BRIEF DESCRIPTION OF THE DRAWINGS
While the specification concludes with claims distinctly pointing out the subject matter that Applicants regard as their invention, it is believed that the invention will be better understood when taken in connection with the accompanying drawings in which:
FIG. 1 is a schematic diagram of an air separation plant employing a plate-fin heat exchanger in accordance with the present invention;
FIG. 2 is a perspective view of a plate-fin heat exchanger employed in FIG. 1;
FIG. 3 is schematic, plan view of a layer of the plate-fin heat exchanger illustrated in FIG. 2 that is used for the passage of an incoming compressed and purified air stream to be rectified in the air separation plant shown in FIG. 1;
FIG. 4 is a schematic, plan view of a second layer that is used for providing a flow passage for a nitrogen-rich stream that is produced in the air separation plant illustrated in FIG. 1;
FIG. 5 is a schematic, plan view of a third layer utilized in the heat exchanger shown in FIG. 2 for providing a flow passage for a refrigerant stream; and
FIG. 6 is a schematic, plan view of a fourth layer employed in the heat exchanger illustrated in FIG. 2 that is utilized to provide a flow passage for a waste stream to be expanded in the air separation plant shown in FIG. 1 to refrigerate the plant.
DETAILED DESCRIPTION
With reference to FIG. 1 an air separation plant 1 is illustrated that is used to generate nitrogen. Such an air separation plant is known as a nitrogen generator.
A feed air stream 10 is compressed at a compression unit 12 that may be a multistage compressor having inter-stage cooling between stages. The compressed and purified air stream is then introduced into a purification unit 14 that is well known in the art. Prepurification unit 14 that can be a temperature swing adsorption unit having beds of alumina or molecular sieve type adsorbent operating out of phase to remove the lower boiling components of the air such as water and carbon dioxide. The resultant compressed and purified stream 16 is cooled to at or near its dew point in main heat exchanger 18 and introduced as a compressed, purified and cooled stream 20 into a distillation column 22.
The introduction of compressed, purified and cooled air stream 20 into distillation column 22 initiates the formation of an ascending vapor phase that becomes evermore rich in nitrogen as it ascends distillation column 22 to produce an oxygen-rich liquid column bottoms 24 and a nitrogen-rich column overhead 26. A first nitrogen-rich vapor stream 26 is condensed within a condenser 28 to return a liquid reflux stream 28 to distillation column 22. The return of liquid reflux stream 29 initiates the formation of a descending liquid phase 29 that becomes evermore rich in oxygen as it descends column 22.
The ascending vapor phase and the descending liquid phase are contacted by mass transfer contact elements 30 and 32 that can be a known structured packing, a random packing or known sieve trays.
An oxygen-rich column bottoms stream 34 is expanded to a lower temperature within an expansion valve 36 and then introduced into a shell 38 of condenser 28 for partial vaporization thereof against the liquefaction of the first nitrogen-rich vapor stream 26. The partially vaporized oxygen-rich liquid column bottoms produces a waste stream 40 that is partially warmed within main heat exchanger 18 and then introduced as a partly warmed waste stream 41 into a turboexpander 42 to produce a refrigerant stream 44 that is fully warmed within main heat exchanger 18 and discharged as a fully warmed waste stream 46. This action adds refrigeration to air separation plant 1 to maintain it at cryogenic temperatures. Part of the work of expansion can be employed in powering compression unit 12. A second nitrogen-rich vapor stream 48 is fully warmed within main heat exchanger 18 to produce a product nitrogen stream 50.
Thus, the incoming compressed and purified air stream 16 is fully cooled through indirect heat exchange with waste stream 40, the refrigeration stream 44 and the second nitrogen-rich vapor stream 48.
With reference to FIG. 2, a plate-fin heat exchanger 18 is illustrated. Plate-fin heat exchanger 18 has layers stacked one on the other that are illustrated in FIGS. 3, 4, 5 and 6. Each of the layers is formed between two parting sheets and side bars and end bars separating the parting sheets. As illustrated, plate-fin heat exchanger 18 has a length “L”, a width “W” and a height “H”. The layers are stacked to form the height “H” of heat exchanger 18 and the outer periphery formed by the side and end bars defines the length “L” and the width “W”. As illustrated the length “L” is longer than the width “W”.
