December 1, 2007 (Vol. 27, No. 21)
Efficiency, Economy, and Safety Are Premier Considerations
From roller bottles to large-scale plants, cell culture has developed tremendously over the last few decades. Today it is not uncommon for proteins and vaccines to be extracted from cell cultures in the scale of several thousand liters. A typical large-scale plant for protein production from cell culture with six 15,000 L production reactors requires the design, construction, and finally, the operation of 18 bioreactors and over 60 vessels.
The planning and engineering of such a biopharmaceutical production plant is a considerable task. A variety of factors must be weighed and a multitude of decisions made. Throughout all of this, compliance with regulations is paramount.
Processes must be known in detail during the planning phase to ensure trouble-free operation of the plant. Energy calculations and economic considerations as well as the duration of process steps and the geometry of the periphery play a central role in the design of the plant.
Typically the process consists of the cultivation of cells in bioreactors and the manufacture of product in production reactors. This process is composed of many steps, which are classified as either upstream or downstream.
The upstream process includes the preparation and supply of culture media, the cultivation of cells, and their extraction. Vessels and appliances for buffer preparation, supply, and transfer as well as filter and chromatography equipment are required at this stage.
In the downstream process, the product is purified through chromatographic methods. Filters, chromatography equipment, and supplies for buffer preparation and transfer are required at this stage.
To avoid contamination, cell culturing must be operated as a sterile process. All vessels and transfer lines involved have to be sterilized. After completion of one production batch, the entire plant is cleaned in an automated clean in place (CIP) process.
Each process step, whether physical (heating, cooling, liquid transfer, sterilization), chemical (cleaning), or biological (cultivation), demands optimum conditions to yield the highest possible efficiency. Sterilization, production and cleaning, dimensioning of the devices, and a sensible order of all process steps must be planned with precision. For all of these considerations, time and money are always decisive factors.
In order to determine the most favorable process conditions, energy balances that include the consumption of CIP-solutions, steam, cooling and heating fluids, and gases must be developed. Initially each process step is calculated separately for each vessel; subsequently, interactions of different vessels must be considered.
Heating and Cooling
Temperature control is necessary for the regulation of the ideal cultivation conditions, sterilization, and maintaining the temperature of buffers, media, and cleaning fluids.
Vessels of over 1 m3 are indirectly heated and cooled by fluids pumped through a half coil welded to the vessel surface. In order to design the optimum temperature control, diameter and length of the half coil, type and temperature of the utilized liquids and flow rate and design of the heating circuit are evaluated. Variables to be considered during the design of the heating circuit are the type of half coil, the number of heat exchangers, and the temperature of heating and cooling liquids.
The design of heat exchangers and the temperature difference between heatingcooling fluids and media is calculated via the heat transfer coefficient. Since the process, by definition, must yield the highest possible efficiency, even the duration of heating and cooling periods must be evaluated. Excessive durations could delay the process or damage the product.
The optimal balance between precision and speed as well as complexity and economy must be found to create an efficient, high-tech process. In a large-scale production plant, it is not sufficient to build the heating circuit to adequately maintain the temperature in the vessels. Customer specifications must also be satisfied.
In plants with media preparation, the bioreactors are sterilized empty with clean steam. Sterilization is conducted in three steps. Initially the vessel is heated and degassed with clean steam. Degassing is necessary because only with saturated steam can the sterilization temperature be reached. In the second step, the actual sterilization, clean steam is introduced and the outlet valve is pulsed with clean steam to ensure regulated pressure drop and condensate draining. Clean air then breaks the vacuum in the concluding cooling step.
Steam generator and pipe diameters are further design considerations. Heat loss by convection and radiation must be calculated since uninsulated vessel parts on the outside can reach sterilization temperature, while the insulated parts don’t. Heat loss should be incorporated into the calculations for the steam generator as a correction factor. Air conditioning must also be considered early on, otherwise overheating of the production room during sterilization might occur. More often than not, pipe diameter optimization calculations result in a requirement for nonstandard piping. Engineers and their customers must then decide if the next bigger or the next smaller standard piping should be selected. Piping with smaller diameters has reduced condensate formation, but lower flow rates. However, with higher flow rates come increased corrosion hazards and noise. If valves with reasonable costs and precision are available for these pipe sizes, they must also be considered.
Clean in Place
Cleaning is an integral part of the process and must be conducted in compliance with applicable regulations. It should be as automated as possible. Cleaning solutions are prepared and heated in the CIP-kitchen and then transported to the respective vessels.
Correct preparation of cleaning solutions, assessment of the completeness of the cleaning process, and recycling of fluids are achieved via monitoring of total organic carbon, conductivity, and pH. The cleaning process is terminated with a water-for-injection rinse and a blow-out with clean air.
Since cleaning of each vessel and all piping in contact with product is necessary, the CIP-kitchen must be correctly dimensioned, and all transfer lines between the CIP-kitchen and vessels have to be sensibly designed to ensure an efficient cleaning process. Hydromechanics, temperature, chemical composition, and time are the main factors affecting this process. Hydromechanic effects, for example, influence the design of the CIP-kitchen, the supply units, the piping, and the spraying devices.
Pipe size, in turn, has an impact on the flow rate, which has to be high enough for effective cleaning. With high flow rates, the pressure drop in the piping must be counteracted.
The maximum consumption of all energy including electrical, water, gases, and steam must be determined for all of the equipment that will be used. At every point of the process, several reactors and vessels operate simultaneously. To design CIP and SIP processes, it must be known how many reactors are sterilized and cleaned at the same time. Oxygen, nitrogen, and carbon dioxide demand is determined by the aeration rate of the reactors. Air consumption is not only dependent on aeration, but also on the breaking of vacuum after sterilization. These are just a few examples of the interactions to be considered.
A time schedule for the entire process must be established in the planning phase. The information on the schedule will be vital for the dimensioning of the gas supply, the air conditioning, the steam generator, and the pumps. The pressure drop in piping and heat exchangers during operation caused by armatures, bends, reductions, or roughness is one of the reasons for the calculations. Pumps must be designed to withstand the demands of the respective circuit.
Dependent on process flow, the geometry of transfer lines and their connection to the vessels must be thought out in advance. For example, the link between the CIP-kitchen and the bioreactors can be realized via a transfer panel or via a transfer group. The decision for one or the other influences the degree of automation, the number of reactors to be cleaned simultaneously, pipe geometries, and significantly, operational safety.
Planning and design of a large-scale biopharmaceutical plant is a path with many crossroads where the right decisions concerning efficiency, economy, and safety have to be made. A large-scale biopharmaceutical plant can only be planned and designed to perfection with experience and a holistic approach.
Karin Koller is scientific assistant
Phone: 41 55 256 81 11.