DOE Big Hill atmospheric fractionation qualification

Issue #3305 separates refinery validation into explicit quality gates. The first DOE Big Hill increment qualified refinery-assay bookkeeping. These benchmarks advance the next gate by sending public assay representations through NeqSim’s rigorous DistillationColumn and checking process-level conservation, boiling-order separation, and repeatability.

Public source and evidence boundary

The source is the U.S. Department of Energy Strategic Petroleum Reserve Big Hill Sweet, sample MLI 009, assay date 1998-05-04:

The DOE manual documents ASTM D2892 atmospheric/light-vacuum distillation followed by D5236 for the residuum. It states that the distillation fractions are measured on a mass-percent basis and that volume-percent values are calculated using each fraction’s specific gravity.

This benchmark uses the measured mass-yield basis for the five intervals that have finite lower and upper boiling boundaries in the public table:

Cut range (degF) Weight % of whole assay SG 60/60 F
175-250 8.6 0.7815
250-375 15.2 0.8305
375-530 15.2 0.8623
530-650 11.1 0.9226
650-1050 30.3 0.9477

The five bounded cuts are normalized on their own basis. The C5-/175 degF light tail and 1050 degF+ vacuum residuum are not included because assigning finite boiling limits to those open-ended fractions would invent data that the published table does not provide.

Accordingly, this first tier is an integration and numerical-robustness qualification, not an independent validation of full crude-column product yields. It does not claim that the normalized five-cut slate represents the complete Big Hill crude.

Column benchmark

DoeBigHillAtmosphericFractionationTest performs the following workflow using public NeqSim APIs:

  1. Build an SRK system from the normalized measured DOE mass yields, cut specific gravities, and finite boiling ranges using OilAssayCharacterisation.
  2. Generate the five real NeqSim TBP pseudo-components.
  3. Feed the resulting broad-boiling slate to an eight-tray DistillationColumn with a reboiler, partial condenser, and one liquid side draw.
  4. Solve at near-atmospheric pressure with the residual-monitored MESH_RESIDUAL column solver; the solved state must satisfy the active full-MESH residual gate and guarded fallback products are not accepted.
  5. Re-run the same initialized column to qualify repeatability rather than accepting a one-off solution.

The operating point is deliberately a reproducible screening case, not a reconstruction of a proprietary or historical refinery design:

Quantity Benchmark value
Feed flow 5000 kg/h
Feed temperature 550 K
Feed pressure 1.5 bara
Internal trays 8
Feed tray 4, bottom-up
Top pressure 1.2 bara
Bottom pressure 1.5 bara
Condenser mode Partial
Condenser temperature Solved; no fixed set point
Reboiler temperature 600 K
Condenser reflux ratio 1.0
Liquid side draw 10% of tray-4 liquid traffic

The condenser remains partial and no fixed condenser-temperature specification is imposed. Top pressure and reflux ratio define the terminal controls while the top temperature remains a solved equilibrium/energy-balance result. Fixing both condenser temperature and reflux ratio overconstrains this broad-boiling screening case and can leave a thermally converged-looking profile with open tray material balances. The 600 K equilibrium-reboiler set point lies inside the bounded assay range and below the representative normal-boiling temperature of the heaviest cut, preserving both vapor traffic and a positive heavy bottoms product.

MESH_RESIDUAL uses inside-out initialization followed by rigorous residual monitoring. Interior tray temperatures remain solver variables governed by the stage energy balances. No tuning to a commercial process simulator is used.

Acceptance contract

The regression requires all of the following on each accepted solve:

The 5% process-balance gates are screening tolerances for this first broad-boiling integration case. They are intentionally much looser than the 1e-10 pseudo-component creation mass-closure gate because the column itself is an iterative process solver. Tighter refinery-specific balance gates should be introduced only after this public heavy-slate case establishes a stable baseline.

Complete modeled slate benchmark

DoeBigHillCompleteAtmosphericFractionationTest adds a second integration tier using the reusable DoeBigHillSweetAssay factory. Its primary sources are the official DOE SPR Big Hill Sweet comprehensive assay, reported 24 September 2021, and companion PIANO workbook.

The complete modeled feed contains all 12 components qualified by the characterization campaign:

The light-end allocation and zero sulfur or nitrogen values used where DOE leaves a cell blank remain explicit modeling assumptions. They are not additional measurements.

The complete-slate screening point deliberately reuses the proven bounded-column topology:

Quantity Complete-slate benchmark value
Feed flow 5000 kg/h
Feed temperature 550 K
Feed pressure 1.5 bara
Internal trays 8
Feed tray 4, bottom-up
Top pressure 1.2 bara
Bottom pressure 1.5 bara
Condenser mode Partial
Reboiler temperature 650 K
Condenser reflux ratio 1.0
Liquid side draw 10% of tray-4 liquid traffic

The test requires all 12 components to reach the column, a non-fallback MESH_RESIDUAL solution, positive overhead/side-draw/bottoms products, external mass closure, enforced energy closure, internal tray material closure, and per-component molar conservation. It also requires C2-C4 enrichment toward the overhead, 1050 degF+ enrichment toward the bottoms, composition-weighted normal-boiling-point ordering of overhead < side draw < bottoms, and 1% repeated-solve agreement for product flows and boiling descriptors. A 120 s timeout fails closed on a solver stall.

The complete-slate case is still an integration qualification. The DOE workbooks do not publish a matching atmospheric-column tray count, feed condition, pressure profile, furnace duty, stripping steam, reflux, pump-around duties, or product specifications. Therefore agreement with the DOE assay cut table would not by itself validate simulated plant product yields.

What this benchmark advances

This increment exercises, in one regression:

DOE assay facts -> reusable complete modeled slate -> standard and TBP components -> Stream -> rigorous DistillationColumn -> three refinery-style product draws

That closes an important integration gap between the characterization foundation and the refinery fractionation workstream. It also protects against future changes that would make heavy pseudo-components impossible to use in a near-atmospheric column even when assay bookkeeping still passes.

Remaining scientific gaps

These benchmarks do not validate:

The next scientific gate should use a public atmospheric operating case that includes enough column design and operating information to compare product yields and boiling ranges without tuning to an under-specified target. Only after that gate should #3305 advance to vacuum fractionation or conversion-unit models.