With additional reference to FIG. 3, compressed and purified air stream 16 is introduced into a first layer 60 of plate-fin heat exchanger 18 through an inlet provided by a central header 62 situated at a central location of plate-fin heat exchanger 18 between the ends thereof that is in flow communication with an opening 63 provided in a side bar 64 of first layer 60. First layer 60 is partitioned by a partition formed by a bar 65 which causes the compressed and purified air stream to be divided into subsidiary compressed and purified air streams 66 and 68 that can thus be seen in FIG. 1. The two streams are distributed by distributor fins 70 and 72 into two opposed sections 74 and 76 of first layer 60 having fins 78 to increase effective heat transfer area to which indirect heat exchange can take place and also to provide internal support within the plate-fin heat exchanger. The subsidiary compressed and purified air streams 66 and 68 flow outwardly from the central location provided by the inlet to layer 60. Flow is then redirected by provision of distribution fins 80 and 82 to an outlet provided by opposed outlet headers 84 and 85 in flow communication with openings 86 and 87 defined within end bars 88 and 89, respectively. Two compressed, purified and cooled air streams 88 and 90 are thereby discharged from headers 84 and 85 that combine to form compressed, purified and cooled air stream 20.
With additional reference to FIG. 4, a second layer 92 is illustrated that is designed to warm second nitrogen-rich stream 48 and thereby to produce the product stream 50. In order to accomplish this, nitrogen-rich stream 48 is divided into subsidiary nitrogen-rich streams 48a and 48b. Subsidiary nitrogen-rich streams 48a and 48b enter opposed inlets to second layer 92 located at the ends of plate-fin heat exchanger 18 that include headers 94 and 95 in flow communication with openings 96 and 97 provided within end bars 98 and 99, respectively. The two streams then flow into distribution fins 100 and 101 and are redirected to flow inwardly, along fins 102 and 106 provided within opposed sections 106 and 108 of second layer 92 separated by a gap 109. It is to be noted that the term “gap” as used herein and in the claims means a region that is not used to effect heat transfer. Such term does not necessarily mean a void. In fact, although not illustrated, any gap mentioned herein can be filled with fins and sealed along the sides by bars for structural support. The flow is then redirected by provision of distribution fins 110 and 112 to outlets provided by openings 113 and 114 defined within a side bar 115 of second layer 92 and headers 116 and 117 in flow communication with openings 113 and 114, respectively. Subsidiary product streams 50a and 50b are discharged from headers 116 and 117 and are combined to form product stream 50. As can best be seen in FIG. 2, headers 116 and 117 are located adjacent to the central header 62 on the same side of plate-fin heat exchanger 18.
With additional reference to FIG. 5, a third layer 118 is provided for flow of the refrigerant stream 44. Refrigerant stream 44 is split into subsidiary streams 44a and 44b that are introduced into opposed inlets formed at the end of third layer 118. Opposed inlets are formed by headers 120 and 121 in flow communication with openings 122 and 123 defined within end bars 124 and 125, respectively. Subsidiary streams 44a and 44b are then redirected by distribution fins 126 and 127 that produce flow of such streams in an inward direction towards the aforesaid central location within two opposed sections 128 and 130 separated by a gap 131. The two opposed sections 128 and 130 are provided with fins 132 and 134 to provide an enhanced heat transfer area and structural support. The two flows are then redirected by distribution fins 136 and 138 to outlets provided by openings 139 and 140 defined within side bar 141 and headers 142 and 144 in flow communication therewith. Subsidiary fully warmed waste streams 46a and 46b are discharged from headers 142 and 144; and as can best be seen in FIG. 1, are combined to form fully warmed waste stream 46. With brief reference to FIG. 2, headers 144 and 142 are situated at locations adjacent the central location and on a side of plate-fin heat exchanger 18 opposite to the side at which central header 62 and adjacent headers 116 and 117 are attached.
