STAGED HEAT AND MASS TRANSFER APPLICATIONS

Abstract: An apparatus including at least one heat transfer zone (400) and at least one mass transfer zone (300). Two applications following the concept of staged heat and mass transfers are given. The first one relates to wastewater stripper overhead. The second one relates to recovering both heat and condensate from a steam boiler deaerator steam vent (35) by retrofitting the vent with an economizer.

Description: Patent Application of Peter Tung For Staged Heat and Mass Transfer Applications Background- Field of Invention This invention relates generally to processes involving heat and mass transfer applications. In a specific respect, the invention relates to the segregation of operating zones in a heat / mass transfer apparatus, such as a distillation column, to effect efficient separations, heat and product recovery with or without chemical reactions. Two applications illustrating the concept of staged heat and mass transfers are given. One relates to applying the segregation concept on wastewater treatment to improve corrosive vapor handling. The other relates to recovering both heat and condensate by retrofitting a steam boiler deaerator steam vent with an economizer. Background-Discussion of Prior Art In unit operations that involve heat and mass transfer, it is sometimes desirable to segregate the process into a number of specific zones so that the actual process mechanism can be better understood. This approach helps analyze process problems and turn them into improvement opportunities. One such scenario can be found in wastewater treatment handling facility. Briefly, the challenge in removing process heat by conventional heat removal methods like shell and tube exchangers is as follows. As the process stream's dew point is reached, the first drop of condensate begins to appear. The acid gases start to dissolve rapidly into the freshly formed condensate droplet. This results in a highly corrosive environment. Even exotic material like Titanium alloys gives limited run lengths. U.S. Pat. No. 5,676,802 Fig 1 taught the use of spray quenching inside a column to avoid the use of overhead condenser. Process condensate, after contacting the process vapor, is pooled in an upper reservoir in the column. Heat is removed by recirculating the quenched process condensate around a cooler as shown in Fig 1. Excess process condensate is siphoned back to the feed drum, mixed in with the feed, and recycled back to the column with the feed stream. While this prior art provides a tangible method to avoid conventional overhead condenser, the single stage equilibrium reached by mist contact has quite a few drawbacks. 1) The one stage quenching provides little or no stripping capability. Consequently, the process condensate contains considerable amounts of acid gases in solution, making it highly corrosive for the cooling and the pumping equipment. 2) The countercurrent arrangement risks mist entrainment as process vapor load increases. 3) Extra piping and control means is required to maintain the level in the reservoir and recycle the process condensate as feed. Another scenario can be found in steam boiler deaerator operations. Typically, the steam vent on a deaerator is around half to one percent of the steam generation load. Dissolved gases in fresh make up water are purged out in this vent stream in dilute form with the balance in steam. It would be beneficial to recover the lost heat and steam condensate. U.S. Pat. No. 5,476,525 proposed the use of a bare piece of pipe extending two feet from the deaerator to effect heat loss by natural convection. A theπnostatic steam trap is mounted at the end of this extended pipe to let out steam that is contaminated with gases like oxygen. Obviously, this method offers some condensate recovery and is better than the common "straight venting" method. It is far from efficient. U.S. Pat. No. 5,728,200 by the same inventor presented two other methods to recapture the vented steam. One method is to contact feed water and return condensate directly with the vent stream, thereby partly condensing the steam into condensate and recovering heat content as well as the condensate. The other method is to "bubble" the vent stream through a pool of feed water to capture the same. While the inventor has tried to improve on his previous invention, these proposed methods still have major drawbacks. 1) The one stage spray arrangements provides no efforts of separation. In fact, a portion of the gases removed by the deaeration process is recaptured and recycled back into the feed water, increasing the overall deaerator load. 2) The method proposed, using temperature or pressure in the vent steam recovery side as indication of feed steam flow requirement, presents a fundamental control objective problem. Steam flow to deaerator varies according to heat recovery operation and not deaeration requirement. The quality of deaerator operation could therefore be compromised. 