Lower current efficiency decreases the 
quantity of NaOH produced (mt), and higher voltage increases the quantity 
of KWH used; thus the smaller the "energy requirement" value KWH/mt, the 
better the performance of the cell. 
CELL TYPE USED IN EXAMPLES 
The examples set forth below were run in laboratory size cells like that 
depicted in the drawing. These cells had an anolyte compartment 10 and a 
catholyte compartment 12. These two compartments were separated by a 
vertically disposed, permselective cation exchange membrane 14. The 
membrane was sealed between anode frame 20 and cathode frame 22 by gaskets 
(not shown) located on either side of membrane 14. Gasket 6 represents the 
gasket sealing means used between anolyte compartment 10 and catholyte 
compartment 12. Near membrane 14 was disposed a vertical, parallel, 
flat-shaped anode 16. On the opposite side of membrane 14 was disposed a 
vertical, parallel, flat-shaped cathode 18. Anode 16 was an expanded-metal 
sheet of titanium having a TiO.sub.2 and RuO.sub.2 coating. Cathode 18 was 
made of woven-wire mild steel. Of course, other type cathodes can be used 
such as low overvoltage cathodes. During regeneration, it is very 
important to protect these low overvoltage cathodes from corrosion such as 
by the method employed in Invention Example 4 on its 257th day as 
described below. 
The mechanical supports and D.C. electrical connections for anode 16 and 
cathode 18 are not shown as they would serve more to obscure the drawing. 
Suffice it to say that anode 16 and cathode 18 were mechanically supported 
by studs which passed through the cell walls and to which were attached 
D.C. electrical connections necessary to conduct current for electrolysis. 
The electrical power passed through the cell was capable of being 
regulated so that a constant current density per unit of electrode 
geometrical area--i.e., amperes per square inch (ASI)--could be maintained 
during normal cell operation. 
Also not shown are the flow devices used to control the cell flow rates. 
The cells were equipped with a glass immersion heater (not shown) in the 
anolyte compartment in order to maintain the cell at an elevated 
temperature. 
Basically the cell frame was made of two types of materials. The anolyte 
side 20 was made of titanium so as to be resistant to the corrosive 
conditions inside the anolyte compartment 10. The catholyte side 22 was 
made of acrylic plastic so as to be resistant to the corrosive caustic 
conditions inside the catholyte compartment 12. The necessary entry and 
exit ports for introducing brine and water and for removing H.sub.2, 
Cl.sub.2, spent brine, and caustic soda are shown in the drawing. 
Anolyte side 20 has port 24 for the brine feed to the cell anolyte chamber 
10. Port 26 provided an outlet for the chlorine generated in the anolyte 
compartment 10, while port 28 provided an exit for spent brine to leave 
the anolyte compartment 10 during normal cell operation. 
Catholyte side 22 of the cell had port 30 as an inlet for water to the 
catholyte compartment 12. Outlet port 32 provided an exit for the hydrogen 
gas generated in the catholyte compartment 12, while port 34 provided an 
exit for liquid caustic also generated in catholyte compartment 12 during 
normal cell operation. 
During normal cell operation the cell in each of the following examples 
electrolyzed brine at a constant current density, a constant temperature, 
and a constant caustic concentration during the long electrolysis step(s) 
before (and between) the membrane regeneration step(s). These conditions 
however, were not the same in each example, nor was the membrane used the 
same in each example. When concentration percentages are given, they are 
intended to be weight percentages. 
PRIOR ART EXAMPLE #1 
A lab cell like that described above was operated at 1.0 ASI, 80.degree. 
C., 12-13 wt. % NaOH in the catholyte, 18-19 wt. % NaCl in the anolyte, 
and at an anolyte pH of about 4.0-4.3. This cell was operated with brine 
that contained from 0.4 to 0.9 gram/liter (gpl) Na.sub.2 CO.sub.3. Use of 
brine with this high a carbonate ion concentration is representative of 
prior art operations, but it is not representative of the method of the 
present invention. 
