Patent Application
20120145368
This invention generally relates to a process for transferring heat or for modifying a tube in a heat exchanger.
Vertically-oriented heat exchangers can be used in many processes, including hydrocarbon processes. Often, a vertically-oriented exchanger may be used to preheat a mixed phase of a liquid hydrocarbon feed and a gas rich in hydrogen. Typically, a vertically-oriented exchanger is used as a combined feed and effluent (hereinafter may be abbreviated “CFE”) exchanger where a mixed phase of a hydrocarbon liquid and a gas are preheated with the effluent from a reactor. Increasing the performance of the CFE exchanger may have an important impact on the energy usage of the process unit. Particularly, additional heat recovered from the CFE exchanger can reduce the energy required for a charge heater and the reactor products condenser. Moreover, the tube side performance of the CFE exchanger may often limit the size and overall performance of the exchanger, particularly for catalytic reforming units.
Often, a liquid hydrocarbon feed and a gas, often a recycle gas including hydrogen, are mixed and introduced on the tube side. Generally, the mixture requires good lift to pass upwards through the vertically-oriented heat exchanger. However, achieving proper lift in the tubes can be difficult due to low inlet temperatures and low recycle gas flow. As a result, the number of tubes may be limited for use, thereby limiting the size and performance of CFE exchanger. Generally, poor liquid lift is typically due to low velocities at the tube inlet resulting in poor liquid-vapor distribution in the tubes, poor heat transfer, and increased tube side fouling. As a result, the liquid lift constraints can impact the overall performance of the CFE exchanger because tube lengths are often limited to no more than about 24 meters due to fabrication shop and tube availability limitations. What is more, the tube side heat transfer coefficient can often be the primary factor in the heat transfer performance of the CFE exchanger. These heat transfer deficiencies of the CFE exchanger can restrict charge through the unit.
As a consequence, there is a desire to improve the heat transfer characteristics of new or existing vertically-oriented heat exchangers utilized in hydrocarbon processing.
One exemplary embodiment can be a process for transferring heat to a first stream from a second stream in a hydrocarbon process. The process can include passing the first stream through at least one generally vertically-orientated tube in an exchanger. An interior surface of the at least one generally vertically-orientated tube may form one or more curved irregularities where the first stream, prior to entering the at least one generally vertically-orientated tube, may include a mixture of a gas including hydrogen and at least one or more C1-C3 hydrocarbons, and a liquid including one or more C4-C13 hydrocarbons.
Another exemplary embodiment may be a process for modifying a tube for a generally vertically-orientated exchanger in a hydrocarbon unit. The process can include introducing an insert into the tube where the insert may form one or more curved irregularities for modifying an interior of the tube.
A further exemplary embodiment can be a process for transferring heat from an effluent to a first stream in a reforming process. The process can include passing the first stream through at least one generally vertically-orientated tube in an exchanger. Generally, an interior surface of the tube can form one or more curved irregularities and the first stream, prior to entering the at least one generally vertically-orientated tube, may include a mixture of a gas including at least about 60%, by volume, hydrogen and a liquid including one or more C4-C12 hydrocarbons.
The embodiments disclosed herein can provide a tube for a vertically-oriented heat exchanger that has one or more curved irregularities within the tube. Particularly, the tube can form helical grooves and/or ridges that increase the heat transfer from a fluid inside the tube to a fluid in a shell of an exchanger by improving the liquid lift and the liquid-vapor distribution of the tubes. Moreover, the tube can also form or contain external fins to increase heat transfer. Additionally, an existing tube can be retrofitted to receive an insert having one or more curved irregularities formed therein. Thus, the liquid lift and liquid-vapor distribution of the tubes may be improved, and the heat transfer of an existing heat exchanger can be increased.
As used herein, the term “stream” can include various hydrocarbon molecules, such as straight-chain, branched, or cyclic alkanes, alkenes, alkadienes, and alkynes, and optionally other substances, such as gases, e.g., hydrogen, or impurities, such as heavy metals, and sulfur and nitrogen compounds. The stream can also include aromatic and non-aromatic hydrocarbons. Moreover, the hydrocarbon molecules may be abbreviated C1, C2, C3 . . . Cn where “n” represents the number of carbon atoms in the one or more hydrocarbon molecules.
As used herein, the term “substantially” can mean at least generally about 90%, preferably about 99%.
As used herein, the term “rich” can mean an amount of at least generally about 50%, and preferably about 70%, by mole, of a compound or class of compounds in a stream.
As used herein, the term “vapor” can mean a gas or a dispersion that may include or consist of one or more hydrocarbons.
