Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Several industrial processes are based on the electrolysis of
aqueous salt solutions. One important application is that of the electrolysis
of sodium chloride brine to produce sodium hydroxide and chlorine. In this
electrolysis, as in others involving electrolysis of aqueous salt solutions,
one method is to separate the anolyte and the catholyte compartments of the
cell using a porous separator. Hereinafter the term "diaphragm" will be used
for a porous separator which permits electrolyte to pass through without any
significant change in composition
In this method hydrogen and caustic soda are produced at the
cathode which is within the catholyte compartment of the cell while chlorine
is produced at the anode which is within the anolyte compartment. The salt
brine passes through the diaphragm from the anolyte to the cathalyte compart- -
ment. The cathodes are usually of iron screening while the anodes are of
graphite or platinized titanium. The diaphragm is usually asbestos.
Diaphragm cells require a flow of solution sufficient to insure
that the back-diffusion of sodium hydroxide into the anolyte is avoided or
minimized. This is necessary to avoid chlorate in the anolyte and loss of
current efficiencyO When using the minimum amount of brine flow within the
diaphragm to avoid chlorate formation, only about half of the sodium chloride
is converted. The spent anolyte must then be evaporated to concentrate the
caustic soda and to crystallize the salt. Finally the salt is filtered or
centrifuged from the caustic solutlon. It is the usual practice to purify
the brine feed to the process to reduce the content of impurities which clog ~`the diaphragms; also it is usual to renew the diaphragms at regular intervals.
Attempts have been made to build diaphragm cells using permselec-
tive membranes in place of the diaphragms. This does not provide a solution
to the principal problem. All of the membrane which offer reasonably high ~`electrical conductivity and chemical resistance are also subject to consider- `able back-diffusion and electromigration of hydroxyl ions, the rate of which
increases with the concentration and temperature of the catholyte.
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A second type of cell or chlorine and caustic soda production is
the mercury cell in which electrolysis of the brine results in production of
a sodium amalgam at the cathode and chlorine at the anode. The amalgam is
reacted with water, producing a salt-free solution of caustic soda together
with hydrogen. In this method a diaphragm is not required because caustic
soda solution is formed in a separate piece of apparatus or compartment from
the one containing brine solution and chlorine.
In one embodiment of a mercury cell, purified sodium chloride
brine is fed into a slightly sloped horizontal trough on the bottom of which
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the cathode mercury flows concurrently with the brine. Above the mercury and
within the brine are horizontal anodes of graphite, or platinum-, or platinum
family-coated titanium. These anodes are suspended from gas tight covers of
the trough. Current is supplied to the anodes by rods suspended from, and
sealed in, holes in these covers.
The brine is confined within the trough by mercury syphons at the
ends of the trough. Typically the concentration in the cell is reduced from
315 to 275 g/l NaCl and the brine leaves the trough by an overflow. Conven-
tionally, it is ed to the trough by a valve and rotameter. After leaving
the cell, the brine is dechlorinated by a combination of HCl addition and
vacuum and air stripping, resaturated, purified and returned to the cell with
its pH at about 7Ø
Electricity is led into the mercury by connections to the steel
trough bottoms with bus-work. Anode rods that project above the cover, and
usually have to be vertically adjustable, are connected to the main bus-work
by pigtails, clamps, soldered rods or similar hardware. To stop operation of
each trough, short circuit switches that connect the anode bus-work to the
cathode bus-work are usually provided.
The covers, sidewalls, end boxes, seals and, at times, ~ost of the
bottom of the trough are covered with corrosion resistant material, usually
hard rubber. The life, before repair or replacement becomes necessary, hardly
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ever exceeds five years and is often much less. This assemblage of components
is commonly called the primary cell.
The mercury flowing out of the primary cell contains sodium, plus
impurities such as calcium, magnesium and iron that may not have been fully
removed in the prepurification of the brine. To remove the sodium~ thus mak-
ing caustic soda, the mercury is washed with distilled water. This operation
is performed in contact with graphite, leading to the production of caustic
solution, hydrogen gas and rela~ively sodium-free mercuryl which is pum~ped
back to the inlet box of the primary cell. The apparatus for this is conven-
tionally called the secondary cell or denuder. In present practice, it is
either a horizontal trough with graphite grids, or a short tower with graphite ~-
packing. If it is a trough, it is arranged either alongside or under the pri-
mary cell; if it is a tower it is usually arranged on the discharge end of the
primary cell, with the pump underneath the tower and a long pipe under the
primary cell that conducts the mercury back.
To understand the function of the mercury cathode in a mercury
chlorine-caustic cell, consider the electrolysis of a sodium chloride
solution between a steel cathode and a graphite anode. Chlorine is evolved
at the anode and hydrogen at the cathode, ~hile caustic soda is formed
simultaneously at the latter according to the reaction:
2NaCl + 2H2 = 2NaOH + C12 + H2
With no separation of the anolyte and catholyte, the following secondary
reactions occur:
2NaOH + C12 = NaOCl + NaCl + H20
3 NaOCl = NaC103 + 2NaCl
C (Graphite) + 2NaOCl = 2NaCl + CO2
C12 + H2 = 2~Cl (Explosion)
Obviously such a cell does not serve the purpose of producing
caustic soda and chlorine. On the other hand in a mercury cell, and utiliz-
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ing a mercury cathode, i~ the mercury is relatively pureJ hydrogen will not
be evolved on the cathode preferentially to the discharge of sodium, and the
mercury will become sodium amalgam, in accordance with the following:
2NaCl ~ 2Na(~lg) l C12
because the hydrogen overvoltage on a mercury surface is higher than the vol-
tage required to deposit sodium into such a surface. The overvoltage is the
voltage of an electrode above that theoretically required to discharge a gas ~ -
on its surface.
If the mercury contains more than a few tenths of one percent of
sodium, or traces of magnesium, nickel or similarly low hydrogen overvoltage
metals, hydrogen and caustic soda rather than sodium amalgam will form to a
greater or lesser extent. When this happens, the current efficiency drops, ;~graphite consumption increases and the chlorine gas in the cell, or in the un-
condensable gas left after most of the chlorine is liquefied, becomes explo-
sive because of hydrogen admixture.
The reverse effect is desired when making caustic and hydrogen from
amalgam. If clean sodium amalgam is placed in a beaker and water or caustic
is poured over it, little or no reaction takes place, because hydrogen does
not readily discharge from a mercury surface. If a piece of graphite is part-
ly dipped into the mercury, hydrogen bubbles can be observed rising from the
graphite very near the mercury surface, the water of caustic become more alka-
line and the amalgam is denuded of its sodium. Thus, under operating condi-
tions the denuder is a short-circuited battery in which the amalgam is the
anode and ~he graphite the cathode.
