Patent Grant
9914881
In refinery operations such as a vacuum distillation unit and similar services, the degree of vacuum separation attainable may be limited by various factors and concerns. One concern is for excessive coking of the vacuum column charge heater. To avoid excessive coking, limits may be placed on the system, such as a limit on the outlet temperature of the charge heater, or a limit of a specified degree of vaporization at the heater outlet. Limiting the outlet temperature of the charge heater relates to the time-at-temperature that the liquid film on the process side of the heater tubes experiences, with higher time-at-temperature being correlated to increased coking/fouling. The liquid film temperature can be significantly hotter than the bulk process temperature. Limiting the degree of vaporization at the heater outlet relates to the propensity for a dry-point to form inside heater tubes, wherein the liquid film vaporizes and comes to an end. As the liquid film shrinks and ceases to exist, both its temperature and the heat flux through it increase until it exits the charge heater, vaporizes, or cokes on the heater tubes. Such limits on the vacuum column charge heater outlet temperature or degree of vaporization can negatively impact the amount of lift attainable in the vacuum unit.
In other services, the process may be even more sensitive to such limits. For example if a process involves separation of a mixture of 80 wt % light hydrocarbons and 20 wt % heavy hydrocarbons, the processing may be severely restricted by a vaporization limit. If the heavy hydrocarbons are very heavy, such as streams containing a significant amount of heavy poly-aromatics, the charge heater outlet temperature might also be limited to avoid exposing these components to excessive time-at-temperature.
Furthermore, in existing processes, the feed to a vacuum distillation unit has typically already been through distillation at atmospheric pressure in a refinery's crude distillation unit (CDU). As a result, simple inclusion of a flash drum upstream or downstream of the vacuum column charge heater would not be justified. Flash drums are usually included in a design to remove non-condensables or components substantially above their critical points as to be clearly located in the vapor phase. The amount of such components present following atmospheric distillation is insufficient to justify inclusion of the flash drum and associated equipment downstream of the vacuum column charge heater. In addition, the inclusion of a flash drum downstream of the vacuum column charge heater would have other negative effects including the loss of vapor traffic to the column and material that would act in a stripping service, decreased resolution of product separation, and the requirement of a higher heater outlet temperature, which is undesirable with regard to feed cracking and coking, and the same limits described previously. The disadvantages of such configurations apply whether or not the flash drum is coupled to the vapor space in the vacuum column or to the vacuum column overhead.
Therefore, there is a need for improved vacuum separation processes with high vaporization.
One aspect of the invention is a method of vacuum separation. In one embodiment, the method includes heating a feed comprising a mixture of light hydrocarbons and heavy hydrocarbons in a first heating zone. The heated feed is flashed in a flash drum to form a liquid stream and a vapor stream. The liquid stream is heated in a second heating zone. The heated liquid stream is introduced into a vacuum distillation column through a first inlet. The vapor stream from the flash drum is introduced into the vacuum distillation column through a second inlet located above the first inlet, the vapor stream of the flash drum being in fluid communication with the vacuum distillation column.
Another aspect of the invention is a vacuum distillation apparatus. In one embodiment, the apparatus includes a feed line; a first heating zone in thermal communication with the feed line, the first heating zone having an inlet and an outlet; a flash drum having an inlet, a liquid outlet, and a vapor outlet, the inlet of the flash drum in fluid communication with the feed line and located downstream of the outlet of the first heating zone; a second heating zone in thermal communication with the liquid outlet of the flash drum, the second heating zone having an inlet and an outlet; and a vacuum distillation column located downstream of the outlet of the second heating zone, the vacuum distillation column having at least two inlets and an outlet, the first inlet of the vacuum distillation column being in fluid communication with the liquid outlet of the flash drum, the second inlet of the vacuum distillation column being in fluid communication with the vapor outlet of the flash drum, the second inlet located above the first inlet.
The present invention overcomes these problems by utilizing low pressure flash drums in novel configurations. The flash drum is located between two heating zones. In some embodiments, the heating zones are separate heaters, while in other embodiments, the heating zones are the radiant and convection heating zones of a single heater.
The liquid stream from the flash drum is heated in the second heating zone and sent to the vacuum distillation column. The section of the vacuum distillation column receiving this heated liquid is generally known as the column flash zone.
