Note: Descriptions are shown in the official language in which they were submitted.
1066655
The invention relates to a method and apparatus
for concentrating an aqueous sulfuric acid solution. More
specifically, the invention relates to the regeneration
of aqueous sulfuric acid of concentration in excess of
75 weight per cent by means of electrolysis in an electro-
lytic cell utilizing platinum electrodes.
The problem of satisfactorily, efficiently and
economically removing water and other impurities, e.g.,
organic compounds, from aqueous sulfuric acid solutions
containing more than about 7S per cent by weight H2SO4
has been a long standing enigma. For example, evaporation
of the water by distillation requires the use of high
temperatures at which the hot concentrated acid is highly
corrosive and is difficult to contain. Attempts to utilize
electrolysis to concentrate sulfuric acid in these high
concentration ranges have been frustrated by the great
speed at which electrodes and container materials are degraded
by the attack of the acid. Platinum electrodes or platinum-
-coated electrodes are generally required, being the most
resistant to sulfuric acid corrosion of all the electrode
materials.
However, previous attempts to utilize platinum
electrodes in such an electrolytic cell have demonstrated
that in addition to the production of hydrogen gas and o~ygen
gas thereby decomposing and removing water from the acid,
cathodic reduction of the acid itself occurs forming solid
elemental sulfur which deposits on the cathode znd dis-
perses into the acid. Such sulfuric acid, contaminated
- with solid sulfur, is generally unsuitable for use in
industrial applications. This has made the eIectrolytic
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10~6655
method appear to be unsatisfactory for providing high con-
centrations of sulfuric acid on a commercial basis, in that
without being able to use platinum electrodes, electrode
replacement becomes necessary so often that the electrolysis
process becomes economically objectionable.
For example, U. S. Patents 1,992,308 and 1,992,310
utilize a lead, rather than a platinum, cathode to remove
sulfur-containing organic compounds from hydrocarbon fluids
;~ by electrolysis. U. S. Patents 2,793,181 and 2,793,182
disclose electrolytically regenerated spent alkylation sul- -
furic acid solutions in a cell containing platinum electrodes.
However, free sulfur was formed in the cathode compartment
of the cell. U. S. Patent 3,616,337 teaches that the use of
a platinum cathode in an electrolytic sulfuric acid regen-
eration process was not satisfactory in that sulfates were
reduced to free sulfur thereon.
A method has now been discovered that permits
platinum to be used for either or both the anode or the
cathode in an electrolytic cell which is used to concentrate
aqueous sulfuric acid solutions wherein no objectionable
sulfur build-up results.
The invention resides in a method of concentrating
an aqueous solution of sulfuric acid of concentration
greater than about 75 per cent H2SO4 by electrolysis in an
electrolytic cell containing at least one corrosion resis-
tant anode and at least one corrosion resistant cathode,
comprising the step of providing a quantity of persulfate
ions in the sulfuric acid solution in a region immediately
surrounding and communicating with said cathode sufficient
to prevent a build-up of elemental sulfur thereon during
electrolysis.
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The invention further resides in a method of :~
reducing the H2O content of a wet fluid by intimately con- ~
tacting the wet fluid with a sulfuric acid solution having ~.
a concentration greater than about 75 weight per cent H2SO4
. 5 whereby the H2O from the wet fluid is absorbed by the sul-furic acid solution to produce a dry fluid which is sub-
stantially free of H2O, said fluid being incapable of form-
ing a chemical reaction with the concentrated sulfuric acid
solution, being immiscible with and insoluble in the acid
solution and being readily separable from the acid solution
after contact therewith, separating the wet sulfuric acid
solution having the H2O absorbed therein from the fluid,
and thereafter reducing the H2O content of the wet sulfuric
acid solution, the improvement which comprises reducing the
H2O content of said wet sulfuric acid solution by elec$rol- -
ysis in an electrolytic cell having at least one corrosion
resistant anode and at least one corrosion resistant cathode
while providing a quantity of persulfate ions in the sul-
furic acid in the region immediately surrounding and communi-
cating with said cathode to prevent a build-up of elemental
sulfur thereon during electrolysis.