The permselective membrane employed was Nafion.RTM. 324 obtained from E. I. 
duPont de Nemours & Co., Inc. This membrane was a composite of two layers 
of sulfonic acid polymer and a reinforcing scrim. Similar membranes are 
described in U.S. Pat. No. 3,909,378. 
The sodium chloride brine was obtained from brine wells located near Clute, 
Tex. This brine was treated so that it was 25.5 wt. % NaCl and contained 
1-2 ppm hardness (calcium and magnesium content expressed as ppm Ca). 
This brine was treated by what is referred to as conventional brine 
treatment, i.e. that type of brine treatment which has conventionally been 
used in preparing brine for electrolysis in asbestos diaphragm-type 
electrolysis cells for the past many years. Conventional brine treatment 
comprises adding Na.sub.2 CO.sub.3 and NaOH to the brine in amounts such 
that the Na.sub.2 CO.sub.3 is in a stoichiometric excess of at least about 
0.4 gpl (grams per liter) with respect to the calcium present in the brine 
and such that the NaOH is in a stoichiometric excess of at least about 0.2 
gpl with respect to the Mg in the brine. Addition of these excesses of 
Na.sub.2 CO.sub.3 and NaOH cause substantially all of the Ca and Mg to 
form the insolubles, CaCO.sub.3 and Mg(OH).sub.2. These insolubles are 
then removed from the brine feed, usually by settling and filtration 
techniques, leaving in the brine the excesses of Na.sub.2 CO.sub.3 and 
NaOH as well as a small residual of Ca and Mg as hardness. (This small 
residual of hardness is on the order of from about 1 ppm to about 5 ppm, 
expressed as ppm Ca). 
In this example, the brine was treated by this conventional brine process 
to reduce the brine hardness to a level of 1-2 ppm expressed as Ca. The 
procedure followed to obtain this hardness level was as follows: Na.sub.2 
CO.sub.3 and NaOH were added to the untreated brine at the well-sight. The 
brine was then settled and filtered to reduce the hardness to about 1-2 
ppm Ca. The Na.sub.2 CO.sub.3 was added in stoichiometric excess with 
respect to the Ca present, so that the filtered brine contained about 0.4 
to 0.9 gpl (grams per liter) Na.sub.2 CO.sub.3. The NaOH was added in 
stoichiometric excess to the Mg present, so that the filtered brine pH was 
about pH 10-12. Normal electrolysis was started and continued for about 
282 days using this brine. 
On the 283rd day after initial start-up, the membrane was regenerated in 
situ according to the following procedure. Cell voltage was reduced by 
turning the cell operating current completely off. Aqueous HCl was added 
to and mixed with the feed brine to obtain an acidified brine with a pH of 
0.1 to 1.0. This acidified-brine was fed to the anolyte compartment of the 
cell at a flow rate that was the same as that during normal electrolysis 
(approximately 9 milliliters per minute). The same water flow rate as used 
during normal cell operation was fed to the catholyte compartment 
(approximately 33/4 milliliters per minute). The membrane in this cell was 
regenerated in this manner for 20 hrs. at a room temperature of 25.degree. 
C. The cell was then restored to normal operation at 1.0 ASI, 80.degree. 
C., 12-13% NaOH, 18-19% NaCl in the anolyte, and an anolyte pH of 4.0-4.3. 
The data in Table I summarize the cell performance before and after the 
membrane regeneration procedure. 
In this and the following tables, "DOL" indicates the number of days on 
line, which is approximately equivalent to the number of days that the 
cell was operated. A few times the cells were shut down because of loss of 
electrical power, and a hurricane evacuation caused a two day shut-down. 
Thus DOL is not exact. "Cell Volts", "NaOH Efficiency" and "Energy 
Requirement" are the same as defined earlier. "Salt in Caustic" is the 
weight percent NaCl in the caustic soda product expressed on a 100% NaOH 
basis. For example, all the data in this table are at about 12 wt. % NaOH, 
and 100% NaOH divided by 12% NaOH, multiplied by the actual wt. % NaCl in 
this 12% NaOH equals the wt. % NaCl on a 100% NaOH weight basis. 
