US3616353A - Maintaining electrolytic cell in standby condition - Google Patents

Abstract

An electrolytic cell wherein a porous electrode is employed is maintained in a standby condition during periods when the cell is not in operation by maintaining a small but effective flow of electric current through said cell during said periods the cell is not in operation. Said small flow of electric current is sufficient to substantially reduce invasion of said porous electrode by the electrolyte contained in the cell.

Description

United States Patent Inventors Appl. No. Filed Patented Assignee MAINTAINING ELECTROLYTIC CELL IN [56] References Cited UNITED STATES PATENTS 2,909,471 10/1959 Nies 204/196 3,298,940 1/1967 Ashley et al. 204/59 Primary ExaminerT. Tung Allorney- Young and Quigg ABSTRACT: An electrolytic cell wherein a porous electrode STANDBY CONDITION 9Claims3Dmwing Figs. is employed is mainta ned in a standby condit on during periods when the cell 15 not in operation by maintaining a US. Cl 204/147. small but effective flow of electric current through said cell 2 during said periods the cell is not in operation. Said small flow Int. Cl B0lk 3/00 of electric current is sufficient to substantially reduce invasion Field 01 Search 204/147, f aid porous electrode by the electrolyte contained in the 148,196, 197 cell.

TO CATHODE BUS 0 ANODE BUS 24 ELECTROLYTE LEVEL MAINTAINTNG ELECTROLYTIC CELL IN STANDBY CONDITION This invention relates to maintaining an electrolytic cell in standby condition when said cell is not in operation.

Electrolytic processes for preparing or converting a wide variety of materials are well known in the art. Generally speaking, these processes usually involve immersing an electrode element in an electrolyte and passing an electric current through said electrolyte between said electrode and an oppositely charged element, e.g., another electrode immersed in said electrolyte or the cell body which can serve as said other element or electrode. These processes include processes described as anode processes, e.g., processes wherein the desired reaction is carried out at or in the region of the anode, and also processes described as cathode processes, e.g., processes wherein the desired reaction is carried out at or in the region of the cathode. In recent years electrolytic processes have been developed wherein porous electrode elements are employed. In one process employing porous electrodes a feedstock to be converted is bubbled through the porous electrode into the electrolyte.

Recently it has been discovered that the reaction in an electrochemical conversion operation can be carried out within the confines of the porous electrode element itself. This type of operation is of particular utility with many feedstocks because it provides or makes possible a simple one-step route, at relatively high conversions, to partially converted products which had previously been difficult to obtain. This is particularly true with respect to electrochemical fluorination processes. Carrying out the fluorination reaction within a porous anode, in addition to making possible the direct production of partially fiuorinated products, also allows operation at high rates of conversion and without substantial formation of cleavage products generally produced by the older methods when operating at high conversion rates. The feed to be fluorinated can be introduced into pores of the porous anode at a point near its bottom and the fluorinated mixture removed from said pores at the top of the anode, generally above the electrolyte level. Passage of the feed into the bulk of the electrolyte is avoided.

One problem which has been encountered in processes employing porous electrodes is excessive invasion (or flooding) of the porous electrode with electrolyte during periods when the cell is not in operation. This is particularly true when the electrode is immersed in the electrolyte to depths greater than about 6 inches. This creates problems when the operation of the cell is resumed. For example, excessive filling of the pores of the porous electrode with electrolyte reduces the feedstock or reactant flow capacity of the electrode and tends to restrict the feed rate, the level and rate of conversion, and/or the efficiency of the process in general for considerable periods of time when cell operation is resumed. Furthermore, in cells provided with porous electrodes having lateral feed passages (described hereinafter) said feed passages become clogged or at least partially filled with electrolyte. This results in uneven feed distribution within the electrode.

The present invention provides a solution for the abovedescribed problems. It has now been discovered that by maintaining a small flow of electric current through the cell during periods when the cell is not in operation, a substantial reduction in electrolyte invasion of the porous electrode(s) can be obtained. Thus, broadly speaking, the present invention comprises maintaining a small but eflective flow of electric current between the electrodes in an electrolytic cell during periods when said cell is not in operation, said small flow of electric current being sufi'icient to prevent excessive invasion of the porous electrode(s) by the electrolyte in the cell.

