WO2006053350A1

WO2006053350A1 - Treatment of high molar mass hydrocarbon streams

Info

Publication number: WO2006053350A1
Application number: PCT/ZA2005/000167
Authority: WO
Inventors: Alex Philip Vogel; Herman Gerhardus Nel
Original assignee: Sasol Technology (Pty) Ltd
Priority date: 2004-11-10
Filing date: 2005-11-08
Publication date: 2006-05-18
Also published as: AU2005304604B2; RU2007117197A; BRPI0516430A; NO20072339L; GB2434589B; RU2388792C2; GB0710985D0; AU2005304604A1; ZA200703773B; GB2434589A

Abstract

The invention provides a process for the removal of contaminants from a high molar mass hydrocarbon stream in hydrocarbon procesing, said method including at least two distinct steps: formation and growth of particles which include contaminant, said particles being of sufficient size to facilitate filtration thereof, said formation and growth being promoted by treating said hydrocarbon stream with an aqueous fluid optinally including an acid; and removal of at least some of the particles from the process stream by particle removal unit operations.

Description

TREATMENT OF HIGH MOLAR MASS HYDROCARBON STREAMS

Field of the Invention

The invention relates to the treatment of high molar mass hydrocarbon streams for the removal of contaminants therefrom. These streams might comprise synthetic waxes such as those produced from synthesis gas via the Fischer-Tropsch reaction.

Background to the Invention

F-T derived product streams, such as waxes, contain oxygenates and to a certain extent metals and/or metal species. Ketones, aldehydes, alcohols, esters and carboxylic acids are the main constituents of the oxygenate fraction. Carboxylic acids and alcohols are able to form under appropriate conditions carboxylate and/or alkoxide complexes and/or metalloxanes with the metals and/or metal species present. These metal carboxylates and/or alkoxides and/or metalloxanes may form deposits in processing equipment and catalyst beds. In addition, fine particulates of less than 1 μm in diameter may be stabilized by surface-active compounds (such as the oxygenates) allowing them to remain in suspension. However, when this surface layer is disrupted, the particulates precipitate forming deposits. Eventually the deposits in the catalyst beds may grow to such an extent that shutdowns of reactors are inevitable.

Metals that might be part of the metal species of concern comprise aluminium, and/or silicon, and/or titanium, and/or zirconium, and/or cobalt, and/or iron, and/or alkaline earth elements such as calcium and barium etc.

The identified problem may be .summarized as the plugging of downstream processing catalyst by a constituent of said product streams or a reaction product of a constituent of said product streams. In this specification, whenever the terms contaminate, contaminant, or related words are used, it is intended to convey the concept of an undesired constituent or a reaction product thereof and not necessarily an external impurity, unless the contrary is clearly indicated by the context.

Summary of the Invention

The invention provides a process for the removal of contaminants from a high molar mass hydrocarbon stream in hydrocarbon processing, said method including at least two distinct steps: formation and growth of particles which include contaminant, said particles being of sufficient size to facilitate removal thereof, said formation and growth being promoted by treating said hydrocarbon stream with an aqueous fluid optionally including an acid; and removal of at least some of the particles from the process stream by one or more particle removal unit operations.

The particle removal unit operations may be filtration.

The process may include maintaining the contaminated hydrocarbon stream with the aqueous fluid at conditions of elevated temperature. The aqueous fluid may be admixed with said hydrocarbon stream.

The high molar mass hydrocarbon stream might be a synthetic wax such as that produced from synthesis gas via the Fischer-Tropsch reaction.

The aqueous fluid may be a water stream.

The water stream may be a process water stream.

The aqueous fluid may be mixed with the hydrocarbon stream so that water constitutes from 0.25 mass% to 2 mass% of the hydrocarbon stream and the acid constitutes from 0.005 mass% to 0.5 mass% of the hydrocarbon stream. Preferably the acid may constitute 0.01 mass% of the hydrocarbon stream and water may constitute 0.5 mass% of the hydrocarbon stream.

The acid may be an organic acid.

The aqueous fluid may include water and an organic acid.

The aqueous fluid may include water and maleic acid.

The aqueous fluid may be pumped into the hydrocarbon stream.

The hydrocarbon stream with admixed aqueous fluid may be passed through a mixer to homogenize the stream. The mixer may be an in line mixer.

The temperature of the hydrocarbon with admixed aqueous fluid may be maintained at a level above at least 160⁰C, typically around 170⁰C. Operation at higher temperatures is also possible.

The hydrocarbon stream with admixed aqueous fluid may be maintained at elevated temperature for a minimum of 1 minute, typically from 10 minutes to 30 minutes to allow for the formation and growth of the particles prior to filtration.

