Expert Knowledge
Wax appearance temperature WAT
Definition of wax appearance temperature WAT
The Wax Appearance Temperature (WAT) is the temperature at which the first wax crystals form in a crude oil or oil product. The wax crystals initially remain finely dispersed in the liquid as a suspension. The WAT therefore indicates when the first wax crystals form, but it does not indicate at what temperature and in what quantity the wax crystals are deposited on cooler surfaces.
Determination of WAT & WDT
The determination of the wax appearance temperature WAT and the wax deposition temperature WDT must be carried out as accurately as possible in order to assess the transport stability. The wax appearance temperature of a crude oil or an oil product is independent of the ambient conditions. Wax crystallization is a physical process. The wax deposition temperature, on the other hand, can vary greatly for a product, as it depends on several influencing factors, e.g. temperature difference between liquid and surface, surface properties, flow velocity, etc.
When determining the wax appearance temperature, the tempering process must be as slow as possible so that the thermal equilibrium state is maintained and crystallization has sufficient time. Cooling too quickly leads to delayed crystallization. The WAT measurement result obtained is then too low and is therefore falsified. The wax deposition temperature, on the other hand, must be determined under realistic ambient conditions during transportation or storage. The different measurement methods therefore consider different parameters.
Measuring methods for detecting the Wax Appearance Temperature (WAT)
There are various measurement methods for determining the wax appearance temperature (WAT). The methods differ mainly in terms of accuracy and the time, personnel and equipment required. The required sample quantity is very small for all methods. The following overview is sorted in descending order of accuracy. It contains the Cross Polarization Microscopy CPM, the Optical WAT Detector OWD and the Differential Scanning Calorimetry DSC.
Cross Polarisation Microscopy CPM
The CPM method is a microscopy with polarized light. It uses the effect that wax crystals that form change the plane of polarization of the light and can therefore be detected.
Specifically, a melted sample is applied as a thin layer to a transmitted light microscope in a tempering cell. Polarization filters 3 are set below and above the sample so that no more light passes through the filter arrangement. This is the case when the polarization planes of the filters are at a 90° angle to each other. The light, which would normally pass through the sample unhindered, is now first polarized at the first filter (primary filter), then passes through the sample and reaches the second filter (secondary filter, analyser), where it cannot pass due to the possible vertical plane of oscillation.
The CPM makes use of the fact that crystals in solutions can rotate the plane of polarization of light, i.e. change it (optical anisotropy). If crystals are formed when the sample cools down, these crystals rotate the plane of oscillation of the polarized light. Points of light now appear in the previously dark secondary filter at the points where wax crystals are located. The first appearance of these points of light represents the wax appearance temperature. Due to the high magnification of the microscope, even the smallest crystals are clearly visible. This method is very accurate, but also time-consuming and labor-intensive, as it usually involves direct optical observation.
Advantages and disadvantages of the CPM method
Advantages
- most accurate method for measuring WAT
- Microscope can be used for other applications
- Crystal sizes and quantities can be determined
Disadvantages
- Most expensive method due to high acquisition costs
- Complex sample preparation
- High time expenditure for measurement and evaluation
Optical WAT Detector OWD
The OWD method works with polarized light, using the light cross-polarization principle and a highly sensitive light sensor instead of a microscope. Like the CPM method, it uses the property of wax crystals that they change the polarization of light and are thus detected.
A few drops of the sample are placed on the tempering cell of the Optical WAT Detector. Then the two polarization filters, which are installed above and below the sample, are manually aligned so that all light is eliminated and the light sensor no longer receives any light. The OWD now heats the sample to the specified starting temperature, then begins the measurement and cools the sample slowly and continuously until the final temperature is reached. As soon as the first wax crystals form, they change the polarization plane of the light and lead to an increase in light intensity on the light sensor. This first increase in intensity, i.e. the start of crystal formation, is determined as the wax appearance temperature. This makes the method very accurate and comparable with the CPM method.
The software records the temperature curve and the light intensity, displays them as a graph and thus enables a quick and precise evaluation of the measurement.
Advantages and disadvantages of the OWD method
Advantages
- Very accurate method for measuring WAT
- Quick and easy measurements
- Low acquisition costs
- Mobile use, laboratory-independent
- Can be used for all crystallization processes
Disadvantages
- New measuring method
- No standard yet
- Largely unknown method
Optical WAT Detector
The Optical WAT Detector OWD allows fast and precise measurement of the formation of wax crystals in oil samples. This is done with the integrated heating and cooling unit and the highly sensitive light sensor.
Differential Scanning Calorimetry DSC
Differential scanning calorimetry measures the amount of heat that a sample absorbs or releases in comparison with a reference sample that is subjected to the same temperature profile. If the difference in the amount of heat between the sample and the reference changes, this is an indication that the thermal behavior of the sample has changed. Changes in the thermal behaviour of the sample can be caused by a variety of physical and chemical processes, e.g. chemical reactions, crystallization, evaporation, etc.
The process of crystallization is of interest for determining the wax appearance temperature. This is always associated with a change in heat flow. Phase changes such as crystallization are always endothermic or exothermic, i.e. “heat-consuming” or “heat-generating”. This leads to a slight deviation from the thermal behavior of the reference sample and can be determined as WAT. However, since a certain amount of wax must crystallize in order to measure a signal, the result shifts towards lower wax appearance temperatures compared to the optical measurement methods.
Advantages and disadvantages of DSC method
Advantages
- approved test procedure
- very broad range of applications
Disadvantages
- expensive method due to high acquisition costs
- cannot be used for oils with a low wax content
- only starts when a large number of wax crystals are present
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