The vapor stream from the flash drum is also sent to the vacuum distillation column. The vapor space of the flash drum is in fluid communication with the vacuum distillation column. The vapor stream enters the vacuum distillation column above where the liquid stream enters, typically in the top ½ of the column, depending on the heat integration of the total system. The vapor stream can be sent directly to the vacuum distillation column, or it can be cooled and flashed a second time, with the second vapor stream being sent to the vacuum distillation zone. The second liquid stream can be recovered and/or recycled. Condensing the flash vapor stream reduces the load on the column.
The processes can be used to recover solvent from a mixture with heavy hydrocarbons.
The processes achieve greater lift with significantly less equipment than is otherwise required, e.g., two vacuum columns. The processes also provide an additional degree of design freedom, the flash temperature, for the process. The added design freedom is particularly useful in solvent recovery from mixtures containing heavy hydrocarbons, such as de-ashing processes for pitch.
One specific example where both the limitation on the outlet temperature and the limitation on the vaporization at the heater outlet might be encountered is in a process for de-ashing thermally hydrocracked un-converted oil, or pitch. One such de-ashing process is described in U.S. application Ser. No 14/534,729, entitled PROCESSES FOR PRODUCING DEASHED PITCH, filed Nov 6, 2014, which is incorporated herein by reference. The de-ashing process involves mixing pitch with fluid catalytic cracking (FCC) light cycle oil (LCO) or another suitable solvent, a series of mechanical separations to remove the pitch ash content, and then a separation of the LCO (or other solvent) from the pitch using vacuum distillation to effect solvent recovery. The pitch is a very heavy hydrocarbon, and it is necessary to limit its time-at-temperature, in this case to a charge heater outlet temperature of about 385° C. (725° F.). The proportions of LCO (or other solvent) and pitch may vary, and it could be up to about 85 wt % LCO and 15wt % pitch. Part of the pitch is also volatile, so in some cases a vaporization above 90wt % may be desirable to achieve fractionation. In this instance, in an embodiment with 68 wt % (84 vol %) vaporization, if a dry-point design limit is set at 75 vol % vaporization, then the attainable lift in the vacuum column would be considerably decreased. Due to such limitations, using a conventional approach, two or more columns and charge heaters (or other heat sources) in series may be needed to achieve the desired lift. However, by using the process of the present invention, a single vacuum distillation column can be used.
In the first process, the flash drum vapor is routed directly to the vapor space in the vacuum column. The preferred receiving vapor space location of the vacuum column would depend on the temperature of the flash drum vapor, for the purposes of efficient heat integration. Typically, this may be in the upper ½ of the column. Since this would result in either a larger vacuum column or an overhead vacuum producing section, it may be a non-ideal arrangement. However, this configuration may be useful in traditional vacuum operations by conducting a flash in between a charge heater's convection and radiant zones. This may allow the radiant zone to be operated at higher temperature.
The first process 100 is illustrated in
The light hydrocarbons and/or solvent include, but are not limited to, VGO (including light and heavy), LCO, the light portions of a reduced crude stream, reformate, toluene, mixed xylenes, furfural, Hi-Sol 15 (available from Jamson Laboratories, Inc., and combinations thereof.
The heavy hydrocarbons include, but are not limited to, thermally hydrocracked pitch, coal tar pitch, de-asphalting unit pitch, and the heavy portions of a reduced crude stream, and the like.
The first heating zone 110 can be any suitable heating zone, including, but not limited to, a heat exchanger, a fired heater, the convection zone of a fired heater, a steam heater, a hot oil heater, an electric heater, or combinations thereof.
The incoming feed 105 will depend on its source. It can be at any suitable temperature. For example, depending upon a process unit's heat exchanger network, it may usually be at a temperature of about 204° C. (400° F.) to about 316° C. (600° F.), e.g., about 260° C. (500° F.). The feed 105 is heated to a temperature of about 260° C. (500° F.) to about 371° C. (700° F.), or about 316° C. (600° F.) to about 343° C. (650° F.), e.g., about 343° C. (650° F.) in the first heating zone 110.
The heated feed 115 is sent to a flash drum 120 where it is flashed into a vapor stream 125 containing primarily the light hydrocarbons, e.g., VGO and optional solvent (if any), and a liquid stream 130 containing primarily the heavy hydrocarbons, e.g., the pitch.
