PAC Process for Melt De-colorization in Sugar Refinery
Sugar refining is a meticulous process aimed at removing the colouring matter and purifying raw sugar to achieve high-quality refined sugar products suitable for consumption.
In the melt decolorization process, several methods exist for 1st decolorization process and 2nd decolorization process.
Briefly understand melt decolonization methods is the follows
Among the various techniques employed, powder activated carbon (PAC) is one of the technology in the secondary stage melt decolorization process. This article explores in detail the utilization of PAC process in sugar refineries, encompassing its process overview, equipment and technology, benefits, challenges, and environmental considerations.
Process Overview
The PAC process in sugar refining involves several systematic steps designed to remove colorants from the melt liquor efficiently. Here’s an elaboration on each key stage of the process
Mixing with Powdered Activated Carbon: Melt liquor, typically at 60 to 65º Brix and temperatures of 80°C to 85°C, is first mixed with powdered activated carbon slurry. This initial stage is critical as it allows the carbon particles to adsorb colorants and organic impurities present in the liquor. The mixture undergoes agitation for 20 to 30 minutes to ensure thorough contact between the activated carbon and the sugar liquor.
Transfer to Filter Feed Tank: The treated liquor is transferred to a filter feed tank. Here, additional components such as filter aids and sometimes additional PAC are added to improve the filterability of the liquor and optimize the filtration process.
Filtration Process: Initially, the filter is pre-coated with filter aid and after pre coating the liquor will be filtered in the filter. The treated liquor passes through the heat exchanger where the liquor is heated up to 85 Deg. C. and then proceeds to the filtration stage using a membrane filter press. This filtration system integrates several functions, including coarse and fine filtration, de-sweetening, squeezing, drying, and cake discharge, all within a single unit.
De-sweetening and Drying process involves multiple wash cycles and air squeezing to achieve a dried cake with minimal sugar loss nearly less than 2.0% and moisture content below 40%.
The entire PAC process is typically fully automated, leveraging advanced control systems to monitor and adjust parameters such as PAC dosing, filtration pressure, and de-sweetening cycles. Automation enhances operational efficiency, consistency, and product quality while minimizing the intervention.
pH Adjustment: Powdered activated carbon tends to be acidic, necessitating pH adjustment with lime before or after treatment to prevent acid inversion in the treated liquor. Maintaining proper pH levels is crucial for ensuring the quality and stability of the final product.
Filtrate Quality: The filtrates from carbon-treated liquor filters must be free from any carbon particles to avoid contamination of massecuite and final sugar products. This requires careful supervision of the filtration process and rigorous check filtration to ensure the purity of the filtered liquor.
The PAC process requires the following equipment:
Pre-coat preparation tank and Pre-coat dosing tank with agitators: These tanks are designed with an agitator for preparing pre-coat slurries. These slurries are essential for pre-coating filter cloths to improve filtration efficiency.
Carbon preparation tank and dosing tank with agitator: These tanks are designed to prepare and dose powdered activated carbon slurry into the sugar liquor in the carbon mixing tank.
Carbon mixing tank & Carbon liquor tank: This mixing tank allows the carbon particles to adsorb colorants and organic impurities in the melt liquor. The mixture is retention with agitation for 20 to 30 minutes and then transferred to the filter feed tank
Filter feed tank: In this tank add filter aid and powder activated carbon as per requirement and the preset dosage to improve the filterability.
Melt heat exchanger: Melt liquor heated up to 85 oC by using a heat exchanger before pumping to the membrane filter from the filtered feed tank. It is essential for reducing the viscosity and increasing the penetration ability of melt liquor in membrane filters.
Membrane filter press: An automatic membrane filter press serves as the core filtration unit in the PAC process. It features membrane filter plates that apply pressure to the filter cake, facilitating efficient separation of solids and liquids. The filter press integrates multiple functions such as filtration, de-sweetening, squeezing, drying, and automated cake discharge
Filter Operation Sequence
Pre-coating: A slurry of filter aid, typically at a concentration of 0.5%, is prepared and pumped into the filter for about 15-20 minutes to pre-coat the membrane cloth.
Production Recycle: Once pre-coating is completed, the production cycle starts, and the liquor is recycled for 5-10 minutes until it is free from any carbon particles.
Production: The production phase begins with a higher flow rate and lower pressure, gradually reducing as the filtration pressure increases. The cycle continues until the pressure reaches the pre-set maximum, at which point the production cycle stops.
