The purpose with performing in situ experiments was to verify whether the citrate rich waste stream from the lime processing factory could be directly extracted on concentrated by electrodialysis. Two weeks off a three month stay as a guest researcher at IPN, México City, was relocated to the factory located at Tecomán in the State of Colima, México. During these two weeks, experiments were performed on the fresh lime juice at the local laboratory/kitchen facility.
For evaluation of optimal process parameters, stripped lime juice at different pH-values should be attempted concentrated.
2.3.2 Experimental
2.3.2.1 Methods and equipment (IPN)
Some model experiments were performed at IPN, México City, to test the electrodialysis equipment. Some of the equipment brought from Denmark had to be supplemented by local utilities due to local power conditions (the power grid yielded 110 V as opposed to 220 V).
The primary equipment necessary for the electrodialysis experiments was designed by the Membrane Group and constructed at the workshop at the Department of Chemical Engineering at DTU. It consists of an electrodialysis stack with 6 acrylic flow-spacers (6 mm), that can be arranges as the experiments requires, and two endpieces holding platinum electrodes.
The ion-exchange membranes are kindly supplied by Tokuyama Europe GmbH (Neosepta CMB, AMX and BP-1 membranes) and by Mitsubishi International GmbH, Germany, (Selemion APS-3 and CMV, Asahi Glass, Japan).
Pumps (EHEIM 1048 09 993, EHEIM, Germany) were easily obtained locally. Unfortunately, the acquired DC power supply did not function for as long as it took to complete an experiment, and spend most time in the repair shop than. For replacement, the DC power supply for a laptop computer (AC Adapter (Input 100-240V∼, 1.10-0.55A, 50-60Hz, Output 16-10V=, 2.2-3.2A)) was utilized as a last resort. This power supply could regrettably not be adjusted. Conductivity was measured by a Conductronic CL8 (Conductronic S.A., Puebla, México). Voltage and current density was initially measured by a FLUKE 83 III Multimeter (John Fluke Mfg. Co., Inc., Everett, Washington), and later by a LCD Auto Range Digital Multimeter 22-163 (Radio Shack).
The citrate concentration was detected in model solution by measuring the Brix index (on a Refractometer No. 16171, ERMA Optical Works, LTD., Tokyo, Japan) and comparing with standard curves.
The membrane setup was similar to the setup sketched in Figure 2.7.
2.3.2.2 Influence of pH and flow velocity
Some model solution tests with 4% sodium citrate solutions were performed at the University.
pH = 1.2 pH = 3.9 pH = 8.3 FV = 1 g/s Exp. M3.8 Exp. M3.4 Exp. M3.5 FV = 5 g/s Exp. M3.6 Exp. M3.7 Exp. M3.9 FV = 10 g/s Exp. M3.1 Exp. M3.3 Exp. M3.2
Table 2.8 Experimental plan for comparing responses as function of sodium citrate feed pH and flow velocity (FV).
Due to several incidences, not previously encountered by the author in Scandinavian research work1, the research plan was not completed.
Some responses as result of the completed experiments are presented in appendix 7.1.1. The less accurate measurements of citrate concentration by Brix index than standard HPLC are evident from these results and must be taken at most as relative measurements. The only clear trend in the results is increasing energy consumption with increasing feed pH.
2.3.2.3 Methods and equipment (Tecomán)
The electrodialysis equipment and pumps were similar to the equipment utilized at IPN. The DC power supply was still the laptop power supply. Current density was measured by the LCD Auto Range Digital Multimeter 22-163 (Radio Shack) and voltage by a HC-213 voltmeter (Hung Chang, P.R. of China). Brix index was measured by a handheld refractometer (II Index Instruments, Ramsey, Cambs.). A VIKING Digital Thermometer 1000 (Frode Pedersen & Co. AS, Allerød, Denmark) was utilized for measuring temperature, and pH was measured by an IMPO type 13.50 pH-meter (IMPO Electronic a/s).
For rough filtration, Cadelite 4187 was utilized. A filter made of a paper napkin was filled by the kieselguhr and lime juice waste was poured through it. The permeated solution was then filtered again by a new paper filter to remove kieselguhr remains.
For calcium removal, a cation-exchange resin, Puralite C100, was obtained. The resin was first charged with hydrogen ions in hydrochloric acid (200g resin to 500 ml demonized water and 12 ml conc. HCl for 15 minutes). Then the acid was removed, and the resin dried. Then the resin was mixed with lime juice (200g resin to 1 liter juice) for 20 minutes or until pH was stable in the mixture.
