Author: Andrzej Jopek, Process Analytical Middle East WLL
Acknowledgements: Pressure Tech UK, ASaP nl, Dirk Horst, David Tamez, Tony Waters.
Process analyzers are an expensive investment but can only deliver valid results if presented with a representative sample during calibration and online use. In instances where heated regulators are deployed to vaporize liquid hydrocarbons it is critical to recognize the chances of sample fractionation / evaporation /condensation and how cumulative errors can impact the final result.
In such cases, an expensive analyzer installation can be held hostage to an in-expensive heated regulator. Today, our understanding of these problems coupled with modern manufacturing techniques has allowed for a new design of regulator that is better suited for vaporizing liquid hydrocarbons whilst maintaining the same familiar packaged product. The following discussion re-focuses on understanding the working design principles of heated regulators with a historical background on how they were introduced to analytical systems.
Heated regulators were introduced in the early 1980’s. Construction consists of a pressure regulator assembly with a centrally located heating element that is uniquely designed to address various problems whilst handling vapor samples. This design still remains valid today and is found in many brands of heated regulators that provide an inexpensive solution to pressure reduction. The heated regulator transitioned to vaporizing liquid hydrocarbons in the days when Liquid Sample Injection Valves (i.e. LSV – flash vaporizers) used on process gas chromatographs could not handle high pressures or were not as reliable as they are today. As heated regulators found their way into sample handling systems they became a low-cost generic solution for many vapor and liquid applications including cryogenic installations such as LNG Loading- thus becoming commonly known as the continuous flow vaporizing regulator.
As installations evolved, so have many different problems associated with vaporized samples such as unstable readings, polymerization, burned out heating elements or plugged vaporizers mostly as a result of misapplication or incorrect temperature control settings. Today we know that using existing designs of vaporizing regulators have very limited uses on liquid hydrocarbons. This has led to new innovative developments in flash vaporizers (specifically on LNG applications) and improved LSV arrangements in other equipment. It should be noted that, flash vaporizers inject or pass a microlitre-sized amount of liquid sample into a fixed volume, temperature-controlled chamber. This attains instant homogenous vaporization resulting in accurate analysis results. Continuous flow vaporizing regulators operate in an opposite manner to flash vaporizers yet expected to deliver the same performance. Most vaporizing regulators have a heating element located within the sample inlet path. This can have a significant negative impact on liquid sample integrity thus becoming nothing more than a fractionating regulator. The illustration below shows what happens to a liquid hydrocarbon sample as it passes over a heating element inside a typical heated regulator. This effect is more evident in liquid samples having a wide boiling point, typically C1 to C4+. Obtaining any reasonable balance of outlet pressure, flow and temperature can be a major challenge.
Heated Regulators for Vapor Samples.
Heating is achieved by an electrical heating element positioned centrally within the regulator body. The position of this element allows for a desired heat conductance path to the regulator body in a radial manner extending upwards. The vapor sample is passed through the inlet towards the heated chamber and around the heater to ensure the vapor is kept above its dewpoint whilst eliminating droplets or mists. As pressure is reduced via the poppet and seat arrangement a Joule-Thompson (J/T) effect is formed within the diaphragm and orifice locations.
The heater temperature is adjusted to overcome the J/T effect whilst providing a relative limited amount of heat to keep the whole regulator body above the sample dewpoint under normal flow conditions. The amount of heat required will vary depending on stream composition, process pressure, sample flow and surrounding temperatures for the given application.
The sample exits from around the diaphragm towards the outlet which is in close proximity to the heated chamber ensuring the sample remains warm. After a while, an overall equilibrium temperature is reached which may require further optimization during use.
Certain high pressure vapor applications can exhibit a significant amount of J/T effect during pressure reduction where there is simply not enough heat available to overcome J/T or icing effects during operating conditions. This leads to vapor samples condensing to liquids at the outlet as well as internal mechanical diaphragm failures resulting in fugitive gas releases through the bonnet vent port. These problems cannot be overcome via a single heating element but may require flash vaporizers, multistage pressure reduction or a dual element heated regulator whilst maintaining minimal flow rates.
Heated Regulators for Vaporizing Liquid Samples.
Liquid hydrocarbons need to be kept below their bubble point and away from any heat source.
The following design incorporates an insulated inlet made from a Polyimide material having a low thermal conductivity of 0.12 W/(mK) compared to that of 16.3W/(mK) for 316 stainless steel.
The polyimide insulation material provides thermal insulation of the liquid sample from point of entry into the vaporizer through to the vena contracta where the pressure is the lowest and fractionation is greatest. By ensuring the integrity of the insulation is maintained up to this point, a supercritical liquid is transformed to a supercritical vapor avoiding fractionation. (Note: Not all liquids are suitable for such vaporization therefore application dependent).
Bore and CV sizes can be made to suit specific liquid densities and flow requirements. Various sized Service Kits are available for easy retrofit.
The outlet heater is offset and biased towards applying heat in the post pressure reduction area. A heater cartridge sits within a solid spiral insert having a large surface area to promote good heat transfer to the
outward flowing sample. By using a spiral insert, the electrical power density is increased by 11.4% to 4.93 W/cm2 on a 100w heater. The heater temperature is adjusted to overcome any J/T effects and to keep it
above the sample dewpoint during operational use. Higher flowrates are achievable due to the regulator having a large body mass enabling it to retain temperature whilst transferring heat to the sample.
Typical applications: Refinery, chemical and ethylene plants, liquefied propane, butane, labs and limited cryogenic samples.