. The invention further resides in an apparatus for
reducing the H2O content of a wet fluid comprising a means
`. for contacting the wet fluid with a sulfuric acid solution
having a concentration of greater than about 75 weight per
cent H2SO4 whereby the H2O from the wet fluid is absorbed by
the sulfuric acid solution to produce a dry fluid which is
substantially free of H2O, said fluid being incapable of
~- forming a chemical reaction with the concentrated sulfuric
30 acid solution, being immiscible with and insoluble in the .. .
- acid solution and being readily separable from the acid after
: contact therewith; a means for separating the wet sulfuric
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1066655
acid solution having the H2O absorbed therein from the fluid, an electro-
lysis cell for electrolyzing at least part of the H2O content of said
separated sulfuric acid solution, said electrolysis cell being equipped
with at least one corrosion resistant anode and at least one corrosion
resistant cathode, a first conduit means for conveying the separated wet
sulfuric acid solution from the contacting means to the electrolysis cell,
a second conduit means for conveying the concentrated dry sulfuric acid
solution from the electrolysis cell to the contacting means, the improve-
ment comprising means for circulating the sulfuric acid solution in a closed
. ..
loop cycle from said contacting means through the first contuit means to
the electrolysis cell, through the second conduit means, and back to the
contacting means, and means for providing a sufficient quantity of per-
sulfate ions in the region immediately surrounding and in communication
with the cathode to prevent build-up of elemental sulfur on the cathode
during electrolysis.
Figure 1 is a view in vertical section of an embodiment of an
apparatus for practicing the method of the present invention.
Figure 2 is a view in vertical section of another embodiment of
apparatus used in practicing the method of the present invention in which
the oxidizing agent is persulfate ion generated in situ and the H2 and 2
gases formed during electrolysis are segregated.
Figure 3 is a top view in horizontal section, mostly broken away,
of yet another embodiment of apparatus used according to a method of the
present invention permitting the impure sulfuric acid solution to be sub-
jected to electrolysis action while being flowed through an electrolysis
cell in a continuous manner.
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106665S
- Figure 4 is a schematic illustration of a system
for drying sulfuric acid by electrolysis according to the ~ :
.,. ~ . .
present invention as an integral part of a fluid drying
process.
The operation of a basic embodiment of the pre~
sent invention will be better understood upon becoming
. familiar with the following description, reference being
had to the accompanying drawings. ~ -
; Referring now to Figure 1, a container 11 is
shown partially filled with aqueous sulfuric acid solution
~' 12 which serves as an electrolyte in which a cathode 13 and
an anode 14 are partly immersed. A source 15 provides
direct current electricity to the anode 14 and the cathode
13. Hydrogen is generated at the cathode 13 and oxygen is
, 15 generated at the anode 14. The mixture of gases is removed , ~,
:. ' '
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1066655
through a port 16 in the upper wall of container 11. The
aqueous H2SO4 to be concentrated is conducted into the
container 11 by a pipe 17 extending through the sidewall of
the container 11 and which is fitted with an inlet valve 18.
When the desired acid concentrati~n is attained, the acid
is removed from the container 11 via the pipe 19 which
communicates with the floor of the container and which is
- fitted with an outlet valve 20. A mixing device 21, such
as a motor driven propeller, the shaft 22 of which extends
through a wall in the container 11 and into the acid, may
be provided to increase electrolyte circulation.
It has been found that the passage of an electro-
lytic current through the aqueous sulfuric acid electrolyte
for the primary purpose of electrolyzing the water present
also generates reduction products of sulfuric acid such
as sulfite ions and sulfur at the cathode together with
oxidation products of sulfuric acid such as persulfate ions
at the anode of the cell or vessel in which the electrolysis
is being conducted. When a sufficient amount of persulfate
ion is supplied to the cathode, all or substantially all
of the reduction products formed on electrolysis of H2SO4
and other sulfur compounds are oxidized, preventing the
build-up of elemental sulfur.
Providing a sufficient quantity of persulfate ions
1 25 at the cathode depends on (1) supplying persulfate ions to
; the solution at a sufficient rate and (2) effecting sufficient
diffusion or transport of the persulfate ions from their
point of supply to the cathode. Persulfate ions may be added
from an external source in salt form, e.g., sodium persulfate,
or they may be generated in situ by maintaining an anodic
17,454-F -5_
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~066655
current density of between 0.5 and 5 amps/sq. in. (0.078
to 0.78 amps/cm2). The rate of supply of persulfate from
an external source depends on the rate at which the persulfate
source is added to the solution and its solubility therein.
