NeqSim’s distillation package provides equilibrium-stage columns, shortcut design, hydraulic rating, internal recycles, side products, and a separate rate-based packed column model for absorption and stripping. The main implementation lives in neqsim.process.equipment.distillation.

Capability Map

Area Main APIs Notes
Rigorous staged columns DistillationColumn, SimpleTray, Condenser, Reboiler Equilibrium-stage MESH-style model with tray-by-tray flash calculations.
Solvers SolverType Direct, damped, inside-out, adaptive matrix inside-out, Wegstein, sum-rates, Newton temperature correction, Naphtali-Sandholm, MESH residual, and AUTO with candidate tracing.
Formal specifications ColumnSpecification, convenience setters Product purity, component recovery, reflux ratio, product flow rate, duty specifications, and staged specification homotopy for difficult product targets.
Side products setGasSideDrawFraction, setLiquidSideDrawFraction, addSideDrawFlowSpecification Side draws are external product streams and are included in outlet stream and mass-balance reporting.
Pumparounds addLiquidPumparound Internal liquid draw/return circuits solved as column tear variables.
Hardware modes CondenserMode, ReboilerMode Partial or total condenser, fixed liquid reflux split, equilibrium reboiler, and vapor boilup ratio mode.
Hydraulics and sizing calcColumnInternals, enableHydraulicPressureDropCoupling Rates tray or packing hydraulics and can couple total pressure drop back into the pressure profile.
Efficiency setMurphreeEfficiency, setMurphreeEfficiencies Column-wide and per-stage Murphree vapor efficiency correction.
Shortcut design ShortcutDistillationColumn, initializeFromShortcut Fenske-Underwood-Gilliland estimates and rigorous-column initialization.
Tray optimization findOptimalNumberOfTrays, findEconomicOptimalTrayConfiguration Searches tray count and feed tray, optionally with economic ranking.
Dynamics runTransient, DynamicColumnModel.EXPERIMENTAL_EULER Explicit-Euler holdup screening model. It is not a rigorous DAE dynamic column model.
Rate-based packed columns RateBasedPackedColumn Segment-based packed absorption/stripping with film mass transfer, heat transfer, packing hydraulics, and JSON diagnostics.

Tray Numbering

The DistillationColumn constructor is:

DistillationColumn column = new DistillationColumn(name, simpleTrayCount, hasReboiler,
    hasCondenser);

The simpleTrayCount argument excludes the optional reboiler and condenser. Tray indices used by addFeedStream, side draws, pumparounds, and per-stage efficiency are internal bottom-up stage indices:

Hardware Internal index convention
Reboiler present Index 0 is the reboiler.
Simple trays Above the reboiler, increasing upward.
Condenser present Last internal index is the condenser.

For example, new DistillationColumn("T-100", 10, true, true) creates 12 internal stages: reboiler, 10 simple trays, and condenser.

Basic Usage

import neqsim.process.equipment.distillation.DistillationColumn;
import neqsim.process.equipment.stream.StreamInterface;

DistillationColumn column = new DistillationColumn("Deethanizer", 10, true, true);
column.addFeedStream(feedStream, 5);
column.setTopPressure(25.0);
column.setBottomPressure(26.0);
column.setCondenserTemperature(273.15 - 10.0);
column.setReboilerTemperature(273.15 + 105.0);
column.setSolverType(DistillationColumn.SolverType.INSIDE_OUT);
column.run();

StreamInterface overhead = column.getGasOutStream();
StreamInterface bottoms = column.getLiquidOutStream();

Use getInletStreams() and getOutletStreams() for process-topology introspection. Outlet streams include the terminal products plus any configured non-zero side-draw product streams.

