Refinery Units at a Glance

Refinery Units at a Glance#

A one-page map of the difflow_refinery plugin: every unit, what it takes and makes, what model it is, and how far it has been validated. The full treatment (equations, specs, references) is in Refinery Unit Operations; each row below links to its section there.

How the units connect#

                 crude assay (TBP, SG, S, N, CCR, Ni+V)
                              │  characterize + composition
                              ▼
 tank ─► preheat train ─► desalter ─► preflash ─► furnace + atmospheric column (CDU)
                                                    │
     ┌──────────────┬──────────────┬────────────────┼────────────────┐
  light ends     naphtha        kerosene / diesel          atmospheric residue
     │              │                    │                          │
  gas plant     ┌───┴────────┐       hydrotreater             vacuum unit (VDU)
     │       light naphtha  heavy naphtha  │                  ┌─────┴─────┐
     │          │              │           │                 VGO       vacuum residue
     │    isomerization   reformer ◄─ H2 ─►│                ┌─┴────────┐
     │          │              │           │               FCC     hydrocracker
     │          │              │           │                │          │
     │          │              │           │      C3/C4 olefins    jet / diesel
     │          │              │           │           │
     │          │              │           │      alkylation
     ▼          ▼              ▼           ▼           ▼
   LPG     isomerate      reformate    ULSD / jet    alkylate ──► blend pools
                                                                 (gasoline, jet,
                                                                  ULSD, fuel oil)

Every arrow is a differentiable connection: a product property has an exact gradient with respect to the assay data, the specs and the operating variables upstream of it.

The hydroskimming part of the map runs end to end in examples/40_refinery_flowsheet.ipynb: CDU, gas plant, naphtha hydrotreater and its fractionator, reformer, distillate hydrotreater and its fractionator, residue desulfurizer, the hydrogen header with a hydrogen plant, and the four pools. Every connection there is now a library one (#326 to #333). The example also takes one gradient across the naphtha hydrotreater, the reformer and the gasoline pool with difflow_refinery.plant (#334), checked against central differences. In that refinery the jet, ULSD and fuel oil (0.31 wt% S) make spec, and the gasoline is 7 RON short (no isomerization unit).

Shared foundations#

These are not units, but every unit reads them.

Piece	Module	What it does
Assay and characterization	`Assay`, `characterize`	A TBP curve and gravity cut into pseudo-components (Twu 1984 critical properties by default). With a `HeavyEnd`, the curve extends into the vacuum range, closed by a residue lump, with sulfur, nitrogen, CCR, Ni+V and asphaltenes carried per cut. One characterization serves the CDU, the VDU and the blend pool. Details
Composition	`difflow_refinery.composition`	Per pseudo-component: paraffin/naphthene/aromatic/olefin volume fractions, hydrogen content, five sulfur classes and basic/non-basic nitrogen. Estimated by Riazi-Daubert or n-d-M; measured PIONA, SARA or hydrogen data overrides the estimate per cut. Details
Correlations	`difflow_refinery.correlations`	Twu, Riazi-Daubert, Lee-Kesler, Kesler-Lee and Maxwell-Bonnell, each written once.
Column thermodynamics	`ColumnThermo`	Raoult’s law with Lee-Kesler vapour pressures and ideal-gas-path enthalpies, vectorised over stages and components.
Stage-network column	`vacuum.StageColumn`	The equation-oriented column the VDU, the gas plant and the hydrotreater’s stripper are built on. Details
Hydroprocessing blocks	`difflow_refinery.hydroprocessing`	A trickle-bed reactor around any kinetic model, a Peng-Robinson high-pressure separator, the recycle-gas loop and a steam stripper. Shared by the hydrotreater and the hydrocracker. Details
Chaining units	`difflow_refinery.plant`	`Chain`, `Stage` and `AD_MODES`: library units composed into one differentiable function. It uses one `jax.jacfwd`/`jacrev` when every unit supports that mode, and the chain rule by unit Jacobians when they do not (the reformer is forward-only, a default hydrotreater reverse-only). The AD-mode table of every unit and the measured compile costs are under Details.

The units#

Palette means the unit is registered with the editor and can be placed in a Flowsheet as an operation. A library unit is called from Python (it is still differentiable and usable inside a Flowsheet function); it has no palette entry.

Planning names the function that wraps the unit as a block for delta-base planning (difflow.planning).

