Upstream vapor condensation changes the absorber feed before absorption begins. When a hot gas mixture is cooled in an upstream heat exchanger, condensable vapor becomes liquid and leaves the gas phase. The absorber therefore receives a lower total gas flow, a different average molecular weight, and revised solute concentrations, so its material balance, gas-to-liquid ratio, removal efficiency, and sizing calculations must all be updated.

The absorber balance must be based on the gas that remains after condensation, not on the original hot-gas feed. Calculate the condensed quantity at the heat-exchanger outlet conditions, remove it from the gas-phase stream, and then convert the remaining gas flow to the absorber operating temperature and pressure.

Why Condensation Changes the Absorber Feed

Condensation removes material from the gas phase

At the heat-exchanger exit temperature and pressure, a condensable component may exceed its equilibrium vapor pressure. The excess leaves the gas phase as liquid.

For a simple calculation, the condensed amount is determined from the inlet vapor quantity and the vapor remaining at the outlet equilibrium condition:

[ n_{\text{condensed}} = n_{\text{vapor,in}} - n_{\text{vapor,out}} ]

The outlet vapor quantity should be based on the heat-exchanger exit temperature and pressure, rather than on the inlet composition alone.

The total gas molar flow decreases

The post-cooling gas flow is:

[ n_{\text{gas,out}} = n_{\text{gas,in}} - n_{\text{condensed}} ]

This reduced flow is the actual gas feed to the absorber, assuming the condensed liquid is removed before entering the column.

The change may be substantial when the original gas contains a large amount of water vapor or another condensable carrier component.

Gas composition is re-normalized

Removing liquid from one component changes its gas-phase mole fraction and changes the relative concentration of every component remaining in the gas.

For component (i):

[ y_{i,\text{absorber}} = \frac{n_{i,\text{gas,out}}} {n_{\text{gas,out}}} ]

This means the absorber feed composition cannot be obtained by simply carrying the original inlet mole fractions downstream.

How to Perform the Sequential Balance

Start with the hot-gas inlet

Define the inlet stream on a consistent basis, such as kilomoles per hour:

[ n_{\text{in}} = n_{\text{condensable,in}} + n_{\text{absorber-solute,in}} + n_{\text{inert,in}} ]

The absorber solute might be ammonia, an acid gas, or another chemically reactive species. Inert gases and noncondensable components generally remain in the gas phase during ordinary upstream cooling.

Determine the vapor remaining after cooling

Use the heat-exchanger outlet temperature and pressure to determine how much condensable vapor can remain in equilibrium with the liquid phase.

For water, for example, the outlet vapor partial pressure cannot exceed the saturation pressure at the outlet temperature when liquid water is present:

[ p_{\text{water}} = y_{\text{water}}P ]

If the calculated partial pressure exceeds the saturation pressure, condensation occurs until the equilibrium condition is satisfied.

In a more rigorous design, the calculation may require vapor-liquid equilibrium data for the complete gas mixture, especially when ammonia, acid gases, or other soluble species are present.

Remove the condensed liquid

The condensed quantity is removed from the gas stream:

[ n_{\text{liquid}} = n_{\text{condensed}} ]

The remaining gas-phase component amounts are then used as the absorber inlet. The liquid product rate can also be converted to mass flow:

[ \dot{m}{\text{liquid}} = n{\text{liquid}}MW_{\text{liquid}} ]

This liquid may be water-rich, but it can also contain dissolved or entrained absorber solute.

Recalculate average molecular weight

The gas average molecular weight changes after condensation:

[ \overline{MW}_{\text{gas}} = \sum_i y_iMW_i ]

This affects mass-flow conversion, gas density, superficial velocity, and equipment calculations.

A calculation that retains the original average molecular weight after removing a major vapor component can produce inconsistent mass and volumetric flow rates.

Convert the gas flow to absorber conditions

The gas flow entering the absorber must be adjusted from the heat-exchanger outlet basis to the absorber operating conditions:

[ \dot{V}_{2}

\dot{V}{1} \left(\frac{T{2}}{T_{1}}\right) \left(\frac{P_{1}}{P_{2}}\right) ]

For nonideal gases, include the compressibility-factor correction:

[ \dot{V}_{2}

\dot{V}{1} \left(\frac{T{2}}{T_{1}}\right) \left(\frac{P_{1}}{P_{2}}\right) \left(\frac{Z_{2}}{Z_{1}}\right) ]

Temperatures must be absolute, and pressures must use a consistent absolute basis.

How the Absorber Balance Is Affected

The gas-side material balance starts at a new feed condition

For a solute transferred from gas to liquid:

[ n_{\text{solute,in}}

n_{\text{solute,out}} + n_{\text{solute,absorbed}} ]

The inlet term must represent the solute flow after upstream condensation. If the solute itself remains gaseous, its absolute molar flow may be approximately unchanged, but its mole fraction rises when a larger amount of carrier vapor has been removed.

The gas-to-liquid ratio changes

The absorber gas-to-liquid ratio is based on the revised gas flow and the actual absorbent flow:

[ \frac{G}{L}

\frac{\text{gas flow entering absorber}} {\text{liquid absorbent flow}} ]

Condensation usually lowers (G), unless the condensate is intentionally combined with the absorber liquid. This changes the operating line and can affect the required solvent circulation rate.

