Increasing Lyophilization Productivity, Flexibility, and Reliability Using Liquid Nitrogen Refrigeration–Part 1

ABSTRACT

Because lyophilization dries a product from the frozen state under temperature controlled conditions, the refrigeration system's process capabilities and performance are critical to a successful commercial lyophilization operation. Part 1 of this article outlines recent trends in pharmaceutical manufacturing and their impact on the evolution of refrigeration technology in lyophilization. It also explains the advantages and disadvantages of choosing a refrigeration system that uses liquid nitrogen instead of mechanical compressors, and its affect on the overall operation. Part 2 of this article, to be published in the December 2007 issue of BioPharm International, will detail design and performance considerations for cryogenic nitrogen refrigeration in a freeze-dryer. The relative cost affect of choosing a cryogenic versus a mechanical refrigeration system will also be discussed as related to the lyophilization process.

Lyophilization (freeze-drying) is increasingly used to gently stabilize pharmaceutical and biopharmaceutical products, and intermediates.^1-4 Its recent growth is being driven by the escalating global demand for aseptic packaging and preservation of parenteral drugs, as well as by the rise in the production of biologics, including protein-based therapeutics and vaccines.^2-4 According to industry experts, the corresponding increase in lyophilization capacity has been fueling double-digit growth in global cGMP freeze-drying equipment sales, which have reached approximately $250 million per year. The global installed base is estimated to be in excess of 3,000 cGMP production units.

During lyophilization, most of the solvent (e.g., water or alcohol) is removed from a product after it is frozen and placed under vacuum. The process actually consists of three separate, but interdependent steps: 1) freezing, 2) primary drying (ice sublimation), and 3) secondary drying (moisture desorption). During primary drying, more than 90% of the solvent changes directly from solid to vapor phase through sublimation. The residual solvent remains adsorbed on the product as moisture. Some of this remaining solvent is desorbed during secondary drying to attain a moisture level too low to permit biological growth or chemical reactions, while still preserving the activity and integrity of the freeze-dried product.^3-5

Key advantages driving the growth of lyophilization as a preferred fill-and-finish step include the enhanced stability of freeze-dried powder, the ability to remove solvent with minimal heating and concentration effects, the relative ease of aseptically processing a liquid in a freeze-dryer, and rapid and easy dissolution of the product upon reconstitution. All these advantages facilitate minimizing the time to market of a novel therapeutic agent, establishing an early marketing lead, and collecting increased revenues. Disadvantages of this method include increased handling and processing time, the need for a sterile diluent upon reconstitution, and the cost and complexity of related equipment, including its operation and maintenance.^1,3,5,6

REFRIGERATION TRENDS IN FREEZE-DRYERS

During lyophilization, the product is first frozen, then dried from this frozen state under precise temperature- and pressure-controlled conditions. The refrigeration system's process capabilities, flexibility, reliability, and performance are all critical to a successful commercial lyophilization operation. Historically, most freeze-dryers have used mechanical refrigeration. Even though approximately 80% of lyophilizer service problems arise in the mechanical refrigeration system,⁷ these compressor-based units have accounted for 90–95% of freeze-dryer installations.

Key Components of a Lyophilizer

However, since the early 1990s, cryogenically refrigerated freeze-dryers have been claiming an increasing market share.^7–11 These reliable, flexible, and well-proven systems use liquid nitrogen (LN₂) or cold gaseous nitrogen (GN₂) to cool the components of the freeze-dryer.

In recent years, both the efficiency and flexibility of cryogenic refrigeration systems for freeze-drying have further increased, and their cost of ownership has decreased.^8–11 With these improvements, LN₂/GN₂ systems are ready for mainstream processes. In this article and the subsequent cryogenic lyophilization article, we outline key considerations to help users make informed decisions about what type of refrigeration is best for a particular lyophilization situation.

REFRIGERATION REQUIREMENTS IN LYOPHILIZATION

There are two key considerations in providing refrigeration to a process: 1) the refrigeration temperature required, and 2) the maximum cooling power required. First, the refrigeration temperature required by the process determines the type of refrigeration system needed. Commercially available refrigeration technologies have different fundamental thermodynamic limitations in terms of operating temperature, cooling rate capability, efficiency, and cooling power. Second, the peak and turn-down capacities of the chosen type of system are determined by the refrigeration load profile over time.

Figure 1. Typical shelf cool-down capability of large commercial freeze-dryers with >20 m2 shelf-space. Note the inferior performance of mechanical versus cryogenic liquid nitrogen refrigeration systems in terms of cool-down rate, cooling rate sustainability, and lowest temperature.

