Catalytic distillation process for hydroprocessing Fischer-Tropsch liquids

Abstract

The invention provides a catalytic distillation system which combines two fundamental unit operations, chemical reaction and distillation in a single piece of equipment for hydrocracking and hydrotreating of Fischer-Tropsch liquids.

Description

PRIOR RELATED APPLICATIONS

Not applicable.

FEDERALLY SPONSORED RESEARCH STATEMENT

Not applicable.

REFERENCE TO MICROFICHE APPENDIX

Not applicable.

FIELD OF THE INVENTION

The invention relates to a method of producing transportation fuels derived from the products of a Fischer-Tropsch synthesis. More specifically, the invention relates to hydroprocessing Fischer-Tropsch liquids using a catalytic distillation tower.

BACKGROUND OF THE INVENTION

Synthetic transportation fuels are increasingly in demand because they contain no sulfur or aromatics and typically have high cetane numbers. The Fischer-Tropsch process used to make synthetic transportation fuels, however, results in a syncrude product containing oxygenates (“FT oxygenates”) and heavy boiling point components. The FT oxygenates typically include primary and internal alcohols, which constitute the major portion of the total FT oxygenate, as well as aldehydes, ketones and acids. The heavy boiling point components typically include paraffin waxes (n-paraffin and mono-methyl branched isoparaffins). The presence of FT oxygenates and olefins in synthetic fuel products is not desired because of thermal stability issues and regulatory constraints.

The FT oxygenates, olefins, and heavy boiling point components may be removed through a combination of hydroprocessing and hydrotreatment. The term “hydroprocessing” as used herein means hydrocracking, hydroisomerization, hydrodewaxing, or a combination of two or more of these processes. The FT oxygenate and olefin content is generally higher in the lower boiling range distillation cuts of the Fischer-Tropsch product and declines precipitously above a 600° F. cut point. One method of avoiding the negative impact of the FT oxygenates on the hydrocracking catalysts is to bypass the lower boiling range distillation cuts around the hydrocracking unit. The lower boiling range distillation cuts, including any FT oxygenate content therein, are then re-blended with the hydrocracked higher boiling range distillation cut to form the product fuel. Catalytic hydroprocessing catalysts of noble metals are well known, some of which are described in U.S. Pat. Nos. 3,852,207; 4,157,294; 3,904,513 the disclosures of which are incorporated herein by reference. Hydroprocessing utilizing non-noble metals, such as cobalt catalysts, promoted with rhenium, zirconium, hafnium, cerium or uranium, to form a mixture of paraffins and olefins has also been used. By providing multiple processes, Fischer-Tropsch liquids upgrading/refining units are often large and involve large capital costs.

SUMMARY OF THE INVENTION

Embodiments of the invention meets these and other needs by providing a catalytic distillation system which combines two fundamental unit operations, chemical reaction and distillation in a single piece of equipment for hydrocracking and hydrotreating of the FT olefins and high boiling point components.

An embodiment of the invention provides a process for upgrading Fischer-Tropsch liquids including hydroprocessing the Fischer-Tropsch liquids in at least one catalytic distillation tower. Hydroprocessing means separating the Fischer-Tropsch liquids into a stream of heavier components and a stream of lighter components and hydrocracking the heavier components and hydrotreating the lighter components. In an alternate embodiment, the Fischer-Tropsch liquids are preheated prior to hydroprocessing. In a preferred embodiment, the process is performed in a single catalytic distillation tower. In an alternate embodiment, the process is performed in two catalytic distillation towers. The heavier components are C₁₀₊ hydrocarbon components. The lighter components are C₁₀₋ hydrocarbon components. Products include diesel boiling point components and naphtha boiling point components. In alternate embodiments, the diesel boiling point components are further steam distilled and a portion of the naphtha boiling point components is recycled back to the catalytic distillation tower. If there are any unconverted heavier components, they are recycled to the catalytic distillation tower.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram for a standard hydroprocessing method.

FIG. 2 is a flow diagram of the inventive hydroprocessing method.

