Technical Field

The present invention generally relates to a system, an apparatus and a method for the solidification of hydrocarbon wax from a liquid hydrocarbon mixture containing said hydrocarbon wax and non-wax hydrocarbons.

Background

During onshore crude oil production, the temperature of crude oil decreases as it is transported upwards through the production pipeline from the underground oil reservoir at subsurface levels to the relatively cooler earth surface where the oil extraction facilities are located. Similarly, the temperature of crude oil in offshore crude oil production decreases as it is transported upwards through the production pipeline from the oil reservoir at subsurface levels to the relatively cooler ocean floor. Subsequently, the temperature of the crude oil decreases as it is transported through the ocean to the earth surface.

The temperature at the onset of crystallization of hydrocarbon compounds typically increases with an increase in molecular weight and carbon number. Accordingly, waxy high carbon compounds in crude oil crystallize at higher temperatures than compounds with a lower number of carbons.

Therefore, when crude oil is transported from warmer to cooler. zones, the waxy high carbon compounds crystallize before the compounds with a lower number of carbons do. However, because crude oil is a complex mixture of hydrocarbon compounds, the onset of crystallization of waxy high carbon compounds, known as the wax appearance temperature (WAT) of the crude oil, is unique for each crude oil sample. Whenever the temperature of the zone falls below the WAT of the crude oil, the wax in the oil film near the inner wall of the pipeline crystallizes because the temperature of the crude oil is the lowest at the pipeline wall and increases radially inwards from the pipeline wall to the centre of the pipe. This liquid film is like a jelly layer and contains both liquid oil and floating wax crystals. The jelly layer is more viscous than the crude oil containing no wax crystals flowing through the centre of the pipe and is therefore almost a no-flow zone. If the jelly layer is not removed, it grows on the inner walls of the pipeline and constricts the cross-sectional flow of the pipeline, resulting in a decrease of the production rate of crude oil. This flow constriction may also contribute to the temperature drop due to the Joule Thomson effect if gas is present in the crude oil.

Many oil fields produce heavy crude oil which has a high amount of waxy high carbon compounds. Therefore, the decrease in production rate due to wax deposits in these oil fields can cost significant economic losses.

Methods used to treat wax deposits include chemical treatment, thermal treatment and mechanical treatment. Chemical treatment methods, such as the use of pour point depressants and wax inhibitors, alter the natural properties of the crude oil to reduce the formation of wax crystals. However, the chemical additives are relatively expensive and a high amount of additives are needed due to the frequency of chemical treatment.

In thermal treatment methods, hot oil or hot water is flushed into the pipeline to melt the wax deposits. However, thermal treatment requires plant shutdown, which translates into higher economic losses.

Mechanical treatment methods include the use of pipeline inspection gauges (PIGs) and wax cutting. PIGs are inserted into surface pipelines . to clean residue deposited on their inner walls and are pushed along the line by the pressure of the oil flow. However, thick wax deposits that narrow the inner diameter of the pipeline may cause the PIGs to get stuck. Intelligent pigging may be used to remove the stuck PIG, but intelligent PIGs on the market are currently still very expensive. It is also difficult to identify the section of the pipeline that is plugged.

There is therefore a need to provide a method of treating wax deposits that overcomes, or at least ameliorates, one or more of the disadvantages described above .

Summary

According to a first aspect, there is provided a system for the solidification of hydrocarbon wax from a liquid hydrocarbon mixture containing said hydrocarbon wax and non-wax hydrocarbons, the system comprising: a solidification zone for containing the hydrocarbon mixture therein; a thermal-contact surface disposed in the solidification zone for thermally contacting said liquid hydrocarbon mixture while in said solidification zone; and temperature control means in thermal communication with the thermal-contact surface and being configured to maintain the temperature of the thermal-contact surface below the wax appearance temperature of said liquid hydrocarbon mixture to thereby solidify said hydrocarbon wax while in said solidification zone. The system may further comprise scraping means which are capable of contacting the thermal-contact surface to at least partially remove said hydrocarbon wax that has solidified on the thermal contact surface.

According to a second aspect, there is provided a method for the solidification of hydrocarbon wax from a liquid hydrocarbon mixture containing said hydrocarbon wax and non-wax hydrocarbons, the method comprising the step of maintaining the liquid hydrocarbon mixture at a temperature below the wax appearance temperature to thereby solidify said hydrocarbon wax while in a solidification zone.

