Heat exchange – Flow passages for two confined fluids – Interdigitated plural first and plural second fluid passages
Reexamination Certificate
2000-06-05
2002-01-15
Leo, Leonard (Department: 3743)
Heat exchange
Flow passages for two confined fluids
Interdigitated plural first and plural second fluid passages
C062S903000
Reexamination Certificate
active
06338384
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a downflow reboiler-condenser. More specifically the present invention relates to a downflow reboiler-condenser having liquid distributing means for distributing and introducing a fluid to be evaporated (a vaporizing fluid) uniformly to evaporation passages of a plate fin type heat exchanger core having condensation passages and the evaporation passages juxtaposed alternately via parting sheets, and particularly to a plate fin type downflow reboiler-condenser suitably used in a double distillation column of an air separation plant.
BACKGROUND ART
According to air separation using a double distillation column, liquid oxygen present at the bottom of a low-pressure distillation column (hereinafter referred to as low pressure column) or in a vessel communicating with the low pressure column is subjected to indirect heat exchange with an overhead nitrogen gas of a high pressure distillation column (hereinafter referred to as high pressure column) in a heat exchanger located at a middle part of the double distillation column to effect vaporization of a part of the liquid oxygen to form an ascending agas in the low pressure column and also condensation of the nitrogen gas into a liquid to form a reflux in these two distillation columns. Such heat exchanger is generally referred to as a reboiler-condenser.
As the reboiler-condenser, those using plate fin type heat exchanger cores are generally used. The plate fin type heat exchanger core has a multiplicity of heat exchange passages composed essentially of condensation passages and evaporation passages arranged adjacent to one another via parting sheets, and a fluid to be condensed or condensing fluid (i.e., nitrogen gas) which is introduced in the form of gas and a fluid to be evaporated or evaporating fluid (i.e., liquid oxygen) which is introduced in the form of liquid are subjected to indirect heat exchange with each other to effect condensation of the former fluid into a liquid which is withdrawn to a lower part of the heat exchanger and also to effect vaporization or gasification of a part of the latter fluid into a gas which is withdrawn to a lower part or to a lower part and an upper part of the heat exchanger.
FIG. 1
shows a reboiler-condenser using a submerged plate fin type heat exchanger core (i.e. a submerged reboiler-condenser) utilizing the thermosyphon effect. This reboiler-condenser
1
is used as submerged in an evaporating fluid (liquid oxygen LO) collecting in a reservoir
2
a
located at the bottom of a low pressure column
2
. In the reboiler-condenser
1
, the inlet ends and outlet ends (the upper ends and the lower ends) of heat exchange passages (evaporation passages) for the evaporating fluid (liquid oxygen LO) are open, and an overhead nitrogen gas GN in a high pressure column
3
is introduced via an upper header
1
a
into the condensation passages. The liquid nitrogen formed by the condensation in the condensation passage is withdrawn from a lower header
1
b.
The liquid oxygen in the evaporation passages is subjected to indirect heat exchange with the condensing fluid (nitrogen gas GN) in the adjacent condensation passages to be vaporized partly to form oxygen bubbles which ascend along the evaporation passages. The ascending force of this oxygen gas and the difference in the density of the vapor and that of the liquid in the vapor-liquid mixture bring about the thermosyphon effect and form a circulatory flow in the liquid oxygen LO inside and outside the reboiler-condenser
1
. Of the oxygen assuming the form of vapor-liquid mixture withdrawn as an ascending stream, the liquid oxygen which did not vaporize returns to the reservoir
2
a
, whereas the oxygen gas forms an ascending gas in the low pressure column
2
, and a part of the gas is withdrawn as a product through a line
4
.
Meanwhile, the nitrogen gas GN introduced into the condensation passages is condensed into liquid nitrogen by the indirect heat exchange with the liquid oxygen and is withdrawn from the bottom of the reboiler-condenser
1
. While the thus withdrawn liquid nitrogen is introduced as a reflux to the above two columns, it is occasionally withdrawn partly as a liquid product.
The submerged reboiler-condenser
1
utilizing the thermosyphon effect, as described above, is a counterflow type heat exchanger where the condensing fluid and the evaporating fluid form a downward flow and an upward flow respectively. Since the reboiler-condenser
1
, as used, is submerged entirely in liquid oxygen, the liquid head of the liquid oxygen isubcools the liquid oxygen flowing from the bottom of the reboiler-condenser
1
to the evaporation passages.
Accordingly, some distance is necessary for the liquid oxygen until it starts boiling or until the temperature of the liquid oxygen is heated by the indirect heat exchange with the condensing nitrogen gas to reach the saturated temperature. This distance occasionally amounts to 20 to 30% of the height of the heat exchanger. That is, the submerged reboiler-condenser
1
has not enough heat transfer surface area to use the heat transfer surface area over the entire height of the heat exchanger.
Further, the liquid head of the liquid oxygen as an evaporating fluid causes a rise in the boiling point of the liquid oxygen as the evaporating fluid, and the temperature difference &Dgr;T between oxygen and nitrogen is reduced (temperature pinch) as shown in
FIG. 2
, to lower the quantity of heat to be exchanged on the designed heat transfer surface area. Therefore, it is now necessary to maintain the temperature difference &Dgr;T at a fixed level in order to maintain the heat load. As a technique for achieving this, the pressure of the condensing nitrogen gas or the operating pressure of the high pressure column is generally increased in such an amount as to cope with the elevation of the boiling point of the liquid oxygen, leading to an increase in power consumption.
In addition, a large amount of liquid oxygen must be stored to allow the reboiler-condenser
1
to function duly, and it takes a long time to start up the system, or a large amount of liquid oxygen is discharged when the reboiler-condenser
1
is stopped, causing waste of power and personnel cost.
In order to eliminate such inconvenience in the submerged reboiler-condenser utilizing the thermosyphon effect as described above, there is proposed a reboiler-condenser utilizing a concurrent heat exchanger, in which an evaporating fluid is vaporized as it flows down from the top of each evaporation passage in the heat exchanger. This type of reboiler-condenser is generally referred to as a downflow reboiler-condenser.
FIG. 3
shows a downflow reboiler-condenser
5
using a plate fin type heat exchanger. A liquid oxygen LO flowing down from a distillation section
2
b
of a low pressure column
2
further flows down from the top of the reboiler-condenser
5
together with the liquid oxygen supplied by a pump
6
from a reservoir
2
a
located at the bottom of the low pressure column and is subjected to indirect heat exchange with a nitrogen gas flowing concurrently in adjacent condensation passages to be vaporized partly. The thus obtained oxygen gas is withdrawn from the bottoms of the evaporation passages into the low pressure column
2
, while the liquid oxygen which did not vaporize is withdrawn from the bottoms of evaporation passages to collect in the reservoir
2
a
located at the bottom of the low pressure column. The thus collected liquid oxygen is returned to the top of the reboiler-condenser
5
for circulation by the pump
6
. Since the nitrogen side is of the same configuration as described above, the same and like elements are affixed with the same reference numbers respectively, and detailed description of them will be omitted.
As described above, since the downflow reboiler-condenser
5
forms no liquid head in the liquid oxygen to be evaporated, the heat exchanger comes to have substantially uniform temperature difference &Dgr;T over the entire height thereof, causing evaporation of th
Hashimoto Hideyuki
Ohya Junichi
Sakaue Seiichi
Armstrong, Westerman Hattori, McLeland, and Naughton, LLP
Leo Leonard
Nippon Sanso Corporation
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