# Offshore Methanol Process Design — Methodology

**Project:** Conceptual production of methanol from methane obtained on an offshore platform  
**Institution:** Escola Politécnica da Universidade de São Paulo — Department of Chemical Engineering  
**Course:** PQI3501 — Process Synthesis and Design  
**Team:** Group T3  
**Reference year:** 2026

## 1. Purpose and engineering boundary

This document describes the reproducible methodology behind the website and the accompanying process-model workbook. The work is a **steady-state conceptual process design** developed from an Aspen Plus flowsheet, preliminary equipment sizing, CAPCOST/Turton economic estimation, uncertainty analysis and an order-of-magnitude environmental assessment.

The work is not FEED or detailed engineering. It does not include a HAZOP, SIL determination, QRA, LOPA, dynamic control study, catalyst-aging model, detailed hydraulic network, mechanical design, vendor quotation, offshore plot plan, structural/weight assessment or externally audited life-cycle assessment.

## 2. Design basis

The process treats a methane-rich gas stream supplied at approximately 40 °C and 10.1 bar. The reconciled feed basis is 600 kg/h with 94.27 mol% CH₄, 5.00 mol% CO₂ and 0.73 mol% H₂O. The feed is assumed to be previously treated for contaminants outside the modeled battery limit.

Fresh water is pumped from approximately 1 bar to 10 bar and mixed with internally recovered water. The modeled flows are 756.64 kg/h fresh water and 1,188.46 kg/h recycled water, producing a combined water stream of 1,945.10 kg/h. The report states a steam-to-carbon ratio of 2.78; this value is retained as a reported design assumption and remains an open independent cross-check.

The target product specification is 99.86 wt% methanol. The reconciled product stream is 1,025.368 kg/h, equivalent to 8.203 kt/y at 8,000 operating hours per year.

## 3. Thermodynamic method

Peng–Robinson is used as the principal property method. The project validates the selected model against pure-component heat capacity, vapor-pressure and density data for CO₂, CO, H₂, H₂O, CH₄ and CH₃OH. The H₂O–CH₃OH binary system is checked against NIST dataset 160. The validation is used to justify the property method across the gas-phase reforming/synthesis sections and the methanol–water separation section.

The validation is a model-selection exercise, not a formal parameter-regression study. Deviations reported for hydrogen vapor pressure do not materially affect the modeled operating region because hydrogen remains in the gas phase.

## 4. Reaction system

### Steam reforming and water-gas shift

- CH₄ + H₂O ⇌ CO + 3H₂
- CO + H₂O ⇌ CO₂ + H₂

Equilibrium screening is performed with an RGibbs representation to understand the influence of temperature, pressure and steam addition. The selected kinetic reformer is modeled in Aspen Plus with RPlug/PFR using the reported catalyst properties and a distributed heat-flux profile.

The reconciled reformer geometry uses 48 parallel tubes, each 6 m long and 0.1016 m in diameter. The website and workbook use 0.1016 m because the detailed catalyst-volume calculation controls over the conflicting 4 m narrative value. The catalyst density is 2,355.2 kg/m³ and the bed porosity is 0.528. The calculated catalyst mass is approximately 2,595.6 kg.

The reformer feed is heated to 550 °C and the simulated outlet reaches 855.6 °C. The reported methane conversion is 95.5%. The applied heat flux is front-loaded along the reactor to support the strongly endothermic reaction.

### Methanol synthesis and reverse water-gas shift

- CO + 2H₂ ⇌ CH₃OH
- CO₂ + 3H₂ ⇌ CH₃OH + H₂O
- CO₂ + H₂ ⇌ CO + H₂O

Equilibrium screening is performed with REquil to select an initial operating region near 40 bar and 200 °C. The kinetic section uses three adiabatic RPlug reactors with interstage cooling. The reactor train is represented as R-201, R-202 and R-203. The overall reactive-carbon conversion used in the website is 75.5%; the 75.8% narrative value is treated as rounding.

Per-reactor conversion fields in the source report are displayed individually and are not summed. The third reactor is identified as a capital hotspot because of its reported 3 m diameter and 12 m length.

## 5. Syngas conditioning and compression

The reformer effluent is cooled through heat recovery and then to 45 °C. A flash vessel removes approximately 1,188.46 kg/h of water for internal recycle. The dry syngas stream is 1,356.63 kg/h and contains approximately 74.47 mol% H₂, 15.07 mol% CO, 8.56 mol% CO₂, 1.05 mol% CH₄ and 0.85 mol% H₂O.