This process produces a pure concentrated caustic, normally about
50%, compared to about 11% concentration for the diaphragm cell. However, the
circulation of mercury, the exposure of the mercury surface to salt brine
where it is always denuded to a greater or lesser extent, and other problems
inherent in the technology have always forced the design into large, expen-
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sive, complicated equipment occupying large buildings and leading to inevit-
able mercury pollution from skimming operations and the like. Furthermore,
mercury cells are extremely sensitive to impurities in the sodium chloride
solution as these increase the denuding of the amalgam that takes place during
electrolysis, resulting in a high and frequently explosive hydrogen content
of the chlorine.
The problems of circulation of mercury may be avoided if the con-
figuration is one in which the mercury is used as a membrane, one face of
which is the cathode in the brine which undergoes electrolysisl the other face
of which is in contact with the caustic soda solution. In this manner the
deposition of sodium in the amalgam occurs simultaneously with denuding at
the opposite face.
Attempts to achieve this have invariably led to a design wherein
the mercury is either supported in syphon-like superimposed channels or on
porous or woven materials, essentially on a diaphragm. lYhen the mercury is
supported in syphons, the long path that the sodium metal must travel through
the mercury results in overconcentration of sodium at the cathode surface
accompanied by hydrogen evolution on the cathode surface and into the chlorine
and, if the mercury is supported on a diaphragm, the eventual result has
always been that the resistance increases because gas bubbles become en-
tangled in the pores. Furthermore, deposition of metallic impurities such as
iron in the pores results in wetting of these impurities by mercury accom-
panied by mercury leaking through the diaphragm and being lost.
A variation in the mercury cathode cell is disclosed by Rosenbloom
in United S~ates Patent No. 2,749,301. The mercury cathode is supported on a
porous diaphragm of a woven plastic fabric or asbestos cloth. The brine flows
beneath the cathode on the anode surface. A very large, and thus uneconomical,
flow of brine must be pumped through the space between the anode and the
diaphragm to avoid the blanketing of the diaphragm by gas bubbles. However,
even with a high flow rate, bubbles of chlorine as well as hydrogen from the
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mercury layer above the diaphragm slowly become entangled in the diaphragm
and reduce the efficiency of the process.
The present invention provides novel processes and apparatus for
the electrolysis of alkali metalr~solutions. It is one feature of this inven~
tion to overcome certain deficiencies which exist in presently used electroly-
sis cells and processes.
These and other features will be set forth more fully in the follow-
ing detailed description.
This invention is directed to processes and apparatus for the
electrolysis of aqueous solutions containing alkali metal ions~ as illustrated
by the electrolysis of sodium chloride solution to form chlorine~ sodium
hydroxide and hydrogen. The alkali metal ions will be in solution with anions
of the mineral acids, and/or hydroxyl ion, and/or anions of organic acids.
Examples of such aqueous solutions are solutions of the chlorides, bromides,
sulfates, sulfites, phosphates, acetates and hydroxides of sodium and potas-
sium.
In one embodiment of the invention a composite membrane is used to
separate the cathode products from the electrolyte. The composite membrane~
is characterized by a membrane (which comprises a solid polymer)facing the ~;
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anode, and a layer of metal permeable to alkali metal in intimate contact with
the membrane. The term membrane is used for a material which exhibits high -
ion permeability, and relatively low permeability with respect to the anolyte.
In a further aspect of this invention the presence of a gas phase in the
electrolyte adjacent to the compo~ite membrane is avoided, either by operation ~ -
above atmospheric pressure under conditions wherein normally produced gaseous
products are liquefied or dissolved in the electro]yte, or by operation with ; ;
the anode covered with an electrolyte-permeable diaphragm or an anion-permeable
membrane and with removal of the anode products through the anode. In still
another aspect of this invention, cell modules, consisting of electrolytic
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and denuding compartments separated by composite membranes are stacked to
provide distributed series flow of electrical curren~ from cell to cell, while
fluids flow uniformly to and from the cell modules within electrically non-
conducting channels, tubing or piping.
~t is characteristic of the composite membrane that in its
application to the electrolysis of an aqueous alkali salt solution, alkali
metal ions migrate from the aqueous solution through the membrane into the
layer of metal and then pass out of the metal layer.
This invention is, therefore, directed towards a process for the
electrolysis of an aqueous solution containing the chloride or bromide of
sodium and/or potassium wherein said ~lectrolysis would normally produce a
gaseous anode product at atmospheric pressure in said solution, which comprises
passing an electric current through said solution which is in an anolyte
chamber disposed between an anode and a composite metal-polymer membrane of a -
polymeric membrane facing said anode and a cathodic layer of metal permeable
to alkali metal in intimate contact with said polymeric membrane at its sur- ;~
face facing away from said anode, at above-atmospheric pressures under condit-
ions whi`ch produce said anode product in a substantially liquefied state or
dissolved within said solution.
This invention is furthermore directed towards electrolysis apparatus
comprising one or more electrolytic cells, each cell having an anode3 a com-
posite metal-polymer membrane comprising a polymeric membrane facing said
anode and a cathodic layer of metal permeable to alkali metal in intimate con-
tact with said polymeric membrane at its surface facing away from said anode,
an anolyte chamber in contact with said anode and said polymeric membraneg
and means to mai~tain the electrolyte in the cell under pressure.
For the most advantageous utilization of this invention it is
obvious that the electrical resistance to the electromigration of alkali metal
ions within the membrane should be low and that the transport capacity of the
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metal layer for the alkali metal should be high to provide the maximum unit
capacity for the composite membrane and the minimum requirement of electrical
energy. Furthermore, the composite membrane is designed to pro~ide direct
interfacial contact between the polymeric membrane and the metallic layer in
order that the alkali metal ions may be discharged directly from the poly-
meric membrane into the metallic layer to be conducted therethrough as alkali
metal atoms, or as alkali metal ions together with free electrons.
A preferred metallic layer is liquid mercury which forms liquid
amalgams with the alkali metals. This characteristic is, of course, well
known in the prior art in which mercury cells are widely used for the product-
ion of electrolytic chlorine and caustic soda. In the following description
mercury will be used in referring to the metal layer of the composite membrane
of this invention.
In a further embodiment of this invention the interface between the
polymeric membrane and the mercury layer can be increased above that of a
planar interface by corrugating or dimpling the membrane into the mercury
layer. It is advantageous also to effect an extension of this interface by
depositing mercury within the polymeric membrane structure, for example, by an
initial electrolysis of a mercury salt solution or by otherwise depositing
mercury within the membrane structure. It has also been found that resistance
can be decreased by applying a swelling agent~ usually a polar solvent such
as e~hanol or glycolJ to the polymeric membrane either alone or in combina-
tion with the mercury impregnant. -
The polymeric portion of the composite membrane should have the
characteristics of low electrical resistivity and high chemical resistance to
chlorine and brine under the operating conditions employedO
The polymeric portion of the composite membrane may comprise a solid
perfluorocarbon polylner having pendant sulfonic acid or sulfonate groups,
or sulfonic acid and sulfonate groups. (The term "sulfonic groups" is used
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to refer generically to the sulfonic acid and/or sulfonate groups.) Said
perfluorocarbon polymer has the pendant groups attached either directly to
the main polymer chain or to perfluorocarbon side chains attached to the
main polymer chain. Either or both the main polymer chains and any side chain
may contain oxygen atom linkages ~i.e. ether linkages). The perfluorocarbon
polymer from which the polymeric portion of the composite membrane of the
invention is prepared includes perfluorocarbon copolymers with said pendant
groups as well as perfluorocarbon polymers having mixed chlorine and fluorine
substituents where the number of chlorine atoms is not more than about 25%
of the total chlorine and fluorine atoms, with said pendant groups. The
polymeric portion may optionally be reinforced, for example, by using a screen
of a suitable metal or a cloth of polytetrafluoroethylene or other reinforc-
ing material as disclosed in Canadian Patent 982,467. The perfluorocarbon
polymers used for the membrane portion of the composite membrane may be
prepared as disclosed in United States Patents~ 3,041,317; 3,282,875 and
3,624,053.
