Food, Sugar, & Starch

ELECTRODIALYSIS IN THE SUGAR INDUSTRY AS A PURIFICATION TECHNOLOGY
by Dr. Florence Lutin
(paper presented at The SPRI sugar conference in Porto, Portugal - April 2000)

1. Introduction

Improvements in both the structure of anion-exchange membranes and the design of electrodialysis stacks have allowed electrodialysis to be considered as one of the technologies that can be introduced in the sugar industry to partially replace ion exchange resins for the demineralization and purification of sugar syrups.

Until a few years ago, the two main limitations of electrodialysis in the sugar industry were the short membrane life, especially for the anion-exchange membranes, and the low operating temperature that had to be maintained below 40°C.

The recent development of a new anion-exchange membrane, (the NEOSEPTA® AXE 01 from TOKUYAMA Corp.) that can be cleaned with a high NaOH concentration and operated at temperatures up to 60°C, has made it possible to minimize organic fouling. Consequently, the operating costs are reduced.

2. Improvement of the electrodialysis technology

2-1 A new Neosepta anion-exchange membrane

To economically operate ED in the sugar industry, one of the key challenges is to use an anion-exchange membrane that is resistant to organic fouling. Until recently, the AFN Neosepta membrane was the most suitable membrane thanks to a low cross-linking, allowing easy transport of organic anions of molecular weight lower than 300. However, this membrane showed a weak mechanical resistance when cleaned with NaOH at high temperature.

The AXE 01, a new anion-exchange membrane, has been developed by Tokuyama Corporation for sugar applications to overcome the above limitations.
Thanks to its higher burst strength, this membrane is easier to handle in industrial ED stacks.

2-1-1 Alkali resistance

To increase membrane life, it is necessary to avoid irreversible organic fouling by opening the polymeric channel with a low DVB content. In addition, it is beneficial to have the possibility of cleaning with a high concentration caustic solution to allow membrane swelling and remove organic molecules trapped inside the membrane. Indeed, when cleaning ion-exchange membranes with caustic, a risk exists of reducing the exchange capacity and the mechanical resistance of the membrane.

The AFN and AXE 01 membranes have been soaked in 1wt% NaOH + 1wt% NaCl solution during 5 days at 60°C. The exchange capacity and burst strength were measured (Table 2).


TABLE 1

 The new membrane features a high alkali resistance with only a 10% E.C. and a 22% burst strength decrease, compared to 90% and 67% for the AFN.

Operating at temperatures higher than 40°C minimizes the growth of microorganisms, especially in the sugar industry where fermentation can start very rapidly. Together with conventional cation-exchange membranes, the AXE 01 can be operated at up to 60°C. In the meantime, new spacers have been developed with new polymeric materials offering perfect mechanical stability at up to 60°C.

Several pilot trials have been carried out with cane and beet sugar syrups in order to evaluate and optimize the demineralization rate that can be achieved with ED.

• Feed : 30 Bx and 50 Bx molasses
  
• Operating temperature : 50°C
  
• Pilot stack : EUR2B-10 - 0.2 m2 - NEOSEPTA AXE 01 and CMX membranes
  

The purpose of this trial was to evaluate the demineralization rate for syrup concentrations at 30 and 50 Bx. At 30 and 50 Bx, for current and conductivity as a function of the treatment time, we have obtained a good reproducibility over 4 runs at each concentration (figures 1-2-3-4).

In both cases, cane sugar molasses can be demineralized by electrodialysis at up to 60%.

The overall demineralization rate is independent of the concentration of the molasses. At the same voltage, the applied current density is directly proportional to the cations concentration in the molasses (Table 3). The demineralization capacities are 3.5 eqh-1m-2 at 30 Bx and 4.5 eq.h-1.m-2 at 50 Bx.

TABLE 2

 
The divalent cations (calcium and magnesium) appear more easily removed from molasses at 30 Bx than at 50 Bx. In the meantime, the potassium and sodium removal rates are higher for 50 Bx molasses than for 30 Bx molasses.

