Patent Publication Number: US-2015075450-A1

Title: Heat recovery from a high pressure stream

Description:
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 
     BACKGROUND OF THE INVENTION 
     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. 
     BRIEF SUMMARY OF THE INVENTION 
     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. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       A preferred embodiment of the present invention is described herein with reference to the drawing wherein: 
         FIG. 1  is an example of an embodiment of the present process for recovering heat from a high pressure stream within a hydrocracking unit. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     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  FIG. 1 , the present process is shown as being incorporated into a hydrocracking unit. As hydrocracking units are known to those of ordinary skill in the art, only those process flows and components related to the present process are shown and described, as it should be clear to one of ordinary skill in the art how the present process can be incorporated into a hydrocracking unit. Also, it should be noted that the present process is not limited to hydrocracking units, but can instead be provided into other types of hydroprocessing units, as well as into processing units of other types in which heat recovery from a high pressure stream is desired. 
     Turning again to  FIG. 1 , this figure shows an embodiment in which a high pressure vapor stream  10  is fed from a hot separator  12  to a first process vessel used as a first steam generator, such as first cooler  22 . Other embodiments are also contemplated, such as embodiments including shell and tube exchangers arranged in parallel which exchange heat with boiler feedwater flowing by natural circulation from a vessel mounted above the shell and tube exchangers. This vessel acts as a disengaging space to separate the steam generated from the circulating boiler feedwater. In this way, multiple services generating steam at the same pressure share a common separation vessel. 
     In an example of the  FIG. 1  embodiment, the pressure of the stream  10  could be within the range of 500 psig (34.5 barg) to 2800 psig (154 barg), and the temperature could be within the range of 400° F. (200° C.) to 700° F. (370° C.). Of course, in other configurations, the stream would be at a different pressure and temperature. 
     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 10/13 th  of the tube side design pressure in this embodiment. 
     The present embodiment of  FIG. 1  also includes a second steam generator, such as second cooler  32 . Exit stream  60  from the first cooler  22  is used as the heat source for creating steam within the second cooler  32 . Of course, the temperature of the stream  60  exiting the first cooler  22  will be lower than that of stream  29  entering the first cooler  22  because some of the heat has been extracted to create the steam of the stream  24  from the boiler feed water. 
     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  FIG. 1  embodiment, stream  62  is first routed to an exchanger  66 , which may be associated with a recycle gas stream, and then to a heat exchanger  68 , which receives a feed from a cold flash drum (not shown). Of course other configurations are also contemplated, depending on the various temperature and pressure parameters and the other components of the processing unit. 
     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.