The preferred perfluorocarbon polymers are prepared by copolymeriz-
ing a vinyl ether having the formula FS02CF2CF20CF (CF3)CF20CF=CF2 and
tetrafluoroethylene followed by converting the -S02F group to either -S03H
of sulfonate (for example alkali metal sulfonates) or both. The equivalent
weight of the preferred copolymers range from 950 to 1350 where equivalent -~
weight is defined as the average molecular weight per sulfonyl group. The
preferred thickness of the polymeric membrane portion is 0.001 to 0.010 inch.
Thus the cells of this invention have an anode and a composite
membrane comprising a polymeric portion comprising perfluorocarbon polymers ;-
having pendant sulfonic acid and/or sulfonate groups and a cathodic layer of ~`~
metal permeable to alkali metal ions in intimate contact with said polymeric
portion.
Although the metal layer of the composite membrane has been describ-
ed as mercury, other metals may be used ~n its place. The nature of the
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metal chosen depends upon the cation of the electrolyte, the permeability
of the metal layer to the cation, and the interaction of the metal layer with
the cation. For example, thin films of silver and/or lead or combinations of
these metals with mercury could be considered for this purpose. Since the
electrolytic cell of the present invention is operable under high pressures
and elevated temperatures, normally solid metals and alloys can be used in
their molten state. A very thin layer of a solid metal which exhibits sodium
diffusion characteristics would permit the use of a solid metallic component
in the composite membrane. This technique could also be used in combination
with a liquid metal. These and other embodiments would facilitate a configur-
ation in which the composite membrane is used in a position other than
hori~ontal.
The anodes of the electrolytic cell can be made~of any material
that is s~itable for the electrolytic process intended, such as the platinum
group metals and their oxides either alone or as a coating over titanium or ~ ~-
tantalum. The anode may be of any suitable configuration such as sheets, ex-
panded or perforated metal, smaller segments of such forms, or other configur-
ation that does not lead to danming or entrapment of the anode product.
The short circuited èlectrodes in the denuding compartments con-
2Q sist of graphite or similar material having a relatively low hydrogen over-
voltage. These electrodes are distributed in a pattern over the surface of
the mercury in the denuding compartments and are partly immersed in the
mercury. Bubbles of hydrogen gas are evolved at the graphite surface, while ~;
hydroxyl ions are formed as sodium ions enter the aqueous solution from the
mercury layer.
The short-circuited graphite electrodes may serve also to
carry the cathodic current to the mercury, but i-t is preferable to use metal-
lic leads from the anode of the cell next above. It has been found that
graphite fabric intertwined between and around these metallic leads is effect-
ive in causing a rapid and thorough denuding of the amalgam, while permittinga close spacing of the metallic leads.
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31.(3~ r
The invention will be described in greater detail in con-
junction with the accompanying diagrammatic representations. It should be
understood that the means of carrying out the inventions exemplified by the
figures are not limiting and that various modifications of constructive and
operational de*ail could be used by a person skilled in the art.
Figure 1 is a partial section of a single element of a cell
using a composite membrane~ a sheet type anode and graphite fabric in the
denuder, intended to produce chlorine under pressure as a dissolved gas.
Figure 2 is a partial section of a single element of a cell
using a composite membrane, an anode (shown in the alternative forms of a
perforated anode and a button anode), and graphite fabric in the denuder, in-
tended to produce liquid chlorine under pressure.
Figure 3 is a partial section of a single element of a cell
using a composite membrane and a drained anode compartment using a screen
anode, and graphite fabric in the denuder, intended to electrolyze, for
example, sodium sulphate.
Figure 4 shows a section through a stack of cell elements in-
side a pressure containing shell intended to produce chlorine under pressure.
Figure 5 shows a flow sheet of the brine and chlorine system
for a cell stack composed of cells using composite membranes to ~roduce
liquid chlorine under pressure.
Figure 6 shows a flow sheet of the brine and chlorine system and a ~ ~ -
cell stack composed of cells using composite membranes to produce chlorine dis-
solved in brine under pressure.
Figure 7 shows a flow sheet of the water, caustic and hydrogen
systems for a cell stack in which the caustic is cooled by recirculation.
Figures 1, 2, 3 and 4 are directed to a cell stack within
which the individual elements occupy an essentially annular space with the
feed of electrolyte and water coming from outside the annulus while the cy-
lindri~cal core at the center of the annulus is used for insulating conduitsfor spent electrolyte. Other geometrical forms, such as rectangular elements,
can also be employed. For each of FIGURES 1, 2 and 3 reference is made in
the following description to the top portion of the cell which is identical
to the top portion of next adjacent lower cell, as shown in the figures.
~ eferring to FIGURE 1 J 1 is the outer anode ring, 2 is the outer
denuder shell, 3 is the water feed pipe, ~ is the anolyte, 5 is the outer
anolyte ~ustle, 6 is the anolyte regulator, 7 is the anolyte feed pipe, 8 is
the anode, 9 is the hydrogen space, 10 is the caustic space, 11 is the graph-
ite fabric tape, 12 is a current lead, 13 is the inner anode ring, 14 is the
inner denuder shell, 15 is the caustic and hydrogen outlet pipe~ 16 is the
inner anolyte bustle, 17 is the inner membrane ring, 18 is the anolyte over~
flow pipe, and 19 is the outer membrane ring. The layers A and B together
form the composite membrane; wherein A is the polymeric membrane layer and
B is the metal layer, namely mercury.
The electrolytic element is confined by the anode 8, the outer
anode ring 1, the inner anode ring 13, the outer denuder shell 2, the inner ---~
denuder shell 1~, and the bottom of the next anode 8'. Within the elements
are the composite membrane AB, the graphite tape 11, and the current leads
12.
The anode 8 is a plate that can be made of steel or nickel with a `
thin layer of titanium bonded in intimate electrical contact to its upper
surface. The upper surface of this titanium layer would, in turn, be coated
with a member of the platinum metal family or one of their oxides, such as
ruthenium oxide. On the under side of the anode are attached the leads 12.
