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Vacuum Distillation for Industrial Oil

Purification with the Best Available

Technology

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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

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