The method may further comprise the step of providing a thermal-contact surface disposed in said solidification zone to solidify said wax thereon. The fluid flow conditions within said solidification zone may be maintained in the turbulent flow regime.

According to a third aspect, there is provided an apparatus for the solidification of hydrocarbon wax from a liquid hydrocarbon mixture containing said hydrocarbon wax and non-wax hydrocarbons, the apparatus comprising: a solidification chamber for containing the hydrocarbon mixture therein; a thermal-contact surface disposed in the chamber for thermally contacting with said liquid hydrocarbon mixture while in said solidification zone and being configured, when in use, to be in thermal communication with a temperature controller for maintaining the temperature of the thermal-contact surface below the wax appearance temperature of said, liquid hydrocarbon mixture to solidify said wax from said liquid hydrocarbon mixture while in said solidification chamber. De initions

The following words and terms used herein shall have the meaning indicated:

The term "hydrocarbon (s) " refers to organic material with molecular structures containing carbon bonded to hydrogen. Hydrocarbons may also include other elements, such as, but not limited to, halogens, metallic elements, nitrogen, oxygen, and/or sulfur. The source of the hydrocarbons may be from crude oils, gas oils, crude oils, and refined petroleum.

The term "hydrocarbon wax" is to be interpreted broadly to include hydrocarbons of high molecular weight, including but not limited to, mineral waxes, such as paraffin wax and microcrystalline wax. Hydrocarbon wax typically comprises alkanes and may contain large amounts of iso-alkanes. More particularly, in the context of the present specification, high carbon hydrocarbon wax refers to hydrocarbons having typically more than 40 carbon atoms, while low carbon hydrocarbon wax refers to hydrocarbons having typically between 20 to 40 carbon atoms .

The term "wax appearance temperature", as used within the context of the present specification, refers to a temperature wherein hydrocarbon wax as defined above present in a mixture of liquid hydrocarbons begins to crystallize. The wax appearance temperature is dependent on the composition of the liquid ^' hydrocarbon mixture because of the complex thermodynamic influences of each component in multi-component systems. Thus, the wax appearance temperature of each mixture is unique.

The word "substantially" does not exclude "completely" e.g. a composition which is "substantially free" from . Y may be completely free from Y. Where necessary, the word "substantially" may be omitted from the definition of the invention.

Unless specified otherwise, the terms "comprising" and "comprise", and grammatical variants thereof, are intended to represent "open" or "inclusive" language such that they include recited elements but also permit inclusion of additional, unrecited elements.

As used herein, the term "about", in the context of concentrations of components of the formulations, typically means +/- 5% of the stated value, more typically +/- 4% of the stated value, more typically +/- 3% of the stated value, more typically, +/- 2% of the stated value, even more typically +/- 1% of the stated value, and even more typically +/- 0.5% of the stated value.

, Throughout this disclosure, certain embodiments may be disclosed in a range format. It should be understood that the description in range format is merely for convenience and brevity and should not be construed as an inflexible limitation on the scope of the disclosed ranges. Accordingly, the description of a range should be considered to have specifically disclosed all the possible sub-ranges as well as individual numerical values within that range. For example, description of a range such as from 1 to 6 should be considered to have specifically disclosed sub-ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to , from 2 to 6, from 3 to 6 etc., as well as individual numbers within that range, for example, 1, 2, 3, 4, 5, and 6. This applies regardless of the breadth of the range.

Detailed Disclosure of Embodiments

Exemplary, non-limiting embodiments of a system for the solidification of hydrocarbon wax from a liquid hydrocarbon mixture will now be disclosed. The hydrocarbon wax solidifies as crystals and these crystals are suspended in the interfacial layer of liquid hydrocarbons adjacent to the thermal-contact surface in the solidification zone, thus appearing as a jelly layer. In the disclosure herein, the terms "interfacial layer" and "jelly layer" are used interchangeably to refer to the thin film of liquid hydrocarbons adjacent to the thermal- contact surface that contain wax crystals present in suspension.

The size of wax crystals in the jelly layer depends on the number of carbon atoms present in the hydrocarbon wax. Specifically, the higher the number of carbon atoms in the wax, the smaller the size of the wax crystals.