A Centra GP8 centrifuge (IEC - Int. Eq. Com., MA, U.S.A.) assisted in the pretreatment of the juice with kind assistance from Danisco Ingreedients, Tecomán, México. The samples were treated at 2000 RPM for 5 minutes.
The local Danisco Ingreedients laboratory also kindly measured the clarity of the juice at 650 nm by a Spectronic 20D (Milton Roy Company, U.S.A.).
Experimental samples was intended to be collected and measured by HPLC upon return to IPN or DTU for more accurate measurements of citrate and other organic matter content. Regrettably, the samples were lost by the courier upon return to the IPN.
1 Including among other things a bomb scare, a gas contamination alert and an earthquake.
2.3.2.4 Experiments with stripped lime juice
It was very quickly established that pretreatment of the stripped lime juice was necessary. Initial experiments with decanted juice demonstrated fouling to a degree that ended every experiment after 15 minutes due to excessive electrical resistance.
By filtering the lime juice through kieselguhr, the juice became a lighter color but was still cloudy.
It was thought that the dust particles from the kieselguhr were responsible, and repeated paper filtrations of the juice seemed to clarify it.
This clarified juice solution was tested in the first experiment that lasted longer than 15 minutes, and is summarized below:
Operation mode: Batch ED, 3 cell pairs, constant voltage Driving Force: 16.0 V (cv)
Feed: Stripped lime juice, kieselguhr-filtrated (500 ml) Concentration side: same as feed solution (500 ml)
Electrode rinse: 0.1M Na2SO4 (1000 ml)
Membranes: Tokuyama CMB and AMX
pH: 2.4
Since the concentration samples were lost, the Brix index during the experiment can be regarded as an expression of the concentration of biological matter. Since the amount of citric acid is very high compared to other acids and sugars remain relatively unaffected, Figure 2.17 suggests the concentration is possible though slow at the low pH.
Separation
0,0 2,0 4,0 6,0 8,0 10,0
0 60 120 180 240
Time (min)
RI (%) Feed
Conc
Figure 2.17 Brix index measurements during a concentration experiment on kieselguhr-filtrated lime juice.
Figure 2.18 shows the electrical current changes during the experiment. Since the power supply fixes the voltage at approximately 16V, the figure shows a steady increase in electrical resistance.
This is due to a combination of reduced conductivity in the dilution circuit and the built-up of fouling.
0 50 100 150 200 250 300 350
0 60 120 180 240
Time (min)
I (mA)
Figure 2.18 Electrical current (I) during experiment.
After the initial experiment, more stripped lime juice was filtered and clarified. According to the first experiment, 1 liter juice was used for each experiment with 500 ml initially in dilution and concentration circuit, respectively. This time, the juices were pH-adjusted by adding concentrated sodium hydroxide. The juices became darker upon pH-adjustment, transforming the color of the initial orange-yellow liquid to a darker red color.
The experiments that had the same basic process parameters as in the previous experiments, expects as noted below:
I - Decanted stripped lime juice, kieselguhr-filtrated, adjusted by NaOH to pH 7.2.
II - Decanted stripped lime juice, kieselguhr-filtrated, adjusted by NaOH to pH 9.5.
III - Decanted stripped lime juice, kieselguhr-filtrated, ion-exchanged, adjusted by KOH to pH 8.5.
IV - Decanted stripped lime juice, kieselguhr-filtrated, ion-exchanged, adjusted by KOH to pH 5.9, added 10g sodium bisulfite.
The results are all very similar as demonstrated in Figure 2.19.
0 200 400 600 800 1000 1200 1400 1600
0 5 10 15
Time (min)
I (mA)
I II III IV
Figure 2.19 Electrical current during initial minutes of experimental concentrations of pH-adjusted, kieselguhr-filtered lime juice.
The electrical resistance increased manifold during the first couple of minutes which indicates strong fouling. It seems that the pH-adjustment catalyses a precipitation of some protein in the juice, which could possibly be either pectin or lignin.
By centrifugation of the lime juice the clarity of the juice was measured to change from 17.6 to 88.6. pH-adjustment of the juice separated from the solid phase after centrifugation still showed precipitations.
