Electrodialysis

Jiří Kratochvíla, Petr Pánek, Roman Kodým, Dalimil Šnita

Introduction Electrodialysis (ED) represents a modern progressive electromembrane separation technology gaining recently an increasing attention in various branches of industry. Especially in the field of brackish water desalination, which represents the largest application of this technology, the ED is nowadays competitive to the conventional reverse osmosis process [1-4]. There can be also increasing trend noted in application of ED within the waste water recovery and desalination of process streams in the pharmaceutical and food industry. It is because the electromembrane separation process doesn’t endanger health and nutritious properties of the final product e.g. by adding coagulants or regenerating agents. A demineralization of milk whey can be mentioned as a representative example.

Fig. 1: Schema of the ED process with detail on one membrane pair and the spacer net, D – diluate compartments, K – concentrate compartments, e1 and e2 – electrode compartments, AM and CM – anion and cation selective membrane, respectively, (+) – cation, (-) – anion.

The electrodialyzer represents a typical plate-and-frame filter-press type device. The main core of ED unit is membrane stack consisting of the planar plate anion (AM) and cation (CM) selective membranes. The individual membranes are separated by electrochemically

inactive net-like spacers serving mainly as a mechanical support of the stack and determining the space for the process solutions flow in between the membranes. Generally, there are two independent hydraulic circuitries considered: a diluate stream (D) and concentrate stream (C), supplying diluate and concentrate solutions into the diluate (DC) and concentrate compartments (CC). A set of one AM and CM with corresponding compartments (DC and CC) then represents a repeating motive of the ED unit often denoted as a membrane pair (MP). For more detail, see the expanded scheme of the ED unit illustrated in Fig. 1. Beside the two discussed hydraulic circuitries auxiliary hydraulic circuit is used to wash the electrode compartments. The driving force of the separation process represents electric field imposed on the membrane stack between two terminal electrodes. Cations move towards the cathode, while anions towards the anode. However, the ions meet ion-selective membranes on this way. Cations permeate across the CMs, otherwise they are rejected by AMs. On the contrary, anions are allowed to cross through AMs, while CMs are not permeable for anions. By a suitable combination of AMs and CMs in the membrane stack and electric field orientation, it is possible to induce one-way transport of salt from diluate compartments into concentrate compartments. The basic principles of this process are summarized in the literature [1-5]. If ions have to be removed below a specific concentration level, the D is a product (e.g. production of drinking and process water from brackish water or sea waters) and, on the contrary, if ions are required to be concentrated above a specific level, the C is a product (e.g. recovery of useful materials from effluents in metallurgy or salt production).

Theory

Material balance of ED unit

The single membrane pair has to fulfil a material balance of dissolved salts in the diluate stream, see Eq.(1), and concentrate stream, see Eq.(2). First and second terms of these equations represent mass inlet and outlet, respectively, and the third term corresponds to transport of salt from D into C. Minus and plus sign indicate inlet and outlet, respectively.

Q�,���� c�,�� − Q�,���� c�,�� − ηIFz = 0

(1)

Q�,���� c�,�� − Q�,���� c�,�� + ηIFz = 0

(2)

Here Q is volumetric flow rate, where subscripts D and C corresponds to diluate and concentrate, respectively, subscripts in and out indicate inlet and outlet, respectively, superscript mp corresponds to one membrane pair, I symbolizes the electric current load and F represents Faraday’s constant. The other parameters are related to a type of salt dissolved in the process solution. In the case of brackish or sea water desalination, the real solution is often substituted by equivalent solution of NaCl with same equivalent molar concentration. It is possible because NaCl represents prevailing dissolved components in these waters. c in Eqs. (1) and (2) thus denotes molar concentration of equivalent salt (NaCl) and z is charge number of equivalent salt (number of electrons equivalent to transport of one molecule of equivalent salt from D into C, note that for NaCl is z=1). An important process characteristic is current efficiency of the electrodialysis η (utilization of the electric current supplied for transfer of equivalent salt from D into C). η ranges in close interval from 0 to 1, where η�= 1 corresponds to a theoretical situation of ideally efficient process. Lower current efficiency can

be caused (a) primarily by by-pass (parasitic) current passing through the external hydraulic circuitry, (b) by backward diffusion flux of salt from C into D due to high concentration gradient, (c) and by migration flux due to the fact, that the membranes are not ideally selective for cations or anions.