Nevertheless, for industrial plants, the most important limiting factor regarding product concentrations is the difference of syrup viscosity between 30 and 50 Bx, and consequently the pressure drop through the stacks. For this reason, working with 30 Bx seems more convenient, if it is acceptable from the point of view of the overall energy balance.

This pilot test is interesting with respect to the minerals and organics removal rate.

Beet sugar syrup can be demineralized by ED at up to 80%. However, to avoid inversion, the pH must be maintained above 7, and, therefore, the demineralization rate must be limited to 68 %. The ion removal capacity is 3.6 eq.h-1.m-2 .

Operating conditions:
·       Pilot stack : EUR2B-10 - 0.2 m2 - Membranes : AXE 01 / CMX
·       Temperature : 25°
·       Brix : 30 Bx - pH: 8.6 - initial conductivity : 6.8 mS/cm - Final cond : 2 mS/cm
Ions Removal Rates

Kinetics of Transport

The kinetic of potassium transport is higher than sodium transport (Figure 5). Figure 6 shows the removal of organic acids through ED. Malate and citrate anions can be transported easily through the anion-exchange membranes. The transport rate of malates is the same than for sulfates.

A fructose syrup has been demineralized up to 50%. Operating conditions:


• Pilot : EUR2B-7 - 0.14 m2 - Membranes : AXE 01 / CMX

• Temperature : 30°C

• Brix : 30 Bx - initial conductivity : 8 mS/cm - Final

• cond : 4 mS/cm

In these conditions, the demineralization capacity is 18 L.h-1m-2 with a 53% conductivity decrease.

At higher temperature 50°C and same voltage, current density capacity becomes 31 L. h^-1m^-2.

The syrup purity is improved from 85% to 90 % (Table 4). The sugar losses into the brine are lower than 0.5 % of the initial sugar content, it means that sugar recovery reaches 99.5 %.

If we increase the demineralization up to 70 %, purity increases from 90% to 93%

The first commercial ED plant in the European sugar industry in has been operating since 1996. The target was to double the existing capacity of the ion exchange resins without any increase of the pollution load.

Electrodialysis has been chosen to demineralize the juice at up to about 55 % and keep the same volume of existing resins as a "polishing" step.

The feed at 12 or 24 Bx is demineralized with 12 stacks arranged in 3 lines in parallel (each line has 4 stacks in series), thus achieving a 55 % conductivity reduction.

Each line treats 20 m3/h. With 4 stacks EUR20-440 in series, the conductivity is reduced from 8 down to 4 mS/cm. The maximum pressure drop through the four stacks operated in a single pass is 4 bars.

The throughput is 5 eq.h-1.m-2. The single pass reduces the residence time and avoids any bacterial development. For the 1999 sugar campaign, the new AXE 01 membranes and new spacers have been installed to operate at 55°C: during this campaign, no increase of pressure drop was observed while the demineralization rate was dramatically improved.

Since then, the EUR40, a new size of ED stack, is available: such a stack would result, if used for demineralization at 60%, in only 2 stacks in series per line.

The sugar losses are less than 0.5 % of the sugar production. The electrical consumption is 1.1 KWH/m3 for ions transport and pumping. CIP consumes only 0.045 L/m3 of HCl at 0.4% and 0.001 L/m3 of NaOH at 0.4%.

The waste flow rate is approx. 26 m3/h with a minerals content of 0.1 eq/l. The capital cost for a capacity of 40 m3/h is 1 700 000 Euro with a membrane replacement cost lower than 3000 Euro/m3/year.

 

With new available technology that can operate at up to 60°C, electrodialysis can be considered as one of the technologies that can contribute to improve the cost effectiveness of sugar plants, since it can help to:
• control organic fouling,
  
• reduce waste effluents and pollution load,
  
• improve sugar yields,
  
• reduce the volume of molasses,
  
• save capital costs.
  

As another example, the results of a large scale pilot plant at a sugar mill in Morocco are presented below.

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