This attachment must again be in intimate electrical contact. For instance,
the leads 12 can be nickel wires attached to the anode plate by electron ~ -
beam welding. These leads are not necessarily straight wires but could be ;~
hair pin loops or any other suitable configuration that will conduct current
between the mercury and the anode next above and make good electrical contact
between them without tending to blanket the space between the mercury surface
or the interface between the mercury B and the membrane A. One skilled in the
art can, of course, calculate the electrical conductivity of these elements.
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~ very good distribution of the current in the mercury is necessary. Whether
this is accomplished by a large multiplicity of thin leads or by fewer thicker
leads equipped with thinner distribution leads within the mercury, such as a
screen, is not important as long as the basic requirements listed above are
met.
The elements in contact with anolyte and chlorine, the anode rings
1 and 13 and the lower portions of the denuder rings 2 and 14, the membrane
rings 17 and 19 and the requisite gaskets that are not shown for the sake of
simplicity must be of such material and construction as will withstand the
1~ corrosive environment encountered. Furthermore~ the construction and bolting
must be such as not to cause short circuits even if protective material should
fail. It is therefore preferred that all these elements be constructed oP
nonconducting materials rather than of coated metals. The fluorocarbons, the
higher density polyolefins, some of the polyesters and in the case of materi- ~;
als facing only the denuder, the epoxies are suitable materials. The current
leads 12, the inner lip of 2 and the outer lip of 14 should be metallic and
amalgam wettable, preferably iron or nickel. This is desirable to prevent the
seepage of solution around the mercury in the case of the denuder rings and
poor electrical contact in case of the leads 12.
The graphite fabric tape 11 serves as a very efficient denuding ele-
ment, but graphite tubes around the leads 12 or other forms of graphite placed
among these leads would also be suitable. However, it is preferred to use the
graphite fabric wrapped around the leads 12 in such a manner as to form flow
channels and thereby facilitate flow and mixing of the water and caustic, to
avoid stratification and poor denuding. The water feed pipe 3 and the caustic
and hydrogen pipe 15 are merely conventional pipe or tubing which are either
made of suitable insulating materials or carry suitable insulating joints.
The brine feed pipe 7 carries flow regulating device 6 to insure
that the proper quantity of electrolyte reaches each element.This anolyte
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regulator 6 can be merely an orifice of suitable size or a control valve to-
gether with a flow sensing means. If a stack of these elements is fed from
a common header, it is preferred to regulate the flow and, if desired, measure
it, by means of a flow regulating device system designed to feed a constant
proportion of the total flow to a number of vertically disposed vessels.
The flow of the individual electrolytic units should be uniform in
spite of the different hydrostatic heads existing up the cell stack. This
can be achieved by various distribution means such as individual feed pots
fed on a time cycle together with flow-regulating orifices.
A preferred method of maintaining controlled and substantially e-
qual volume flow of brine in each of the brine streams despite the differences
in hydraulic pressures is to deliver each stream vertically upward in a tube
with tapered bore. In this tube is a rotor, or float, which is suspended in
the moving stream. The gravitational force on the rotor (less the buoyancy)
is then balanced by an equal and opposite force exerted on the rotor by the `
moving stream. This force is independent of the stream flow rate and it is
equal to the pressure differential times the maximum transverse section area
of the rotor. The pressure differential is therefore also independent of the ;
flow rate.
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The float is an axially symmetrical body which is either a sphere
or~ preferably, a body having a center of gravity substantially below the
cross section at maximum area. It is designed to be self centering in the
moving stream and it may be described as plumb-bob-shaped. Its vertical pos-
ition in the tapered tube is variable with the flow rate. When this position ~
is measured on a linear scale as a measure of this flow, the device is known ` -
as a rotameter.
The parameters of this device are related as follows:
Sf (~ P)= Vf (df--d) (1)
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q/S = C 2gVf ~df-d~ 1/2
- (2)
Sfd _
where: ~-
~P is the pressure differential;
Sf is the maximum cross-sectional area of the float;
Vf is the volume of the float;
df is the density of the float;
d is the density of the fluid;
C is an orifice constant; ;
g is the gravitational constant;
S is the area of the annular space between the float and the tube
wall at the maximum cross section; and -
q is the volume rate of flow.
These equations indicate that the pressure differential, ~P, which is imposed
on the stream may be controlled by selection of the float parameter,
Vf (df-d)/Sf~ i.e. by suitable selection of the float geometry and apparent
density, which together are referred to as the specific weight.
This invention is utilized for the purpose of maintaining an equal ~ -
flow in the several streams as follows. The several streams may be designat-
ed numerically by 1, 2, 3~ .. n, and the hydrostatic pressures may be desig-
nated P1~ P2~ P3~ Pn~ where the subscripts denote the stream numbers. The
float parameter, as indicated above, in the float o~ each stream is adjusted
so that the pressure differentials ~P1, ~P2, ~P3, ... ~Pn satisfy the condi- ~ -
tion: -
P1+4P1--P2+~P2=P3+~3 = -- = Pn+~Pn
With adjustment of the Moat parameters as indicated above, the
flow distribution remains equali~ed regardless of the total brine flow to the
system.
In the foregoing description of this aspect of the present invention
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the flow of brine in the several s-treams is equalized and the flow in the sev-
eral tubes is upward against the downward force at a float which is heavier
than the fluid. It is possible however to utilize this invention to maintain
the proportionality of flow at streams which are not equal by the use of floats
suitably proportioned and weighted. A means to adjust the ratio of flow be-
tween streams, such as a valve in each stream, may be used in conjunction with
the float control means whereby the ratio of flow between the streams ~i.e. as
set by a valve) is maintained regardless of total flow from header. It ~s
also within the scope of this invention to utilize a float which is lower than
the fluid in mean specific gravity. For this case the flow of fluid in the
tapered tube is downward; the taper at the tube is expanding downward and the -
center of gravity of the float is above the cross section at maximum area.
There is the additional advantage that in this flow control system,
the vertical position of the float is related to the annular area S and the ~ ~ -
volume flow q, assuming that all of the parameters on the right hand side of
equation (2) remain constant. By varying the upstream pressure Pl at the com-
mon manifold it is possible to obtain a proportional change in flow rate of the
liquid stream to each cell while maintaining the differentials of hydrostatic
pressure. It is a matter of great advantage in chlorine-caustic production to
be able to control the solution flow to the individual cells proportionately
by means of a control in a common manifold. This is for the reason that flex-
ibility in production is essential to meet fluctuations in demand without pro-
viding very large storage facilities. If desired, these flow-balancing devices
can be used as sensors of flow information when coupled with a capacitative or
inductive position sensing means.
This flow control system is also of value in systems other than the
chlorine cell disclosed herein. This type of flow control has general applic-
ability to any system of multiple feeds from a common header where it is de-
sired to maintain equal or proportionate flow between the multiple feed streams
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:. : : , ,
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113 ~
despite different upstrec~m pressures.
The anolyte overflow pipe 18 should be high enough so that the hydro-
static head which results from its height compensates for the weight of the
mercury, of the membrane, and of the caustic, and provides the force necessary
to keep the membrane A firmly pushed up against the bottom of the leads 12.