TABLE I 
______________________________________ 
Cell NaOH Salt in 
Energy 
DOL Volts Efficiency Caustic 
Requirement 
______________________________________ 
20 3.13 88 0.081 2380 
280 3.70 90 0.046 2750 
283 Membrane Regenerated 
288 3.42 88 0.094 2600 
350 3.70 89 0.053 2790 
______________________________________ 
Of particular interest in the data of this table is the amount of decrease 
in NaOH efficiency observed as occurring from just before to just after 
the membrane regeneration. In this prior art example, the efficiency 
declined by two percentage points. As will be shown in examples of the 
present invention this undesirable side effect of membrane regeneration 
can be eliminated. (See Invention Example 1 below). 
PRIOR ART EXAMPLE #2 
A lab cell like that described in Prior Art Example #1 was operated and the 
membrane regenerated. Cell operation and membrane regeneration differed 
from Prior Art Example #1 in the following ways. The membrane was of the 
same type, but the lot number and date of manufacture were different. This 
difference alone can account for some small differences in cell 
performance and should be considered when comparing data from various 
tables. 
Cell operation was at an anolyte pH of about two instead of 4.0-4.3. This 
difference was obtained by adding aqueous HCl to and mixing it with some 
of the same type conventionally treated brine as prepared and described in 
Prior Art Example #1, and then feeding a combination of some of this 
acidified-brine and some of the conventionally treated brine to the 
anolyte chamber. The acidified-brine solution contained a NaCl 
concentration of about 25 wt. %, an HCl concentration of about 3 wt. % 
HCl, a CO.sub.2 content of only about one ppm, and a total hardness of 1-2 
ppm as Ca. The acidified-brine made up only about 25% of the total brine 
fed to the cell. Because the resulting combined mixture of acid-brine and 
conventionally treated brine contained in excess of 100 ppm CO.sub.2, this 
type cell operation is not representative of the present invention. 
Normal electrolysis was started and continued for about 227 days using the 
above described mixture of acid-brine and conventionally treated brine. On 
the 228th day after initial start-up, the membrane was regenerated in situ 
according to the following procedure. Cell voltage was reduced by reducing 
the operating current from 1.0 ASI to 0.03 ASI. Acid-brine similar to the 
3% HCl acid-brine described above, but containing 0.13 wt. % HCl, was fed 
to the anolyte compartment at a flow rate slightly higher than the normal 
brine flow rate used during the days of normal electrolysis. The water 
feed to the catholyte was increased above the flow rate used during normal 
electrolysis so as to maintain a caustic concentration of about 0.4 wt. % 
NaOH during the membrane regeneration step. Cell temperature was 
maintained at about 60.degree. C. and air was bubbled into the anolyte 
compartment to provide mixing. Membrane regeneration was continued in this 
manner for 20 hours. Then the cell was returned to normal electrolysis 
conditions of 1.0 ASI, 80.degree. C., 12-13% NaOH, 18-19% NaCl in the 
anolyte, and an anolyte pH of about two. 
The data in Table II summarize the cell performance before and after the 
membrane regeneration procedure. 
TABLE II 
______________________________________ 
Cell NaOH Salt in 
Chlorate 
Energy 
DOL Volts Efficiency 
Caustic 
in Caustic 
Requirement 
______________________________________ 
26 3.04 88 0.134 2 ppm 2310 
225 3.23 87 0.078 23 2490 
228 Membrane Regenerated 
231 3.11 86 0.280 43 2420 
251 3.25 86 0.160 12 2530 
______________________________________ 
In the table "DOL", "Cell Volts", "NaOH Efficiency", and "Energy 
Requirement" are the same as defined earlier. "Chlorate in Caustic" is the 
ppm NaClO.sub.3 impurity in the caustic on a 100% NaOH weight basis. 