Builder Pattern

The fluent builder covers the common rigorous-column setup path. Use the constructor when only one end of the column has hardware, because the builder currently exposes withCondenserAndReboiler() as the hardware shortcut.

import neqsim.process.equipment.distillation.ColumnSpecification;
import neqsim.process.equipment.distillation.DistillationColumn;

DistillationColumn column = DistillationColumn.builder("Deethanizer")
    .numberOfTrays(15)
    .withCondenserAndReboiler()
    .topPressure(25.0, "bara")
    .bottomPressure(26.0, "bara")
    .temperatureTolerance(1.0e-3)
    .massBalanceTolerance(1.0e-2)
    .maxIterations(100)
    .insideOut()
    .internalDiameter(2.5)
    .addFeedStream(feedStream, 8)
    .topProductPurity("ethane", 0.95)
    .bottomSpecification(new ColumnSpecification(
        ColumnSpecification.SpecificationType.PRODUCT_PURITY,
        ColumnSpecification.ProductLocation.BOTTOM, 0.98, "propane"))
    .build();
Builder method Description
numberOfTrays(int) Number of simple trays, excluding condenser and reboiler.
withCondenserAndReboiler() Adds both terminal hardware items.
topPressure(double, String), bottomPressure(double, String), pressure(double, String) Sets endpoint pressures.
temperatureTolerance(double), massBalanceTolerance(double), tolerance(double) Sets convergence tolerances.
maxIterations(int) Sets requested solver iterations.
dampedSubstitution(), insideOut(), autoSolver() Selects common solver strategies.
relaxationFactor(double) Starting damping factor for damped substitution.
internalDiameter(double) Internal diameter in metres.
addFeedStream(StreamInterface, int) Adds a feed to an internal tray index.
topProductPurity(String, double) Adds a top product purity specification.
bottomSpecification(ColumnSpecification) Adds any bottom-end specification object.
build() Creates the configured column.

Operating Specifications

Direct operating specifications are applied before the inner column solver runs.

column.setTopPressure(25.0);
column.setBottomPressure(26.0);
column.setCondenserTemperature(263.15);
column.setReboilerTemperature(378.15);
column.setCondenserRefluxRatio(3.0);
column.getCondenser().setHeatInput(-5.0e6);
column.getReboiler().setHeatInput(6.0e6);
column.setReboilerBoilupRatio(2.5);

Product-quality and recovery targets use ColumnSpecification. Purity and recovery targets are dimensionless fractions from 0 to 1; product-flow-rate specifications are evaluated internally in mol/hr.

column.setTopProductPurity("ethane", 0.95);
column.setBottomProductPurity("propane", 0.98);
column.setTopComponentRecovery("ethane", 0.99);
column.setBottomComponentRecovery("propane", 0.99);
column.setBottomProductFlowRate(1000.0, "mol/hr");

ColumnSpecification topFlow = new ColumnSpecification(
    ColumnSpecification.SpecificationType.PRODUCT_FLOW_RATE,
    ColumnSpecification.ProductLocation.TOP, 500.0);
topFlow.setTolerance(1.0e-3);
topFlow.setMaxIterations(30);
column.setTopSpecification(topFlow);

For iterative specifications, NeqSim wraps the selected inner solver in an outer adjustment loop and uses condenser or reboiler temperature as the manipulated variable where possible. Product purity, component recovery, and product-flow specifications can be staged with setSpecificationHomotopySteps(steps). Values above one ramp the effective target from the current product value to the final user target, leaving the stored ColumnSpecification unchanged. When AUTO is selected and an adjustable product specification is active, NeqSim uses three homotopy stages by default unless the user has configured another stage count.

Condenser and Reboiler Modes

column.setCondenserMode(DistillationColumn.CondenserMode.PARTIAL);
column.setCondenserMode(DistillationColumn.CondenserMode.TOTAL);
column.setCondenserLiquidReflux(500.0, "kg/hr");
DistillationColumn.CondenserMode condenserMode = column.getCondenserMode();

column.setReboilerMode(DistillationColumn.ReboilerMode.EQUILIBRIUM);
column.setReboilerVaporBoilupRatio(1.8);
DistillationColumn.ReboilerMode reboilerMode = column.getReboilerMode();

setCondenserLiquidReflux(value, unit) configures the LIQUID_REFLUX_SPLIT mode. Use it instead of calling setCondenserMode(LIQUID_REFLUX_SPLIT) directly because the fixed reflux flow is required.