The ED unit plate-and-frame construction enables easy way to increase the capacity of the unit simply by assembling more membrane pairs into the electrodialysis stack. The electric current then passes in between the terminal anode and cathode through each MP in the stack. The applied electric current therefore participate in the transport of salt from D into C in each of MPs. The total amount of salt transported from D into C in the ED stack can be obtained by multiplying the mass balance by number of membrane pairs Nmp, see Eqs.(3) and (4).

Q�,��c�,�� − Q�,��c�,�� − ηIN��Fz = 0

(3)

Q�,��c�,�� − Q�,��c�,�� + ηIN��Fz = 0

(4)

Generally, the total inlet and outlet volumetric flow rates are equal and after rearranging the material balances then obtain form of Eqs. (5) and (6), where QD and QC corresponds to total volumetric flow rate through diluate and concentrate compartments, respectively.

�c�,�� − c�,��� = +ηIN��Q�Fz

(5)

�c�,�� − c�,��� = −ηIN��Q�Fz

(6)

An important characteristic of the electrodialysis process is degree of desalination ϕ representing a measure of inlet solution desalination. It is traditionally defined by Eq.(7).

φ = �1 − c�,��c�,�� � ∙ 100[%] (7)

A combination of Eqs. (7) and (5) provides an expression for degree of desalination in a form of Eq.(8).

φ = ηIN��c�,��Q�Fz ∙ 100[%] (8)

The maximum theoretical degree of desalination for given current load is then given by Eq.(9).

φ�!" = IN��c�,��Q�Fz ∙ 100[%] (9)

The dependence of current efficiency on the current load can be calculated from the outlet concentration of the salt in D according to Eq.(10). This equation results from combination of Eqs. (7) and (9).

η = φφ�!" = Q�Fz

IN�� �c�,�� − c�,���

(10)

Faraday’s law

The electric current is applied in to the system via electrochemical reactions represented by water electrolysis. At the surface of the terminal anode oxygen is formed, while hydrogen is produced at the terminal cathode. The electrodes are washed by sodium sulphate solution in the electrode chambers, which form a different hydraulic circuitry separated from the diluate and concentrate streams. Faraday’s law representing a fundamental relationship of electrochemistry can then be applied for the calculation of amount of gasses evolved during the entire electrodialysis process. The Farady’s law is given by Eq.(11).

mi=IMitFn

(11)

Here m represents weight of the developed i-th gas, I is electric current, M is a molecular mass and t denotes operational time of the electrodialysis. Finally, n corresponds to number of electrons exchange during the individual electrode reaction.

Goals of the laboratory work 1. Perform experimental parametrical studies with the pilot plant electrodialysis unit.

Vary stepwise total voltage U from 0 to 50 V under several different values of volumetric flow rates Q. Q is identical for diluate and concentrate streams. Record the values of U, I, specific conductivity of inlet and outlet solutions and temperature.

2. Compute the values of the inlet and outlet solutions concentration using linear interpolation of empirical data of electric conductivity in dependence on temperature and molar concentration.

3. Read the volume of produced oxygen and hydrogen and compute the charge passed through the device (by means of Faraday’s law) and compare this value with real total charge provided by the electric current source.

4. Make following plots: - dependence of U on I for various Q. - dependence of desalination degree on current load for various volumetric flow

rates. Add to this plot the dependence of the maximum theoretical desalination degree (max obtained under a condition of η = 1 for the same values of I and Q.

- dependence of desalination degree on the electric energy input defined by P = UI [W] for various Q. - dependence of the efficiency of electrodialysis process η on the current load for

different I and Q.

Experimental part

Description of the pilot plant ED unit

The photography of the electrodialyzer and the supporting fixtures can be seen in Fig. 2 and corresponding schema in Fig. 3.

Fig. 2: Photography of the pilot plant electrodialysis unit installed in ICT Prague, (A) membrane stack, (V2 through V5) rotameters for diluate and concentrate streams, (V1 and V6) rotameters for electrode solutions, (B) and (C) cylindrical reservoirs for gasses produced at the electrodes, (D, E,F, G) conductometers (diluate inlet (D), concentrate inlet (E), diluate outlet (F) and concentrate outlet (G)).

