-
Problem 4. Diffuser recycle rate
Problem 4. Diffuser recycle rate
Industry:
Sugar Cane Processing
Industrial Representative:
Richard Loubser, Sugar Milling Research Institute NPC, University of KwaZulu-Natal, Durban.
Moderator:
Student Moderator:
Problem Statement:
Sugar is extracted from shredded sugarcane using a counter current leaching process in a process unit called a diffuser. Shredded sugarcane with high sugar concentration enters the diffuser at one end and is drawn along the length of the diffuser, usually dragged by an arrangement of moving chains located under the bed of cane. Hot water, called imbibition, is poured onto the cane bed at the other end of the diffuser and percolates through the cane bed, leaching sucrose and other dissolved solids from the shredded cane.
It leaves the bed through a screen at the bottom and is collected in a tray. The juice is pumped forward to sprays above a point in the bed that is closer to the cane entry end, where it percolates through the bed and is collected in a tray adjacent to the first tray. The juice typically passes through 12 to 14 such stages, depending on the design of the diffuser. The concentration of sugar in the juice increases with each pass through the bed.
The objective of the diffuser design is to achieve flow that is as close to the ideal plug flow as possible. There are, however, limitations on how closely this can be achieved due to operational factors such as unsteady conditions and design factors that require a degree of recycle. Healthy cane will typically have 15% dissolved solids, 15% fibre (insoluble solids) and 70% water. Factories aim to add less than 300 % imbibition on fibre (i.e. 3 tonnes added water per tonne fibre in cane) to extract the sugar, such that the total water added is less than half the total mass of cane added. Shredded cane can absorb water so that its total water content is 85% of the total mass. A 300% imbibition rate, however, will only raise the water on cane to 79%. Therefore, without the total amount of water in the bed being increased through recycle, all the imbibition will be absorbed by the cane and little or no juice would be extracted without an additional mechanical squeezing step.
Mass transfer of sugar between the cane and the juice can only occur when there is good contact between the cane and the juice. Mass transfer is improved by increasing the liquid hold-up in the cane bed to increase the amount of cane surface in contact with liquid at any time. This is achieved by increasing the total amount of juice added to the top of the bed at each stage. Recycling of juice at a stage level is employed to increase the total juice flow per stage without increasing the total imbibition added to the diffuser. The more recycle used, the greater the amount of juice in each stage. However, recycle results in lateral dispersion of dissolved solids, thereby reducing the efficiency of counter-current extraction. Too much recycle may result in flooding of the cane bed, where the rate of juice addition exceeds the rate at which juice can percolate through the bed, leading to pooling and lateral flow of juice on the top surface of the bed, with further negative consequences to the dissolved solids concentration profile. The operational object for the diffuser is to optimise the dissolved solids extraction through maintaining a sufficiently high percolation rate without compromising the dissolved solids concentration profile in the juice along the length of the diffuser. The flow rate of the juice in the bed needs to be maximised while minimising the degradation of the plug flow requirement.
The amount of recycle depends on several factors:
Controllable
- Height of the bed
- Horizontal bed speed
- Position of the sprays
Non-controllable
- Percolation rate (permeability)
- Length of the stage
- Amount of imbibition
A value for recycle of 30% is typically recommended but no scientific justification can be found for this value.
Questions to be explored:
- What is optimum recycle fraction that should be used as a target for setting and controlling a diffuser?
- Can a relationship between the controllable variables and noncontrollable variables be derived that will enable the factory to achieve the optimum recycle?
https://www.sugarprocesstech.com/mills-diffusers-technology-sugar-industry/
Presentation
MISG 2021 Study Group Problem 4 Presentation
Report-back Presentation
