The process of designing an industrial electrochemical system is multifaceted, typically specialized to the application, and iterative. Our description is limited and focuses on the key trade-off between size and efficiency. This balance, illustrated in Figure 14.6, dominates the design process. Economic considerations are at the heart of the design of industrial electrolytic processes. As we’ll explore in more detail, low current densities correspond to high efficiencies and low operating costs for electricity. On the other hand, lower current densities require larger electrode areas, and the increased size leads to greater capital costs.
Figure 14.6 Trade-off between size and efficiency is essential part of design.
As noted earlier, there are three important performance measures for electrolytic processes: the faradaic efficiency , space–time yield (Y), and the energy efficiency . These are design variables—the engineer has a hand in selecting these to meet the needs of the application. In contrast, quantities such as exchange-current densities and electrical conductivity are considered physical properties. Additional variables that underlie the performance measures are shown in Table 14.1. A starting point for reactor selection and sizing is the desired production rate of the material, , typically expressed on an annual basis. The production rate is determined by the opportunity in the market and will include a target price that can be used for profitability analysis. As we know, Faraday’s law is used to convert the production rate to a current for use in reactor sizing. For the purposes of this text, the production rate is provided as an input. Detailed costing and profitability analysis is beyond the scope of this text and, if this is your objective, you should refer to a design text that covers economic analyses of processes. Both the current density and cell potential appear in Table 14.1. Of course, the current density and potential are coupled through the polarization curve. This relationship is central to the analysis of electrolytic systems.
Table 14.1 Eight Key Variables for an Electrochemical Reactor
Variable Units Description Comments
kg·s−1 metric tons·yr−1 Rate of production of desired material Size scales with production rate
i A·m−2 Current density These three along with the configuration of the cell would be optimized simultaneously
Vcell V Potential of an individual cell
ηenergy – Energy efficiency
A m2 Total electrode area, sometimes referred to as the separator area Follows directly from Faraday’s law once i is established
Vs V Voltage of the DC power system
m – Number of cells that are connected in series Follows directly,
Ac m2 Area of individual electrodes Cells may be placed in parallel
When designing an industrial electrochemical process, there are many questions to be answered. At what current density should the system operate? Is it preferred to have a few very large cells, or would more, smaller cells be better? What shape should the electrodes take? How should these individual electrodes and cells be arranged? How will flow of reactants be distributed within each cell and between multiple cells? We will discuss these topics in the sections that follow; you should remember, however, that the topics are all interrelated. Before going on, let’s look at a quick example where the current density and production rate are given.
ILLUSTRATION 14.6
Design an electrolysis reactor by determining the total cell area and number of cells required to produce 6800 metric tons of chlorine per year. The electrolyzer is to operate 360 days per year at a current density of 4000 A·m−2. The electrodes are 1 m × 2 m in size.
SOLUTION:
We first determine the total current required to produce the desired amount of chlorine. The faradaic efficiency is assumed to be one.
Next, we calculate the total area required (anode area or cathode area).
Finally, the number of cells:
Industrial electrolytic cells can be thought of as electrochemical reactors that are used to generate products from raw materials with use of electricity. Some types of reactors can be used for a number of different processes, while others, such as the Hall–Héroult cell used for aluminum production, are tailored for one application. As mentioned previously, a wide variety of reactor types have been developed; in fact, designing electrodes and configuring them to accomplish the chosen reactions efficiently has been an active area of applied and theoretical research. A detailed treatment of a broad spectrum of reactors is beyond the scope of this text. It will be sufficient for us to examine just two basic electrode structures and two means of assembling electrode pairs into an electrochemical reactor. Basic electrode structures of importance here are planar (2D) and porous (3D) electrodes. Configurations will be restricted to (i) assemblies with parallel plate electrodes, which may be 2D or 3D, and (ii) plug-flow reactors with porous electrodes. In the first instance, anode and cathode plates are placed parallel to each other and separated by a fixed distance that is filled with electrolyte as shown in Figure 14.3. When the exchange-current density for the reaction is low, these electrodes are often made porous to increase the specific interfacial area. Almost all the applications that we examine will use these assemblies of parallel plate electrodes. The second approach is limited to porous electrodes or packed beds where reactant streams flow through the electrodes.
