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New Anion Exchange Membranes for the Electrodialytic Treatment of Acids.

Patrick Altmeier*, Henning Bolz*, Günter Schwitzgebel**

*PCA GmbH; Donatusstr. 43;
D-66822 Lebach

** Universität Saarbrücken;
Im Stadtwald; D-66123 Saarbrücken

Paper presented at the Eleventh International Forum on Electrolysis in the Chemical Industry November 2-6, 1997, Clearwater, FL

Abstract

New anion exchange membranes on the basis of alipathic polyethers demonstrate the ability to tailor membranes with high conductivity for organic anions or membranes with high permselectivity in mineral acids. This paper provides data on the chemical composition and on electrochemical properties relevant to potential end users. The possibility of tailoring the membranes is shown by the example of membranes developed for the transport of large organic anions. It is stated that the permeability of such anions is limited by a sieving effect of the membrane matrix and a concept is presented to minimize this effect. The properties of the membrane type PC 400D developed by this method are demonstrated by the example of lactobionic acid.

Introduction

Electrodialysis is a technique which has developed since the early 1950s mainly for desalination purposes [1]. Later, advanced applications with participation of acids and bases as products or educts became of interest. Examples are concentrating acids by electrodialysis and production of caustic and acid solutions with the aid of bipolar membranes. In these applications membrane properties are the key factor and key problem. The membrane type limiting product concentration and product yield of the whole process are the anion exchange membranes.

There are many types of ion exchange membranes, most of them consist of an homogenious structure [2]: An inert polymer and the polymer which bears the ion exchanging groups. This is mainly prepared from an active polymer, which is often transformed to the final polymer after the membrane forming step. Thus, inert and active polymers are worked together to form the precursor membrane.

Chloromethylated polystyrene or polysulfones are mainly used as active polymers, other membranes are developed on the basis of derivatives of polyvinylpyridine. These membranes must be chloromethylated within a production step by means of chloromethyl methylether, which is one of the most potent carcinogenic substances. Furthermore, the obtained active polymers contain space filling aromatic rings a fact that makes membrane tailoring difficult.

Membranes produced by this way are used in many industrial applications, which are affected by the anion exchange membrane properties.

Organic acids are often produced by fermentation. Normally, they are isolated as salts and are transferred into the acid by adding sulfuric acid. But, the isolation of the acid can also be performed by electrodialytic salt splitting. This process requires a membrane which allows the permeation of the anion of the desired organic acid.

Conventional anion exchange membranes exhibit very low conductivities for large organic anions because channels in the crosslinked matrix of the ion exchanger are too narrow for their passage. Efforts are made to overcome this problem by reducing the degree of crosslinking or by elongating the crosslinking units [3]. On both ways, the results are limited because the mechanical strength of membranes decreases when the crosslinking degrees is reduced.

These problems induced PCA to develop ion exchange membranes on a different basis.

High Permeability for Large Organic Anions

If membranes are desired with high permeability for large organic anions, experimental results show that a reduced permeability is obviously induced by a sieving effect. A model for the description of the anion permeability is presented and membranes were prepared according to the requirement.

For the membranes made out of crosslinked polymer chains (Fig. 3), the model must take into account the different forms of loops which are responsible for the sieving effect. The aim of the development of anion exchange membranes with high permeability for large organic anions is to considerably enlarge these loops.

This cannot effectively be done by lenghtening the crosslinking units but by increasing the distance of the crosslinking points in the polymer chain. This concept of widening the loops in its statistical contribution by doubling the length of the monomer unit of the polymer chain is done at PCA by using a copolymer of epichlorohydrin and ethylene oxide. The copolymer (information about tacticity and copolymer distribution is given in [5]) can be regarded as epichlorohydrin with a double distance between each chloromethylene group compared with the homopolymer.

Fig.3

Fig. 3: Schematic network of a crosslinked polymer with loops limiting the permeation of large ions.

Fig.4

Fig. 4: Model of functionalized and crosslinked polymer chains

As shown in Fig. 4, a polymer network with double diameter of loops is obtained at the same ratio crosslinks per fixed ions. Due to the roughly doubled polymerization degree (one CCO- unit is one monomer unit) of the copolymer, the amount of crosslinks per polymer chain is also the same. The result is a membrane matrix with an open structure permeable for large anions with a satisfactory swelling and mechanical strength. Furthermore, the polymer matrix produced from the copolymer has a lower ion exchage density resulting in more space per counterion which is required by the large anions.

Experimental

Conductivity of Membranes in Correlation of the Molar Mass of the Counter Ion

Conductivities are measured according [6]. Fig. 5 shows the conductivities of ion exchange membranes in different salts in correlation to the molecular mass of the anion. It is clearly visible that very dense membranes such as acid blockers are almost impermeable for large anions because the polymer network is tailored as densely as possible. Membranes optimized for a more loose structure, taken as a tightly sieve, show a higher permeability for large anions. These membranes do not show a sharp cut-off characteristic.

 

Fig.5

Fig. 5: Conductivities of anion exchange membranes in 0,2 m solutions of sodium salts: Chloride (35), acetate (59), lactate (89), gluconate (195) and lactobionate (357). (): molecular mass of the non hydrated anion. The membrane PC 400 D is developed according the model described below.

 

Salt Splitting of Sodium Lactobionate

Lactobionic acid production from sodium lactobionate was chosen as an example since this acid production takes maximum advantage of the membrane type PC 400 D.