The liquid stream 130 is sent to the second heating zone 135. The second heating zone 135 can be any suitable heating zone, including, but not limited to, fired heaters, the radiant zone of a fired heater, electric heaters, and the like. The second heating zone 135 heats the liquid stream 130 to about 371° C. (700° F.) to about 482° C. (900° F.), or about 385° C. (725° F.) to about 482° C. (900° F.), or about 427° C. (800° F.) to about 482° C. (900° F.), or about 385° C. (725° F.) to about 454° C. (850° F.).
The heated liquid stream 140 is sent to vacuum distillation column 145 for separation.
The vapor stream 125 is sent directly to the vacuum distillation column 145. The vapor stream 125 preferably enters the vacuum distillation column 145 at a point higher than the heated liquid stream 140, likely in the middle half of the column, dependent on the heat integration. The vapor stream 125 may be at a pressure of about 100 kPa (gauge) or less, or more typically it may be at vacuum pressure, i.e. 101.325 kPa (absolute) or less, or about 10 kPa(a) or less.
The heated liquid stream 140 and the vapor stream 125 are separated in the vacuum distillation column 145 into product streams which could include, but are not limited to, a light overhead cut, one or more cuts from the various light hydrocarbons and/or solvent, and pitch.
In the second process, the vapor stream from the flash drum is cooled and flashed before being sent to the vacuum distillation column.
In the process 200 shown in
The first heating zone 210 can be any suitable heating zone, such as those described above.
The feed 205 is heated from a temperature of from 204° C. (400° F.) to about 316° C. (600° F.), e.g., about 260° C. (500° F.), to a temperature of 260° C. (500° F.) to about 371° C. (700° F.), or about 316° C. (600° F.) to about 343° C. (650° F.) in the first heating zone 210.
The heated feed 215 is sent to a flash drum 220 where it is flashed into a vapor stream 225 containing primarily the light hydrocarbons, and a liquid stream 230 containing primarily the heavy hydrocarbons.
The liquid stream 230 is sent to the second heating zone 235. The second heating zone 235 can be any suitable heating zone, such as those described above. The second heating zone 235 heats the liquid stream 230 to a temperature of to about 371° C. (700° F.) to about 482° C. (900° F.), or about 385° C. (725° F.) to about 482° C. (900° F.), or about 427° C. (800° F.) to about 482° C. (900° F.), or about 385° C. (725° F.) to about 454° C. (850° F.), e.g., about 385° C. (725° F.).
The heated liquid stream 240 is sent to vacuum distillation column 245 for separation, as described above.
The vapor stream 225 is sent to a heat exchanger 250 where the temperature of vapor stream 225 is reduced from about 260° C. (500° F.) to about 371° C. (700° F.), or about 316° C. (600° F.) to about 343° C. (650° F.) to a temperature of about 204° C. (400° F.) or less, for example. The vapor stream 225 may be at a pressure of about 100 kPa (g) or less, or it may be at vacuum pressure, i.e. about 100 kPa (a) or less, or in some cases preferably about 30 kPa(a) or less.
The cooled vapor stream 255 is sent to a second flash drum 260 where it is separated into a second vapor stream 265 and a second liquid stream 270. The second vapor stream 265 is sent to the vacuum distillation column 245. The second vapor stream 265 preferably enters the vacuum distillation column 245 at a point higher than the heated liquid stream 240. The receiving vapor space location of vacuum distillation column 245 would depend on the temperature of the second vapor stream 265, but may be immediately below the top section of the column, contemplating appropriate heat recovery in heat exchanger 250.
The second liquid stream 270, which is primarily solvent or other light hydrocarbons, can be recovered and/or recycled.
In another embodiment of the process, the flash drum is located between the convection and radiant sections of a single fired heater and is tied to the vacuum column upper sections or overhead. This effects the lift of a large portion of the solvent and some of the volatile pitch. Thus the degree of vaporization in the radiant section of the fired heater is reduced. The shift in composition of the heavy feed to the vacuum column, e.g., that which is heated in the fired heater radiant section, also increases the lift of heavy material at constant heater outlet temperature. In addition, in some embodiments, the solvent lifted in the flash drum is of sufficient purity to be recycled to the process. The condensation of the solvent is a source of high value heat. After condensation only a small amount of vapor is routed to the vapor space of the vacuum distillation column, so that the vacuum distillation column and the associated equipment can also be smaller.
The heated feed 315 is sent to a heating zone 320, such as a fired heater, which includes a convection heating zone 325 and a radiant heating zone 330. The heated feed 315 enters the convection heating zone 325 where it is heated to a temperature of about 316° C. (600° F.) to about 427° C. (800° F.), or about 316° C. (600° F.) to about 371° C. (700° F.), or about 316° C. (600° F.) to about 343° C. (650° F.).
