Natural Gas Processing¶

In this tutorial you will model a natural gas dew-point control unit in DWSIM's Classic UI: a multi-component gas stream is chilled, and heavier hydrocarbons (LPG and condensates) are separated as liquid in a flash drum.

What you will learn

How to model a multi-component natural gas mixture
How to set up dew-point control with a Cooler and Separator Vessel
How to read C3+ recovery and gas-phase composition

Prerequisites

Completed Phase Envelope and Simple Flash Drum

Process Overview¶

Raw natural gas contains methane, ethane, propane, butanes, and heavier hydrocarbons. Pipeline specs require low hydrocarbon dew point to prevent liquid dropout. The dew-point control unit chills the gas to condense heavier components, which are separated and processed as LPG/NGL.

Process Flow Diagram¶

graph LR
    F["Wellhead Gas<br/>320 K, 70 bar<br/>C1-C5"] --> CH["CH-1<br/>Chiller<br/>240 K"]
    CH --> SEP["SEP-1<br/>Separator"]
    SEP -->|Sales Gas| G["Sales Gas"]
    SEP -->|NGL| L["NGL Liquid"]

Key Design Parameters¶

Parameter	Value
Compounds	Methane, Ethane, Propane, n-Butane, n-Pentane
Property Package	Peng-Robinson
Feed	100 mol/s, 320 K, 70 bar, 80% C1, 10% C2, 5% C3, 3% nC4, 2% nC5
Chiller outlet T	240 K (-33 °C)

Step-by-Step in the Classic UI¶

1. Set up¶

File > New Chemical Process Model:

Compounds: Methane, Ethane, Propane, N-butane, N-pentane
Property Package: Peng-Robinson

Why Peng-Robinson for natural gas?

PR is the de-facto industry standard for natural gas mixtures: accurate VLE for the C1-C5 hydrocarbon range and reliable for gas separations down to cryogenic temperatures. SRK is a similar option; activity-coefficient models do not apply at these conditions.

2. Build the flowsheet¶

Drag and configure:

Material Stream Wellhead-Gas: T=320 K, P=70 bar (7000000 Pa), molar flow=100 mol/s, mole fractions as in the parameters table
Material Stream Chilled (empty)
Cooler CH-1: outlet T=240 K, ΔP=0, η=100%; Inlet=Wellhead-Gas, Outlet=Chilled, create energy stream
Material Stream Sales-Gas (empty)
Material Stream NGL (empty)
Separator Vessel SEP-1: Inlet=Chilled, Vapor Outlet=Sales-Gas, Liquid Outlet=NGL

Why 240 K (-33 °C)?

Below the dew point of the heavier components (C3+, C4+) at 70 bar, but above the freezing point of any residual water or CO2. This temperature provides good NGL recovery without incurring the higher refrigeration cost of cryogenic operation.

Natural gas dew-point control flowsheet

3. Solve¶

F6 ON → Solve.

4. Inspect results¶

Sales-Gas Results: methane mole fraction > 0.85 (enriched), molar flow ~80-90 mol/s
NGL Results: C3+ enriched, molar flow ~10-30 mol/s
CH-1 Results: chiller duty (large negative number = significant cooling required)

You can also use Utilities > Phase Envelope on the Wellhead-Gas to visualize where 240 K / 70 bar lies on the envelope and confirm two-phase region operation.

Results and Validation¶

Variable	Expected
Sales gas C1 fraction	> 0.85
NGL C3+ fraction	> 0.50
Sales gas flow	70 - 90 mol/s
NGL flow	10 - 30 mol/s
C3+ recovery in NGL	> 80%

Expected results

Sales gas enriched in C1/C2; NGL captures most of C3-C5. C3+ recovery exceeds 80%.

Understanding the Results¶

Hydrocarbon dew-point separation exploits the wide range of vapor pressures across C1-C5. At 240 K and 70 bar:

Methane (Tc = 191 K) - supercritical, gaseous
Ethane (Tc = 305 K) - mostly gaseous, partially absorbs
Propane (Tc = 370 K) - condenses substantially
n-Butane and n-Pentane (Tc > 400 K) - condense almost completely

Lower chiller T → more C3+ recovery, but more refrigeration energy. Industrial designs balance NGL revenue against energy costs.

Automating This Tutorial¶

Files in this repository

Python script: examples/advanced/04_natural_gas.py
Pre-built flowsheet: examples/saved/natural_gas.dwxmz

FluentAPI (Python)MCP Server (JSON-RPC)LLM Prompt

See examples/advanced/04_natural_gas.py in the DWSIM.Tutorials repository.

Standard dwsim.unitop.add for Cooler + Separator, then connect and solve.

Output may vary

Results depend on the LLM's reasoning quality and tool-use accuracy. Always verify the simulation matches your intent before relying on the numbers.

Use DWSIM (via the MCP server) to build the following simulation:

- Create a flowsheet called "NaturalGasProcessing"
- Add Methane, Ethane, Propane, N-butane and N-pentane as compounds;
  set the property package to "Peng-Robinson"
- Add a material stream "Wellhead-Gas" at 320 K and 7000000 Pa
  (70 bar), molar flow = 100 mol/s, mole fractions: Methane = 0.80,
  Ethane = 0.10, Propane = 0.05, N-butane = 0.03, N-pentane = 0.02
- Add a Cooler "CH-1" with outlet T = 240 K, ΔP = 0, efficiency = 100%;
  inlet = Wellhead-Gas, outlet = Chilled, with an energy stream
- Add a Separator Vessel "SEP-1" with inlet = Chilled, vapor outlet
  = Sales-Gas, liquid outlet = NGL
- Solve the flowsheet
- Report the methane mole fraction and molar flow of Sales-Gas, the
  C3+ mole fraction and molar flow of NGL, the C3+ recovery in NGL,
  and the chiller duty

Exercises

Increase chiller T to 270 K. How does C3+ recovery change?
Reduce feed pressure to 30 bar. Does separation improve at 240 K?
Add 5% CO2 and 3% N2 to the feed. How do they distribute?

Next Steps¶

In Ethanol Plant, you will simulate a fermentation process.