DEHAZING A LUBES PRODUCT BY INTEGRATING AN AIR SEPARATION UNIT WITH THE DEHAZING PROCESS

FIELD OF THE INVENTION

[001] The present invention relates to the integration of an Air Separation Unit (ASU) into a lubes dehazing process. More specifically, the present invention relates to the use of waste nitrogen from an air separation unit of a Gas - to Liquid (GTL) conversion plant in the process of dehazing a dewaxed hydrocarbon base oil.

BACKGROUND OF THE INVENTION

[002] There are various technologies available for producing lubricant base oils from gaseous carbon-containing feed stocks such as methane. Typically, these gas to liquid (GTL) processes include syngas generation, hydrocarbon synthesis and product upgrading. As is known, syngas is a gas mixture containing primarily carbon monoxide and hydrogen. Generally, the production of syngas involves both partial oxidation and reforming reactions of natural gas oxygen and water sources into hydrogen, and carbon monoxide and possibly carbon dioxide. Syngas processes also include catalytic partial oxidationand autothermal reforming or a combination thereof.

[003] Hydrocarbon synthesis, in the context of the present invention, is the catalytic conversion of syngas into waxy paraffinic hydrocarbons by the Fischer- Tropsch reaction.

[004] Product upgrading in the context of the present invention refers to the process of making a lubricant base oil from a waxy feed by one or more treatments such as hydrodewaxing, hydroisomerization and hydrofϊnishing.

[005] As is known, in the art, some lube oil streams contain constituents which form a haze in the base oil at ambient or lower temperatures, especially if kept at a low temperature for an extended time period. Base oils that do not form a haze are regarded as being of higher quality, and hence, various dehazing processes are employed in producing higher quality lube base oils. Typically dehazing processes include the catalytic hydrogenation of haze-forming constituents in the oil or the treatment of the oil at ambient or reduced temperatures with an adsorbent such as gamma alumina or the like.

[006] In the autothermal or partial oxidation method oxygen is chemically combined with a hydrocarbon (natural gas) to form a syngas for the hydrocarbon synthesis process. The oxygen used is separated from air in an air separation unit (ASU). As is known in the art, an ASU typically comprises a chiller tower for cooling compressed air fed to the ASU for separation. The ASU also includes prepurification vessels containing an adsorbent such as a molecular sieve or activated alumina for removing water and CO ₂ , thereby providing a dried air stream. Finally, an ASU is equipped with a cryogenic air separator or separators for distilling the dry air stream to separate the oxygen from the nitrogen and argon in the air. An air separator can also be operated to provide a nitrogen- enriched stream, a nitrogen-depleted stream and, optionally, an argon stream.

[007] It is an object of this invention to integrate an ASU with lube dehazing in an energy efficient manner. It is another object of the invention to integrate an ASU of a GTL plant with haze removal of product oil. Other objects of the invention will become apparent from the description which follows.

SUMMARY OF THE INVENTION

[008] The present invention provides for integrating a cryogenic ASU into a lube oil dehazing process. The ASU cryogenically separates air into an oxygen stream and two different nitrogen streams (one being a nitrogen-enriched stream (utility nitrogen) and the other, a nitrogen-reduced stream (waste nitrogen). According to the invention, the waste nitrogen stream from the ASU is used to cool a base oil to be dehazed to promote the separation and agglomeration of haze-forming components in the oil. v

[009] In a preferred embodiment, the ASU is part of a GTL plant.