Solver Options

Solver type Strategy Typical use
DIRECT_SUBSTITUTION Classic tray-by-tray substitution. Default for simple, well-posed columns.
DAMPED_SUBSTITUTION Sequential substitution with an initial fixed relaxation factor. Stiffer cases where direct substitution overshoots.
INSIDE_OUT Inside-out style flow correction with K-value tracking and polishing. General deethanizer/depropanizer and multi-feed work.
MATRIX_INSIDE_OUT Adaptive matrix warm start plus rigorous inside-out polishing. Larger hydrocarbon fractionators where matrix setup cost is justified.
WEGSTEIN Accelerated successive substitution after warm-up. Well-conditioned fixed-point problems.
SUM_RATES Flow-corrected tearing method. Absorbers, strippers, and flow-sensitive columns.
NEWTON Tray-temperature Newton accelerator. Difficult temperature convergence. It is not full simultaneous MESH Newton.
NAPHTALI_SANDHOLM Guarded simultaneous correction of MESH blocks after inside-out warm start. Residual-driven hydrocarbon fractionators.
MESH_RESIDUAL Inside-out initialization plus full residual auditing. Material, equilibrium, summation, energy, product-draw, and spec residual checks.
AUTO Runs a feasibility pre-screen, initializes a copied candidate, solves a relaxed damped base case, probes candidate strategies on column copies, and accepts the first solved non-fallback candidate or the best valid fallback. Agent workflows and uncertain cases where robust automatic selection and diagnostics are useful. 
column.setSolverType(DistillationColumn.SolverType.AUTO);
column.run();
DistillationColumn.SolverType selected = column.getLastSolverTypeUsed();

AUTO keeps the requested solver type as AUTO, while getLastSolverTypeUsed() reports the concrete solver that completed the run. Inspect getLastAutoSolverSummary() or getConvergenceDiagnostics() to see the feasibility pre-screen, candidate list, residuals, iteration counts, solve times, and fallback notes. For product-specification cases, also inspect getLastSpecificationHomotopyStepCount() to confirm whether staged continuation was used.

Side Draws

Side draws withdraw a fraction of tray vapor or liquid traffic. They are true external product streams: getOutletStreams() includes them, and getMassBalance(unit) subtracts them from the feed-product balance.

column.setGasSideDrawFraction(6, 0.05);
column.setLiquidSideDrawFraction(4, 0.10);

StreamInterface gasSideDraw = column.getSideDrawStream(6,
    DistillationColumn.SideDrawPhase.GAS);
List<StreamInterface> allSideDraws = column.getSideDrawStreams();

For target side-product flow rates, use side-draw flow specifications. The column adjusts the corresponding tray split fraction as an outer tear variable.

DistillationColumn.ColumnSideDrawSpecification spec = column.addSideDrawFlowSpecification(6,
    DistillationColumn.SideDrawPhase.GAS, 100.0, "kg/hr");
spec.setTolerance(1.0e-4);
spec.setMaxIterations(15);
column.setMaxColumnTearIterations(20);
column.setColumnTearTolerance(1.0e-4);
column.run();

double actualDraw = spec.getLastActualFlowRate();
double drawResidual = spec.getLastRelativeResidual();
boolean tearConverged = column.isLastColumnTearConverged();

If the requested side-draw flow is physically impossible, the split is bounded by available tray traffic and the latest tear-variable diagnostics report non-convergence.

Pumparounds

Liquid pumparounds are internal draw/return circuits. They are not external products and do not appear in getOutletStreams().

DistillationColumn.ColumnPumparound pumparound = column.addLiquidPumparound("PA-1", 4, 6,
    0.15, 10.0);
column.setMaxPumparoundIterations(12);
column.setPumparoundTolerance(1.0e-4);
column.run();

StreamInterface returnStream = pumparound.getReturnStream();
double latestChange = column.getLastPumparoundRelativeChange();

The temperatureDrop argument is in Kelvin. Positive values cool the returned liquid; negative values heat it. A non-finite or below-zero-K return temperature fails explicitly.

Hydraulics and Pressure-Drop Coupling

calcColumnInternals() evaluates tray or packing hydraulics for the latest column state.

import neqsim.process.equipment.distillation.internals.ColumnInternalsDesigner;

column.setInternalDiameter(2.5);
ColumnInternalsDesigner designer = column.calcColumnInternals("sieve");
double totalPressureDropPa = designer.getTotalPressureDrop();

Hydraulic coupling is opt-in. When enabled, the column rates internals, converts total hydraulic pressure drop to a linear pressure profile between top and bottom, and re-solves until the hydraulic tear variable is within tolerance.

column.enableHydraulicPressureDropCoupling("sieve");
column.run();
double coupledPressureDropPa = column.getLastHydraulicPressureDropPa();
double hydraulicResidual = column.getLastHydraulicPressureDropResidual();

Supported internals names include common tray types such as "sieve", "valve", "bubble-cap", and packed-column mode "packed", depending on available internals data.