In this Prior Art Example there was a substantial increase in both salt and 
chlorate impurity in the caustic after the membrane regeneration step. A 
salt concentration of 0.28 wt. % and a NaClO.sub.3 concentration of 43 ppm 
represent unacceptably high levels of these impurities. Above 0.20 wt. % 
NaCl and above 25 ppm NaClO.sub.3 are considered unacceptable. Also as 
noted in the table, cell voltage returned to an unacceptably high level 
after only 23 days. As will be shown later in the following examples, the 
method of the present invention results in a significant improvement in 
long term cell performance, and it also provides the following: less 
frequent membrane regeneration steps are required to maintain a given 
level of cell performance and caustic product purity is maintained at 
acceptable levels after the membrane regeneration step (see Invention 
Examples 1, 2, 3, and 4 below). 
INVENTION EXAMPLE 1 
A lab cell like that described in Prior Art Example #1 was operated and the 
membrane regenerated as required to maintain acceptable cell performance. 
The major difference in operation between the cell in Prior Art Example #1 
and the cell in this example was the level of CO.sub.2 ("carbon oxide") in 
the brine which was fed to the anolyte compartment. 
In order to reduce the CO.sub.2 content of the brine solution which was fed 
to the anolyte compartment of the cell during normal electrolysis, the 
following procedure was used. The same conventionally treated brine as 
used in Prior Art Example #1 was acidified using aqueous HCl. The brine 
was mixed and sparged with nitrogen to aid in the removal of entrained 
CO.sub.2 for a period of about 16 hours. The resulting acidified brine 
contained about 25.5 wt. % NaCl, 0.65 wt. % HCl, about 1 ppm Ca total 
hardness, and less than 1 ppm CO.sub.2. This acid-brine was then fed to a 
cell containing a Nafion.RTM. 324 membrane which was operated at 1.0 ASI, 
80.degree. C., 12-13 wt. % NaOH, and 18-19 wt. % NaCl in the anolyte, and 
at an anolyte pH of about 1.5-3.0 during normal electrolysis. Normal 
electrolysis was started and continued for 209 days. 
On the 210th day after initial start-up, the membrane was regenerated in 
situ using a procedure similar to the one in Prior Art Example #1. Cell 
voltage was reduced by turning the cell operating current completely off. 
The same acid-brine used during normal electrolysis was fed to the anolyte 
compartment at the same flow rate as used during normal electrolysis. 
Water at the same flow rate as used during normal cell operation, was 
continuously fed to the catholyte compartment. The membrane in this cell 
was regenerated in this manner for 24 hours and at a room temperature of 
25.degree. C. The cell was then restored to normal electrolysis operation 
at 1.0 ASI, 80.degree. C., 12-13% NaOH, 18-19% NaCl in the anolyte, and an 
anolyte pH of 1.5-3.0. 
The following table summarizes the cell performance before and after the 
membrane regeneration procedure. 
TABLE III 
______________________________________ 
Cell NaOH Salt in 
Chlorate 
Energy 
DOL Volts Efficiency 
Caustic 
in Caustic 
Requirement 
______________________________________ 
5 3.01 88 0.188 1 ppm 2290 
209 3.09 88 0.082 3 2350 
210 Membrane Regenerated 
220 3.02 88 0.141 11 2300 
250 2.97 88 0.140 6 2270 
______________________________________ 
By operating a cell according to the present invention, cell voltage was 
reduced by the membrane regeneration step with essentially no reduction in 
NaOH efficiency as shown by the data in Table III. 
The cell in this example continued to operate and the membrane was 
regenerated two more times using the same procedure as used in the first 
regeneration set out above. The table below summarizes the cell 
performance before and after these two further membrane regeneration 
steps. 