The rate at which persulfate ions are generated at the anode
depends on anodic current density, electrolytic temperature,
and on the concentration of H2SO4 in the aqueous acid
electrolyte.
The diffusion or transport of persulfate ion from
anode to cathode depends on electrode spacing and on the
degree of mixing provided in the electrolyte. The persulfate
ions are believed to contact the cathodic reduction products,
rather than the cathode itself, due to the electrostatic
repulsive forces existing between the cathode and the per-
sulfate ions. However, for simplicity of explanation in
the present application, the persulfate ions will be said
~4~ ~crm
~i s 3 to ~contact~ the cathode, by which ~e~ the interaction
just described will be denoted. ;~
The cathodic reduction products are believed to
include sulfite ions, sulfur and intermediates such as
thionate ions, thiosulfate ions, hyposulfite ions, poly-
sulfide ions and the like. In the practice of the present
method it is believed that the intermediates along the path
to formation of free sulfur are actually oxidized before
sulfur is allowed to form, although the sulfur itself, if
formed (e.g., by suspension of the method of the present
invention) will be oxidized back into solution (e.g. to
sulfate ion) by the method of the present invention.
When the method of the present invention is
utilized, a slight threshold film of sulfur is sometimes
17,454-F -6-
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1066655
found to form on the cathode. At the lower anodic current
densities, e.g., about 0.5 amps/sq. in. (0.078 amps/cm ),
this amount may be .0001 gm. sulfur/sq. in. (.000016 gm/cm2)
of cathode surface area. At higher current densities, e.g.,
4 amps/sq. in. (0.62 amps/cm ), this ~ilm is virtually
undetectable, i.e., less than about .0001 gm/sq. in.
After formation of this slight initial film
however, any further increase in the amount of sulfur on
the cathode is prevented by the practice of the present
invention. The term build-up is used in this application
to denote the further increase in the amount of sulfur
on the cathode, which increase may be controlled accord-
ing to the present method to limit it to zero or any
other predetermined rate less than that which would occur
absent the practice of the present invention.
As an example of the build-up of sulfur occurring
absent the practice of the present invention, an electrolysis
was ~onducted on an 85.2 per cent by weight H2SO4 solution
with an anodic current density of 0.15 amp/sq. in. (0.023
amp/cm ); sulfur built-up on the platinum cathode at the
rate of about 0.11 gm. sulfur/gm. H2O removed from the
acid solution (Table II, Comparison 1).
i The electrolyte used herein is the aqueous
sulfuric acid solution to be purified, from which all
or a portion of the water and organic compounds present,
if any, are to be removed. Aqueous sulfuric acid solutions
; of any concentration may be further concentrated by the
method of the present invention. However, acid solutions
of initial concentration ranging upward from about 75 weight
per cent H2SO4 are the acid solutions wherein sulfur
17,454-F -7-
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1066655
build-up has been found to be a problem and thus is defined
as the range of applicability of the present invention.
In the course of being concentrated by the present
method, the aqueous acid solution may be purified of virtually
any organic compound which is oxidizable by persulfate ions
in a medium of aqueous sulfuric acid. The persulfate ions
generated herein in the aqueous sulfuric acid solution are
such a powerful oxidizing agent that almost any organic
material is oxidized. The extensively fluorinated organic
compounds are less responsive to oxidation by persulfate
ion; perfluorinated compounds such as e~}4~ are found not ~-
to be oxidized to a noticeable degree. However, the presence
of non-oxidized perfluorinated organic compounds in the
aqueous sulfuric acid solution is not generally deleterious
due to the relative inertness of such compounds. Exemplary,
though not exhaustive of organic compounds which are well
, oxidized are butylene glycols, phenolics, sulfonated phenols,
polystyrene derivatives, e.g., sulfonated polystyrenes, and
halohydrocarbons.
~ Any initial concentration of organics may be
treated, the higher concentrations requiring a greater
quantity of electricity to be utilized to attain a particuiar
- final organic compound concentration. This in turn requires
a greater electrolytic current flow or the use of a greater
period of time to achieve the particular final concentration
desired.
