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Vacuum Distillation for Industrial Oil
Purification with the Best Available
Technology
WHITE PAPER
© 2017 AGC Refining & Filtration LLC

AGC REFINING & FILTRATION
VACUUM DISTILLATION FOR INDUSTRIAL OIL PURIFICATION WITH THE BEST AVAILABLE TECHNOLOGY 2
Contents
Abstract 3
General Principles of Vacuum Distillation 3
Vacuum Concepts 6
The Allen Vacuum Distillation System 10
The Purification of Transformer Oil 13
The Purification of Gas Compressor Oil 15

Abstract
Can oil last indefinitely?
We do not really know how long oil can be purified and reused.
In the sixty-year history of manufacturing oil purification equipment, AGC Refining & Filtration LLC has yet
to experience a situation where, with routine and thorough purification with our equipment the oil could
not be re-used for a very long time.
We consider oil a renewable resource.
In order to achieve the most effective design of equipment for oil purification and re-use, Allen Filters, Inc.
has consistently applied the Best Available Technologies (BAT) such as computational hydrodynamic
modeling followed by pilot testing to constantly improve the performance of its equipment.
AGC Refining & Filtration LLC has also consistently equipped their systems with state-of-the-art electronic
process controls to produce equipment that operates in a safe, consistent, reliable and stable manner.
World-wide in 123 countries our equipment is operational, in several cases after more than 60 years
rendering reliable and trouble-free service to utilities, refineries, oil and gas plants, chemical plants and
hydroelectric plants among others.
Some have yet to waste their initial oil quantity.
General Principles of Vacuum Distillation
Distillation is the process most frequently used for separating multi-component liquids into individual
compounds.
A good understanding of mass-transfer and pressure drop fundamentals is required to arrive at the
optimum equipment design.
Mass transfer occurs due to a difference in concentration, in which the random motion of molecules
causes a net transfer of mass from high to low concentration. The amount of mass transfer can be
calculated using mass transfer coefficients.
A distillation column can use either trays or packing to facilitate mass-transfer while keeping the pressure
drop throughout the column to a minimum.
Mass transfer characteristics differ between trays and packing, but the key for both is achieving a steady-
state equilibrium by supplying a large interfacial surface area.
This interfacial area results from the passage of water vapor through the open areas of the packing
column and the spreading of liquid over the surface of the packing.
While initially, AGC Refining & Filtration LLC designed the process of vacuum distillation around stacks of
trays, recently, through the use of advanced computer-aided simulations and determination of mass
transfer coefficients, the possibilities for optimizations were identified that lead to a more effective and
efficient design of vacuum vessel internals such as using a column of packed Raschig® Rings. (See
Figure 1 and AFI-104 A Fourth Generation of Random Packing)
The optimization objectives were:
1 maximizing the number of theoretical stages included in the height of the packing,
2 minimizing the pressure drop for each theoretical stage of separation, and
3 maximizing the operating range of the column to achieve the best performance

Figure 1: Raschig Rings
With a choice between random packing (packing is dumped into the vessel) and structured packing, when
examined repeatedly in computer simulations and pilot tests, the random packing proved to be the most
efficient. Design data for standard random packing is widely available in the public domain.
Several pilot studies performed by Allen Filters, Inc. involving different configurations of packing proved
the theoretical models to be correct.
Within a packing type, elements are available in a wide variety. These are geometrically similar and differ
only in their specific dimensions. Computer programs were used to predict mass transfer efficiency and
calculate the precise packed bed height for the most common stripping and heat transfer conditions.
Mass-transfer occurs at the interface between vapor and the liquid film that covers the packing surface.
The mass-transfer is significantly affected by the specific area of the packing which was the reason for
experimenting with packing rings of different nominal size.
However, as long as the diameter of the vessel is very large compared to the specific dimension of the
rings, the effect of nominal size of the rings is negligible. The packing rings that are dumped into the
vessel arrange themselves in an irregular manner and as long as the bed height does not exceed the
column diameter by a certain amount, liquid distribution will be even.

Figure 2: Typical Allen Vacuum Distillation Vessel Internals
In the distillation column, the liquid flows from the distributor ring onto the packed ring stack as a series of
streams. Good wetting, resulting in a substantial fraction of the packing surface area being wet, requires
that the packing rings distribute these streams into a combination of rivulets and films that fully migrate
throughout the packed column cross-section.
Figure 3: Performance of a Packed Column
FLOW THROUGH THE COLUMN
Figure 3 illustrates the performance of a packed column with respect to throughput. The performance
remains relatively constant and stable, except at the low and high flow rates. At low flow rates, sheets of

