Exploring Autoclaves and Their Temperature Profiles #2

In this article, we will continue what we have started in the previous article continuing the rest of Moist heat cycles types to have the full understanding of Autoclaves.

Please go back to the article "The Superheated Water Sterilization Cycle: A Step-by-Step Approach" to catch the info represented in this article.

Saturated steam sterilization — Active air removal

Saturated steam sterilization is one of the most widely used and reliable methods for terminal sterilization across a variety of industries, from healthcare to pharmaceutical manufacturing. Within this broad category of steam sterilization, the pre-vacuum process represents the most common technique employed.

The pre-vacuum approach involves the use of a mechanical vacuum pump to actively remove air from the sterilization chamber prior to the introduction of steam. This air removal step is a critical component, as the presence of residual air pockets can significantly impede the penetration and distribution of the saturated steam throughout the load.

The pre-vacuum process is particularly well-suited for sterilizing loads that have a tendency to trap air, such as surgical instruments with complex geometry, porous materials like textiles and dressings, and the components of filling and packaging equipment used in the pharmaceutical industry. By eliminating these air gaps, the steam can make direct contact with all surfaces of the load, ensuring complete and consistent sterilization.

An alternative technique used extensively in pharmaceutical manufacturing is the vacuum pulsing process. This method involves a series of alternating vacuum and steam pulses applied to the load prior to the sterilization exposure. Each vacuum pull can remove approximately 90% of the residual air within the chamber, corresponding to about a 1-log reduction in air content. By performing three successive vacuum pulses, 99.9% of the air can be eliminated - a highly effective air removal process.

Additional positive pressure pulses, where steam is introduced above atmospheric pressure, may also be incorporated into the cycle. This helps to further condition the load, enhancing air removal efficiency and ensuring intimate steam contact with all surfaces. The precise number, duration, and pattern of these vacuum and positive pressure pulses is determined during the comprehensive development and validation of the sterilization cycle.

The pre-vacuum and vacuum pulsing methods are preferred for products and applications where thorough air elimination is critical to the success of the sterilization process. This includes porous materials, instruments with internal channels or cavities, packaged goods, and surface sterilization applications. By creating a steam-rich environment and eliminating air pockets, these techniques help ensure the complete and consistent sterilization of challenging load types.

The video below from Belimed, a well-known company, explains the concept in a very effective manner.

The complete pre-vacuum sterilization cycle consists of six main Phases:

1. Air Removal Phase: Air is extracted from the chamber and the load using either a deep vacuum or a combination of vacuum and steam pulses. This is a crucial step for ensuring intimate steam contact.
2. Heating Phase: Steam is introduced into the chamber, raising the temperature and pressure to the specified sterilization levels.
3. Sterilization Phase: The validated sterilization temperature and pressure are maintained for the predetermined duration, killing any microorganisms present.
4. Exhaust Phase: The steam is removed from the chamber, and a target vacuum level is attained to prepare for the drying stage.
5. Drying Phase: For dry loads, the temperature in the jacket surrounding the chamber and the vacuum within are held for a set period to facilitate the removal of residual moisture.
6. Vacuum Relief Phase: Air is gradually admitted back into the chamber until atmospheric pressure is reached, allowing for the safe removal of the sterilized load.

Figure 1 shows the Temperature-Pressure Profile of this type of cycle

Air Overpressure: A Critical Consideration in Sterilization

Before going deeper into the various sterilization cycle types, it is essential to first understand the concept of air overpressure. This specific term plays a crucial role in ensuring the success and integrity of the sterilization process, particularly for liquid-containing load items.

In nearly all liquid containers used in the pharmaceutical and healthcare industries, such as prefilled syringes, glass bottles or vials, plastic bags, and semi-rigid containers, there is a headspace filled with gas (air, nitrogen, or other inert gas) above the liquid contents. As the liquid is heated during the sterilization cycle, the headspace gas expands, leading to an increase in pressure within the container.