The heated feed 335 is sent to flash drum 340 where it is flashed into a vapor stream 345 and a liquid stream 350.
Liquid stream 350 is then sent to radiant heating zone 330 where it is heated to a temperature in the range of about 371° C. (700° F.) to about 482° C. (900° F.), or about 385° C. (725° F.) to about 482° C. (900° F.), or about 427° C. (800° F.) to about 482° C. (900° F.), or about 385° C. (725° F.) to about 454° C. (850° F.)., e.g., about 385° C. (725° F.).
The heated liquid stream 355 is sent to vacuum distillation column 360 for separation, as described above.
The vapor stream 345, which is at a temperature of about 260° C. (500° F.) to about 427° C. (800° F.), or about 316° C. (600° F.) to about 343° C. (650° F.), is sent to heat exchanger 310 where the temperature of vapor stream 345 is reduced in an aspect by heat exchange with feed 305. The cooled vapor 365 is sent to a second heat exchanger 370 for further temperature reduction to a temperature of about 204° C. (400° F.) or less, for example. The vapor stream 345 may be at a pressure of about 100 kPa (g) or less, or it may be at vacuum pressure, i.e. about 100 kPa (a) or less, or preferably about 50 kPa(a) or less.
The cooled vapor stream 375 is sent to a second flash drum 380 where it is separated into a second vapor stream 385 and a second liquid stream 390. The second vapor stream 385 is sent to the vacuum distillation column 360. Stream 385 will typically join the vapor space of the vacuum distillation column 360 either just below the top fractionation section or at the overhead vapor outlet, depending on the degree of condensation achieved in heat exchanger 370. Preferably, a high degree of condensation is achieved in heat exchanger 370 and the second vapor stream 385 enters the vacuum distillation column 360 at a point higher than the heated liquid stream 355.
The second liquid stream 390, which is primarily solvent and/or light hydrocarbons, can be recycled or recovered as product.
The next embodiment is similar; however, the balance of heat available in the vacuum column charge heater convection and radiant zones require the inclusion of another heater to achieve the desired flash temperature. The additional heater is located upstream of the fired heater convection zone. The heater can be a process exchanger, another fired heater, a hot oil heater, or any other suitable heat source.
The heated feed 415 is sent to a hot oil heater 417 where it is heated to a temperature of e.g., about 327° C. (620° F.).
The heated feed 419 is sent to a heating zone 420, such as a fired heater, which includes a convection heating zone 425 and a radiant heating zone 430. The heated feed 415 enters the convection heating zone 425 where it is heated to a temperature of e.g., about 343° C. (650° F.).
The heated feed 435 is sent to flash drum 440 where it is flashed into a vapor stream 445 and a liquid stream 450.
Liquid stream 450 is then sent to radiant heating zone 430 where it is heated to a temperature in the range of e.g., about 385° C. (725° F.).
The heated liquid stream 455 is sent to vacuum distillation column 460 for separation, as described above.
The vapor stream 445, which is at a temperature of e.g., about 343° C. (650° F.), is sent to heat exchanger 410 where the temperature of vapor stream 445 is reduced in an aspect by heat exchange with feed 405. The cooled vapor 465 is sent to a second heat exchanger 470 for further temperature reduction to a temperature of about 204° C. (400° F.) or less, for example. The vapor stream 445 may be at a pressure of about 100 kPa (g) or less, or it may be at vacuum pressure, i.e. about 100 kPa (a) or less, or preferably about 50 kPa(a) or less.
The cooled vapor stream 475 is sent to a second flash drum 480 where it is flashed into a second vapor stream 485 and a second liquid stream 490. The second vapor stream 485 is sent to the vacuum distillation column 460. Preferably stream 485 will have a very small flow during normal operation and will typically join the vapor space of the vacuum distillation column 460 either just below the top fractionation section or at the overhead vapor outlet, depending on the degree of condensation achieved in heat exchanger 470. Preferably, the second vapor stream 485 enters the vacuum distillation column 460 at a point higher than the heated liquid stream 455.
The second liquid stream 490, which is primarily solvent and/or light hydrocarbons, can be recycled or recovered as product.
For the processes illustrated in
By “about” we mean within 10% of the value, or within 5%, or within 1%.
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.
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