Designing for High Product Yield
An optimized TFF system will ensure that all product can be recovered from the system after processing. One key to achieving high product yield is minimization of system volume, which increases the likelihood of enabling a buffer recirculation step for enhanced product recovery.
In addition, a system with low minimum working volume simplifies product recovery since a larger proportion of the final product pool ends up in the recycle tank and is easily recovered by pumping, and a lower proportion of the pool is distributed through the process lines and membranes. Finally, careful design of the process lines to ensure that they all slope toward a low-point product recovery point will enhance the volumetric recovery from the system.
Testing was performed on the Mobius FlexReady TF-2 system, with a model protein solution at concentrations ranging from 23 to 275 g/L and a starting pool volume of 2.5 L. A four-step product recovery procedure was implemented wherein 1) product was pumped from the tank to the low-point recovery port, 2) air or buffer was introduced at a high point in the process tubing to displace the product remaining in the membranes, membrane holder, and feed line into the product recovery container, 3) a small bolus of air or buffer was introduced at the high point to push any remaining product from the retentate line into the recycle bag, and 4) the feed pump was used again to move any last remaining product from the tank and feed line into the recovery vessel.
As shown in Figure 2, the Mobius FlexReady TF-2 System enables very high protein recovery over the full range of protein concentrations, using the buffer-displacement method. Additionally, this high recovery is achieved with minimal dilution of product (<10%), indicating excellent plug flow of the buffer through the tubing and membranes to the recovery port. Similar results are observed using the air blowdown method, which yields 98–100% recovery of product from solutions at 25–275 g/L without requiring any dilution of the product pool (data not shown).
Maintaining Product Quality
One way to maintain high product quality is by avoiding protein-damaging air-liquid interfaces, particularly during mixing in the recycle tank. In the Mobius FlexReady systems, a recycle bag that includes a vortex breaker enables low working volumes with no air entrainment or vortexing, while the inclusion of a bottom-mounted mixer enhances mixing and a retentate diverter plate inhibits turbulence and short-circuiting of the retentate stream as it re-enters the bag (data not shown). Air-product interaction is further reduced through a sub-surface retentate return to the recycle bag and careful sizing of the feed pump inlet tubing to avoid cavitation.
As shown in Figure 3, the Mobius FlexReady TF-2 system can operate efficiently over a wide range of volumes (0.5–40 L) at a range of mixing speeds (100–700 RPM) without generating turbulence, with larger volumes allowing higher RPM before turbulence is seen.