Fig.6

Fig. 6: schematic view of the experimental unit [7].

In a four chamber electrolysis apparatus (Fig. 6), sodium lactobionate in chamber II was next to the cathode compartment I. When a current is applied, lactobionate anions and water were transported to chamber III next to the anode compartment IV. Protons from the anode compartment were also transported into this chamber III to produce the acid. Within this experiment the amount of acid produced in chamber III is analyzed in correlation to the applied charge.

Discussion

Energy Consumtion for the Production of Lactobionic Acid

The current efficiency and the transported water are calculated from the amount and concentration of acid produced. The results are shown in Tab. 1.

The amount of energy utilized is determined by the performance of the cell components. The membranes used in the cell determine the current efficiency of the process and are apart from the electrolyte solutions an important resistive element in the circuit which contributes to the overall power utilization.

Salt concentration

/ mass %

Acid concentration

/mass %

Current efficiency

/ %

Water transport

g H2O / g acid

Voltage drop

/ V

15%

5%

>95%

1,3

1,8

15%

15%

>95%

1,3

1,2

 

Tab. 1: Electrochemical properties of the membrane PC 400 D for the transport of lactobionate anions from the sodium lactobionate compartment to the lactobionic acid compartment. See Fig. 6 for the experimental arrangement

 

Fig.7

Fig. 7: Summation of the voltage drop of the particular elements of a salt splitting process for the production of lactobionic acid at a current density of 0,5 kA m-2. For a description see text. The contributions of the electrodes are taken from [8], the contribution of the bipolar membrane is taken from [9]

 

The voltage drop of each component of the system is shown in Fig. 7i. To predict the voltage drops in a system with a hydrogen consuming anode, the overvoltage of the oxygen evolving anode is replaced by this one of the hydrogen consuming anode (See Fig. 7ii). In Fig. 7iii the expected voltage drop of a bipolar salt splitting unit is predicted. The energy consumption of the processes can be calculated from the voltage drop and the current efficiency. The results are given in table 2.

Although the voltage drops compared with standard processes are relatively high due to small conductivities of the solution, the specific energy consumption is low due to the high molecular weight of the products. These results show that it is possible to separate large anions economically by electromembrane processes including special anion exchange membranes.

 

Arrangement Fig. 7

Specific Energy consumption

KWh / t product

electrolysis with activated electrodes

i

600

electrolysis with hydrogen consuming anode

ii

500

electrodialysis with bipolar membranes

iii

430

 

Tab. 2: Energy consumption of the splitting from sodium lactobionate into lactobionic acid and Caustic at a current density of 0,5 kA m-2 at 25-28 C. Calculated from the values of Tab. 1 and Fig. 7.

 

Treatment of Mineral Acids

The production of mineral acid and bases by electrodialytic salt splitting is a promising technology which was intended as a competitive process for the production of sodium hydroxide. Today, some problems of this process are evident as far as concentration, yield and quality of the product are concerned. The product yield is mainly determined by the anion exchange membrane as long as they show a reduced current efficiency in concentrated mineral acid solutions because of proton leakage. The properties of the polymers used in PCA ion exchange membranes are especially suited for tailoring membranes with high permselectivity in mineral acids [7]. PCA now produces a membrane family PC Acid for concentration of mineral acid:

PC Acid 35 for concentrating hydrochloric acid or for the production of hydrochloric acid from sodium chloride.

PC Acid 70 for the concentrating nitric acid or nitric acid/hydrofluoric acid pickling acids.

Membranes as Solid Polymer Electrolyte in non Aqueous Solution

The use of solid polymer electrolyte (SPE) in electrochemical cells is a upgrowing field with many possibilities and advantages. Up to now, mainly cation exchange membranes are used because of their wide applicability. Anion exchange membranes can be advantageous if anions are the object of interest e.g. at the Kolbe electrolysis. Often, such processes are performed in non aqueous solution due to product and educt solubility. For this purpose PCA develops membranes with high conductivity to take advantage of the electrochemical transport to the reactive anode.

Conclusion

A new series of anion exchange membranes giving the ability to manipulate the structure of the membranes was developed by PCA GmbH. This results in membranes that can be adjusted to match specific applications by balancing out properties such as:

- customized membrane features

- high conductances in organic acids

- high conductive solid polymer electrolytes in non aqueous solution

- high current efficiencies in mineral acids

- wide variety of possible reinforcements

References

[1] H.K. Londsdale, J. Membr. Sci., 10(1982)98-101

[2] H. Kawate, K. Tsuzura, H. Shimizu in K. Dorfner, Ion Exchangers S. 597-598, W. de Gruyter, 1991, Berlin, New York

[3] W. Gudernatsch, Ch. Krumbholz, H. Strathmann; Desalination, 79 (1990) 249-260

[4] P. Altmeier, WO 95/06083

[5] H. Cheng, ACS Symp. Ser. 496 (1992) 157-69

[6] P. Altmeier, G. Schwitzgebel, Poster presented at the annual meeting of DECHEMA 1996

[7] P. Altmeier, A. Konrad, Proceedings of ICOM'96 p559

[8] S. Holze, J. Jörissen, C. Fischer, H. Kalvelage; Chem. Eng. Technol. 17(1994) 382-389

[9] T. Kobayashi, A. Tomita and F. Hanada; Proceedings of ICOM 1993, Heidelberg

 

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