TABLE IV 
______________________________________ 
Cell NaOH Salt in 
Chlorate 
Energy 
DOL Volts Efficiency 
Caustic 
in Caustic 
Requirement 
______________________________________ 
250 2.97 88 0.140 6 2270 
305 3.06 88 0.117 2 2330 
307 Membrane Regenerated 
358 3.02 88 0.138 2 2300 
388 Membrane Regenerated 
390 3.08 88 0.142 1 2345 
430 3.06 88 0.145 2 2330 
______________________________________ 
After more than 400 days of operation long-term cell performance was 
maintained at an acceptable level of energy increase. At the same time, 
efficiency was maintained at essentially a constant level of 88% and 
impurities in the caustic were maintained at acceptably low levels. 
INVENTION EXAMPLE 2 
A lab cell like that described in Prior Art Example #1 was operated and the 
membrane regenerated. The membrane in this cell was an unreinforced 
sulfonamide type membrane. Similar membranes are described in U.S. Pat. 
No. 3,969,285. Membranes of this type with a reinforcing scrim have been 
sold commercially by E. I. duPont de Nemours and include membranes such as 
Nafion.RTM. 214 and Nafion.RTM. 227. 
The brine feed to this cell was the same as the brine feed to the cell in 
Invention Example 1, except for the amount of total hardness. In order to 
further reduce the hardness of the brine the conventionally treated brine 
of Prior Art Example #1 was further treated by passing this brine through 
a column containing DOWEX* A-1 chelating resin made by The Dow Chemical 
Company. Next, the brine was acidified and the CO.sub.2 removed. The 
resulting acidified brine contained about 25.5 wt. % NaCl, 0.65 wt. % HCl, 
only about 0.2 ppm Ca total hardness, and less than 1 ppm CO.sub.2. 
FNT *Trademark of The Dow Chemical Company 
This brine was fed to the lab cell containing the sulfonamide membrane 
described above and this cell was operated at 1.75 ASI, 80.degree. C., 
28-31% NaOH, 20-21% NaCl in the anolyte, and at an anolyte pH of 3-4 
during normal electrolysis. Normal electrolysis was started and was 
continued for about 194 days. 
On the 195th day after initial start-up, the membrane was regenerated in 
situ using the following procedure. The cell current was turned off and 
the current leads disconnected. Both anolyte and catholyte were drained 
from the cell. An acid solution of 0.5 wt. % HCl and water was added to 
the anolyte compartment. An acid solution of 1.0 wt. % formic acid and 
water was added to the catholyte compartment. Each compartment was filled 
with their respective acid solutions. Mixing of the acid solutions was 
provided by sparging a stream of nitrogen gas into the bottom of each cell 
compartment. The acid solutions were heated by an immersion type heater 
and maintained at a temperature of about 75.degree. C. During the 
regeneration procedure the acid solutions were drained from the anolyte 
and catholyte compartments. Respective, fresh acid solutions as described 
above were used to refill each compartment. The drain and refill step was 
repeated three more times during the five hour regeneration procedure. The 
acid wash solutions removed from the cell were analyzed for pH and for Mg, 
Ca, and Fe content. The results of these analyses are tabulated in Table 
V. 
TABLE V 
______________________________________ 
Sample pH ppm Mg ppm Ca ppm Fe 
______________________________________ 
Anolyte #1 1.2 114 114 3000 
Anolyte #2 1.3 80 28 5200 
Anolyte #3 1.3 74 22 5000 
Anolyte #4 1.2 44 22 3600 
Catholyte #1 
4.6 4 26 2600 
Catholyte #2 
3.9 5 22 2200 
Catholyte #3 
3.8 2 22 2200 
Catholyte #4 
3.6 1 22 2000 
______________________________________ 
The cell was then restored to normal operation at 1.75 ASI, 80.degree. C., 
28-31% NaOH, 20-21% NaCl in the anolyte and a pH of 3-4. The data in Table 
VI summarize the performance of this cell before and after the membrane 
regeneration procedure. 
