Neqsim Pseudocomponent Split Characterization

Pseudocomponent Split-Factor Characterization

Use this skill to represent a reservoir fluid with a small, controllable number of adjustable factors instead of building a bespoke characterization for every fluid. It provides three plant-agnostic, dependency-free building blocks:

1. A Whitson three-parameter gamma molar split of a plus fraction (C7+), governed by a single split/characterization factor alpha.
2. A lumping split factor computed from a detailed reference composition.
3. A delumping reconstruction that turns a lumped composition back into detailed components using that split factor.

These are screening-level helpers. For design-grade work, move to the rigorous NeqSim neqsim.thermo.characterization Java classes described below.

When to Use

• When a fluid must be described by detailed light components plus a heavy pseudocomponent set, and you want one factor to control the heavy-end split.
• When you have a reference fluid and want to generate representative or synthetic fluids by adjusting the split factor.
• When a lumped composition (few components) must be delumped back to a detailed composition using the internal distribution of a reference fluid.
• When you need a transparent, reproducible split before running the rigorous NeqSim characterization for design-grade work.
• When a fluid is represented on a universal Paraffinic-Aromatic (P/A) set (fixed 10 light + N paraffinic + N aromatic heavy lumps) and its heavy-end character is carried by a single split factor S (the paraffinic fraction of each heavy lump), per the Uleberg (2026) universal characterisation.

Inputs

• z_plus: total mole fraction of the plus fraction, in (0, 1].
• m_plus: average molar mass of the plus fraction (g/mol).
• boundaries: increasing molar-mass boundaries (g/mol); n+1 values give n pseudocomponents. The last boundary may be math.inf.
• alpha: gamma shape / split factor (> 0; 1.0 = exponential heavy end).
• eta: minimum molar mass of the distribution (g/mol).
• full_composition and lumping_scheme for the split-factor / delumping path.

Outputs

• GammaSplitResult: per-pseudocomponent mole fractions and average molar masses.
• SplitFactorResult: per-component split factors and per-lump totals.
• Delumped detailed composition (tuple of mole fractions).

Engineering Method

Whitson gamma split

The heavy end is described by the three-parameter gamma probability density

$$ p(M) = \frac{(M-\eta)^{\alpha-1},\exp!\left(-\frac{M-\eta}{\beta}\right)}{\beta^{\alpha},\Gamma(\alpha)}, \qquad \beta = \frac{M_{+}-\eta}{\alpha} $$

where M is molar mass, eta the minimum molar mass, alpha the split factor, and M+ the plus-fraction average molar mass. Mole fractions in each molar-mass interval are obtained from the regularized lower incomplete gamma function P(alpha, y), and each pseudocomponent's average molar mass from P(alpha+1, y). alpha = 1 yields an exponential (Pedersen-like) distribution; higher alpha narrows the distribution and lightens the tail.

Lumping split factor and delumping

For each lump, the split factor of a detailed component is its mole fraction divided by the lump total (each lump's factors sum to 1). A lumped composition is delumped by multiplying each lump value by its component split factors. This mirrors FluidMagic's EOSConverter.calculate_split_factor / _delump.

Universal Paraffinic-Aromatic (P/A) split factor S

The universal characterisation of Uleberg (2026) keeps one fixed component set (10 light + N paraffinic + N aromatic heavy lumps) and carries each feed's heavy-end character in a single split factor. For heavy lump i with total mole fraction Z_i, the split factor S_i (the paraffinic fraction) divides it into a paraffinic copy P_i = S_i Z_i and an aromatic copy A_i = (1 - S_i) Z_i, so moles are conserved. The recommended operational form is a single constant S applied to every heavy lump; a general two-endpoint form interpolates S_i linearly in molecular weight between S1 (lightest heavy lump) and Sn (heaviest), clipped to [eps, 1-eps]. For a feed with no separator calibration, S is assigned provisionally from the screening correlation S ~= 1.3298 - 0.003531 * MW_C7+ (MW in g/mol; clipped). S calibrates the heavy-end stock-tank-oil density (and only weakly GOR); the light/heavy molar ratio — hence the GOR — is set by the feed's own composition (its ZI), not by S. The fixed critical properties, acentric factors, volume shifts and BIPs of the paraffinic and aromatic families live in the universal EOS; this skill only performs the molar split.