Solute concentration may increase or decrease

If only a carrier vapor such as water condenses, the remaining gas contains a higher mole fraction of the noncondensable solute. This can increase the apparent solute loading entering the absorber even though the absolute solute molar flow has not changed.

If the solute also condenses, dissolves into the condensate, or is removed by entrainment, its gas-phase molar flow decreases. The absorber balance must then include that upstream removal explicitly.

Absorber sizing can change

Gas flow affects column diameter through superficial gas velocity and flooding limits. A lower gas volume generally reduces the required cross-sectional area, but the revised solute concentration and thermodynamic driving force may change packing height, tray requirements, solvent circulation, or heat-removal needs.

The correct sizing basis is therefore the gas flow and composition at the absorber inlet, not the original hot-gas condition.

Interaction With Heat-Exchanger Performance

Condensation contributes to heat duty

When vapor condenses, the heat exchanger removes both sensible heat and latent heat:

[ Q = Q_{\text{sensible}} + Q_{\text{latent}} ]

The latent contribution is approximately:

[ Q_{\text{latent}} = n_{\text{condensed}}\Delta H_{\text{vap}} ]

Ignoring this term can underpredict the required cooling duty and misrepresent the achievable outlet temperature.

The liquid outlet must be accounted for

The heat exchanger may produce two outlet phases: a cooled gas and a condensate. A downstream separator, knockout drum, or properly designed drain is needed if the condensate is not intended to enter the absorber.

If liquid carryover occurs, the absorber feed is not a dry gas stream. The balance must include the liquid entering the column and any dissolved solute it contains.

Condensation affects pilot-plant measurements

Flow meters may report flow at different reference conditions, while wet-gas and dry-gas measurements may use different bases. Condensation between measurement points can create apparent discrepancies unless every stream is placed on the same temperature, pressure, and moisture basis.

For pilot work, record the heat-exchanger outlet temperature, pressure, condensate rate, gas flow basis, and whether the measured gas is wet or dry.

Understanding the Trade-offs

Assuming all condensable vapor is removed

Cooling below a component's saturation temperature does not necessarily mean complete removal. The outlet gas retains the equilibrium vapor amount unless a further condenser, separator, or drying step removes it.

Use the outlet equilibrium condition to calculate the residual vapor.

Treating the condensate as pure solvent

Condensate can contain absorbed ammonia, acid gas, dissolved salts, or entrained droplets. Treating it as pure water may overstate the solute flow reaching the absorber and understate the liquid product concentration.

Analyze or estimate condensate composition when the solute is highly soluble or reactive.

Using volumetric composition without a consistent basis

Wet-basis mole fractions, dry-basis mole fractions, actual volumetric flow, and standard volumetric flow are not interchangeable. Mixing these bases can invalidate both the heat-exchanger and absorber balances.

Perform the material balance in molar units first, then convert to mass or volume at the required conditions.

Assuming condensation always improves absorber performance

A smaller gas flow can reduce column size and solvent demand, but a higher solute concentration can increase local loading and alter the absorption driving force. Condensation can also create plugging, corrosion, flooding, or liquid-distribution problems if the liquid is not properly separated and managed.

The downstream equipment must be evaluated using both the revised flow rate and the revised composition.

Ignoring absorber temperature effects

The absorber may operate at a different temperature from the heat-exchanger outlet. Solubility, reaction equilibrium, vapor pressure, and gas volume can all change between these locations.

The absorber balance should therefore begin with the actual absorber inlet conditions, including any reheating, further cooling, evaporation, or liquid addition between units.

How to Apply This to Your Pilot Plant

Use a sequential calculation that follows the physical path of the stream from the heat exchanger to the absorber.

If your primary focus is heat-exchanger duty: Calculate the condensed vapor from the outlet equilibrium condition and include its latent heat in the cooling-duty balance.
If your primary focus is absorber sizing: Remove the condensate, recalculate gas molar and volumetric flow at absorber conditions, and use that revised flow for hydraulic design.
If your primary focus is solute removal: Track the solute in the gas, condensate, and absorber liquid separately so that upstream dissolution or condensation is not incorrectly credited to the absorber.
If your primary focus is pilot-plant data reconciliation: Express every measurement on a consistent molar, temperature, pressure, and wet/dry basis before comparing streams.
If your primary focus is liquid-product recovery: Measure or estimate condensate composition and include any dissolved absorber solute in the liquid product balance.

A reliable absorber design begins with the stream that actually reaches the absorber after upstream condensation, not with the original hot-gas feed.

Summary Table:

Area	Effect of Upstream Condensation	Required Action
Gas flow	Total gas molar flow decreases	Remove condensed vapor before calculating absorber feed
Gas composition	Remaining solute mole fractions may increase	Recalculate gas-phase composition
Molecular weight	Average gas molecular weight changes	Update mass flow, density, and velocity
Gas-to-liquid ratio	Gas flow and solvent demand may change	Recalculate the operating line and solvent rate
Absorber sizing	Diameter, packing height, or tray requirements may change	Use flow and composition at absorber inlet conditions
Heat-exchanger duty	Condensation adds latent heat duty	Include sensible and latent heat in the balance
Liquid management	Condensate may contain dissolved solute or entrained droplets	Use a separator and track condensate composition
Pilot measurements	Wet/dry and reference-condition differences can distort results	Reconcile all streams on a consistent molar basis

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