Critical Point Drying (CPD) - Transitional Stage

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5. Transitional Stage

Table of Contents:

1. Introduction
2. The Critical Phenomena
3. Critical Point Drying
4. Intermediate Stage
5. Transitional Stage

It is apparent from previous, (Figure 1) the conditions for which the Critical Point Passage is obtained, (for CO₂ - 31.1^oC and 1072psi). However, it must be remembered that these Isothermals are obtained from a fixed mass of gas and an applied pressure for a series of constant temperatures. In the application to C.P.D. we have a fixed volume, which is filled with the Transitional Fluid, some typical examples of which are given in Figure 4. (Water is +374^oC and 3212psi)

Figure 4.

Pressure is obtained by the effect of applying heat and while it can be readily appreciated that we can take a liquid from below its Critical Temperature and obtain the Transition to gas above its Critical Temperature, an understanding of the relevant 'start' and 'end' points and the cycle involved is required in evaluating the Design and Performance of C.P.D. equipment. It is still useful however, to utilise these CO₂ Isothermal as indicated in Figure 5, with the 'superimposed arrows' showing differing conditions for the C.P.D. device.

Figure 5.

It is already acknowledged that this is not for the exact comparable circumstances, i.e. (for C.P.D. we would fill at CO₂ cylinder pressure and ambient temperature: not at the saturated vapour pressure at the lower temperature ... the let down and decrease in pressure is as a result of venting and subsequent reduction in mass of gas: not reduction in externally applied pressure ... the relative volume is referring to the initial level of liquid in relationship to the total free volume available, this being the chamber plus ancillary fittings, associated with a practical system: not the variation in volume the substance experiences).

If we consider 'X' with the liquid CO₂ more than half filling the total available volume and we heat from 10^oC to 35^oC then we will make the transition from liquid to gas. The pressure rise will be rapid as the liquid will expand and the level will increase before the Critical Temperature is reached. This is termed 'going around' the Critical Point; usually a pressure relief valve is employed to prevent excessive pressure increase.

For condition 'Y' with approximately a half full condition, the liquid level will remain relatively constant, its density decreasing and that of the vapour increasing, becoming the same at the Condition of Critical Temperature having been reached, together with the corresponding Critical Pressure.

The condition 'Z' with less half-full condition, the level will fall, vaporisation occurring before the Critical Temperature is reached, and the specimen may be uncovered and subjected to evaporation. Ideally, we wish to have a situation where the liquid fills the specimen chamber, while still only accounting for approximately 50% of the total volume available. This is to ensure that the specimens are not uncovered during initial flushing stages and in addition this should enable Critical Constants of Temperature, Pressure, and Density to be achieved relatively simultaneously without the occurrence of either excessive pressure or evaporation conditions occurring.

Figure 6.

The Chamber and Manifold arrangement of the K850 allows simultaneous achievement of Critical Temperature and Pressure.

Figure 7: K850 Critical Point Drier

It is also advisable to maintain a temperature somewhat above the Critical Temperature during the pressure letdown stage, to avoid the possibility of gas re-condensing and also to control the letdown rate itself as there is evidence that time for pressure equalisation is advisable to avoid damage to the specimen.

Figure 8: Scanning Electron Micrograph of Critically Point Dried specimen showing Bacteria adhering to the tip of a fungus (Erysiphe Graminis) which itself was infecting a Barley leaf. Magnification X 8000.

While the CPD process is not as critical as its name might suggest, it is not unreasonable to attempt to achieve ideal circumstances while considering practical implications.

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