As Table 5 indicates, Scenario 1 results in positive NPVs ranging from $320,000 at the lowest capacity to over $1.6 million
for the upside scenario, suggesting this alternative be incorporated into our design. However, an examination of operating
costs indicates the costs are higher for the single-use scenario for all 10 years. This suggests the savings derive from combining
the two seed fermentations into a single fermenter, and not from using single-use systems. Based on this information we proposed
Scenario 2, which combines the 50-L and 250-L seed fermenters into a single 250-L stainless-steel fermenter. This scenario
produces lower less capital savings than scenario one but lifecycle savings are greater, making it the clear winner between
the two scenarios.
Scenario 3 examines replacing the stainless-steel production fermenter with a single-use fermenter. Lifecycle costs for this
option are very poor and there is a large technical risk because single-use fermenters of the required size are not yet available
on the market. The negative economics result from the high cost of the single-use fermenter bags, which are close to $10,000
Scenario 4 examines replacing three large stainless-steel vessels, one of them agitated, and depth filters in the recovery
process with single-use systems. At this volume, the cost of single-use mixing bags is quite high, driving the economics to
favor stainless-steel. In Scenario 8, we examine the same recovery process but replace just the hold vessels with single-use
bags, keeping the stainless-steel agitated vessel. The economics for this option are favorable.
Scenario 5 examines replacing fixed stainless-steel buffer and media-preparation tanks with single-use mixing systems. Here
again, the optimum design for single-use systems is very different than for stainless steel. The stainless-steel design includes
separate media and buffer preparation suites with separate gowning and material airlocks and separate air handlers. In the
single-use design, we combine media and buffer preparation into a single suite because the risk of cross contamination is
eliminated. We also reduce the number of preparation systems from seven to four, reflecting the higher productivity of the
single-use mixing equipment. This resulted in significant savings in capital costs, operating personnel, and HVAC costs. However,
these savings were dwarfed by the high cost of single-use mixing bags, resulting in negative NPVs for all capacity scenarios.
Scenario 6 is the buffer hold case study described earlier, and Scenario 7 uses single-use bags to hold intermediate product
after various purification unit operations. Both of these scenarios use relatively inexpensive single-use storage bags and
result in positive lifecycle economics.
Table 5 shows the importance of evaluating lifecycle costs in determining the optimum mix of stainless-steel and single-use
systems. In our case study, if we look at capital savings only, we might decide to incorporate all of the single-use scenarios
listed in Table 5. The result is a rough order of magnitude capital savings of $26 million. However, the NPV for these scenarios
is a loss of $7 to $13 million. A more optimum solution is to combine scenarios 2, 6, 7, and 8. This results in a respectable
capital savings of $18 million and positive lifecycle savings of $11 to just over $16 million.
Impact on Project Timeline
A final and very important financial consideration is the impact of single-use systems on project timeline. In many instances,
the financial impact of getting to market faster dwarfs the lifecycle savings we have been discussing so far. The problem
is that it is often very difficult to determine the impact of replacing a given piece of stainless-steel equipment with single-use
systems on the overall project timeline. The reduced delivery time, installation, commissioning, and qualification of single-use
systems shortens project timeline. However, the overall project timeline might still depend on long lead times for stainless-steel
equipment for which there is no suitable single-use replacement.
For projects where time-to-market is an important consideration, we recommend performing the analysis described in this article,
then evaluating the alternatives with negative NPVs based on their potential to affect the overall project schedule. For example,
alternative five shows a negative NPV for single-use media and buffer preparation. Typically, stainless-steel media and buffer
preparation equipment is not on the critical project path and replacing it with single-use mixing bags is not likely to accelerate
the overall project timeline. Therefore, alternative five might still be rejected. However, stainless-steel fermenters are
often on the critical project path, so alternatives one and three might be reconsidered if speed to market were an important