The membrane A is shown as a planar fil~, but it is preferred to have an ex-
tended surface with corrugations or dimples extending up into the mercury, to
extend the interface between it and the mercury. This reduces the electric
resistance of the interface and of the membrane itself, and reduces the quan-
tity of mercury in the system. The hydrostatic head to be provided by 18
should preferably be at least two inches of water column above that required
for hydraulic balance. The anolyte pipes should also be designed to insulate
electrically. Of course, some or all of the pipes attached to the cell could
be replaced by passages within the cell structure.
Referring to Figure 2, 20 is the cell~housing, 21 is an anode and 22
and 23 are gaskets. All other elements of the cell described in Figure 2 have
the function of similar elements in Figure 1. The anode 21 together with the
current leads fulfill the same function as the anode 8 and the current leads
12 of Figure l, but facilitate run-off of liquid chlorine. ~ -
Two alternative anode constructions are shown in Figure 2. On the
left hand side the top of the current leads 211 are enlarged to form a button.
This results in an anode that consists of a multiplicity of small pieces with
channels between them for the run-off of the liquid chlorine. It is obvious
that more than one lead can be connected to a single button in this manner and
that the lead could be run through the top of the cell housing and t~e button
placed entirely on top of the cell housing rather than having the button run
through the top of the cell housing. This construction has the advantage that
it lends itself to molding techniques, to wire and screw machine metal working ~
techniques and permits the use of brittle anode materials that cannot readily ~;
-t7-
.. ~
-
~t~
be fabricated to cover a large surface in a thin section. In this manner a
choice of materials is available and the current lead and the button can be
one material or two quite different materials.
The anode design shown on the right hand side, wherein the anode is
a perforated sheet 21"~ lends itself to a different set of fabrication tech-
niques. Perforated sheet~ screens or expan~ed metal can be electrically con~
nected to the current leads~ either by running each c~rrent lead through the
top of the cell housing or by Joining several current leads to each other with- ~;
in the denuder compartment and bringing only one single connection from such
a bundle of current leads to the anode sheet. By positioning the anode sheet
somewhat above the top of the cell housin~space for liquid chlorine is pro-
vided.
The top face of either the buttons or the anode sheet must have a
configuration such that liquid chlorine does not blanket it but tends to run
off onto the top of the cell housing. If found desirable for corrosion re-
sistance, the top of the cell housing can be protected by a layer of fluoro-
carbon held in place by shoulders on the leads or buttons. -
The gaskets 22 and 23 seal the polymeric membrane to the cell hous- ~ ~ ~
. . .
ings and adjacent housings to each other. When using perfluorsulphonic acid
polymers as a membrane~ it is possible to seal this material to other fluoro- "~
carbons or to leave its edge in the sulphonyl fluoride form and to build up -
gasket structures in this manner. Similarly, this material, or its sulphonyl -
fluoride form can be used as is~ or attached to fluorocarbon linings of other
cell components, thereby leading to a variety of means of attachment and cor-
rosion protection.
The bustle construction of Figure 2 is somewhat different from the
bustle construction of Figure 1. This i5 due to the fact that the cell of
Figure 1 is intended to produce chlorine dissolved ~mder pressure while the
c~ll in Figure 2 is designed to produce liquid chlorine which should drain ~`
-18-
": ` , ~
-
freely through the perforations or channels in the anode along the top of the
cell housing and into the inner bustle structureO
Referring to Figure 3, 24 is a diaphragmJ 25 is an anode screen, 26
is an anode pan and 27 is an anolyte and anode gas outlet.
The cell of Figure 3 is essentially the same as the cell in Figure
1 with a fundamentally different anode construction. Using the electrolysis
of sodium sulphate solution to produce caustic soda, sulfuric acid, hydrogen
and oxygen as an example, it is obvious that the anode gas, oxygenJ is neither
soluble nor liquefiable under any practical conditions within a cell. There-
fore, to carry on such an electrolysis, one can either accept the disadvan-
tages of gas bubbles in the anolyte, which requires a large anolyte flow to ~ ~
sweep the gas bubbles from the cell but still results in a higher cell re- -
sistance, or one can operate a cell with a composite membrane at the cathode
and a diaphragm protected drained anode, leaving the intervening electrolyte
substantially free of gas bubbles. The electrolyte can either percolate
completely through the anode diaphragm or can be circulated partially through
the cell while part of it is permitted to flow through the diaphragm. This ~-
can be c~ntrolled by choosing a diaphragm of proper resistance to flow and by
controlling the differential pressure between the main electrolyte flow and
the gas phase in the anode pan by means well known in the artO
The diaphragm can be replaced by an anion-premeable membrane or an
anion-permselective membrane, so that, for example in the case of sodium
sulphate electrolysis, the anolyte will contain mostly sulphuric acid with a
minimum of sodium sulphate which would drain at the anode. The sodium sulphate
circulating stream can then be merely resaturated without the need of an external
crystalization step to separate the sodium sulphate from the sulphuric acidO
Since one of the main applications for sodium sulphate electrolysis is the
recovery of rayon spin baths, such a crystallization can be carried out by
circulating
-19-
. . . ~ . . . . ............ . . . . . . .
". . . . , :
~a~73s
the solution from the cell through the spin bath recovery~
A cell of this nature can also be used for the electrolysis of sod-
ium chloride by choosing suitable diaphragm and anode materials, in which
event the cell can be operated at nearly atmospheric pressure.
Although Figure 3 shows a screen anodeJ other draining forms of
anodes can be employed as well, thus permitting the use of materials such as -
lead-silver-alloy, magnetite and others.
Referring to Figure 4, 1 is a pressure shell, 2 is a caustic or wat-
.
er distributor,3 is a caustic or water feed, 4 is a flexible cathode bus, 5
is a caustic or water feed tube, 6 is an insulating pressure fluid, 7 is a
cathode end cap, 8 is an anolyte feed header, 9 is an anolyte regulator, 10
is an anolyte disengagement core, 11 is a cell element, 12 is an anolyte over-
flow pipe, 13 is an insulating ring, 14 is an anolyte feed, 15 is an anode end
cap~ 16 is the caustic and hydrogen outlet, 17 is the anolyte and chlorine
outlet, 18 is the cathodic rectifier bus connection and 19 is the anodic rec-
tifier bus connection.
This figure shows a schematic section of a stack of cell elements
producing chlorine as liquid or dissolved gas. The bottom of the stack is
the end anode which also carries mostif not all piping connections, since
2Q the water or caustic connection 3 could also be carried in the base. This
base is also connected to the positive side of the rectifierO Mounted over
the stack and fastened in a pressure tight manner to the base is pressure -
shell 1, and the space between the pressure shell and the cell stack is filled
with insulating pressure fluid 6. On top of the cell stack is the cathode end
cap 7. This end cap is connected by means of flexible bus work to the shell 1 ~ `~
which acts as the vertical bus leading the negative current from the rectifier
which is connected at its bottom to the top of the cell stack. On top of the -
cell stack is also located the water or caustic distributor 2 which feeds thc
denuders of each element. The liquid volume in each denuder is relatively
- 20 -
''~ . ` ` ` ` ~ ,.,`'' :,
.