Efficiency and Internals

Murphree vapor efficiency can be set globally or per stage.

column.setMurphreeEfficiency(0.70);
column.setMurphreeEfficiency(3, 0.65);
double stageEfficiency = column.getMurphreeEfficiency(3);
column.clearPerStageMurphreeEfficiency();

Use mechanical-design classes and calcColumnInternals() for actual-tray counts, HETP-style height estimates, flooding, weeping, entrainment, downcomer backup, and pressure-drop checks. The findEconomicOptimalTrayConfiguration methods also use tray efficiency to convert theoretical stages to actual trays for cost ranking.

Shortcut Initialization and Tray Optimization

ShortcutDistillationColumn provides Fenske-Underwood-Gilliland estimates. A rigorous column can use those estimates directly as a starting design.

DistillationColumn.ShortcutInitializationResult init = column.initializeFromShortcut(feedStream,
    "ethane", "propane", 0.98, 0.98, 1.3);
if (init.isFeasible()) {
  column.setSolverType(DistillationColumn.SolverType.INSIDE_OUT);
  column.run();
}

Search utilities are available for tray count, feed tray, and economic ranking.

column.setMaxTrayOptimizationCandidates(200);
column.setMaxTrayOptimizationTimeSeconds(20.0);
int trays = column.findOptimalNumberOfTrays(0.95, "ethane", true, 30);
DistillationColumn.EconomicTrayOptimizationResult economic =
    column.findEconomicOptimalTrayConfiguration(0.95, "ethane", true, 30);

Optimization mutates the column to the selected feasible candidate. Capture or copy the model first if the current tray count must be preserved.

Dynamic Screening Model

The current distillation dynamics are explicitly experimental: getDynamicColumnModel() returns DynamicColumnModel.EXPERIMENTAL_EULER, and isDynamicColumnModelExperimental() returns true.

column.setDynamicColumnEnabled(true);
column.setDynamicEnergyEnabled(true);
column.setTrayWeirHeight(0.05);
column.setTrayWeirLength(1.0);
column.setTrayDryPressureDrop(200.0);

The transient model uses explicit-Euler tray holdup updates, Francis-weir liquid overflow, optional per-tray energy tracking, and a simplified vapor hydraulic relation. Treat it as a screening tool for qualitative inventory response, not as a replacement for commercial DAE dynamic simulators in control-system design or safety-critical trip studies.

Rate-Based Packed Column

Use RateBasedPackedColumn for counter-current packed absorption and stripping when equilibrium stages are not the right model.

import neqsim.process.equipment.distillation.RateBasedPackedColumn;

RateBasedPackedColumn absorber = new RateBasedPackedColumn("CO2 absorber", gasIn, liquidIn);
absorber.setColumnDiameter(1.2);
absorber.setPackedHeight(6.0);
absorber.setNumberOfSegments(12);
absorber.setPackingType("Pall-Ring-50");
absorber.setTransferComponents("CO2");
absorber.run();

StreamInterface treatedGas = absorber.getGasOutStream();
String report = absorber.toJson();

The rate-based model exposes segment profiles, component-transfer totals, pressure-drop and flood fraction diagnostics, film/heat-transfer model choices, and equation-oriented residual diagnostics.

