Adding a Second Production Train
The first example is to show the impact on a facility's WFI system when the demand from a second, identical production train is added. This example also demonstrates how decisions made with respect to the production schedule can be adjusted to reduce this impact. WFI is one of the more expensive utilities required for bioprocesses and, as such, is usually one of the first to run out and become a bottleneck when the demand on a facility increases.
Two basic properties define the capacity of a WFI system: generation rate and operating capacity in the storage tank. The operating capacity is not the working volume of the tank, but the range between the normal maximum and normal minimum levels during facility operation. In determining the generation rate and operating capacity, you can trade one off for the other and still meet the facility's WFI demand. A small operating capacity can be combined with a large generation system, or a larger operating capacity can be used with a smaller generation system.
There may be several other considerations that go into sizing the generation system and operating capacity, such as emergency reserve volumes or floor footprint limitations, but you can typically explore a set of generation rate and operating capacity combinations. The model considers the WFI system characteristics or the combinations of generation system size and operating capacity that can support the WFI demand.
Consider the base case, a single production train of a hypothetical process. In Figure 1a, the red line shows the WFI usage profile for a single production train over four, 8-hour shifts. The average WFI usage rate over this time period is shown in blue. Note that the maximum instantaneous draw is 20 kg/min and the average draw is approximately 6.5 kg/min.
Now, consider the combinations of generation rate and operating capacity that can be used. Assuming the generation rate is fixed, we used the model to calculate the WFI system characteristic curve depicted as line A on Figure 2, which shows the relationship between the generation rate and tank operating capacity in the context of meeting the demand profile in Figure 1a. This line represents the minimum requirement or boundary for the WFI system that can support the single production train.
In this single train scenario, if the generation system is producing WFI at 10 kg/min, the storage tank must have an operating capacity of at least 1,000 kg. From the graph you can also see that the smallest generation system that can support the single train is one that produces WFI at about 6.5 kg/min, equivalent to the average WFI draw. The accompanying operating capacity would have to be above 1,350 kg. As the generation rate approaches the maximum facility draw rate (~20 kg/min), the operating capacity needed approaches zero.
Now that a base case has been established, look at what happens to the WFI system when we change the operations. When a second identical production train is added to the facility, the average WFI usage rate doubles. Figure 1b shows the WFI usage profile when both trains are running simultaneously. In this case, the maximum instantaneous draw also doubles. In Figure 2, the calculated WFI system characteristic curve labeled B depicts the generation rate and operating capacity relationship for this demand profile.
The system requirements for this scenario are much greater than that for the base case. Note, that the range in production rate for WFI system characteristic curve B is bound by the average draw (13 kg/min) and the maximum draw (40 kg/min).
Though the impact of the second train on the WFI system seems obvious, with the appropriate production strategy we could mitigate or even eliminate the increase in systems requirements. The model can also be used to look at how changing the schedule of the two trains changes the WFI usage profile and therefore the WFI system characteristics. Figure 1c shows the WFI usage profile for both production trains, but with a 50-minute difference between start times. The average and peak draws are the same as in the simultaneous production profile; however, the shapes of the profiles are different.
The WFI system characteristic curve labeled C in Figure 2 shows how this shift in schedule affects the requirements on the WFI system. Above a generation of 20 kg/min, the operating capacity requirement is cut in half as compared to line B. Below a 20 kg/min generation rate the operating capacity requirement is still lower, but converges with line B. Figure 1d and its corresponding WFI system characteristic curve D tell a similar story but for an 80-minute shift in start times.
When the start times of the two trains are shifted by 100 minutes, the WFI demand profiles cease to overlap and the maximum draw is back down to 20 kg/min (Figure 1e). The WFI system characteristic curve labeled E in Figure 2 is now bound by this maximum draw, so as the generation rate approaches 20 kg/min, the operating capacity goes to zero.
Finally, in Figure 1f and WFI system characteristic curve F, with a 150-minute shift the requirements of the WFI system are almost equivalent to what they were in the single train scenario. However, there is a difference between the A and F curves, because though the maximum draws for both scenarios are the same, the averages of the two WFI usage profiles are different. Curve F is limited between 13.5 and 20 kg/min, but A has a range between 6.5 and 20 kg/min.
By selecting the production schedule in the F scenario, the WFI system used for the single production train is sufficient to handle the second train as long as the WFI system has a generation rate greater than 13.5 kg/min. If the other resources can handle this schedule, no modifications to the WFI system are necessary in order to reach the production goal in this example.