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Background Knowledge Wax and Paraffin

Wax and paraffine in crude oil

The groups of waxes and paraffins are often combined when crude oil is concerned. Under the same circumstances both groups lead to the same problems. Thus, the terms "wax" and "paraffin" are used synonymous.

Crude oil is a very heterogeneous alloy of several thousands of single components of mainly hydrocarbons. The amount of heteroatoms like sulfur, oxygen, nitrogen and metalorganic compounds differs from oil to oil. The crude contains many different fractions from soluted gases (gaseous under standard conditions = atmospheric pressure and room temperature) up to high molecular compounds like resins and asphaltenes which can be classified and separated by their chemical and physical properties.

Paraffines are a part of the homologeous series of alkanes - the saturated series of hydrocarbons - with the general formula CnH2n+2. Paraffines are classified as the part from about C18. Due to the numerous possibilities of branched iso-alkanes the physical properties of paraffins are wide-spreaded.

Waxes are a very heterogeneous group of higher hydrocarbons without a specific composition, often heteroatoms are contained. Waxes are defined by their mechanical and physical properties 1.

 

Problems with wax in crude oil

The crude oil composition varies strongly from reservoir to reservoir. The spectrum of extracted oil reaches from thin, low-viscous and straw-coloured oils with very low amounts of wax to deep black, highly viscous (solid at room temperature) oils with high amounts of wax. A classification in paraffinic-waxy and naphtenic oils helps to estimate the behaviour of the oil but for accurate informations about the oil extensive tests under realistic conditions has to be executed.

If crude oil is cooled - in the borewell, at oil conditioning or during pipeline transport - below its WAT wax crystals start to form. These crystals can form deposits when coontacting any kind of surface like walls, valves etc. Over short periods of time these deposits can plug the equipment. During a continouos oil prodution they can be hold at a nearly constant level. Shear forces on the bounding surface of the wax layers - transmitted by the transpoted fluid - are increasing if the diameter is reduced by deposits. The soft wax layers are sheared of. A varying or stopping prodution flow can lead to formation of massive wax plug. These plugs can only be removed with high efforts: physically (by heating or by pressure pulses) or chemically (by using dissolvers). The economic costs and the production loss very high are in each case.

 

Control and avoidance of wax deposits

Considering that the removal of wax deposits is a very time-consuming and expensive process the prevention of the formation or if prevention is not possible the control of wax deposits is a main objective. This can be done like the removal in three different ways:

  • mechanical: deposits in pipelines can be removed by pigs (technical devices for inspection and cleaning of pipes) and at borewells special inserts for deposit removal are in use. These devices can be used regularly if certain parameters (time, dropping feed rate, pressure loss etc) are exceeded.
  • physical: due to the fact that the wax deposition is a temperature-dependant process, the oil or the production devices can be heated to stay above the WAT of the oil. So the wax crystal formation is avoided.
  • chemical: by adding chemicals already at the borewell the chemical properties of the crude can be influenced. The WAT can be shifted to higher temperatures that are near or better above the operating temperature. Also the growing of wax crystals can be reduced by using kinetic inhibitors which do not suppress the crystal formation but reduce the speed of the crystal growth rate. Another way is to allow the formation of crystals but to interfere in the agglomeration process with anti-agglomerats, the wax does crystallise but the crystals can not combine to bigger particles.

 

Wax Appearance Temperature (WAT) and Wax Deposition Temperature (WDT)

In crude oil paraffins and waxes are soluted in the low-molecular, liquid components. If the oil is cooled down the limit of solubility of these high-molecular compounds is under-run at a specific temperature. The component whose solubility is reached starts to crystallise, first micro-crystals will begin to formate. This process leads to a phase separation, the initial single phase alloy changes to a two-phase mixture. If the composition is not altered, crystallisation is a temperature-only process. Pressure dependency is also influencing the crystallisation, but it is of much lesser influence than temperature, so under test conditions with only small pressure changes it can be neglected.

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.

The Wax Deposition Temperature (WDT) 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.

 

Measuring methods For detecting the Wax Appearance Temperature (WAT)

Cold Finger: A temperature controlled metal cylinder (finger) is inserted in the sample and is cooled down slowly. At fixed time intervals the finger is taken out and examined for wax depositions. By this simple design many tests can be run simultaneously with only little effort. The accuracy of this method is low compared to others and the expenditure of time for a single test is high, but for screening tests this method has its advantages. Another feature of the cold finger is the measurement of the total amount of wax in a sample.

