Electrosynthesis is a method for the production of chemical reaction(s) that is electrically driven by passage of an electric current, typically a direct current (DC), in an electrochemical cell through an electrolyte between an anode electrode and a cathode electrode from an external power source. The rate of production is proportional to the current flow in the absence of parasitic reactions. For example, in a liquid alkaline water electrolysis cell, the DC current is passed between the two electrodes in an aqueous electrolyte to split water, the reactant, into component product gases, namely, hydrogen and oxygen where the product gases evolve at the surfaces of the respective electrodes.
Water electrolysers have typically relied on pressure control systems to control the pressure between the two halves of an electrolysis cell to insure that the two gases, namely, oxygen and hydrogen produced in the electrolytic reaction are kept separate and do not mix.
In the conventional mono-polar cell design in wide commercial use today, one cell or one array of (parallel) cells is contained within one functional electrolyser, cell compartment, or individual tank. Each cell is made up of an assembly of electrode pairs in a separate tank where each assembly of electrode pairs connected in parallel acts as a single electrode pair. The connection to the cell is through a limited area contact using an interconnecting bus bar such as that disclosed in Canadian Patent No. 302,737, issued to A. T. Stuart (1930). The current is taken from a portion of a cathode in one cell to the anode of an adjacent cell using point-to-point electrical connections using the above-mentioned bus bar assembly between the cell compartments. The current is usually taken off one electrode at several points and the connection made to the next electrode at several points by means of bolting, welding or similar types of connections and each connection must be able to pass significant current densities.
Most filter press type electrolysers insulate the anodic and cathodic parts of the cell using a variety of materials that may include metals, plastics, rubbers, ceramics and various fibre based structures. In many cases, O-ring grooves are machined into frames or frames are moulded to allow O-rings to be inserted. Typically, at least two different materials from the assembly are necessary to enclose the electrodes in the cell and create channels for electrolyte circulation, reactant feed and product removal.
WO98/29912, published Jul. 9, 1998, in the name of The Electrolyser Corporation Ltd. and Stuart Energy Systems Inc., describes such a mono-polar cell electrolyser system configured in either a series flow of current, in a single stack electrolyser (SSE) or in a parallel flow of current in a multiple stack electrolyser (MSE). Aforesaid WO98/29912 provides details of the components and assembly designs for both SSE and MSE electrolysers.
As used herein, the term "cell" or "electrochemical cell" refers to a structure comprising at least one pair of electrodes including an anode and a cathode with each being suitably supported within a cell stack configuration. The latter further comprises a series of components such as circulation frames and gaskets through which aqueous electrolyte is circulated and product disengaged. The cell further includes a separator assembly having appropriate means for sealing and mechanically supporting the separator within the enclosure and an end wall used to separate adjacent cells blocks. Multiple cells may be connected either in series or in parallel to form cell stacks and there is no limit on how many cells may be used to form a stack. A cell block is a unit that comprises one or more cell stacks and multiple cell blocks are connected together by an external bus bar. Aforesaid PCT application WO98/29912 describes functional electrolysers comprising one or more cells that are connected together either in parallel, in series, or a combination thereof.
Depending on the configuration of such a cell stack electrochemical system, each includes an end box at each end of each stack in the simplest series configuration or a collection of end boxes attached at the end of each cell block. Alternative embodiments of an electrolyser includes end boxes adapted to be coupled to a horizontal header box when both a parallel and series combination of cells are assembled.
In the operation of the cell stack during electrolysis of the electrolyte, the anode serves to generate oxygen gas whereas the cathode serves to generate hydrogen gas. The two gases are kept separate and distinct by a low gas permeable membrane separator. Some desirable properties of separators include: high electrical resistivity, low ionic resistivity, low gas permeability, good mechanical integrity, and low cost.
The flow of gases and electrolytes within cells are conducted via circulation frames and gasket assemblies which also act to seal one cell component to a second and to contain the electrolyte in a cell stack configuration in analogy to a tank.