liquid are not stable and rivulets do not spread evenly to wet the entire packing surface. The result is poor
mass-transfer.
As the flow through the column increases, there is a large stable region where packing is well wetted. At
this higher flow rate portion of the stable region, the mass-transfer improves. This improvement is caused
by an increase in interfacial area due to liquid waves and entrainment.
At the higher flow rates the performance decreases rapidly due to substantial liquid entrainment in the
vapor being carried upwards through the column. The entrainment degrades the composition profile and
can result in liquid and vapor flow re-distribution.
As the flow through the column increases beyond the stable operating region, the pressure drop begins to
rise much more quickly due to more liquid entrainment in the vapor and a greater interstitial vapor velocity
resulting from higher liquid holdup. In addition, mass-transfer begins to drop as increasing amounts of
entrained liquid flow up through the column. Because the packing is relatively open to cross-flow, liquid
and vapors tend to re-distribute themselves, resulting in a substantial decrease in mass-transfer
efficiency.
The R & D Department at Allen Filters, Inc. has performed exhaustive studies to verify the results
obtained by Computational Hydrodynamics Simulation. The efficient configurations that are implemented
in our Vacuum Distillation Systems are the result of these experiments.
Vacuum Concepts
The most important component of our Vacuum Distillation System is the vacuum system. The pumps
used in the Allen Filters Inc. systems are of the transfer type vacuum pumps.
These pumps force gas molecules in a preferred direction by positive displacement or momentum
exchange. Ultimately the gas is compressed to slightly above atmospheric pressure when it is ejected into
the atmosphere through a mist eliminator.
Transfer pumps are typically built for a long service life and are used for relatively high loads of dirty gas.
Operating pressure ranges
The mechanics of a vacuum pump dictate the pressure range at which the pump is able to operate. The
industry recognizes the following pressure ranges:
Coarse Vacuum 760 to 1 Torr
Rough Vacuum 760-10-1
Torr
Low Vacuum 10-2
Torr
High Vacuum 10-4
Torr
Ultra High Vacuum 10-9
to 10-12
Torr

Figure 4: Vacuum Levels for Various Types of Pumps
Ultimate Pressure and Pumping Speed
A pump’s ultimate pressure is the value measured when the pump is built. It is measured by blanking the
pump’s inlet with a pressure gauge, operating the pump for some time, recording the pressure achieved
and calling that the ultimate or blank-off pressure. The ultimate pressure influences the dehydration
capacity of the system. (See Figure 15)
Pumping speed is defined as the volume of gas (at any pressure) that is removed from the system by the
pump per unit time. Thus pumping speed is a measure of the pump’s capacity to remove gas from the
distillation column.
Corrosive Gases
Pumping corrosive gases can severely damage the pump’s internals. For that reason all Allen Vacuum
Distillation Systems have a means of condensing the vapors and disposing of them before they can reach
the vacuum pump.
This increases the useful life of the pump considerably, with dual-stage rotating piston pumps lasting as
long as 40 years and longer.
Rotary Vane Pumps
In rotary vane pumps, gas enters the inlet port and is trapped between the rotor vanes and the pump
body. The eccentrically mounted rotor compresses the gas and sweeps it towards the discharge port.
When gas pressure exceeds atmospheric pressure, the exhaust valve opens and gas is expelled. Oil is
used as a lubricant, coolant and sealant for the vanes. Single stage rotary vane pumps have an ultimate
pressure of around 10
-2
(rough vacuum).

Rotary vane pumps are used primarily as backing pumps. They are less expensive than rotary piston
pumps, but they do not have the same reliability. Over time, the vane clearances wear out and the pump
efficiency drops off.
Figure 5: Rotary Vane Vacuum Pump
Rotary Piston Pumps
These function best at pumping high gas loads at pressures lower than 10
-3
Torr. The mechanism is
rugged and can withstand severe duty. Gas from the condensate collection tank enters the pump body
through a sliding sleeve valve. An eccentrically mounted cylinder orbits around the inside of the pump
body without rotating itself. It compresses the gas and expels it through the exhaust valve into a de-
mister, which traps any oil vapor and into the atmosphere.
Rotary piston pumps are reliable and have a proven long lifetime in severe service.
They are an Allen Filters, Inc. standard component on all Vacuum Distillation Systems.

Figure 6: Rotary Piston Vacuum Pump
Vacuum Pump Size
The required size of a vacuum pump must be carefully calculated. It is a function of the following
parameters:
 the amount of contaminants present in the fluid,
 the partial pressures of the individual contaminants as well as that of the combined mixture,
 the molecular weight distribution of the contaminants from light to heavy components,
 the fluid temperature at the pump inlet, and
 the degree of vacuum required at the pump inlet.
Allen Filters, Inc. uses proprietary computerized simulation programs to arrive at the proper choice of
vacuum pump.
A common design fault is to under- or over-design the vacuum pump. If the pump is under-designed, the
vacuum will be insufficient for the mass-transfer required to boil off the contaminants.
If the pump is over-designed, the excessive vacuum will remove significant quantities of liquid entrained
in the vapor. This will eventually destroy the vacuum pump, in spite of any condensate trap.

