In many hydrocarbon processing applications the stream composition is liquid. The calibration mixture for process control instrumentation must also therefore be a liquid.
In hydrocarbon processing applications such as crude oil refining, it is common to use liquid mixtures to calibrate analytical instrumentation which measures the composition of process streams that would be liquid at ambient temperature.
At Coregas, we have a broad range of hydrocarbons that can be used to produce liquid hydrocarbon mixtures in cylinders. A liquefied calibration gas mixture is maintained as a liquid in the cylinder with either the vapour pressure of the hydrocarbon components or with a head pressure of helium gas.
To ensure that the liquid phase sample is withdrawn from the cylinder, a dip tube will be fitted to a dual port cylinder valve. The bottom of the dip tube will be immersed in the liquid at the bottom of the cylinder; withdrawal of the liquid phase from the cylinder is therefore possible. For such mixtures note that when the liquid level falls below the dip tube, the product withdrawal from the cylinder will be from the gas phase. Furthermore, the cylinder must be kept in an upright position to ensure that the dip tube remains immersed in the liquid.
A supply of helium to the dual port valve gas side will allow continuous withdrawal of liquid. Helium 5.0 grade is ideal for this purpose. Since the pressure in the liquid cylinder will be maintained constant with the helium to pressure, the cylinder pressure will not fall as the contents are depleted. The easiest way to determine the cylinder contents is to mount the cylinder on a weighing scale. An alarm can be set when 50% of the mixture has been used to ensure timely re-ordering of a replacement cylinder.
Withdrawal of the gaseous phase is also possible using the gas port of the dual port valve. However, it will be of a different composition to the liquid phase, due to the fractionation of the components in the cylinder and the natural vapour liquid equilibrium of the gas and liquid phases.
We also supply a wide range of gaseous process control calibration mixtures for other hydrocarbon processing applications such as, biogas production, natural gas processing, natural gas custody transfer and LNG production.