Establishment of Operating Current Density
Understanding the relationship between current density and voltage is essential to setting the design current density. There are several factors that influence this choice. One strategy would be to operate at the highest current density possible, which would be at the limiting-current density. As seen from Equation 14.2 for the space–time yield, higher current densities increase Y and result in a smaller reactor volume for a fixed rate of production. A smaller reactor (smaller electrodes and less separator area) corresponds to lower initial costs. However, operation near the limiting current has its drawbacks.
As the limiting current is approached, Vcell increases rapidly. Both the voltage efficiency and the energy efficiency decrease with increasing current density, see Equation 14.5.
Operation at the limiting current may lower the current efficiency, especially for multistep reactions. Thus, the faradaic efficiency, ηf, is reduced. A lower faradaic efficiency reduces both the energy efficiency and the space–time yield. This situation is therefore particularly bad—a larger reactor that is less efficient.
The limiting-current density may exceed the capability of the available electrode materials or separator membranes and may therefore unacceptably shorten the lifetime of these materials. For example, high current operation may lead to excessive cell temperatures that damage the physical components of the cell.
Constant current operation at the limiting current may not be robust from an operational standpoint, since a change in the inlet conditions or in the operating conditions may reduce the limiting current and lead to undesirable side reactions if operation at the same value of the current is continued.
In the end, the current density will be selected so that the profitability of the electrolytic process is maximized. The choice of current density is both important and complex. Data on reaction rates, mass-transport rates, ohmic losses, current efficiencies, and heat removal are needed for design purposes. For the problems that we will consider in this chapter, you will either be given the current density, instructed to operate at the mass-transfer limit, or be given sufficient information to establish the relationship between the cell voltage and current in order to determine operation below the limiting current. In all cases, we will assume a uniform current density for simplification purposes.
While simplification is necessary for our initial treatment of the topic, let’s not forget that we, the electrochemical engineers, have many tools at our disposal to alter the polarization curve. The flow rate of reactants can be increased. Higher flows lead to increased rates of mass transfer and greater limiting-current densities. The gap between electrodes can be reduced, resulting in a lower ohmic drop and better energy efficiency. We may be able to change the concentration of reactants, which directly affects the limiting-current density. Catalysts can be added to reduce kinetic polarizations and improve faradaic efficiency. Porous electrodes may be used to increase the specific surface area of the electrodes, and the temperature of the process may be changed.
Electrical Configurations
Once the current density is fixed, the electrode area follows directly from Faraday’s law and the production rate. From our experience with electrochemical systems, we know that the potential of individual cells will be on the order of a few volts, and certainly less than 10 volts. The scale of industrial processes requires enormous amounts of electrical power. Most often the electrical power to drive the electrolytic process comes from high voltage alternating current (AC) that is then rectified. It’s not practical to supply vast quantities of direct current (DC) at a potential of just a few volts. The solution is to place cells electrically in series to build voltage. The same approach is used for batteries, double layer capacitors, and fuel cells for high-power applications. The number of series connections is established from the system voltage and potential of an individual cell.
(14.17)
ILLUSTRATION 14.7
Aluminum is produced in an electrolytic process. The rectified electrical power supplied is 1200 VDC. If the individual cells operate at 4.2 V, how many cells are connected in series?
Determining the electrode area and the number of cells connected in series is not the end of the story. When a system contains more than a single anode and cathode pair in series, there are two general methods for making electrical connections: monopolar and bipolar. These terms have the same meanings as they did for batteries and fuel cells. In the monopolar configuration (Figure 14.7a), a separate electrical connection is made to each electrode. The current through the cell is divided among the electrodes electrically connected in parallel, . Many electrode pairs can be combined in a single cell. In the individual cell, all of the anodes in a cell are at the same potential, as are all of the cathodes. The voltage between each anode and cathode pair is the same and equal to the cell voltage. Both surfaces of each electrode are active. These cells can then be connected in series as needed to add voltage.
Figure 14.7 Monopolar (a) shown with a separator and bipolar (b) configurations. Current flow is shown for electrolysis. Cell pitch is the number of electrode pairs per unit length.