'7~
large so that a discontinuous feed is acceptable, provided the average feed
volume is accurate. Thereforeg measurement of the total feed 3 to the stack
accompanied by accurate distribution by a device such as a rotating valve,
small gang type pluager pumps, and the like, is suitable. The distribution
device should be on top of the stack so that the individual feed tubes drain
empty into the denuders to prevent current lea~age through the set of feed
tubes 5. If the insulating fluid 6 provides perfect insulation? the pressure
shell except for the insulating ring 13 can be bare metal. However, the dan-
ger of leaks from the cell stack, in spite of pressure balance controls be-
tween the cell stack and the fluid 6, is eYer present and a major short can
develop. Therefore~ it is preferred that the inner surface of the pressure
shell 1 be lined with a suitable insulating material which can be rubber or
any other plastic material compatible with the fluid 6 and the operating temp-
erature. In addition, the area at the bottom of the shell and the top of the
anode end cap 15 should be shrouded in leak tight combination with ring 13 as
the total differential voltage of the stack exists at that point.
The shell configuration described above assumes that the main out-
put from the rectifier or other direct current source is at or near ground
level. Should this not be the case, the insulating joint in the shell can be
anywhere along the height of the shell and the current connections would then
be placed on either side of the insulating joint.
Referring to Figure 5, 1 is a cooler or heat recovery unit, 2 is a
cell stack~ 3 is an anolyte circulation pump~ 4 is a spent anolyte cooler~ 5 --
is a resaturator~ 6 is a chlorine decanterl 7 is a de~hlorination system~ 8
is a chlorine drying system, 9 is a slurry tank, 10 is a slurry pump~ 11 is
salt feed~ 12 is chlorine product, 13 is an anolyte bleed straam, and 14 is
a purified anolyte bleed return stream.
Figure 5 is a schematic flow sheet for the brine and chlorine sys-
tems surrounding one or several cell stacks producing liquid chlorine directly
-21-
. .
'' : '' '
~3s~7~5
in the cells. The operation of the process depicted in this flow sheet is
obvious therefrom to one skilled in the art. Briefly, it comprises the fol-
lowing. The spent anolyte passes from the cell 2 into the heat exchanger 4
where its temperature is reduced. The cooled anolyte then passes into resat-
urator 5 where its salt content is increased. The anolyte then passes into
the chlorine decanter 6 from which most of the anolyte is recirculated through
pump 3 back into the cell 2. Some of the anolyte from chlorine decanter 6
passes into the dechlorinator 7 from which it is discharged as stream 13.
Purified anolyte, stream 14 and salt 11 are fed into a slurry tank 9 and
passed by slurry pump 10 into the resaturator 5. The chlorine removed from
the chlorine decanter 6 and dechlorinator 7 are passed through the chlorine
dr~yer 8 and discharged as stream 12. The heat transfer fluid from the cell
2 passes through heat exchanger 1 and the spent anolyte cooler ~ from which
it is then passed back into the cell stack 2.
The theoretical decomposition voltage of sodium chloride is about
2.3~ and a stack operates commercially at a voltage of about 2 Yolts above
this theoretical voltage per element. This excess voltage appears as heat
and corresponds to a rate of about 60 KW per daily ton of chlorine produced.
Most of this heat will appear as a temperature rise in the circulating elec-
trolyte and in the caustic produced and some will result in a temperature
rise of the insulating fluid 6 of Figure 4. This heat must be removed. This
can be accomplished by simple heat exchange and the heat rejected to air or
water. However, if an adequate difference in temperature ex~ists between op-
erating temperature and the heat sink, a substantial amount of the electrical ~ -
energy above that theoretically required can be recovered for power generation,
process heat, water desalination and the like. For power generation it would
be beneficial to use a fluid such as Freon with boiling characteristics of
temperature and pressure close to the conditions of the cell stack. This
then makes the coolers and the cell stack into a boiler. The boiling insulat-
-22-
.' ,~,
... . . . . .
.- ' ~ ~ . ' ' . . `
i' . ~ .
73~;
ing fluid can be used to drive a turbine a~ter which it can be condensed and
returned to the coolers 4. Otherwise, distilled water is quite satisfactory
as the insulating fluid.
Liquid chlorine has an unusually large coefficient of thermal ex-
pansion. At room temperature liquid chlorine is much heavier than the ano-
lyte but at higher temperatures the specific gravity of chlorine approaches
or becomes less than that of the anolyte.
The chlorine must therefore be separated from the anolyte either -
by decantation beneath the anolyte or removed at the top of the anolyte. The
temperature conditions in the decanter~ by control if necessary, should en-
sure that there is an adequate gravity difference for separation. Furthermore,
the peculiar characteristics of chlorine indicate that the cell configuration
and operation must be such that they provide against chlorine blanketing of
the membrane when the chlorine is lighter than the electrolyte and against
chlorine flooding of the anode when chlorine :is heavier than the electrolyte.
The dechlorination system 7 shown in Figure 5 is a dechlorination
system that permits bleeding of a relatively small portion of the recirculat-
ing anolyte for two purposes, maintaining impurities at an acceptable level
and slurrying the salt feed to the system. The rate of circulatio~ of the
anolyte may be between about 5 and 10 gallons per minute per daily ton of
chlorine while the bleed stream 15 about 0.3 to 0.5 gallons per minute. The
dechlorination under pressure is accomplished by heating the bleed stream
with suitable heat ~xchange means to recover heat. If a small use for gas- ~ -
eous chlorine, such as the manufacture of hypochlorite or hydrochloric acid
is available, the dechlorination can be accomplished by simply flashing the ~- ;
": :
bleed stream to atmospheric pressure and then dechlorinating by air blowing --
or vacuum in the conventional manner.
Referring to Figure 6, 1 is a cooler or heat recovery unit, 2 is
a cell stack, 3 is an anolyte cooler~ 4~is a resaturator~ 5 is an expansion
; ' : ,
: . ' ' . : '
engine, 6 is a chlorine flash separator, 7 is an anolyte circulation pump, 8
is a dechlorination system, 9 is a chlorine condenser, 10 is a chlorine drying ~system, 11 is a slurry pump, 12 is a slurry tank, 13 is salt feed, 14 is a pur- -
ified anolyte bleed return stream, 15 is chlorine product~ and 16 is an anolyte
bleed stream.
