Engineering Evaluation Case: Two Parallel Scrubbers Share One Recirculation Tank but Do Not Receive Equal Liquid Flow

Parallel packed scrubbers may share:

• one sump;
• one pump;
• one chemical-control system.

This can reduce equipment count.

It also creates a hydraulic network where the two towers may not receive equal liquid flow.

Even identical towers can behave differently.

Project Situation

Consider two parallel packed absorbers treating the same gas service.

The liquid system includes:

• one common recirculation tank;
• one pump;
• a header splitting to Tower A and Tower B.

Both towers use the same:

• packing;
• bed height;
• distributor type.

The design assumption is simple:

each tower receives 50% of the liquid.

In operation, one tower runs well while the other develops poor outlet performance.

The actual liquid split should be investigated.

Equal Pipe Diameter Does Not Guarantee Equal Flow

Branch flow depends on the complete resistance of each path.

Differences may include:

• pipe length;
• elbows;
• valve position;
• elevation;
• distributor resistance.

One branch can receive significantly more liquid even when the piping looks symmetrical.

The Distributor Adds Its Own Resistance

Suppose Tower A has partially blocked distributor holes.

Its branch resistance rises.

More liquid may then flow toward Tower B.

The common pump can still show the expected total flow.

The problem remains hidden if only total pump flow is measured.

The Two Towers Can Drift Chemically

If liquid from both towers returns to one tank, their chemistry is mixed.

This can be beneficial because:

• pH is averaged;
• reagent dosing is centralized.

But it can also hide differences in tower duty.

One tower may receive:

• higher contaminant gas load;
• lower liquid flow.

Its outlet gas can deteriorate even though common sump pH looks normal.

Gas Split and Liquid Split May Not Match

Parallel towers may also receive unequal gas flow.

If Tower A receives 60% of the gas but only 40% of the liquid, its local L/G can be much lower than intended.

The combined plant flow totals may all look correct.

Tower-level values do not.

Common Chemistry Can Spread Contamination

Suppose one scrubber handles more:

• dust;
• oil;
• soluble metal.

Those contaminants return to the shared tank and then circulate through both towers.

A local process contamination problem can therefore become a system-wide packing or distributor problem.

Pump Control Can Mask Branch Imbalance

A flow-controlled pump may maintain the total circulation set point.

That does not guarantee correct distribution between branches.

The system should consider:

• branch flow measurement;
• balancing valves;
• distributor differential pressure.

One Tower Can Starve During Partial Operation

If one scrubber is taken offline, the liquid network changes.

The full pump flow may try to enter the remaining tower unless the system is controlled.

The remaining distributor may then experience:

• excessive pressure;
• excessive liquid loading.

Operating one of two towers is therefore a separate case.

Chemical Dosing Point Matters

If reagent is added to the common tank, adequate mixing is important before the flow splits.

Otherwise, one branch may receive temporarily stronger chemical than the other.

This can occur when the dosing point is close to one pump suction or branch connection.

What Should Be Measured?

A useful diagnostic set includes:

• liquid flow to Tower A;
• liquid flow to Tower B;
• gas flow to each tower;
• differential pressure of each bed;
• outlet contaminant from each tower;
• common sump pH and chemistry.

Only then can the two towers be compared correctly.

Maintenance of One Tower Can Affect the Other

Cleaning one branch can send:

• solids;
• chemical flush solution

into the common tank.

The operating tower may then receive contaminated liquid.

Isolation and maintenance procedures should therefore be part of the system design.

When Separate Liquid Loops May Be Better

A common loop reduces equipment cost.

Separate loops provide stronger control when the two towers have:

• different duties;
• different chemistries;
• highly variable loads.

The choice depends on the process.

Engineering Takeaway

Two identical packed towers do not automatically operate identically just because they share one pump.

Gas and liquid distribution must be checked at the branch level.