3) The steam-recapturing portion is so integrated with the rest of the deaerator unit that retrofitting to existing deaerator will be cumbersome. This invention will demonstrate applications of staged heat and mass transfer concept to provide solutions to the above challenges. Objects and Advantages Accordingly, several objects and advantages of this invention are: The present invention focuses on overcoming the difficulties in analyzing the common process of integrated heat and mass transfer by separate zones analysis. The present invention takes the approach of built-in design flexibility so that the resulting apparatus and control method can cover a wide range of operating scenarios. It is another object of this invention to further improve the operation of a process wastewater stripper overhead to provide reliable sour gas removal and acid scrubbing capability. It is another object of this invention to recover both usable heat and condensate from a steam boiler feed water deaerator' s vent stream. It is another object of this invention to demonstrate that the boundary of the zones as well as the operating targets can be moved to accommodate process needs. It is yet another objective of this invention to illustrate the method and opportunity of applying this invention to retrofitting existing deaerators and wastewater stripping columns. Further objects and advantages of this invention will become apparent from a consideration of drawings and ensuing description. Description of Drawings Fig. 1 shows internal spray quench in wastewater treatment, prior art. Fig. 2 shows simplified conceptual staged heat and mass transfer zones, General Example. Fig. 3 shows preferred arrangement of wastewater stripper overhead. Fig. 4 shows preferred arrangement of deaerator vent economizer. Summary The summary of this invention is the application of the concept of staged heat transfer and mass transfer zones to effect efficient separations, heat and product recovery. Two applications following the concept of staged heat and mass transfers are given. The first one relates to wastewater stripper overhead and improvements in sour gas handling. The second one relates to recovering both heat and condensate from a steam boiler deaerator steam vent by retrofitting the vent with an economizer. Description of Invention This invention relates to applying the concept of segregating heat transfer zone and mass transfer zone to facilitate process analysis and to improve process control robustness. In order to clarify the concept of staged heat and mass transfer zone, two typical scenarios are given below. 1) Let us focus on a distillation tower that has an overhead condenser and a reflux drum. Often times, the condensed vapor is further sub-cooled by the cooling medium when condenser flooding is used for controlling heat transfer area. Consequently, the reflux is cooler than the vapor exiting at the top tray. Most process control applications calculate the internal reflux by adding the sensible heat contribution to the external reflux (reflux as read) to arrive at a L D ratio. L /D ratio is the internal reflux to overhead distillate ratio. It is a measure of the degree of rectification. While the concept is heat balanced, the assumption of instantaneous thermal equilibrium is an over simplification. In reality, up to a few trays' mass transfer ability could be lost in exchange for thermal equilibrium before mass transfer can take place. 2) It is a common practice in oil refinery operations, such as atmospheric crude oil distillation, to remove heat along the length of a column by using pumparounds (PA). A typical PA takes a process stream, cools it, and returns it back to the column at a usually higher location. This common approach is taken for two main reasons. One is because of better usable heat quality (hotter) and the other is to optimize vapor loading. A common practice is to allow a few trays, typically three trays, for heat transfer. The sizing is mostly based on empirical data with some trial and error. In heat transfer, area available is an important factor. Again, because of back mixing, the trays that are providing the area for heat transfer have very little contribution to mass transfer, i.e. separation. Essentially the heat transfer zone operates at a homogeneous composition profile. It is worthwhile pointing out that the source of the cooled stream does not have to be originated from the same column as given by this example. Instead, it can be an external source as shown in Fig.2. This invention focuses on applying the above concept to segregate heat transfer and mass transfer zones so that each zone's function and effectiveness can be identified and possible improvements made. The following is a General Example of zone segregation, General Example Fig. 2 shows a countercurrent stripping of sub-cooled condensate by a vapor stream, containing condensable components, using a packed column arrangement 500. Two zones are shown. They are symbolically separated by a boundary (imaginary or physical) into a heat transfer zone 400 above and mass transfer zone 300 below. For clarity, stream numbers are changed to reflect the transitional boundary between the two zone's. Vapor stream 25 changed to vapor stream 35 and liquid stream 150 changed to liquid stream 160 after crossing the boundary. Vapor stream 25, containing condensable components, enters the bottom of distillation structure 500, later to be also called zone G, and make its way up to the mass transfer zone 300. Liquid stream 160, containing soluble components like dissolved gases, exiting from the heat transfer zone 400, is in countercurrent contact with vapor stream 25 at mass transfer zone 300. Soluble components, such as dissolved gases, in liquid stream 160 are transferred to vapor stream 25, utilizing mass transfer gradient between the two streams, along mass transfer zone 300 before exiting as liquid stream effluent 170. Likewise, some condensable components in vapor stream 25 are also transferred to liquid stream 160 depending on the dew point of the vapor stream 25. Mass transfer zone 300 operates like a stripping zone with the liquid steam 160 and vapor stream 25 at thermal equilibrium with each other. Vapor stream 25 turns into vapor stream 35 after crossing the boundary, enters heat transfer zone 400 where it is put to countercurrent contact with liquid stream 150. In heat transfer zone 400, latent heat in vapor stream 35 is exchanged for sensible heat in liquid stream 150, raising the temperature of liquid stream 150 as it moves down the heat transfer zone 400. Heat