15/02/2010Sterilisation of liquids in closed containers. Cycles with differential pressure control

When creating sterilisation cycles for liquids in closed containers defining the process for a new product can be an intimidating task due to the high number of parameters to which values must be assigned. This article aims to provide help in the correct selection of parameters.

Requirement for pressure control

Sterilisation cycles with differential pressure control between the interior and exterior of the container are used for the sterilisation of liquids in closed containers.

Sterilisation can be performed in autoclaves with a superheated water shower (sometimes referred to as a raining water system) or with a mixture of air and steam using fans to homogenise the environment.

In addition to the liquid, there is always a certain amount of air inside the containers being sterilised. The pressure inside the container before commencement of sterilisation process is the atmospheric pressure at the location where the container was filled and sealed, which is assumed equal to the pressure of the autoclave chamber before commencement of the sterilisation cycle.

Heating

The first stage of the sterilisation cycle consists of heating the product. To minimise heating time, it is beneficial to increase the chamber temperature as quickly as possible. This consequently establishes a temperature differential between the chamber and the product, and as a result the product will heat up more rapidly. When the chamber temperature is increased, the chamber pressure increases simultaneously due to the expansion of air and the increase in water vapour pressure. With respect to autoclaves operating with a mixture of steam and air, this is due to the direct introduction of steam to the chamber, whereas with water shower autoclaves it is due to the evaporation of a portion of the re-circulating water.

The container receives heat energy from the chamber atmosphere and its temperature increases; its temperature during this stage is lower than the temperature of the chamber. The increase in temperature also results in an increase in the pressure within the container due to the vapour pressure and the expansion of air. Other reasons that contribute to the variation of the interior pressure of a rigid container will be explained later in this article.

Since the temperature inside the container is lower than the temperature of the chamber, if no action were taken he chamber pressure would be higher than the theoretical value in the container during the entire heating process.

In order to avoid an excessive pressure difference between the chamber and the interior of the container, it may be necessary to decompress the chamber to reduce its pressure, depending on the type of container.

In reality, if the container is rigid (normally made of glass) the interior pressure increase as explained can be calculated if the liquid temperature is known. This is achieved by use of a sample container, identical to those being sterilised but having a temperature sensor inserted into the liquid.

If the container is semi-rigid or soft (for example plastic) the anticipated increase in pressure is lower and an increase of the container's volume also occurs due to the deformation of the plastic.

Sterilisation

Once the product has reached the selected sterilisation temperature, the control system must maintain the chamber temperature and pressure constant until completion of the set sterilisation period. The chamber and product temperatures are the same, and consequently the theoretical pressure inside the container will be the value calculated at the sterilisation temperature and the chamber pressure should be the same.

Cooling

During the cooling process performed after sterilisation, it is also necessary to maintain control of the chamber pressure in relation to the product temperature. During the heating process there are no abrupt temperature and pressure variations, whereas at the start of the cooling process, due to the abrupt condensation of the steam present in the chamber, a sharp decrease in pressure may occur that could put the integrity of the container at risk.

The objective of a cycle at a controlled differential pressure is to maintain pressure in the chamber at a level that prevents the rigid containers from exploding and the non-rigid containers from becoming deformed. The ideal condition would be to have the chamber always at the same pressure of the container if it were rigid. This is easy to achieve during the heating and sterilisation process, but is more difficult when beginning the cooling process for the reason stated previous. The reduction in vapour pressure in the chamber must be compensated by and increase of air pressure. However, the loss of vapour pressure in the chamber can be so abrupt that it can be difficult to provide sufficient airflow to compensate. The solution is to address the problem before it arises by increasing the chamber air pressure after completing sterilisation but before beginning the cooling process.

Types of container

Rigid containers

A glass container can support differences in pressure in both directions, in other words, the external pressure can be higher or lower than the internal pressure. If the external pressure is greater than the internal pressure, under normal circumstances nothing will happen. However, if the internal pressure is greater, problems may arise.

Under the action of a specific pressure differential, the outward deformation of the rubber stopper can be sufficient to break or deform the security tab of an aluminium cap. If the differential is very substantial, depending on the container size, the thickness of the glass and its mechanical resistance, it is possible that it may not be able to support the pressure increase and will explode. In addition, when a vial explodes, the neighbour vials will typically explode as well.

Normally, during the sterilisation process a slightly higher internal pressure than external pressure can be tolerated. The aluminium cap is normally the limiting factor.

Inside the container there are two volumes. One volume is occupied by liquid and the other, which -is referred to as the head- space, is filled with a mixture of air and water vapour. When heating the glass container, five physical phenomena simultaneously occur:

• The glass expands, increasing the total volume of the vial. Consequently, the head-space increases and the pressure decreases.
• Part of the water evaporates, which increases the pressure.
• Although only slight, the volume of liquid water decreases because part of it evaporates, increasing the head-space and lowering the pressure.
• The water that has not evaporated expands, increasing its volume, and consequently the head-space decreases and the pressure increases.
• The air expands therefore increasing the pressure.

As the water coefficient of expansion is much higher than the coefficient of the glass, and the amount of evaporated water is very low, the result of the combined action of the five phenomena is an increase in pressure that is higher the value calculated without taking into account the expansion of the glass and the water, which in turn will depend on the proportions of liquid and head-space.

The autoclave control system typically calculate the theoretical pressure inside the container using the temperature read from the sample container, but since the relative proportions of liquid and head-space are not known, the calculation only takes into account the effects of water vapour and the expansion of air.

