Dephlegmator system and process

Details Dephlegmator system and process Dephlegmator system and process

2000-09-15

2002-02-26

Capossela, Ronald (Department: 3744)

Refrigeration

Cryogenic treatment of gas or gas mixture

Separation of gas mixture

C062S903000, C165S166000

Reexamination Certificate

active

06349566

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATIONS
Not applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not applicable.
BACKGROUND OF THE INVENTION
Dephlegmators are widely used in the process industries for the separation of gas mixtures, particularly those which contain components with sub-ambient boiling points. Such separations require significant amounts of low temperature refrigeration and are thus highly energy intensive. Dephlegmators offer simple, reliable, and efficient operation for such gas separations.
The characteristic feature of dephlegmator operation is the utilization of simultaneous heat and mass transfer in a group of generally vertical flow channels or passageways in indirect heat transfer communication with other flow channels containing heating or cooling fluids. A dephlegmator thus combines both heat transfer and mass transfer in a single operating system. Heat and mass transfer in process streams within dephlegmator channels can occur in either a condensation or vaporization mode.
In the condensation or rectification mode of operation, a feed gas mixture is cooled and partially condensed within a group of flow channels by indirect heat transfer with one or more refrigerants or colder fluids flowing in adjacent channels. The resulting condensed liquid flows downward while exchanging heat and mass with the remaining vapor, which flows upward. A liquid stream enriched in higher boiling components and a vapor stream enriched in lower boiling components are withdrawn from the feed flow channels. Rectification occurs in this operation, and a dephlegmator operating in this mode is often called a rectifying condenser or rectifying dephlegmator. This type of dephlegmator can be used for rejecting nitrogen from natural gas (U.S. Pat. Nos. 4,732,598 and 5,802,871), producing refrigerated liquid methane (U.S. Pat. No. 5,983,665), recovering helium from natural gas (U.S. Pat. Nos. 5,017,204 and 5,329,775), purifying synthesis gas (U.S. Pat. No. 4,525,187), recovering C
4
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hydrocarbons (U.S. Pat. No. 4,519,825), and for recovering olefins from hydrocarbon-hydrogen mixtures such as cracked gases, refinery offgases, and petrochemical plant offgases (U.S. Pat. Nos. 5,361,589, 5,377,490, 5,379,597, and 5,634,354).
In the vaporization or stripping mode of operation, a liquid feed mixture is heated and partially vaporized within a group of flow channels by indirect heat transfer with one or more warmer fluids flowing in adjacent channels. The vaporizing liquid flows downward while exchanging heat and mass with the generated vapor, which flows upward. Stripping action is promoted by the upward flowing vapor. A liquid stream enriched in higher boiling components and a vapor stream enriched in lower boiling components are withdrawn from the feed channels. This type of dephlegmator, often called a stripping dephlegmator, is described in representative U.S. Pat. No. 5,596,883.
Some condensing type dephlegmators utilize an upward-flowing boiling liquid to provide refrigeration in a group of flow channels which remove heat by indirect heat exchange from a condensing stream in adjacent channels. The refrigeration channels are open at the lower end, and usually at the upper end as well, and the dephlegmator may be partly or completely submerged in the boiling liquid. This type of refrigeration circuit is called a thermosiphon heat exchanger and is discussed further below.
A combined mode of operation also is possible in which a vapor is condensed in a first group of flow channels while a liquid is vaporized in a second group of channels, wherein the first and second groups of channels are in heat transfer communication. Heat to vaporize the liquid in the second group of channels is provided by the condensing vapor in the first group of channels, rectification occurs in the first group of channels, and stripping occurs in the second group of channels. This type of dual-mode dephlegmator is used for air separation as described in U.S. Pat. Nos. 5,592,832 and 5,899,093.
Dephlegmators are constructed with multiple flow channels or passageways which are grouped and manifolded to segregate process stream(s) from heating or cooling stream(s) while allowing indirect heat transfer between the streams. More than two groups of channels can be used to process multiple streams in the same dephlegmator. Plate and fin heat exchangers, also known as core-type exchangers, are widely preferred for dephlegmator service. These are typically of brazed aluminum construction, but any appropriate metals can be used. Shell and tube heat exchangers have utility as dephlegmators, but are less favored than the plate and fin configuration.
In the operation of dephlegmators used for the separations described above, the proper distribution of the process feed stream into the multiple flow channels and the withdrawal of vapor and/or liquid product streams from the multiple flow channels are necessary for efficient operation. Of particular importance in a widely-used type of condensing dephlegmator described below is the proper introduction of feed gas into the bottom end of a group of flow channels while withdrawing condensate from the bottom end of the same flow channels.
Several methods have been proposed to introduce feed vapor into and remove condensed liquid from the bottom of a brazed aluminum, core-type dephlegmator. U.S. Pat. Nos. 5,333,683, 3,992,168, 3,983,191 and 3,612,494 disclose the use of two separate headers, one for the vapor to enter the bottom of the dephlegmator core and the other for the liquid to drain from the bottom of the core. These designs require distribution fins, both to distribute the vapor into the core and to collect the liquid draining from the core. These distribution fins, particularly the vapor distribution fins, reduce the fluid-handling capacity of the core below that which could otherwise be attained in the full cross-section of heat/mass transfer flow channels used in the main body of the dephlegmator core.
U.S. Pat. Nos. 5,144,809, 3,568,462 and 3,568,461 show the use of integral dome headers which enclose the entire bottom end of the dephlegmator core and allow vapor to enter the core and liquid to drain from the core without obstruction. However, to have adequate mechanical strength, these dome headers are restricted to relatively low pressure applications or cores of relatively small cross-section.
Other methods have been proposed to separate vapor and liquid exiting a conventional core-type heat exchanger or for the input or output of fluids from core-type heat exchangers.
U.S. Pat. Nos. 5,765,631, 5,321,954 and 4,599,097 show various types of integral domes and other integrated vessels which can be used primarily to separate mixtures of vapor and liquid entering or leaving a conventional core-type heat exchanger in order to individually distribute them into the core or remove them from the core. Some of these devices alternatively could be used for input or output of fluids from dephlegmator cores, but they are also restricted to use in relatively low-pressure applications or with cores of relatively small cross-section.
U.S. Pat. No. 5,385,203 discloses a conventional core-type heat exchanger mounted inside a partitioned vessel such that the several separate chambers formed by the partitions provide a multi-stage thermosiphon-type heat exchanger with different boiling refrigerants in each of the separate chambers. Circulation of the boiling refrigerants is obtained by the submergence of appropriate sections of the core in the refrigerant liquids contained within each of the chambers. The thermosiphon boiling refrigerants in the open circuits of the core serve to cool a process gas stream contained within a closed circuit of the core.
Integral domes and other vessels mounted on a conventional core-type heat exchanger as shown in U.S. Pat. No. 4,330,308 provide a similar multi-stage thermosiphon-type heat exchanger with different boiling refrigerants in each of the separate sections of the core. Circulation of the boiling refrigerants is obtai

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