Experiments simulating continuous extraction of lactate from fermentation broth

To start simulating the conditions when lactate is extracting in conjunction with a continuous fermentation, the titration device was added to the setup as shown on Figure 4.11 to simulate the continuous addition of fresh broth to the stack. A pH-sensor was introduced in the feed tank, and the titration device was programmed to maintain the pH of the feed at 5.5 by adding additional fermentation broth to the feed. Since the initial feed solution was not removed in these experiments, added broth was concentrated in respect to lactic acid (80 g/l) and had a pH of 2.1. Thus, it was only necessary to add a small volume to the feed to preserve to pH while also adding small amounts of bio-matter. The membrane setup was not changed and the methods and equipments utilized are also identical to what was used in previous experiments.

When lactate is continuously removed from a fermenter, a low lactate concentration with some unconverted sugar is more realistic than the 7% lactate concentrations utilized in initial experiments. The feed solutions in the following experiments all contained 2% lactic acid adjusted to pH 5.5 by potassium hydroxide. Since the pH of the feed solution is preserved, the alkaline concentration in the base was increased from 0.1 M to 0.5 M. This gives the alkaline solution a much higher capacity for extracted lactate. Sodium hydroxide was replaced by potassium hydroxide, since agricultural waste is more probable to contain free potassium ions and these ions are almost identical in behavior under the present circumstances.

4.2.3.1 REED experiment 7: Retention of calcium

Since retention of “hard” ions like calcium is crucial to subsequent purification steps, the calcium

Two experimental runs tested the retention of calcium: one using the special grade ACS membrane and one using the standard AMX membrane.

The feed solution was 500 ml 2% potassium lactate at pH 5.5 in both cases, and the alkaline solution 500 ml 0.5 M KOH. About 200 ppm calcium hydroxide was added to the feed.

The current was reversed every 30 seconds in these experiments.

The extraction experiments lasted for four hours each. Pure lactic acid was added to the feed tanks through titration, keeping the feed pH at 5.5. The results of one of the experiments (utilizing AMX membranes) are depicted in Figure 4.22.

0 4 8 12 16 20

0 60 120 180 240

Time (min.)

Lactate concentration (g/l)

0 0,0003 0,0006 0,0009 0,0012 0,0015

Lactate flux (mol/m2 s) Lactate (feed)

Lactate (base) Flux

Figure 4.22 Lactate concentrations in feed and base and the lactate flux during the experimental run utilizing AMX membranes.

This figure demonstrates how the lactate concentration is maintained in the feed tank through the constant titration, which maintains a stable lactate flux and thus, the lactate is easily extracted to the alkaline solution. Much higher lactate content can be sustained in the alkaline solution without significant transport of lactate back to the feed due to the increased base concentration of 0.5 M.

The results of the two experimental runs were almost identical.

The calcium retentions of both membranes are both sufficiently high to accommodate the restrictions set for bipolar membranes, as can be seen from results in Table 4.1.

Neosepta AMX Neosepta ACS

Time (min) [Ca²⁺]Feed [Ca²⁺]Base [Ca²⁺]Elec [Ca²⁺]Feed [Ca²⁺]Base [Ca²⁺]Elec

0 179 0.08 0.52 170 0.07

30 182 0.22 172 0.12

60 186 0.27 173 0.18 0.35

120 183 0.46 0.49 176 0.41

180 180 0.67 185 0.47

240 181 0.59 197 0.64 0.20

Table 4.1 Calcium concentrations in ppm in feed and base tank and electrode rinse during two experiments with different anion-exchange membranes.

No significant difference in calcium retention could be observed for the two membrane types. The build-up of calcium in the base is insignificant and will not be of consequence in a continuous process.