While the invention is not to be limited by any theories as to its operation, it is presently believed that in electrochemical conversion processes carried out within the pores of a porous electrode, e.g., porous carbon, a three-phase interface exists between (a) the pore surfaces, (b) the converting species from or carried by the electrolyte, and (c) the reactant to be converted. Thus, some invasion of the electrode pores by the electrolyte is believed desirable. However, excessive invasion must be avoided for the reasons given above. it is desirable that the fraction of the electrode pores (pore volume) filled by the electrolyte not exceed about 0.7, preferably about 0.5, more preferably about 0.04.

An object of this invention is to provide an improved electrolytic process. Another object of this invention is to provide a method of operating an electrolytic cell having disposed therein one or more porous electrodes. Another object of this invention is to provide a method of maintaining an electrolytic cell in standby condition during periods when said cell is not in operation. Another object of this invention is to provide a method of protecting porous electrodes employed in an electrolytic cell when said cell is not in operation. Another object of this invention is to provide a method of preventing excessive electrolyte invasion of a porous electrode employed in an electrolytic cell. Other aspects, objects, and advantages of the invention will be apparent to those skilled in the art in view of this disclosure.

Thus, according to the invention, there is provided a method for maintaining an electrolytic cell in standby condition during a period when said cell is not being used, said cell being provided with at least one porous electrode which is at least partially immersed in a current-conducting electrolyte which excessively invades said porous electrode during said periods when said cell is not in use, which method comprises: maintaining a small flow of electric current, sufiicient to substantially reduce said electrolyte invasion, between said porous electrode and another electrode element during said period when said cell is not in use.

In the practice of the invention, the amount of current flow maintained on the cell during periods when said cell is not in operation can be any small but effective amount which is sufficient to substantially reduce invasion of the porous electrode(s) by the electrolyte. Generally speaking, said current flow can be within the range of from 0.0l to 2 milliamps per square centimeter of electrode geometric area. In many instances, a presently preferred range of current flow is within the range of 0.01 to l, more preferably 0.0l to 0.5, milliamps per square centimeter of electrode geometric area. However, it is within the scope of the invention to employ amounts of current flow outside said ranges. For economic reasons it is preferred that the amount of current flow employed be as small as possible, commensurate with the desired results.

The invention is applicable to porous electrodes of any operable size, shape, or configuration, and/or manner of disposing the electrode in the electrolyte. For example, the porous element of the electrode can be cylindrical or rectangular (including square). Said porous element can be fabricated from any suitable current-conducting porous material which can be employed in the electrolyte being used in a particular electrolytic process, e.g., nickel, iron, various metal alloys, and carbon. Porous carbon impregnated with a metal such as nickel can also be used. Porous carbon is a presently preferred electrode material for many electrolytic processes. Various grades of porous carbon are available commercially. Porous carbons having average pore diameters within the range of from 1 to 150, preferably 40 to 140, and frequently more preferably 50 to 120, microns can be used in making electrodes for use in various electrolytic processes. Said porous electrodes can be employed in any convenient cell arrangement or cell body.

The invention is applicable to any electrolytic process wherein a porous electrode or electrodes can be employed, and wherein excessive invasion of said electrode(s) occurs during periods when the cell is not in operation. The invention is particularly applicable to electrochemical conversion processes in which porous electrodes can be employed and the feedstock is introduced into the pores of an electrode in essentially vapor phase. Some examples of such processes are electrochemical halogenation, electrochemical dehalogenation, electrochemical cyanation, and cathodic conversions such as the reduction of alcohols to hydrocarbons, the reduction of organic ozonalysis products, or the reduction of organic acids to alcohols. One electrochemical conversion process in which the invention is particularly valuable is the electrochemical fluorination of fluorinatable materials in the presence of an essentially anhydrous liquid hydrogen fluoride-containing electrolyte. Thus, for purposes of convenience, and not by way of limitation, the invention will be further described in terms of being employed in the electrochemical fluorination of fluorinatable materials when using said hydrogen fluoridecontaining electrolyte.

Various processes for carrying out electrochemical fluorination reactions are known. In one presently preferred process a current-conducting, essentially anhydrous liquid hydrogen fluoride electrolyte is electrolyzed in an electrolysis cell provided with a cathode and a porous anode (preferably porous carbon), a fluorinatable feedstock is introduced into the pores of said anode and at least a portion of said feedstock is at least partially fluorinated within the pores of said anode, and fluorinated products are recovered from the cell.