Exemplary of normally encountered industrial
waste streams are those containing about 200 ppm or more
total organic carbon (TOC). The present method is usable
on these streams as well as on streams wherein the
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10666~5
concentration of organic matter is in excess of 2500 ppm
TOC. As the organic compounds are in the process of being
- removed, the electrolytic current also breaks water down
into hydrogen and oxygen, resulting in an increase of
the H2SO4~concentration in the aqueous solution. This
concentrated solution may be diluted by adding pure
water to obtain a yet lower concentration of organic
impurities while simultaneously returning the solution
to its original concentration of H2SO4.
The aqueous sulfuric acid may be electrolyzed
to 100 weight per cent H2SO4 or higher by practicing
the method of the invention. Alternatively, the acid
may be withdrawn from the electrolytic cell at any
desired concentration between the initial value and
100 per cent.
The electrolytic cell or container may be
made of any material that does not adversely react
with the electrolyte under conditions encountered
in the electrolysis, e.g., the presence of a strong
oxidizing ager.t, electric current flow, and elevated
temperature. Representative, though not exhaus/ive
of the usable materials are glass, ceramics, te~lon,
and platinum, or other materials of construction
having a protective coating formed of one of these
acid resistant materials.
Since the electrodes are likewise in contact
with the strongly oxidizing concentrated sulfuric
acid, platinum is used as the electrode material of
construction. Platinum is the element which is most
resistant to corrosive degradation, and therefore,
., ' '
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needs to be replaced the least often and contaminates
the solution to the least degree possible. Alternatively,
platinum may be utilized as a coating on other substrates,
such as steel or iron, for example, so long as adequate
corrosion resistance is provided.
In the practice of the present electrolytic method,
corrosive degradation of a platinum anode is found to be very
slow and to involve the substantial redeposition, i.e., about
90 per cent, of the platinum lost from the anode onto the
cathode. Accordingly, the flow of electrolytic current may
be reversed periodically to permit the platinum anode to
regain approximately 81 per cent of its platinum loss dur- -
; ~ ~g ing the cycle. The bipolar design ofrelectrolytic cell (Fig.
3) using platinum electrodes is particularly suitable
: 15 for the electrolysis of aqueous sulfuric acid because
the platinum lost from the anodic surfaces of the interior
electrodes, i.e., electrodes between the ends of the array,
is substantially regained on the cathodic surfaces of
adjacent interior electrodes.
In controlling the build-up of any noticeable -
amount of sulfur deposit on the cathode, the persulfate
ion may be supplied continuously or intermittently, so
long as there is at all times a sufficient amount of
` persulfate ion in contact with the cathode to reoxidize
the reduction products of H2SO4 which are formed at the
cathode. In some situations temporary build-up of sulfur
deposits on the cathode may be tolerable, in which event
` persulfate ions need be supplied to the cathode in suffi-
cient amount to reoxidize the H2SO4 reduction products only
at the point when the sulfur deposits must finally be removed.
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1 ,454-F -10-
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1066655
As indicated above, persulfate ions may be
supplied either from an external source, e.g., a persulfate
salt, or they can be generated in situ by maintenance of
an appropriate anodic current density. Since it is
the simpler and more efficient technique, the in situ
generation of persulfate ions is the preferred technique.
Overall, the rate at which in situ generated persulfate
ions are provided to the cathode is a function of the
anodic current density, electrolyte temperature, H2SO4
concentration in the aqueous acid electrolyte, inter-electrode
separatio~ distance, and degree of electrolyte mixing.
In the preferred practice of this embodiment of the present
invention, the aqueous acid will be encountered at a given
I
concentration and temperature. Optimum electrode spacing
is chosen, an adequate degree of mixing is provided, and
; then a current density at least as great as the minimum
anodic current density sufficient to prevent the build-up
of elemental sulfur on the cathode is applied to the ;
electrolyte. For example, with electrolyte temperature
, 20 50C, electrode separation of 1/4 inch (0.64 cm), H2SO4
i concentration 80 per cent by weight, and with amounts
;~ of mixing varying from mild to vigorous, anodic current
densities of from about 0.5 to about 5 amperes per square
inch (0.078 to 0.78 amp/cm2) are found suitable. Higher
current densities permit faster electrolysis of the aqueous
H2SO4 solution and are effective in preventing sulfur
formation, but may cause greater materials corrosion
and a greater density of gas bubbles to evolve. These
bubbles reduce electrolytic conductivity and are thus
detrimental to current efficiency. -
17,454-F -11-
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1066655
The electrolyte may be maintained at any
temperature above its freezing point. Higher temperatures
are advantageous in that the cathode reduction products
are oxidized faster and the electrolyte conductivity
increases with temperature, thus pef~itting persulfate ~-
ions to be generated at a faster rate. However, higher
temperatures are disadvantageous in that persulfate ion
decomposes more rapidly, and corrosivity and resulting
materials of construction problems are increased. Presently
operating temperatures from about 10C to about 80C
are preferred, although use of higher temperatures may
become feasible if appropriate materials of construction
are developed.
