Patent Application
20150075450
The present invention relates generally to hydroprocessing units, and more particularly to a process for recovering heat from a high pressure stream. In one embodiment, the high pressure stream providing the heat is a vapor stream from a hot separator, which is used to generate both a medium steam and a low pressure steam that can each be used in further processing, such as being used as stripping steam within components such as in a stripper, a product fractionator, and/or a diesel side stripper
Energy optimization for hydroprocessing units, such as hydrocracking units, has become very important, and there is a drive towards minimum utilities and maximum heat recovery. The present inventors have realized that one way to achieve this is via steam generation using the hot separator vapor. However, the present inventors also realize that since the hot side is reactor effluent that is at a very high pressure, safety is a big concern. Hence, steam generation with the required intrinsic safety becomes important. The scheme developed by the present inventors, an example of which is described below, achieves this requirement.
Briefly, in certain embodiments, the present process is a process for recovering heat from a high pressure stream during hydroprocessing, where one embodiment of the process includes serially introducing a high pressure stream from a hot separator into a first steam generator and a second steam generator; using the first steam generator to generate a medium pressure stream of steam, and then using the medium pressure stream as stripping steam. The process also includes using the second steam generator to generate a low pressure stream of steam, and then using the low pressure stream as stripping steam.
Also, in certain embodiments, the present process is for recovering heat from high pressure steam during hydroprocessing includes the steps of using a hot separator to create a high pressure vapor stream, and then extracting heat from the high pressure vapor stream to generate both medium pressure steam and low pressure steam. In certain embodiments, the medium pressure steam is routed to a stripper, where the medium pressure steam is used as stripping steam, and the low pressure steam is routed to at least one of a product fractionator and a diesel side stripper, where the low pressure steam is used as stripping steam.
Finally, certain embodiments of the present process for recovering heat from high pressure steam during hydroprocessing involve routing a high pressure stream to a first process vessel and routing a first feed water stream to the first process vessel. The process continues by extracting heat from the high pressure stream within the first process vessel to create a medium pressure stream of steam from the first feed water stream. The process also involves routing the high pressure stream from the first process vessel to a second process vessel and routing a second feed water stream to the second process vessel. Finally, the process involves extracting heat from the high pressure stream within the second process vessel to create a low pressure stream of steam from the second feed water stream.
A preferred embodiment of the present invention is described herein with reference to the drawing wherein:
Briefly, in certain embodiments of the present process, which can be used in a hydroprocessing unit (such as a hydrocracking unit), two different pressure levels of steam are generated using hot separator vapor, where one of the steam streams is intended for use as stripping steam for a stripper (medium pressure steam) MLP and the other of the steam streams is intended for use as stripping steam for a product fractionator and a diesel side stripper (Low pressure steam). Each steam generator will produce exactly the amount needed for stripping at the required level. Additional high pressure steam from the header will be used to makeup the medium pressure steam requirement, and additional medium pressure steam from the header will be used to makeup the low pressure steam requirement when needed, such as during start-up and in other cases when steam generation is insufficient for the process requirements. If the resulting steam generation is more than that required, a pressure controller will close the streams of makeup steam from the header.
However, this closure causes the pressure in the steam generator(s) to increase, which increases the temperature of the steam being generated. The result is that the temperature difference between the hot side fluid and the water from which the steam is generated will decrease and less steam will be generated. This will tend to self regulate the steam generation. Normally, the steam generators operate at a pressure lower than the respective header that supplies steam during startup. To prevent contamination of the respective supply header, if the steam pressure increases over a certain level, a high pressure switch closes an isolation valve to prevent the flow of steam back to the header. This type of valve closure prevents contamination of the steam header if there is a tube rupture or leak from the high pressure side. Such a tube leak or rupture could otherwise cause hydrogen sulfide and other non-condensibles to enter the steam header, thereby contaminating it.
