Abstract:
This is a process and apparatus for using a passive desuperheater for passively desuperheating a superheated gas stream before the stream is transmitted to a heat exchanger. A spent gas stream of a liquid condensate is accumulated in the passive desuperheater. An incoming superheated gas stream comes into the passive desuperheater below the liquid level of the liquid condensate in the passive desuperheater for maximum direct contact heat transfer between the incoming steam and the liquid condensate. An outgoing stream of saturated gas exits the desuperheater above the level of the condensate.

Description:
TECHNICAL FIELD 
     This invention relates to the use of superheated steam systems for energy input to process exchangers. The passive desuperheater incorporates the desuperheating operation within the exchanger condensate drum by direct contact between incoming superheated steam and the subcooled condensate draining from the exchanger. 
     BACKGROUND ART 
     Utility steam is typically available at superheated conditions for heat transfer applications. Superheated steam is less efficient for heat transfer than saturated steam. Superheated steam requires more exchanger surface area than an appropriate level of saturated steam to achieve the same energy input. 
     A refinery typically operates several levels of utility steam headers. The high pressure steam level is nominally 600 psig and is superheated to ˜700° F. These conditions are too severe for direct application as reboiler heat source for several distillation tower applications in the refinery. For instance, the steam is too hot for use in debutanizer reboiler service. The high temperature steam can be cooled by injecting water. However, traditional desuperheaters are complex, expensive, and suffer from poor reliability. 
     Traditionally, utility steam is de-superheated by the controlled injection of condensate to reduce superheat prior to use in heat exchangers. These injection type desuperheaters require a high pressure condensate source (typically requiring a new pump), an in-line injection nozzle and control valve, and are prone to reliability problems in field service. 
     SUMMARY OF THE INVENTION 
     The passive desuperheater of this invention revises the traditional configuration of typical equipment used in steam driven exchangers to perform the desuperheating service without external condensate injection. The passive desuperheater incorporates the desuperheating operation within the exchanger condensate drum by direct contact between incoming superheated steam and the subcooled condensate draining from the exchanger. This system eliminates the need for a separate condensate source and pump, the condensate injection nozzle, and the desuperheating control station. 
     Using superheated steam for process heat transfer is relatively space inefficient, since exchanger area required for desuperheating the steam transfers only a small fraction of the energy to the receiving stream. The portion of the exchanger dedicated to desuperheating often occupies a relatively large fraction of the overall high-pressure heat exchanger (i.e., desuperheater and condenser) area. This inefficiency results because the desuperheating operation has a low internal heat transfer coefficient due to the heat transfer mechanism during the normal operation of such a system. In comparison, the condenser portion of such an exchanger has a relatively high internal heat transfer coefficient. When the entire high-pressure heat exchanger functions as a condenser, the exchanger can be made smaller to achieve the same heat transfer specification. 
     The passive desuperheater of this invention introduces superheated steam below the associated condense pot liquid level. Intimate contact between in the incoming steam and the condensate is ensured in this way. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustrating a typical installation of the passive desuperheater of this invention. 
         FIG. 2  is another schematic illustration of another typical service using the passive desuperheater of this invention. 
         FIG. 3  is a schematic illustrating the passive desuperheater vessel and nozzle schedule. 
         FIG. 4  illustrates the sparger of this invention in greater detail. 
         FIG. 5  illustrates a passive desuperheater incorporated within a heat exchanger. 
         FIG. 6  illustrates a prior art desuperheater. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Heat exchangers using steam as the heat source are most efficient when the condition of the steam is saturated vapor, somewhat hotter than the target temperature of the fluid being heated. If the steam source is saturated, but too hot, process side “film boiling” can occur which impairs heat transfer efficiency. If the steam is not hot enough, excessive surface area is required due to the low driving force temperature difference across the exchanger. 
     In a steam heater, steam is introduced to one side of a shell and tube exchanger and process fluid is routed through the other side of the exchanger. For example: heavy naphtha from the bottom of a debutanizer communicates directly with the shell side of a shell and tube reboiler. Superheated 600 psig steam is routed to the tube side of the exchanger. As the steam condenses on the walls of the tubes, naphtha is heated and boiled on the shell side of the tubes. The condensed steam flows, by gravity, through the tubes and out the bottom of the exchanger into a condensate pot. The partially vaporized naphtha on the shell side is forced out the top of the shell side of the exchanger, back to the tower. The steam side condensate pot is normally drained on level control to maintain back pressure on the steam side of the exchanger. 
     The passive desuperheater utilizes the accumulating condensate to desuperheat incoming steam to saturated conditions by direct contact. Incoming steam is introduced below the liquid condensate level through a sparger to maximize direct contact heat transfer between the incoming steam and condensate. The incoming steam is cooled by the resident condensate as it bubbles through the liquid level. Steam leaving the condensate pot is at saturation point, somewhat warmer than the condensate drained from the exchanger. 
     The condensing temperature of steam is controlled by the pressure of the system. The pressure of the steam side with the passive desuperheater design is controlled by throttling incoming steam flow. Effectively, the condensing temperature on the steam side of the exchanger is varied to provide more or less driving force in the exchanger. As the driving force temperature difference changes, more or less heat transfer occurs to achieve the desired process outlet temperature. 
     The system is designed with the exchanger elevated above the condensate drum to allow condensate to free drain in to the drum. Steam is fed through a sparger into the bottom of the condensate drum, below the normal liquid level. Steam flow is modulated by a control valve, typically based on process outlet temperature. Condensate is drained from the drum via a control valve typically based on drum level. Additional system instrumentation would typically include flow indication on the incoming steam, level indication on the condensate drum, pressure indication on the condensate drum, and temperature indication on the steam line leaving the condensate drum. 