It is found that use of current densities in the
upper end of the preferred range causes a noticeable
temperature increase in the aqueous H2SO4 electrolyte.
If the temperature increase becomes objectionable, e.g.,
from a corrosivity point of view, e.q., at about ~aoc,
a cooling device may be provided in or around the electrolyte
to prevent the electrolyte from exceeding a desired operating
. temperature limit. Such cooling devices are well known in
the art of electrolytic cell construction and may be -
adapted to the particular embodiment of the present invention
practiced.
Positioning the electrodes close to one another
` not only enhances diffusion of persulfate ions from the
anode to the cathode, but also reduces the amount of ohmic
resistance encountered by the electrolytic current. This
in turn reduces the power required to electrolyze the
quantity of water present.
17,454-F -12-
106665S
However, spacing the electrodes too close to one
another is to be avoided because the bubbles impair the
conductivity of the electrolyte, increasing the power
required to electrolyze the water present.
Use of high current densities causes extensive
bubble formation and, to maintain sufficient electrolyte
conductivity, the electrodes must be spaced further apart
than when low current densities are used. It is found that
with current densities in the range of O.S to 5 amperes
per square inch (0.078 to 0.78 amp/cm2), separation distances
of about 1/4 inch (0.64 cm) are preferred. From the
; foregoing it will be apparent to those skilled in the art
how to utilize greater or lesser separation distances as may
be appropriate to a given system.
, 15 Although the evolution of bubbles of hydrogen and
oxygen gas during the electrolysis effects substantial
mixing of the electrolyte, it is generally desirable to
provide additional mixing, e.g., by means of a stirrer
as indicated in Figures 1 and 2. Almost any other known
means of mixing, such as an inert gas sparge, may also
be used. Mechanical mixing so vigorous that cavitation
~ and aeration of the electrolyte results is to be avoided
A due to the reduced electrical conductivity caused by
excessive gas bubble formation and entrainment.
Though not essential to the invention, it may be
found desirable to provide a liquid permeable membrane
; between the electrodes to separate hydrogen and oxygen as
they are evolved, e.g., where they are to be utilized
separately or where there exists a risk of ignition of
the mixture of H2 and 2 If a membrane is used, it
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- 1066655
must not impede the diffusion of persulfate ion from the
anode to the cathode. A generally suitable membrane
may be constructed, e.g., of a woven fiberglass net
; with open spaces approximately 0.5 mm. x 0.5 mm. Such
an embodiment is illustrated in Figure 2, in which like
elements common to both Figuresl and 2 are identified
by like reference numerals. Herein a membrane 25 is
placed between the electrodes under the surface of the
electrolyte. A partition 26 extends from the upper
wall of the container 11 to a depth beneath the surface
of the electrolyte to join the membrane 25 so as to form
a barrier. Hydrogen evolved at the cathode 13 is now
removed through port 27 in the upper wall of the container
11 and oxygen evolved at the anode 14 is removed through
-' 15 port 28, also in the upper wall of the container 11, thus
segregating the gases and eliminating the explosive
combustion hazard.
A preferred embodiment of the present invention
is illustrated in Figure 3 as a top view, mostly broken
away and sXown in horizontal section. Aqueous sulfuric
acid solution may be concentrated therein by removal of
water and/or organic compound impurities on a continuous
basis. Aqueous acid is fed into a bipolar electrolytic
cell through an entry pipe 31. The acid is directed
past bipolar electrode baffles 32 to flow along the path
indicated by the arrows before leaving the cell through
the exit port 33. Against the entry end of the cell is
placed a cathode 34. Against the exit end of the cell
is placed an anode 35. The end electrodes 34 and 35
are supplied with electricity by a source of direct
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1066655
current electricity 36. Hydrogen gas evolves at
the cathode 34 and at the baffle plates 32 on the
surface which face toward the anode. Oxygen gas
evolves at the anode 35 and at the baffle plates 32
on the surfaces which are closer to the cathode.