The thermal-contact surface may be mounted on a support member that at least partially extends into said solidification zone. The scraping means may also be mounted on the support member. The support member may be longitudinal member, which is capable of rotating about a longitudinal axis extending concentrically through the support member.

In one embodiment, the thermal-contact surface may be diametrically opposed to the scraping means and may remain relatively immobile with respect to the scraping means. Accordingly, the scraping means may be configured to rotate along with the support member within said solidification zone. Advantageously, the movement of the scraping means may facilitate better homogenous heat exchange between the thermal-contact surface and the liquid hydrocarbon mixture.

The speed of rotation of the support member may be configured to maintain the fluid flow conditions within said solidification zone of said hydrocarbon mixture in the turbulent flow regime. In one embodiment, the speed of rotation of the support member is configured to maintain the fluid flow conditions within said solidification zone of said hydrocarbon mixture at a Reynolds number in the range of about 200 to about 10, 000. Preferably, the fluid flow conditions may be maintained at a Reynolds number in the range of about 200 to about 5, 000, or about 300 to about 10,000, or about 500 to about 10,000. Most preferably, the fluid flow conditions may be maintained at a Reynolds number in the range of about 500 to about 5,000.

If the Reynolds number is lower than 200, the shear rate of the flow may not be sufficient to induce turbulent burst actions and remove the larger crystals of lower carbon wax compounds from the jelly layer. If the Reynolds number is higher than 10,000, the excessive shear of the turbulent flow will totally remove the jelly layer into the main liquid hydrocarbon flow, preventing an accumulation of undesirable smaller wax crystals of high carbon wax compounds in the jelly layer.

Advantageously, the burst activity of the turbulent flow causes the larger crystals of lower carbon wax compounds to mix with the relatively warmer liquid hydrocarbon mixture and melt, thereby re pining the main liquid hydrocarbon flow. Consequently, the undesirable, smaller crystals of high carbon hydrocarbon wax may be progressively concentrated in the jelly layer resulting in an accumulation of high carbon wax. Advantageously, the ability and ease of the disclosed system to selectively accumulate high carbon wax from a hydrocarbon mixture is an improvement over conventional mechanical treatment methods.

The turbulence of the hydrocarbon mixture also causes effective agitation so that heat transfer becomes more efficient and uniform in the solidification zone, ensuring that a majority of the hydrocarbon wax is crystallized.

The support member may comprise plural arms that extend from said support member at an angle that is offset from said longitudinal axis. The plural arms may be generally perpendicular to the longitudinal axis. In one embodiment, the thermal-contact surface is disposed on the plural arms that extend radially from the support member. Advantageously, the plurality of arms maximizes the contact surface area for heat transfer between the arms and the hydrocarbon mixture. Further, the plurality of arms maximizes the surface area for the crystallization of hydrocarbon wax.

In another embodiment, the scraping means is disposed on the plural arms. Advantageously, the arms help to agitate the hydrocarbon mixture as the support member is rotated, ensuring that a desired level of turbulence is reached.

The support member may be an annular pipe. The annular pipe may extend through the solidification zone. The annular pipe may be in fluid communication with the plurality of arms and the thermal-contact surface. Advantageously, the annular pipe may extend through the solidification zone to provide a larger surface area for effective heat transfer.

The pipe may contain a liquid coolant that is capable of imparting to the thermal-contact surface a temperature lower than the wax appearance temperature. The liquid coolant may flow within the plurality of arms along the support member to cool the thermal-contact surface. In one embodiment, the flow of the liquid coolant may be controlled such that the . temperature of the thermal- contact surface is maintained below the wax appearance temperature of the liquid hydrocarbon mixture. In one embodiment, the liquid coolant is chilled water. In another embodiment, the liquid coolant is glycol.

In one embodiment, the scraping means may be mounted on the support member and is capable of rotating, thereby causing abrasion between the scraping means and the thermal-contact surface. The rotary movement of the support member causes abrasion between the scraping means and the thermal-contact surface, resulting in the removal of the concentrated high carbon wax. Advantageously, the smaller crystals of high carbon wax that have been concentrated in the interfacial layer adjacent to the thermal-contact surface may be intermittently dislodged and separated from the bulk liquid hydrocarbon mixture, thereby preventing the liquid hydrocarbon flow in the solidification zone from plugging. Advantageously, the wax crystals may be constantly removed from the thermal- contact surface, thereby permitting continued effective heat transfer between the thermal-contact surface and the liquid hydrocarbon mixture and further permitting continued solidification of wax on the thermal-contact surface .