transfer zone 400 operates like a quenching zone. At steady state, material balance will result in an equilibrium concentration of acid gases dissolved in the liquid phase across the heat transfer zone 400. This acid gases concentration in turn forces the acid gases to be removed via the vent stream 45. Because of the quenching effect, vapor stream's 35 flow rate decreases as it moves to the top and exits as the concentrated vapor stream 45. Liquid stream's 150 flow rate increases as it moves down. At the boundary, vapor stream 35 and liquid stream 150 are in thermal equilibrium with each other, meaning the two streams are at the same temperature. This quenching zone operates like a direct contact heat exchanger having temperature gradient (delta T), area (A) for heat transfer and some form of heat transfer coefficient (U). The equation for heat transfer determining rate of heat transfer (Q). Q=U*A*deltaT The above is a gross simplification of the actual simultaneous heat and mass transfer but the concept can be applied to the following example: A sour gas stream loaded with steam as vapor stream 25, exiting the stripping section of a wastewater stripper at atmospheric pressure, say at 210 deg F with half mole percent Hydrogen sulfide. A cooled process water stream 150, acting as the quenching stream. Those who are skilled in the art would recognize that the boundary is movable depending on operating parameters like heat balance, flow rates, etc as well as design parameters like total heat transfer area, stages and so on. The above concept provides a quenching and stripping combination that can be further refined and exploited for applications that are more specific. With proper design, this combination can provide the required quenching duty as well as the required stripping duty. The following is one adaptation of the concept to tailor the streams to fit within an operating envelope. The above General Example is now referred to as zone G, the same as 500. Wastewater Treatment Application In an overhead wastewater treatment application as shown in Fig. 3, two more operating zones is now added to the General Example, zone G, given above. The first zone, the pretieatment zone 200, reduces the excess steam to allow proper processing of the off gas through zone G, as described in the General Example above. Process vapor is concentrated after passing through zone G and vented to downstream processing units as described above. The second zone, the cleanup zone 100, cleans up the quenching medium to protect the pump and the inline cooler from dissolved acid gases. The following is a detail description of the two additional zones. The pretreatment zone 200 primarily functions as heat transfer zone and the vapor stream 15 and the quench liquid stream 40 operate cocurrently. This arrangement offers the advantage of lower pressure drop as the vapor volume in vapor stream 15 is continuously being reduced when it comes in contact with the quench liquid stream 40. Even at very high column loading as dictated by process requirement, flooding is hardly a consideration, unlike countercurrent arrangement. Pressure drop across and capacity through the pretreatment zone 200 are not major concerns. However, the temperature at the pretreatment zone 200 outlet is very important. This temperature is controlled to a target such that the majority of the excess steam in the exiting process vapor stream 25 is condensed and yet the off gases are not appreciably dissolved in the exiting liquid effluent stream 50. This stream 50 is to be cleaned up in the cleanup zone 100 below. The pretreatment zone 200 essentially prepares the process vapor stream 15 for zone G processing. Since the process vapor 25 is a substantially reduced feed vapor stream 15 after passing through the pretreatment zone 200, flooding through zone G can be positively prevented. Through zone G, the H2S vapor passes through while the steam content is further condensed and recovered as described above. Quench effluent streams 50, 170 from both the pretreatment zone 200 and zone G, 500 respectively, are processed by the cleanup zone 100. The cleanup zone 100 functions as a stripping zone. The vapor stream 5 and liquid streams 50, 172 are countercurrent to one another as shown. The vapor stream 5 contains low concentration of acid gases because steam is present in excess. Therefore, this vapor stream can be used to process the circulating quench water 20 as long as adequate stages are provided. Excess condensate 10, which is quite clean, overflows downward. This setup eliminates process condensate reprocessing as suggested by prior art. The circulating pump 700 takes the process condensate 20 from cleanup zone 100 and cools it through air finned cooler 800. Part of the cooled process condensate 30 is diverted as pretreatment quench flow 40 through control valve 101 and enters pretreatment zone 200. The balance of the cooled process condensate 30 passes through control valve 102, further cooled by cooling water heat exchanger 900, and enters zone G 500 as quench stream 150. Liquid stream 170 from zone G 500 can be partly removed as a slipstream 171 for further processing or fed to cleanup zone 100 as liquid feed stream 172. The above arrangement allows full operation flexibility because the pretreatment zone 200 is provided to protect zone G 500 from flooding. The heat balance control loop varies quench water circulation rate by adjusting control valve 101 to maintain exit temperature of vapor stream 25. As the off gas load changes, the quench flow 140 cooling duty in zone G 500 will adjust to maintain the heat and mass transfer zones boundary's temperature target. The secondary cooler 900 can be deleted if the main cooler 800 maintain adequate temperature approach. The combination is