With reference to FIG. 6, a fourth layer 142 is illustrated having two opposed sections 144 and 146. The waste stream 40 is divided into subsidiary waste steams 40a and 40b (see FIG. 1) that are in turn introduced into inlets formed by headers 150 and 151 and openings 152 and 154 defined within side bar 155. The flow is then inwardly directed within sections 114 and 146 by distribution fins 156 and 157. The flow of the subsidiary waste streams 40a and 40b partly warmed within the fourth layer 142 as they flow inwardly along fins 158. After having been partly warmed, waste streams 41a and 41b flow into distribution fins 160 and 162 to be redirected to outlets formed by openings 164 and 166 defined within side bar 167 and in flow communication with headers 168 and 170. As can best be seen in FIG. 2, headers 150 and 151 are located adjacent the ends of plate-fin heat exchanger 18 and on the same sides as headers 142 and 144. Headers 168 and 170 are located on the opposite side of plate-fin heat exchanger 18 in an outlying relationship to the central location and central header 62 and adjacent headers 116 and 117.
With reference again to FIG. 1, subsidiary partly warmed waste streams 41a and 41b, discharged from headers 168 and 170, are combined into partly warmed waste stream 41 and then introduced into turboexpanders 42.
In practice, the aforementioned layers will be patterned in the stack used in forming plate-fin heat exchanger 18 in a manner known by those skilled in the art. For example, for a nitrogen generator 1 illustrated in FIG. 1 the layers 60, 92, 118 and 142 will be included in approximately 120 layers having a repeating pattern that is designed in a manner well known in the art. For example, the pattern could be (given by the reference numbers of the layers alone) of 142, 60, 142, 60, 118, 60, 92, 60, 142, 60, 118, 60, 142, 60, 92, 60, 118, 60, 142, 60, 142. The important features of any pattern are that it is symmetrical (to prevent gross thermal gradients across the stack), and that the heat loads (energy transfer/unit time) for each individual layer are comparable (to prevent local excess or lack of energy available for transfer from hot to cold). Naturally there are many layer patterns which could meet these criteria, in addition to the layer pattern used in the preceding example.
As can be appreciated, since all the flows are split so that they flow in two sections of each layer, the cross-sectional flow area per layer and therefore the entire cross-sectional flow area for heat exchanger 18 has been increased. As a result, there is a potentially lower pressure drop due to the increase cross-sectional flow area. The lower pressure drop allows the heat plate-fin heat exchanger 18 to be constructed in a conventional manner in a conventional size to produce the lower pressure drop or with a greater fin density to produce a greater heat transfer capacity. Additionally, plate-fin heat exchanger 18 might be made more compact with higher density fins to achieve a particular heat transfer duty that would normally require a longer heat exchanger.
Although the two opposed sections making up each layer are illustrated as being symmetrical given the central location being equidistant between the ends of the plate-fin heat exchanger 18, the central location could be off dead center. In such case, the unequal pressure drops produced in the opposed sections of each layer that would not be symmetrical could be compensated by varying the fin density. Hence, the term, “central location” as used herein and in the claims means any location between the ends of the plate-fin heat exchanger that can be at the geometric center thereof.
Additionally, although there exists counter current flow within heat exchanger 18 between the fluid to be cooled, namely compressed and purified air stream 16 and the streams to be warmed, for example, waste stream 44, second nitrogen-rich stream 48 and waste stream 40, this should not be seen as a limitation. For example, a plate-fin heat exchanger could be constructed in accordance with the present invention in which all flows were co-current or in the same direction. It is to be further pointed out that although a preferred embodiment has four different layers, it is possible to construct a heat exchanger in accordance with the present invention with only two layers, for example, layer 60 and layer 92 or more than four layers should there be more flows to be subjected to heat exchange.
While the invention has been described with reference to a preferred embodiment, as will occur to those skilled in the art, numerous, changes, additions and omissions can be made without departing from the spirit and the scope of the present invention that is recited in the appended claims.
Patent Application
20070295027
