Electrolytic cell

Details Electrolytic cell Electrolytic cell

2002-09-04

2004-09-28

King, Roy (Department: 1742)

Chemistry: electrical and wave energy

Apparatus

Electrolytic

C204S280000, C204S284000

Reexamination Certificate

active

06797136

ABSTRACT:

The present invention relates to a method for retrofitting a conventional hydrogen evolving cathode cell to a gas diffusion electrode cell. The invention also relates to the retrofitted electrolytic cell and the use thereof.
BACKGROUND OF THE INVENTION
The electrolytic production of alkali metal hydroxides is today of considerable importance, although a large amount of energy is consumed in the electrolysis processes. Many attempts have been made to lower the energy consumption, e.g. by using a gas diffusion cathode in the electrolytic cell which today is believed to have the highest capability of saving electric energy and lower the production costs.
If the production of sodium hydroxide from brine in a conventional hydrogen evolving cathode cell in which the electrolytic reaction (1) occurs instead is performed in a gas diffusion cathode cell, the reaction (1) is replaced by reaction (2).
2NaCl+2H
2
O→Cl
2
+2NaOH+H
2
, E
0
=2.21 V  (1)
2NaCl+
½O
2
+H
2
O→Cl
2
+2NaOH, E
0
=0.96 V  (2)
Thus, by converting a hydrogen evolving cathode cell to a gas diffusion electrode cell, the cell voltage is reduced from 2.21 V to 0.96 V, thus an energy saving of about 60% becomes possible. Accordingly, various investigations have been conducted for converting conventional cells for production of e.g. chloralkali electrolysis to gas diffusion electrode cells.
U.S. Pat. No. 5,693,213 describes briefly a method for converting a conventional salt water electrolytic cell provided with a conventional hydrogen evolving cathode to a gas diffusion cathode cell in which the cathode chamber is partitioned into a solution chamber and a gas chamber by providing a gas diffusion cathode to the original cell. However, this conversion requires a large reconstruction cost due to the complicated cathode chamber structure needed for controlling the pressure arisen between the gas diffusion electrode and the ion exchange membrane.
The present invention intends to solve the above-mentioned problems.
THE INVENTION
The present invention relates to a method for retrofitting an electrolytic cell comprising an anode and a cathode compartment, a separator partitioning the compartments, said cathode compartment comprising a hydrogen evolving cathode, wherein the method comprises making at least one substantially horizontal slit in the hydrogen evolving cathode resulting in plural cathode members, bending the edge of the cathode member at the slit away from the separator, arranging a gas diffusion electrode to at least one of the cathode members on the side facing the separator, and arranging an electrolyte layer to the gas diffusion electrode.
It has been surprisingly found that the present invention can provide advantageous operation to a retrofitted gas diffusion electrode cell. The invention enables a simple retrofitted gas diffusion electrode cell by converting a conventional hydrogen evolving cathode cell at a low cost. The invention further ascertains low cell voltage and stable operation. It can thus enable homogeneous connection between the separator and the gas diffusion electrode. The relatively short distance between the separator and the gas diffusion electrode minimises the cell voltage and makes the cell operation almost as energy-saving as a zero gap cell. The invention also ascertains a safe operation minimising flooding of electrolyte in the cathode compartment.
By “retrofitting an electrolytic cell” is generally meant equipping or converting an already existing electrolytic cell with new parts.
By the term “separator” is meant any separating mean, such as an ion exchange membrane, a diaphragm or other suitable means. Suitable membranes may be made of perfluorinated, sulphonated or teflon-based polymers, or ceramics. Also polystyrene-based membranes or diaphragm of polymers or ceramics may be used. There are several commercially available membranes suitable for use such as Nafion™ 324, Nafion™ 550 and Nafion™ 961 available from Du Pont, and Flemion™ available from Asahi Glass.
By the term “slit” is meant a long straight incision or opening, suitably a through-line, in the hydrogen evolving cathode.
According to one embodiment, the distance between the slits is from about 100 to about 600 mm, preferably from about 200 to about 400 mm. The slits suitably are from about 2 to 10 mm, preferably from about 3 to about 5 mm wide. The edge of the cathode member can be bent in any direction as long as it allows the electrolyte layer to pass through the slit, but preferably, the edge is bent downwards away from the separator.
By the terms “existing cathode” and “cathode members” is meant any originally existing hydrogen evolving cathode or members of an existing hydrogen evolving cathode used in a conventional electrolytic cell.
By the term “electrolyte layer” is meant a hydrophilic layer capable of retaining electrolyte substantially deriving from the anode compartment, e.g. in the production of sodium hydroxide in which process sodium ions carrying water molecules are transported over the separator. The electrolyte layer is arranged between the separator and the gas diffusion electrode and suitably comprises a carbon cloth, e.g. a graphite cloth, nonwoven cloth filter of fluorinated resins, ceramic fiber cloth, ceramic fluor resin cloth, or a ceramic coated carbon cloth retaining the electrolyte between the gas diffusion electrode and the separator. The carbon cloth suitably extends through the slits made in the cathode members as further described herein. The electrolyte can in this way be drained from the electrolyte layer avoiding flooding of the gas diffusion electrode. The electrolyte can thus leave the cathode compartment in a controlled way at the oxygen-containing gas-supplied side of the cathode compartment.
According to one embodiment, the electrolyte layer is from about 0.1 to about 2 mm thick, preferably from about 0.2 to about 1 mm.
According to one embodiment, the gas diffusion electrode comprises several electrode members. The electrode members are suitably shaped as belts or in patchwork configuration, preferably in patchwork configuration, suitably in which configuration the gas diffusion electrode members are substantially square-shaped. Suitably, the electrode members have a length of from about 2 to about 40 cm, preferably from about 10 to about 30 cm in the vertical direction. If the vertical length is less than about 2 cm, the manufacturing of the electrode members can be complicated. If the vertical length is longer than about 40 cm, the lower portion of the electrode member may be exposed to higher electrolyte pressure than the upper portion, which can reduce the rate of the electrolytic reaction taking place at the gas diffusion electrode member due to difficulties in supplying oxygen-containing gas. The size of the gas diffusion electrode is suitably dimensioned relative to the pressure difference of the electrolyte in the vertical direction. In this way, an optimised homogeneous gas supply can be provided. According to one embodiment, the gas diffusion electrode can pass through the formed slits together with the electrolyte layer. However, even though this embodiment results in extra electrode reaction area, it is preferred that the gas diffusion does not pass through the slits because the extra electrode reaction area will be further away from the separator than the rest of the electrode area which will increase the ohmic loss at said extra electrode reaction area.
Suitably, the gas diffusion electrode members have a length of from about 2 to about 40 cm, preferably from about 10 to about 30 cm in the horizontal direction. The space between adjacent electrode members in the vertical direction may be from about 1 to about 5 mm, preferably from about 2 to about 3 mm.
Preferably, a space is provided between adjacent gas diffusion electrode members in the horizontal direction. Thereby, the gas diffusion electrode members do not necessarily continue over the whole horizontal direction in the cell, but may be

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