Cycle Time Reduction and Debottlenecking

The equipment item that limits the batch size is known as “size bottleneck”. To identify this bottleneck, for each piece of equipment of a process we calculate the “Equipment Capacity (Size) Utilization” that represents the fraction of an equipment’s capacity utilized during an operation. This can be easily illustrated with a vessel operation (see figure below). If the red line represents the max allowable liquid level in the vessel during the operation and the blue area represents the actual level, obviously only a fraction of the vessel’s capacity is utilized during that operation, which is equal to the ratio of “Liquid Volume” over “Max Liquid Volume”. The vessel with the highest size utilization is the “Size Bottleneck” and it determines the max batch size. However, if that equipment has a short cycle time and can operate for multiple cycles within a batch, then, that must be taken into account in the estimation of the max batch size.

To identify throughput bottlenecks, in addition to size utilization we must also consider the time utilization of each piece of equipment. This is done by calculating the “Equipment Uptime”, which is defined as the total time a piece of equipment is utilized per batch divided by the Plant Cycle Time (PCT). PCT, also known as recipe cycle time, represents the time between consecutive batches (see figure).

To identify throughput equipment bottlenecks in the entire recipe (flowsheet), we simply calculate and plot the Combined Utilization for each equipment item. For a piece of equipment, its Combined Utilization is defined as the product of its Capacity (Size) Utilization times its Equipment Uptime. In the chart below, the blue bars represent capacity utilization, the orange represent time utilization, and the green represent combined utilization.

The equipment with the highest combined utilization will become the first throughput bottleneck as we try to increase plant throughput. How do we eliminate throughput bottlenecks? Elimination of equipment sharing and installation of extra capacity are the common ways. Changes in operating conditions (e.g., level of dilution for protein refolding that affects size utilization and batch size) may also eliminate certain throughput bottlenecks. Does this methodology work 100%? The answer is no. Real world limitations in time utilization may lead to situations where a procedure that does not have the max Combined Utilization is the true bottleneck under practical conditions. For instance, if you have a plant with a PCT of 24 hr and a certain vessel procedure has a cycle time of 16 h, it is very difficult under practical conditions to utilize the remaining 8 h. If this vessel operates at full capacity (size), then, it is the current true bottleneck even if some other procedure has a higher combined utilization.

To address the shortcomings of the “Combined Utilization” methodology for identifying throughput bottlenecks, we have developed an alternative methodology that is based on the max batch throughput potential of each procedure. To have a common basis of comparison, the batch throughput potential of each procedure is expressed in equivalent final product. SuperPro calculates three different batch throughput potentials (the Conservative, Realistic, and Theoretical) and generates a chart that displays those values for each procedure (see figure below). The procedure with the lowest conservative batch throughput potential is identified as the Conservative throughput bottleneck (P-10/C-103 in the case). Similarly, the procedures with the lowest realistic and theoretical batch throughput potentials are identified as the Realistic and Theoretical throughput bottlenecks, respectively. Our objective with this methodology is to identify the TRUE bottleneck based on the value of the realistic bottleneck. For more info on how the throughput potentials are calculated, please check our literature on the subject.

Resources that cannot be stored (e.g., labor, steam, power) can become bottlenecks if the available capacity cannot meet the instantaneous demand at some point in time. For instance, the figure below displays the labor requirement (blue lines) for four consecutive batches. The red line represents the limit (7 operators available). Notice that for short periods of time there is a need for eight operators. If that need cannot be met, then certain operations will need to be delayed to accommodate that constraint. Oftentimes, such resource constraints become time bottlenecks that determine the maximum number of batches that can be processed over a period of time.

A storable resource (e.g., demand for a material) can become a bottleneck if the cumulative demand (shown with the green line on the figure below) over a period of time exceeds the cumulative capacity during the same period.

A storable resource may also become a bottleneck if there limited storage capacity and a high demand for a short period of time. For instance, the figure below shows the level of WFI (water for injection) in the storage tank. The blue line (the values correspond to the y-axis on the right-hand-side) represents the level of WFI in its storage tank. The red lines represents the limits of the tank (full and empty). If for a short period of time the blue line hits the bottom (the tank runs dry), the operation that requires WFI at that moment must be delayed and that delay may create a new time bottleneck.

It is clear from the previous analysis that, if our goal is to increase plant throughput, we should make changes that increase the batch size and/or reduce the plant cycle time. In general, we recommend the following strategy:

1. Increase batch size until at least one cyclical step operates at 100% capacity utilization. If its uptime is low, try increasing its number of cycles per batch. This may create opportunities for additional increases in batch size. A side benefit of increased batch size is the reduced cost for quality control (QC) and quality assurance (QA), which depends on the number and not the size of batches.
2.  If a process operates at its maximum batch size, focus on plant cycle time reduction through elimination of time bottlenecks. Long process steps and equipment sharing are the causes of time bottlenecks. Bottlenecks created by equipment sharing are eliminated by installing extra equipment that reduces sharing. The size of the new equipment should be chosen in a way that creates opportunities for batch size increase (sizing based on the most demanding step). Rearrangement in the order of equipment use (for shared equipment) may also create opportunities for cycle time reduction and sometimes for batch size increase. If the time bottleneck is caused by a step that has a very long cycle time, the new equipment should be operated in a staggered mode based on the cycle time of the next time bottleneck. Sometimes a time bottleneck can be eliminated by moving secondary operations from bottleneck to non-bottleneck equipment. For instance, instead of heating material in a vessel, the heating can be done using an external heat exchanger during the charge/transfer of the material into the vessel. Please note that whenever a recommendation is made to buy new equipment, the final decision should be based on overall project economic evaluation criteria, not simply on throughput considerations.