Referring to
Referring to
Typically, the tube 140 can have an interior 164 and an exterior 168. Generally, one or more fins 172 can be formed on the exterior 168 while one or more curved irregularities 180 can be formed on the interior 164. Generally, the curved irregularities can be formed by any suitable process, such as grinding, rolling, or extruding. As a result, one or more grooves 182 may be formed between one or more ridges 184 forming a helical pattern, although any suitable pattern may be formed. Although the one or more curved irregularities 180 can be one or more grooves 182 or one or more ridges 184, preferably a combination of such structures are formed. Procedures for making grooves and/or ridges inside a tube are disclosed in, e.g., U.S. Pat. No. 2,181,927, U.S. Pat. No. 3,559,437, U.S. Pat. No. 3,847,212, and US 2005/0145377 A1. Thus an exchanger can contain any number of tubes 140 to facilitate heat transfer.
The length of the one or more curved irregularities 180 can extend about 5-about 40% of the total tube length, with about 10-about 30% being preferred to minimize additional pressure drop while providing desired liquid-vapor distribution, improved vertical flow regime, and improved heat transfer in a two-phase region. The one or more curved irregularities 180 can be formed near the inlet of a feed stream having a mixed phase, or encompass the entire length of the tube. However, often the one or more curved irregularities 180 only extend a portion of the tube 140 because inserts would be retrofitted into the tubes of an existing exchanger. The one or more curved irregularities 180 may only extend a portion of the length of the tube to minimize unnecessary pressure drop.
Referring to
As discussed, the exemplary tubes utilized in an exchanger can be utilized in any desirable service for processing hydrocarbons. Particularly, the hydrocarbon processes can include reforming naphtha, isomerizing xylene, converting aromatics, and dehydrogenating paraffins. Such processes are discussed in, e.g., Dachos et al., UOP Platforming Process, Chapter 4.1, Handbook of Petroleum Refining Processes, editor Robert A. Meyers, 2nd edition, pp. 4.1-4.26 (1997), and Silady, UOP Isomer Process, Chapter 2.5, Negiz et al., UOP Tatoray Process, Chapter 2.7, and Pujadó, UOP Pacol Dehydrogenation Process, Chapter 5.2, Handbook of Petroleum Refining Processes, editor, Robert A. Myers, 3rd edition, pp. 2.39-2.46, 2.55-2.63, and 5.11-5.19 (2004).
Usually, the one or more liquid hydrocarbons provided to the exchanger are combined with a gas that may include make-up and/or recycle gas. Any suitable hydrocarbons, such as hydrotreated naphtha, one or more xylenes, toluene and benzene, and/or paraffins, may be provided to the exchanger. Generally, these hydrocarbons can include one or more C4-C13 hydrocarbons. Any suitable gas, including one or more C1-C6, preferably C1-C3, hydrocarbons as well as hydrogen, may be combined with the liquid hydrocarbons to form a mixed-phased feed of one or more liquids and gases. Hydrogen comprised in the feed can be generally at least about 30%, preferably at least about 40%, and optimally at least about 60%, by mole, based on the total moles of liquids and gases in the feed. After mixing the liquids and gases prior to entering the tubes, the feed may pass upward therein. On the shell side of the exchanger, any suitable reactor effluent can be utilized including one or more C1-C13 hydrocarbons and hydrogen. Often, the reactor effluent can include one or more paraffins, xylenes, toluene, benzene, and olefins. Generally, the maximum pressure drop from an inlet to an outlet of a tube may be about 41-about 83 kPa and the feed side pressure drop may preferably be about 27-about 56 kPa. Typical parameters for several exemplary processes are depicted in Table 1 below:
Utilizing the one or more curved irregularities can improve the flow characteristics at the inlet on the tube side of the exchanger. Thus, the heat transfer coefficient can be improved along at least a part of the length of the tube. Generally, the one or more curved irregularities on the inside surface of the tubes can induce swirling to avoid a plug-flow regime, improve liquid-vapor distribution, improve lift, and thus enhance heat transfer. In addition, the one or more tubes may include one or more fins to improve heat transfer on the outside of the tubes.
Generally, the embodiments disclosed herein allow for the use of additional tubes with corresponding lower velocities in the heat exchanger compared to designs without one or more irregularities while maintaining acceptable lift characteristics for the liquid portion of the fluid traveling upwards in the tube. The tubes can be used in combination with tubes not forming one or more curved irregularities on their inside surface. So a combination of grooved and ungrooved tubes may be used.
In addition, there can be a synergy between modifications to the tube and the shell for increasing the heat transfer characteristics of the exchanger because the shell-side-improvements would no longer be limited by the heat transfer deficiencies of the tubes. The exemplary shells with baffles disclosed above, as well as others, may be utilized.
Thus, the improved heat transfer can improve unit operations. By improving the two-phase vertical flow regime, the lift of the liquid portion of the fluid can be improved and thus can lower flow requirements of the recycle gas. Moreover, such improvements can allow an increased charge of feeds through the unit.
Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The preceding preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever.
From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention and, without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions.
