Reverse Osmosis Desalination¶

In this tutorial you will model a seawater desalination plant in DWSIM's Classic UI using a Reverse Osmosis membrane unit. RO is the dominant technology for producing fresh water from seawater (>60% of global desalination capacity).

What you will learn

How to model an electrolyte system (Na+, Cl-, water)
How to use the Reverse Osmosis unit operation (Plus / Patreon Edition)
How to read fresh water (permeate) and brine (reject) properties

Prerequisites

Completed Heater and Cooler
DWSIM Plus / Patreon Edition (the RO operation is part of the Plus add-on)

Plus feature

Reverse Osmosis requires DWSIM Plus / Patreon Edition activation. Without activation, the operation is not available in the Object Palette.

Process Overview¶

Pressure higher than the natural osmotic pressure is applied to the salty side of a semi-permeable membrane. Water passes through; salts are rejected. Output: low-salinity permeate + high-salinity brine.

Seawater (35 g/kg TDS) has osmotic pressure ~25 bar. Industrial RO operates at 60-80 bar with 40-50% recovery.

Process Flow Diagram¶

graph LR
    SW["Seawater<br/>298 K, 1 atm<br/>35 g/kg salt"] --> P["P-1<br/>HP Pump<br/>70 bar"]
    P --> RO["RO-1<br/>Membrane"]
    RO -->|Permeate| FW["Fresh Water"]
    RO -->|Reject| BR["Brine"]

Key Design Parameters¶

Parameter	Value
Compounds	Water, Sodium ion, Chloride ion
Property Package	Sour Water (or Electrolyte NRTL)
Feed	1 kg/s, 25 °C, 1 atm, 35 g/kg salinity
Pump outlet pressure	70 bar
Salt rejection	99.5%
Water recovery	45%

Step-by-Step in the Classic UI¶

1. Set up¶

File > New Chemical Process Model:

Compounds: Water, Sodium ion, Chloride ion (the ions are listed in the Electrolytes database)
Property Package: Sour Water (or Electrolyte NRTL if Plus is activated)

Why an electrolyte/aqueous property package?

Sodium-chloride solutions are strongly non-ideal due to ionic interactions. Standard NRTL or PR cannot capture salt rejection or osmotic pressure properly; electrolyte models (electrolyte-NRTL, Pitzer) are required for full ionic behavior at industrial salinities.

2. Build the flowsheet¶

Drag and configure:

Material Stream Seawater: T=298.15 K, P=1 atm, mass flow=1 kg/s; mass fractions: Water=0.965, Sodium ion=0.0138, Chloride ion=0.0212
Pump P-1: outlet pressure=70 bar (7000000 Pa), efficiency=75%; create energy stream W_pump

Why a high-pressure pump (70 bar)?

RO works by overcoming the osmotic pressure of the feed. Seawater has osmotic pressure around 25-28 bar; you need to feed at 60-80 bar to drive water through the membrane while leaving salt behind, with enough margin to maintain useful permeate flux.

Material Stream Pumped (empty)
Reverse Osmosis RO-1: salt rejection=0.995, water recovery=0.45 (find these settings on the unit's editor)
Material Stream Fresh-Water (empty)
Material Stream Brine (empty)

Connections: Seawater → P-1 → Pumped → RO-1 → (Fresh-Water as port 0, Brine as port 1).

RO desalination flowsheet

3. Solve¶

F6 ON → Solve.

4. Analyze¶

Fresh-Water Results: salinity should be < 0.5 g/kg (drinking water quality)
Brine Results: salinity ~60-80 g/kg (about 2× seawater)
W_pump Energy: pumping power - divide by permeate flow to get specific energy (kWh/m³)

Typical specific energy: 2-4 kWh/m³ for modern RO at 45% recovery.

Results and Validation¶

Variable	Expected
Permeate salinity	< 0.5 g/kg
Brine salinity	60-80 g/kg
Recovery	0.45
Specific energy	2-4 kWh/m³

Expected results

Permeate is fresh water; brine is concentrated. Specific energy near the thermodynamic minimum.

Understanding the Results¶

RO is a membrane separation. Membrane salt rejection and water permeability determine product quality and flux. Key economic factors:

Higher recovery → smaller brine volume but higher pressure required
45% recovery / 70 bar → ~3 kWh/m³, near thermodynamic minimum
Pretreatment is critical to prevent scale and biofouling

Brine disposal back to the sea creates a dense plume that can damage marine ecosystems - a major environmental concern.

Automating This Tutorial¶

Files in this repository

Python script: examples/advanced/06_reverse_osmosis.py
Pre-built flowsheet: examples/saved/reverse_osmosis.dwxmz - requires DWSIM Plus; run the script locally to generate

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

See examples/advanced/06_reverse_osmosis.py in the DWSIM.Tutorials repository.

dwsim.unitop.add with type Pump and ReverseOsmosis, 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 "ReverseOsmosis"
- Add Water, Sodium ion and Chloride ion as compounds; set the property
  package to "Sour Water" (or "Electrolyte NRTL" if available)
- Add a material stream "Seawater" at 298.15 K and 1 atm with mass flow
  = 1 kg/s and mass fractions Water = 0.965, Sodium ion = 0.0138,
  Chloride ion = 0.0212
- Add a Pump "P-1" with outlet pressure = 7000000 Pa (70 bar) and
  efficiency = 75%; energy stream = W_pump
- Add a Reverse Osmosis unit "RO-1" with salt rejection = 0.995 and
  water recovery = 0.45; permeate outlet = Fresh-Water, retentate
  outlet = Brine
- Connect: Seawater → P-1 → RO-1 → (Fresh-Water, Brine)
- Solve the flowsheet
- Report the salinity (g/kg) of Fresh-Water and Brine, the recovery
  ratio, the pumping power W_pump, and the specific energy
  (kWh per m³ of permeate)

Exercises

Reduce salt rejection to 95%. How does permeate salinity change?
Increase pump pressure to 80 bar. Does specific energy improve?
Increase recovery to 60%. What pump pressure is needed?

Next Steps¶

In Methanol Synthesis, you will build a complete syngas-to-methanol process with recycle.