In one embodiment, the scraping means is capable of abutting the thermal-contact surface, so that complete or near complete removal of the solidified wax is achieved.

The system may further comprise separation means configured to remove the wax crystals from the bulk liquid hydrocarbon mixture. In one embodiment, the separation means is filtration. In another embodiment, the separation means is a cyclone separator.

In one embodiment, the solidification zone is a pipe.

In one embodiment, there is disclosed a method for the solidification of hydrocarbon wax from a liquid hydrocarbon mixture. Advantageously, the step of maintaining the thermal-contact surface at a temperature below the wax appearance temperature of the liquid hydrocarbon mixture triggers the formation of wax crystals in the interfacial layer adjacent to the thermal-contact surface .

The method may further comprise the step of providing a liquid coolant to maintain the temperature of the thermal-contact surface. The method may further comprise the step of providing an annular pipe in fluid communication with the thermal-contact surface within the solidification zone for the liquid coolant to pass through .

The method may further comprise the step of providing scraping means which is moveable relative to said thermal- contact surface.

In one embodiment, the scraping means is rotated and the speed of rotation is configured to maintain the fluid flow conditions within said solidification zone of said hydrocarbon mixture at a Reynolds number in the range of 200 to 10,000.

The method may further comprise the step of solidifying the hydrocarbon wax on the thermal-contact surface in the solidification zone. Advantageously, the high carbon hydrocarbon wax crystals form and grow in the interfacial layer adjacent to the thermal-contact surface.

The method may further comprise the step of removing the solidified hydrocarbon wax using the scraping means. In one embodiment, the solidified hydrocarbon wax comprises mainly high carbon hydrocarbon wax.

The method may further comprise the step of separating the solidified hydrocarbon wax from said liquid hydrocarbon mixture. Brief Description Of Drawings

The accompanying drawings illustrate a disclosed embodiment and serves to explain the principles of the disclosed embodiment. It is to be understood, however, that the drawings are designed for purposes of illustration only, and not as a definition of the limits of the invention.

Fig. la is a schematic diagram of a system for the solidification of hydrocarbon wax from a liquid hydrocarbon mixture.

Fig. lb is a schematic diagram of a unit in the system of Fig. ia.

Fig. lc is a process flow diagram of the method for the solidification of hydrocarbon wax from a liquid hydrocarbon mixture.

Fig. 2a is a graph showing the effect of overall concentration of wax in a particular crude oil composition on WAT of the crude oil composition.

Fig. 2b is a graph showing the overall effect of wall shear stress on the jelly layer.

In the figures, like numerals denote like parts.

Detailed Description Of Drawings

Referring to Fig. la, a system for the solidification of hydrocarbon wax from a liquid hydrocarbon mixture in accordance with an embodiment of the present disclosure is shown.

The system 100 is located in a crude oil production pipeline 107 found either at subsurface levels, within the ocean or at the earth' s surface, where the crude oil flows in the direction of arrow 110. The system 100 comprises a plurality of disks 102 and scraper disks 104 disposed along the length of a . central annular pipe 106. The annular pipe 106 is disposed on a longitudinal axis represented by dashed line 105. The scraper disks 104 rotate around the longitudinal axis 105 of the central annular pipe 106 with the arrow shown by arrow 109 and the speed of the scraper disks 104 is controlled by an external motor (not shown) . The central annular pipe 106 is in fluid communication with disks 102 and contains glycol. The glycol flows into the system 100 along arrow 108. Each disk 102 is fixed adjacent to each scraper disk 104 and a blown-up view of each unit 103 is shown in Fig. lb. Fig. lb also shows the flow of glycol from the central annular pipe 106 into disk 102 along the direction of arrow 112 and from disk 102- into the central annular pipe ^' 106 along the direction of arrow 113 in each unit 103.