These evolved gases are removed through an exit port 37
placed in the upper wall 38 of the vessel 30.
- The considerations relating to choice of materials
for a bipolar electrolytic cell for continuous treatment are
very similar to those enumerated for the basic embodiment
of the invention as illustrated in Figures 1 and 2. The
` electrodes and baffles are all constructed from an electrically
conductive material that withstands the corrosivity of the
medium, e.g., platinum. The baffles are uniformly separated
from the end electrodes and from one another at a distance
ot about 1/4 inch (0.64 cmJ, depending on various other para-
m^ters as outlined above. The baffles, as mounted, must -
; be elctrically insulated from the side walIs unless the
~ walls are made of nonconductive material. An externally
; 20 driven mixer is ordinarily not required; the flow of the
electrolyte as directed by the baffles and by the evolved
gas bubbles provides sufficient electrolyte circulation.
Aqueous sulfuric acid is fed continuously through
the concentrating vessel, generally at as great a rate as
is acceptable in terms of the amount of water removal
required. This rate will thus depend on the initial
concentration of the acid, the current through the vessel
and the final concentration of acid desired. For example,
with an initial 56 per cent by weight H2SO4 concentration
and using an anode current density of about 3 amps/sq. in.
. I , .
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~066655
(0.47 amp/cm2), a final concentration of 96 per cent
H2SO4 was obtained.
A particularly useful application of the present
invention is in a closed system drying process for fluids
containing water. The H2O content of the fluid is substan-
tially removed by intimately contacting the fluid with con-
centrated sulfuric acid in cocurrent or countercurrent fashion
as is well understood in the art. The H2O is absorbed by
the sulfuric acid which is separated from the fluid and
then is reconcentrated by subjecting the aqueous sulfuric
acid to electrolysis according to the practice of this in-
vention.
; The fluid to be dried may be either a liquid or
a gas and should not be subject to deleterious reaction with
, lS concentrated aqueous sulfuric acid, and it should not be
miscible with or soluble in the acid or else it may not
be readily separable from the acid after contacting.
Examples of fluids advantageously dried using the present
integrated system include the halogens and hydrocarbon
liquids and gases. The extent to which H2O must be removed
from the H2SO4 solution by the present method depends on
the nature of the fluid and the extent to which the H2O-
content therein is to be reduced for subsequent use of the
acid.
me entire fluid drying process may be operated
on continuously recycling one initial charge of sulfuric
acid, with occasional small additions of make up acid.
Figure 4 presents the integrated fluid drying
process in schematic form. Referring to the drawing in
Figure 4, the wet fluid stream 41 to be dried is contacted
- with concentrated sulfuric acid furnished by supply stream
'' .,
17,454-F -16-
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10666~;5
42 in contacting device 43, wherein the fluid gives up a
substantial part of its H2O content to the concentrated
sulfuric acid. The fluid stream leaves the contacting
device 43 as exit stream 44 substantially reduced in H2O
; 5 content. The sulfuric acid which has absorbed H2O in the
contacting device leaves the contacting device as exit
stream 45 which is conducted into dilute acid reservoir 46.
Periodically the dilute acid content in reservoir 46 is
; emptied by a feed stream 47 into an electrolytic cell 48.
This cell is provided with a persulfate oxidizing agent
according to any of the practices or modifications of the
present invention and an electric current is supplied to
effect electrolysis which removes water from the aqueous
acid without elemental sulfur formation or build-up.
Periodically, after the electrolysis has proceeded
to the point that the acid in a relatively simple electrolysis
cell has been concentrated to the desired percentage of
H2SO4, electrolysis is suspended and the acid content of the
electrolytic cell is removed by a pump 49 and is conducted as
stream 50 to a concentrated acid reservoir 51. The content
of this reservoir is fed continuously as a supply stream
42 back to the contacting device 43 to be brought into contact
with the fluid stream 41 to be dried.
' Alternatively the continuous flow electrolysis
apparatus illustrated in Figure 3 utilizing a bipolar cell
is employed, generally making unnecessary the reservoirs
46 and 51. This modification of the integrated system
permits continuous cycling of the aqueous sulfuric acid
solution between the contacting device and the electrolytic
cell.