For most liquid load applications, it is necessary to increase the pressure within the sterilization chamber to minimize the differential pressure between the internal container pressure and the chamber pressure. Maintaining this balance is critical to preserving the shape and closure integrity of the containers, as well as ensuring the proper positioning of components like syringe stoppers.

The specific air overpressure required can vary significantly, depending on the type of container being sterilized. For example, the overpressure needs for glass bottles may differ considerably from those for plastic bags.

To provide the necessary air overpressure, it is common to use oil-free compressed air. The quality of this air supply is dependent on the specific application and requirements. In some cases, a microbial-retentive air filter may need to be installed in the air supply line to ensure the sterility and purity of the overpressure gas.

Carefully considering and controlling the air overpressure is a critical aspect of the sterilization process, as it helps maintain the integrity and functionality of the liquid-containing load items throughout the cycle. This understanding of air overpressure lays the foundation for the discussions of the various sterilization cycle types and their unique characteristics.

Figure 2 shows the concept of Air over Pressure

Steam-Air Mixture (SAM)

The steam-air mixture (SAM) sterilization process is a specialized technique used to achieve effective sterilization while accounting for the presence of air within the sterilization chamber. In this method, air is intentionally introduced into the steam environment to create a total pressure above the saturation pressure of the steam at the specified temperature.

The addition of air to the steam, although necessary for the process, does result in a reduced heat transfer rate compared to using saturated steam alone. To ensure effective and uniform sterilization, the steam-air mixture must be continuously circulated within the chamber. This circulation serves two critical purposes:

1. Preventing Stratification and Cold Spots: Proper circulation helps avoid the formation of stagnant zones or cold spots within the load, which could otherwise compromise the sterilization efficacy.
2. Mitigating Steam Depletion: Constant circulation of the steam-air mixture prevents the depletion of steam next to the cold container surfaces, where the steam could condense, affecting the overall heat transfer and sterilization process.

Fans are typically employed to circulate the steam-air mixture throughout the sterilization chamber, ensuring a homogeneous environment.

The steam-air mixture sterilization cycle consists of three major Phases:

1. Heating Phase: During this initial Phase, the process is similar to a vented system, with steam entering the chamber until the prescribed sterilization temperature is reached. However, if the product integrity can be affected by the rising steam pressure, venting is precluded. In cases where the partial pressure from the entrapped air is insufficient to protect the product, compressed air is introduced to provide the necessary overpressure.
2. Sterilization Phase: Once the desired sterilization temperature is attained, circulation and temperature maintenance are sustained for the prescribed exposure time.
3. Cooling Phase: After the sterilization phase, the product is cooled using various methods, such as cooled compressed air, heat exchangers, or chilled water sprays. During this cooling stage, it is crucial to prevent damage to the contained product from rapid depressurization of the chamber. Compressed air is used to maintain the required pressure in the chamber until the product has been sufficiently cooled, at which point the chamber is vented to atmospheric pressure.

The steam-air mixture sterilization process requires careful control and monitoring of the air-to-steam ratio, circulation patterns, and cooling strategies to ensure the effective and consistent sterilization of the load while preserving the integrity of the contained products.

Figure 3 shows the Temperature-Pressure Profile of this type of cycle

Superheated Water process by Water Spray

The superheated water sterilization process is a highly efficient method for the sterilization of liquid-filled containers. This technique utilizes recirculating superheated water to effectively and consistently sterilize a wide range of products.

There are several variations of the superheated water sterilization process, with the most common approach involving a continuous recirculation system. In this system, a pump is used to continuously recirculate water from the bottom of the sterilizer, below the load zone, to a series of spray nozzles positioned above the load. This recirculating water can be heated and cooled using various methods, including direct injection of steam and cooling water or indirect heating and cooling via sanitary heat exchangers.

The use of indirect heating and cooling methods is often preferred, as it allows for the use of almost any type of steam or water on the non-sanitary side of the heat exchanger. This is advantageous because the cooling water that directly contacts the sealed containers has been sterilized along with the product, ensuring the final product's microbial quality and safety.