Differential Scanning Calorimetry, DSC 2: With this method the amount of heat that is emmitted or absorbed by a sample can be compared with a reference sample that is subjected to the same temperature profile.

The process of crystallisation is always accompanied by a change in heat flux. Phase changes like crystallisation are always exothermic (heat releasing) or endothermic (heat absorbing). This effect leads to a slight variance in the thermal behaviour of the sample compared to the inert reference. These changes in heat flux by crystallisation process allow a very accurate measurement of the WAT.

Cross Polarisation Microscopy, CPM: A completely molten and temperature controlled sample is applied as a thin layer on the sample holder of a light-optical microscope. Below and over the sample light-polarizers3 are installed. The polarizers are adjusted in way that no light can penetrate the second filter. This is achieved if the planes of polarisation are at right angle. The light is polarised at the first filter (primary polariser), penetrates the sample and is blocked at the second filter (secondary polariser, analyser) due to the fact, that polarised light can only pass through a polarizer if the planes of polarition are equal.

The CPM takes advantage of the fact that many crystals can turn the plane of polarisation of light. This effect is called optical anisotropy. In the slowly cooled sample crystallisation starts when the WAT is reached. The emerging crystals are anisotropic to light and turn the plane of polarisation. On the before dark secondary polariser now little points of light start to appear. This method is very accurate but time- and work-consuming. 

This method is used by our Optical WAT Detector. Instead of a microscope it uses an ultra-high sensible light sensor.

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.

 

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 Optical WAT Detector OWD method

AdvantagesDisadvantages
  • very precise method for measuring WAT
  • new measuring method
  • quick and easy measurements
  • no standard yet
  •  low acquisition costs
  • largely unknown method
  •  mobile use, laboratory independent
 
  • can be used for crystallization processes
 

 

 

Differential Scanning Calorimetry, DSC-method

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 the Differential Scanning Calorimetry, DSC-Methode

AdvantagesDisadvantages
  • approved test procedure
  • expensive method due to high acquisition costs
  • very broad range of applications
  • cannot be used for oils with a low wax content

 

  • only starts when a large number of wax crystals are present

 

Measuring methods for detecting the Wax Deposition Temperature (WDT)

Approved measuring methods for determining the Wax Deposition Temperature WDT are the Cold Finger and the Wax Flow Loop.

 

Measuring principle of the cold finger

The measuring principle of the cold finger simulates the temperature difference between the crude oil sample and an external wall, e.g. pipeline. This simulates the ambient conditions.

The cold finger laboratory device is constructed as follows: a metallic cooling finger, the cold finger, is placed in a stirred and tempered crude oil sample. The wax contained in the oil begins to deposit on the surface of the cold finger. The amount of wax deposits is weighed. The cold finger is removed either once or several times at fixed intervals and the amount of wax deposits is weighed.

 

The cold finger setup: classic in a water bath or modern in a dry bath

The devices differ significantly when it comes to tempering the oil sample. The original/classic cold finger method uses an open water bath. This has limitations in terms of temperature range, handling and workplace safety.

PSL Systemtechnik revolutionized the design of the cold finger in 2011. With the Multi-Rack Cold Finger CF15, the method was modernized and adapted to the state of the art. PSL Systemtechnik thus created a new standard in cold finger measurement for petroleum laboratories. One part of the innovation is the dry bath, an electrical temperature control of the sample. This enables a wider temperature range with higher work safety. In addition, the samples have been integrated into drawers. This significantly simplifies handling and allows further measurement options.

 

Aims 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 deposition temperature WDT, 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 deposition temperature WDT
  • Determination of the wax appearance temperature WAT - for practitioners, see excursus WAT in practice
  • 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.

 

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 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.

An exchangeable test pipeline with different diameter and length options allows measurements over a wide range of flow conditions. Studys show that the nature of the inner surface (texture, material) has only very little (if any) effect on the wax deposit formation.

Use of the terms Wax Appearance Temperature (WAT) and Wax Deposition Temperature (WDT) in practice

In practice, a precise distinction is often not made between wax appearance temperature and wax deposition temperature, because in practice it is not the first appearance of wax crystals that leads to problems in the processes, but the wax deposits. By equating the two terms, the Cold Finger and Wax Flow Loop measuring methods are therefore also used to determine the wax appearance temperature. Ultimately, however, these are devices that simulate the ambient conditions in the transportation or storage of crude oils and determine, among other things, the temperature at which the waxes are deposited; the first appearance of wax crystals cannot be measured in this way.

 

 


Footnotes:
  1. Wax (Wikipedia)
  2. DSC (Wikipedia)
  3. Polarizer (Wikipedia)

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