BRIEF DESCRIPTION OF THE DRAWINGS

[010] Figures 1 to 4 are schematic illustrations of various embodiments of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[011] The present invention embodies various combinations of several integration improvements between an ASU and an oil dehazing process in which the oil to be dehazed is cooled to below ambient temperatures and thereafter the haze-forming constituents are separated from the oil. According to a preferred embodiment of the present invention, air is cryogenically separated in an ASU into a oxygen and two different nitrogen streams, one being a nitrogen-enriched stream (utility nitrogen) and the other a nitrogen-reduced stream (waste nitrogen). The utility nitrogen is a substantially rich nitrogen stream, i.e., it will contain greater than about 99 mole% nitrogen. The waste

nitrogen stream will comprise preferably less than about 99 mole% nitrogen and typically will comprise up to about 5 mole% argon and up to about 6 mole% oxygen. At least a portion of the waste nitrogen stream is used to cool a lube base oil to be dehazed to thereby promote the agglomeration and separation of haze-forming components in the oil. The remainder of the waste nitrogen stream may be used for cooling within the ASU in the chiller tower as well as to regenerate the adsorbent in the prepurification vessels. The utility nitrogen can be utilized within the GTL plant to provide a nitrogen blanket during catalyst loading, for purging instruments, for purging the cold box and similar uses.

[012] Figure 1 illustrates this first embodiment of the invention. In the embodiment, a compressed air stream 10 is fed to an ASU 12 where it is cooled in a chiller tower, treated in prepurification vessels to remove water and CO ₂ and then cryogenically separated into two nitrogen streams, one being a utility nitrogen stream 11 and the other a waste nitrogen stream 14, and an oxygen-rich stream 15. Optionally, the ASU may be operated to also separate an argon-rich stream 26. A portion of the waste nitrogen stream 14 is directed via line 16 to haze incubation unit 17 to cool a base oil in unit 17 to promote the agglomeration and separation of haze- forming constituents in the oil.

[013] The haze incubation unit 17 may comprise a vessel containing cooling coils through which the waste nitrogen from the ASU flows. Other useful haze incubation units such as shell and tube heat exchangers or double pipes heat exchangers may be employed to cool the base oil to be dehazed by the ASU waste nitrogen, thereby promoting nucleation of haze constituents.

[014] After the agglomeration of the haze-forming constituents, they are separated from the base oil by any suitable means such as filtration, adsorption, decantation or the like. In the embodiment shown in Figure 1, the incubated oil

is transferred via line 20 to haze separation unit 21, and the dehazed oil is removed via line 22.

[015] The waste nitrogen stream from the haze incubation unit 17 may be vented via line 18 to the atmosphere.

[016] Waste nitrogen 19 from the ASU not used in dehazing may be used for cooling within the ASU, vented via line 19 to the atmosphere, used for power generation or the like, or some combination thereof.

[017] The pressure of the compressed air stream 10 may vary widely depending upon the size and type of the ASU and the use to which the separated oxygen is to be put. In one embodiment of the invention, the pressure of the compressed air stream will be between about 50 psia (about 350 kPa) to about 1500 psia (about 10500 kPa) and preferably between about 95 psia (about 650 kPa) to about 1200 psia (about 8400 kPa). Under these conditions a nitrogen enriched gas stream 14 would be generated at temperatures in the range of about

35 ⁰ F (about O ⁰ C) to about 12O ⁰ F (about 5O ⁰ C) and pressures of about 15 psia (about 105 kPa) to about 20 psia (about 140 kPa).

[018] The base oil to be dehazed by the process of the invention generally will be a dewaxed lubricating oil base stock that has been upgraded by any one or more of a series of steps including , hydroisomerization hydrofinishing and the like. Preferably, the lubricating oil base stock will be one derived from Fischer-Tropsch (F-T) waxy hydrocarbons and/or contain a high amount of linear paraffinic compounds comprising mostly Cs ₊ paraffins (e.g., C ₅ , - C ₂₀₀ ) and preferably C ₁₀₊ paraffins. The present process is particularly advantageous for removing haze-forming constituents from heavy (i.e., high boiling) base

stocks such as those boiling above about 86O ⁰ F (460 ⁰ C) , preferably above about 98O ⁰ F (527 ⁰ C) and more preferably above about 1080 ⁰ F (582 ⁰ C).

