Patent Application
20170241722
Field of the Invention
The specific technical challenge this invention is intended to address is to improve the process of rapidly cooling or quenching thermally cracked hydrocarbon gasses to allow the production of olefins (or like desired products) which are a desired product used in the production of many other chemicals in a more economical manner. Slower cooling of the cracked gasses produces more waste products, and rapid cooling of the cracked gas by injection of water or oil droplets into the cracked gas stream wastes the heat of the gas and makes more difficult (due to dilution) the recovery of the desired product from the cooled gas stream. Currently the shell and tube heat exchangers and other types of heat exchangers discussed in the below referenced patents are used in industry to cool the cracked gas along with injection of water droplets in some older facilities. Where using heat exchangers with a plurality of gas tubes in a bundle, such as shell and tube exchangers, the gas flows from a plenum chamber with hot cracked gases, through the tube-bundle where the gasses are cooled. This creates a plurality of different gas flow paths, and the possibility of substantially different quench times for gas flow in different gas tubes, which would necessarily lead to sub-optimal product production rates.
Such problems have been noted where significant variations in mass flow rate of crack gas is seen from tube to tube leading to substantially different quench times and so net product production rate variation from tube to tube within the tube-bundle. In a standard design system the gas flows in from a single pipe to an expansion trumpet then into the tube-bundle through an inlet tube-sheet. The flow then separates into individual flow through each of the gas tubes of the tube-bundle, the mass flow rates in each are largely dependent on how gas pressure is distributed on the inlet tube-sheet. On the cold side of the tube-bundle, the gas flows are then merged again at the gas tube exits the tube-bundle and flows into another plenum chamber Obviously some minimum time to quench exists for a given mass flow rate and tube-bundle geometry which maximizes the total production of the product. It is also obvious that if the mass flow rate per gas tube can be equalized in the tube-bundle, that this optimal production can be achieved in the whole of the tube-bundle. The inlet flow of gas into the exchanger has substantially greater energy both in a thermal sense, and in a kinetic energy sense than the outlet flow of gas as the gas at the inlet is hotter, and consequently has larger specific volume, and consequently greater velocity, and kinetic energy for a constant cross-sectional area and mass flow rate. Attempts to control the flow of the gas through the tube-bundle at either the inlet or in the body of the tube-bundle will tend to cause greater waste of flow energy that would a design that controls this gas flow at the tube-bundle outlet.
The present invention provides practical means to control relative gas flow in individual tubes of a commercial transfer line exchanger at the exit of the tube bundle. This method is substantially superior to prior art.
Prior Art
Bowser in his 1938 patent (U.S. Pat. No. 2,178,095) teaches using two plates, one each within the inlet and outlet cone of the tube fluid. These plates were made adjustable in distance from the tube-sheet. The intent of this is to diffuse the inertial momentum of the inlet and outlet fluid such that the flow through the tube-bundle will vary less than would be the case otherwise from tube to tube. This patent is generally applicable to all shell and tube exchangers, though Bowser indicates the tube fluid to be a liquid. It is obvious similar results could be had using a gas. Vollhard in his 1964 patent (U.S. Pat. No. 3,144,080) teaches a heat exchanger primarily for the purpose of quench cooling thermally cracked hydrocarbon gasses to allow the production of olefins. The heat exchanger of this patent rather than using a conventional shell and tube design uses a bundle of individual tube in tube heat exchangers in arrangements where the cracked gas passes through the inner tube and the annular portion has water and steam to cool the cracked gas. These annular volumes are manifolded together in patterns shown in that work to use common water inlet tubes and common steam and water outlet tubes. In a later 1967 patent Vollhardt (U.S. Pat. No. 3,348,610) teaches how to make a shell and tube heat exchanger for the same purpose. That being quench cooling of thermally cracked hydrocarbon gasses to allow the production of olefins. To the knowledge of the inventor this latter design has not been very commercially successful while variations of the former have been very successful. However the former design is expensive in that it requires a great many welds and is rather large in volume for the effective amount of heat exchange area created.