TABLE VI 
______________________________________ 
Cell NaOH Salt in 
Energy 
DOL Volts Efficiency Caustic 
Requirement 
______________________________________ 
4 3.48 88 0.034 2650 
194 3.54 88 0.027 2700 
195 Membrane Regeneration 
204 3.34 88 0.072 2540 
285 3.40 86 0.052 2650 
______________________________________ 
From the analysis of the anolyte acid solutions in Table V, it was apparent 
that substantially less Ca than Mg was present in these solutions. This 
unexpected result was exactly reversed from the normal Ca and Mg content 
of anolyte acid regeneration solutions for membrane cells operated and 
regenerated like those described in Prior Art Examples #1 and #2. The fact 
that the Mg concentration was higher than the Ca concentration may be 
attributed to the fact that Mg(OH).sub.2 is more insoluble than 
Ca(OH).sub.2 at the high pH's encountered at the anolyte face of the 
membrane and within the membrane. Although CaCO.sub.3 is much more 
insoluble at a high pH than Mg(OH).sub.2 this calcium precipitate was 
substantially prevented from forming apparently because essentially all 
the CO.sub.2 (or other "carbon oxide" forming compounds) in the feed brine 
had been removed. The present invention takes advantage of these facts, 
and the result is reduced energy consumption and an improvement in the 
amount of impurities in the caustic when membrane regeneration becomes 
necessary in order to maintain and prolong long-term cell performance. 
As shown by the data in Table VI, energy consumption at the cell was 
reduced after the membrane regeneration step, salt in the caustic remained 
acceptably low, and cell performance after 285 days of operation was 
essentially equal to the level of performance that was obtained when the 
membrane was new. 
Also note in Table V, the high concentration of Fe present. This iron was 
corrosion coming from the cathode, among other Fe sources, as a visual 
inspection of the cathode showed. Control of this corrosion is shown in 
Invention Example IV below. 
INVENTION EXAMPLE 3 
A lab cell like that described in Prior Art Example #1 was operated and the 
membrane regenerated. The membrane in this cell was Nafion.RTM. 324. The 
acid brine feed to the cell was the same as described in Invention Example 
#2. The cell was operated at 1.0 ASI, 80.degree. C., 17-18 wt. % NaOH, 
19-20% NaCl in the anolyte, and at an anolyte pH of 1.5-3.0. Normal 
electrolysis was started and continued for 529 days. 
On the 530th day after initial start-up, the membrane was regenerated in 
situ using the following procedure. The cell was turned off and was then 
flushed with conventionally treated brine of the same type as described in 
Prior Art Example #1. This was done to remove the strong caustic from the 
catholyte and the acid-brine solution from the anolyte compartment. Both 
cell compartments were then drained. The anolyte compartment was then 
filled with a 0.5 wt. % HCl and water solution. The cathode compartment 
was filled with a 1.0 wt. % HCl and water solution which also contained 
1000 ppm of ANCOR.RTM. OW.RTM.-1 corrosion inhibitor, 1000 ppm isopropyl 
alcohol, and 220 ppm TRITON.RTM. X-100 wetting agent. ANCOR.RTM. OW.RTM.-1 
is a registered trademark of Air Products and Chemicals, Incorporated, and 
ANCOR.RTM. OW.RTM.-1 corrosion inhibitor is a commercial product available 
from that company. It is composed of a group of acetylic alcohols, a major 
portion of which is 1-hexyn-3-ol. TRITON is a trademark of Rohm and Haas 
Company, and TRITON X-100 is a commercial product available from that 
company. TRITON X-100 is a cogeneric mixture of isooctyl phenoxy 
polyethoxy ethanols. 
The corrosion inhibitor and wetting agent were added in order to protect 
the cathode from corrosion during the regeneration procedure. Actually 
this corrosion technique did not work as well as the cathodic protection 
method described in the next example, Invention Example 4. 