Lyophilization is a unique process from a refrigeration point of view, not only for requiring ultralow-temperature refrigeration (below –50 °C),¹² but also because the load is extremely variable, often requiring a system turn-down in excess of 10:1.⁷ Both these key requirements favor cryogenic refrigeration over mechanical systems.

Chamber shelves (and sometimes walls) need to be cooled down to between –40 °C to –60 °C. The actual target temperature may vary from product to product, but it must always be set below the eutectic temperature of the solution to be lyophilized. The eutectic temperature is the lowest value at which a mixture of materials will melt. Meanwhile, the lowest temperature in the condenser typically needs to be between –60 °C and –80 °C, and sometimes as low as –100 °C, to make sure the solvent condenses out at a rate that will maintain an appropriate vacuum in the chamber. These temperatures depart from the comfortable realm of "industrial refrigeration," defined as refrigeration from –35 °C to –50 °C.¹² Thus, the requirements of lyophilization mainly reside in the "ultra low-temperature refrigeration" space, defined as –50 °C to –100 °C.¹² The efficiency and reliability of mechanical systems deteriorates as refrigeration temperature drops. Cryogenic systems, in contrast, provide practically constant cooling power throughout the temperature ranges of any lyophilization cycle.

Table 1. Overview of the properties of some low-temperature heat transfer fluids (HTFs)12,13

Depending on the effectiveness of the condenser, the condenser surface is kept at a temperature approximately 10–20 °C lower than the shelves, i.e., –50 to –80 °C during drying. It is critical to ensure that the temperature of the accumulating ice remains cold enough to condense out the solvent vapors. If not, the vacuum in the chamber can be lost, leading to loss of process control and possible destruction of valuable product. Meltdown of the cake occurs when the temperature of the product rises faster than the removal of the moisture or solvent. In addition, vacuum pump seal fluids may become contaminated by the solvent coming in through the condenser. Vacuum levels are typically controlled by adding refrigeration to the condenser, thus causing further condensation of the solvent vapors. Some experts therefore, view the condenser as a vacuum pump operated by refrigeration. Reliable and flexible cooling of the condenser is also crucial to lyophilization.

Refrigeration Temperature and Cool-down Rate

Operating temperatures below –50 °C negatively affect the performance, efficiency, and reliability of mechanical systems. However, such operating temperatures have no impact on cryogenic systems driven by liquid nitrogen (which has a normal boiling point of –195.8 °C). The cooling rate and efficiency of a mechanical compressor-based system starts to deteriorate below –20 °C.^7,8 Figure 1 shows the typical shelf cool-down of large commercial freeze-dryers equipped with mechanical compressors versus cryogenic heat exchangers. Cryogenic systems are capable of providing a rapid, constant cool-down rate throughout the entire ultralow temperature range. Mechanical refrigeration systems, on the other hand, cannot maintain their initial cool-down rate. This is probably the reason why original equipment manufacturers (OEMs) of freeze-dryers relying on mechanical compression equipment typically specify the cool-down rate in terms of overall time to reach a certain temperature. Citing an average rate can mask a deteriorating cooling rate over time. Only cryogenic systems can maintain cool-down rates of 1 °C per minute or higher over the entire temperature range of a lyophilization cycle.

Figure 1 also shows that LN₂ systems can reach –55 to –70 °C setpoint for the heat transfer fluid (HTF) inlet temperature to the shelves one to two hours faster than comparable mechanical units reaching a –50 °C setpoint. If the lyophilization cycle requires rapid cooling, this means increased productivity in terms of cycle time reduction. The manufacturers of sensitive products, such as vaccines, attain product viability benefits from rapid cooling. In addition, LN₂/GN₂ systems can easily go to even lower temperatures if required. Their lowest operating temperature is limited by the characteristics of the HTF, not those of the refrigeration system.

In summary, LN₂ systems offer a wider processing window of operation leading to added flexibility and productivity benefits. They do not suffer from the fundamental thermodynamic limitations of mechanical refrigeration systems, such as deterioration of efficiency and cool down rate, or limits on operating temperatures.

Refrigeration Load Profile

Lyophilization has special demands due to the extreme variability of the refrigeration load requirements. There are two main cooling circuits in a freeze-dryer: one for the shelves and another for the condenser. The shelf cooling circuit needs high-peak refrigeration power for the relatively short time (two to three hours) required to cool down the freeze-dryer and its contents, and to freeze the entire batch. This peak load on the shelf circuit is followed by a relatively longer period (one to three days) requiring significantly lower refrigeration power for the condenser circuit, but at a lower temperature. This load serves mainly to condense out the ice, which is slowly sublimating and desorbing from the product during primary and secondary drying. The corresponding refrigeration power required to run the condenser circuit is typically an order of magnitude lower than that required by the shelves for initial cool down and freezing.