The second means of connecting multiple electrode pairs is the bipolar stack. Here, assemblies of electrode pairs separated with a solid conductive plate are stacked like a deck of cards. Current in a bipolar stack flows straight through the stack and eliminates the need for connections to each internal electrode; the current distribution tends to be more uniform in the bipolar arrangement. The use of narrow-gap cells is considerably easier in bipolar stacks. There are, however, a couple of important disadvantages. Because the current flows from cell to cell through a bipolar stack, failure of one cell results in failure of the entire stack. In contrast, electrodes in a monopolar arrangement function independently. Also, because the difference in potential from one end of the bipolar stack to the other is large, it is possible for a portion of the current to skip one or more cells and flow directly to another cell downstream (see Figure 14.7b). This phenomenon is referred to as a bypass or shunt current. Bypass currents reduce the faradaic efficiency. Electron-transfer reactions are still needed for the bypass currents, and these reactions may be undesired or destructive to the cell, as is the case for corrosion reactions. Often this damage is more important than the small loss of efficiency. Bypass currents can be reduced by eliminating bypass pathways, but this can be difficult to do in an industrial cell where, for example, electrolyte from different cells flows into a common manifold. Because of these characteristics, bipolar stacks are standard in fuel cells and redox-flow batteries, but used less frequently in industrial electrolysis.
Another key reactor characteristic that needs to be determined is whether or not a divided cell should be used. A divided cell uses a separator to create distinct anolyte and catholyte solutions. If possible, we prefer not to have a separator since it represents an extra resistance to current flow between the electrodes. However, as we have noted previously, the anode is at a higher potential than the cathode in an electrolytic cell. Therefore, a product or by-product produced at the anode can be reduced spontaneously at the cathode. Similarly, a product or by-product produced through reduction at the cathode can be oxidized at the anode. In addition, soluble products may react with each other in solution. Thus, a principal purpose of the separator is to prevent loss of faradaic efficiency by minimizing or eliminating transport of reaction products in order to prevent undesirable reactions. For example, the diaphragm in a chlor-alkali cell helps to keep Cl2 that is dissolved in the electrolyte from reaching the cathode where it would react. Consequently, the faradaic efficiency is increased by preventing Cl2 reduction to Cl− at the cathode. A separator can also maintain purity of the anolyte and catholyte solutions. For example, the diaphragm in a chlor-alkali cell helps to reduce the amount of Cl− in the catholyte, which increases the value of the liquid NaOH product. Finally, separators can prevent the formation of explosive mixtures such as H2/Cl2. The following questions may be useful in considering whether or not to use a separator:
To what extent is the desired product likely to react at the opposite electrode?
Are there undesirable solution phase reactions that may be prevented through the use of a separator?
Are there safety issues that can be addressed through the use of a separator?
Will use of a separator to create distinct anolyte and catholyte solutions enhance the value of product streams or avoid an expensive downstream separation process?
Flow Configurations
Industrial electrochemical reactors are usually flow reactors. Streams of reactants into and out of the reactor are an essential aspect of both continuous and semicontinuous operation. Reactors can also incorporate internal convection to improve rates of transport, as well as to improve the concentration and temperature distributions. Flow can be important for the removal of gases evolved in the reactor in order to minimize the resistance as discussed in Section 14.3. For assemblies of parallel plate electrodes, the flow is principally coplanar across the electrode surface; whereas with plug-flow reactors, the flow is in void spaces of the porous electrode.
Flow patterns are often unique to the application and too numerous to categorize succinctly. Nonetheless, we will identify the basic flow arrangements for an assembly of cells. For multiple electrode pairs that are housed together in one assembly (a cell for monopolar or a stack for bipolar), there are two principal flow arrangements: parallel flow and series flow. Figure 14.8 illustrates the difference between series and parallel flow for a monopolar design. It is clear that parallel flow will have a lower pressure drop. Series flow enables a greater fraction of the reactants to be converted in a single pass through the assembly, at the expense of a larger pressure drop. Problems associated with gas evolution can be exacerbated with series flow as bubbles accumulate along the flow path. Hybrids of parallel and series are also possible. Industrial practice favors parallel flow where conversion can be increased by placing cell stacks in series with respect to flow, or by separation and recycle of reactants.
