Carbonation Process in Sugar Refinery

The carbonation process is a widely used method for refining sugar from raw sugar melt. It involves the reaction of calcium hydroxide and carbon dioxide to form calcium carbonate, which precipitates and removes impurities and colorants from the sugar solution. The carbonation process has several advantages, such as low capital and operating costs, high color and turbidity removal efficiency, low sugar loss and environmental friendliness.

In this blog post, we will explain the main steps and equipment involved in the carbonation process, as well as some tips and best practices to optimize its performance.

The goal of the carbonation process is to eliminate impurities that make raw melt liquor cloudy. Carbonation is typically used in refineries before any decolorizing process. It has a positive effect on sugar liquors, reducing color by 40-50% and ash content by 20-25%. Lime and carbon dioxide are added to the sugar liquor to create calcium carbonate precipitates that absorb impurities and color. These impurities are then removed through filtration.

Reactions in Carbonation Process

Ca(OH)2+CO2=CaCO3+H2O

The carbonation process has two main steps. The first step is the creation of a large, gel-like precipitate from the reaction between Calcium Hydroxide and CO2. The second step involves conditioning the precipitate to improve its ability to be filtered. Impurities are absorbed and trapped by the calcium carbonate particles formed from the reaction between carbon dioxide and calcium hydroxide. This calcium carbonate traps wax, gums, polysaccharides, colorants, ash and also destroys invert sugars. The precipitate containing most impurities is then removed through filtration.

The carbonation process consists of four main stages: liming, first carbonation, second carbonation and filtration.

Liming:

In this stage, a slurry of calcium hydroxide (lime) is added to the raw sugar melt in a liming tank to increase its pH to 10.5-11. The liming tank is equipped with an agitator to ensure proper mixing and suspension of the lime and sugar solution. The liming stage helps to neutralize any acids present in the raw sugar melt and to prepare it for the carbonation reaction.

First carbonation:

In this stage, the limed sugar solution is transferred to a first carbonator tank, where scrubbed carbon dioxide gas is injected into it. The carbon dioxide gas reacts with lime in the sugar solution to form calcium carbonate, which precipitates as a fine and gelatinous solid. The calcium carbonate also adsorbs and entraps impurities and colorants present in the sugar solution, such as sulphates, starch, gums, wax and invert sugars. The first carbonator tank is operated at a pH of 9.5-9.6 and a temperature of 80-85°C. About 75-85% of the total carbon dioxide gas required for the process is added in this stage.

Second carbonation:

In this stage, the partially carbonated sugar solution is transferred to a second carbonator tank, where more carbon dioxide gas is added to complete the reaction. The second carbonator tank is operated at a lower pH of 8.5-8.8 and a lower temperature of 70-75°C. The remaining 15-25% of the total carbon dioxide gas required for the process is added in this stage. The second carbonation stage helps to condition the calcium carbonate precipitate and improve its filterability.

Third Carbonation: At this stage, only more carbon dioxide gas and MOL is added for further reaction but no external heat is provided. The reaction between Calcium Hydroxide and Carbon Dioxide is exothermic. Heat generated from this reaction compensates for the heat.

Key-points

1. After completion of gradual Carbonation at each Carbonator body, liquor flows to the next by gravity.
2. When liquor exits the third body it is called Carbonated Liquor and stored in a buffer tank.
3. Carbonator 1st and Second Body have Calandria inside. The third body of Carbonator doesn’t have any Calandria inside as it compensates for required heat from exothermic reaction as mentioned in the previous passage.
4. These Calandrias facilitate exchange of heat for reaction. Steam is used as a heat source. Calandria is Shell and Tube type Heat exchanger. Steam is fed in shell side of Calandria and liquor is fed in Tube side from inner side of Carbonator body. Steam condensate is either returned to the hot water tank or to the Boiler section to be used as Process Water or in Boiler for steam generation.
5. Carbonation is a Critical Control Point(CCP) for Sugar Refinery. In case of unexpected liquor quality due to process hazards, carbonated liquro is sent back to melter section.

Filtration:

In this stage, the fully carbonated sugar solution is filtered through rotary vacuum filters or pressure leaf filters to separate the clear sugar liquor from the calcium carbonate cake. The clear sugar liquor is then sent to further decolorization and crystallization stages, while the calcium carbonate cake is washed with water and discharged as a by-product or waste.

Some tips and best practices to optimize the performance of the carbonation process are:

1. Use good quality lime with high purity and reactivity.

1. Control the lime dosage according to the raw sugar quality and pH target.

2. Use scrubbed carbon dioxide gas from boiler flue gas or other sources to reduce costs and emissions.

3. Control the carbon dioxide gas flow rate according to the pH target and reaction kinetics.

4. Maintain optimal temperature and residence time in each carbonator tank to ensure complete reaction and precipitation.

5. Monitor and adjust the pH, turbidity and color of the sugar solution at each stage of the process.

6. Use efficient filters with adequate capacity and surface area to ensure clear filtration and low moisture content of the cake.

7. Recycle the filtrate water from the cake washing to reduce water consumption and improve sugar recovery.

The carbonation process is a simple, effective and economical method for refining sugar from raw sugar melt. It can produce refined sugar with low color, turbidity and ash content, as well as low environmental impact. By following the steps and tips described above, you can optimize your carbonation process and achieve high quality and yield of refined sugar.