Compression is split into four stages with intercooling:

1. 10 → 16 bar
2. 16 → 24 bar
3. 24 → 32 bar
4. 32 → 40 bar

Intercooling limits compressor discharge temperature and reduces power demand relative to a single-stage compression.

## 6. Recycle, purge and downstream purification

The synthesis effluent is cooled and flashed at high pressure. The gas phase is split into approximately 96% recycle and 4% purge. The purge flow is 98.79 kg/h and the recycle flow is 2,370.90 kg/h. The purge is necessary to limit accumulation of hydrogen and inerts; it requires controlled handling such as flare, thermal oxidation or a future recovery route.

The crude liquid is pressure-reduced and sent to a second flash to remove remaining light gases. Final methanol–water separation is carried out with a RadFrac column. A DSTWU shortcut model supplies initial sizing before the rigorous RadFrac specification. The 0.36 reflux ratio belongs to the shortcut calculation; the final RadFrac reflux ratio is 1.03829.

The final product is 1,025.368 kg/h at 57.4 °C and 1 bar. The reported product specification is 99.86 wt% methanol, while the product stream contains 99.89 mol% methanol. These are different composition bases and are retained separately.

## 7. Mass, water and energy balances

The stream table is the controlling source for material-flow reconciliation. The website highlights the streams that govern the process but the complete report remains the primary source for the full 37-stream model.

The reported overall energy efficiency is 50.2% and the process heat-transfer rate is approximately 1,025 kW. The report identifies heat-recovery duties and additional exchanger-match opportunities. These are treated as preliminary optimization opportunities rather than guaranteed whole-plant savings.

## 8. Preliminary equipment sizing

Equipment dimensions are taken from Aspen Plus design calculations, shortcut methods and report appendices. They support conceptual costing only. The sizing does not include detailed mechanical design, materials verification, nozzle loads, offshore installation factors, fatigue, vibration, corrosion allowance or vendor confirmation.

## 9. Economic method

The economic model uses CAPCOST and Turton-style factors. The base case assumes:

- 8,000 operating hours per year;
- three construction years with a 30% / 40% / 30% capital distribution;
- ten operating years;
- 16% after-tax discount rate;
- 35% marginal tax rate;
- methanol price of $400/t;
- 10% residual value of FCIL;
- methane purchase cost of zero inside the modeled battery limit.

The reconciled headline values are:

- fixed capital investment: $7.570m;
- working capital: $3.605m;
- annual revenue: $3.277m/y;
- COMd: $3.213m/y;
- after-tax operating cash flow: $0.279m/y;
- base-case NPV: −$6.603m.

The project therefore meets the technical product target but does not achieve financial viability under the adopted assumptions.

## 10. Uncertainty and sensitivity

The reported Monte Carlo analysis uses 50,000 samples and varies FCIL, methanol price, working capital, marginal tax, discount rate, raw-material cost and residual value. The distribution remains entirely negative:

- minimum NPV: −$11.859m;
- P5: −$9.122m;
- median: −$6.420m;
- mean: −$6.434m;
- P95: −$3.773m;
- maximum: −$0.619m;
- probability of positive NPV: 0%.

The website uses the reported summary statistics. It does not generate random samples at runtime.

## 11. Preliminary environmental method

Direct and indirect emissions are kept separate. The report estimates approximately 116 kg CO₂/h associated with steam generation and 138 kg CO₂/h from the reformer furnace. The values are order-of-magnitude estimates based on energy demand, furnace efficiency and literature emission factors. They do not constitute a verified cradle-to-gate carbon footprint.

Water recycling is presented as an internal process benefit: 1,188.46 kg/h, or approximately 61.1% of the combined process-water flow.

## 12. Data-governance hierarchy

When narrative, table and appendix values disagree, the following hierarchy is used:

1. controlling stream table or explicit equation;
2. detailed equipment/economic appendix;
3. narrative text and rounded executive values;
4. qualitative interpretation of figures.

Every adjustment is recorded in `QA_REPORT.md` and in the QA sheet of the workbook.

## 13. Reproducibility package

The download package contains:

- the original full conceptual-design report in PDF;
- the structured support workbook in XLSX;
- this methodology;
- the data dictionary;
- the QA and reconciliation report;
- the local JSON datasets used by the website;
- a snapshot of the website source code.

The workbook is a transparent support model derived from the report and website datasets. It is not the original Aspen Plus binary case file.
