BENZENE RECOVERY PROCESS

This application claims priority to United States provisional patent application 60/946,890, filed June 28, 2007, and is hereby incorporated by reference as if written herein in its entirety.

FIELD

[0001] The selective removal of benzene from a wide range of liquid feedstreams containing benzene by using a combined membrane pervaporation and solvent extraction process.

BACKGROUND

[0002] Benzene is a building block in plastics and basic chemicals. Benzene is used in the synthesis of styrene and cumene, two important intermediaries for polymers and other chemical derivatives, hi refineries and chemical plants, two major sources of benzene are reformate streams (i.e., the product stream from a catalytic reformer) and pyrolysis gasoline streams (i.e., a byproduct of steam cracking in olefins production). Both of these streams are rich in "BTEX" (benzene, toluene, ethylbenzene, and xylenes). Both streams also contain other components, such as normal paraffins (in virgin naphtha), isoparaffins (in reformate), and straight and branched olefins (in pyrolysis gasoline).

[0003] In order to be used in the synthesis of more valuable chemicals, benzene must be recovered and purified to greater than 98% by volume; however, since many of the commingled aliphatic components have boiling points in the same range as benzene, separation to this purity by simple distillation is not feasible. Some other separation process is required as a complement.

[0004] The most common method of removing benzene from mixed streams that contain both aromatic and aliphatic compounds with similar boiling points is by distillation combined with a solvent extraction step. In one method, the feed stream rich in BTEX is first distilled to produce a narrow boiling range cut (called a "heart cut") that contains benzene and aliphatic compounds that have the same boiling range. The heart cut stream is then subjected to solvent extraction using a solvent that is selective for the removal of aromatics, thereby producing an essentially pure benzene product, hi another

method, the BTEX feedstream is first subjected to solvent extraction to provide a product stream of mixed aromatics. The mixed aromatics stream is further separated by distillation to produce a highly purified benzene product, since there exists a large difference in the boiling points of benzene and the BTEX component having the closest boiling point, toluene. Both of these methods involve high capital expenditures and are energy intensive.

[0005] A membrane process known as "pervaporation" has been shown to be a less energy intensive process for aromatic/aliphatic separation. The term pervaporation comes from the words "permeation" and "evaporation" - two of the steps in the process. The pervaporation separation process uses a non-porous membrane which, when contacted with a multi-component liquid feed such as a mixed aliphatic and aromatic feedstream, selectively absorbs one or more of the species from the feedstream based upon chemical affinity. The absorbed species then permeate across the membrane under the influence of a concentration gradient that is produced by evaporating the absorbed molecules from the product side of the membrane using a vacuum or sweep gas. The permeate vapor emerging on the other side of the membrane is then condensed and recovered as a liquid. Temperature and pressure on the product side of the membrane are factors in the ability of a given species to "evaporate" from the membrane. Higher temperatures and better vacuums (less than about 25 mm Hg to about 50 mm Hg) favor evaporation of all species that permeate through the membrane, while lower temperatures and poorer vacuums (greater than about 25 mm Hg to about 50 mm Hg) favor selective evaporation of the lower boiling components from the product side of the membrane. In effect, there is a critical pressure for a given temperature that may allow selective evaporation of components. It is also well known that the glass transition temperature (Tg) of a polymeric membrane may determine its selectivity toward various species. For instance, in rubbery membranes (i.e. those operating above their Tg), solubility of the feed components into the membrane polymer may govern permeation. Rubbery membranes tend to swell at elevated temperatures making them more effective for the separation of aromatics from non-aromatics but less effective for the selective separation of one aromatic compound (like benzene) from another (like toluene). In glassy membranes (i.e. those operating below their Tg), diffusivity tends to govern permeation, and lower molecular weight species may diffuse faster than higher molecular weight species.

Therefore, in glassy membranes (or membranes that have a glassy component) it may be possible to selectively separate benzene from toluene.

[0006] Schucker (U.S. Patent 6,273,937, hereinafter the '937 patent), incorporated herein by reference in its entirety, has disclosed an improved pervaporation process in which the vacuum is produced according to Bernoulli's equation by the circulation of a high boiling fluid through a converging-diverging nozzle. While the process of the '937 patent is capable of separating aromatic molecules from aliphatic molecules at better vacuums (less than about 25 mm Hg to about 50 mm Hg), the process is not capable of selectively separating benzene from other BTEX aromatics. Even with the use of aromatic extraction solvents as working fluids (e.g., sulfolanes, tetraethylene glycol or propylene carbonate), further processing via distillation is required to separate the benzene from other aromatics.