Separation and feed preparation#

Unit	Main class (palette name)	In → out	Model	Planning	Validation	Example
Preheat train	`PreheatedCrudeUnit` (palette: `Desalter`, `PreflashDrum`, `CrudeUnitWithPreheat`)	crude from the tank → crude at the furnace inlet	Exchanger train heated by the column’s pumparounds and products, solved together with the column; desalter; three-phase preflash drum; Ebert-Panchal fouling	`cdu_block`	Drum and exchangers against IDAES 2.10 unit models on the same thermo	37
Crude distillation unit	`CrudeUnit` (palette: `CrudeDistillationUnit`)	crude → naphtha, kerosene, diesel, AGO, residue	Fired heater solved with an equation-oriented MESH column; side strippers, pumparounds, steam	`cdu_block`	Against an independent Pyomo/IPOPT column and IDAES property packages (details); characterization, crude thermodynamics and a side-draw column against DWSIM 9.0.5 (details)	35, 36, 40
Vacuum unit	`VacuumColumn` (palette: `VacuumColumn`)	atmospheric residue → LVGO, HVGO, slop, vacuum residue	Stage-network column at vacuum with packed beds; contaminants carried per cut	—	Against an independent Pyomo/IPOPT model, equilibrium and Murphree beds (details); vacuum feed flash and heavy-crude characterization against DWSIM 9.0.5 (details)	34, 36
Saturated gas plant	`GasPlantColumn`, `GasCompressor`, `AmineTreater` (all three on the palette)	light ends + naphtha → fuel gas, LPG, C3/C4 splits, stabilized naphtha	Cubic-EOS (PR or SRK) stage columns, staged compressor with knock-outs, amine treating as a removal fraction	`gasplant_block`	Debutanizer against IDAES `TrayColumn`; splitter against IDAES flashes (details); PR flashes against DWSIM 9.0.5 on the same constants (details); debutanizer, C3/C4 and naphtha splitter columns, compressor train and TVP/RVP against DWSIM 9.0.5, the splitter column-level for the first time (details)	38, 40

Conversion#

Unit	Main class	In → out	Model	Planning	Validation	Example
C5/C6 isomerization	`IsomerizationReactor`, `IsomerizationUnit` (both on the palette)	light naphtha → isomerate	Adiabatic approach-to-equilibrium bed on ideal-gas thermochemistry; optional deisopentanizer and deisohexanizer recycle	`isom_block`	Equilibrium layer against IDAES `GibbsReactor` (details; stale since #339 moved the constants, pending regeneration); thermochemistry and adiabatic equilibrium against DWSIM 9.0.5 (details); rate constants illustrative	39
Hydrotreater	`Hydrotreater` (library)	naphtha, kerosene or diesel → treated product, wild naphtha, off-gas; optionally jet / diesel (or light / heavy naphtha)	Trickle-bed HDS by sulfur class (LHHW, H2S-inhibited), HDN, aromatics saturation with equilibrium; charge-heater duty; HP separator, H2 recycle, stripper; optional TBP-split product fractionator (`res.fractionate`); diesel and naphtha (`NAPHTHA_HDT_PARAMS`) constant sets	`hdt_block`	Balances and gradients only; no literature cross-check; constants illustrative. Heats of reaction and aromatics-saturation equilibrium against DWSIM 9.0.5 (details); hP-separator flash of a solved effluent against DWSIM 9.0.5 PR78 on the same constants, and the dissolved H2/H2S under DWSIM’s data (details)	40
Hydrocracker	`Hydrocracker` (library)	VGO → LPG, naphtha, kerosene, diesel, unconverted oil	Pretreat bed (hydrotreating kinetics) then cracking bed on continuous lumping or discrete lumps, organic-N inhibition; TBP-split fractionator; UCO recycle	`hcu_block`	Balances and gradients only; no literature cross-check; constants illustrative	—
Fluid catalytic cracker	`FCCUnit` (library)	VGO → dry gas, C3, C4, gasoline, LCO, slurry, flue gas	3-, 4- or 5-lump riser and coke-burning regenerator solved together for the heat balance; TBP-split main fractionator	`fcc_block`	Balances and gradients only; no literature cross-check; constants illustrative (details); regenerator coke-burn heat against DWSIM 9.0.5 (details)	—
Catalytic reformer	`CatalyticReformer` (library)	hydrotreated heavy naphtha (`NaphthaFeed.from_hydrotreater`, #327) → reformate, net H2, LPG, fuel gas	29 lumps by carbon number, equilibrium from Gibbs energies; three adiabatic beds with fired heaters; PR separator and H2 recycle; component-split stabilizer; feed sulfur to H2S and reformate S (trace)	`reformer_block`	Balances and gradients only; no literature cross-check; constants illustrative. Reaction thermochemistry, equilibria and one bed’s energy balance against DWSIM 9.0.5 (details)	40
Alkylation	`AlkylationUnit` (library)	C3-C5 olefins + isobutane → alkylate, propane, n-butane	Sauer-Colville-Burwick yield and octane correlations; per-olefin stoichiometry; shortcut DIB, depropanizer and debutanizer; isobutane recycle	`alky_block`	The correlation layer reproduces the GAMS `process.gms` optimum (profit 1161.3366); heats of alkylation against DWSIM 9.0.5 (details)	—
Hydrogen network	`HydrogenNetwork` (library)	reformer net gas, H2 plant, import → hydrotreater / hydrocracker makeup, fuel gas, export	Header balance by species; optional PSA (recovery, product purity); ordered swing sources with capacities; purity and makeup partial-pressure specs; makeup purity fed back into the hydrotreaters by substitution	`h2_block`	Balances close by construction; gradients against finite differences; PSA defaults illustrative	—
Residue desulfurizer	`ResidueDesulfurizer` (library)	atmospheric residue → desulfurized residue (VLSFO base), distillate, gas	Trickle beds: HDS by sulfur class plus refractory residue sulfur (LHHW, H2S-inhibited), HDM of Ni+V onto the catalyst, CCR reduction, small 538 C+ conversion; once-through treat gas, ideal product split; `fuel_oil_blend` to a 0.5 wt% S pool	—	Balances and gradients only; no literature cross-check; constants illustrative. Its per-H2 heats (benzothiophene HDS, benzene saturation, cracking) against DWSIM 9.0.5 (details)	—