Diagnostics and Results

Getter Description
solved() Current convergence flag.
getLastSolverTypeUsed() Concrete solver that completed the latest run, especially useful when requested solver is AUTO.
getLastSolveStatus() Strict solve status: rigorous convergence, reconciled products, fallback products, failure, or not run.
getLastSolveStatusReason() Concise explanation for fallback or rejected candidate states.
getLastAutoSolverSummary() Candidate trace from AUTO, including the feasibility pre-screen and per-candidate residual metrics.
getLastIterationCount() Inner solver iteration count.
getLastSolveTimeSeconds() Latest solve wall time.
getLastTemperatureResidual() Average tray-temperature residual in Kelvin.
getLastMassResidual() Relative mass-balance residual.
getLastEnergyResidual() Relative enthalpy-balance residual.
getLastTopSpecificationResidual(), getLastBottomSpecificationResidual() Endpoint spec errors.
getLastSpecificationResidual() Maximum absolute endpoint spec error.
getSpecificationHomotopySteps() Configured number of staged continuation targets for adjustable product specifications.
getLastSpecificationHomotopyStepCount() Number of specification continuation stages completed by the latest solve.
getLastColumnTearIterationCount() Outer side-draw, pumparound, and hydraulic tear iterations.
getLastColumnTearResidual() Maximum outer tear residual.
isLastColumnTearConverged() Whether active side-draw, pumparound, and hydraulic tear variables met tolerance.
getLastPumparoundRelativeChange() Maximum latest pumparound return-flow change.
getLastHydraulicPressureDropPa() Latest coupled hydraulic pressure drop in Pa.
getLastHydraulicPressureDropResidual() Relative pressure-profile change from hydraulic coupling.
getLastMeshResidualNorm() Full scaled MESH residual infinity norm.
getLastMeshMaterialResidualNorm() Component material residual norm.
getLastMeshEquilibriumResidualNorm() Phase-equilibrium residual norm.
getLastMeshSummationResidualNorm() Mole-fraction summation residual norm.
getLastMeshEnergyResidualNorm() Tray energy residual norm.
getLastMeshProductDrawResidualNorm() Terminal product-draw residual norm.
getLastMeshSpecificationResidualNorm() Active endpoint specification residual norm.
getLastMeshResidualVector() Copy of the complete scaled residual vector.

The MESH residual gate is diagnostic-only for legacy sequential solvers by default. It is effective by default for NAPHTALI_SANDHOLM and MESH_RESIDUAL; call setEnforceMeshResidualTolerance(true) to make it part of the convergence contract for other solvers.

Common Workflows

NGL Fractionation

  1. Build the feed with an EOS suitable for the hydrocarbon range, set mixing rule, and run the feed stream.
  2. Initialize with ShortcutDistillationColumn or initializeFromShortcut when light and heavy keys are known.
  3. Run INSIDE_OUT or AUTO first; use MESH_RESIDUAL or NAPHTALI_SANDHOLM for residual audit.
  4. Add product purity or component recovery specs.
  5. Rate internals and enable hydraulic pressure-drop coupling only after a stable base case exists.

Absorber or Stripper

Use DistillationColumn without condenser/reboiler for simple equilibrium-stage absorber or stripper studies, or RateBasedPackedColumn for packed mass-transfer studies.

DistillationColumn absorber = new DistillationColumn("Absorber", 10, false, false);
absorber.addFeedStream(gasStream, 0);
absorber.addFeedStream(leanSolvent, 9);
absorber.setSolverType(DistillationColumn.SolverType.SUM_RATES);
absorber.run();

For a stripper with a reboiler and no condenser, use new DistillationColumn("Stripper", 8, true, false).

Troubleshooting

Symptom Recommended checks
No convergence Verify tray numbering, feed condition, endpoint temperatures, pressure profile, and component list. Start with DIRECT_SUBSTITUTION, DAMPED_SUBSTITUTION, or AUTO, then inspect getConvergenceDiagnostics().
Oscillation Reduce aggressive condenser/reboiler specs, set a lower relaxation factor, or use DAMPED_SUBSTITUTION.
Specification does not close Check getLastTopSpecificationResidual(), getLastBottomSpecificationResidual(), feasible product split, and whether the required condenser/reboiler exists. For sharp purity, recovery, or product-flow targets, use setSpecificationHomotopySteps(steps) or AUTO.
Side-draw spec reports non-converged The target may exceed available tray traffic or feed component inventory. Inspect getSideDrawStream(...), side-draw fraction, and getLastColumnTearResidual().
Pumparound fails Check draw fraction, tray numbers, and return temperature. A return below 0 K is rejected.
Hydraulic coupling fails Run without coupling first, set a positive internal diameter, and verify calcColumnInternals(...) succeeds for the selected internals type.
Unexpected dynamic result Confirm the model is acceptable for screening. The current formulation is EXPERIMENTAL_EULER, not a rigorous industrial DAE.