17,454-F -17-
1066655
Example 1
In tests 1-3 and comparative run 1-3 the effect
of electrode spacing and electrolyte mixing were evaluated.
In tests 1-3, electrolysis of an aqueous H2SO4 solution was
conducted according to the method of the present invention
in a U-shaped borosilicate glass tube, into one arm of
which were placed a platinum sheet anode and a platinum
sheet cathode, and into the other arm of which was placed
a glass stirrer. The platinum electrodes were 4 mils thick
- 10 (0.0016 cm), had a surface area of 1.43 sq. in. (9.21 cm2)
each, and were separated by 1/4 inch (0.64 cm). Stirring
was accomplished by rotation of a stirring rod, provision
of a N2 stream to bubble beneath and upward past the
electrodes, and also by provision of a bridge between the
two arms of the U tube at a point above the electrodes.
In comparative runs 1-3 no mixing was provided -
except that due to the rising bubbles of H2 and 2 formed
by the electrolysis. A borosilicate glass U tube was
employedt however, this time the electrode separation distance
was 1 inch, the anode being placed in one arm of the U tube
and the cathode in the other arm. The platinum electrodes were
4 mils (.0016 cm) thick and had a surface area of 0.71 sq.
in. (4.58 cm2) each.
By comparison of the tests and their individual
comparative runs, it can readily be seen that providing
effective mixing of the electrolyte and close electrode
spacing effects a definite reduction in the amount of sulfur
formed during the electrolysis. The difference in surface
area between the electrodes in the tests and comparatives is
not considered material to the results obtained. The results
of Example 1 are summarized in Table I.
i, ,
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1066655
Example 2
In tests 1-3 and in comparative runs 1-3 sulfuric
acid was concentrated by electrolysis utilizing the apparatus
illustrated in Figure 1. In comparative runs 1-3 the
anodlc current density used was insufficient to generate
enough persulfate ions to prevent sulfur build-up on the
cathode. In tests 1-3 the method of the present invention -
was practiced in that sufficiently great anodic current
densities were utilized to generate enough persulfate ions ~ ~-
to prevent the build-up of sulfur deposits on the cathode.
The cell was constructed of borosilicate glass. The
electrodes each were constructed of platinum and had a surface
area of 1.4 sq. in. (9.0 cm ). They were placed 1/4 inch
(0.64 cm) apart. There was no membrane or wall between the
cathode and the anode. The starting concentration of the
sulfuric acid in each case was 85.2 per cent by weight.
The effects of various electrolyte temperatures
and anodic current densities were tested. The parameters
and results are presented in Table II.
By comparing comparison runs 1, 2 and 3 with
one another and tests 1, 2 and 3 with one another, it is
seen that conducting the electrolysis at lower temperatures
is beneficial in reducing the amount of sulfur formed.
By comparing test 1 with comparison 1, test 2 with comparison
2, and test 3 with comparison 3, it is seen that conducting
the electrolysis at higher anodic current densities permits
attaining of the desired H2SO4 concentration in considerably
shorter time periods, during which substantially less sulfur
is formed. Furthermore, comparing the set of comparative
runs 1, 2 and 3 with the set of tests 1, 2 and 3 shows
17,454-F -19-
: ~ ..... .. ., : .
1066655
that utilization of the appropriate anodic current density
in conjunction with sufficiently close electrode spacing
essentially eliminates sulfur build-up (i.e., prevents
further deposit of sulfur after the initially deposited
trace amount) during concentration by electrolysis between
about 85 per cent H2SO4 and 95 per cent H2SO4.
TABLE I
,
Concentration Current Sulfur
H SO in Density Formed
Wt2Pe~ Cent amps/ (~ps/ (mgs/g
Run Initial Final Sq. In. cm ) H2O)
Test 1 85.0 92.6 0.12 0.019 0.37
Comparison 1 85.0 94.1 0.13 0.020 4.8
Test 2 85.3 94.5 0.25 0.039 0.66
Comparison 2 85.0 94.2 0.27 0.042 0.87
Test 3 85.0 94.7 0.67 0.104 0.039
Comparison 3 85.0 95.9 1.44 0.22 0.39
,
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