FIG. 3 is a detailed flow diagram of the inventive hydroprocessing method.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The term “Cx”, where x is a number greater than zero, refers to a hydrocarbon compound having predominantly a carbon number of x. As used herein, the term Cx may be modified by reference to a particular species of hydrocarbons, such as, for example, C5 olefins. In such instance, the term means an olefin stream comprised predominantly of pentenes but which may have impurity amounts, i.e. less than about 10%, of olefins having other carbon numbers such as hexene, heptene, propene, or butene. Similarly, the term “Cx+” refers to a stream wherein the hydrocarbons are predominantly those having a hydrocarbon number of x or greater but which may also contain impurity levels of hydrocarbons having a carbon number of less than x. For example, the term C15+ means hydrocarbons having a carbon number of 15 or greater but which may contain impurity levels of hydrocarbons having carbon numbers of less than 15. The term “Cx-Cy”, where x and y are numbers greater than zero, refers to a mixture of hydrocarbon compounds wherein the predominant component hydrocarbons, collectively about 90% or greater by weight, have carbon numbers between x and y inclusive. For example, the term C5-C9 hydrocarbons means a mixture of hydrocarbon compounds which is predominantly comprised of hydrocarbons having carbon numbers between 5 and 9 inclusive, but may also include impurity level quantities of hydrocarbons having other carbon numbers.

Unless otherwise specified, all quantities, percentages and ratios herein are by weight.

Liquid products from the Fischer-Tropsch reaction include hydrocarbons ranging from methane (CH₄) to high molecular weight paraffinic waxes containing more than 100 carbon atoms. Examples of Fischer-Tropsch systems are described in U.S. Pat. Nos. 4,973,453; 5,733,941; 5,861,441; 6,130,259, 6,169,120 and 6,172,124, the disclosures of which are herein incorporated by reference.

Referring to FIG. 1, the Fischer-Tropsch Reactor liquid product (FTRL) 1 is distilled in a fractionator 2 into two or more streams. A Light Fischer-Tropsch Liquid (LFTL) 3, and a Heavy Fischer-Tropsch Liquid (HFTL) 4 are recovered from fractionator 2.

All or part of the LFTL 3, which is comprised primarily of C₂ to C₂₄ hydrocarbons, is fed into a hydrotreater 5. Hydrogen 6 is also added to the hydrotreater 5. All or part of the HFTL 4, which is comprised primarily of C₁₀ to C₅₀₊ hydrocarbons, is fed into a hydrocracker 7. Hydrogen 8 is also added to the hydrocracker 7. Product 9 from the hydrotreater 5 and product 10 from the hydrocracker 7 are combined into a feed stream 11. The feed stream 11 is fed into a light ends separator 12 to remove hydrogen 13 which is sent to a compressor for recycle. Also produced from the light ends separator 12 is a product stream 14, which is fed into a products fractionator 15. The products from the product fractionator 15 include unstabilized naphtha 16, diesel 17 and wax recycle 18. The wax recycle 18 is combined with the HFTL 4 and fed to the hydrocracker 7. The unstabilized naphtha 16 consists mainly of n-paraffins in the C₅-C₉ range. The unstabilized naphtha 16 is a good steam cracker feedstock producing more light olefins compared at the same operating severity. The diesel 17 has a broad boiling distribution and can be further fractionated into kerosene/jet fuel and a narrower boiling distribution diesel. The high level of n-paraffins in the diesel 17 provides it with significantly higher cetane numbers than conventional petroleum-based diesel. The ratio of iso-paraffins to n-paraffins in the diesel 17 dictates its pour point and other low temperature properties.

The term “hydrotreating” as used herein refers to processes wherein a hydrogen-containing treatment gas is used in the presence of suitable catalysts which are primarily active for saturating olefins and aromatics. Suitable hydrotreating catalysts for use in the present invention are any known conventional hydrotreating catalysts. Examples of such hydrotreating catalyst include, for example, those comprised of at least one Group VIII metal, preferably iron, cobalt and nickel, more preferably cobalt and/or nickel on a high surface area support material, such as alumina. Other suitable hydrotreating catalysts include zeolitic catalysts, as well as noble metal catalysts where the noble metal is selected from palladium and platinum. More than one type of hydrotreating catalyst may be used in the present invention. Typical hydrotreating temperatures range from about 400° F. to about 900° F. with pressures from about 500 psig to about 2500 psig. Olefin saturation with noble metal catalysts may be performed at milder conditions, with temperatures as low as 100° F. and pressures as low as 1 atmosphere.