Koontz, in his 1983 patent (U.S. Pat. No. 4,397,740) discusses the subject of equalization of flow in the tube-bundle, and the consequent formation of coke by using smaller diameter tubes toward the center of the tube-bundle, and larger ones in the outer part of the tube-bundle. Practical experience which the inventor shares with Koontz, shows that typically the trumpet or gas inlet cone distributing the flow of cracked gas will have higher inertia and so pressure form in the center of the inlet tube-sheet. That will necessarily leads to higher mass flow rates in the central tubes of the tube-bundle. Koontz addresses this issue by having smaller diameter tubes toward the center of the tube-bundle, and larger tubes in the outer part of the tube-bundle. The smaller tubes will have higher pressure drop, the larger lower pressure loss for a given mass flow rate. While this is a possible solution to the root problem, it is an expensive one, and not adjustable. Shen-Tu in a 1988 patent (U.S. Pat. No. 4,785,877) teaches the use of inserts into the individual tube inlets. This design will reduce direct thermal load on the tube-tubesheet welds, and reduce abrasion by coke particles on the same. The patent also asserts that this will reduce deposition of coke on the tube-sheet and tube inlets. It is possible, but not argued in the patent that variation of the tube-insert design by location of the tube in the tube-bundle might mitigate the problem of uneven flow, but this is not addressed by that patent. Brucher. & Lachmann teach a method to solve the problem of use of a shell and tube heat exchanger in this application by suspending a thin tube-sheet from a thick structural tube-sheet either by structural members called slabs in their 1989 U.S. Pat. No. 4,858,684 or webs in their 1991 U.S. Pat. No. 5,035,283 This basic design in practice restricts the shell and tube exchanger designer to a rectangular arrangement of tubes which is less than ideal in efficient use of space and requires the addition of a second thick shell wall shown in FIG. 1 of their 1991 patent and FIG. 1 of their 1989 patent (item 10 in that patent). These restrictions raise the cost of fabrication of the heat exchanger designed for a given heat load over what might be achieved in less demanding service, however the expense and ratio of heat exchanger area per unit volume tends to be superior to that of the tube in tube Vollhard design of the 1964 patent, and this design is commercially successful. All of these designs in general use enhanced natural circulation which takes advantage of the difference in density of the water with no steam coming down from a steam drum in down-comer pipe(s) and a mixture of steam and water rising in the riser(s) from the exchanger to the steam-drum. Other means might be used but this is the most common. The pressure difference induced by natural circulation is small, typically significantly less than one barr. Albano et. al. In their 1995 patent (U.S. Pat. No. 5,464.057) modify the design of Vollhard in that they use a bundle of individual tube in tube heat exchangers, but rather than the typical roughly cylindrical tube-bundle, in the patent drawings they show two rows of tubes, and a heavily modified set of gas inlet trumpets. In a classical design as discussed above, heater tubes come together using “y” joints, to a “transfer line”, and then to the trumpet also called an inlet cone discussed above. In the design of Albano et. al. in their drawings each of the heater tubes are manifolded into four of the transfer line exchanger gas tubes using a refractory four forked “y” type fixture. If sufficient care is taken with the design of the “y” fixture fairly even flow can be expected. In their later patent Albano et. al. In their1998 patent (U.S. Pat. No. 5,816,322) further develop this fundamental idea and come up with a design that allows use of a traditional transfer line, but creates a trumpet that has an internal cone shape that forces the gas flow to match up to the inlet of a circular tube-bundle with one row of tubes at a constant radius. As with the earlier Albano et. al. design, it uses a tube in tube type exchanger rather than a shell and tube exchanger. While this method will tend to create more even flow, this is at the cost of the loss of volume of tubes, and heat transfer capacity that could have been placed in the center of the tube-bundle, and in substantially higher fabrication costs. Knight, et. al. in their 2006 patent (U.S. Pat. No. 6,845,813) teach the use of what they call an “INTRA-BODY FLOW DISTRIBUTOR” which is an hydrodynamic shape placed in the inlet cone/trumpet. It is in the inventor's opinion a much more sophisticated version of the Bowser design discussed above, that will get the desired effect at much lower cost in pressure loss of the product stream than the Bowser design would require. The inventor and Knight (who worked together some 20 years ago) had seen variation of the Bowser design used in this application, with clearly less than optimal results.