In the case of a tube rupture, pressure in the steam drum will increase and a high pressure switch will be activated, which will then close shut-off valves in the boiler feed water (BFW) line and the blowdown line, thereby capturing the fluid from the rupture tube within the generator itself and minimizing contamination of the boiler feedwater header. The set pressure at which the switch gets activated is the BFW pump shut-in pressure. In certain embodiments, the steam generator design pressure is preferably set to be 10/13th of the tube side (high pressure) design pressure. After the steam generator has been cut-off from the BFW line, the steam outlet line, the makeup line and the blowdown line, the steam generator is isolated and a pressure safety valve
(PSV) on the generator will open if the pressure reaches the PSV set pressure. The line from the PSV is routed to the relief header, rather than to the atmosphere, since hydrocarbons and hydrogen sulfide will be present in the vapors to be relieved if there is a tube rupture. Since the PSV line is routed to the relief header, there is a chance of leakage of steam to the flare header during normal operation, and hence a rupture disc is also provided upstream of the PSV to eliminate leakage. Otherwise, if steam leakage were to occur during colder temperatures, a blockage of the relief header due to ice buildup could result.
In an example of one embodiment, as shown in
Turning again to
In an example of the
Prior to reaching the first cooler 22, the stream 10 can be passed through other components, such as through one or a series of heat exchangers, in order to remove some of the heat for use in other parts of the process. In this example, the stream 10 first passes through a heat exchanger 14 (such as a shell and tube heat exchanger), which heats one of the process streams, such as fresh feed; it then passes through another heat exchanger (such as another shell and tube heat exchanger) 16, which heats another stream in the process, such as recycle gas; and finally it passes through a heat exchanger 20, which heats another stream in the process, such as feed to the fractionation section. Of course, other configurations are also contemplated, depending on the various temperature and pressure parameters and the other components of the processing unit.
After the stream 10 has passed through the heat exchangers 14, 16, and 20, the resultant stream 29 is routed to the first cooler 22, as mentioned above, wherein it used to provide heat to generate steam from the boiler feed water that enters cooler 22 through boiler feed water (BFW) line 25. The liquid level within the first cooler 22 is monitored by a liquid level controller (LIC) 17 that is associated with a flow indicator controller 19 and a valve 21, for regulating and controlling the amount of boiler feed water routed to cooler 22 through the boiler feed water (BFW) line 25.
The boiler feed water is turned into steam within the first cooler 22 by extracting heat from stream 29, resulting in a resultant stream 24 of saturated steam. The resultant stream 24 could, for example, be at a pressure within the range of approximately 100 to approximately 400 psig (7 to 28 barg) in other embodiments. After the resultant stream 24 leaves the first cooler 22, it is fed to a superheater 26. As the steam passes through superheater 26, the saturated steam is superheated and leaves as stream 27, which can ultimately be fed to a stripper (not shown) through line 31, after passing through flow control valve 30. Valve 30 is controlled by an associated flow indicator controller that regulates and monitors the flow of the stream to the steam stripper column.
However, prior to going through line 31 to be used as stripping steam in the stripper, the superheated steam is mixed with a stream 33 of high pressure steam from the header. In order to arrive at the desired pressure for the stripper (which in this case is the medium pressure steam), this embodiment uses a control valve 35 associated with a pressure indicator controller (PIC) 37, as well as an additional control valve 39 associated with an additional PIC 41. In particular, the PIC 37 monitors the pressure of stream 27 at a point after this stream passes through superheater 26, but before being combined with another stream, and if the pressure of stream 27 needs to be increased (or decreased) in order to arrive at the desired pressure for entering the stripper through line 31, PIC 37 opens (or partially or fully closes) valve 35 so that more (or less) high pressure stream 33 is mixed with stream 27.
In this embodiment, the medium pressure stream of line 31 could be any preselected pressure value between approximately 100 psig (7 barg) and 400 psig (28 barg).
If PIC 41 determines that the pressure in line 43 is above a predetermined value (such as, for example, a predetermined value between 140 psig (10 barg) and 300 psig (21 barg)), a high pressure switch closes the isolation valve 39 to prevent the flow of steam to the high pressure header. Such a configuration prevents contamination of the steam header if there is a tube leak or rupture on the high pressure side because without the closure of the isolation valve 39, hydrogen sulfide and other non-condensables could enter the steam header during a tube leak or rupture, thereby contaminating the header.
A pressure alarm system 100, which in this case is a pressure alarm (high/high), or PAHH, is associated with the first cooler 22 (first steam generator). As known in the art, such pressure alarm systems, as well as the other controls and controllers mentioned herein, are commonly associated with a computer processor. This first pressure alarm system 100 includes a pressure indicator (PI) 102 that monitors the pressure of stream 24 at a location between first cooler 22 and superheater 26, as well as including shut-off valves 104, 106 and 108. If there is a tube rupture in first cooler 22, pressure within the first cooler 22 will increase, and such an increase will be detected by the pressure indicator 102. Once the pressure reaches a predetermined level (such as, for example, a predetermined value between 140 psig (10 barg) and 300 psig (21 barg)), the controller activates a high pressure switch that closes the following shut-off valves: (a) the shut-off valve 104 (associated with stream 27), (b) the shut-off valve 106 (associated with blow down line 23), and (c) the shut-off valve 108 (associated with the boiler feed water line 25). Thus, with these valve closings, the fluid from the ruptured tube is safely captured within the first steam generator itself.