Products#

Unit	Main class	In → out	Model	Planning	Validation	Example
Product blending	`BlendPool`, `BlendComponent` (library)	components → gasoline, jet, ULSD or fuel oil, with spec margins	Nonlinear blending rules (Ethyl RT-70 octane, RVP index, Refutas viscosity); distillation and cetane index computed from the blend; flash, freeze and smoke points, viscosity and straight-run octane estimated from a stream (`properties`, unverified)	`product_value_block`	Rules tested against published worked examples (RVP index, Refutas)	33, 40

What “illustrative” means here#

Two kinds of number appear in these units, and they deserve different trust.

Physics and published correlations: mass, element and energy balances, equilibrium from thermochemistry, the column equations, and the characterization and blending correlations. These are transferable, and where a unit has been cross-checked it is against an independent implementation of the same model (Pyomo/IPOPT, IDAES), not against a commercial simulator.
Kinetic and yield constants in the conversion units (hydrotreater, residue desulfurizer, hydrocracker, FCC, reformer, isomerization rates, alkylation octane temperature terms): illustrative. They were chosen to give plausible behaviour, not taken from a published parameter set, and they must be fitted to the unit’s own data (for example with difflow.estimation) before the yields are used to plan. The trends and the gradients are meaningful; the absolute yields are not predictions.

Every citation, equation number or coefficient that could not be checked against its source is marked (unverified) in the full documentation and in the code.

One set of formation data. Every unit with reactions takes its ideal-gas heats of formation, entropies and heat capacities from one table, difflow_refinery.thermochemistry (#339). Each species records its source (CODATA, API Technical Data Book, CRC, Yaws, NIST-JANAF and TRC fits) and how far it was checked. Since #338 the hydrotreater (and with it the hydrocracker’s pretreat bed and the residue desulfurizer) reads it too, and its aromatics-saturation equilibria are Cp-integrated: the old constant-dH/dS form made K 3-5x too large at 300-420 °C. Benzene saturation now agrees with DWSIM to 0.016 in ln K and with the reformer exactly. See Thermochemical data. Moving isomerization onto the table changed its C5/C6 equilibria. The iC5 share of the C5s at 450 K went from 0.820 to 0.772 (DWSIM on ChemSep data gives 0.762), and its IDAES reference has not yet been regenerated. The reformer’s numbers did not change.

Known gaps#

Fractionation in the conversion units is simplified. The FCC main fractionator, the hydrocracker fractionator and the hydrotreater’s optional product fractionator are TBP splits, the reformer’s stabilizer is a component split, and alkylation uses shortcut columns. The gas plant’s rigorous columns reached main after these units were built and are not yet wired in.
No literature cross-check for the hydrotreater, residue desulfurizer, hydrocracker, FCC or reformer: the papers named in their issues could not be obtained.
Not built: example notebooks for the hydrocracker, FCC and alkylation (the hydrotreater and the reformer appear only inside the whole-refinery example 40); the 10-lump FCC scheme; mechanistic alkylation kinetics; catalyst-activity tracking wired into difflow.reconciliation.tracking.
Known model defect: with the illustrative reformer constants, a rich (high-naphthene) naphtha makes less net H2 than a lean one, the reverse of commercial experience (details).
Product property estimates are unverified (#330): flash, smoke point, viscosity and straight-run octane come from correlations recalled but not checked against their sources; the freeze point is an n-paraffin solubility model on checked melting points (details). A measured value overrides each.
Boiling ranges are TBP, not ASTM D86, throughout.
Connections between units. The nine gaps that examples/40_refinery_flowsheet.ipynb first ran into (#326 to #334) are closed: gas_plant_feed, NaphthaFeed.from_hydrotreater and hydroprocessed_feed, the hydrotreater’s fractionator, the hydrogen network, the product property estimates and reformer sulfur, the residue desulfurizer, the charge heater and dissolved-gas reporting, and difflow_refinery.plant for gradients across units. The example’s refinery mass balance closes to 1.8e-6, and that remainder is the reported fold of gas_plant_feed. What it still assumes: the crude unit’s offgas H2S (2 mol %; evolved_h2s gives a sulfur-balance basis but no sourced fraction), a once-through treat gas on the residue desulfurizer (its hydrogen demand is a lower bound), and an unlimited hydrogen plant (the notebook’s section 11 lists them).
Chaining units across AD modes costs compile time and memory. A forward-only reformer next to a reverse-only (default) hydrotreater cannot be traced end to end in either mode; Chain(method="chain") uses the unit Jacobians, or the hydrotreater can be put in forward mode. Example 40’s 2-input chain Jacobian compiled in about 7.5 minutes on 4 cores (details).