The present invention provides an improvement over the current state of the art intended to reduce variation in flow between tubes in a tube-bundle of a transfer line exchanger which is used to rapidly cool or quench thermally cracked hydrocarbon gasses to allow the production of olefins, and other applications in which the heat is recovered and used to generate steam. This invention can be used either with a shell and tube exchanger, or a bundle of tube in tube heat exchangers. All attempts to reduce variation of the gas flow between tubes in a tube bundle are based on some form of interference with the gas flow. The present invention does so by interference with the lower temperature and lower velocity and so inertia flow out of the exits of the tubes. Several different variations of this of this invention are possible, some more complex, methods allow adjustment of the flow during operations. Other very simple methods would require knowledge of the pattern of pressure formed on the inlet tube-sheet, and detailed iterative calculation of the gas flow pattern and application of these calculations to construct flow obstructions on the outlets of individual gas tubes. Perhaps the simplest embodiment would be to make washers, with a specific outside diameter significantly larger than the gas tube inside diameter, and inside diameter to be calculated based on the discussed iterative calculations of the gas flow, but generally would be somewhat smaller than the tube inside diameter. These would be affixed centered on tubes at the outlet tube-sheet. Without this obstruction and impediment to flow, the gas mass flow rate from the tubes with higher pressure at the inlet would have higher than average gas mass flow rate than the other tubes. More important the flow from these tubes would have a higher temperature at the outlet.
Consider the transfer line exchanger tube bundle without the present invention. The objective of the quench cooler is to rapidly reduce the temperature of the crack-gas below the level where adverse chemical reactions are seen, but to also process as high a mass flow rate of gas as practical. In such a Transfer Line Exchanger without the present invention, tubes with moderately higher inlet pressures could exhaust at temperatures high enough to significantly reduce ethylene yield, while the tubes with lower inlet pressures cool the crack-gas stream below the optimal temperature. With the simplified present invention consisting of orifice washers affixed to the tube outlets, and with accurate and proper iterative calculation of the crack gas flow, these mass flow rate differences between tubes of the tube-bundle are minimized which in turn minimizes temperature differences. This in turn can be used to improve product output as the outlet temperature is better controlled.
The preferred embodiment of the present invention rather than using the above described orifice plates, uses a rod for each outlet which will act much as a needle or globe valve on the outlet of each tube. In the preferred embodiment the rod is aligned with the center-line of the tube, the tip of the rod is pointed down into the outlet and most commonly would have a generally conical shape with the point aimed down along the center-line of the tube, the outside diameter of the rod and associated cone are generally large enough to plug the tube outlet. A mechanism is provided to move the rod in a linear manner on the axis of the specific heat exchange tube. As the rod is moved up the flow area at the tube exit rises, as the rod is moved down the flow area at the tube exit drops. On the tip of the cone one or more thermocouples are embedded to measure the gas temperature at the tube exit. The motion of the rod can be controlled by any of several means. The method indicated in the drawing is to thread the rod outside of the transfer line exchanger exhaust chamber and have that rod penetrate into the chamber and have a nut at a fixed location from the exit tube-sheet, threaded onto the rod and either a key slot on the rod or a spline on the slot separate from the nut hold the rod to prevent it turning with the nut. The nut is turned by some other mechanism (any of several possible) to advance or retreat the individual rod.
The temperatures read by the thermocouples at the rod tips allow the operator how to adjust the rod intelligently without having to do a detailed flow analysis. If the temperature at the exit of a given tube is higher than the bulk outlet flow, the flow area from that tube should be reduced. If the temperature at the outlet of that given tube is lower than that of the bulk outlet flow of the transfer line exchanger then the flow area at the exit of that tube should be increased. This aspect of the more sophisticated design allows the operator to adjust the system to changing operational conditions in the system.
Other types of flow obstructions of individual tube outlets of the tube-bundle could be used. One need not obstruct the flow of all the tubes in the tube-bundle to substantially control the inlet flow distribution. To control this distribution one should however, obstruct the flow of the tubes in the tube-bundle that will have a larger than average flows. These flow obstructions or impediment causes a higher pressure on the inside of the tube, which in turn causes higher pressure at the inlet tube-sheet which pushes mass flow away from those tubes to others that have lower pressure drop.
For a proper understanding of the current invention, reference should be made to following detailed description taken in conjunction with the accompanying drawings, in which like parts are given like reference numerals, and wherein:
The current invention is a technique and device applied to a shell and tube, or tube in tube bundle type heat exchanger intended primarily for use to rapidly cool or quench thermally cracked hydrocarbon gasses to allow the production of olefins, it may however be used for other purposes. The invention in the preferred embodiment is shown in detail in
As previously mentioned, without use of the present invention, higher pressure in front of a tube inlet means higher mass flow rate in that tube, and necessarily lower flow rates in other tubes, this leads to sub-optimal quenching of the product, and lower product production rates for a given input stream. The present invention addresses this in
An alternative embodiment is shown in
Because many varying and different embodiments may be made within the scope of the inventive concept herein taught, and because many modifications may be made in the embodiments herein detailed in accordance with the descriptive requirements of the law, it is to be understood that the details herein are to be interpreted as illustrative and in a limiting sense.
