Refrigeration – Cryogenic treatment of gas or gas mixture – Separation of gas mixture
Utility Patent
1999-05-14
2001-01-02
Doerrler, William (Department: 3744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Separation of gas mixture
C062S654000, C417S251000
Utility Patent
active
06167724
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a pump, more particularly to a cryogenic rotary pump and to a cryogenic air separation apparatus including the pump.
A cryogenic rotary pump conventionally contains one or more pumping chambers. The or each pumping chamber is, in operation, swept by a rotary pumping member. The rotary pumping members are carried on a shaft which is typically driven directly by an electric motor. The number of pumping chambers and/or pump speed depends on the pressure to which it is required to raise a cryogenic liquid by the pump.
Such cryogenic rotary pumps may be used to perform any one of a number of different duties. Cryogenic rotary pumps are, for example, widely used in cryogenic air separation plants. Such plants or apparatus typically include a double rectification column, for separating the air, comprising a higher pressure column, a lower pressure column and a condenser-reboiler placing an upper region of the higher pressure column in heat exchange relationship with a lower region of the lower pressure column. The condenser-reboiler is typically located in or above a sump in which a liquid oxygen fraction separated in the lower pressure column collects. Conventionally, the reboiling section operates as a thermosiphon. Therefore no external electrical pump is required to urge the liquid oxygen through the reboiler. One disadvantage of a thermosiphon is that liquid head effects result in a temperature difference between boiling liquid and condensing vapour greater than would otherwise be necessary, thereby adding to the thermodynamic inefficiency of the operation of the condenser-reboiler in operation. Accordingly, downflow reboilers are now used as an alternative to thermosiphon reboilers. In such downflow reboilers the liquid to be boiled is distributed to a header at the top of the boiling passages and flows down these passages. In the case of liquid oxygen, it is considered unsafe to operate the reboiler with dry areas on the boiling surfaces. Accordingly, only a portion of the liquid oxygen is boiled and there is a need to pump to the distributor an appreciable flow of liquid oxygen. A cryogenic rotary pump can be used for this function.
Another use for a cryogenic rotary pump in a cryogenic air separation plant is to pump a liquid oxygen product to a relatively high pressure, sometimes above the critical pressure of oxygen. The thus pressurised oxygen is warmed so as to provide an elevated pressure product at approximately ambient temperature. One advantage of such an arrangement is the need for an oxygen gas compressor, the operation of which can be hazardous, is avoided.
Modern air separation plants are increasingly designed to produce an elevated pressure gas oxygen product and with downflow reboilers. Typically two separate cryogenic rotary pumps are employed to perform these functions, although when the pressure of the oxygen product is in the order of 10 bar or less, it is known to reduce the pressure of a sidestream of the pumped liquid oxygen and introduce it into the downflow reboiler. Since the recycle flow to the downflow reboiler can exceed the flow rate of oxygen product out of the plant, such a practice is particularly inefficient.
It is an aim of the present invention to provide a rotary cryogenic pump which can perform a plurality of pumping duties relatively efficiently.
SUMMARY OF THE INVENTION
According to a first aspect of the present invention there is provided a cryogenic rotary pump for pressurising a flow of a cryogenic liquid and for dividing the flow into a first lower pressure stream and a second higher pressure stream, including a plurality of pumping chambers in series with one another, a single rotary drive shaft carrying all rotary pumping members, a liquid receiving chamber intermediate a pair of pumping chambers, a first outlet from the pump for the lower pressure stream, the first outlet being contiguous to the liquid receiving chamber, and a second outlet from the pump for the second higher pressure stream downstream of the series of pumping chambers.
According to the second aspect of the present invention there is provided cryogenic air separation apparatus including a double rectification column for separating the air, comprising a higher pressure column, a lower pressure column, and a condenser—reboiler placing an upper region of the higher pressure column in heat exchange relationship with a lower region of the lower pressure rectification column, wherein the reboiler is of a downflow kind having generally vertical boiling passages communicating with a sump, there being an outlet for liquid oxygen from the sump, characterised in that the outlet for liquid oxygen communicates with a cryogenic rotary pump according to the first aspect of the invention and that the first outlet of the cryogenic rotary pump communicates with an inlet of the reboiler for liquid oxygen and the second outlet of the cryogenic rotary pump communicates with heat exchange means for warming the oxygen.
By appropriate selection of the pumping members and the number of pumping chambers, and/or the pump speed, the first stream can be produced at a pressure not significantly above that required to lift the stream to the top of the reboiler, typically a distance in the range of 10 to 20 meters, and the second stream can be produced typically at a pressure in the range of 10 to 60 bar. An advantage of a cryogenic rotary pump according to the first aspect of the invention is that a single drive shaft (which therefore requires only a single electric or other motor to drive it) carries all the rotary pumping members. Duplication of motors and associated electrical switch gear is therefore avoided. In addition, only one pump inlet liquid line equipped with a shut-off valve is required. When the cryogenic rotary pump according to the first aspect of the invention is used in a cryogenic air separation apparatus according to the second aspect of the invention, the ability to avoid duplication of motors and electrical switch gear and pump inlet liquid lines (equipped with shut-off valves) makes possible a reduction in the size of the insulating housing, known as a “cold box”, in which the cryogenic parts of the apparatus are housed.
There is preferably only one pumping chamber upstream of the liquid receiving chamber. The pumping member in this upstream chamber is typically an inducer comprising a helical blade of constant or varying pitch or other axial or radial pumping member dependent on the required pumping duty. For an inducer, the helix preferably performs 1½ to 2½ complete turns, i.e. extends through an angle in the range of 540 to 900° for low NPSH (net positive suction head) requirements.
The precise pressure at which the first stream leaves the first outlet depends in part on the pitch or diameter of the blade or the pumping member speed. Accordingly, for an upstream pumping chamber of given size, and for a given pumping member speed (which may be dictated by the speed at which downstream pumping members are intended to operate) the outlet pressure of the first stream can be selected from an albeit relatively small range of pressures by appropriate choice of the precise dimensions of the helical blade. Preferably, the or each pumping chamber downstream of the liquid receiving chamber has associated therewith a radial rotary pumping member, typically taking the form of an impeller having blades which urge the fluid being pumped in a generally radial direction.
Preferably, there is an axial or radial diffuser, or three dimensional (axial/radial) diffuser, located downstream of each pumping member. If desired, the blades of the radial diffuser may be of a variable angle kind.
The number of pumping chambers and the rotational speed of their pumping members downstream of the liquid receiving chamber depends on the pressure to which it is desired to raise the second stream. If, for example, the second stream is required at a pressure in the order of 10 to 12 bar, there may be a single radial pumping chamber dow
Doerrler William
Pace Salvatore P.
The BOC Group plc
LandOfFree
Pump does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Pump, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Pump will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2457067