The hydrocarbon stream with admixed aqueous fluid may be passed into a mixing vessel which allows the correct residence time for the particle generation to take place while inhibiting settling or breaking up of formed particles.

The pressure in the system is maintained for particle generation and filtration to prevent water from vapourising and forming a two-phase mixture anywhere in the system, even across the filter. The minimum pressure is therefore set by the operating temperature of the system. Typically the pressure is maintained at a minimum of 6 bar and may be as high as 9 bar or even higher. The level of contaminants in the hydrocarbon stream can be as high as 100 ppm but typically is around 60 ppm. The level of contaminants in the treated hydrocarbon stream is typically below 2 ppm and more often below 1 ppm.

Although an aqueous phase consisting essentially of water does lead to particle generation, far more rapid blocking of the filter material is observed than when acid is present.

Specific Description of the Invention

The invention will now be described, by way of non-limiting example only, with reference to the following example with reference to the flowsheet of Figure 1.

In the process of the example, a contaminated hydrocarbon stream 1 from storage is heated in a preheater 11 to a temperature above 160°C, typically to approximately 170⁰C or even higher. An aqueous fluid 2 is pumped into the hydrocarbon stream while maintaining said process temperature. The resultant stream 3 then passes through a mixer 12, such as an in-line mixer, before entering a mixing vessel 13.

The heated mix of hydrocarbon with admixed aqueous fluid stream 3 is maintained in the mixing vessel 13 at a temperature above 16O⁰C, preferably around 170⁰C, for a residence time of from 10 minutes to 30 minutes, preferably around 30 minutes, to allow for the formation and growth of the particles. The mixing conditions in the mixing vessel 13 are selected to inhibit settling and to avoid breaking up of formed particles.

The aqueous fluid may consist of water and an organic acid, preferably maleic acid .

The aqueous fluid may be mixed with the hydrocarbon stream so that water constitutes from 0.25 mass% to 2 mass% of the admixed hydrocarbon stream and the maleic acid constitutes from 0.005 mass% to 0.5 mass% of the admixed hydrocarbon stream. Preferably the maleic acid constitutes approximately 0.01 mass% of the admixed hydrocarbon stream and aqueous fluid constitutes approximately 0.5 mass% of the admixed hydrocarbon stream.

Other organic acids or short chain oxygenates which may be used include methanol, ethanol, oxalic acid, acetic acid, propanoic acetic, salicylic acid, succinic acid, tartaric acid, lactic acid, malonic acid, glycine acid, citric acid, carbonic acid, fumaric acid, phthalic acid, the anhydrides of these acids (e.g. maleic anhydride) and thermal decomposition products of these acids. Also included are solid acids such as silica-alumina and/or other mixed oxide systems that possess Brønsted acidity.

Although an aqueous phase consisting essentially of water does lead to particle generation, the size of the solid particle generated is substantially smaller causing clogging of the filter material which blocks far more rapidly than when the selected acid is present.

After treatment of the stream with the maleic acid and water mixture, the stream exiting the mixing vessel 13 is filtered using a pressure leaf filter 14. Usually the method makes use. of at least two filters 14 in parallel that are operated alternately to allow the process to run continually.

The filters 14 may be precoated with a suitable material such as cellulose or diatomaceous earth.

Filter cycle times may be improved by adding body feed (filtration aid) from the body feed tank to the hydrocarbon stream prior to filtration. It is convenient to express the Al content of the feed as the equivalent mass of AI(OH)3. For cases where the main contaminant in the feed is Al, the body feed rate may be between 0.5 and 3 kg of body feed / kg AI(OH)₃ in the untreated hydrocarbon, more preferably around 1.5 kg of body feed / kg AI(OH)₃ in the contaminated hydrocarbon. The system pressure is maintained at levels to ensure that there is no water flashing at any point in the process, especially across the filter medium. The pressure is maintained at above 6 bar and may be above 9 bar or even higher, depending on the system temperature.

Velocities in the pipes, especially those down stream of the body feed addition point, are maintained such that deposition does not occur as this could lead to blockages, but must not be so high that the formed particles break up.

Break up of particles causes finer particles to be caught on the filter and results in filter cycle times being drastically reduced.

Example 1 - Treatment of Fischer-Tropsch Wax

Untreated Al-contaminated wax from storage was heated to 170⁰C. An aqueous fluid was pumped into the heated untreated wax so that the mixture contained 0.01 mass% maleic acid and 0.5 mass% water. The mixture was homogenized using an in-line mixer and the homogenized mixture was hereafter maintained at 170⁰C in a mixing tank with a 30 minute residence time. A body feed rate of 1.5 kg body feed/kg AI(OH)₃ complex in the untreated wax was added to the wax leaving the mixing tank.

