Vacuum Degassing Applications

1. Direct answer and purpose

Vacuum degassing removes dissolved, entrained or generated gas and sometimes volatile moisture from a liquid, paste, melt or slurry by lowering gas partial pressure and providing enough interfacial area and residence time for transfer. The engineering task is to name the gas-removal objective and endpoint, throughput and operating range, product properties, temperature, pressure, contacting method, foam and entrainment behaviour, vapour and noncondensable load, and verification method. Vacuum level alone does not establish removal performance.

It does not promise a universal removal percentage. Product-specific process testing or a validated model is required before a guaranteed endpoint is published.

2. Controlled decision and application boundary

The process owner names the material, gas species, starting condition, required endpoint, production rate, temperature and product-quality constraints. The degasser specialist defines vessel or vent geometry, contacting and residence time. The vacuum specialist assesses gas/vapour load, working pressure, protection and pump compatibility.

Draw the boundary from product feed through distribution, spray, film, agitation, packing, screw vent or vessel; disengagement space; demister, separator or trap; vacuum line and control; pump; exhaust; and product discharge. Include heating/cooling, purge, seal liquid, electrical power, drains, cleaning and instrumentation.

3. Define the gas-removal objective

State which gas or volatile is to be removed and why. Possible objectives include dissolved oxygen, carbon dioxide, air, monomer, solvent, moisture or trapped bubbles. “Degas the product” is not an acceptance criterion. Specify concentration, units, sampling point, test method, temperature, pressure and time after processing.

Distinguish dissolved-gas concentration from visible bubbles, foam volume, total volatile loss and vacuum exhaust composition. A clear-looking liquid can retain dissolved gas; a foaming product can have a low dissolved-gas result but unacceptable carryover. If several endpoints matter, schedule each separately.

4. Solubility, equilibrium and mass transfer

Reducing gas partial pressure above a liquid reduces the equilibrium dissolved concentration for systems where the selected thermodynamic relation applies. Actual removal requires transfer from the bulk product to the interface and then to the gas phase. Viscosity, diffusion, interfacial area, agitation, film thickness, bubbles, temperature and residence time determine how closely the process approaches equilibrium.

Henry-law or vapour-liquid-equilibrium data can support a screening calculation only within its applicable composition and temperature range. Reactive, associating, multicomponent, non-Newtonian or high-solids systems may require test data or a more rigorous model. The process owner approves the property basis.

5. Product properties and temperature

Record density, viscosity or rheology, surface tension, solids, particle size, volatility, vapour pressure, heat sensitivity, shear sensitivity, contamination limits and expected property range. Temperature can reduce viscosity and change gas solubility and vapour pressure, improving one part of the process while increasing vapour load or product risk.

Use representative product conditions, not only a water test. If a surrogate is proposed, identify which transfer, foaming, contamination and cleaning behaviours it represents and which it does not.

6. Throughput and residence time

For a steady-flow vessel screen, nominal residence time is working liquid volume divided by volumetric throughput. This is an average inventory time, not proof that every element experiences the same contact. Short-circuiting, dead zones, level variation and changing viscosity can broaden the residence-time distribution.

Record minimum, normal and maximum throughput; batch size and cycle; start-up and recirculation; working volume; level range; and any parallel paths. The case governing pump gas load may differ from the case governing endpoint because a higher throughput increases removal demand and shortens contact time.

7. Contacting method and interfacial area

Gas removal can be promoted through spray nozzles, thin films, packed columns, trays, agitation, recirculation, membranes, extrusion vents or staged vessels. The chosen method must match viscosity, solids, foam, shear limits, cleanability and pressure drop. State droplet or film basis only when supported by vendor or test evidence.

Agitation or sonication may increase mass transfer, but can entrain more gas before vacuum is established or create stable foam. A larger pump cannot correct poor distribution or product short-circuiting.

8. Absolute pressure and vacuum stability

State degassing-vessel pressure in absolute units, the sensor location and normal, limiting and upset values. Working pressure must be compatible with product temperature, vapour pressure and desired volatile retention. Excessively low pressure can boil a carrier liquid or remove wanted aroma, solvent or plasticiser.

Define allowable pressure ripple, recovery after feed changes, control deadband and time at pressure. Stability should be evaluated with actual feed and foam behaviour. Pump-inlet pressure alone may not represent product-space pressure when the separator or line is restrictive.

9. Foam formation and control

Foam can expand rapidly as dissolved gas leaves solution or volatile components flash. Record foam onset pressure, maximum expansion, collapse time, batch or flow sensitivity, and effects of temperature, contamination and antifoam. Provide disengagement volume, level/foam detection, controlled pressure ramp and automatic response.

Antifoam may affect product quality, downstream processing, mass transfer and cleaning; its use belongs to the process owner. Do not assume a generic freeboard percentage. Establish it by product evidence and credible upset conditions.

10. Entrainment, separators and carryover

Droplets, powder, fibres, melt or foam can enter the vacuum line. Define demister, knock-out vessel, filter, condenser or trap duty; clean and fouled pressure drop; drainage; level alarms; safe isolation; and recovery or disposal. Carryover protection must accommodate start-up slugs and foam, not only steady aerosol.

A separator protects equipment but can reduce effective speed when undersized, wet or blocked. Measure pressure on both sides where the gradient is operationally important.

11. Gas, vapour and leakage load

Pump load includes gas released from product, carrier vapour, flashing volatile, deliberate purge, seal gas, air leakage and gases evolved by reaction. Avoid double counting gas measured in the exhaust and gas inferred from product concentration change. State mass, molar or reference flow and actual suction conditions.