Mixing of the acid solutions in their separate chambers 10 and 12 was 
provided by sparging a stream of N.sub.2 gas into the bottom of both cell 
compartments. The acid solutions were heated by an immersion type heater 
and maintained at 75.degree.-80.degree. C. During the regeneration 
procedure the respective acid solutions were added to each cell 
compartment in 75 ml aliquots. This adding of additional fresh acid was 
repeated four times during the 41/2 hour regeneration procedure. Before 
restoring the cell to normal operation both acid solutions were drained 
from the cell, and then the membrane was substantially dried by heating 
with the immersion heater described previously. The drying step was 
carried out at a temperature of between 100.degree. C. to 200.degree. C. 
and required about ten minutes. The cell was then restored to normal 
electrolysis operation. 
Cell performance data obtained before and after the regeneration procedure 
are tabulated in Table VII. 
TABLE VII 
______________________________________ 
Cell NaOH Salt in 
Energy 
DOL Volts Efficiency Caustic 
Requirement 
______________________________________ 
5 3.02 84 0.130 2410 
526 3.18 84 0.031 2540 
530 Membrane Regenerated 
535 3.12 89 0.029 2350 
575 3.15 88 0.027 2400 
______________________________________ 
The data in Table VII shows that after the regeneration procedure, energy 
consumption was reduced, efficiency was surprisingly increased by a 
surprising amount, voltage was reduced, and salt impurity in the caustic 
remained constant. Being able to use a membrane cell for 575 days and 
still have cell performance of this quantity is not to be expected by 
those skilled in the art. Even more unexpected is being able to continue. 
The cell in this example continued to be operated, and a second and third 
regeneration were used at later dates according to the following 
procedure. The cell voltage was reduced to about 2.1 volts. In this way 
the cathode potential was maintained at slightly above the cathode 
decomposition voltage (defined above as the "cathodic protection 
voltage"); therefore, corrosion of the cathode was substantially 
prevented. Normal acid-brine feed was fed to the anolyte compartment at 
the flow rate normally used during cell electrolysis. H.sub.2 O was added 
to the catholyte at an increased rate in order to reduce the catholyte pH 
to about pH 8-9. The membrane was regenerated in this manner at room 
temperature for 25 hours during the 2nd regeneration and for 6 hours 
during the 3rd regeneration. A summary of cell performance before and 
after these regeneration procedures is given in Table VIII. 
TABLE VIII 
______________________________________ 
Cell NaOH Salt in 
Energy 
DOL Volts Efficiency Caustic 
Requirement 
______________________________________ 
575 3.15 88 0.027 2400 
578 3.19 88 0.015 2430 
585 Membrane Regenerated 2nd Time 
591 3.05 87 0.064 2350 
625 3.16 90 0.026 2350 
636 Membrane Regenerated 3rd Time 
638 3.03 87 0.064 2330 
790 3.13 87 0.052 2410 
______________________________________ 
The data in Table VIII indicate that long term cell performance was 
maintained for almost 800 days with essentially the same energy 
consumption and product purity as when the membrane was new. This is, 
indeed, unexpected. 
INVENTION EXAMPLE 4 
A lab cell like that described in Prior Art Example #1 was operated and the 
membrane regenerated using two different procedures. The membrane in this 
cell was Nafion.RTM. 324 and the acid-brine feed was the same as the 
acid-brine used in Invention Example #1. The cell was operated at 1.0 ASI, 
80.degree. C., 12-13% NaOH, 18-19 wt. % NaCl in the anolyte, and at an 
anolyte pH of 1.5-3.0. Normal electrolysis was started and continued for 
166 days. 
On the 167th day after initial start-up, the membrane was regenerated in 
situ using the following procedure. The electric current to the cell was 
turned completely off. The current leads were disconnected from the anode 
and cathode, and the cell remained electrically isolated from ground 
potential. The same type acid-brine used during normal electrolysis was 
fed into the anolyte compartment. Water was fed into the catholyte 
compartment. The flow rates of both the acid brine and the water were the 
same as what they had been during normal cell operation. Samples of 
anolyte and catholyte were taken periodically during this procedure. The 
membrane was regenerated in this manner at a room temperature of 
23.degree. C. for 23 hours. The cell was then restored to normal cell 
operation and continued to be operated up to the 256th day after initial 
start-up. 