Filtration was performed using an 80 micron screen filter mesh precoated with 1 kg/m² of cellulose material (Arbocel BWB 40). A filter flux of 0.5 m³/(m².hr) was maintained for the duration of the run. The run length was determined by measuring the time it took for the pressure drop across the filter to exceed 1 bar at this set filter flux rate.

Feed metal levels in the untreated wax feed was 24.7 ppm Al in solution with other metals below detection limits. The average level of Al in the treated wax over a run length of 27.6 hr was 0.74 ppm at which time the pressure drop across the filter exceeded 1 bar. The average process efficiency was calculated as 97%. Example 2 - Treatment of Fischer-Tropsch Wax

A contaminated wax with an Al level of between 23 and 120 ppm and a Co level of between 1.8 and 6.5 ppm was continuously treated using the same process described in Example 1. The wax was treated at 167⁰C using an aqueous solution of maleic acid (2 wt % concentration).

The treatment performance is reported in Table 1.

Table 1 Performance in Successive Treatment Cycles

It is clear that the process matter of this invention can produce sustained results. The Al separation efficiency increases with an increase in the Al content in the contaminated wax feed.

Claims

1. A process for the removal of contaminants from a high molar mass hydrocarbon stream in hydrocarbon processing, said method including at least two distinct steps: formation and growth of particles which include contaminant, said particles being of sufficient size to facilitate filtration thereof, said formation and growth being promoted by treating said hydrocarbon stream with an aqueous fluid optionally including an acid; and

removal of at least some of the particles from the process stream by one or more particle removal unit operations.

2. A process as claimed in claim 1, wherein the particle removal unit operation is filtration.

3. A process as claimed in claimi or claim 2, which process includes maintaining the contaminated hydrocarbon stream with the aqueous fluid at conditions of elevated temperature.

4. A process as claimed in any one of the preceding claims, wherein the aqueous fluid is admixed with said hydrocarbon stream.

5. A process as claimed in any one of the preceding claims, wherein the high molar mass hydrocarbon stream is a synthetic wax.

6. A process as claimed in claim 5, wherein the synthetic wax is produced from synthesis gas via the Fischer-Tropsch reaction.

7. A process as claimed in any one of the preceding claims, wherein the aqueous fluid is a water stream.

8. A process as claimed in claim 7, wherein the water stream is a process water stream.

9. A process as claimed in any one of claims 5 to 8, wherein the aqueous fluid is mixed with the hydrocarbon stream so that water constitutes from 0.25 mass% to 2 mass% of the hydrocarbon stream and the acid constitutes from 0.005 mass% to 0.5 mass% of the hydrocarbon stream.

10. A process as claimed in claim 9, wherein the acid constitutes 0.01 mass% of the hydrocarbon stream and water constitutes 0.5 mass% of the hydrocarbon stream.

11. A process as claimed in any one of the preceding claims, wherein the aqueous fluid includes water and an organic acid.

12. A process as claimed in any one of the preceding claims, wherein the aqueous fluid includes water and maleic acid.

13. A process as claimed in any one of the preceding claims, wherein the aqueous fluid is pumped into the hydrocarbon stream.

14. A process as claimed in any one of preceding claims 4 to 13, wherein the hydrocarbon stream with admixed aqueous fluid is passed through a mixer to homogenize the stream.

15. A process as claimed in claim 14, wherein the mixer is an in line mixer.

16. A process as claimed in any one of the preceding claims, wherein the temperature of the hydrocarbon with admixed aqueous fluid is maintained at a level above at least 16O⁰C.

17. A process as claimed in claim 16, wherein the temperature is maintained around 17O⁰C.

18. A process as claimed in any one of the preceding claims, wherein the hydrocarbon stream with aqueous fluid is maintained at elevated temperature for a minimum of 1 minute.

19. A process as claimed in claim 18, wherein the temperature is maintained for from 10 minutes to 30 minutes to allow for the formation and growth of the particles prior to filtration.

20. A process as claimed in any one of claims 4 to 19, wherein the hydrocarbon stream with admixed aqueous fluid is passed into a mixing vessel which allows the correct residence time for the particle generation to take place while inhibiting settling or breaking up of formed particles.

21. A process as claimed in any one of the preceding claims, wherein the pressure in the system is maintained for particle generation and filtration to prevent water from vapourising and forming a two-phase mixture anywhere in the system, even across the filter.

22. A process as claimed in claim 21, wherein the pressure is maintained at a minimum of 6 bar.

23. A process as claimed in any one of the preceding claims, wherein the level of contaminants in the hydrocarbon stream is as high as 100 ppm and the treated hydrocarbon stream is below 2 ppm.

24. A process as claimed in any one of the preceding claims, wherein the level of contaminants in the hydrocarbon stream is 60 ppm and the treated hydrocarbon stream is below 1 ppm.