For continuous processing, a material-balance screen is gas removal per liquid volume multiplied by throughput. This gives removed-gas mass, not suction volumetric flow. Convert with composition, absolute pressure and temperature, then add other loads and pressure losses.

12. Equipment and materials compatibility

Candidate systems may use liquid-ring, dry, oil-sealed, Roots or ejector arrangements depending on pressure, gas/vapour load, liquid carryover, solids, materials and utilities. Check lubricant or seal-liquid interaction, corrosion, swelling, polymerisation, contamination and cleaning chemistry.

Define warm-up, flush, purge, drain, standby and shutdown. Exhaust destination and permissible backpressure must match composition and environmental requirements. Product-contact requirements need project evidence rather than generic cleanliness claims.

13. Batch, continuous and recirculating modes

A batch vessel can provide controlled hold time and endpoint sampling but may see an early gas peak and severe foam. Continuous equipment must maintain endpoint across throughput, feed variability and level. Recirculation can increase cumulative contact while adding heat, shear and residence-time uncertainty.

Name the acceptance basis for each mode: one batch, first-pass outlet, recirculated tank average or product after a defined hold. Do not compare results taken at different locations or delay times without accounting for reabsorption.

14. Instrumentation and sampling

Measure product-space and pump-inlet absolute pressure, product temperature, flow or batch mass, vessel level, foam/high-level condition, separator differential pressure, pump status, valve position and relevant utilities. Use dissolved-gas, residual-volatile, density, bubble or product-quality instruments appropriate to the named endpoint.

Sampling must prevent air contact, pressure change, heating or delay from altering the result. Record sample point, container, preservation, elapsed time, method, calibration and uncertainty. Online analysers require representative flow and response-time checks.

15. Required asset - Degassing Application Requirement Matrix

16. Operating-state matrix

17. Worked case A - lower throughput

Assume a hypothetical aqueous product enters at 20 mg/L of a named dissolved gas and must leave at or below 5 mg/L using an approved test. Throughput is 2.0 m3/h, or 2,000 L/h. A vessel has 0.50 m3 working liquid volume. Nominal residence time is 0.50/2.0 = 0.25 h = 15 min.

Required concentration reduction is 15 mg/L. The material-balance removal rate is 15 x 2,000 = 30,000 mg/h = 0.030 kg/h. This is the named dissolved-gas load only. Carrier vapour, purge and leakage are separate.

18. Worked case B - higher throughput

Keep the same 0.50 m3 working volume, 20 mg/L inlet and 5 mg/L endpoint, but increase throughput to 5.0 m3/h, or 5,000 L/h. Nominal residence time becomes 0.50/5.0 = 0.10 h = 6 min. Required gas removal becomes 15 x 5,000 = 75,000 mg/h = 0.075 kg/h.

Compared with case A, the gas mass rate is 2.5 times larger while nominal residence time falls from 15 to 6 min. The same pressure and pump cannot be assumed to deliver the same endpoint. A test or validated mass-transfer model must demonstrate the outlet result at 5.0 m3/h.

19. Interpretation and bounded comparison

The cases show why throughput cannot be sized only by pump displacement. At higher flow the vacuum system handles more released gas, while the product has less contact time. Enlarging the pump may stabilise pressure but may not correct insufficient interfacial area or short-circuiting. Enlarging the vessel can increase nominal residence but does not prove good contacting.

All numbers are hypothetical. They exclude gas reabsorption, carrier evaporation, foam, leakage, purge, measurement uncertainty and equipment margin. No removal guarantee follows from the calculation.

20. Gas-load and suction-volume method

After establishing gas species and mass or molar flow, calculate suction volume at actual absolute pressure and temperature with an appropriate property method. Add product vapour, purge and leakage as separate lines. Check separator and piping pressure drop at the combined load and at credible foam or liquid-carryover conditions.

Do not convert dissolved concentration directly into pump m3/h without molecular weight, pressure, temperature and composition. Do not add incompatible standard and actual volumetric flows.

21. Measurement methodology

Synchronise feed and outlet dissolved-gas results with throughput, product temperature, vessel and pump-inlet pressure, level, agitation or contacting state, recirculation, separator differential pressure, condensate/drain collection, valve positions, pump status and utilities. Capture transients during feed entry and pressure ramps.

Run cases at the agreed product-property and throughput range. Preserve raw trend files and sample records. Reconcile product concentration change with exhaust or collection evidence where practical, and investigate differences rather than assigning them to an arbitrary efficiency.

22. Diagnostics

23. Commissioning and acceptance

Verify boundaries, tags, materials, utilities, drains, exhaust, fail positions, interlocks and calibration. Establish empty pump-down and pressure-rise baselines. Wet commission with approved material, starting at controlled low throughput and progressing through agreed maximum flow, temperature and property cases.

Acceptance must name product/grade, gas species, starting concentration, endpoint and method; throughput; batch or flow mode; temperature; absolute pressure and stability; contacting and residence-time basis; foam and entrainment evidence; gas/vapour load; pump-inlet conditions; raw trends; sampling; deviations and rejected runs. Pass only when the endpoint is demonstrated at the stated case.

24. Upsets and change control

Function-test or simulate allowed responses to foam/high level, liquid slug, separator blockage, feed surge, loss of cooling, pump trip, valve or sensor fault, exhaust backpressure and loss of purge. Define heat/feed isolation, pressure recovery, safe drainage and restart.

Revalidate after changes to product, additive, viscosity, temperature, throughput, working volume, contactor, pressure programme, antifoam, separator, piping, pump, sampling or endpoint. A throughput increase is a process change, not merely a scheduling change.