Patent Application
20160115392
This invention relates generally to the recovery of a diesel stream from a fractionation column, and more particularly to processes to increase the recovery of the diesel stream.
Refiners are keen to increase the distillate product yields, mainly diesel, from their existing hydroprocessing units. New generation highly active catalysts have enabled incremental diesel yields within the reactor section. Additionally, improvements in the hydroprocessing flow schemes may also lead to increased diesel yields within the reactor section of the hydroprocessing unit.
Despite these increases in the yield of diesel from the reactor section of the hydroprocessing unit, the increased yield may not be realized by the refiner because of the separation equipment and schemes used to recover diesel (and other hydrocarbon streams) from the hydroprocessed effluent. For example, it has been found that existing fractionation column feed heaters and the diesel draw sections in the fractionation column are limiting the ability of the refiners to recover the additional diesel. This results in slippage of diesel into the bottom unconverted portion of material from the fractionation column. Thus, although the reactor section is able to produce more diesel, the fractionation column is unable to recover all of it. This is a revenue loss for the refiner.
As will be appreciated, increasing the size of the fractionation column to recover the increased diesel yield may not be feasible due to plot space restrictions or excessively high capital and operating costs.
Therefore, there remains a need for an effective and efficient process for recovering the increased diesel.
One or more process have been invented in which a vacuum column is used downstream of the main fractionation column to enable recovery of this additional diesel from the fractionation column bottoms. The vacuum column is integrated with the main fractionator and does not need a separate overhead system to maintain the vacuum level in the column.
A first aspect of the invention, the invention may be characterized as a process of increasing the yield of diesel from a fractionation column downstream of a hydroprocessing zone by: separating a bottoms stream from a fractionation column in a vacuum column into a vapor stream, a diesel stream, and a bottoms stream; removing the vapor stream from the vacuum column with an eductor; and, compressing the vapor stream from the eductor to the fractionation column.
In at least one embodiment, the process also includes separating a feed stream in the fractionation column into one or more transportation fuel streams and the bottoms stream.
It some embodiments, the process further includes separating a feed stream in the fractionation column into at least a diesel stream and the bottoms stream. It is contemplated that the process includes combining the diesel stream from the fractionation column with the diesel stream from the vacuum column.
It some embodiments, the process further includes removing a sidedraw stream from the fractionation column; and, passing the sidedraw stream through the eductor.
It various embodiments, the process further includes heating the bottoms stream from the fractionation column. It is contemplated that heating the bottoms stream from the fractionation column comprises passing the bottoms stream from the fractionation column to a charge heater to provide a heated bottoms stream, and, passing the heated bottoms stream to the vacuum column. It is also contemplated that heating the bottoms stream from the fractionation column comprises heating the bottoms stream from the vacuum column in a reboiler to provide a heated vacuum bottoms stream, and, passing the heated vacuum bottoms stream to the vacuum column.
In at least one embodiment, the process further includes cooling a portion of the diesel stream from the vacuum column, and, passing the cooled portion of the diesel stream back to the vacuum column.
In one or more embodiments, the process further includes passing a first portion of the diesel stream one back to the vacuum column, and, recovering a second portion of the diesel stream as a product stream.
In a second aspect of the present invention, the invention may be characterized as a process of increasing the recovery of diesel from a fractionation column downstream of a hydroprocessing zone by: separating a feed stream in a fractionation column into at least one naphtha stream, a diesel stream, and a bottoms stream; separating the bottoms stream from the fractionation column in a vacuum column into a vapor stream, a diesel stream, and a bottoms stream, wherein vacuum column includes at least three packed sections and an inlet for the bottoms stream from the fractionation column disposed above a first packed section and below two other packed sections; removing the vapor stream from the vacuum column with an educator; and, compressing the vapor stream of the vacuum column from the educator to the fractionation column.
In various embodiments of the present invention, the process further includes stripping diesel from the bottoms stream of the fractionation column in the vacuum column by passing steam into the vacuum column. It is contemplated that an inlet in the vacuum column for the steam is disposed below the inlet for the bottoms stream from the fractionation column. It is further contemplated that the process also includes passing a portion of the bottoms stream from the vacuum column to a reboiler to provide a heated vacuum column bottoms stream, and, passing the heated vacuum column bottoms stream to the vacuum column. It is still further contemplated that the vacuum column further comprises an inlet for the heated vacuum column bottoms stream disposed below the three packed venturi jet nozzle in the vacuum column.
In at least one embodiment, the process also includes heating the bottoms stream from the fractionation column in a charge heater to provide a heated bottoms stream; and, passing the heated bottoms stream to the vacuum column.
In at least one embodiment, a motive fluid for the venturi jet nozzle comprises steam. It is contemplated that a motive fluid for the venturi jet nozzle comprises a stream from the fractionation column. In some embodiments, the motive fluid for the venturi jet nozzle comprises a sidedraw pumparound stream from the fractionation column.
In at least one embodiment, a portion of the vapor stream from the vacuum column comprises diesel.
Additional objects, embodiments, and details of the invention are set forth in the following detailed description of the invention.
The drawings are simplified process diagrams in which:
As mentioned above, one or more process have been invented to increase a recovery of diesel from a fractionation column. In this various embodiments, the existing equipment, such as the heater and fractionation column, may be operated within their design limits. Some diesel is allowed to slip into the fractionation column bottoms product, which is then routed to the vacuum column. In the vacuum column, diesel is separated and recovered as a product stream and can be blended with the diesel stream from the fractionation column. The pressure of the vacuum column is maintained with venturi jet nozzle, preferably a liquid jet eductor which utilizes a diesel pump around stream from the main fractionator as the motive fluid.