Python Usage Pattern

import math
from pseudocomponent_split import (
    gamma_mole_split,
    calculate_split_factor,
    delump_composition,
)

# 1. Split a C7+ plus fraction into 4 pseudocomponents with split factor alpha.
split = gamma_mole_split(
    z_plus=0.05,
    m_plus=220.0,
    boundaries=[90.0, 140.0, 200.0, 300.0, math.inf],
    alpha=1.0,
    eta=90.0,
)
print(split.mole_fractions, split.molar_masses)

# 2. Split factor from a detailed reference fluid, then delump a lumped fluid.
full = [0.70, 0.10, 0.06, 0.04, 0.03, 0.07]
scheme = [[0], [1, 2], [3, 4, 5]]
sf = calculate_split_factor(full, scheme)
detailed = delump_composition(list(sf.lump_totals), sf.split_factors, scheme)

Universal P/A split factor S

from pseudocomponent_split import (
    c7plus_split_correlation,
    constant_pa_split,
    apply_constant_split_to_vector,
)

# Provisional constant S when a feed has no separator calibration.
S = c7plus_split_correlation(mw_c7plus=210.0)   # ~0.588, clipped to [0.02, 0.98]

# Split heavy-lump totals into paraffinic / aromatic copies (P_i, A_i).
core_totals = [0.02, 0.015, 0.01, 0.005]
paraffinic, aromatic = constant_pa_split(core_totals, S)

# Re-split a fluid already on a universal P/A set (names end in P / A) with a
# new constant S; light components pass through unchanged, moles conserved.
names = ["N2", "C1", "C7P", "C7A", "C8P", "C8A"]
zi = [0.02, 0.90, 0.03, 0.01, 0.03, 0.01]
zi_new = apply_constant_split_to_vector(names, zi, split_factor=0.5)

Map a feed's own composition (its ZI) onto the universal P/A cores by nearest molecular weight (paper Section 2.2), so the light/heavy ratio — hence the GOR — comes from the feed, not from a template:

from pseudocomponent_split import map_source_to_pa_core_totals

m = map_source_to_pa_core_totals(
    source_names=["C1", "CO2", "C7", "C10", "C30"],
    source_fractions=[0.80, 0.02, 0.10, 0.05, 0.03],
    source_molecular_weights=[16.0, 44.0, 96.0, 140.0, 400.0],
    light_names=["N2", "CO2", "C1", "C2", "C3"],
    core_labels=["C7", "C10-14", "C31-50"],
    core_molecular_weights=[96.0, 150.0, 500.0],
)
# m.light_fractions -> light slots; m.core_totals -> heavy-lump Z_i to split by S

Related NeqSim Functionality

For rigorous, design-grade characterization use the NeqSim Java classes in neqsim.thermo.characterization:

• PlusFractionModel — gamma molar distribution with alpha, eta, and a Watson Kw-based auto-estimate of alpha (estimateAlpha, setAlpha, setEta, setGammaParameters).
• PlusCharacterize / Characterise — drive the plus-fraction split from SystemInterface composition and plus-fraction molar mass / density.
• TBPCharacterize / TBPfractionModel — true-boiling-point pseudocomponents.
• LumpingModel + LumpingConfigBuilder and PseudoComponentCombiner — lumping of detailed components into pseudocomponents.
• Recombine — recombine separator gas and oil to a reservoir fluid.

In Python these are reachable through the neqsim package (for example from neqsim import jneqsim).

Validation Checklist

• Plus-fraction mole fraction (z_plus), molar mass (m_plus), eta, and the molar-mass boundaries are stated with sources.
• The split factor alpha is documented, and the resulting pseudocomponent mole fractions sum to z_plus.
• Per-pseudocomponent average molar masses increase monotonically and the mole-fraction-weighted average is close to m_plus.
• For delumping, each lump's split factors sum to 1 and a round-trip reconstruction reproduces the reference composition.
• Assumptions and limitations are recorded and qualified PVT review is planned.

Common Mistakes

• Choosing boundaries below eta, or a non-increasing boundary list (raises).
• Setting m_plus at or below eta (the mean must exceed the minimum).
• Treating the screening split as a tuned EOS; it sets a distribution, not critical properties.
• Reusing one lump's split factor for a different reference fluid whose internal distribution differs.

Limitations

• The gamma split and split factor are screening-level. They do not tune an EOS, set critical properties, or guarantee phase-behavior accuracy.
• Boundaries, eta, and alpha must be chosen with engineering judgement.
• Results must be reviewed by a qualified PVT engineer before design or operational use.

References

• Uleberg, K. (2026). Legacy Common-Slate vs. Universal Paraffinic-Aromatic Fluid Characterisation in Process Simulation: A Sleipner A Multi-Feed Case Study, Equinor ASA (universal P/A split factor S, Eqs. 3-4, Section 2.2).
• Whitson, C.H., Brulé, M.R. (2000). Phase Behavior, SPE Monograph 20.
• Pedersen, K.S., Christensen, P.L., Shaikh, J.A. (2015). Phase Behavior of Petroleum Reservoir Fluids, 2nd ed.
• NeqSim: https://github.com/equinor/neqsim