4.2.3.2 REED experiment 8: Retention of unconverted sugars, calcium, and magnesium in brown juice

To make sure, calcium and magnesium would not enter a complex compound in a feed containing bio-matter that was able to penetrate the anion-exchange membranes, the previous experiment was repeated with brown juice as feed solution. The brown juice contained 700 ppm calcium and 400 ppm magnesium. This experiment was also an investigation of the diffusion flux of unconverted sugars from the feed to the base. Since most of the feed solution from a fermentation broth is recycled back to the fermenter, it is important to retain the unconverted sugars, too, so the conversion efficiency of the fermenter is preserved. The stack was equipped with ACS membranes.

During a 2 hour period, only very small amount of divalent cations was detected in the base and electrode rinse as indicated in Table 4.2.

Time (min) [Ca²⁺]Feed [Ca²⁺]Base [Ca²⁺]Elec [Mg²⁺]Feed [Mg²⁺]Base [Mg²⁺]Elec

0 667 0.15 x 394 0.01 x

60 737 0.09 x 425 0.01 x

120 705 0.13 0.19 403 0.02 0.05

Table 4.2 Calcium and magnesium concentrations in ppm in feed, base, and electrode rinse during REED experiment 8.

(x) marks samples where no traces could be detected.

The leakage of divalent cations is even smaller than in the previous experiments even though the added concentrations are much higher. Unlike, in the model solutions where calcium and magnesium were free ions, the majority of the cations were probably bonded to the biological material in the brown juice and thus more easily retained by the membranes.

The unconverted sugars in the brown juice were all sufficiently retained. Only insignificant amounts of sugars were detected in the base. Some diffusion of sugars is to be expected, but commercial anion-exchange membranes with excellent retention of sugars are easy to come by.

4.2.3.3 REED experiment 9: Shorter frequency of reversal during long time operation

The last experiment with the reverse electro-enhanced dialysis setup that is disclosed in this chapter is a fouling experiment that lasted for 7 hours. The electrical current was reversed every 10 seconds in this experiment, to investigate whether the shorter intervals would result in lower overall cell resistance, and consequently lower power consumption.

The feed consisted of 250 ml brown juice with 2% lactic acid. Brown juice with high lactic acid content was titrated to maintain the pH-value of the feed. The alkaline solution was 250 ml 0.5 M KOH.

After four hours, the alkaline solution was replaced with a fresh solution to continue operations

The nature of the voltage drop across the central cell pair changed during the experiment as shown on Figure 4.23.

-2 -1 0 1 2

0 60 120 180 240

Time (sec.)

Ecell (V)

-6 -4 -2 0 2 4

13800 13860 13920 13980

Time (sec.)

Ecell (V)

Figure 4.23 Comparison between the voltage drop across the cell pair in the beginning and in the middle of REED experiment 9.

Between each current reversal, the familiar build-up of resistance was obvious as demonstrated in the two graphs in Figure 4.23. The increase in cell resistance between reversals was much more pronounced later in the experiment than in the beginning. It also seems that one membrane surface was more receptive to the fouling than the opposite surface in the feed chamber, giving the graph an asymmetrical look. This could possibly be contributed to uneven flow distribution in the spacers.

The overall voltage drop of the cell increased from 1.5 V in the beginning of the experiment to 1.9 V near the end. Between reversals, the 1.9 V increased to 2.8 V or 8 V, respectively, depending on the current’s direction.

The lactate concentration in the feed was kept relatively constant due to the constant titration as shown in Figure 4.24. The figure also shows the lactate concentration in the two batches of base.

0 5 10 15 20

0 60 120 180 240 300 360 420

Time (min)

Lactate concentration (g/l)

Alkaline Solution Fermented Brown Juice

Figure 4.24 Lactate concentration in feed (fermented brown juice) and base (alkaline solution).

The flux of lactate was lower than expected from earlier experiments. Moreover, the current efficiency was found to be 3%, which is unacceptable. The low efficiency and fluxes were later

discovered to be the result of the short intervals between current reversal. As will be demonstrated in the subsequent modeling section later in this chapter, it is necessary for the membrane profiles inside the membranes to shift before acceptable current efficiencies can be reached, which takes some time. During this buffer time, only low efficiencies can be obtained as seen in this case.

The self-cleaning mechanism allowed the process to run for the full 7 hours of operation.