One of the key features of the superheated water sterilization process is the use of controlled air overpressure. The minimum overpressure required is determined by several factors, including the temperature being used, the pressure needed to maintain the desired product characteristics, and the overpressure necessary to keep the recirculation pump primed.

Compared to other steam sterilization methods, the superheated water process offers several advantages. Notably, the heat-up and cool-down rates are easier to control and, when properly set up, are not significantly influenced by variations in product load or utility supply. This consistent control over the heating and cooling stages is a crucial factor in ensuring the effectiveness and efficiency of the sterilization process.

The primary quality attribute of the water used in the superheated water process is its microbial content. The water may be sterilized within the chamber along with the load, sterilized in a separate vessel, maintained at elevated temperatures, or chemically treated to maintain the desired low microbial levels.

One of the significant benefits of the recirculating water approach is the efficient cooling of the terminally sterilized products, which can increase the overall throughput of the sterilization system. This rapid cooling may also be necessary to help ensure the long-term stability and integrity of the sterilized products.

This video Below shows the concept in a clear way.

The sterilization cycle consists of four major Phases:

1. Fill Phase: At the beginning of the sterilization cycle, a quantity of water is introduced into the sterilizer or produced as condensate from the steam. This water is then sprayed over the product to initiate the process.
2. Heating Phase: The sterilizing temperature is achieved through one of two methods whether Introducing air and steam into the circulating system to heat the water or heating the water through a heat exchanger and introducing compressed air into the chamber.
3. Sterilization Phase: The circulation system is operated, and the circulating water, along with the contained product, is maintained at the required sterilizing temperature for the desired duration.
4. Cooling Phase: The pressure in the chamber is maintained by compressed air, and the contained product is cooled as the temperature of the circulating water is lowered at a controlled rate. The chamber is de-pressurized once the contained product has been reduced to a safe temperature.

Figure 4 shows the Temperature-Pressure Profile of this type of cycle

Superheated Water process by Water Immersion

Another variation of the superheated water sterilization process involves the complete submersion of the product in water.

This approach follows a similar sterilization cycle to the water spray system, consisting of the following Phases:

1. Fill Phase
2. Heating Phase
3. Sterilization Phase
4. Cooling Phase

Figure 5 shows the Temperature-Pressure Profile of this type of cycle

The key difference in this submerged process is that the contained product is totally immersed in the water throughout the cycle. This complete immersion helps maintain the shape and integrity of the product during the sterilization process.

By submerging the product in the superheated water, the sterilization is more uniform and consistent, as the product is entirely surrounded by the sterilizing medium. This can be particularly beneficial for certain types of products that require additional support or protection during the high-temperature sterilization process.

Overall, the submerged superheated water sterilization approach follows the same fundamental stages as the water spray system, but the complete product immersion provides an alternative method for ensuring effective and controlled sterilization while preserving the product's physical characteristics.

This video Below shows the concept in a clear way.

Recap

In this in-depth exploration, we have covered a wealth of information surrounding the principles and mechanisms of autoclave sterilization. Let's dive deeper into the key topics discussed across these articles:

• The Fundamental Principle of the Autoclave
• The Physical Design of the Autoclave
• The Main Phases of the Sterilization Cycle
• The Concept of Air Being Heavier than Steam
• The Five Distinct Moist-Heat Sterilization Cycles

By delving into these comprehensive details, we have gained a deeper understanding of the science, technology, and best practices surrounding autoclave sterilization. This knowledge equips us to make informed decisions, ensure the safety and efficacy of sterilization processes, and continually improve upon these critical technologies that play a pivotal role in maintaining healthcare and research standards.

Please go back to the article "The Superheated Water Sterilization Cycle: A Step-by-Step Approach" to catch the info represented in this article.

Stay Tuned

In our forthcoming article, we will explore different types of steam in an effort to gain a deeper understanding of what steam truly is.

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