The result is fairly close to the actual situation if the initial head-space is substantial. A container that has 50% head-space will reach a pressure that will only be 2% higher than the pressure calculated, taking only the vapour pressure and the expansion of air into account. However, if the head-space is decreased to 15% the actual pressure will be 17.3% higher than the pressure calculated and the head-space is reduced to 10%, the actual pressure will be 37.7% higher than the pressure calculated. In other words, a glass bottle or vial that is 90% full of liquid in which the normally calculated theoretical pressure is 3.34 barA will actually be 4.59 barA.

It can therefore be concluded that in rigid containers, the chamber pressure can be maintained slightly above the theoretical value calculated without representing a danger to its integrity.

Semi-rigid containers

These are the most problematic, since in order to provide a certain level of rigidity; they moulded features on the sides and base. This asymmetry, together with the variation in plastic thickness at the corners and other parts, results in expansion properties and mechanical resistance that are not constant over the surface.

If the internal pressure is substantially higher than the external pressure, the base of cylindrical-type containers (vials and ampoules of round and oval cross-section) tends to deform outwards and after cooling does not return to its original shape. As a consequence, they will no longer stand upright when placed on a flat surface. The photos displayed here show that the bases of such containers before and after incorrect sterilisation have clearly deformed outwards.

Those that have a rectangular shape tend to have very weak corners (trihedrons) where the three sides meet. The injection and blowing process results in the plastic being thinner in those areas than in other parts of the surface and consequently excess external pressure can easily deform the corners inwards, as shown in the photograph to the right.

The ideal processing conditions for all of these containers are those that maintain the chamber pressure equal to the theoretical pressure inside the container at all times. As a result, when the internal and external forces are equal, the container will always retain its shape.

Soft containers

Normally these are bags and as the material is very flexible, the theoretical increase in internal pressure causes the material to stretch and the bag to inflate.

If the external pressure is greater than the internal pressure, the volume of the head-space will reduce due to compression of the air. It is difficult for a high pressure differential in this direction to damage the bag because it does not stretch the material or compromise its welded joints.

However, a low chamber pressure will cause the bag to inflate which, if it results in stretch beyond the elastic limit, will lead to non-recoverable deformation or failure at a joint.

Consequently, the ideal processing conditions for this type of product is to always maintain the chamber pressure a little above the theoretical pressure inside the container.

Temperature control

If the process requires that the product be heated to 121º and the chamber is set to 121º, then the product with either never reach temperature or a very long time will be necessary. In order to rapidly heat the product to the selected sterilisation temperature, the chamber is initially set to a temperature that is slightly higher than the required product temperature.

As the product temperature approaches the sterilisation temperature, continuing with the same temperature control would cause the product temperature to over-shoot. Consequently, from a certain product temperature value onwards the chamber temperature should be reduced. The closer the product is to the sterilisation temperature, the lower must be the differential between the sterilisation temperature and the chamber temperature. The objective is for the product to reach the sterilisation temperature (or some tenths of a degree above it) and for the chamber to reach some tenths of a degree above the product to compensate for the thermal losses of the equipment.

To achieve this control profile, three temperature parameters related to the sterilisation temperature must be appropriately selected:

• Increased the chamber temperature compared to the sterilisation temperature during the heating process.
• Temperature of the product from which the chamber temperature begins to reduce.
• Increased chamber temperature compared to the sterilisation temperature during the sterilisation process.

In a water shower sterilisation autoclave, there is an amount of water re-circulating in the chamber which, depending on the chamber size and load, can represent 45 - 75% of the product volume. Suppose that in order to speed up the heating process the chamber water has been heated to 124ºC. This water has a high calorific value. If at a point when, the product is close to the sterilisation temperature (say 120.5ºC) heating of the chamber water is stopped, the product will continue to be heated through the thermal inertia of this water and may reach a temperature 121.6 - 122ºC. Depending on the type of product, this may or may not represent a problem.

The amount of water introduced into the chamber of a particular water shower sterilisation autoclave is always the same. Due to the geometry and distribution of the load in the trays, the greater the size of the product container, the greater the total amount of product that can be accommodated within the same chamber. Consequently, the thermal inertia of the water in the chamber will have a decreased effect on the final phase of product heating with a batch of large containers when compared to another batch where the containers are smaller.

When using a sterilisation autoclave with a mixture of steam and air, the amount of surplus heat in the chamber as the product temperature approaches the sterilisation temperature is of little importance due to the low density of the steam. The thermal inertia is much lower than that in a water shower autoclave.

The product heats up via heat transferred across the container, with the layers of liquid in contact with the container heating first followed gradually by the centre of the liquid. If an excess in temperature would damage the product, the overheating temperature of the water cannot be very high otherwise the surface layers of liquid in contact with the container could be damaged.

A low volume container heats and cools quicker than a high volume container due to the relationship between the surface and the volume of the container. Comparing two similar shaped containers of different sizes, the smaller container has a greater surface to volume relationship than the larger one. The heat to be supplied must pass through the surface to heat the entire volume and consequently, a complete liquid sterilisation cycle will be shorter when small containers are processed as opposed to large containers.

In addition, the shape of the container influences its heating and cooling time. The lesser the distance from the centre of the container to a point on the surface of the container, the more rapidly it will heat up.

Only an empirical study with each type of product and batch will correctly confirm these parameters, which, with the help of the guidelines included herein, should be easier to specify.