How Startup and Shutdown Conditions Can Damage Mist Eliminator Performance

Mist eliminators are usually selected using normal and design operating conditions.

But some of the most severe stresses on the separator can occur during startup and shutdown.

During these transient periods, gas flow, liquid loading, temperature, pressure, and upstream hydraulics may change rapidly.

A mist eliminator that performs well during stable operation can experience:

• liquid slugs;
• temporary flooding;
• thermal distortion;
• vibration;
• abnormal carryover.

This is why startup and shutdown should be considered as part of the operating envelope.

Stable Operation Is Not the Whole Story

Normal process calculations assume reasonably steady conditions.

During startup, this assumption may not hold.

Examples include:

• gas flow increasing before drainage stabilizes;
• spray systems starting suddenly;
• liquid inventory moving through the vessel;
• condensation forming during heat-up;
• foam forming temporarily.

Shutdown can create the opposite sequence.

Gas velocity may fall rapidly while liquid remains in the separator.

Temperature may change quickly.

These transients can produce conditions that are not represented by one steady-state design point.

Startup Can Create Temporary High Liquid Loading

When pumps, spray systems, or circulation loops start, liquid may reach the separator before the process reaches normal hydraulic balance.

A sudden surge can create heavy liquid loading.

If the mist eliminator is already exposed to increasing gas flow, the combination can reduce drainage margin.

The separator may temporarily flood.

This can produce a short burst of downstream carryover even though normal operation later becomes stable.

Condensation During Heat-Up Can Add Unexpected Liquid

During startup, hot vapor may contact cooler vessel walls and internals.

Condensation can occur.

This creates liquid in locations where the normal process may not generate much liquid.

If condensate reaches the mist eliminator, it increases temporary liquid holdup.

In some cases, operators may think the demister is leaking or underperforming when the liquid is actually generated by startup condensation.

Shutdown Can Leave the Separator Fully Wet

When gas flow is reduced, the aerodynamic force decreases.

This often improves drainage.

But if the process shuts down quickly, a large amount of liquid may remain inside the separator.

If the unit restarts before the pad has drained fully, the next startup begins with an already wet demister.

This reduces available void space and can increase initial pressure drop.

Repeated fast cycling can therefore create more severe conditions than long steady operating periods.

Liquid Slugs Are Especially Dangerous

Mist eliminators are designed primarily for dispersed droplets.

They are not intended to behave like bulk-liquid handling devices.

A large slug of liquid can:

• saturate mesh rapidly;
• overload vane drainage;
• create strong mechanical loads;
• move flexible pad sections.

If the upstream process can generate slugs during startup, the separator may need protective design measures or operating procedures.

A conventional efficiency rating says little about this type of event.

Thermal Expansion Can Shift Components

Metal and plastic demisters expand and contract with temperature.

During startup and shutdown, temperature may change quickly.

Repeated thermal cycling can affect:

• frames;
• support grids;
• fasteners;
• segment joints;
• edge fit.

Plastic systems are particularly sensitive because thermal expansion can be relatively large.

A separator that fits correctly when cold may experience compression or movement at operating temperature.

Vibration Can Be Stronger During Transitional Flow

Gas flow may be unstable during startup.

Pulsating flow or rapid changes in velocity can excite flexible components.

Potential effects include:

• pad movement;
• vane vibration;
• loosening of hold-down components;
• wear at contact points.

The steady-state gas velocity may be acceptable, but transient forces can still create mechanical problems.

This is especially important in large unsupported spans.

Startup Foaming Can Change Mist Duty

Some processes foam more strongly during startup than during normal operation.

Feed composition may not yet be stable.

Liquid level and reaction conditions may also fluctuate.

The separator can therefore receive a much finer and heavier mist load temporarily.

Repeated carryover during startup does not automatically mean the normal demister design is inadequate.

The transient process condition must first be understood.

Instrument Readings Can Be Misleading

Differential pressure may change rapidly during startup.

Part of the increase may come from:

• temporary liquid holdup;
• changing gas density;
• unstable flow;
• condensation.

Operators should be careful not to interpret every startup DP spike as permanent fouling.

The more useful question is whether the pressure drop returns to its normal baseline once operation stabilizes.

Operating Procedure Can Protect the Separator

Some startup problems can be reduced through procedure rather than equipment changes.

Possible actions include:

• gradual gas-flow increase;
• controlled spray startup;
• allowing drainage before full load;
• monitoring differential pressure;
• avoiding rapid restart after wet shutdown.

The exact sequence depends on the process.

The important principle is that the mist eliminator should not be exposed unnecessarily to simultaneous extreme gas and liquid loads.

When Design Changes May Be Needed

If startup upsets are frequent or unavoidable, engineering changes may be justified.

These can include:

• additional drainage capacity;
• stronger support;
• larger active area;
• upstream bulk-liquid separation;
• staged mist elimination.

The solution should target the specific transient problem.

Final Engineering Perspective

Mist eliminators operate through the entire process cycle—not only at steady state.

Startup and shutdown can create temporary combinations of liquid loading, temperature change, vibration, foaming, and condensation that are more severe than normal conditions.

A robust design and operating procedure should therefore consider transient hydraulic and mechanical loads as well as steady-state separator performance.