[019] The base stock to be dehazed normally will be transferred directly from a hydrofinishing unit to the haze incubator unit 17 and will be at a temperature in the range of about 4O ⁰ F (5 ⁰ C) to about 25O ⁰ F (125 ⁰ C). In principle, of course, the base stock may be transferred from any source such as storage tanks or the like and may be at ambient temperatures or higher. The rate of cooling is not critical and is dictated largely by practical considerations, such as, desired rate of throughput, the starting temperature of the oil, and the like. In general, a cooling rate will be in the range of about 0.001°C/min. to about 10°C/min and will be conducted for a time sufficient to promote the agglomeration and separation of haze-forming constituents. The point at which haze incubation is terminated will vary depending upon numerous factors including the nature of the haze- forming constituents in the oil. Suffice it to say that turbidity analysis and similar conventional techniques may be used as a suitable guide for determining when to terminate the incubation period.

[020] After haze incubation of the base stock, the dehazed oil is separated from waxy haze-forming constituents, for example, by filtration through a suitable medium. The waxy haze-forming constituents may be recovered and recycled to the hydroisomerization step or may be put to other uses.

[021] The dehazed base oil may be subjected to optional finishing treatments such as treatment in a typical guard unit with an adsorbent capable of removing fines that are non-waxy.

[022] In another embodiment of the invention, after filtering the haze-forming constituents from the base oil, the filter is regenerated, at least in part, by purging the filter with a portion of the waste nitrogen stream from the ASU. Thus, as shown in Figure 2, a portion of waste nitrogen stream 14 is sent via line 23 to the haze removal unit 21 for purging the filter therein, thereby regenerating it. Optionally, a combination of solvent treatment and nitrogen purges may be employed to regenerate the haze removal filter.

[023] In another embodiment of the invention, the integration of an ASU 12 with a haze incubation unit 17 is further enhanced. As shown in Figure 3, at least a portion of the waste nitrogen from the haze incubation unit 17 is sent via line 24 when needed to provide supplemental heat for regeneration of the prepurification vessels (not shown) within the ASU 12.

[024] In an especially preferred embodiment of the invention, the integration of an ASU with a dehazing process is further integrated with a GTL plant. Thus, as is shown in Figure 4, oxygen-rich gas stream 15 from ASU 12 is transferred to syngas generator 30 for conversion therein to a syngas having a H2:CO in the range of about 0.5:1 to about 4: 1 but preferably from about 0.7:1 to about 2.5: 1. The syngas produced is sent via line 31 to hydrocarbon conversion unit 32 where it is catalytically converted into hydrocarbons. It is preferred to conduct the process under reaction conditions that result in the formation of more of higher molecular weight hydrocarbons. Typical conditions in a slurry hydrocarbon synthesis process employing a cobalt supported catalyst include temperatures, pressures and hourly gas space velocities in the range of from about 320 - 85O ⁰ F (160-450 ^O Q, 80 - 600 psi (550-4100) and 100 - 40,000 V/hr/V, expressed as standard volumes of gaseous CO and H2 mixture (O ⁰ C, 1 atm) per hour per volume of catalyst respectively.

[025] The product from the hydroconversion unit 32 typically comprises the lower boiling materials 33 and a higher boiling waxy fraction 34.

[026] The waxy fraction 34 is transferred to upgrading unit 35. Here the waxy feed may be hydrodewaxed over a combination of catalysts or a single catalyst at temperatures in the range of about 5O ⁰ F (1O ⁰ C) to about 1500 ⁰ F (816 ⁰ C) at pressures ranging from 500 to 20,000 kPa. The process may be operated at hydrogen partial pressures in the range of about 600 to 6000 kPa.

[027] A heavy fraction of the upgraded product, i.e., a fraction boiling above about 98O ⁰ F (527 ⁰ C) typically will require dehazing. In any event, a base stock 36 requiring dehazing is transferred to haze incubation unit 17 for further treatment in accordance with the invention.