Electric Heating Of Natural Gas To Prevent Freezing After Throttling

Electric circulation heaters are widely used for heating natural gas in a variety of industrial applications. These include:

1. Boiling gaseous natural gas off of a liquid natural gas (LNG) storage tank;
2. Heating the natural gas feed into a steam-methane reformer, to provide the energy required for the endothermic reforming reaction;
3. Heating natural gas used as seal gas in a cryogenic petrochemical process;
4. Preheating natural gas before pressure reduction (throttling) to overcome temperature reduction resulting from the Joule-Thomson effect.

Among the four applications listed, perhaps the most surprising is the preheating required to prevent freezing. Natural gas whose pressure is reduced across a throttling valve undergoes a sudden temperature decrease as a results of the Joule-Thomson effect. If the temperature of the gas drops below the freezing point of water, any water present in the gas may freeze. Ice may clog the fine control surfaces of control valves and regulators, leading to a breakdown of the natural gas supply system. To prevent this from happening, natural gas is preheated by electric heaters before it is throttled, raising the temperature by an amount equal to the temperature drop during subsequent throttling.

In this article we discuss the Joule-Thomson effect as it applies to natural gas, and explain how to size a flanged electric heater for this application. Specifically, we calculate the heater power required to preheat natural gas just enough so that its temperature will return to its original value after throttling.

The Joule-Thomson Effect

When a gas is throttled in a valve, the gas performs no work and exchanges no heat with the environment, so that its enthalpy is unchanged. It was discovered by Thomson, following the work of Joule, that the temperature however is changed during throttling according to the relationship where:

• T: Temperature;
• P: Pressure;
• h: Enthalpy;
• c_p : Specific heat at constant pressure;
• v: Specific volume.

μ, the rate of change of temperature with respect to pressure, is called the Joule-Thomson coefficient. The Joule-Thomson coefficient varies widely with natural gas pressure, temperature and composition. However, in most engineering applications, throttling occurs from an initial pressure of the order of 1,200 psig (8.27 MPag) down to a final pressure about equal to atmospheric pressure, and near the room temperature of 68 ^oF (20 ^oC). For these conditions, the average Joule-Thomson coefficient may be taken as 7 ^oF/100 psi (5.6 ^oC/MPa). Given the Joule-Thomson coefficient and the pressure drop, the temperature drop may be calculated. The amount of preheating required so that natural gas will come back to its initial temperature after throttling is simply the throttling temperature drop times the specific heat capacity times the mass flow rate.

Example. A pipeline near a chemical plant transports natural gas at 1200 psig (8.27 MPag), 68 ^oF (20 ^oC). The plant draws 10000 pounds per hour of natural gas that has been throttled down to 15 psig. Find the temperature drop due to throttling and the preheating electric heater power required so that the temperature of the gas will be back to its original value after throttling.

We see that, had there been no preheating, the temperature would have fallen to -15 ^oF (-26.1 ^oC), which would very likely have frozen the natural gas supply system.

In practice, it is usual to add a 25% safety margin to the calculated heating power required, to account for wall heat loss and uncertainties in measurement and control. Thus the preheater for this application would have a design power of 160 kW.

Finally, we emphasize that the natural gas must be heated before throttling, not after, as the aim is to prevent the possibility of freezing, not to repair any damages from freezing once they have occurred.