The pH of the stripped lime juice was raised and caused the now familiar precipitation. This clouded solution was kieselguhr-filtrated two times and then paper filtered several times to reasonably clarity, but still demonstrated fouling.
In reverse electrodialysis operations (RED), a normal ED membrane setup is utilized (Figure 2.7).
But with intervals, the manifold system is switched so feed enters the spacers previously occupied by concentrate and concentration stream enters the spacers previously supporting the feed stream.
The direction of the current is also switched, and a new period is initiated. This way, the fouling built up on the membrane surfaces in the feed compartments is now located in the concentration compartments. Since the current direction is reversed, the fouling layer is often washed away in the first minute after reversal, and by discarding this first amount of concentrate emerging from the stack, the membrane fouling can be disposed of. A new fouling layer starts to built up in the new feed compartments until a critical level of electrical resistance is reached, where the reversal is again instigated. Since the juice had a high degree of visible fouling, RED operations was tested.
Operation mode: Batch RED, 3 cell pairs, constant voltage Driving Force: 15.0V (cv)
Feed: Twice kieselguhr-filtrated, ion-exchanged, stripped lime juice (400ml) Concentration side: same as feed solution (350 ml)
pH: 7.5 (adjusted by solid pellets of NaOH)
ρJuice: 1.044 g/ml
Flow feed: 10.5 ml/s (11.0 g/s) Flow conc: 11.1 ml/s (11.6 g/s)
The reversals were only performed every 60 minutes, due to the relatively large loss in citrate recovery by every reversal. Since the equipment still contains relatively large amounts of liquid compared to the total amounts of feed and concentrate held by the external tanks, the high citrate amount in spacers and manifold is transferred back to the feed solution when the manifolds are switched. Likewise, the concentrated citrate solution is diluted by the citrate depleted feed solution still contained inside the spacers after reversal.
This is evident from Figure 2.20 and Figure 2.21 that demonstrates the Brix index and pH-changes of feed and concentrate during two RED experiments.
5,0 6,0 7,0 8,0 9,0 10,0 11,0 12,0 13,0 14,0
0 30 60 90 120 150 180
Time (min)
RI (%)
0 1 2 3 4 5 6 7
pH
RI - feed RI - conc pH - feed pH - conc
Figure 2.20 Brix index and pH-changes in feed stream and concentrate during a RED operation with pretreated lime juice.
6,0 7,0 8,0 9,0 10,0 11,0
0 50 100 150
time (min)
RI (%)
0,00 2,00 4,00 6,00 8,00 10,00 12,00
pH
RI - feed RI - conc pH - feed pH - conc
Figure 2.21 Brix index and pH-changes in feed stream and concentrate during a RED operation with pretreated lime juice.
The two experiments demonstrate that it is possible to run for extended periods through this operation mode. The fouling built-up between reversals is evident from Figure 2.22 that shows the reduction of electrical current between reversals (compare with Figure 2.20).
0 500 1000 1500 2000 2500
0 30 60 90 120 150 180
t (min)
I (mA)
Figure 2.22 The loss of electrical current during each reversal period, demonstrating the fouling built-up.
The results shown in Figure 2.20 and Figure 2.22 originate from an experiment, where the first reversal was a simple manifold switch, which demonstrated a great step back in the separation (when considering the Brix index an indication of acid concentration). The second reversal after 120 minutes included an attempt to empty the equipment for a much liquid as possible, before switching the manifolds. As can be observed in Figure 2.20, the second reversal had only insignificant effect on the concentration.
2.3.2.5 Conclusion on experiments
The conclusion on the in situ experiments demonstrated the need for further knowledge of the fouling content of the stripped lime juice, and indicated the need for an effective pretreatment. The concentration process is possible at lowest pH, but the conductivity is very low, and thus, the citrate flux is low and the energy consumption high. For an economical feasible process, model experiments indicate that a medium range feed pH is most optimal, but the stripped lime juice has revealed a heavy fouling caused by precipitating bio-matter when the pH is raised, even after intensive pretreatments including kieselguhr-filtration, ion-exchange and centrifugation. Operations with reverse electrodialysis demonstrated potential, but the experiments were carried out in batch mode, meaning that new fouling material was not added during operation. In the real application, fresh lime juice would be continuously added to the process, resulting in increasing fouling.
Whether the RED operation can handle this fouling in real application remains to be seen.