Very few organic compounds are resistant to fluorination. Consequently, a wide variety of feed materials, both normally liquid and normally gaseous compounds, can be used as feedstocks in said process. Organic compounds which are normally gaseous or which can be introduced in gaseous state into the pores of a porous anode under the conditions employed in the electrolysis cell, and which are capable of reacting with fluorine, are presently preferred as starting materials. Generally speaking, desirable organic starting materials which can be used are those containing from 2 to 8, preferably 2 to 6, carbon atoms per molecule. However, reactants which contain less than 2 or more than 6 or 8 carbon atoms can also be used. Some general types of organic starting materials which can be used include, among others, the following: alkanes, alkenes, alkynes, amines, ethers, esters, mercaptans, nitriles, alcohols, aromatic compounds, and partially halogenated compounds of both the aliphatic and aromatic series. it will be understood that the above-named types of compounds can be either straight chain, branched chain, or cyclic compounds. The presently more preferred starting materials are the normally gaseous organic compounds, and particularly the saturated and unsaturated hydrocarbons, containing from 2 to 4 carbon atoms per molecule. Normally liquid feedstocks which can be vaporized under cell-operating conditions are also preferred starting materials.

The hydrogen fluoride electrolyte can contain small amounts of water, such as up to about weight percent. However, it is preferred that said electrolyte be essentially anhydrous, e.g., contain not more than about 0.l weight percent water. Commercial anhydrous liquid hydrogen fluoride containing up to about 1 percent by weight of water can be used. Thus, as used herein and in the claims, unless otherwise specified, the term essentially anhydrous liquid hydrogen fluoride" includes liquid hydrogen fluoride which can contain water not exceeding up to about l percent by weight. As the electrolysis reaction proceeds, any water contained in the hydrogen fluoride electrolyte is slowly decomposed and said electrolyte concomitantly approaches the anhydrous state. Pure anhydrous liquid hydrogen fluoride is nonconductive. To provide adequate conductivity in the electrolyte, and to reduce the hydrogen fluoride vapor pressure at cell-operating conditions, an inorganic additive can be incorporated in the electrolyte. Presently preferred additives for this purpose are the alkali metal fluorides and ammonium fluoride. Said addi tives can be utilized in any suitable molar ratio of additive to hydrogen fluoride within the range of from l:4.5 to 1:], preferably I14 to lz2.

Generally speaking, the fluorination process can be carried out at temperatures within the range of from 80 to 500 C. at which the vapor pressure of the electrolyte is not excessive, e.g., less than 250 mm. Hg. It is preferred to operate at temperatures such that the vapor pressure of the electrolyte is less than about 50 mm. Hg. A presently preferred range of temperature is from about 60 to about l20 C.

Pressures substantially above or below atmospheric can be employed if desired, depending upon the vapors pressure of the electrolyte as discussed above. Generally speaking, the process is conveniently carried out at substantially atmospherlC pressure.

Current densities within the range of 30 to I000, or more, preferably 50 to 500, milliamps per square centimeter of anode geometric surface can be used. The voltage which is normally employed will vary depending upon the particular cell configuration employed and the current density desired. Under normal operating conditions, however, the cell voltage or potential will be less than that required to evolve or generate free or elemental fluorine. Voltages in the range of 4 to l2 volts are typical. Generally speaking, the maximum normal voltage will not exceed 20 volts per unit cell. The term "anode geometric surface refers to the outer geometric surface area of the porous element of the anode which is exposed to the electrolyte and does not include the pore surfaces of said porous element.

