Cloud-Point Detection Using a Portable Thickness Shear Mode Crystal Resonator
Authors: Mansure A.J. , Spates J.J. , Germer J.W. , Barker K.M. 
Published: December 2007

The Thickness Shear Mode (TSM) crystal resonator monitors the crude oil by propagating a shear wave into the oil. The coupling of the shear wave and the crystal vibrations is a function of the viscosity of the oil. By driving the crystal with circuitry that incorporates feedback, it is possible to determine the change from Newtonian to non-Newtonian viscosity at the cloud point. A portable prototype TSM Cloud Point Detector (CPD) has performed flawlessly during field and lab tests proving the technique is less subjective or operator dependent than the ASTM standard. The TSM CPD, in contrast to standard viscosity techniques, makes the measurement in a closed container capable of maintaining up to 100 psi. The closed container minimizes losses of low molecular weight volatiles, allowing samples (23 ml) to be retested with the addition of chemicals. By cycling/thermal soaking the sample, the effects of thermal history can be investigated and eliminated as a source of confusion. The CPD is portable, suitable for shipping to field offices for use by personnel without special training or experience in cloud point measurements. As such, it can make cloud point data available without the delays and inconvenience of sending samples to special labs. The crystal resonator technology can be adapted to in-line monitoring of cloud point and deposition detection.

Introduction

This paper describes a new technique for measuring cloud point using a quartz crystal resonator. The resonator consists of a thin, highly polished disk with circular electrodes on both sides (Figure 1). when excited at resonance frequency, the crystal vibrates in a thickness shear mode (TSM), the faces moving with in-plane displacement (Figure 2).

The TSM resonator is instrumented as a sensor by incorporating it as the frequency control element of an oscillator circuit. This oscillator tracks the resonant frequency as it responds to changes in the fluid contacting the crystal. The oscillator also has level-control circuitry to measure the amplitude of the oscillation voltage. The level control compensates for changes in resonator damping caused by changes in the viscosity of the contacting fluid and/or deposits that form on the crystal. Changes in the fluid contacting the crystal cause changes in both the stored and dissipated energy in the crystal, leading to a change in resonant frequency and crystal damping. For a Newtonian fluid, both the resonant frequency and damping voltage decrease proportionally with the square root of the density viscosity product.

The crystal is immersed in oil in a small (25 ml) glass sample cup which is placed inside a closed container (see Figure 3). A 100 psi pressure relief valve ensures excessive pressure does not build up inside the container. The container is heated and cooled by four thermoelectric elements. A thermocouple sticks down into the sample cup to measure the temperature of the oil itself and provides input to a temperature controller which switches the power supplies that drive the thermoelectric elements. A fan forces air past the fins of the thermoelectric element heat sinks. Since air is the ultimate heat source/sink for the device, the operating temperature range may be affected by ambient air temperature.

For an ambient air temperature of 70 F, the device can heat the oil in the sample cup to over 190 F and cool the oil to 45 F. Operation of the device has been limited to 185 F to minimize the potential for damage to the oscillator circuitry. Limiting operation to this range reduces the potential to boil any water included in the oil and reduces the potential to reflux the oil. P. 593^