[0007] It is therefore advantageous to have an inexpensive and energy efficient process and apparatus for the selective pervaporative separation of benzene from a mixed hydrocarbon feedstream.

SUMMARY

[0008] In some aspects, the present disclosure describes a process comprised by passing a feedstream to a permeation unit, wherein the permeation unit comprises a non- porous membrane having a first surface in contact with the feedstream and a second opposing surface in contact with a vacuum, wherein the amount of vacuum in contact with the second opposing surface is at a critical vacuum pressure. The process also includes absorbing selectively one or more components from the feedstream into the membrane at the first surface, wherein the one or more components comprises benzene. The process also includes permeating the one or more components from the first surface to the second opposing surface by influence of a concentration gradient across the membrane, wherein the concentration gradient across the membrane is maintained by use of the vacuum in contact with the second opposing surface. The process also includes vaporizing the one or more components at the second opposing surface while under critical vacuum pressure. The process also includes condensing the one or more components into a product stream. The process also includes separating the product stream comprising the one or more components, so as to separate benzene from the other components.

[0009] In other aspects, the present disclosure describes a pervaporation apparatus comprised by a permeation unit, a vacuum source, a fluid pump, a working fluid, a decanter vessel, and a flash drum. The permeation unit in the pervaporation apparatus is further comprised by a non-porous polymeric membrane having a first surface and a second opposing surface.

[0010] The foregoing has outlined rather broadly the features of the present disclosure in order that the detailed description that follows may be better understood. Additional features and advantages of the disclosure will be described hereinafter, which form the subject of the claims.

BRIEF DESCRIPTION OF DRAWINGS

[0011] The foregoing summary as well as the following detailed description of embodiments will be better understood when read in conjunction with the appended drawings. It should be understood, however, that the disclosure is not limited to the precise arrangements and instrumentalities shown. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present disclosure.

[0012] The disclosure may take physical form in certain parts and arrangement of parts. For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions to be taken in conjunction with the accompanying drawings, in which:

[0013] Figure 1 illustrates a schematic of an embodiment of the process apparatus described in the disclosure.

DETAILED DESCRIPTION

[0014] The principles of the presented embodiments and their advantages are best understood by referring to Figure 1.

[0015] In the following descriptions and examples, specific details are set forth such as specific quantities, sizes, etc., to provide a thorough understanding of the presented embodiments. However, it will be obvious to those of ordinary skill and creativity in the art that the embodiments may be practiced without such specific details. In many cases, details concerning such considerations and the like have been omitted inasmuch as the details are not necessary to obtain a complete understanding of any and all the embodiments and are within the knowledge and creativity of persons having ordinary skill in the relevant art.

[0016] While most of the terms used herein will be recognizable to those of skill in the art, the following definitions are nevertheless put forth to aid in the understanding of the present disclosure. It should be understood, however, that when not explicitly defined, terms should be interpreted as adopting a meaning presently accepted by those of skill in the art.

[0017] "Fluid catalytic cracking (FCC)," as defined herein, is a process in which large molecules of a high-boiling hydrocarbon feedstock fluid are vaporized and broken into smaller molecules by contacting the feedstock with a fluidized powdered catalyst at high temperature and moderate pressure.

[0018] "Naphtha," as defined herein, is a mixture of hydrocarbons obtained in the refining of petroleum, in some instances by FCC. Naphtha is often referred to as petroleum ether.

[0019] "Pyrolysis gasoline," as defined herein, is the byproduct of steam cracking.

[0020] "Reformate," as defined herein, is the product stream from a catalytic reformer.

[0021] In an embodiment of the process shown in Figure 1, a feedstream comprised of one or more components, wherein one of the components comprises benzene, may be

