gSOLIDS
Application example: Sizing of recycles
The sizing of recycles involves determining the optimal trade-off between capital and operational costs
The sizing of recycles typically involves determining the optimal trade-off between capital and operational costs.
gSOLIDS's ability to model and solve the process flowsheet to a high level of accuracy, coupled with its optimisation capabilities, make it possible to determine economically-optimal design and operating points.
Need for sizing recycles
There are significant costs associated with recycling material, both in terms of :
- capital expenditure (CAPEX), which is a function of the size and capacity of units
- operating expenditure (OPEX), which is a function of transportation and energy costs.
The relationship between process capacity and recycle flows is not always linear. In some instances, increasing process capacity will proportionately decrease recycle flows while in other cases this may not be true.
Example: optimising recycle size
The example flowsheet below shows a typical agglomeration process.
- Feed flowrate
- Mixer volume
- Agglomerator capacity
- Mill capacity
- Efficiency of screens
gSOLIDS palette – icons view (click to expand)
Fine particles are agglomerated, then separated by screening into product and recycle streams. Product particles need to be within certain size range; all other particles are recycled, with the oversize crushed in a mill.
In order to optimise the recycle size it is necessary to determine the design that minimises the sum of CAPEX and OPEX for the process. Key factors that could potentially influence recycle flows are shown in the diagram.
Approach and results
The flowsheet is constructed in gSOLIDS using standard unit operations dragged off the palette (right). Key process variables and unit parameters are then specified via specification dialogues.
Having constructed the flowsheet, it is possible to perform steady-state or dynamic simulation or – by adding an objective function and constraints – optimisation.
In this case the relatively simple flowsheet meant that the most effective approach was simply to simulate with varying agglomerator volume.
Results
The figure shows the recycle ratio of fine (blue) and coarse (red) particles and for the sum of the two (green) against agglomerator volume. The chosen volume is situated at the minimum total recycle.
It is possible to perform a more rigorous analysis by adding cost information and performing an optimisation against an economic objective function.
Sensitivities
In addition to knowing the optimal design, it is essential to understand how the optimal solution varies with uncertainties in the model parameters.
Recycle rate sensitivities to feed rate, agglomerator capacity and mill capacity
The figures below show the sensitivity of recycle flow rates with respect to design parameters such as feed flow rate and agglomerator and mill size.
A similar analysis can and should also be done for the sensitivity with respect to model parameters, such as rate constants for agglomeration, mass transfer, reaction and crystallisation.
Effect of feed rate
As feed flow increases, less agglomeration takes place, meaning that smaller particles are produced. This leads to an increase in fines recycle rates, and a decrease in coarse recycle rate. The result is an overall decrease in recycle ratio.
Effect of agglomerator capacity
The effect of agglomerator capacity on recycle ratio is very non-linear.
Small agglomerators may not provide enough size enlargement, meaning that a large amount of fines have to be recycled. On the other hand, large agglomerators can produce too many large particles, meaning that a large amount of coarse particles have to be recycled.
Effect of mill capacity
As mill capacity increases, the greater capacity allows coarse particles to be broken up into increasingly smaller particles. An overall reduction in recycle ratio is achieved.
Combined sensitivities: feed rate and mill capacity (top) and feed rate and agglomerator capacity (bottom)
Combined effects: feed rate and mill capacity
From the plot on the right, mill capacity can be seen to be the dominant factor.
As the mill capacity increases, the recycle ratio increases. It is necessary to balance the cost of increasing mill capacity and increasing recycle flows.
Combined effects: feed rate and agglomerator capacity
The effect of increasing agglomerator capacity is similar for different flowrates. This indicates that the main consideration is sizing of the agglomerator.
Privacy policy | Accessibility | Company registration
Copyright © 1997-2012 Process System Enterprise Limited
All rights reserved