for optimizing the dry and wet bulb temperatures and cooling water usage. Suitable material of construction for zone G can be fiberglass, high temperature polymer, ceramic packing, other non-corrosive materials or even stainless steel. The rest of the zones are not under severe operating conditions. Those skilled in the art would have no trouble specifying the proper material of construction. Deaerator Steam Vent Application Dissolved gases, like oxygen, in make up water needs to be removed by a process commonly known as deaeration before the makeup water is suitable for use as boiler feed. Typically, the steam vent on a deaerator is around half to one percent of the steam generation load. By applying the same concept of this invention, a deaerator steam vent apphcation is shown in Fig. 4. The same numbering system as used in the General Example is repeated here to reflect the application of the exact same concept. In Fig. 4, two zones can now be readily identified. The first zone is the mass transfer zone 300. It allows countercurrent contact of steam vent 25 to strip out dissolved gases in the incoming makeup water 160. This mass transfer zone 300 operates like a stripper. The makeup water 160 is partly deareated and exits this stripping zone 300 as preheated feed flow 170 to deaerator. The second zone above 400 allows countercurrent contact of cold makeup water 150 running down to scrub out part of the remaining steam content in the steam vent 35 moving up. The non-condensibles are eventually being vented 45 while the incoming cold makeup water 150 is brought to thermal equilibrium with the steam vent 35 at the boundary between the two zones. As can be seen, this application is identical to zone G above, either as a stand-alone General Example or as part of the wastewater treatment stripping section application. However, the straight adaptation of zone G method is favored here because of the following specific process criteria: 1) The vent stream 25 flow rate is relatively constant and seldom needs adjusting. 2) The quench flow 150 is only a fraction of the total makeup water flow to the deaerator. Those who are skilled in the art of column hydraulics would have no problem designing such an economizer, after determining the specific process requirements. Suitable elevation can be provided, including proper liquid seal, to allow the economizer effluent to enter a pressurized deaerator receiver tank in stead of an atmospheric deaerator receiver tank. Suitable material of construction like fiberglass, high temperature polymer, stainless steel or other non-corrosive materials can be used. The key to successful application is to be able to adjust the heat balance so that zone boundary temperature can be conttolled. For example, quench flow to the economizer can be controlled such that the temperature at midway measures 75 deg C. It is important to note that this location is not necessarily the boundary location. However, once the temperature is fixed, the zone boundary is more or less determined as well. This way, the vent stream 45 exits to atmosphere with only the equilibrium amounts of saturated water vapor at close to the makeup water temperature. The economizer also strips the makeup water of dissolved gases resulting in reduced deaeration load. This application of staged heat and mass transfer zone can be added to any existing deaerator vent stream without imposing any restriction on the original venting requirement. It simply makes the deaerator operate so much more efficiently. The economizer unit can be sized and elevated sufficiently to allow feed by gravity. Heat and condensate recovery will be the ongoing savings. As a variation, the control method employed in this invention can be enhanced by taking advantage of making the zone boundary movable as opposed to a stationary boundary as indicated above. The alternate control method is to preset the quench water flow but cycles the flow in an on/off fashion with a timer. This way, the economizer is being purged out periodically with intermittent quench flow. Additional Ramifications The concept of this invention can be extended to retrofitting existing process units as given by the deaerator vent example above. Any quenching process that experiences difficulty in hydraulics or unstable operation can be analyzed by this method of segregating the operating zones into staged heat / mass transfer zones. Boundaries of these zones can be moved to facilitate the finding of a process solution to the problem. Examples can be found in special heat exchange services that can potentially be converted from indirect to direct contact heat exchange services, after taking severity and load into consideration. In the example of acid gas stripping and scrubbing, this invention can be combined with other prior art to incorporate chemical reaction within the zones. Heat and mass transfer zones can then be tailored for pH adjustments, chemical reactions and product removal such as ammonium sulfate. Conclusion, Ramifications, and Scope of Invention Thus, the reader will see that this invention is truly one practical solution of analyzing process problems in a different perspective. In essence, the present invention provides a practical and thermodynamically sound alternative in the field of separation processes. While the above description contain many specificities, these should not be construed as limitations on the scope of the invention, but rather as exemplification on preferred embodiments thereof. Many other variations are possible. Examples can be found in petrochemical processes like the quench tower in cracking furnace applications, pulp and paper industries, cryogenic separations, pharmaceutical manufacturing processes and food and beverage applications and the list goes on.

