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. |