The heart of the device represents membrane stack consisting of 192 membrane pairs. Active area of one membrane is of 0.32 x 0.64 m2. Both sides, left and right, are bordered by electrode compartments equipped with platinized titanium terminal electrodes washed by Na2SO4 solution. Volumetric flow rate of the electrode solutions is controlled by rotameters V1 and V6. There are two transparent plastic columns, where hydrogen and oxygen from water electrolysis are accumulated. There are four rotameters V2 through V5 controlling volumetric flow rate of concentrate and diluate solutions. NaCl solution with equal concentration is used for both concentrate and diluate. The set of four conductometers are used for measuring the electrical conductivity of diluate and concentrate inlet and outlet.

Fig. 3: Scheme of the pilot plant electrodialysis unit, V – valves, H – reservoirs, B and C – storage columns for gasses, C – conductometer

Measurement with ED pilot plant unit

Values of the voltage and volumetric flow rate, which will be measured, are written in the protocol which you obtain in the laboratory.

1. Set the volumetric flow rate of diluate (V2), concentrate (V3) and electrode solutions (V1, V6) only WHEN THE ELECTRICAL SOURCE IS SWITCHED OFF and voltage and passing current are equalled to zero.

2. After the assistant control and under his supervision the current source is switched on and required voltage is set.

3. After each change of the voltage record three times every 5th minute the values of electric conductivity, temperature, voltage and electric current to the protocol until the steady state is reached (approx. 5 min).

4. After the measurements, reduce the voltage to zero and switch off the current source. 5. Change the values of volumetric flow rates and repeat the steps from 2 to 4. 6. Finally, set the voltage to zero and switch off the current source and call the assistant

who switch off the whole device. 7. Determine the volume of oxygen produced using graduated cylinder.

Summary

1. Assistant control 2. Switch on the pumps 3. Set the first value of volumetric flow rate 4. Switch on the electrical source

5. Set the voltage 6. 5 min to the steady state, read and write the first set of values of the measured quantities 7. Reset the voltage 8. After 5 min. read and write the second set of values of the measured quantities 9. Repeat the step 7 and 8 10. Increase the voltage to the next measured value and repeat the steps from 5 to 9 11. Decrease the voltage to zero 12. Set the second value of volumetric flow rate, repeat the steps from 3 to 11 13. Decrease the voltage to zero and switch off the source 14. Switch off the pumps 15. Determine the volume of oxygen using cylinder

Safety Keep on mind that it is worked with electrical – conductive solutions under relatively high values of current and voltage. According to EU regulation the safety DC current is I < 10 mA and safety DC voltage is U < 25-60 V.

Manipulate only with valves mentioned within this instruction (V1 – V6).

A short speech about the safety will take place right before the work starts. Please listen carefully and ask immediately if you need.

References 1. Strathmann H, Giorno L, Drioli E (2006) An introduction to membrane science and

technology, Institute on Membrane Technology, CNR‐ITM, Italy 2. Baker R W (2004) Membrane Technology and Applications, 2nd ed., John Wiley & Sons,

ISBN 0‐470‐85445‐6 3. Baker et al. (1991) Membrane separation systems, Noyes Data corp., USA, ISBN 0‐8155‐

1270‐8 4. Strathmann H: Electrodialysis, a mature technology with a multitude of new applications

(2010) Desalination 264(3):268‐288 5. Tanaka Y (2007) Ion exchange membranes fundamentals and applications, 1st ed.,

Elsevier, Amsterdam

Symbols c molar concentration of equivalent salt [mol m-3] F Faraday’s constant, 96,485 C mol-1 I electric current [A] m weight [kg] M molecular mass [kg mol-1] n number of electrons exchange in the electrode reaction Nmp number of membrane pairs, 192 pairs Q volumetric flow rate [m3 s-1] Qel electrical source charge [C] Qmt mass transport charge Qi oxygen respectively hydrogen production charge t time [s] U total voltage [V]

-. volumetric flow rate [m3 s-1] z charge number of equivalent salt φ degree of desalination [%] η electrodialysis process efficiency

Subscripts and superscripts

D diluate C concentrate in inlet out outlet mp membrane pair max maximum theoretical value

Questions 1. What was the aim of this laboratory work? Which quantities were measured and which

were set? 2. Under which conditions the electric source could be switched on? 3. Check the results and estimate and afterwards compute the desalination degree and

maximum theoretical desalination degree when volumetric flow rate is 5,000 dm3 h-1 and applied voltage is 50 V. Assume the corresponding dependences to be linear if it is necessary.