Process Description
The organic acids are usually weak acids that are not very conductive and it is usually not practical to work with the standard three-compartment configuration. Therefore, as seen in the general description for bipolar membrane ED, two main cell stack configurations can be considered: two-compartment with bipolar and cation-exchange membranes only or two-compartment with bipolar and anion-exchange membranes only (see schematics). The preferable configuration depends on the nature of the cations and the source of the organic salt.
SCHEMATIC OF TWO-COMPARTMENT CELL WITH CATION-EXCHANGE MEMBRANES
SCHEMATIC OF TWO-COMPARTMENT CELL WITH ANION-EXCHANGE MEMBRANES
In either case, it is critical to implement the appropriate pretreatment process for each fermenter product. Eurodia / Ameridia has developed an expertise in the optimum pretreatement sequence for common fermentation streams. For instance, suspended solids must be filtered to ppm levels and a 1 micron particle size. Ca and Mg multivalent cations that tend to precipitate at high pH levels have to be removed to a level as low as 1 ppm by chelating ion exchange resins (especially with the two-compartment cation configuration).
If the organic salt concentration out of the fermenter is low and the feed contains many impurities, it would be worthwhile to use conventional electrodialysis before bipolar membrane ED to simultaneously purify and concentrate the salt.
Two-compartment Configuration with Bipolar and Cation-exchange membranes.
With concentrated organic salts from chemical synthesis and with salts of strong bases such as NaOH or KOH, the two-compartment configuration with cation-exchange membranes can be used to remove the cations from the organic acid and produce the base. In this case, the DC current moves the cations across the cation-exchange membranes into the base compartment. In the meantime, the H+ ions from the water splitting reaction replace the cations that were moved to the base compartment. There, the Na or K cations combine with the OH- anions and form the strong base KOH or NaOH.
This process is highly efficient due to the weakly dissociated nature of most organic acids, reducing the loss of H+ ions across the cation-exchange membranes. However, due to the low conductivities of the organic acids, some salt must remain remain in the acid. Typically, this amount is about 1-2 wt% to be removed by conventional methods, corresponding to a minimum conductivity of ~10 mS/cm. Depending on the feed initial conductivity, about 90-95% conversion is possible. In this case, most of the water (and of the impurities) will remain in the acid product, except for the water transported with the cations, and it is not possible to control the concentration of the acid product. The base product concentration can be controlled by water addition to be up to 10 wt% (for NaOH).
Two-compartment Configuration with Bipolar and Anion-exchange Membranes
This configuration works well with fermentation processes using ammonia as the neutralizing base. Many process benefits can be obtained by removing the acid product across the anion-exchange membranes while simultaneously concentrating and purifying the acid. The ammonium cations are converted to ammonium hydroxide that is recycled to the fermenter to control pH. The fermenter itself can now be operated at relatively low product concentrations to assure high productivity. The product acid concentration is typically at much higher concentration (2-4 N) if no water is added to the acid loop, except for the water transported with the acid. In this case, the conductivity of the acid would be sufficient due to the unavoidable diffusion of neutral ammonia into the acid loop: this contamination can be up to 10 % (on a molar basis) unless the ammonia diffusion is minimized by ammonia gas stripping.
In addition the post-purification sequence should be simplified, as many of the impurities from the fermentation is also rejected by the anion membrane and separated from the acid product. However, for the same reason, the anion-exchange membranes can be easily fouled by any impurities remaining in the feed. Therefore, the practicality of the required pretreatment sequence often will determine whether this configuration is suitable.
Technical Considerations:
The following table summarizes the process guidelines required for an efficient operation of bipolar membrane electrodialysis stacks.
General Process Guidelines for Bipolar Membrane Electrodialysis
Feed Salt Solutions :
- Soluble and clear salts
- Starting conductivity >35 mS/cm
- Multivalent metals <2 ppm >
- Minimize high MW organics (i.e. above 500)
Acid Products:
- Caution with poor solubility acids
- Typical acid concentration: 1-2 N for strong acids, up to 5 N for weak acids
- Base Products: Typical concentration: 2 to 5 N (up to 12 w% NaOH)
General
- Temperature up to 40°C
- No oxidizing chemicals, organic solvents
The feed salt must have a sufficient conductivity to achieve a sufficient conversion since there is a minimum final conductivity of about 10 mS/cm for practical operation. In general, the current density of bipolar membrane ED is higher than for regular ED (i.e. 500-1000 vs. 200-400 A/m2) because of current efficiency considerations. Therefore, to minimize the voltage across the stack (and the heat generated), the minimum conductivity is higher than for conventional ED. Also, the feed must be soluble and free of suspended solids (1 micron or less) to avoid membrane fouling and stack plugging. Eurodia/ Ameridia has experience in the pretreatment sequence that is often required.
The amount of multivalent cations, such as Ca, Mg, Fe, etc., in the salt feed have to be limited to a few ppm (the lower the better). These they will form insoluble hydroxides in the presence of OH- and precipitate in the cation-exchange membranes to increase cell voltage and reduce membrane life. Often, this is has to be satisfied by the use of chelating ion exchange resins columns.
Another design consideration is the concentration of the acid and base products. For strong acids, such as HCl or H2SO4, the maximum concentration that can be obtained is 2N. This is because, at higher concentrations, there would be too many small hydrogen ions leaking through the anion-exchange membranes into the base loop. Therefore, the current efficiency drops very quickly at concentrations above 2N. For weakly dissociated acids, such as most organic acids, and bases the concentration can be as high as 5N. However, one has to keep in mind that there must be enough water to diffuse inside the bipolar membrane. |
