Ion Exchange Process—Batch Type
Ion Exchange Resin consists of perfectly spherical beads of around 0.5mm in diameter and for sugar use they would be based on a styrene or acrylic polymer. The beads can be considered as sponges and the sugar solution will enter the beads and the colour will remain attached to the resin bead active sites.
The resin has the property that when the liquor is fed through a bed of it in a vessel, the colour attaches itself to the resin. After a certain time in operation most of the active sites are full and not much more colour is being removed from the sugar. Then the resin has to be taken out of service, desweetened with water to remove all of the sugar and then regenerated with 10% Salt Solution containing a small amount of Sodium Hydroxide. On regeneration the colour molecules detach themselves from the resin and go into the salt solution. Once regenerated it is then ready to go back on line and remove more sugar colour.
An ion exchange plant consists several resin vessels, called cells, usually operating in pairs with one pair off line at any time for regeneration. The resin is desweetened, regenerated and sweetened back on line all in the same vessel. This makes the vessel a complicated and hence expensive design. The rest of the equipment is for regeneration with tanks for taking in solid salt, dissolving it and adding it to the resin vessel at a controlled rate for regeneration. There is also equipment for restoring the condition of the resin by backwashing it with hot water and compressed air.
Generally the resin plant will be designed for a certain percentage of colour removal so that the required refined sugar colour is obtained. The level of colour removal can be as high as 80% removal of the colour entering the station but it can be designed to give lower values as required. Though the high level of capital investment makes this a process that is generally only installed when high levels of colour removal are required.
In simple terms the design basis is based on the volume of resin and the number of resin cells installed. This provides a balance between capital cost and operating costs of alkaline salt used. This is because a set of resin cells will have to be regenerated once the colour coming off the station is too high to make the correct colour of refined sugar.
So when an Ion Exchange Station is installed, it will be designed for a certain percentage of colour removal, for example 75% and the number of Resin Cells and the regeneration equipment will be installed to achieve this. It may be possible to improve the performance slightly by regenerating more often. A lower level of performance can be obtained by regenerating less often but the large capital cost has already been installed and so this is the big expense of the installation.
Ion Exchange Decolourisation is the key in Refining sugar. Although refining improves in many ways, colour is one property that is immediately obvious and can easily be measured. There for color is often specified, hence one of the principle control in every refinery.
Although the word ‘color’ means visual appearance, it’s interpreted in sugar work as an index of amount of impurities is causing, the visual appearance. When the sugar chemist says ‘colour’ he means ‘’colorant’ the material causing the color.
Generally three type of colorant are recognized according to their origin.
Plant pigments of sugar cane
Melanodins type material (Reaction between amino acid & reducing sugar).
Colorant type material formed by the thermal degdration of sugar.
Types of Colourant –
Type 1 – Ionic / Aromatic
This type can be removed by adsorption with ion exchange function or aromatic surface
Type 2 – Nonionic / Aromatic
This presence is highly aromatics in nature. There is no ionic group present. This type of colorant is most effectively removed by adsorption with aromatic nature. i.e. carbon, to lesser extent by polystyrene resin.
Type 3 – Ionic & Non Aromatic
It contains ionic functional group & aliphatic molecular structure with conjugated double bonds due to its aliphatic nature. This group is most effectively removed by ion exchange mechanism, & some extend to ppt. If the double bond is conjugated extensively they can also be adsorbed by aromatic adsorbent.
Type 4 – Non Ionic & Non Aromatic
It contains small population of colorant & very difficult to remove by currently available adsorbent unless non aromatic fraction is highly conjugated.
Comparison of three different dcolorization resin primarily being used today-Macro porous acrylic, Macro porous styrene & granular styrene.
Now a days two principle type of strong Macro porous anionic resin are used for color removal in cane sugar refinery, Styrene & Acrylic.