passed into a permeation unit 14 through a line 10. The feedstream may be reformate in one embodiment. In another embodiment, the feedstream may be pyrolysis gasoline. In still another embodiment, the feedstream may be FCC naphtha. The permeation unit 14 may contain a non-porous, polymeric selective membrane 11 possessing a first surface 11a and an opposing second surface lib. A portion of the feedstream that has a chemical affinity to the selective membrane 11 may come into contact with the selective membrane 11 at first surface 11a, absorb into membrane 11, and permeate across membrane 11 from the first surface 11a to the opposing second surface lib. A concentration gradient maintained by a vacuum condition on the side of the opposing second surface lib may drive the absorbing and permeation process. The vacuum condition may be a critical vacuum condition for benzene in an embodiment. In certain embodiments, the membrane is operated at a temperature below the membrane's glass transition temperature (Tg). In some embodiments, the operating temperature of the membrane is about O ⁰ C to about 300 ⁰ C. Upon crossing through the selective membrane 11, the permeate may become volatilized from the second surface lib due to the vacuum condition generated on the opposing side surface lib of the permeation unit 14. The vacuum condition may be a critical vacuum pressure. The vaporous permeate may exit the permeation unit 14 through line 13 driven by the pressure differential of the partial vacuum. The lean retentate may exit permeation unit 14 through line 12. The vaporous permeate may optionally be cooled in line 13 by a chilled condenser operating at a temperature below the boiling point of the permeate at the operating vacuum in an embodiment.

[0022] A partial vacuum may be generated by a Venturi nozzle 15 connected to the permeation unit 14 by line 13. Venturi nozzles are well known in the art. The pressure produced in a Venturi nozzle is defined by Bernoulli's equation (Formula 1):

(P ₁ -P ₂ ) = ¹ ^ p (V ₂ ² - V ₁ ² ) (1)

where P ₁ = the pressure at the entrance of the nozzle

P ₂ = the pressure at the narrowest point of the nozzle

p = the density of the working fluid

V ₁ = the velocity of the working fluid at the entrance of the nozzle

V ₂ = the velocity of the working fluid at the narrowest point of the nozzle

Extremely high vacuums (<2 torr) may be achieved by the use of working fluids with very high boiling points (i.e., very low vapor pressures). High flux across a pervaporation membrane may be produced when a vacuum of less than about 5 torr is present. A Venturi nozzle 15 may be a circular-arc with a converging-diverging pathway. The converging-diverging pathway is known in the art to cause an increase in the velocity of flow of a working fluid as it passes through the restriction in the nozzle, thereby creating a corresponding decrease in fluid pressure. The decrease in fluid pressure creates a suction, or vacuum, for drawing a fluid connected to the nozzle. In some embodiments, the temperature of the working fluid operates in the range of about O ⁰ C to about 15O ⁰ C. In other embodiments, the working fluid operates in the temperature range of about 0 ⁰ C to about 100°C. In still other embodiments, the working fluid operates in the temperature range of about 25 ⁰ C to about 15O ⁰ C.

[0023] The working fluid may be pumped by fluid pump 21 through line 22 into the Venturi nozzle 15 at a velocity that produces a desired vacuum and therefore a pressure gradient across the non-porous membrane 11. hi some embodiments, the vacuum obtained by passing the working fluid through the Venturi nozzle may reach a value that is a critical vacuum. The critical vacuum in this disclosure is defined as a vacuum pressure that lies between the vapor pressure of benzene and the vapor pressure of toluene at the conditions existing on the second surface lib of the membrane. A vacuum at the critical vacuum on the second surface side lib of the non-porous membrane 14 advantageously may favor the desorption of benzene from the opposing second surface lib of the membrane 11 over other aromatics such as toluene.

[0024] Operation at a critical vacuum may allow benzene to volatilize from the opposing second surface lib of the non-porous membrane 11 and inhibit the volatilization of toluene and other aromatics. A critical vacuum value may be determined using a mathematical model derived by Ji and co workers (W. Ji; S. K. Sikdar, and S. T. Hwang, "Modeling of Multicomponent Pervaporation for Removal of Volatile Organic Compounds from Water," J. Mem. Sd., 93(1994), 1-19): (Formula 2).

^j _i =%rr _i - _l ^b -p- ) (2)

wherein:

Ji = permeate flux of i (mol/m ² s)

Qi = intrinsic permeability of membrane for i (mol-m/m ² kPa s)

Pi ^sat = saturated vapor pressure of i (Pa)

Yi = activity coefficient for i

Xi = mole fraction of i in the polymer at the second opposing surface of the membrane

pi = partial pressure of i in the permeate vapor (Pa)

t = membrane thickness (m).

Therefore, the value of the critical vacuum for the selective separation of benzene from a feedstream mixture containing toluene and other aromatics is: (Formulas 3 and 4)

pi ^sat Yi ^χ i > Pi ^b for benzene (3)

and

Pi ^sat Yi ^χ i ^< Pi ^b f° ^r other aromatics (4).

Combining Formulas 3 and 4 gives the condition for the critical vacuum as: (Formula 5)

(Pi ^Sat %iXi Wene > P* > (pi^X. )toluene (5).

