Why distillation runs under vacuum
Lowering the pressure lowers boiling points. Products that would decompose, polymerise or discolour at their atmospheric boiling point can be separated at a lower temperature. Vacuum is used for edible oil deodorising and fatty acid distillation, solvent recovery in pharmaceutical plants, essential oils and fine chemicals, lube oil and residue fractionation in refineries, and glycerine and bio-diesel refining.
The process design itself, including stages, reflux ratio, reboiler duty and product specifications, belongs to the process licensor or the plant’s process engineers. This page covers the interface that the utility and vacuum engineers must close: what the vacuum system sees, what it needs from the plant, and how it is controlled.
What the vacuum system actually sees
Overhead vapour from the column goes first to the condenser. Most of it condenses and returns as reflux or product. What leaves the condenser towards the vacuum system is:
- Non-condensable gas: air leaking in through flanges, seals, instruments and valve glands; gas dissolved in the feed; any inert gas purge; and gas produced by slight decomposition.
- Vapour carried with the non-condensables, at a partial pressure close to the saturation pressure at the condenser outlet temperature.
- Occasional liquid droplets if the condenser or separator is undersized.
The saturated vapour carried with the gas can be several times the mass of the air, especially if the condenser outlet is warm. Sub-cooling the gas at the condenser outlet, or adding an after-condenser, reduces this sharply. That is why the condenser outlet temperature is one of the most important numbers on the vacuum enquiry.
The enquiry schedule the vacuum supplier needs
| Item | What to state | Supplied by |
|---|---|---|
| Control pressure | Absolute pressure required and where it is measured: column top, condenser outlet or pump inlet | Process owner |
| Operating states | Start-up evacuation, normal, turndown, product change, shutdown and upset | Process owner |
| Non-condensable load | Air leakage estimate, dissolved gas, purge flow, in kg per hour | Process owner with vacuum specialist |
| Condensable vapour at the pump | Composition and flow leaving the condenser in each state | Condenser specialist |
| Condenser outlet temperature | With coolant type and worst-case coolant temperature | Condenser specialist |
| Pressure drop | Column top to condenser outlet to pump inlet | Process and piping engineers |
| Chemistry and hazards | Corrosive, toxic or flammable components; materials of construction | Process owner and safety |
| Discharge conditions | Exhaust destination, back-pressure, emission controls | Environmental and piping |
| Utilities available | Cooling water, chilled water, steam, power, nitrogen, instrument air | Plant utilities |
Where a value is uncertain, give a range and say how it will be confirmed. A hidden margin in the leakage figure, added again by the pump supplier, can double the installed size.
Estimating air leakage
Air leakage is often the largest non-condensable load. It rises with system volume, the number of flanged joints, rotating seals and instrument connections, and with the quality of maintenance. Industry practice, such as the Heat Exchange Institute’s guidance for steam jet systems, relates design leakage to system volume and operating pressure; vacuum suppliers apply similar methods. On an existing plant, a pressure-rise test with the column isolated gives a measured figure that is far better than any estimate.
Keep leakage low by using welded joints where practical, good gaskets, double mechanical seals on pumps under vacuum, and by leak-testing after every shutdown. Every kilogram of air that leaks in drags saturated vapour with it to the pump.
Choosing the vacuum equipment
| Option | Typical use in distillation | Interface notes |
|---|---|---|
| Steam ejectors with inter-condensers | Refinery and oils service, large loads, deep vacuum, dirty or hot gas | Needs reliable motive steam and cooling water; condensate is contaminated and must be treated |
| Liquid ring pump | Rough vacuum, wet and solvent duty | Seal liquid can be the process solvent; its temperature limits achievable pressure |
| Dry screw pump | Solvent recovery, pharmaceutical and fine chemical columns | No seal liquid, so solvent can be recovered at discharge; purge gas and temperature control needed |
| Roots booster on a backing pump | Lower pressures and higher flows | Bypass or variable speed protects against overload during start-up |
| Hybrid ejector and liquid ring | Deep vacuum with lower steam use | Ejector carries the low-pressure stage, liquid ring the high-pressure end |
Controlling column pressure
Stable column pressure keeps temperatures, separation and product quality stable. Common control methods are:
- Bleed control: admitting a small controlled flow of air or inert gas at the pump suction. Simple, but it adds load.
- Recycle control: returning part of the pump discharge gas to its suction.
- Pump speed control: varying the speed of a dry or liquid ring pump with a drive. Saves power but has a limited range.
- Condenser control: adjusting coolant flow or temperature; this changes vapour partial pressure but responds slowly.
Place the pressure transmitter where the process needs control, usually the column top, and make sure its tapping is protected from condensate. Slope the impulse line back to the process, keep it warm if vapour can condense in it, and use an absolute pressure transmitter rather than a gauge-pressure instrument, so that changes in weather do not appear as changes in column pressure.
Utilities the vacuum package depends on
- Cooling water at a known worst-case temperature. Cooling towers in Telangana and Andhra Pradesh can deliver 32–35 °C water in summer, which raises condenser outlet temperature and the vapour load at the pump.