The Allen Vacuum Distillation System
A process flow diagram of a typical Allen Vacuum Distillation System is shown in Figures 16 and 17. The
main components of the system are:
1. the Inlet and Outlet pumps,
2. the Vacuum Vessel,
3. the Heater,
4. the Filters,
5. the Condensate Tank,
6. the Vacuum Pump, and
7. the Control Panel.
The Inlet and Outlet Pumps
Figure 7: A Typical Positive Displacement Gear-Type Pump
These pumps are of the positive displacement gear-type, which are ideal for relatively low-capacities and
high pressure and variable motor speed applications.
The pumps have mechanical or lip seals, are direct-coupled to C-flange motors and have internal
pressure relief valves.
The pumps are rugged and have a history of long service life.
The pumps are controlled by a Programmable Logic Controller (PLC) and each has an optional Variable
Frequency Motor Drive, which is used to adjust the motor speed to oils of various fluid viscosities.

The Vacuum Distillation Vessel
Figure 8: A Typical Vacuum Distillation Vessel
The vacuum vessel is where the contaminants are vaporized by high temperature and high vacuum,
using the commonly applied chemical engineering process of Vacuum Distillation.
The vacuum lowers the boiling point of the contaminants and promotes vaporization a lower temperatures
than without a vacuum.
The vacuum vessel internals consist of a distribution manifold that applies the fluid uniformly over the bed
of Raschig Rings. The latter provide a large surface area that promotes mass-transfer that vaporizes the
contaminants.
Four level switches control the level of the clean fluid in the bottom part of the Vacuum Vessel.
An optical foam detector is mounted in the top of the vacuum vessel. When the detector senses foam
rising in the vessel, it automatically allows a momentary release of the vacuum that is sufficient to subside
the foam. Foam generation is common when the water content of the fluid is high. As soon as moisture is
eliminated, the foam generation will stop.
The Heater
Allen Systems use low watt-density immersion heaters as shown in Figure 9. These are rugged industrial
type heaters with stainless steel elements welded to flanges to provide superior rigidity and strength.
Heavy-duty jumper straps and terminal posts insure permanent tightness of the connections.

Watt-density on the heating coil is designed for low watt-density operation by increasing the coil diameter
and its length to provide maximum coil life.
A back-up adjustable thermal over-temp thermostat is located in the top of the heater.
The heater is also controlled by the Programmable Logic Controller, which allows the oil to cool the
elements after a system shut down. This prevents hot spots and carbonization of the fluid.
Figure 9: A Typical Low Watt-Density Immersion Heater
The Filters
The pre-Filter and any other optional solids filters consist of cylindrical vessels with removable covers,
which are held down by swing-bolts for easy removal.
The filter elements are high-efficiency, high dirt loading disc-type filter elements. The discs give the
elements an extremely large filtering surface.
Figure 10: A Two-Element Filter Vessel Figure 11: Disc-Type Filter Element

The Purification of Transformer Oil
The Allen Vacuum Distillation Systems can be used to purify and re-use mineral as well as synthetic oil.
Its applications include electric insulating oils such as transformer and circuit breaker oil as well as steam
and gas turbine and hydrocarbon compressor lubrication, seal and control oil. It removes the following
contaminants:
 100% of free water,
 100% of emulsified water,
 dissolved water down to 5 ppm or less total water,
 gases down to below detectable limits, and
 solids down to ISO requirements (usually less than 5-microns).
In doing so, the process restores the viscosity and flash point as well as acidity to acceptable levels.
Note: For an explanation of how contaminants interact please see AFI-106 and 107, Maintenance of
Transformer Oil, Part I and II.
The purification of electric insulation oil such as that used in transformers can be accomplished in two
ways:
 a Single-Pass Purification System or
 a Multiple-Pass Purification System.
A Single-Pass System purifies the oil during each pass through the purifier. It takes from the transformer
a quantity of oil equal to the system’s capacity and eliminates the contaminants down to the final clean oil
parameters. In other words, if the water content of the dirty oil is 1000 ppm and the capacity of the system
is 100 gallons per hour, then every hour 100 gallons is processed, and purified down to 5 ppm total water.
A Multiple-Pass System purifies the oil to a lesser extent during each pass through the purifier, until the
entire transformer volume is clean.
The two methods require different Vacuum Distillation Systems. The main difference between the
systems is in the vacuum pumping system.
The Single Pass Purification System
The single-pass purifications system requires a vacuum pump with a high pumping speed and a deep
vacuum. For this a two stage system is used consisting of:
 a high-volume blower and
 a two-stage rotary piston main vacuum pump.
A rotary piston vacuum pump and a high capacity booster combination achieves both lower pressures
and higher capacities than a single vacuum pump. The booster rapidly reduces the pressure, while the
low inter-stage pressure created by the piston pump minimizes power consumption and limits the heating
of the gas flowing through the booster.
The booster is protected by a high temperature switch to avoid damage when the booster is operating
continuously at the cut-in pressure.
Also, since the booster is water-cooled and there is no metal-to-metal contact or lubricant in the pumping
chamber, it is extremely reliable.
It is important that the selection of the booster be matched to the system characteristics of the application
The Single-Pass System is used in two ways:

