Expert Knowledge
Wax Precipitation Temperature WPT
Definition of Wax Precipitation Temperature WPT
The Wax Precipitation Temperature (WPT) is the temperature at which the first wax deposits form on surfaces from a crude oil or oil product. The wax deposition temperature is always below the wax appearance temperature. It is these deposits that lead to severe impairments and even clogging of the pipeline, particularly during transportation and storage of crude oil and oil products.
Determination of WAT & WPT
The precise determination of the wax appearance temperature WAT and the wax precipitation temperature WPT is essential for evaluating the transportation stability of crude oil and oil products. While the wax appearance temperature is independent of external conditions, the wax deposition temperature is influenced by several factors. These include the temperature difference between the liquid and the surface, the nature of the surface and the flow rate.
The wax appearance temperature indicates the point at which the first wax crystals form in the oil. As this is a physical process, it is important to reduce the temperature slowly so as not to disturb the thermal equilibrium.
In contrast, the wax precipitation temperature is not constant, but varies depending on the conditions. The measurement should be carried out under realistic transport or storage conditions in order to obtain reliable values. Different measurement methods take various parameters into account in order to provide the most accurate results possible.
Measuring methods for detecting the Wax Precipitation Temperature WPT
Approved measuring methods for determining the Wax Precipitation Temperature WPT are the Cold Finger and the Wax Flow Loop.
Measuring principle of the cold finger
The cold finger measuring principle simulates the temperature difference between a crude oil sample and a cooler environment, such as a pipeline wall. This temperature difference leads to the deposition of wax contained in the oil, which simulates realistic ambient conditions in the laboratory.
This works as follows with the Cold Finger laboratory device: The metallic cold finger is placed in stirred and tempered petroleum samples. The wax contained in the crude oil is deposited on the surface of this cold finger. To determine the amount of deposits, the cold finger is removed and weighed either once or at fixed intervals.
The cold finger setup: classic in a water bath or modern in a dry bath
The oil sample can be tempered in different ways. The classic method uses an open water bath, but this has limitations in terms of temperature range, handling and work safety.
This is why PSL Systemtechnik developed a modern alternative in 2011 with the Multi-Rack Cold Finger CF15. The introduction of a dry bath has improved temperature control, increased safety and optimized handling. In addition, the new design enables easier operation and extended measuring options thanks to integrated drawers.
Aim and benefits of cold finger measurements
How does the oil behave under the conditions and what influence do the inhibitors have?
The aims of the cold finger tests are to measure the wax precipitation temperature WPT, the total wax content, the wax appearance temperature WAT for practitioners (see excursus WAT in practice) and the influence of shear on the amount of wax deposited. The comparison of untreated and treated samples, i.e. inhibitor samples, provides information on the effectiveness of the inhibitors used.
The following tests can be carried out with the Cold Finger:
Determination of the wax precipitation temperature WPT
Determination of the wax appearance temperature WAT
Amount of wax deposits over time at a constant temperature
Total wax content of the crude oil sample
Shear effects depending on the flow velocity
Developing, testing and optimizing wax and kerosene inhibitors
Comparison of the measurement results of an untreated and an inhibited sample
Monitoring and avoiding wax deposits
The Cold Finger laboratory instrument thus proves to be an important basis for monitoring and avoiding undesirable wax deposits in pipelines, during storage and in industrial processes.
PSL is your expert for laboratory equipment in flow assurance
You can choose between 4 cold finger models.
Measuring with the Wax Flow Loop
In a test loop an amount of sample is pumped through a thermostated pipeline section. In this section the wall temperature of the pipeline can be adjusted by heating or cooling. Pressure- and temperature measurements before and after the section show changes resulting from wax depositions. If the wall has a temperature below the WAT of the sample fluid wax depositions will occure.
These deposits leads to a reduced diameter of the pipeline and by this to a pressure drop that can be detected. In addition the wax layers on the wall have a thermal insulation effect which is showing in changes in the differential temperature. The accuracy of this method is not as high as the DSC or the CPM but in opposite to those methods the sample can be examined under flow conditions.
Wax Flow Loop WL
The fully automated Wax Flow Loop from PSL Systemtechnik is a compact laboratory system for investigating the precipitation and deposition of waxes, kerosenes and asphaltenes in pipelines.
Wax and Wax Flow Loop
The WAT can statically be measured very accurate and with low effort. In daily work it is observable that falling below the WAT does not automatically lead to problems with wax depositions. The wax layers are soft during their formation and vulnerable to shear forces exerted by the flowing fluid.
With the Wax Flow Loop the behaviour of oil and oil products in the production system can be examined with a flowing medium. The variable flow rates allow simulations of different application-fields. The wide-range temperatured test pipeline allows simulation of extreme environmental conditions like cold regions or sub-sea. Not only the behaviour of the flowing medium can be examined but also inhibitors of any kind can be tested for their performance, compared relatively and classified in absolute ways.
Exchangeable test pipelines enable measurements across different pipe diameters and thus the evaluation under widely varying flow conditions. Further experimental evaluations and comparisons are also possible over different capillary lengths. As investigations have shown, the surface quality (roughness and material) is only of secondary importance, if there are any effects at all on wax deposition.
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