Characteristics of styrene & acrylic resins-
Description of Ion exchange unit-
The sugar solution to be decolorized is pumped form a liquor tank after MBF into the 1st IE Column , usually the sugar solution have 60 -65 brix & 600to 700 IU Color at a temperature of 80-85 C . A safety filter musty be used to avoid plugging of resin bed with suspended solid. Normally check filter is used 10-50µm after DBF. Decolonization is achieved by percolation through one or two resin beds with a melt flow range of 1.5 to 4 BV / hr. Mostly a double pass process is used to reach average decolourization to 85-90 %.
The industrial vessels are designed melt peculating top to bottom in resin beds.
As density of melt is 1.3 & density of resin when hydrated with melt. These decolourization vessels are difficult to operate, because resins have a natural inclination to float in the melt. Considering the relative viscosity of melt 5cp, the possible flow through the resin bed is also soon limited by the pressure drop, & this is therefore difficult to maintain well compacted resin bed with this vessels compacting design. Good compacting of resin bed is necessary for uniform flow distribution in the column without channeling. It is essential to obtain the best performance during decolourization & regeneration.
For this reason most of the latest IE vessels have been installed to peculate melt from bottom to top into resin bed. The resin is compacted or an inert material melts during up flow decolourisation of melt. And against a bottom plate or sand a send material bed during down flow regeneration.
To prevent accidental resin leakage likely to pollute to sugar circuit a resin trap can be install at the outlet of the decolonization plant.
It is recommended that an on line 100-200µm screening filter must be used. In modern sugar refinery all vessels & pipes are making by stainless steel. The finely decolorized collected liquor is called fine liquor is collected ion a separate tank & concentrated up to 75 brix in melt concentrator before being sent to the crystallization.
Between decolourization & regeneration the IE vessels sweetened off & on. The dilute sugar solution is so obtained collected separately is called sweet water. It is off 25-30 brix & used in remelting.
The sweetening on & off & regeneration steps, process water which is either condensate or soft water is used. To reduce process water requirement internal water recycling is done in regeneration process. About 40% of process water can be recycled in this way.
The brine & regeneration effluent station is installed for the regeneration vessels with acid or caustic brine solution. First a saturated brine solution obtains by dissolving NaCl crystals in water. At ambient temp. The saturated brine concentration 330gm of NaCl / litre. As the efficiency of neutral brine regeneration is usually not sufficient for the complete removal of all coloring material from the resins, acid or caustic brine solution must be prepared by using HCL & NaOH dosing pumps. Although the preparation of acid brine of simple, the addition of caustic solution of 35 or 45 % in to the neutral saturated brine must be manage carefully. The impurity contents in commercial NaCl crystal specially Mg, flocculate in the presence of NaOH in to a hazy Mg(OH)2.
Before using the caustic brine solution for the regeneration of the resins, it is necessary to filter it through sand filtration. The design of the brine preparation system depends on the impurities of NaCl Crystals. Some time brine saturation, basification and decantation is carried out in a multiple compartment brine preparation tank.
The regeneration effluents, contain, sugar colorants and water can be divided in to two parts. The first composed primarily of rinse water and back wash effluents, can be sent directly to waste. The second has a PH tank to 12 if caustic brine regeneration is done or low PH 2 to 5 for acid brine regeneration. This effluent must be neutralized before being sent to waste.
All regeneration effluents must be cooled before being sent to waste. This heat recovery can be done in heating the process water.
The diluted brine circuits and internal parts of ion exchange vessels are protected against corrosion by coating a rubber lining in side the vessel.
All modern ion exchange decolonization plants are fully automation and controlled by DCS & PLCS. The automatic valves are installed on the pipe many folds are opened and closed by means of sequences programmed in the process operation cycle. The flows in the various sequences are controlled by flow meters and automatic control valve installed on inlet pipes of the plants. Once the sequence times or volumes and the flow set points have been adjusted on the automation system the plant can run fully automatically with out any special action required from the operator.