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Vacuum Distillation for the Removal of Water and Other Volatile Contaminants

James C. Fitch

Numerous methods have been developed to selectively remove water and other volatile contaminants from hydraulic and lubricating fluids. These methods include absorbent filter media and regenerable adsorbent packings and the like. In many cases, it is not economical or practical to use disposable media, and as a result, continuous scrubbing processes have been developed. These processes are available in a variety of designs and are universally referred to as oil reclamation systems.

The most common scrubbers are derivatives of vacuum distillation processes used in refineries. The process involves expanding oil to produce high surface area to facilitate the vaporous extraction of water and certain other contaminants. Distillation involves heating, vaporization, condensation and cooling of vapors. Distillation separates components of a liquid mixture by partial vaporization and separate recovery of the vapor and liquid residue. The more volatile components, water for instance, convert to the vaporous state while the less volatile components remain as a liquid (the oil). The vapor is subequently condensed or purged to the atmosphere. Completeness of separation depends on properties of the components (e.g., boiling point) and efficiency of the distillation process.

Vaporization is the change from a liquid to a vapor state. The change typically requires the addition of heat energy to the liquid. Heat can be introduced just prior to distillation, or in some cases, the oil's normal operating temperature may be sufficient. Condensation is the change in state from a vapor to a liquid, generally requiring the removal of heat from the vapor in a condenser. Condensation is the reverse of vaporization.

Vacuum distillation is simply distillation at pressures below one atmosphere. Reduced pressure permits vaporization at reduced temperatures. For instance, at atmospheric pressure water boils at 212°F (100°C); but under vacuum (typically around 27" Hg) this boiling point can drop to 135°F (57°C) or lower. This has two distinct advantages:

1. Thermal decomposition and degradation of the fluid and its additives are avoided.

2. The energy requirement for both heating and cooling is reduced.

In-Plant Applications
Typically, vacuum distillation equipment is applied for dehydration purposes. However, other low boiling point contaminants can often be effectively removed as well, including H2S, fuel, refrigerants, solvents and other light hydrocarbons. In common in-plant reclamation processes, each lubricant type is treated separately. This avoids the cross-contamination of fluids with incompatible additive systems and reduces the time and cost of flushing between applications.

In dehydrating and reclaiming used lubricants and hydraulic fluids, vacuum distillation equipment is applied in one of three process configurations:

1. The first application is simply the transfer, in a single pass, through the vacuum distillation unit from one tank to another. Treated oil is not mixed with untreated oil. If the unit is 90 percent efficient, the process leaves 10 percent volatile contaminant in the effluent.

2. The second application involves the recirculation (multi-passing) of fluid through the vacuum distillation unit from a tank or the sump/reservoir of a static, nonoperating fluid system (Figure 1). The purified oil is recombined with the contaminated oil. One pass is completed when the volume of fluid circulated through the unit equals the total volume of fluid in the reservoir. Continued recirculation provides many passes during which, theoretically, some fluid is never treated. Nevertheless, this recirculation configuration is widely applied and can provide adequate contaminant removal and control.

3. The third application involves online recirculation through a vacuum distillation unit on an operating fluid system. This might be a paper machine main lube oil system, turbine-generator lubrication or a hydraulic system. In many cases, the fluid conditioner may be dedicated to the machine for continuous contaminant removal. For dehydration purposes, the rate of water removal should equal the exact rate of water ingression. Such applications may not require the addition of heat to the fluid, as operating temperatures may be adequate to achieve distillation.

Vacuum Distillation Equipment Design
A typical vacuum distillation system for fluid reclamation typically includes the following basic components (Figure 2):

1. Positive displacement pumps are required to transfer fluid to and from the vaporizer (vacuum chamber). In some cases, a vacuum is used to draw fluid into the unit. While this eliminates the input fluid pump, it reduces the capability of flow control.

2. An incoming filter is common to remove solid contaminants and to keep the distillation column clean. Even more common is the use of discharge filtration for the final removal of solid contaminants.

3. In some cases, in order to facilitate vaporization, heat must be applied either electrically or by steam transfer to raise the fluid temperature to the boiling points of the target contaminants. However, as previously mentioned, the inlet fluid temperature may be high enough without additional heating.

4. A distillation column or chamber is required to separate the volatile liquids and gases from the fluid. The primary objective of the distillation column is to create high surface area with the contaminated fluid, allowing ready effervescent vaporization of the contaminants. A number of different processes are used, including: atomizing the oil under pressure, passing the oil over a number of rings or saddles, thinly dispersing oil over a rotating surface, passing oil inside-out through glass-fiber cartridges, and passing oil through a column of reticulated porous media. Some commercial designs allow the water to coalesce first so that it more freely releases from the oil matrix and polar oil constituents (additives, contaminants, etc.).

5. A condenser is used to convert the vapor to a liquid and for cooling the condensate so it will not re-evaporate.

6. A vacuum pump is needed to exhaust trace amounts of noncondensed vapors and noncondensable gases to the atmosphere. A variety of rotary vacuum pumps are used to maintain wide range pressures in the vaporizer depending on the properties of the volatile contaminants.

7. Suitable gauges for monitoring and control must also be provided to maintain efficient operation.

Practical Industry Application
Because vacuum distillation can effectively remove soluable, emulsified and free water from lubricating and hydraulic fluids, it has wide application in industry today. Problems involving damage of the oil or its additives,

especially when lower temperature heating is involved (less than 150°F [65°C]), are rarely reported. And unlike many alternative methods for removing water, vacuum distillation units may represent the lowest cost for systematic dehydration. Several suppliers and service providers offer vacuum dehydrators, including those listed in the Sidebar below.