Setting the vacuum to be the critical vacuum may prevent other aromatics from being desorbed from the non-porous membrane 11 in an embodiment of the process described.

[0025] The passage of working fluid through the Venturi nozzle 15 creates a suction on the permeate in the permeation unit 14 and draws the permeate into the working fluid where the permeate mixes intimately with the working fluid flowing through the Venturi

nozzle to create a product stream. In some embodiments, the working fluid may be an extraction solvent having selectivity for extraction of aromatic compounds over non- aromatic compounds. Working fluid-containing permeate, some dissolved and some entrained, exits Venturi nozzle 15 through line 16. The working fluid/permeate, which comprises a product stream, enters decanter vessel 17, wherein the flow separates into two liquid phase portions: a heavier extract phase 18 and a lighter phase 19, allowing selective separation of benzene from other components in the product stream. In some embodiments, the heavier phase 18 is comprised of the working fluid and dissolved benzene. In some embodiments, lighter phase 19 is a raffϊnate phase, which is comprised of all the other components that are poorly soluble in the working phase liquid. In certain embodiments, separation of 18 from 19 occurs by gravity in the decanter vessel. In other embodiments, the decanter vessel is alternately comprised as a centrifuge for separation of 18 and 19. In certain embodiments, the decanter vessel 17 may operate at about atmospheric pressure. Operation of the decanter vessel 17 either above or below atmospheric pressure may be advantageous in certain instances, and such operation remains within the spirit and scope of the disclosure. The lighter phase raffinate 19 exits the decanter vessel 17 through midline 20.

[0026] The heavier extract phase 18 exits decanter vessel 17 through line 23. The heavier extract phase 18 is then heated by heat exchanger 24 to above the normal boiling point of benzene, whereupon the heavier extract phase 18 exits the exchanger 24 through line 25. The heated heavier extract phase 18 then enters into a flash drum 26, wherein benzene vaporizes away from the working fluid in a flash heating process. In some embodiments, flash drum 26 may be operated at about atmospheric pressure. In other embodiments, flash drum 26 may be operated at a partial vacuum to facilitate the flashing process and reduce the energy needs of heat exchanger 24, as will be evident to one skilled in the relevant art.

[0027] Selection of extraction solvents with higher boiling points as working fluids may facilitate the flash separation process. Examples of working fluids for benzene separation may include, but are not limited to, those selected from the group consisting of propylene carbonate, ethylene carbonate, N-methyl pyrrolidone, tetramethylene sulfone, tetraethylene glycol, N-formyl morpholine, furfural, nitrobenzene, dipropylene glycol,

glycerol, diethylene glycol, ethylene glycol, l-butyl-3-methylimidazolium hexafluorophosphate, and mixtures thereof. These non-limiting examples may beneficially demonstrate affinity for selective extraction of aromatic molecules over non- aromatic molecules.

[0028] Vaporized benzene exits the flash drum 26 through a line 27. The vaporized benzene is condensed by heat exchanger 28 and exits the system as a liquid product stream through line 29. In some embodiments, the heat exchanger 28 is a chilled condenser operated at a temperature below the boiling point of benzene at the operating vacuum of flash drum 26. The lean working fluid exits the flash drum 26 through line 30, is cooled by heat exchanger 31, and is recycled to fluid pump 21 through a line 32. Integration of the heat exchangers discussed in this embodiment may provide advantageous energy cost savings.

[0029] A method of providing a critical vacuum for a pervaporation system that does not use a vacuum pump or a steam ejector is disclosed. This disclosure is based on the use of a converging-diverging (Venturi) nozzle that uses a high boiling point aromatic extraction solvent as the working fluid instead of steam. A permeation membrane module whose permeate side is connected to the throat of the Venturi nozzle is also disclosed. Working fluid passing through the nozzle pulls a vacuum, in some embodiments a critical vacuum, by adjustment of the flow rate of working fluid through the Venturi nozzle that is sufficient to selectively permeate and volatilize benzene from the surface of the membrane. The extraction solvent in an embodiment is one that has selectivity for benzene over other similar boiling point chemicals. Benzene may be removed from the extraction solvent by a flashing process, either at atmospheric or reduced pressure, and collected as a product stream.