Referring to Fig. la, the use of the system 100 for the removal of wax from crude oil will now be described. The system 100 is located in a section 107 of the production pipeline. The flow of glycol into the system 100 is controlled such that the temperature of the surface of disk 102 is lower than the wax appearance temperature of the crude oil. Referring to Fig. 2a, the WAT of a particular crude oil composition decreases with a decreasing amount of wax present in the crude oil. For example, when none of the wax is extracted from the crude oil, the WAT is about 50°C. When about 40% of wax is extracted from the crude oil, the WAT is about 15°C. Accordingly, in surface pipelines where the ambient temperature is about 25°C, the probability of wax deposition for crude oil with a WAT of about 15°C is reduced. Therefore, wax may be extracted from the crude oil until the WAT of the crude oil is lower than the bulk temperature of the main crude flow in the pipeline. Above this point, the hydrocarbon wax in the crude oil is less likely to precipitate out as solid and thus, less likely to cause plugging of the pipeline. Advantageously, the controlled temperature of the surface of disk 102 aids in the crystallization of wax in the interracial oil jelly layer adjacent to the surface.

The speed of the scraper disks 104 is also controlled such that the flow of the crude oil in the section 107 of the production pipeline is maintained in the turbulent flow regime, where the Reynolds number is in the region of 500<Re<5000. Re is calculated using the Taylor-Couette flow configuration where Re=U · h/ (μ/ρ) , wherein U is the average axial and rotational disk velocity, h is the gap between disk 102 and scraper disk 104 and μ/ρ is the kinematic viscosity of the crude oil. Advantageously, the flow regime causes the large crystals of low carbon wax to be removed from the jelly layer on disks 102, thus selectively concentrating the small crystals of high carbon wax in the jelly layer.

Furthermore, as can be seen in Fig. 2b, the turbulent flow of crude oil provides a convenient shear rate of between 2500 < γ< 3500 1/s due to the combined effects of axial crude oil velocity and rotational scraper disk velocity. The shear rate aids in the selective removal of large crystals of low carbon wax, thus progressively concentrating the small crystals of high carbon wax in the jelly layer. Referring to Fig. 2b, it can be seen that at a low shear rate of 200 1/s, the jelly layer cannot be removed and thus, both the large crystals of low carbon wax and small crystals of high carbon wax remain in the jelly layer. However, at a shear rate of more than 5000 1/s, total removal of jelly layer occurs and no hydrocarbon wax can remain crystallized on the surface of disks 102. Accordingly, the optimal shear rate (taking into account average axial oil velocity and rotational scraper disk velocity) should be in the range of 200-4000 1/s.

The scraper disks 104 aid in the intermittent removal of the jelly layer containing small crystals of high carbon wax adjacent to the surface of disks 102 when rotated past disks 102. Further, the scraper disks 104 are also moveable along the longitudinal axis 105, allowing them to abut disks 102. The scraper disks 104 abut disks 102, so that complete or near complete removal of the jelly layer containing solidified high carbon wax is ensured at intermittent intervals when the scraper disks 104 rotate past disks 102. The dislodged layer containing approximately 25% wax and 75% oil is then transported along with the crude oil in the direction of arrow 110.

Referring to Fig. lc, the crude oil stream 122 with the dislodged wax are separated in a separator 120 located downstream of the section 107 of the extraction pipeline. The solid wax deposits leave separator 120 in stream 126 and the crude oil with a relatively lower percentage of waxy high carhon compounds leave separator 120 in stream 124.

Applications

Advantageously, the system does not incur additional production downtime.

Advantageously, the use of chemical additives and pigging operations is reduced, thereby reducing such associated costs.

Advantageously, the system permits the crude oil to have a wax appearance temperature that is lower than the ambient temperature, thereby allowing the continued extraction of liquid hydrocarbons with a lower probability of having plugged flow. Thus, the system has a decreased possibility of flow interruptions.

Advantageously, wax precipitation is localized and is thus easily identifiable.

Advantageously, the precipitated wax is subsequently removed from the crude oil extraction system, reducing the possibility of plugging of the crude oil extraction pipeline, thereby also reducing the economic losses associated with production downtime.

Advantageously, the system is a relatively cheaper alternative for the removal of wax in crude oil.

It will be apparent that various other modifications and adaptations of the invention will be apparent to the person skilled in the art after reading the foregoing disclosure without departing from the spirit and scope of the invention and it is intended that all such modifications and adaptations come within the scope of the appended claims.