ILLUSTRATION 14.8
We calculated the total current, electrode area, and number of individual cells that an electrolysis reactor would need to have to produce 7500 metric tons of chlorine per year in Illustration 14.6. Here we assume membrane-type cells with a cell voltage of 2.95 V. What would be the electrolyzer current and voltage for a completely monopolar and a bipolar configuration of the electrodes?
In a monopolar configuration with each electrode pair connected in parallel, the total current (see Illustration 14.6) would be (75 electrode pairs, each with a 2 m2 area operating at 4000 A·m−2) and the voltage would be 2.95 V.
In a bipolar configuration, the same current passes through each cell in the electrolyzer as it moves from one end to the other. Therefore, the total current is equal to the current from a single cell:
The total electrolyzer voltage is equal to the individual cell voltage multiplied by the number of cells:
The power, of course, should be the same.
Figure 14.8 Basic flow arrangements. (a) Parallel flow. (b) Series flow.
Reactor Volume
The volume of the reactor can be estimated from a knowledge of the electrode area required to meet the desired production rate and the specific area of the reactor, . Referring back to Figure 14.7, we see that for the parallel plate construction, we can estimate the specific area from the cell dimensions, specifically the thicknesses of the electrodes and the width of the gap between electrodes. The calculation is straightforward, and the result can be summarized by a quantity called cell pitch (see Section 10.4). This parameter is simply the number of electrode pairs per unit length when the repeating units are stacked together.
(14.18a)
(14.18b)
where the factor of 2 in Equation 14.18b accounts for the two active faces of the electrode. With porous electrodes, there is a lot of internal surface area inside the electrode, represented by a, the specific interfacial area of the electrode. However, even for porous electrodes, we often speak of the superficial current density rather than the true current density based on the internal area. When using the superficial current density with a porous electrode, it is not necessary to include the internal surface area in our analysis.
ILLUSTRATION 14.9
A process for the electrowinning of zinc uses electrodes in a bipolar configuration. The gap between electrodes is 3.0 cm and each electrode (before deposition of Zn) has a thickness of 1 cm. What is the specific area, ar? If the electrodes operate at 1700 A·m−2 with a faradaic efficiency of 93%, estimate the reactor volume needed to produce 1000 kg·day−1 of Zn. Finally, determine the space time yield.
Remember, the factor of 2 arises because areas of both sides of the anode are counted. The volume can be determined from Equation 14.2, which is rearranged to
Oxygen is evolved at the anode, and at the cathode the reaction is
For the space time yield
Scale-Up
The process of designing a system for industrial electrolysis is sequential but with some iteration as noted previously. This characteristic is best illustrated through an envisioned process to scale-up a reactor. Referring to Figure 14.9, we might start with fundamental electrode studies as described in Chapter 6. Basic kinetic data are obtained and side reactions identified. The effects of temperature and reactant concentration are often examined at this stage. The second stage is a complete system of anode, cathode, and electrolyte, but at a subscale—a small single cell where the reactants are supplied in large excess to each electrode. The electrode area of this subscale cell might be a factor of 10 or more less than Ac. Of course, at this point, the area for an individual electrode and the total area are only estimates. These estimates will be refined at each stage. For this subscale cell, uniform current density is assumed and often there are no mass-transfer limitations.
Figure 14.9 One possible scale-up process from fundamental electrochemistry to prototype reactor.
The next step is to increase the area of the cell to its full size; that is, Ac. At this stage, the flow configuration is set and the design includes the effect of finite utilization (conversion) of reactants, u:
(14.19)
Utilization can be defined for an electrode, a cell, or a cell stack. Finally, these individual electrode pairs are connected together to form a system. As noted previously, electrodes are almost invariably connected together electrically (series–parallel combination) to build voltage. The cell may also be combined to form one or more mechanical assemblies. The flow rates of reactants and products are critical elements of the final cell design.
Finally, we note that there is off-the-shelf hardware available for initial evaluation of a process and prototype development. One example is the so-called plate-and-frame assembly shown in Figure 14.10. An off-the-shelf reactor such as this facilitates the development of new electrochemical processes.
Figure 14.10 Plate-and-frame system that is commercially available for process evaluation. Image provided by ElectroCell A/S.
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