Blowback 1: This stage involves pushing back any residual liquor between the plates.
Cake Wash: The de-sweetening of the cake is done in three stages—center, left side, and right side washes—ensuring thorough removal of sucrose. By the end of this process, the Brix level of the sweet water will be below 2.0%.
Blowback 2: This step pushes back any remaining sweet water between the plates to enhance cake squeezing.
Air Squeezing: Compressed air, applied at 8-10 bar, is used to squeeze out the sweet water from the cake, resulting in a drier cake compared to conventional filter presses.
Cake Drying: Additional drying of the cake is achieved by applying compressed air at around 6-8 bar.
Cake Discharge: After drying, the bottom cover door opens, and the plates are shifted and shaken one by one to discharge the cake using a programmable mechanical system.
Cloth Washing: The filter cloth is washed with high-pressure water (40-50 bar) using an automatic washing mechanism (AWM), which can be programmed for multiple washings as needed.
Check filter: The check filter serves as a final safety measure to ensure that the filtered liquor is free from residual carbon particles before moving on to further processing stages.
Fine liquor tank: The fine liquor tank is a storage vessel for the clarified and decolourized liquor after it has passed through the filtration process. This tank holds the treated liquor before it undergoes further processing or crystallization.
Membrane Filter Press: An automatic membrane filter press serves as the core filtration unit in the PAC process. It features membrane filter plates that apply pressure to the filter cake, facilitating efficient separation of solids and liquids. The filter press integrates multiple functions such as filtration, de-sweetening, squeezing, drying, and automated cake discharge.
Cloth Washing System: Includes pumps and tanks dedicated to washing filter cloths after each filtration cycle. Proper cloth washing is crucial for maintaining filtration efficiency and extending the lifespan of filter cloths.
Operational Parameters:
Powder Activated Carbon (PAC) consumption: 0.1% on sugar solids in liquor for average input melter having at 500 IU
Note: If the input melt colour increases then we have to add the PAC up to 0.15% to 0.2% on sugar solids. However, the increased dosing rate will affect the production rate of the system.
Filter Aid consumption: 0.4-0.5 Kg/ Ton @ raw sugar colour at 500 IU
Pre-Coat consumption: 0.3-0.4 Kg/ Ton @ raw sugar colour at 500 IU (Pre-coat requirement is around 0.6 kg/m2 per filter)
Colour reduction across the system will be around 50 % to 60%
Cycle time: 15-16 Hrs ( It will depend on input melt colour and dosage of PAC)
Washing cycle time: 4 to 6 Hrs
Sugar losses in cake are less than 2.0 % and moisture is less than 40 %.
Benefits of PAC Process
The utilization of PAC in sugar refining offers several significant advantages:
High Decolorization Efficiency: PAC effectively removes colorants and impurities from sugar liquor, resulting in a substantial reduction in color intensity and improved product clarity.
Environmental Sustainability: The PAC process is environmentally friendly, generating zero liquid effluent and requiring minimal water usage compared to other decolorization methods.
Operational Efficiency: Automation and integration of filtration functions in a membrane filter press streamline operations, reducing labor costs and enhancing overall process efficiency.
Superior Product Quality: PAC-treated sugars exhibit superior clarity and glazing of sugar, meeting stringent quality standards and customer expectations in the sugar industry.
Constraints of PAC Process
Despite its advantages, the PAC process in sugar refining does present certain constraints and challenges:
High Operational Costs: Beyond initial investments, the PAC process requires ongoing operational expenses for maintenance, replacement of filter media, chemicals, energy consumption, and labor costs associated with operating and monitoring the filtration equipment. As per the general estimation, the operating cost of the PAC process is 3 to 4 times higher than the IER decolorization process.
Maintenance Requirements: Regular maintenance of filtration equipment, including membrane filter presses and pumps, is essential to ensure continuous operation and optimal performance. This maintenance can involve costs and downtime for refineries.
Operational Sensitivity: Filtration parameters such as carbon dosing, filtration pressure, and de-sweetening cycles must be carefully controlled to maintain process efficiency and product quality. Variations in these parameters can impact filtration performance and require skilled operators for monitoring and adjustment.
Summary: This article explores the secondary decolourization process in sugar refining using powdered activated carbon (PAC). It highlights the flexibility, zero effluent generation, and automated operation of this method. Benefits include lower sweet water generation, and reduced sugar loss in the filter cake, although it does come with high operational costs. The article also covers necessary equipment and technology, emphasizing the process’s environmental friendliness.
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