Dew Point Standards

Standards for calculating water and hydrocarbon dew points of natural gas.

Overview
Water Dew Point (ISO 18453)
Hydrocarbon Dew Point
Usage Examples
Comparison of Methods

Overview

Dew point specifications are critical for:

Pipeline transport (preventing liquid dropout)
Custody transfer specifications
Processing plant design
Sales contract compliance

Available Implementations:

Draft_ISO18453 - Water dew point using GERG-water equation
BestPracticeHydrocarbonDewPoint - Hydrocarbon dew point using SRK-EoS

Water Dew Point (ISO 18453)

Standard

ISO 18453:2004 - Natural gas — Correlation between water content and water dew point

Purpose

Calculate the temperature at which water vapor in natural gas begins to condense at a given pressure.

Implementation

Class: Draft_ISO18453

Uses the GERG-water equation of state which is specifically designed for water in natural gas systems.

Constructor

import neqsim.standards.gasquality.Draft_ISO18453;

// Create standard from any fluid
Draft_ISO18453 waterDewPoint = new Draft_ISO18453(thermoSystem);

How It Works

Converts input fluid to GERG-water EoS if not already
Sets pressure to reference pressure (default 70 bar)
Performs water dew point temperature flash
Returns temperature where water just begins to condense

Key Methods

Method	Description
`calculate()`	Perform dew point calculation
`getValue("dewPointTemperature")`	Get dew point in °C
`getValue("pressure")`	Get reference pressure in bar
`setReferencePressure(P)`	Set reference pressure
`isOnSpec()`	Check against sales contract specification

Example

import neqsim.thermo.system.SystemSrkCPAstatoil;
import neqsim.standards.gasquality.Draft_ISO18453;

// Natural gas with water
SystemInterface wetGas = new SystemSrkCPAstatoil(273.15 + 20, 50.0);
wetGas.addComponent("methane", 0.90);
wetGas.addComponent("ethane", 0.05);
wetGas.addComponent("propane", 0.02);
wetGas.addComponent("CO2", 0.02);
wetGas.addComponent("water", 100e-6);  // 100 ppm water
wetGas.setMixingRule("CPA_Statoil");

// Calculate water dew point
Draft_ISO18453 iso18453 = new Draft_ISO18453(wetGas);
iso18453.setReferencePressure(70.0);  // 70 bar reference
iso18453.calculate();

double wdp = iso18453.getValue("dewPointTemperature");
System.out.printf("Water Dew Point at 70 bar = %.1f °C%n", wdp);

Specification Checking

// Check against contract specification
iso18453.getSalesContract().setWaterDewPointTemperature(-8.0);  // Max -8°C

if (iso18453.isOnSpec()) {
    System.out.println("Gas meets water dew point specification");
} else {
    System.out.println("Gas FAILS water dew point specification");
}

Hydrocarbon Dew Point

Purpose

Calculate the temperature at which hydrocarbon liquids begin to condense from natural gas (cricondentherm).

Implementation

Class: BestPracticeHydrocarbonDewPoint

Uses SRK equation of state with Peneloux volume correction (mixing rule 2) for hydrocarbon phase behavior.

Constructor

import neqsim.standards.gasquality.BestPracticeHydrocarbonDewPoint;

// Create from any fluid (water is automatically removed)
BestPracticeHydrocarbonDewPoint hcDewPoint = 
    new BestPracticeHydrocarbonDewPoint(thermoSystem);

How It Works

Creates new SRK-EoS system from input (excludes water)
Sets reference pressure (default 50 bar)
Performs dew point temperature flash
Returns hydrocarbon dew point temperature

Key Methods

Method	Description
`calculate()`	Perform dew point calculation
`getValue("hydrocarbondewpointTemperature")`	Get dew point in °C
`getValue("pressure")`	Get reference pressure in bar
`isOnSpec()`	Check against specification

Example

import neqsim.thermo.system.SystemSrkEos;
import neqsim.standards.gasquality.BestPracticeHydrocarbonDewPoint;

// Rich natural gas
SystemInterface richGas = new SystemSrkEos(273.15 + 20, 50.0);
richGas.addComponent("methane", 0.85);
richGas.addComponent("ethane", 0.06);
richGas.addComponent("propane", 0.03);
richGas.addComponent("n-butane", 0.02);
richGas.addComponent("n-pentane", 0.01);
richGas.addComponent("n-hexane", 0.005);
richGas.addComponent("n-heptane", 0.003);
richGas.addComponent("n-octane", 0.002);
richGas.addComponent("nitrogen", 0.02);
richGas.setMixingRule("classic");

// Calculate hydrocarbon dew point
BestPracticeHydrocarbonDewPoint hcDP = new BestPracticeHydrocarbonDewPoint(richGas);
hcDP.calculate();

double hcdp = hcDP.getValue("hydrocarbondewpointTemperature");
System.out.printf("Hydrocarbon Dew Point at 50 bar = %.1f °C%n", hcdp);

Multiple Pressures

// Calculate HCDP curve at multiple pressures
double[] pressures = {20, 30, 40, 50, 60, 70, 80};

System.out.println("Pressure (bar) | HC Dew Point (°C)");
System.out.println("---------------|-----------------");

for (double P : pressures) {
    richGas.setPressure(P);
    BestPracticeHydrocarbonDewPoint hcDP = new BestPracticeHydrocarbonDewPoint(richGas);
    hcDP.calculate();
    double hcdp = hcDP.getValue("hydrocarbondewpointTemperature");
    System.out.printf("%14.0f | %16.1f%n", P, hcdp);
}