     A prior art system introduced steam above the liquid level in the bottom of the condensate drum and below a series of internal trays. Condensate from the exchanger was introduced above the internal trays. The trays were intended to promote mixing and heat transfer between the rising steam and the falling condensate. While this works, it does not work well. Direct contact desuperheating occurs but is insufficient due to inefficient contacting in the trays. Passive desuperheaters of this invention eliminate the contact trays inside by introducing superheated steam below the vessel liquid level via a sparger. Intimate contact between in the incoming steam and the condensate produce is ensured in this way. The new units with the revised design provide excellent results. 
     The present invention is illustrated by way of example in the accompanying drawings. 
       FIG. 1  is a schematic illustrating a typical installation of the passive desuperheater.  FIG. 1  shows process stream  10  (naphtha) entering steam driven heater  12  on the shell side. Hot naphtha  14  exits the shell side of the steam heater and passes over the temperature sensor serving temperature controller  18 . Superheated steam  20  enters the unit through optional flow indicator  22  and passes through temperature control valve  24 . Temperature control valve  24  receives its control signal from temperature controller  18 , modulating the in flow of superheated steam to control the final temperature of hot naphtha  14 . Superheated steam from temperature control valve  24  enters passive desuperheater  26  via sparger  28  located below liquid level  30  of accumulated condensate  32  in passive desuperheater  26 . Optional pressure indicator  33  allows monitoring desuperheater conditions. Steam exits sparger  28  and rises through condensate level  30  achieving intimate contact with condensate  32  and undergoing direct contact heat transfer to desuperheat the rising steam. Saturated steam  34  exits the top of vessel  26  and passes over optional temperature indicator  36  en route to the tube side of process heater  12 . Saturated steam condenses in the tubes of the process heater yielding condensate  38  which flows by gravity back into passive desuperheater vessel  26 . Spent condensate  40  is drawn from the bottom of passive desuperheater  26  based on maintaining constant level as measured by level instrument  42 . The level reading from level instrument  42  is used to modulate the condensate level control valve  44  to maintain the system water balance. 
       FIG. 2  is a schematic illustrating another typical use for the passive desuperheater in service on a distillation tower reboiler. Process feed  50  (unstabilized naphtha) enters distillation tower  52 . Accumulated tower bottoms is drawn from the bottom of tower  52  as stream  54  and routed to reboiler  56  where the naphtha stream is heated and at least partially vaporized before returning to the tower as stream  58 . Temperature sensor  60  located a few trays up in the tower monitors tower conditions and sends a signal to temperature control valve  62  to control the flow of superheated steam  64  into passive desuperheater  66 . The operation of desuperheater  66  is identical to the description for  FIG. 1 . Desuperheater  66  provides saturated steam to reboiler  56  and discharges condensate  70  via control valve  72  operated by level control  74 . The distillation tower produces two process stream products, stabilized naphtha  76  from the bottom of the tower, and light overhead  78 . 
       FIG. 3  is a schematic illustrating the passive desuperheater vessel and nozzle schedule. Nozzle  80  is the superheated steam inlet to desuperheater  82 . The steam is introduced through sparger  84 . Nozzle  86  is the condensate liquid drain from the bottom of desuperheater  82 . Nozzles  88  are level bridle connections for hooking up the level sensing instrument. Nozzle  90  is the condensate inlet nozzle for liquid returning from the associated exchanger. Nozzle  92  is the saturated steam outlet feeding desuperheated steam to the associated exchanger service. 
       FIG. 4  illustrates sparger  84  in greater detail. Sparger  84  includes a multiplicity of apertures  94  which allow steam to perculate through condensate  32  of  FIG. 1 . This maximizes direct contact heat transfer between the incoming steam and condensate  32 . Sparger  84  is a perforated hollow tube circumscribing space  96  through which steam passed to aperture  94 . 
       FIG. 5  is a schematic showing an alternate configuration for the passive desuperheater where-in the passive desuperheater is incorporated into the exchanger itself. In this figure, cold process stream  102  enters the shell side of exchanger  104  where the stream is indirectly heated by steam condensing inside the exchanger tubes. The hot process stream  106  leaves the shell side of exchanger  104  and passes over temperature sensor  108  on its way to additional downstream processing. Superheated steam  110  enters tube side  112  of exchanger  104  via temperature control valve  114 . Pressure of tube side  112  is monitored by pressure sensor  116 . Level sensor  118  is used to adjust condensate level  120  in the tube side of the exchanger by manipulating condensate level control valve  122  ensuring adequate desuperheating is occurring. Spent condensate exits via line  124 . 
       FIG. 6  illustrates a prior art desuperheater.  FIG. 6  shows prior art desuperheater  130 . Desuperheater  130  includes traditional trays  132 . Superheated steam enters desuperheater  130  via line  134  and passes upwardly through trays  132  and exits via line  136  as saturated steam. Condensate enters desuperheater  130  via line  138  and passes downwardly over trays  132 . Condensate  140  accumulates in the bottom of desuperheater  130  below steam line  134 . Condensate  140  has a liquid level  142  which also is below steam line  134 . 
     The above detailed description of the present invention is given for explanatory purposes. It will be apparent to those skilled in the art that numerous changes and modifications can be made without departing from the scope of the invention. Accordingly, the whole of the foregoing description is to be construed in an illustrative and not a limitative sense, the scope of the invention being defined solely by the appended claims.