In the Laboratory
The Entrance and Exit Effects in Small Electrochemical Filter-Press Reactors Used in the Laboratory
W
Angel Frias-Ferrer, José González-García,* Verónica Sáez, Eduardo Expósito, Carlos M. Sánchez-Sánchez, Vicente Montiel, and Antonio Aldaz Grupo de Electroquímica Aplicada, Departamento de Química Física, Universidad de Alicante, Ap. Correos 99, 03080 Alicante, Spain; *
[email protected] Frank C. Walsh School of Engineering Science, Lanchester Building, University of Southampton, Highfield, Southampton SO17 1 BJ United Kingdom
The effect of the scale up of an electrochemical reactor on fluid dynamics and mass-transport behavior is a critical aspect in research and project development. To extrapolate the results obtained at laboratory scale, the electrochemical engineer must consider several areas (geometric size and shape, fluid mechanics, concentration distribution, chemical composition, current distribution, and heat transfer) during the subsequent cell designs (1–2). In the scale up, the study of fluid entrance and exit effects is an important aspect, particularly in small reactors. From the hydrodynamic point of view, in small cells the entrance and exit effects can predominate. In these systems it is difficult to reach a fully developed flow, even at low Reynolds numbers (Re). This fact must be taken into account for the possible scale up of the system. It is crucial for the student to quantify and determine whether a cell is working under entrance and exit effects in order to understand its behavior and to ascertain whether extrapolation of the results to larger systems is possible. We present a laboratory experiment that is designed to examine this phenomenon. This experiment is particularly suitable for the early training of chemical engineers and chemists. A short theoretical background is required for the students and the experimental work is concise and repetitive, allowing students to ascertain whether a specific reactor is working under entrance and exit effects and to decide whether the results can be safely extrapolated to a larger scale.
Theory Filter-press type reactors (3, 4) are one of the most important electrochemical reactor geometries. The parallel plate configuration (Figure 1) is widely used. The advantages of such a reactor geometry can be seen from an industrial, research, or practical point of view: wide availability of components, ease of scale up, versatility, and familiarity (1–4). Facile scale up is achieved whenever the system is working under a fully developed flow regime. Fully developed flow regime is related to the geometry and size of the system. Sometimes, a special hydrodynamic flow regime (characterized by a highly turbulent electrolyte flow behavior due to entrance and exit effects) can be obtained for small reactors. The entrance effect arises at the leading edge of the electrode because the velocity profile is horizontal and the mass transport coefficient (km) is infinite. After this point, km will gradually decrease to a value corresponding to the fully developed flow (Figure 2). Similarly, the region approaching the point where the electrolyte leaves has the same behavior. The nonuniformity of flow at the inlet and at the outlet can result in nonuniform reaction rates, mass transport rates, and product distributions. At the inlet of the reactor (Figure 2) the velocity profile is closer to a turbulent profile than a laminar one for any volumetric flow (in this figure, the laminar profile is seen only
seal (EPDM, rubber) electrolyte compartment (PVC) electrolyte outlet luggin electrode plate
flow
electrolyte inlet seal (EPDM, rubber) electrode (copper) end plate (aluminum)
entry length
Figure 1. Construction of one half of the electrochemical reactor assembly. (The turbulence promoter was contained within the electrolyte compartment.)
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fully developed laminar flow
Figure 2. Development of the laminar flow velocity profile inside a parallel plate reactor.
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in the fully developed flow region). This is due to the change of the geometry at the entrance and exit of the system, which generates a high degree of turbulence (Figure 3). The study of entrance and exit flow effects are carried out by means of electrochemical methods that allow the global km to be evaluated by measurement of the limiting current (IL ). This is a well-known procedure that involves a mass transport controlled reaction rate at the working electrode.
At the working electrode, the reaction may be described by
Oxd + ze−
flow
inlet flow
entry length
fully developed length
Figure 3. A possible effect of the change in geometry in the entrance and exit of the reactor.