Hydrotreating catalysts useful in hydrotreater 5 are well known in the art and consist of sulfided or non-sulfided metals which are active to hydrogenation transfer reactions, such as Cobalt, Nickel, Platinum, Palladium. Reactor flow rates of 0.1 to 10 LHSV are typical, and reactor temperatures of 200° F. to 750° F. are characteristic of this hydrotreating process. Hydrogen flow rates of 500 to 10,000 SCF/bbl and hydrogen pressure of 200 to 2500 psig are also typical of this hydrotreating process. Other acceptable hydrotreating conditions known in the art may also be used.

The term “hydrocracking” as used herein refers to a process having all or some of the reactions associated with hydrotreating, as well as cracking reactions, which result in molecular weight and boiling point reduction and molecular rearrangement, or isomerization.

Hydrocracker 7 may contain one or more beds of the same or different catalyst. In some embodiments, when the preferred products are distillate fuels, the preferred hydrocracking catalysts utilize amorphous bases or low-level zeolite bases combined with one or more Group VIII or Group VIB metal hydrogenating components. In another embodiment, when the preferred products are in the gasoline boiling range, the hydrocracking zone contains a catalyst which comprises, in general, any crystalline zeolite cracking base upon which is deposited a minor proportion of a Group VIII metal hydrogenating component. Additional hydrogenating components may be selected from Group VIB for incorporation with the zeolite base. The zeolite cracking bases are sometimes referred to in the art as molecular sieves and are usually composed of silica, alumina and one or more exchangeable cations such as sodium, magnesium, calcium, rare earth metals, etc.

Referring to FIG. 2, the FTRL 1 is fed into a catalytic distillation unit (CDU) 105. Hydrogen 110 is also added to CDU 105. Products from the CDU 105 include hydrogen 115, unstabilized naphtha 120, and diesel 125. Hydrocracking and hydrotreating occur with the CDU 105.

Combining catalytic reaction and product separation in the same reactor can improve the conversion and selectivity for many reactions, reduce capital costs and also enhance catalyst lifetimes. Traditional catalytic distillation unit technology combines a heterogeneous catalytic reaction and product separation in a single reactor. The heterogeneous catalyst acts as distillation packing as well as a catalyst for the reaction. The success of the CDU technology for the production of MTBE has led to great interest in using CDU as a more general reaction technique.

The catalytic distillation system is often preferred to conventional fixed-bed reactor technology for some of following reasons: The combination of reaction and distillation steps (process intensification) reduces capital costs significantly; the integrated heat removal (exothermic heat of reaction absorbed by evaporating liquid) further simplifies the process, and overall utility requirements and operating expenses are reduced; the counter-current contacting of hydrogen and hydrocarbon reactants provides a favorable reaction driving force; continuous washing of the reaction zone with internal liquid traffic in the column results in extended catalyst life.

Some requirements for considering catalytic distillation include distillation must be a practical method of separating the reactants and products, the reaction must proceed at a reasonable rate at the temperature equivalent to the boiling point of the liquid mixture in the column, and the reaction cannot be overly endothermic.

FIG. 3 is a detailed flow diagram of the process described in FIG. 2. In a preferred embodiment, the FTRL 1 is heated to a temperature of from about 200° F. to about 600° F. in a heat exchanger 130. In a preferred embodiment, the heat exchanger 130 uses heat from the diesel product 125 to elevate the temperature. The heated liquid is then further heated to a temperature of from about 400° F. to about 800° F. in a preheater 135. The preheated liquid is then fed to the CDU 105. Alternate embodiments include no heat exchanger, preheater, uses latent heat or hot streams from other processes within the Fischer-Tropsch unit, or use of a heating utility like steam or hot oil. In a preferred embodiment, the preheater is a fuel-burning heater. In alternate embodiments, the preheater is a heat exchanger with high pressure steam or hot oil.

Upon entry into the CDU 105, the preheated FTRL is flashed into a vapor stream and a liquid stream. The vapor stream contains components that are mainly liquids at ambient temperatures. The liquid stream contains components that are mainly solid (wax) at ambient temperatures. The CDU 105 includes a hydrotreating section 105a and a hydrocracking section 105b. Also included in the CDU 105 are fractionation trays interspersed above, below and between the hydrotreating section 105a and the hydrocracking section 105b. Also entering the CDU 105 is hydrogen 110.