With reference to the attached Figures, one or more processes will now be described with the understanding that the process are merely exemplary and are not intended to be limiting.
With reference to
In the fractionation column 12, the feed stream is separated into various transportation fuel streams 16a, 16b, 16c. For example, a heavy naphtha stream 16a, a kerosene stream 16b, and a diesel stream 16c may be separated by the fractionation column 12. As is known typical operating parameters for a fractionation column 12 include a pressure of about 55 to 69 kPag (8-10 psig) and with a temperature range of 315 to 385° C. (600 to 725° F.).
In addition to the transportation fuel streams 16a, 16b, 16c, the fractionation column 12 provides an overhead stream 18 and a bottoms stream 20. The overhead stream 18, as is known, can be passed to condenser 22 and then to a receiver 24 to separate a hydrocarbon stream 26, and a water stream 28, typically wash water. The hydrocarbon stream 26 can be recovered as a light naphtha stream 30, and used as a reflux stream 32 as well.
In order to recover additional diesel from the materials in the bottoms stream 20, the bottoms stream 20 is passed to a vacuum column 34. As shown in
In a preferred embodiment, the bottommost pack section 36c comprises a stripping section 37 of the vacuum column 34. In the stripping section 37, the materials from the bottoms stream 20 from the fractionation column 12 will pass through the packed beds in a downward flow direction. A stripping stream 40, for example, steam, from flow in an upwards direction in the vacuum column 34 and strip the diesel from the material in the stripping section 37.
As shown in
As shown in
With reference both
Continuing upward in the vacuum column 34, the diesel vapor will pass through the chimney tray 38 and into the uppermost packed section 36a of the vacuum column 34. A majority of the diesel vapor will condense in the uppermost packed section 36a of the vacuum column 34 and drop downward onto the chimney tray 38. Liquid diesel can accumulate in the chimney tray 38 and can be removed in a diesel stream 50 via a pump 52. A first portion 50a of the diesel stream 50 can be recovered and processed along with the diesel stream 16c from the fractionation column 12. A second portion 50b of the diesel stream 50 can be used to spray the vapor in the middle packed section 36b. A third portion 50c of the diesel stream can be used to spray the uppermost packed section 36a to condense out the diesel from the vapor in the top section of the vacuum column 34. It is also contemplated that heat may be removed from the third portion 50c via a heat exchanger 54, for example an air cooled or water cooled heat exchanger. Although not depicted as such, heat may also be removed from the second portion 50b of the diesel stream 50.
In order to draw the vapor upwards through the vacuum column 34, the vacuum column 34 has a preferred operating pressure between about 13.3 and 40.0 kPa. To provide the suction for producing the desired pressure conditions, a venturi jet nozzle 55, such as a liquid eductor 56 may be used. Although not depicted as such, instead of the liquid eductor 56, the venturi jet nozzle 55 may comprise a steam injector. Additionally, other configurations may be used, for example, a hot well with the liquid eductor 56, but these embodiments described herein are not as expensive and do not require as much space.
Returning to both
It is preferred that the motive fluid for the liquid eductor 56 is a liquid stream, and most preferred that the liquid stream is taken from the fractionation column 12. As shown in the Figures, a motive fluid stream 60 is withdrawn from the fractionation column 12, preferably a sidecut stream taken at a position above the diesel steam 16c. A pump on line 60 imparts the necessary velocity to the fluid stream to pull the vacuum desired in the vapor stream 58. The fluid in the motive fluid stream 60 can be cooled in a heat exchanger 62, and then passed to the liquid eductor 56. A mixed stream 64 from the liquid eductor 56 (comprised of the vapor from vapor stream 58 of the vacuum column 34 and the fluid from the motive fluid stream 60) is passed back to the fractionation column 12 for separation and recovery of the components. Any diesel that is in the mixed stream 64 (either from the vacuum column 34 in the vapor stream 58 or from the fractionation column 12 in the motive fluid stream 60) can be recovered either in the fractionation column 12 (or in the vacuum column 34). The alternative steam injector utilizes the same venturi principal and passes a gaseous steam stream though the venturi to draw the vapor stream 58 through the injector.
By utilizing the vacuum column 34, the diesel that passes with the materials in the bottoms stream 20 from the fractionation column 12 can be recovered without increasing the size of the fractionation column 12. It is contemplated that the design can be implemented in with existing schemes by blocking a valve 66 for a pump 68 associated with bottoms stream 20 from the fractionation column 12. Thus, the necessary piping and controls can be implemented with the existing equipment allowing for a refiner to increase the recovery of diesel.
The vacuum column 34 allows for a refiner to recover diesel from the bottoms stream 20 from the fractionation column 12 that would typically be lost. The cost of the vacuum column 34 and required equipment is believed to be offset by the improved diesel recovery. Furthermore, these various embodiments can be easily incorporated into various existing systems already in use, minimizing the cost to implement same.
It should be appreciated and understood by those of ordinary skill in the art that various other components such as valves, pumps, filters, coolers, etc. were not shown in the drawings as it is believed that the specifics of same are well within the knowledge of those of ordinary skill in the art and a description of same is not necessary for practicing or understating the embodiments of the present invention.
While at least one exemplary embodiment has been presented in the foregoing detailed description of the invention, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the invention, it being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set forth in the appended claims and their legal equivalents.