The operation of this a~rrangement can be briefly summarized as fol-
lows. The spent anolyte passes into resaturator 4 and then into expansion en-
gine 5 which is driven by the anolyte as it flows to the chlorine flash separ-
ator 6. The major portion of the anolyte passes from the separator 6 into the
anolyte recirculation pump 7 which pumps the anolyte into the anolyte cooler
3 and back into the cell stack 2. A minor portion of the separated anolyte
passes into the dechlorination system 8 from which it is bled from the system
at stream 16~ The chlorine from the flash separator 6 and from the dechlor-
ination system 8 passes into a chlorine condenser 9~ a chlorine drying system
10 and then passes out as product stream 15. Purified anolyte 14 and salt 13
are passed into a slurry tank 12 and directed by slurry pump 11 into the re-
saturator 4. An electrically-insulating heat-transfer fluid is pumped through ;
the cell stack 2 and into a heat transfer unit 1 from which it is passed
through the anolyte cooler and back into the cell stack.
This flow sheet therefore is similar to the flow sheet of Figure
5, except that it is intended to provide a means of operation for a cell
where the entire chlorine production leaves the cell as a dissolved gas. To
obtain the chlorine product, the pressure of the spent anolyte is reduced from
cell operating pressure which results in the evolution of gaseous chlorine in ~;
relatlvely direct relationship to the ratio of operating pressure and pressure
maintained in the chlorine flash separator 6. Since the chlorine separated ~ ;
in 6 is hot and moist, and under some pressure, it is condensable by cooling.
It is preferred to flash at pressures above 100 psi to be able to take advan-
tage of normally available cooling water.
-24-
73~j
Since very large quantities of anolyte must be pumped from flash
pressure to operating pressure, resulting in considerable energy use, it is
preferred to recover a large proportion of this energy by flashing the spent
anolyte and produced chlorine through a type of expansion engine 5.
The flow sheets of Figures 5 and 6 indicate the production of a
wet, liquid chlorine. This liquid chlorine can be washed free of adherent
salt if necessary and then dried. In normal commercial practice, it is es-
sential that the chlorine be dried so that its moisture content is in equil-
ibrium with sulphuric acid of about 95% at 60 F, as otherwise the steel
equipment in which liquid chlorine is conventionally piped and stored will
corrode.
The salt feed to the system will usually have been purified and pre-
pared on the site. The salt feed should preferably be relatively fine to in-
sure rapid dissolution, as otherwise salt crystals might enter the cell and
crode the membranes.
Referring to Figure 7, 1 is a cell stack, 2 is a caustic cooler,
possibly combined with a heat recovery system (not shown) 3 is a caustic se-
parator, 4 is a caustic circulation pump, 5 is a caustic separator flash
tank, 6 is a hydrogen cooler, 7 is a mercury removal system, 8 is a caustic ;
vacuum degassing vessel, 9 is a vacuum pump~ 10 is a hydrogen stream, 11 is
a recovered mercury stream, 12 is a caustic stream and 13 is a water stream. ;~
Hydrogen and caustic aretwithdrawn from the cell stack together and
separated in 3. Since hydrogen is only very sparingly soluble in caustic~
most of the hydrogen will separate as a hot gas under pressure. After cool-
ing this hot gas in 6, the mercury content of the cold gas3 which is already
very low because the hydrogen is under pressure, is lowered further and the
condensed mercury can be recovered. The very slight quantity of mercury vap-
or present in cold hydrogen under pressure can be further removed in 7 by
known techniques such as chlorine water scrubbing, adsorption and the like.
25-
' ~ ,' . ' '' ' '' '
Because the system operates in a completely encapsulated apparatus as dis-
tinct from the conventional mercury cell and because the mercury content of -~
the hydrogen is inversely proportional to the pressure at which the hydrogen
is generated, the mercury pollution that occurs or must be dealt with is min-
imized by a factor of ten or more. The hydrogen issuing at 10 would then no
longer be of ecological consequence because of the efficient removal of mer-
cury. It should also be noted that the chlorine is produced without the pre-
sence of uncondensable gas and there is~ therefore, no sniff gas to be vented
or treated.
After being separated in 3, most of the caustic would be recycled
for cooling through 2. This cooling system together with the cooling system
shown in Figures 5 and 6 can be operated in various ways depending upon the -
results de~ired. It can either be run simply to cool the caustic~ thus remov~
mg the heat content of the caustic stream, or the caustic heat removal can
be so managed that the caustic stream is at a higher temperature than the
brine stream, thus ~enhancing the possibility for heat recovery as distinct
from simple cooling. The net increase in caustic in the system~ that is the
net caustic production, would be permitted to flow from 3 to 5 where it would
be expanded to nearly atmospheric pressure. Dissolved hydro~en as well as ~-~; 20 small hydrogen bubbles would leave the caustic at that point. Depending upon
the configuration of the system1 the hydrogen leaving in 5 may be free of
mercury in which event lt need not be subjected to any treatment other than
the removal of caustic droplets, or it may contain some mercury contamination,
in which event it can be treated in parallel to the main hydrogen stream is~
suing from 3. Hydrogen has a tendency to remain ss~spended as very tiny bub-
bles in caustic when hydrogen pressure is relieved and it may therefore be
desirable to flash the remnant hydrogen bubbles from the caustic by vacuum `
in 8. The small amount of hydrogen that would be pumped from the caustic at
this point can be joined to the hydrogen stream from S and treated as neces~
:: :
7~i
sary. The caustic coming from 8 can then be sent directly to storage with-
out fear of hydrogen explosions in the storage tanks.
The system is operable over a wide range of temperatures and pres-
sures which will produce liquid or dissolved chlorine, in accordance with
their physical characteristics. Within the range of operable conditions, the
selection of a particular pressure and temperature is determined by balancing
the economic factors by well-known optimization techni~ues. Examples of
temperature and pressure range for liquid chlorine production are from about
100 to 1000 psia at from about 60 to 270 F, or 300 to 650 psia at from 150
to 225 F.
In accordance with economic considerations the processAwould gener-
ally be operated at a temperature above the temperature~-of an available heat ~ -
sink. In common industrial practice~ this would be 80 F. This means that
a cell producing liquid chlorine would be operated at pressures above 100
psi and that the flash pressure in the case of a system where the chlorine
is entirely produced as a dissolved gas would also be no lower than 100 psi
to permit liquefaction of the chlorine without mechanical refrigeration.
The upper pressure/temperature limits are again substantially a
matter of economics. The upper limit of course for liquid chlorine is its `~
critical temperature. At very high pressures the equipment becomes extremely
expensive and therefore the process would not normally be operated above a
pressure of 1000 psi. The operating temperatures which are possible within
these pressure ranges can be determined from w~ll'known data. The possibili-
ties of heat recovery from the system are enchanced by operating at high temp-
eratures so as to have a temperature differ~nce between the brine and caustic
temperatures and the heat sink temperature respectively. With liquid chlorine
production directly in the cell~ a major consideration is the boiling point
curve for liquid chlorine. In the case of dissolved chlorine production in
the cell~ the solubility of chlorine decreases with increasing salt concentra-
--27 :
" :' :.. ' .