(1)
where Oxd is an oxidized species in the bulk solution, z is the number of electrons transferred, and Red is a reduced species. Using eq 2, we can obtain the value of the global (averaged) km km =
vortex region
Red
IL Az F c
=
D δN
(2)
S 15 15
L 70 40
where A is the electrode area (m2), F is the Faraday constant (96485 C mol᎑1), c is the bulk concentration of reactant species (mol m᎑3), D the diffusion coefficient of the active species (m2 s᎑1), and δN is the thickness of the Nernst diffusion layer (m). The value of km depends on the thickness of δN. In turbulent systems, there is a higher value of km than in laminar systems because δN will be smaller. In this way, the study of IL at different flow rates for a known system will give us the value of km if we know the process variables of flow velocity, reactant concentration, and electrode area. IL was obtained using an electrochemical method: the potential step technique. The working electrode potential is instantaneously stepped from the system open-circuit value (when I = 0) to a value where the electrochemical reaction is working under mass transport control. At the beginning, the current reaches a high value owing to non-Faradaic charge, immediately decreases, then relaxes to the steady IL value (1). On the other hand, in large-scale reactors, where it is easy to reach the fully developed flow regime, it is common to use turbulence promoters to increase the inherent agitation inside the reactor and therefore to improve km (5, 6) achieving higher current densities (and hence a higher production rate). These turbulence promoters are usually polymeric grids fitted in the reaction compartment. The increase of the mass transport coefficient using turbulence promoters can be used as a test procedure to examine the importance of the entrance and exit effects in small systems. The typical result for this practice in large-scale systems is to achieve greater values of km, usually 1.5 or 2 times greater, than when the system is working with an empty electrolyte channel. In the proposed experiment, a small filter-press reactor with a single compartment is used. This compartment is filled with different turbulence promoters to study the influence of these turbulence promoters as compared to the empty configuration. Experimental Arrangement The cell studied was an in-house filter-press reactor. Figure 4 shows the compartment and the geometrical dimensions. The equivalent hydraulic diameter, de, for this reactor is defined by
40 10
60
de =
B Figure 4. Dimensions of cell compartment (in mm). B, L, and S are used to calculate de equivalent diameter.
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2B S B + S
(3)
where B, S, and L are defined in Figure 4. The working electrode and the counter electrode consisted of two smooth,
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In the Laboratory
2.5-mm thick plates of copper. The electrodes were polished to remove the oxide due to atmospheric corrosion. The reference electrode was a saturated calomel electrode placed near the working electrode via a Luggin capillary. Limiting current experiments were carried out by a potential-step method using an EG&G PARC model 175 generator, an AMEL model 553 potentiostat, and a Philips PM 8133 XY recorder. Figure 5 shows a schematic diagram of the experimental system used to obtain the limiting current values.
lence promoter B, ...) and, in all cases, the experiments are repeated for the same flow rates in each configuration.
Data Analysis After the experiments, students can manipulate the data to determine the value of km for each flow rate in each configuration. The IL value can be obtained from the plateau of the I versus t plot. With this value and eq 2 students can obtain the value of km.
Hazards The use of potentiostats in the laboratory involves the application of potentials and the consequent passage of currents through solutions of electrolytes and as such the risks associated with the combination of an electrical appliance and a conducting fluid medium are always present. The main hazards are those encountered in the use of any electrical appliance but with the added complication of the presence of electrochemically active solutions; namely, the cell design incorporates non-insulated contact points as well as exposed electrode surfaces hence electric shock is a major risk. Death can be the result of the normal voltage of 240 V causing currents of greater than 30 mA to flow through the body for more than 40 ms. Minor shocks may also cause injury following involuntary muscle contraction. Explosion and fire can also be caused by electrical sparks, short circuits or overload heating, old wiring in the presence of flammable material. Cupric sulfate is harmful by inhalation or ingestion. Prolonged exposure may cause dermatitis. Sodium sulfate may irritate eyes. Repeated or prolonged skin contact may lead to irritation. Ingestion of large quantities can result in gastrointestinal irritation and diarrhoea. Experimental Development We use the cathodic reduction of Cu(II) from copper sulfate in 0.5 M sodium sulfate at pH 2 (eq 4) to measure IL : (4)
Cu
The anodic reaction is the oxidation of metallic copper in the other electrode and the experiment is carried out in a undivided cell configuration. The final step potential was –500 mV versus SCE. The I versus t graphs obtained by this potential step are similar to the graphs shown in Figure 6. The concentration of copper was 150 ppm.
