Scale-up and Design Finalization for Clarification

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Scale-up

The goals of scale-up are to create reproducible product yield, contaminant removal, quality, and purity. At this stage in the process, you will need to choose a process time and evaluate safety factors. A process is typically scaled-up by maintaining the same volume to filter area ratio between the various scales being evaluated. However, changes in process time or flow rate can impact this choice.

During the optimization process, your process performance must be tested on representative feedstock. If you have selected TFF for your primary clarification step, one of the most important experiments for characterizing performance for TFF optimization is to determine the critical filtrate flux at several cross flow rates (or pressure drops) and several protein concentrations.

During the TFF scale up process, the control of permeate flow rate during the entire processing time is mandatory to meet basic microfiltration success criteria, achieve contaminant removal targets, and limit fouling effects on the membrane. Permeate control can be achieved by using a permeate pump or a permeate valve.

Best Practices for Characterizing Performance - TTF Determine critical filtrate flux at approximately three different cross flow rates. Ideally, the starting and ending product concentrations should be tested. Always begin with the least fouling conditions and move towards more fouling conditions. For each test point, calculate retention and contaminant removal. Then, generate graphs showing critical flux versus cross flow rates and protein concentrations to determine the optimum critical flux and the optimum feed flow rate.

During the depth filter scale up process, it is best to pilot at two scales. Testing completed at each of the process scales should be executed in a constant-flux mode, recording the pressure increase with load volume. To determine and confirm correct filter sizing and process conditions, run the following test volumes:

1. Process 5-25 L of solution through the depth filter. Testing multiple devices with multiple process samples will ensure that sizing is as accurate as possible.
2. To help ensure reliable scaling and operation, test the process-scale device on volumes greater than 50 L.

As with the testing and implementation of the TFF solution, simulate final process conditions as closely as possible. Since biological process streams are very complex mixtures, their properties could change drastically if any process parameters change.

Process parameters
If the temperature of the fluid is not kept constant, the age of the fermentation/cell culture is changed, or if it is refrigerated overnight, the fluid properties may change. This means precipitants may form, proteins may denature and the sample will no longer be representative of the final process fluid.

Design Finalization

Design finalization is your next critical challenge. You must design a strategy that enables you to increase your flow volumes within the constraints of your physical plant –without violating the safety margins you have already established for your project.

It is key to design a filter configuration that supports a large-scale implementation. When multiple filters are required to process your batch volume, look for a filtration solution to meet your process time requirements and the physical constraints of your plant. You can increase your probability of success through procedural reviews, and assessing operator training and equipment qualifications.

Safety
Once you complete confirmation and scale-up studies for depth filters, you need to address the safety factor to apply for the final system sizing. The selection of the safety factor to be applied is dependent upon multiple factors, but it’s important to note that variability is typically normal. This means that, historically, safety factors of 1.5-2.3 have been used for depth filtration system scale-up. Depending on your unit operation, and information on the process variability that can be expected, this safety factor could be smaller or larger.

Additional Considerations
Other issues to take into account regarding the clarification process include:

Plant-related considerations

• Sanitization
• Hardware requirements
• Disposable flow path opportunities

Operating procedure-related considerations

• SOP feasibility
• Equipment and operator qualification
• Automated and/or manual integrity testing

Design constraints

• Ability for increased batch size
• Robust filtrate control mode
• Safety factors
• Tolerance for process excursions

Merck offers a full line of clarification products and services to help speed your biotech and plasma-derived products to market faster and more cost effectively.