As the liquid components and the hydrogen contact within the hydrotreating section 105a they are hydrotreated to saturate olefins and other unsaturated components. The lighter components, mostly C₅-C₁₀ are condensed overhead in a condenser 160. The condenser 160 separates the lighter components into hydrogen 115, unstabilized naphtha 120 and water 165. A portion of the unstabilized naphtha 120 is recycled to the CDU 105 and a second portion is sent for further processing. The reflux may ensure, but is not limited to, providing good separation, recycling unconverted components, controlling the heat of reaction, reducing the fouling rate and “washing” the catalyst packing sections. Depending on the activity of the hydroprocessing catalysts, the operating pressures of the CDU 105 may be as low as 100 psig or as high as 1000 psig. The temperature required to distill the hydrocarbon stream are dictated by tower distillation pressure and are in the 600° F. to 1200° F. range. In preferred embodiments, the condenser 160 includes a separation vessel and a heat exchanger.

As the wax components contact hydrogen within the hydrocracking section 105b, they are cracked into smaller paraffinic components within the diesel boiling point range. A side stream 140 is withdrawn from a tray directly below the hydrogen feed stage and sent to a side stripper 145. In alternate embodiments, the side draw location is dependant on the target product boiling point range. Steam 150 is added to the side stripper 145 to produce the diesel product 125 by stripping C₁₀₋ light ends 155. The diesel product 125 is sent to storage after heat exchange with the FTRL 100 in the heat exchanger 130. The C₁₀₋ light ends 155 are returned to the CDU 105 above the hydrotreating section 105a.

Any unconverted wax 160 is recycled back to the CDU 105 to a feed point above the hydrocracker catalyst packing stage. In an alternate embodiment, the unconverted wax 160 is combined with the FTRL 1 stream prior to entry of the CDU 105. In a preferred embodiment, the CDU 105 includes a fuel-burning furnace as a reboiler. In alternate embodiments, the CDU reboiler is a heat exchanger with high pressure steam or hot oil.

The hydrotreating section 105a is filled with hydrotreating catalyst as described above. In a preferred embodiment, the hydrotreating catalyst is a noble metal catalyst to allow lower hydrotreating pressures. The hydrocracking section 105b is filled with hydrocracking catalyst as described above. In a preferred embodiment, the hydrocracking catalyst is a dual-functional (cracking and hydrogenation) noble metal catalyst. One skilled in the art is familiar with the arrangement of the catalyst within the tower for optimum performance. In a preferred embodiment, the CDU 105 operating pressure is about 100 psig to about 200 psig enabling successful distillation at hydroprocessing temperatures.

In an alternate embodiment, the CDU 105 may include separate towers for the hydrotreating section and the hydrocracking section. As seen by comparing the FIG. 1 and FIG. 3, the capital costs and “footprint” of embodiments of the present invention are significantly reduced.

While the invention has been described with respect to a limited number of embodiments, the specific features of one embodiment should not be attributed to other embodiments of the invention. No single embodiment is representative of all aspects of the inventions. Moreover, variations and modifications therefrom exist. For example, other processes can be placed in the catalytic distillation tower depending on the makeup of the FT liquid product. Additionally, heat exchangers and preheaters may be designed for maximum heat efficiency. The appended claims intend to cover all such variations and modifications as falling within the scope of the invention.

Claims

1. A process for upgrading fischer-tropsch liquids comprising hydroprocessing the Fischer-Tropsch liquids in at least one catalytic distillation tower; wherein hydroprocessing comprises separating the Fischer-Tropsch liquids into a stream of heavier components and a stream of lighter components and hydrocracking the heavier components and hydrotreating the lighter components.

2. The process of claim 1, further comprising preheating the Fischer-Tropsch liquids prior to hydroprocessing.

3. The process of claim 1, wherein the process is performed in a single catalytic distillation tower.

4. The process of claim 1, wherein the process is performed in two catalytic distillation towers.

5. The process of claim 1, wherein the heavier components comprise C10+ components.

6. The process of claim 1, wherein the lighter components comprise C10− components.

7. The process of claim 1, wherein hydrocracking the heavier components produces a stream of diesel boiling point components.

8. The process of claim 1, wherein hydrotreating the lighter components produces a stream of naphtha boiling point components.

9. The process of claim 7, wherein the diesel boiling point components are further steam distilled.

10. The process of claim 8, wherein a portion of the naphtha boiling point components is recycled back to the catalytic distillation tower.

11. The process of claim 1, wherein any unconverted heavier components are recycled to the catalytic distillation tower.

12. The process of claim 11, wherein the unconverted heavier components are returned to the catalytic distillation tower above the hydrocracking stage.