This type of highly variable refrigeration at temperatures between –40 °C and –80 °C is best served by cryogenic refrigeration. LN₂/GN₂ systems are much more flexible in this temperature range, and are capable of efficient turn-down. Mechanical refrigeration systems are better suited to meeting steady demands. Compressors are ill-suited for short duration peak loads followed by extended operation at low load and ultralow temperatures. Under such conditions, compressors run inefficiently, using a lot of power while providing minimal cooling. They are designed to meet the short period peak load, yet are operated under suboptimal efficiency conditions for most of the lyophilization cycle time. Cryogenic systems, on the other hand, easily meet the variable refrigeration demands of lyophilization. Unlike mechanical systems, cryogenic systems operate with only small changes in thermal efficiency during the entire process cycle.

REFRIGERATION SYSTEMS USED IN FREEZE-DRYERS

All lyophilization refrigeration systems feature an HTF loop, which is the passive component from a refrigeration point of view. The HTF loop includes the fluid, piping, and pumping system with controls. The main differences between refrigeration systems are in 1) the active component(s) that drive the system, and 2) the necessary auxiliary systems. The active refrigeration system cools the low temperature HTF, which in turn refrigerates the shelves. Typically, a separate active circuit needs to provide the cooling for the condenser by direct expansion of a refrigerant, except in some advanced designs.

Low Temperature HTF

The thermophysical properties of the HTF circulating in the freeze-dryer have a significant impact on the unit's performance. Many of these properties are highly temperature-dependent. For example, the viscosity of the HTF can significantly increase as the temperature declines and approaches first the HTF pour point and then the freezing point. Although high pump-around rates ensure low temperature differences between the shelf inlet and outlet temperatures of the HTF, they may also lead to significant frictional parasitic heat generation. Hence, care must be taken in choosing the proper HTF. Table 1 summarizes key properties of some popular HTFs.

Mechanical Refrigeration Systems

In mechanical systems, compressors driven by significant electric power provide the active cooling. In general, ultralow-temperature mechanical systems that match the refrigeration demand at the necessary operating temperatures are increasingly complex and less flexible. They involve multistage or multirefrigerant cascade compression systems.¹² The complex refrigeration package includes compressors, heat exchangers, expansion devices, evaporators, and extensive controls. Necessary auxiliary systems include a cooling water loop, oil lubrication system, and an extra power infrastructure (including an extra supply and backup system) to support the significant power draw of the compressors. The rotating compression equipment involved can be screw or reciprocating types, with a trend towards costlier screw compressors because of their better reliability.^7,10,12 The cooling duty is provided by appropriately chosen refrigerants, such as R-23, R-404a, R-507, and R-508b (an azeotropic mixture of R-23 and R-116).¹² The refrigerants are first vapor compressed, condensed, adiabatically expanded, and evaporated 1) in a heat exchanger to cool the HTF, and 2) directly in the condenser to freeze out the solvent ice. These refrigerants are typically single or multiple component mixtures of hydrofluorocarbons (HFCs), which are often toxic or flammable.

Cryogenic Refrigeration Systems

Cryogenic cooling systems recover the stored cold from liquid nitrogen in specially engineered cryogenic heat exchangers. The necessary auxiliary systems include a liquid nitrogen storage tank, a set of cryogenic valves, and piping from the tank to the refrigeration skid. All of these components are highly insulated to minimize cryogen losses, e.g., by vacuum jacketing or superinsulation. The cryogenic LN₂/GN₂ cools the HTF in an initial cooling circuit. Typically, a second LN₂/GN₂ cooling circuit cools the condenser by direct expansion in the coils or plates. A more advanced cryogenic refrigeration system is noq available that uses a single nonfreezing cryogenic heat exchanger to simultaneously cool both the shelves and the condenser at different temperature set-points using two HTF loops.

SUMMARY

As demand for parenteral and biologically-derived products expands, companies are increasingly using lyophilization to protect and stabilize their sensitive pharmaceutical and biologic products. Freeze-dryer performance is key to achieving the required activity, stability, quality, and shelf-life for the finished products. As products are becoming more complex, cryogenic nitrogen refrigeration is gaining favor over mechanical refrigeration because of its inherent reliability and responsiveness to meet stringent and flexible cooling profiles while achieving ultra-low shelf and condenser temperatures. This will be further discussed in Part 2 of this article, to be published in the December 2007 issue.

Balazs Hunek, PhD, is a senior manager of technology, 630.320.4242, balazs_hunek@praxair.com Alan Cheng, PhD, is a senior development associate, R&D, and John Capettini is a manager of global applications market development,all at Praxair, Inc.

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