 total Purification, including oil and paper insulation (Transformer is shut down) and
 routine purification involving only oil purification (Transformer remains in operation).
Figure 13: Single-Stage Oil Sealed Rotary Piston Pump Figure 14: Two-Stage Oil Sealed Rotary Piston Pump
Total Purification Procedure
1. The transformer is taken out of service.

2. The Vacuum Distillation System (VDS) is used to pump the oil from the transformer to a
temporary storage tank.
3. The vacuum system of the VDS is used to evacuate the empty transformer and maintain a high
vacuum.
4. The vacuum will draw moisture out of the paper insulation (this may take several days depending
on the condition of the paper).
5. After it has been established that no more moisture is being drawn out of the paper, the vacuum
is left on the transformer.
6. The VDS is then used to pump the oil from the temporary storage tank, through the purifier, into
the transformer which is still under vacuum.
The result is a transformer with clean dry oil and paper that has been dehydrated also. Equilibrium
conditions will allow any residual moisture to migrate into the dry oil, thereby dehydrating the paper even
more.
Routine Purification Procedure
The Single-Pass Purification System can also be used for routine transformer oil maintenance. The
system is hooked up to a transformer in a “kidney-loop” format and is operated continuously until the
required oil specifications have been achieved.
Because of the single-pass purification, the routine maintenance will take less time. The benefit of a
routine purification program is that contamination levels are kept to a minimum. This greatly increases the
useful life of a transformer beyond the average life span of 30 years.
The Multiple Pass Purification System
This system purifies the oil partially during each pass. After each pass the partially purified oil is returned
to the transformer, where it is mixed with the dirty oil. This is the reason that a multiple-pass system takes
longer to purify a batch of dirty oil. However, the end result is still the elimination of free, emulsified and
dissolved water down to 2–5 ppm total water as well as evaporation of gases and filtration of solids.
The advantage of a multiple pass system is that is less expensive; the vacuum system consists of a
single multi-stage rotary piston pump and a less expensive heater.

The Purification of Gas Compressor Oil
The Allen Vacuum Distillation System is also used to purify and re-use hydrocarbon gas compressor oil
as well as steam turbine lubrication and control oil.
The Purification of Steam Turbine Lubrication Oil
The system used for removal of contaminants from steam turbine lubrication and control oil is identical to
the system used for purification of transformer oil.
The major contaminants in steam turbine lubrication oil are water and solids. Either a Single-Pass or a
Multiple-Pass System will remove water and solids down to 5 ppm or less total water and solids down to
less than 5-micron.
Control oil which often is phosphate ester oil or other synthetic oil, the vacuum distillation will remove
water and solids (i.e. rust particles etc.) down to the levels required for reliable operation of the trip-
throttle valve.
The Purification of Compressor Oil
Hydrocarbon gas compressors have lubrication oil and seal oil. Some manufacturers combine the
lubrication and seal oil into one system while some separate the two. Seal oil is often the most
contaminated because of leaking shaft seals.
Regardless of the type of oil system, the Vacuum Distillation System has been proven in many refineries
and oil & gas plants to purify even the most contaminated oil, thereby allowing it to be re-used almost
indefinitely.
The vacuum distillation system used for hydrocarbon gas compressor differs from the system used for
transformer oil in that a refrigerated condensing package is added. The refrigerated condensing package
consists of a condenser and a refrigeration compressor. It condenses the hydrocarbon volatile
contaminants, which are collected in the condensate tank for disposal. This prevents the corrosive gases
from reaching the vacuum pump and destroying it.
The vacuum distillation systems for hydrocarbon gas compressors are usually mounted on a trailer to be
used in the entire facility.

Figure 16: A Flow Diagram of a Typical Vacuum Distillation System for Transformer Oil
Figure 17: A Flow Diagram of a Typical Vacuum Distillation System for Compressor Oil
Note the Refrigerated Condensing System

Figure 18: General Assembly of a Typical Stationary Vacuum Distillation System

Figure 20: Programmable Logic Controller Figure 21: Trailer-Mounted Vacuum Distillation Unit

Figure 23: Maintenance of Oil in a Large Transformer by an Allen Trailer Mounted Vacuum Distillation System

www.AGCInternational.com
3045 East Elm Street
Springfield, Missouri 65802, USA
Toll Free: +1 800 865 3208
Phone: +1 417 865 2844

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