Liquefied gas mixtures, or ‘liquid mixtures,’ are often used for calibrations where the application demands calibration with a ‘heavy’ or longer-chain hydrocarbon (see Demystifying hydrocarbon gas mixtures in non-refillable cylinders). Liquid mixtures have the advantage of closely matching the product being analysed (e.g. stream gas), while also offering increased efficiency by allowing an increased number of calibrations with the same cylinder, when compared to a gaseous mixture.
While liquid mixtures do have significant advantages, there are some disadvantages to consider. As is described in our previous blog on Issues with liquefied calibration gases, there are rules that can be followed to mitigate the issues. One of those being choice of cylinder type.
Traditionally, liquefied gas mixtures were supplied in regular high-pressure cylinders with dual port dip tube (DP/DT) valves. Maintaining the mixture in liquid phase is achieved by it being kept under pressure by an overpressure gas, usually helium. While this is the most cost-effective solution, it is not without its faults:
- Inevitably some of the lighter components will evaporate into the headspace.
- Some of the headspace gas will also dissolve into the liquid.
- Both outcomes will irreparably alter the composition of the calibration mixture.
- The emptier the cylinder becomes the more these effects are magnified.
Separate the liquid mixture from the gas headspace entirely.
A physical barrier is the most effective way. This has typically been achieved using a Welker Constant Pressure (CP) cylinder. Now customers also have the option of choosing Effectech’s patented CLV (collapsible liquid vessel) cylinder. Determining which is the most appropriate solution is dependent on the customers’ requirements.
What is a Constant Pressure (CP) Cylinder?
The CP cylinder is designed with internal pistons and seals. The cylinder is pressurised with an inert gas supply, before being turned around and filled slowly from the opposite end. By letting the gas push against the piston, slowly venting the pre-charge gas, the sample is taken at full line pressure from start to finish. While the cylinder is being emptied, full pressure is being maintained and the gas composition is not being altered as a result.
Advantages of CP cylinders:
- The CP cylinder is a historically proven technology. The CP cylinders have been around a very long time, many operators are familiar with using them and the track record of performance is indisputable.
- The CP cylinder can accommodate a much wider variety of compositions.
- The mixers. CP cylinders come with a variety of mixers which can be critical for maintaining homogeneity of the compositions.
What is a Collapsible Liquid Vessel (CLV) Cylinder?
The CLV uses a collapsible bag inside a standard cylinder with conventional dual port dup tube valve to hold the liquid. There is a physical barrier between the liquid (which is inside the bag) and the overpressure which is inside the cylinder. The new, patented technology keeps the LPG / NGLs in a single liquid phase which provides a stable liquid composition over the lifetime of the cylinder.
Advantages of CLV cylinders:
- CLV cylinders require lower upfront cost and no servicing costs.
- The CLV can hold a high volume of liquid, reducing the mixtures cost per litre in addition to reduced handling and procurement costs through needing to order them less frequently.
- Physical size. CLV cylinders can hold large volumes of gas in a cylinder of manageable size.
Which one should you choose?
The choice will come down to several factors as mentioned above but most important is probably the composition of the sample. Contact us today to discuss your specific application or for more information on our specialty gas & calibration gas mixtures.