Feature

White Sugar Mill technology sweetens sugars future

We all know black strap molasses - that sticky, bitter, dark black gunk that is a by-product of the sugar industry. High test molasses is a slightly higher sugar content product which many have heard of. However, if somebody were to talk about white-strap molasses that would probably draw a blank stare with most people.

The pilot scale ultra-filtration plant used to test the WSM technology.

In the hi-tech biotechnology industry, molasses represents an inexpensive source of sugar for fermentations - however, the downside of black strap and high-test molasses is that both contain large quantities of impurities that can interfere with the later extraction of high value biochemicals. Imagine that you are using molasses as a sugar source when making the amino acid lysine - if you use a dark black sugar source, chances are your amino acid will come out looking brown and impure on the other end of the process - a costly defect to repair.

A group of South African companies, Tongaat-Hulett Sugar Ltd. (THS) and its partner S A Bioproducts Ltd. (SABP) have developed and patented a technology known as WSM - an acronym for White Sugar Mill - which will revolutionise modern sugar processing by increasing recovery of sugar, producing a high value white strap molasses free of the problems of black strap molasses, and conserving minerals which are extracted from sugar cane and converting them into fertilizers which can be returned to the soils from which they came.

Ultrafiltration

Samples of juice from various points in the WSM process. As is evident, the final product is water white.

Filtration refers to the process whereby a solution flows through a membrane of some sort and some things are held back by the membrane and others are allowed through. With size exclusion filtration, where the things that go through and the things that stay behind are decided by their size, the filters are described as micro filters if they stop relatively big things but still let large molecules through, ultra-filters if they exclude large molecules but let small ones through. Ultrafiltration of sugar cane juice removes high molecular weight compounds from the juice so that it does not cause complications further down the production process. A number of different membrane types exist which can be used for ultra-filtration. Some of these membranes, such as ceramic and stainless steel membranes are pretty tough and can be vigorously cleaned with powerful cleaning agents that would dissolve other types of membranes. Given the high volumes and relatively low cost of the sugar cane juice, the stainless steel and ceramic membranes were the most sensible and cost effective filters to use.

Sucrose is a sugar made up of one molecule of glucose and one molecule of fructose. If the bond between these two sugars is broken, free glucose and fructose result, a reaction termed inversion. Inversion will typically occur faster at higher temperatures and under acidic conditions.

Ion exchange resins are resins capable of swapping one chemical ion for another. This process unfortunately works better under slightly acidic conditions, hence if the sugar cane juice is to pass through cation exchange resins to be demineralised, the juice must first be cooled to below 15°C to slow down the rate of inversion reactions. This is achieved with a refrigeration unit. Refrigeration is an expensive process, hence cool juice from this section is warmed up for a subsequent process step by hot juice entering the refrigeration section - this demonstrates a clever engineering inclusion to conserve energy and reduce costs.

The demineralisation pilot plant.

De-mineralisation is carried out in two Simulated Moving Bed ion-exchange units, one containing a strong acid cation exchange resin and the other a weak base anion exchange resin. Each unit comprises a patented multi-port distribution valve developed by THS which distributes the various process flows to between 24 and 36 stationery resin vessels. Generally a three pass system is used with cation exchange followed by anion exchange, the liquor flow crossing backwards and forwards between the cation unit and the anion unit. This is a fancy way of saying that both positively and negatively charged ions are removed from the juice.

With ion exchange resins there comes a time when the resin is exhausted - it has swapped everything it contains with stuff in the juice and is now no longer working. At this point a resin needs to be regenerated - it is typically treat with a strong chemical which causes it to release the things it has taken hold of and replace them with new ions which can be swapped at a later stage when the resin is used again.

The cation resin is regenerated with either Hydrochloric or Nitric acid at 7-9% concentration and the anion resin with Ammonia at 4% concentration.

Apart from removing inorganic ionic species, the two de-mineralisation resins also remove at least 80% of the colour in the juice by a combination of ion-exchange and adsorption.

The by-product of the regeneration steps is that the cation resin will for example remove hydrogen ions from the acids in exchange for the metal ions it has adsorbed, such as sodium and potassium. If nitric acid is used to regenerate the resin for example, this would mean that the by-product would be potassium nitrate - a valuable fertiliser. Likewise the anion resin regeneration would yield by products such as ammonium nitrate and ammonium sulphate - both useful fertilisers.

To enable sugar equivalent in quality to refined sugar to be produced, the juice colour needs to be decolourised. This is done in an ion-exchange unit similar in design to those used for de-mineralisation. The resin used is a strong base resin in the chloride form. Regeneration of this resin is by a caustic brine solution.

The WSM process integrates into a conventional sugar factory between the first and second evaporation stages. The vapour from the first effect goes directly to the second effect as in a normal evaporator train.

So what does this technology offer over conventional systems?

Sugar Recovery

A diagram showing the WSM process additions (blue) superimposed on a conventional cane sugar production system (yellow).

If the whole throughput of a mill were to be treated by the WSM Process, then typically an improvement in Boiling House Recovery of approximately 7% will be found.

The crystallisation rate from massecuites after WSM treatment is considerably higher as a result of higher purities and lower viscosities for equivalent purities. This translates to an effective crystallisation capacity increase. Cooling crystallisers will not be needed for the white strikes.

The sugar boiled from the decolourised juice will meet of a high quality, with the 1st and 2nd boiling sugars blended and two recovery boilings remelted and recycled to the decolourised juice.

Reduced Scaling

We have all seen that scale that forms on the element of our kettle. This is a product of ions in the water changing form under heating and precipitating onto the element.

In industrial processes, scaling of heat exchange surfaces is a big problem as it reduces the heating efficiency of heat exchanges and necessitates cleaning cycles which cost time and money.

The demineralisation of the juice in the WSM process has a major impact on the amount of scaling deposited on the heating surfaces in the evaporators from the second effect onwards, reducing it to insignificant levels. This gives a capacity increase as the HTC's in these evaporators will not progressively decrease in service. Cleaning requirements will be drastically reduced saving costs, plant downtime and throughput disruptions. The same effect will be found in the vacuum pans, but as scaling there is generally less of a problem than in the evaporators, it will be less noticeable.

In the production of ethanol from fermentation feedstock that has undergone the WSM process, reduced scaling will be found in the distillation column, improving operation and lowering cleaning costs. This will apply to the separation processing plant for any fermentation product.

Fermentation

The WSM "molasses" obtained after the white sugar boilings is a very clean, high quality fermentation feedstock which improves product yields and simplifies separation processes.

Experimental work done recently in Brazil showed a 2% increase in ethanol yield compared to conventional clear juice. The same work also showed that if the decolourised juice is used as a fermentation feedstock on its own, a 7% ethanol yield improvement compared to clear juice could be obtained.

Fertilisers

Depending on the chemicals used, the spent regenerants from the demineralisation can have considerable value as fertilizer; Potassium is recovered from the juice and Nitrogen from the regeneration chemicals. They can be applied directly to the cane fields, substituting purchased fertilizer, reducing the operating costs of the WSM Process and also minimizing any environmental impact.

Compiled and written by Dr Garth Cambray based on data provided by Mike Cox of Tongaat-Hulett

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