Feed rates which can be employed will preferably be within the range of from 0.5 to 10 ml. per minute per square centimeter of anode geometric surface area. Since the anode can have a wide variety of geometrical shapes, which will affect the geometrical surface area, a sometimes more useful way of expressing the feed rate is in terms of anode cross-sectional area (taken perpendicular to the direction of flow). More preferably, the feed rate will be such that the feedstock is passed into the pores of the anode, and into contact with the fluorinating species therein, at a flow rate such that the inlet pressure of said feedstock into said pores is essentially less than the sum (a) the hydrostatic pressure of the electrolyte at the level of entry of the feedstock into said pores and b) the exit pressure of any unreacted feedstock and fluorinated products from said pores into the electrolyte. Said exit pressure is defined as the pressure required to form a bubble on the outer surface of the anode and break said bubble away from said surface. Said exit pressure is independent of hydrostatic pressure. Under these preferred flow rate conditions, there is established a pressure balance between the feedstock entering the pores of the anode from one direction and electrolyte attempting to enter the pores from another and opposing direction. Essentially all of the feedstock travels within the porous electrode via the pores therein until it reaches a collection zone within the electrode from which it is removed via a conduit, or until it exits from the electrode at a point above the surface of the electrolyte. Broadly speaking, the upper limit on the flow rate will be that at which breakout" of feedstock and/or fluorinated product begins along the immersed portion of the anode. Breakout" is defined as the formation of bubbles of feedstock and/or fluorinated product on the outer immersed surface of the anode with subsequent detachment of said bubbles wherein they pass into the main body of the electrolyte. Broadly speaking, the lower limit of the feed rate will be determined by the requirement to supply the minimum amount of feedstock sufficient to prevent evolution of free fluorine. As a practical guide to those skilled in the art, the feed rates can be within the range of from 3 to 600, preferably l2 to 240, cc. per minute per square centimeter of cross-sectional (taken perpendicular to the direction of flow). Herein and in the claims, unless otherwise specified, for convenience the volumetric feed rates have been expressed in terms of gaseous volume calculated at standard conditions, even though the feedstock may be introduced into the porous electrode in liquid state.

FIG. I is a diagrammatic illustration of one cell and electrode arrangement in which the invention is applicable.

FIG. 2 is a diagrammatic illustration of another type of electrode to which the invention is applicable.

FIG. 3 is a diagrammatic illustration of still another type of porous electrode to which the invention is applicable.

Referring now to the drawings, the invention will be more fully explained. In FIG. I there is illustrated an electrolytic cell, denoted generally by the reference numeral l0. comprising a cell body 11 having an anode 12 disposed therein. As here illustrated said anode comprises a cylinder of porous carbon having a cavity 14 formed in the bottom thereof. A current collector 16 is provided in intimate contact with the upper portion of said anode 12 and is connected to the anode bus of the current supply. It will be noted that the upper end of anode 12 extends above the electrolyte level 18. A circular cathode 20, which can be a screen formed ofa suitable metal such as a stainless steel, surrounds said anode l2 and is connected to the cathode bus of the current supply by a suitable lead wire 22. Any suitable source of current and connections thereto can be employed in the practice of the invention. In the operation of the cell arrangement of HO. 1, a feedstock is introduced into the cavity portion 14 of said anode via conduit 15, travels upward through the pores of said anode, and exits from the upper end of the anode above electrolyte level 18. During passage through said anode at least a portion of the feedstock is electrochemically converted. Conversion products together with any remaining unconverted feedstock, and possibly some electrolyte vapors, are withdrawn via conduit 24 from the space above the electrolyte within cell and pass to a suitable separation means (not shown) for recovery of products. During the introduction of said feedstock an electric current in an amount sufficient to supply the desired operating current density at the anode is passed between the anode and the cathode. During periods when said cell is not in operation, introduction of said feedstock and/or said flow of electric current are terminated. it is during these periods that excessive invasion ofsaid porous anode by electrolyte occurs.

Referring to FIG. 2, there is illustrated another electrode assembly comprising a porous element having the general shape of a hollow tube, closed at one end thereof and open at the other end. The bottom of said porous element is sealed with a suitable resistance cement material 32, such as Flouroseal. Said porous element 30 is mounted onto the lower end of a generally tubular cap 34 by means of the threads shown. A first conduit 36 extends through said cap 34 into the lower portion of the interior of porous element 30 to a point adjacent said closed end thereof. A metal plug 38 is mounted on the lower end portion of said first conduit 16 in a close-fitting relationship with the inner wall of porous element or member 30. Said conduit 16 serves as a feed conduit and, together with said plug 38, can serve as a current collector when connected to a current supply. Said metal plug divides the inner wall of porous element 30 into a lower first surface and an upper second surface. Said lower first surface can be defined as comprising the chamber 40 which is formed at the lower end of conduit 36. Said upper second surface can be defined as the portion 42 of the inner wall of porous element 30 which is in communication with annular space 44. If desired, the region where said porous element 30 joins cap 34 can be covered with an external seal 46, such as Teflon tape. In the operation of the electrode illustrated in FIG. 2, a feedstock is introduced via conduit 36. passes from chamber 40 into the pores of porous element 30, travels upward through said element 30, and exits therefrom via said upper second surface 42, and is removed from the anode assembly via annular space 44 and outlet conduit 48.