Usage Examples

Combined Water and HC Dew Points

import neqsim.thermo.system.SystemSrkCPAstatoil;
import neqsim.standards.gasquality.Draft_ISO18453;
import neqsim.standards.gasquality.BestPracticeHydrocarbonDewPoint;

// Create wet gas with heavy ends
SystemInterface gas = new SystemSrkCPAstatoil(273.15 + 20, 70.0);
gas.addComponent("methane", 0.88);
gas.addComponent("ethane", 0.05);
gas.addComponent("propane", 0.02);
gas.addComponent("n-butane", 0.01);
gas.addComponent("n-pentane", 0.005);
gas.addComponent("n-hexane", 0.002);
gas.addComponent("CO2", 0.02);
gas.addComponent("water", 50e-6);
gas.setMixingRule("CPA_Statoil");

// Water dew point
Draft_ISO18453 waterDP = new Draft_ISO18453(gas);
waterDP.setReferencePressure(70.0);
waterDP.calculate();
double wdp = waterDP.getValue("dewPointTemperature");

// Hydrocarbon dew point
BestPracticeHydrocarbonDewPoint hcDP = new BestPracticeHydrocarbonDewPoint(gas);
hcDP.calculate();
double hcdp = hcDP.getValue("hydrocarbondewpointTemperature");

System.out.println("=== Dew Point Analysis ===");
System.out.printf("Water Dew Point (at 70 bar): %.1f °C%n", wdp);
System.out.printf("Hydrocarbon Dew Point (at 50 bar): %.1f °C%n", hcdp);

Effect of Water Content on WDP

// Analyze water dew point vs water content
double[] waterContents = {10, 20, 50, 100, 200, 500};  // ppm

System.out.println("Water Content (ppm) | Water Dew Point (°C)");
System.out.println("--------------------|--------------------");

for (double ppm : waterContents) {
    SystemInterface gas = new SystemSrkCPAstatoil(273.15 + 20, 70.0);
    gas.addComponent("methane", 0.95);
    gas.addComponent("ethane", 0.03);
    gas.addComponent("CO2", 0.01);
    gas.addComponent("water", ppm * 1e-6);
    gas.setMixingRule("CPA_Statoil");
    
    Draft_ISO18453 waterDP = new Draft_ISO18453(gas);
    waterDP.setReferencePressure(70.0);
    waterDP.calculate();
    double wdp = waterDP.getValue("dewPointTemperature");
    
    System.out.printf("%19.0f | %19.1f%n", ppm, wdp);
}

Comparison of Methods

Water Dew Point Approaches

Aspect	ISO 18453 (GERG-water)	CPA-EoS
Model	GERG-water specific	General CPA
Accuracy	Optimized for NG	Good general accuracy
Speed	Fast	Moderate
Water association	Empirical	Explicit

Hydrocarbon Dew Point Approaches

Aspect	Best Practice (SRK)	PR-EoS	GERG-2004
Heavy ends	Good	Good	Limited
Accuracy	Typical ±2-3°C	Typical ±2-3°C	Best for lean gas
Speed	Fast	Fast	Moderate

Typical Specifications

European Gas Specifications

Parameter	Typical Limit
Water dew point	< -8°C at 70 bar
HC dew point	< -2°C at 1-70 bar

US Pipeline Specifications

Parameter	Typical Limit
Water dew point	< -7°C (20°F) at max operating P
HC dew point	< -4°C (25°F) at cricondenbar

Accuracy Considerations

Factors Affecting Water Dew Point

Water content measurement uncertainty
Pressure accuracy
EoS model selection
Presence of glycols or methanol

Factors Affecting HC Dew Point

Heavy end characterization (C6+ components)
Retrograde behavior near cricondentherm
EoS binary interaction parameters
Pressure at measurement

Recommendations

Use CPA or GERG-water for water dew point
Characterize C6+ fraction carefully for HC dew point
Report dew point with reference pressure
Consider measurement at multiple pressures for HC dew point curve

References

ISO 18453:2004 - Natural gas — Correlation between water content and water dew point
Folas, G.K., et al. (2007). High-pressure vapor-liquid equilibria of systems containing ethylene glycol, water and methane. Fluid Phase Equilibria.
ISO 23874:2006 - Natural gas — Gas chromatographic requirements for hydrocarbon dewpoint calculation
GERG Technical Monograph TM14 (2007) - The GERG-2004 Wide-Range Equation of State for Natural Gases

Dew Point Standards

Comprehensive Java library for thermodynamic, physical property, and process simulation. NeqSim provides tools for PVT analysis, phase equilibria, transport properties, and full process simulation.

Dew Point Standards

Table of Contents

Overview

Water Dew Point (ISO 18453)

Standard

Purpose

Implementation

Constructor

How It Works

Key Methods

Example

Specification Checking

Hydrocarbon Dew Point

Purpose

Implementation

Constructor

How It Works

Key Methods

Example

Multiple Pressures

Usage Examples

Combined Water and HC Dew Points

Effect of Water Content on WDP

Comparison of Methods

Water Dew Point Approaches

Hydrocarbon Dew Point Approaches

Typical Specifications

European Gas Specifications

US Pipeline Specifications

Accuracy Considerations

Factors Affecting Water Dew Point

Factors Affecting HC Dew Point

Recommendations

References