The rigid end boxes can serve several functions which include providing a return channel for electrolyte flowing out from the top of the cell in addition to serving as a gas/liquid separation device. The end box may also provide a location for components used for controlling the electrolyte level, such as, liquid level sensors and temperature, i.e. for example heaters, coolers or heat exchangers. In addition, with appropriate sensors in the end boxes individual cell stack electrolyte and gas purity may be monitored. Also, while most of the electrolyte is recirculated through the electrolyser, an electrolyte stream may be taken from each end box to provide external level control, electrolyte density, temperature, cell pressure and gas purity control and monitoring. This stream is returned to either the same end box or mixed with other similar streams and returned to the end boxes. Alternatively, probes may be inserted into the end boxes to control these parameters. An end box may also have a conduit to provide the two phase mixture to the existing liquid in the end box to improve gas liquid separation. End boxes of like type containing the same type of gas can be connected via a header such that they share a common electrolyte level.
One prior art pressure control system provides a water seal to equalize pressure in the two halves of the cell. This is the approach most often followed in "home-made" electrolysers. Typically, the water seal is a couple of inches deep and so the cell operates a couple of inches WC pressure above atmospheric.
An alternative system provides a membrane separator which can sustain a pressure difference between the two halves of the cell without gas mixing. The PEM cell is the best example of this type of system. The PEM cell can sustain up to a 2500 psi pressure difference without significant loss of gas purity.
A third is an active control system which senses pressure and controls the outflow of gases from the two cells. Control can be achieved in one of two ways: by a mechanical system which relies on pressure regulators, such as a dome-loaded flow regulator to control pressure between the two cells which, for example, might employ the oxygen pressure as a reference pressure to regulate the pressure in the hydrogen half of the cell; and by an electronic system which relies on measurement of the difference in gas pressure between the two cell to control the rates of gas outflow from the two sides of the cell so as to maintain a desired pressure difference of usually zero or with the hydrogen side slightly higher.
Typically, however, for very small commercial hydrogen generators (0.1 Nm.sup.3 /h) PEM type electrolysis cells are favoured. Although the cost of the cell is far higher than for alkaline electrolysers, these costs are more than offset by the controls needed for the alkaline systems using mechanical or electronic actuators, and by the need for higher pressures and, hence, compression in electrolysers using a water seal pressure control system.
Control systems that rely on mechanical actuators are difficult to calibrate and ensure "close to zero" pressure difference on a pressurized cell. In the case of a Stuart cell, one needs to ensure that the pressure difference doesn't force a level difference large enough to expose one side of the cell to the gas phase of the opposite side of the cell as this will reduce cell efficiency or may result in poor gas purity. Electrolysers that rely on mechanical actuators typically have a 1 atmosphere or so pressure difference between the two sides of the cell. Gas purity is maintained by, for example, a woven asbestos or a needle felted polyphenylene sulphide (PPS) membrane.
Control systems which rely on electronic actuators suffer from the weakness that in order to maintain level differences between the two sides in the cell within an inch of height, which is required to insure that the membrane is covered on both sides, sophisticated high resolution pressure measurements are needed. The demands of the measurement and control system are put in perspective when we consider controlling pressure to 2 cm WC in a cell pressurized to 7 bars or 7000 cm WC.
It would be advantageous to provide prior art electrolyser systems with a simple, low cost, in situ level control monitor that can be utilized for systems control. This would eliminate the need for complex, expensive pressure measuring systems that must retains their integrity in a hostile process environment of elevated temperature of, for example, 30-100.degree. C. and concentrated alkali environments of, say, 20-40 wt. % KOH.
It is most important that the liquid levels pressure differential be maintained within well-defined limits in order to reduce the risk of intermixing of product gases, namely, hydrogen and oxygen across the membrane, and to ensure proper fluid management to permit safe and functional operation of the electrolysis cell.
However, there remains a need for a relatively low cost and reliable method of controlling pressure in a pressurized electrolyser.