;
tion and temperature and increases with increasing pressure. Thus, the great~
er the pressure difference between the cell and the flash separator, the more
chlorine per unit of brine f~ow will be produced per pass. The higher the
salt concentration, the lower will be the chlorine production per pass of
brine. The higher the cell operating temperature, the less chlorine will be
dissolved in the brine but the more heat can be recovered from it. Standard
optimization techniques together with known data concerning the boiling point `~
and solubility of chlorine would be used to determine operating conditions for -
any particular case.
All conventional chlorine cells must be housed as they cannot be
practically operated out of doors. The cells of this invention can be operat-
ed out of doors in almost any climate and the cost of building avoided. The
cell design of the invention requires neither bus work from element to element
nor distributing bus work at each element resulting in the total a~sence of
copper or aluminum bus bars around the cells. This not only saves capital
costs but avoids one of the main maintenance costs of the average chlorine
plant. ;
The foregoing descriptions of the invention have been directed to '!'
the electrolysis of sodium chloride and sodium sulphate, producing caustic soda ~-
and hydrogen in each case. This is however not inte~ded to be a limiting fea-
ture of the invention. Solutions containing other ions, particularly the cor-
responding potassium!r ion solutions, can be electroly7ed in~a similar manner.
The invention set forth herein is generally applicable for compounds that are
capable of being electrolyzed in conventional mercury cathode cells. It is
also useful to carry out other known electrolysis processes of solutions con~
taining alkali metal ions. Of course anolytes (or impurities or interfering
ions therein) and denuding media known to interfere with the proper function-
ing of the electrolyt~c proeess should be avoided. Furthermore~ the denuder ~:
can be used, at times without denuding graphite, for the production of sodium
-28_
`,.;'
, . . .. ~ . - :.
, ......... . .
.. . .
-
73~
sulphide by using so~ium polysulphide as the denuding medium, for the pro-
duction of sodium hydrosulphite by using sulphur dioxide solution as the
denuding medium, for the production of alcoholates, for various organic
reductions, dimerizations and similar reactions.
Also, by careful control of current efficiency ratios to avoid the
oxidization of the metal component of the composite membrane, it is possible
to use this metal component as a bipolar electrode, thus carrying out a fur-
ther react.ion step between a final electrode within the denuder chamber and
the anionic side of the now bipolar metal component of the composite membrane.
An example of such a reaction would be the production of sodium. Furthermore,
although the mercury layer is always described as stationary, this does not `-`-
exclude the alternative of circulating mercury from the composite membrane.
Examples
The following examples were conducted in cells containing a platin~
ized titanium anode facing a composite membrane with substantially saturated
brine flowing between the anode and the composite membrane. The chlorine
produced was withdrawn from the cell with the brine stream. The composite
membrane comprised "Nafion" perfluorosulphonic acid membrane made from a co-
polymer of tetrafluoro ethylene and a vinyl ether of the formula FSO2CF2CF20CF
~CF3)CF20CF=CF2 through conventional thermoplastic techniques followed by con-
version of the pendant sulfonyl fluoride group to the acid from the resulting
copolymer having an equivalent weight in the range of 950 to 1350 ~made by
E.I du Pont de Nemours ~ Co., Inc.), with a layer of mercury above it. The
amount of mercury was sufficient to cover the polymeric membrane entirely,
care being exercised that any corrugations or dimples in the polymeric mem-
brane ~ere also covered. The mercury of the composite membrane was in con-
tact with graphite elements and water was passed over the mercury to produce
caustic and hydrogen above the composite membrane. In all examples, by virtue
of the swelling of the membrane either by means of solvent or by the electro-
- 29 -
. .
': . . . ' ~ , 7
lyte, its surface was extended into the mercu~y because the pressure on the
brine was more than the weight of mercury and caustic above the membrane. The
gross cell surface was appro~imately one square decimeter (about 2" x 8~
The flow of water through a denuder was varied so as to produce various con-
centrations of caustic from less than 10% to over 50%, and no effect on cell
operation was observed as a result.
E~ample 1
A cell was operated at atmospheric pressure with the polymeric mem-
brane having a 2 mil nominal thickness. The brine flow rate was approximately
4.2 gallons per min-ute. The cell voltage was 4.9 at 50 alnperes and was 6.6
volts at 80 amperes. The brine flow rate was increased to 6.4 gallons per
minute to improve the sweeping of gas from the cell and the voltage dropped
~o 6.o at 80 amperes. The temperature was 190 to 205 F.
Example 2
The same cell was operated with a membrane that had first been soak-
ed in hydrochloric acid and then in a saturated aqueous solution of mercuric
chloride at 100 C for 24 hours. The mercury in the membrane was then re- `~
duced to metallic mercury in situ by means of hydroxylamine and the cell oper- ;
ated under the same conditions as before using a brine flow rate of 6.4 gal-
lons per minute. The cell voltage was 5.6 at 80 amperes and 4.6 volts at 50
amperes.
Example 3
The same cell was operated under the same conditions but the mem-
brane was swollen with glycol prior to the mercury impregnation described in
Example 2. The cross cell voltage was 0.2 to 0.3 volts lower at 80 amperes
than the voltage without the use of glycol-treated membrane. -
ample 4
A cell with a 3.5 mil nominal thickness polymeric membrane was oper- -
ated at 75 F and 450 psi pressure with a brine.flow of 50 cc per minute.
~3-
:
:
The anode was grooved to permit li~uid chlorine to run off. The elements
above the mercury consisted of nickel pins of 1/16" diameter on 1/4" centers
with a graphite fabric around and between the pins; the graphite and nickel
being in contact with the mercury. At first, at about 10 a~peres and a cell
voltage of 3.64, chlorine dissolved in brine appeared in a sight glass at the
discharge of the cell. As the amperage was raised to 50 amperes the cell vol-
tage increased and then fell as liquid chlorine appeared and the cell voltage
stabilized at 5.1. The liquid chlorine appeared as a separate phase in the
sight glass together with brine containing dissolved chlorine. Liquid chlor~
ine appeared as the amperage was raised since the production of chlorine ex-
ceeded its solubility in brine. At a higher temperature and with a thinner
polymeric membrane a lower cell voltage could be obtained. The sight glass
was arranged so that the brine and liquid chlorine entered through a dip tube
at the top. The sight glass also had an overflow above the bottom edge of the
dip tube. This arrangement resulted~ at these operating temperatures, in set-
tling of the liquid chlorine at the bottom of the sight glass and an overflow
of the brine from the top of the sight glass. Thus the sight glass functioned
as a decanter. From the overflow, the brine containing dissolved chlorine
flowed to a receiving vessel. Subsequently the pressure in the receiving ves-
sel was reduced and the dissolved chlorine bubbled from the brine. Hydrogen
and caustic flowed from the cell into a vessel where the hydrogen and caustic
separated. The entire system had initially been padded with nitrogen and the
hydrogen was subsequently vented together with the nitrogen pad.
This invention has been described in terms of specific embodiments
set forth in detail. Alternative embodiments will be apparent *o those skill-
ed in the art in view of this disclosure, and accordingly such modifications
are to be contemplated within the spirit of the invention as disclosed and
claimed herein.
_31-