Claims: Claims What is claimed as the invention is: 1) An apparatus comprising of at least one heat transfer zone and at least one mass transfer zone operating in series of one another, means of introducing at least one liquid stream to said apparatus for quenching, means of introducing at least one vapor stream to the apparatus for stripping, said liquid stream for quenching operates countercurrently with said vapor stream for stripping, means of manipulating heat balance between said zones and control of the apparatus thereof, to effect migration of condensable components from the vapor stream to the liquid stream before discharge, and migration of soluble components from the liquid stream to the vapor stream before discharge, and means of removing said effluent liquid and vapor streams, 2) The apparatus according to claim 1 where means of manipulating heat balance between the zones is by adjusting the flow rate of the liquid quench stream to target a preset temperature set point at a suitable location along the length of the apparatus, 3) The apparatus according to claim 2 operates as a deaerator steam vent economizer, 4) The apparatus according to claim 1 where said liquid stream to the apparatus for quenching is a closed loop recycled flow after cleanup and cooling, 5) The apparatus according to claim 4 operates as a crude sour gas internal direct contact condenser, 6) The apparatus according to claim 1 further comprises of a pretreatment zone, means of introducing at least one liquid stream to said pretreatment zone for quenching, means of removing said spent quench stream after use, means of removing soluble components from the spent quench stream, cooling the spent quench stream and circulating the spent quench for reuse, means of controlling heat balance of the pretreatment zone, to effect volume reduction of the vapor stream to the apparatus for stripping in claim 1, 7) The apparatus according to claim 6 where means of introducing said liquid stream for quenching is a closed loop recycled flow after cleanup and cooling, 8) The apparatus according to claim 6 where the quench liquid and stripping vapor operates cocurrently to one another, 9) The apparatus according to claim 6 where the means of removing said soluble components from the spent quench stream is by overflowing downward to another mass transfer section below the pretreatment zone, known as a cleanup zone, where the soluble components are stripped by process steam and the resulting liquid effluent from said cleanup zone is circulated and cooled for reuse, and means of removing said spent quench stream after use is by overflowing the cleanup zone excess inventory downward as process condensate to be treated by wastewater stripper below, 10) The apparatus according to claim 6 where means of controlling heat balance of the pretreatment zone is by adjusting the quench stream flow rate to maintain a suitable effluent temperature, 11) The apparatus according to claim 1 where means of introducing said liquid stream for quenching is intermittent, 12) The apparatus according to claim 1 where said heat transfer zone and mass transfer zone contain corrosive resistant packing internals, 13) The apparatus according to claim 6 where the pretreatment zone top elevation matches the top elevation of the heat transfer zone of claim 1 , and the bottom elevation of the pretreatment zone matches the bottom elevation of mass transfer zone of claim 1.

Inventors: TUNG PETER, TUNG, PETER
Applicants: TUNG ELIZABETH, TUNG PETER, TUNG, ELIZABETH, TUNG, PETER
Application year: 2001
Main classification: B01D3
IPC codes: B01D3, B01D19, C02F1, F28C3