- Chilled water or brine for after-condensers or vent condensers where solvents must be recovered.
- Steam at stable pressure and dryness for ejectors.
- Nitrogen for purge, blanketing and breaking vacuum safely on flammable duty.
- Instrument air, clean and dry to the plant’s ISO 8573-1 class, for control valves and actuators; a refrigerated dryer typically gives a pressure dew point around +3 °C, and a desiccant dryer −20 °C or −40 °C where lines are exposed to cold.
- Reliable electrical supply, with a plan for what happens to the column on a power failure.
Commissioning and handover checks
- Leak-test the isolated column and condenser system and record the pressure rise rate.
- Run the vacuum system on blank-off and record its own ultimate pressure.
- Evacuate the column and record the time and pump current against expectations.
- Bring the column to normal duty and record top pressure, condenser outlet temperature and pump inlet pressure together.
- Test control response to a step in boil-up or feed.
- Check the separator, seal pot and exhaust for liquid carry-over and emissions.
- Repeat key readings in the hottest season.
How Reddy Compressors helps
A distillation plant relies on several utilities working together, and we support the ones closest to our work:
- Sell new and used/refurbished vacuum pumps (rotary vane, liquid ring, dry screw, dry claw, scroll and rotary piston), rent them, service them and supply their spares.
- Review your vacuum enquiry, condenser data and pump curves and point out gaps in the non-condensable and vapour loads.
- Supply, rent and service instrument and plant air compressors of all major brands, with refrigerated or desiccant dryers and filtration for the required ISO 8573-1 class.
- Sell, rent and service Roots boosters and blowers, and supply their spares.
- Sell, rent and service nitrogen and booster gas compressors, and supply their spares, for purge and inerting duties.
- Provide rental compressors from our own fleet for shutdowns, commissioning and temporary capacity, serviced every 300 running hours on hire.
We are an independent multi-brand specialist working since 2002, not an authorised distributor of any pump or compressor maker. We have tie-ups with all major and reputed brands. Our service teams cover the whole of Telangana and Andhra Pradesh, and we supply equipment, rentals and spares across India, with project-based site support elsewhere where it is feasible. The distillation process design, hazard studies and final equipment selection remain with the process owner.
Reddy Compressors, Srinilayam, 12-13-200, First Floor, Street No. 1, Tarnaka, Secunderabad 500017. Phone +91 99590 60718 or +91 92468 88812; email reddycompressors@gmail.com.
What to send us
- Column duty, product and solvent list, and operating pressure at the column top.
- Condenser type, coolant and worst-case coolant temperature.
- Estimated or measured air leakage and any purge flow.
- Existing vacuum equipment make, model and nameplate photos.
- Instrument air demand, pressure and dew point requirement.
- Operating problems such as unstable pressure, high steam use or solvent in the exhaust.
Frequently asked questions
Why is my vacuum distillation column pressure unstable?
Unstable column pressure usually comes from varying non-condensable load, such as leaks that open with temperature, from cooling water temperature swings at the condenser, or from a control loop with the transmitter in the wrong place. Check the condenser outlet temperature, run a pressure-rise leak test and review whether bleed, recycle or speed control suits the range you are operating in.
Does the vacuum pump need to handle all the overhead vapour?
No. In a well-designed system, the overhead condenser removes almost all of the vapour, and the pump handles the non-condensable gas plus the vapour saturated in it at the condenser outlet temperature. The pump only sees the full vapour load if the condenser is bypassed, flooded or short of cooling, so those cases should be stated and protected against.
How much air leakage should I design for in a vacuum system?
Design leakage depends on system volume, number of joints and seals, and operating pressure. Industry guidance, such as the Heat Exchange Institute’s method for steam ejector systems, links leakage to volume and pressure, and vacuum suppliers use similar rules. On an existing plant, a measured pressure-rise test gives a far more reliable figure than any rule of thumb.
Steam ejector or liquid ring pump for distillation?
Steam ejectors suit large, hot, dirty or corrosive loads and deep vacuum, and have no moving parts, but they need reliable steam and produce contaminated condensate. Liquid ring pumps suit rough vacuum and wet duty with lower utility demand. Many plants combine them, with an ejector for the low-pressure stage and a liquid ring pump behind it.
Why does warm cooling water affect vacuum distillation?
Warmer cooling water raises the condenser outlet temperature, which raises the vapour partial pressure leaving the condenser. More vapour then travels with the non-condensable gas to the vacuum system, which may not cope, so column pressure rises. In summer this is a common cause of lost vacuum. Chilled after-condensers or better cooling tower performance help.
What instrument air quality does a distillation plant need?
Instrument air should be clean, dry and oil-free enough for control valves and positioners, usually specified as an ISO 8573-1 class by the plant. The dew point must stay below the lowest temperature the air lines see. A refrigerated dryer typically gives a pressure dew point around +3 °C, while exposed or critical systems often use desiccant dryers at −20 °C or −40 °C.
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Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer, Reddy Compressors. Last updated 17 October 2026. How our engineering content is reviewed.