[0030] The present disclosure is also embodied as a pervaporation apparatus which is demonstrated schematically in Figure 1. The pervaporation apparatus may be configured to separate benzene from a mixture of other components. The pervaporation apparatus may be comprised by a permeation unit 14, a vacuum source, a fluid pump 21, a working fluid, a decanter vessel 17, and a flash drum 26. A mixture of one or more components, one of which comprises benzene, enters permeation unit 14 through line 10. The permeation unit 14 may be comprised by a non-porous, polymeric membrane 11 having a

first surface 11a and an opposing second surface lib in an embodiment. The operating temperature of the polymeric membrane 11 is maintained below the membrane's glass transition temperature (Tg) in some embodiments. In other embodiments, the operating temperature of the polymeric membrane 11 is maintained from about O ⁰ C to about 300°C. In certain embodiments, the vacuum source is comprised by a Venturi nozzle 15, through which the working fluid flows to create a vacuum. The pervaporation apparatus may contain a line 12 in an embodiment to remove the mixture of components not permeated across membrane 11.

[0031] After exiting permeation unit 14 through line 13, the mixture of one or more components, one of which comprises benzene, enters the working fluid in Venturi nozzle 15. For separation benzene from mixed component feedstreams, the pervaporation apparatus advantageously may utilize a working fluid demonstrating affinity for selective extraction of aromatic molecules over non-aromatic molecules, wherein the working fluid is selected from the group, including but not limited to, propylene carbonate, ethylene carbonate, N-methyl pyrrolidone, tetramethylene sulfone, tetraethylene glycol, N-formyl morpholine, furfural, nitrobenzene, dipropylene glycol, glycerol, diethylene glycol, ethylene glycol, l-butyl-3-methylimidazolium hexafluorophosphate, and mixtures thereof.

[0032] In an embodiment of the pervaporation apparatus, Venturi nozzle 15 is operated to maintain the second opposing membrane surface lib at a reduced pressure condition, hi certain embodiments, the reduced pressure condition may be a critical vacuum for benzene. In an embodiment of the apparatus, a mixture of one or more components, wherein one of the components comprises benzene, travels from membrane surface lib into the working fluid inside the Venturi nozzle 15 through line 13. The working fluid exits the Venturi nozzle 15 through line 16 and contains the one or more components of the permeate, wherein one of the components comprises benzene. The working fluid may operate within a temperature range of about O ⁰ C to about 150°C in some embodiments. In other embodiments, the working fluid may operate within a temperature range of about 0°C to about 100°C. In still other embodiments, the working fluid may operate within a temperature range of about 25 ⁰ C to 150°C. Working fluid is supplied to Venturi nozzle 15 from fluid pump 21 through line 22.

[0033] Upon leaving the Venturi nozzle 15, the working fluid containing a mixture of one or more components, one of which comprises benzene, travels through line 16 and collects in a decanter vessel 17 and may separate into a mixture of two immiscible phases 18 and 19. The decanter vessel 17 may be alternately be embodied as a centrifuge in certain instances. Decanter vessel 17 may optionally be heated, cooled, or operated under a reduced pressure condition in some embodiments.

[0034] Separation of the working fluid and one or more components, one of which comprises benzene, away from one or more components insoluble in the working fluid is accomplished in decanter vessel 17. In an embodiment of the pervaporation apparatus the lighter phase 19 may be comprised by one or more components insoluble in the working fluid, and the heavier phase 18 may be comprised by the working fluid and one or more components, one of which comprises benzene. Lighter phase 19 may be removed from decanter vessel 17 through midline 20 in an embodiment. The heavier phase 18 comprised by the working fluid and one or more components, one of which comprises benzene, may travel through lines 23 and 25 to flash drum 26 after passing through heat exchanger 24. Heat exchanger 24 may be used to heat the working fluid solution of one or more components, one of which comprises benzene. In an embodiment of the apparatus, heat exchanger 24 may heat the working fluid solution of one or more components, one of which comprises benzene, to above the normal boiling point of benzene. Flash drum 26 may be operated at atmospheric pressure in an embodiment. In another embodiment, flash drum 26 may be operated at a reduced pressure condition to advantageously lower the temperature of the flashing process. Following flashing, vaporized benzene travels though line 27 before being condensed by heat exchanger 28. The condensed benzene may be collected as a product stream through line 29. Lean working fluid may be passed back to fluid pump 21 after passing through heat exchanger 31 and line 32.

[0035] Persons of ordinary skill and creativity in the art will recognize that many modifications in the process and apparatus described herein are possible, including but not limited to, integration of additional heat exchangers for optimum heat and energy utilization. The embodiments described hereinabove are meant to be illustrative only and

should not be taken as limiting of the scope of the disclosure, which is defined in the following claims.