In FIG. 3, porous element 50 has the general shape of a rectangular block. A first lateral passageway 52 extends longitudinally into and substantially across said block adjacent the lower end thereof. The surface of said passageway 18 comprises a first surface for the introduction of a reactive feedstock into the pores of porous element 50. Preferably, said reactive feedstock will be introduced into said passageway 52 at about the midpoint thereof via conduit 54. If desired, the plug 56, shown in one end of passageway 52, can be removed and said reactive feedstock introduced via conduit 58. A second lateral passageway 60 extends longitudinally into and substantially across said plug 50 adjacent the upper end thereof. Said second passageway comprises a collection lateral and the surface thereof provides a second surface for withdrawing products and unreacted feedstock from within the pores of said porous element 50. Effiuent conduit 62 is preferably connected into about the midpoint of passageway 60 as shown. Current collectors 64, comprising metal bars, extend into the upper end of porous element 50.

EXAMPLE Electrode invasion or flooding was measured by immersing porous carbon electrodes in an electrolyte under controlled test conditions including time, current flow, removal of the electrode from the electrolyte, removal of excess electrolyte from the exterior of the electrode, cooling of the electrode so that electrolyte within the pores thereof would solidify, cutting the electrode horizontally into a number of vertically oriented segments (classifiable by depth of immersion) and, by means of volume and weight measurements, determining the density of the electrode segments. The carbon electrodes, generally of circular or square cross section, had an initial apparent density of about l.05 grams per cubic centimeter, and a true density of about 2.2 grams per centimeter. Therefore, the extent of pore invasion was readily evaluated by interpolation of the measured density of the electrode section between these values by means of the equation X=dl.05/ 1.15 where X= fraction of pores filled by electrolyte, and

d= density, grams per centimeter, ofanode section.

For example, two identical porous carbon electrodes having rectangular dimensions of l2 inch were immersed overnight in an essentially anhydrous hydrogen fluoride-containing electrolyte having an approximate molar composition of KF-2HF at a typical cell-operating temperature of to C. with no electrical current flowing. One electrode was then removed, wiped clean, cooled, and cut into sections on which densities were determined. The other electrode was then connected to serve as an anode in the cell. A low-voltage direct current capable of causing a low current fiow of l to 2 milliamps per cm of anode geometric surface was applied to said anode in the cell for about 48 hours. Said anode was then removed from the cell, wiped clean, cooled, and cut into sections on which densities were determined. Said electrodes were made from National Carbon Company grade-45 carbon (NC-45) having a pore volume of about 0.5 centimeter per gram with pore diameters ranging from about 10 to 100 microns, and an average pore diameter of about 58 microns. The results ofthe test runs are tabulated below:

Depth 0! N 0 current flow Current flow immersion,

inches (1 X (1 The above data show the substantial decrease in the invasion or flooding of electrode pores with electrolyte at the deeper immersions, e.g., greater than 4.5 inches, which approximate commercial-scale operation. Application of the invention to those electrodes having reactant flow channels therein will be especially beneficial by keeping said channels unobstructed during cell shutdown and standby. it is also evident from the above table that electrolyte invasion of the upper portion of the anode to which current was passed was actually increased toward a desirable value of pore invasion, e.g., about 30-40 percent invasion (X=0.30 to 0.40).

A number of other test runs were made in which the current densities were reduced to as low as 0.03 milliamperes per square centimeter of anode geometric surface area. Representative of these is the following run made at a current density of 0. l milliamps per cm of anode geometric area.