Limiting Current, IL The Cu(II) solution is introduced into the reservoir. Once the electrolyte flow rate and the temperature are stabilized, the potential step experiment can be done to obtain the values of IL. The limiting current will be measured as a function of the linear flow rate for different flow rates. It is advisable to repeat the experiment at least three times for each flow rate to get values with a high degree of agreement. The potential step experiments will be repeated with several configurations for the reactor (empty mode, turbulence promoter A, turbu-
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Figure 5. Diagram of experimental arrangement. (1) reservoir, (2) thermometer, (3) centrifugal pump, (4) electrochemical reactor, (5) flowmeter, (6) valves, (7) heat exchanger, (8) potentiostat, (9) Luggin.
-140
-120
I / mA
− Cu2+ + 2e
-100
-80
I L value is obtained from this plateau
-60
-40 0
5
10
15
20
25
30
35
t/s Figure 6. Current versus time curves showing the determination of the limiting current plateau.
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km
10ⴚ4
empty reactor promoter A promoter B
10ⴚ5 100
1000
Re Figure 7. Log–log plot of mass transport number versus Reynolds number for the electrochemical filter-press reactor (with and without turbulence promoters) for the cathodic reduction of Cu(II) ions in 0.5 M sodium sulfate at pH 2.
when the stream leaves the manifold and expands itself inside the reactor compartment. The incorporation of turbulence promoters serves to dampen inherent turbulence. In addition the promoters can produce channeling effects that also diminishes the mass transport rates. Thus the promoter does not create extra agitation and it can produce electrolyte channelling phenomena reducing the efficiency of the reactor. The behavior observed with this smaller cell is opposite for those observed with electrochemical reactors at industrial scale: the entrance and exit effects can be neglected in comparison with the effect produced by the turbulence promoters on the large electrode area. In large scale, the insertion of turbulence promoters increases the values of km related to those obtained with empty configuration. In this fashion, from this reactor submitted to study it is not possible to extrapolate the results to big reactors because the hydrodynamics inside it will not be the same as in a larger reactor. W
A plot of the values of km versus Re in a log–log plot for each configuration will give a straight line as a result of the dependence of the mass transport on the Re km ∝ Re
Re =
a
(5)
v ρ de µ
(6)
where a is an empirical exponent, v is the mean linear velocity of electrolyte in the compartment (m s᎑1), ρ is the liquid density (kg m᎑3), de is the equivalent hydraulic diameter (m), and µ is the dynamic viscosity of the electrolyte (Pa s). The results for these experiments are shown in Figure 7. Conclusions As shown in Figure 7, it is better to obtain high values of km working in the empty configuration because the insertion of turbulence promoter causes no additional effect in the system behavior. The behavior of the relatively short cells used in this study are dominated by the turbulence produced by the inlet jets and the sudden changes in flow direction
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Supplemental Material
Instructions for the students and notes for the instructor are available in this issue of JCE Online. Literature Cited 1. Walsh, F. C. A First Course in Electrochemical Engineering; The Electrochemical Consultancy: Romsey, United Kingdom, 1993. 2. Goodridge, F.; Scott, K. Electrochemical Process Engineering; Plenum Press: London, 1995. 3. Walsh, F. C.; Robinson, D. Chemical Technology Europe 1994, May兾June, 16–23. 4. Pletcher, D.; Walsh, F. C. Industrial Electrochemistry; Chapman & Hall; London, 1993. 5. González-García, J.; Frías, A.; Expósito, E.; Montiel, V.; Aldaz, A.; Conesa, J. Ind. Eng. Chem. Res. 2000, 39, 1132–1142. 6. Frías-Ferrer, A.; González-García, J.; Conesa, J. A.; GadeaRamos, E.; Expósito, E.; García-García, V.; Montiel, V.; Aldaz, A. Characterization of an Electrochemical Industrial Size Filterpress Reactor by Hydrodynamic and Mass Transport Studies. In Récent Progrès en Génie des Procédés: Tracers and Tracing Methods; Leclerc, Jean Pierre, Ed.; Societé Française de Génie des Procédés: Paris, 2001;Vol. 15 No. 79, pp 205–212.
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