Downstream Processing
The generated biomass and other suspended compounds are removed using a disc-stack centrifuge (DS-101). During this step,
roughly 2% of the product is lost in the solid-waste stream. The bulk of the contaminant proteins are removed using a 62-L
protein-A affinity chromatography column (C-101) that operates in four cycles. The product yield for this step is 90%. The
protein solution is then concentrated 5x and diafiltered 2x (in P-21/DF-101). The total membrane filtration area for the diafilter
is 2.6 m2. The product yield is 97%. The concentrated protein solution is then chemically treated for 1.5 h with polysorbate 80 to
inactivate viruses (in P-22/V-111). An ion-exchange (IEX) chromatography step follows (P-24\ C-102) with a product yield of
90%. The IEX column has a volume of 28 L and the batch is processed in three cycles. Ammonium sulfate is then added to the
IEX eluate (in P-25\V-109) to increase the ionic strength for the hydrophobic interaction chromatography (HIC) step (P-26\C-103)
that follows. The recovery yield of the HIC step is 90%. The HIC column has a volume of 25 L and it processes a batch in three
cycles. A viral exclusion step (DE-105) follows. This is a dead-end type of filter with a pore size of 0.02 μm. Finally, the
HIC elution buffer is exchanged for the product bulk storage buffer (PBS) and concentrated 1.5-fold in DF-102. Approximately
100 L of the final protein solution is stored in a 200-L disposable storage bag (DCS-101). The amount of purified product
produced per batch is 2.5 kg. The overall yield of the downstream operations is approximately 63%.
Buffer and Media Preparation
Several buffers and media are required at different quantities. These have to be prepared and transferred to the use points
in time for processing. Two alternatives for the buffer and media preparation activities are investigated.
The first option uses single-use preparation and storage systems. Media are prepared in 200-L single-use bags and transported
to the use point. The various buffers are prepared in 500-L and 1,000-L bags (using a skid) and then transferred through sterile
filters into 200-L storage bags. The 200-L bags are subsequently moved to the point of use. Specifying single-use bags in
SuperPro Designer is a simple process. The first step involves specifying the type of bag, capacity, purchase, and disposal
costs and other properties in the consumables database. The bag can then be allocated to a unit procedure that represents
buffer preparation or storage. The tool calculates the number of bags required per batch and campaign during simulation. Other
consumables, such as chromatography resins and membrane cartridges, are specified and calculated in a similar manner. In this
case study, the three types of bags used have working volumes of 200 L, 500 L, and 1,000 L and their assumed purchase costs
are $300, $400, and $570 per item, respectively.
Table 1. Media and buffer prep and holding tanks for the stainless-steel option
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The second option uses traditional stainless-steel tanks for buffer and media preparation and holding. Media are prepared
and fed using a single tank (dedicated) per bioreactor. The buffer preparation area includes a number of preparation tanks
connected to a group of holding tanks using a set of transfer panels. The holding tanks are connected with the main process
using buffer delivery lines. The amount and number of buffers that need to be prepared determines the size and the number
of the tanks. Table 1 displays the required tanks and their sizes. Three tanks are required for media preparation (MP-101,
102, and 103); five tanks are required for buffer preparation (PV-101, 102, 103, 104, and 105) and eight tanks are required
for buffer holding (HV-101 to HV-108). Because the stainless-steel tanks are reused, CIP is essential after every buffer preparation
batch.
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