Section Depth d X A 0-0.5 1.11 0.05 B 0.5-1.5 1.14 0.08 C l.5-2.5 1.16 0.10 D 2.5-3.5 1.20 0.13 E 3.5-4.5 1.24 0.17 F 4.5-5.5 1.21 0.14 G 5.5-6.5 1.24 0.17 H 6.S7.5 1.34 0.25 l 7.5-8.5 1.43 0.33

While certain embodiments of the invention have been described for illustrative purposes, the invention is not limited thereto. Various other modifications of the invention will be apparent to those skilled in the art in view of this disclosure. Such modifications are within the spirit and scope of the disclosure.

We claim:

1. In a method for the electrochemical conversion of a reactant feedstock in an electrolytic cell provided with at least one porous electrode which is at least partially immersed in a current-conducting electrolyte; wherein during periods when said cell is in operation for production of conversion products, said feedstock is introduced into the pores of said porous electrode, an electric current is passed between said porous electrode and another electrode at a normal operating current density at said porous electrode, and conversion products and any unconverted feedstock are withdrawn from said cell; wherein during periods when said cell is not in operation for production of conversion products, introduction of said feedstock and/or said flow of electric current are terminated; and wherein said electrolyte excessively invades said porous electrode during said periods when said cell is not in operation; the improvement of maintaining said cell in standby condition when it is not in operation for production of conversion products, which comprises:

terminating operation of said cell for the production of said [0 of electric current maintained while said cell is not in operation is within the range of from 0.01 to 2 milliamps per square centimeter of porous electrode geometric surface.

3. A method according to claim 2 wherein said electrolyte invasion of said electrode is reduced to a value not exceeding about 0.7 of the pore volume of said electrode.

4. A method according to claim 2 wherein said porous electrode is an anode, said electrolyte is a current-conducting essentially anhydrous hydrogen fluoride electrolyte, said electrochemical conversion process is an electrochemical fluorination process, and said normal operating current density is within the range of from 30 to 1000 milliamps per square centimeter of anode geometric surface area.

5. A method according to claim 4 wherein said small How of electric current is within the range of from 0.01 to 0.5 milliamps per square centimeter of anode geometric surface area.

6. A method according to claim 4 wherein said electrolyte invasion of said anode is reduced to a value not exceeding about 0.7 of the pore volume of said anode.

7. A method according to claim 4 wherein said electrolyte invasion of said anode is reduced to a value not exceeding about 0.5 of the pore volume of said anode.

8. A method according to claim 1 wherein said normal operating current density is within the range of from 30 to 1000 milliamps per square centimeter of porous electrode geometric surface, and said small reduced flow of electric current is within the range of from 0.01 to 2 miliiamps per square centimeter of porous electrode geometric surface.

9. A method according to claim 8 wherein operation of said cell for the production of conversion products is terminated by terminating the introduction of said feedstock into said porous electrode.

Claims (8)

1. 2. A method according to claim 1 wherein: said small flow of electric current maintained while said cell is not in operation is within the range of from 0.01 to 2 milliamps per square centimeter of porous electrode geometric surface.
2. 3. A method according to claim 2 wherein said electrolyte invasion of said electrode is reduced to a value not exceeding about 0.7 of the pore volume of said electrode.
3. 4. A method according to claim 2 wherein said porous electrode is an anode, said electrolyte is a current-conducting essentially anhydrous hydrogen fluoride electrolyte, said electrochemical conversion process is an electrochemical fluorination process, and said normal operating current density is within the range of from 30 to 1000 milliamps per square centimeter of anode geometric surface area.
4. 5. A method according to claim 4 wherein said small flow of electric current is within the range of from 0.01 to 0.5 milliamps per square centimeter of anode geometric surface area.
5. 6. A method according to claim 4 wherein said electrolyte invasion of said anode is reduced to a value not exceeding about 0.7 of the pore volume of said anode.
6. 7. A method according to claim 4 wherein said electrolyte invasion of said anode is reduced to a value not exceeding about 0.5 of the pore volume of said anode.
7. 8. A method according to claim 1 wherein said normal operating current density is within the range of from 30 to 1000 milliamps per square centimeter of porous electrode geometric surface, and said small reduced flow of electric current is within the range of from 0.01 to 2 milliamps per square centimeter of porous electrode geometric surface.
8. 9. A method according to claim 8 wherein operation of said cell for the production of conversion products is terminated by terminating the introduction of said feedstock into said porous electrode.

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