If a molecular distillation system reaches deep vacuum when empty but the pressure rises after feeding starts, this does not automatically mean the vacuum pump is faulty.
The most common reason is simple: once feed enters the wiped-film evaporator, water, solvent and other volatile components begin to evaporate. This creates a vapor load that the condenser, cold trap and vacuum system must handle continuously.
That is why no-load vacuum and operating vacuum are different conditions.
Before comparing equipment quotations, we should first understand how vacuum specs are stated.
Why Does Feeding Cause the Vacuum to Rise?
An empty system mainly needs to remove residual gas and leakage.
Once material enters the evaporator, the situation changes.
The wiped film process creates a continuously renewed heated film. As volatile material evaporates, the amount of vapor entering the vacuum system increases.
If vapor is generated faster than it can be condensed or removed, system pressure rises.
In simple terms: More vapor load → higher operating pressure
Does Water or Residual Solvent Affect Vacuum?
Yes. This is often one of the biggest causes of vacuum deterioration after feeding begins.
Feeds may contain:
- Water
- Ethanol
- Other solvents
- Light volatile components
- Dissolved gases
These materials can evaporate quickly when the feed enters a hot evaporator under vacuum.
Even a relatively small amount can create a large vapor load compared with normal leakage into an empty system.
For this reason, pretreatment or a first-stage devolatilization step may sometimes be useful before the main molecular distillation stage.
Can Feed Rate Be Too High?
Yes.
Higher feed rate means more material enters the evaporator per hour. If the proportion of volatile material remains the same, vapor generation also increases.
This is why process parameters such as feed rate, temperature and vacuum should be adjusted together.
If the feed rate is too high, several things may happen:
- Operating pressure rises
- Separation performance falls
- Condenser load increases
- Cold trap fills faster
- Product purity becomes unstable
Reducing feed rate is therefore a useful diagnostic step when vacuum worsens after feeding starts.
What Does the Cold Trap Do?
The cold trap captures condensable vapors that pass beyond the internal condenser.
It protects the vacuum pump and helps maintain stable pressure.
Vacuum performance can deteriorate if:
- The trap temperature is too high
- Cooling capacity is insufficient
- The trap contains too much condensed liquid
- Vapor load exceeds its capacity
If vacuum gradually worsens during a long production run, checking the cold trap is often worthwhile.
Is a Larger Vacuum Pump Always the Solution?
No.
A pump with a lower ultimate pressure does not automatically provide better operating vacuum during production.
Pump selection should consider:
Ultimate Pressure + Pumping Speed + Vapor Load
A molecular distillation system may reach an excellent empty-system vacuum but still struggle under feed if the pump cannot handle the gas load at the actual operating pressure.
The complete vacuum system should therefore be selected together with the evaporator and condenser.
A HJCHEM Example
HJCHEM molecular distillation systems typically specify:
Standard No-Load Vacuum: 0.1–1 Pa
High-Vacuum Option: 0.01–0.1 Pa
These values describe the vacuum capability of the system without process vapor load.
Once material is introduced, actual operating pressure depends on:
Feed Composition + Feed Rate + Temperature + Condenser Load + Vacuum Capacity
So a supplier stating: “Vacuum: 0.01 Pa”, without explaining whether that value is measured empty or under feed does not provide enough information to predict production performance.
This is why HJCHEM now clearly identifies these values as no-load vacuum in equipment specifications.
What Should Buyers Check Before Comparing Vacuum Performance?
Before choosing a short path molecular distillation system, ask:
- Is the stated vacuum measured with or without feed?
- What operating pressure is expected during production?
- Does the feed contain water or solvent?
- What feed rate is used for the estimate?
- What condenser area is provided?
- Is a cold trap included?
- What vacuum pump configuration is used?
- Can the vacuum system handle the expected vapor load?
These questions are more useful than comparing one vacuum number alone.
The Key Point
Vacuum usually worsens during feeding because the system is no longer evacuating an empty vessel—it is handling a continuous vapor load.
The main factors are: Feed Volatility + Feed Rate + Condenser Capacity + Cold Trap + Vacuum Pump + Leakage
A good no-load vacuum is useful, but it does not guarantee the same pressure during actual molecular distillation.
For equipment selection, HJCHEM recommends evaluating operating vacuum together with feed properties and throughput, rather than comparing ultimate vacuum values alone.