The present invention relates to the monitoring of the saturation point of a gas, and more particularly to the control of the amount of a liquid fuel that is vaporized and added to a fuel gas in accordance with the saturation point of the fuel gas.
In certain applications in which a gaseous fuel is burned it is sometimes desirable to add a condensible fuel to a fuel gas to increase the quantity or the heating value of that fuel gas. For example, in refineries fuel gas (consisting of a mixture of natural gas and process gas) is often burned to provide the heat needed in various refining steps. However, the fuel gas available may sometimes be insufficient and in these cases butane is added to the fuel gas mixture. The butane is normally stored in liquid form at ambient temperature, and can be vaporized prior to adding it to the other fuel gas.
It is desirable to be able to maximize the amount of butane that is added to the fuel gas in order to minimize use of costlier fuel oil and/or to avoid reducing production due to lack of fuel. However, if too much butane is added to the fuel gas, the gas will become saturated and the excess butane will condense. Liquid in the fuel gas can cause burner flameouts and accidental fires leading to injuries and serious mishaps.
The amount of butane that can be safely added to the fuel gas is determined by monitoring the butane saturation point of the gas. In the past, the dew point of the fuel gas was measured, using either of the following methods, as an indicator of the saturation point. One of these methods uses a chilled mirror that is in contact with the gas. The mirror is cooled in a controlled fashion until condensate is detected on its surface, either visually or by means of an electro-optical device, at which point the temperature is recorded. This process can be repeated in a cyclic fashion, or the temperature can be regulated to keep a predetermined level of condensate on the mirror. Examples of this test method can be found in U.S. Pat. No. 4,276,768 and the "Standard Test Method for Water Vapor Content of Gaseous Fuels by Measurement of Dew-Point Temperature" issued by the American National Standards Institute under ASTM Designation D 1142.
Although the chilled mirror provides a direct indication of dew point temperature, in actual practice this approach has not been satisfactory as a technique for enabling the maximum amount of butane to be added to a fuel gas. One reason for this is that such systems tend to be rather complex and require continual maintenance to keep them operational.
The other method for determining the dew point of the fuel gas is to calculate it. Unlike water vapor in air, in which the dew point is strictly a function of ambient and adiabatic saturation temperatures (as well as pressure), the dew point of a fuel gas is also dependent upon the composition of the gas. Typically, some of the components of the fuel gas are soluble in the butane, and hence their concentration levels in the gas will affect the dew point. A chromatograph is used to provide a composition analysis of the fuel gas from which the dew point temperature of the gas is calculated by a computer. The main drawbacks of this method are high cost and complexity.