Portable Cloud Point Detector Provides High Resolution, High Accuracy Measurements Of Opaque Oil Samples
Crude oil and crude oil products contain substantial amounts of petroleum waxes. These waxes or paraffins have a limited solubility in oil and tend to precipitate out at a temperature called the cloud point. The cloud point is of practical importance because precipitation and deposition of paraffins results in a narrowing of production pipelines making crude oil recovery difficult.
Thickness shear mode (TSM) resonator technology developed at Sandia National Laboratories has recently been used to monitor crude oil cloud point. The resonator in contact with a test fluid is sensitive to changes in the liquid parameters of viscosity and density which change at the cloud point. An AT-cut quartz resonator excited into mechanical resonance interacts mechanically with a contacting fluid. This coupling of the resonator to the test fluid results in both a decrease in resonant frequency of the TSM and resonance damping. When the resonator is used as the frequency control element of an oscillator circuit and changes in frequency are measured, changes in the liquid parameters relating to cloud point are easily seen. The TSM resonator Based cloud point detector has a number of advantages:
A portable prototype TSM Cloud Point Detector (CPD) has performed flawlessly during field and lab tests proving the technique is less 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 pressures up to 100 psi. The closed container minimizes losses of low molecular weight volatiles, allowing samples to be retested with the addition of chemicals. By cycling/thermal soaking the sample, the effects of thermal history can be investigated. The CPD is portable, suitable for shipping to field offices for use by personnel without special training or experience in cloud point measurements. 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. The resonator consists of a thin, highly polished, quartz disk with circular electrodes on both sides. When excited at it’s resonant frequency, the crystal vibrates in a thickness shear mode (TSM), the faces moving with in-plane displacement.
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 maintain 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. A damping voltage derived from the level control circuitry is made available for measurement purposes. 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 damping voltage. 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 an oil sample contained in a small (25 ml) glass sample cup which is placed inside the detector (see Figure 1). The closed detector volume prevents the loss of volatiles from the container.
|Figure 1. Portable CPD with TSM resonator, oscillator circuit, thermocouple, sample cup, stir bar and driver, thermoelectric elements, heat sinks, and fan.|
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 measures the temperature of the oil sample and provides input to a temperature controller that switches the power supplies that drive the thermoelectric elements. A fan forces air past the fins of the thermoelectric element heat sinks. 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.A stir bar and stir bar driver are provided.
The stir bar can be driven to rotate at up to 1,600 rpm. By monitoring both the resonant frequency and damping voltage, the device gives two related determinations of the cloud point. The change in liquid properties at the cloud point causes a sudden change in these measurements that can easily be used to visually identify the cloud point (see Figure 2).
|Figure 2. Change in resonant frequency and damping voltage at cloud point.|
The nature of this change is normally sharp enough that the cloud point can be automatically picked by a mathematical algorithm. The data shown in Figure 2 was taken with the portable CPD recording at a rate of once every 6 sec. with a cooling rate of 1° F per minute: approximately one data point every 0.1° F of temperature change. The figure shows that the cloud point can be easily determined to within 0.2° F. The repeatability of the device is excellent.
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