Further, once the shut-off valves 104, 106 and 108 have been closed and the first steam generator (including the first cooler 22 in this embodiment) is isolated, a pressure safety valve (PSV) 110 is configured and arranged to open if the pressure reaches the PSV set pressure. The stream from the pressure safety valve 110, when opened, is routed through stream 112 to a relief header (not shown) because, in this embodiment, hydrocarbon and hydrogen sulfide will also be released during a tube rupture. However, since in this embodiment the stream 112 is routed to the relief header (not shown), there is a chance of leakage of steam to the relief header, and accordingly this embodiment also preferably includes a rupture disc 114, or other equivalent device, in series with the PSV 110 to eliminate such steam leakage. If such steam leakage were to occur during colder temperatures, a blockage of the flare header could result.
The steam generator design pressure, which is the same as the PSV set pressure of this first steam generator (including first cooler 22) is preferably set to be 10/13th of the tube side design pressure in this embodiment.
The present embodiment of
After stream 60 passes through the second cooler 32 and is used to generate steam within the second cooler, an exit stream 62 from the second cooler 32 can be passed through one or more heat exchangers, or other components, before a resultant stream 64 is routed to a product condenser for further processing, which processing is known to those of ordinary skill in the art. In the
Finally, the line associated with the blowdown stream 23 from the first cooler 22 includes a valve 76, in addition to the valve 106 of the first pressure alarm system 100 discussed above. This valve 106 is used to control the flow of the blowdown stream 23, which is then designated as stream 77 after passing through the valve 106, and stream 77 is routed to a blowdown drum (not shown). In this example, the refinery blowdown network is designed for low pressure, so the blowdown drum will act as a vessel with a PSV where a pressure break can be achieved.
The second steam generator (second cooler) 32 operates in a similar manner to that of the first steam generator (first cooler) 22, and thus will not be described in great detail, except to discuss any significant differences between the two steam generators (coolers). Additionally, components and flows associated with the second cooler 32 that correspond to those of the first cooler 22 will be designated with like reference numerals, except those associated with the second cooler will include a single prime (′) or a double prime (″) designation.
One difference between the flows of the steam generated with the second cooler 32 and those associated with the first cooler 22 is instead of having the resultant medium pressure stream 31 being passed to a stripper (as with the first steam generator with the first cooler 22), the resultant stream from the second steam generator (with the second cooler 32) is routed in parallel through two streams, designated as low pressure stream 31′ and low pressure stream 31″. In this embodiment, the stream 31′ is routed to a product fractionator (not shown) and the stream 31″ is routed to a diesel stripper (not shown). The steam of streams 31′ and 31″ is used as the stripping steam in the product fractionator and the diesel stripper, respectively. In this embodiment, the streams 31′ and 31″ are preferably configured to be at a specific predetermined pressure that is between 15 and 20 psi, but other pressures are also contemplated, depending on the intended use of the streams.
An additional difference between the flows associated with the first and second steam generators relates to the supplemental steam being provided arrive at the desired pressure for the medium pressure stream 31 (associated with the first steam generator, including first cooler 22) and the low pressure streams 31′ and 31″ (associated with the second steam generator, including second cooler 32). In particular, with regard to the medium pressure stream 31, this stream can be mixed with the appropriate amount of high pressure steam from the header through stream 33 to arrive at the predetermined pressure via various valves and controls, as discussed above. A similar control process is followed for the low pressure streams 31′ and 31″, except instead of receiving supplemental high pressure steam from the header through stream 33, as needed, the low pressures streams 31′ and 31″ in this embodiment receive supplemental medium pressure steam from the header, as needed.
Other than the differences noted above, the components associated with the second steam generator (including the second cooler 32), such as the second pressure alarm system 101′, the second superheater 26′, etc., operate in essentially the same manner as the corresponding components of the first steam generator (including the first cooler 22). According, such components need not be discussed further.
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 is 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.