On the 257th day the membrane was again regenerated using the same 
procedure as was used during the first regeneration except for the 
following changes. Cell current and voltage were reduced and cell voltage 
was then maintained at 2.1 volts by passing a small current through the 
cell during the entire regeneration procedure. This step was done in order 
to maintain the cathode potential at slightly above the decomposition 
voltage in order to substantially prevent corrosion of the cathode. 
Additional water flow to the catholyte compartment was also used in order 
to further reduce the catholyte pH. After about 10 minutes into the 
regeneration procedure the rate of water addition was reduced to the same 
flow as used during normal electrolysis. Samples of the anolyte and 
catholyte were taken periodically during the regeneration procedure. A 
summary of the analyses of the electrolyte samples taken during the 1st 
and 2nd membrane regeneration procedures are given in Tables IX and X, 
respectively. A summary of cell electrolysis performance before and after 
each regeneration is given in Table XI. 
TABLE IX 
______________________________________ 
1st REGENERATION 
Hours Regeneration 
ppm ppm ppm 
Sample in Progress Mg Ca Fe pH 
______________________________________ 
Anolyte #1 
1 &lt;2 &lt;2 &lt;2 1.7 
Anolyte #2 
3 6.4 &lt;2 4.4 0 
Anolyte #3 
5 6.7 &lt;2 2.6 0 
Anolyte #4 
6 6.8 &lt;2 77 0 
Anolyte #5 
6-22 composite 
4.9 &lt;2 97 0 
Anolyte #6 
23 3.0 &lt;2 87 0 
Catholyte #1 
1 &lt;4 &lt;4 &lt;4 14 
Catholyte #2 
3 &lt;4 &lt;4 &lt;4 13.8 
Catholyte #3 
5 &lt;4 &lt;4 &lt;4 12.4 
Catholyte #4 
6 &lt;4 &lt;4 58 4.2 
Catholyte #5 
6-22 composite 
&lt;4 &lt;4 55 -- 
Catholyte #6 
23 &lt;4 &lt;4 58 4.0 
______________________________________ 
TABLE X 
______________________________________ 
2nd REGENERATION 
Hours Regeneration 
ppm ppm ppm 
Sample in Progress Mg Ca Fe pH 
______________________________________ 
Anolyte #1 
1 20 5.8 &lt;1 1.2 
Anolyte #2 
3 11 9.7 4.7 0 
Anolyte #3 
6 7.5 2.4 2.3 0 
Anolyte #4 
23 7.3 2.2 1.2 0 
Catholyte #1 
1 &lt;1 &lt;1 &lt;1 12.8 
Catholyte #2 
3 &lt;1 &lt;1 &lt;1 -- 
Catholyte #3 
6 &lt;1 &lt;1 &lt;1 4.0 
Catholyte #4 
23 &lt;1 2 &lt;1 8.1 
______________________________________ 
TABLE XI 
______________________________________ 
Cell NaOH Salt in 
Energy 
DOL Volts Efficiency Caustic 
Requirement 
______________________________________ 
12 3.04 88 0.190 2310 
128 3.01 88 0.183 2290 
165 3.11 88 0.085 2370 
167 Membrane Regenerated 1st Time 
171 3.06 88 0.168 2330 
214 3.03 89 0.126 2280 
256 3.18 90 0.053 2370 
257 Membrane Regenerated 2nd Time 
260 3.02 89 0.132 2270 
______________________________________ 
The results of the analyses of samples taken during the membrane 
regeneration procedures confirm that by using the 2nd regeneration method, 
essentially no corrosion of the cathode occurred. The data in Table XI 
demonstrate that long term cell performance and acceptable caustic purity 
can be maintained by using brine containing only low amounts of CO.sub.2 
("carbon oxide") and suitable membrane regeneration procedures.