Note: Descriptions are shown in the official language in which they were submitted.
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F~L~ r ~ 3~ 'r''~, if
I
Process for the adsorptive separation of air
The present invention relates to an improved process for the separation of oxygen
or nitrogen from air by Vacuum Swing Adsorption (VSA) or Pressure Vacuum
5 Swing Adsorption (PVSA) with the use of a vacuum pump system optimised for the desorption of the adsorbed air component.
The direct production of oxygen from air at ambient temperatures is already carried
out extensively in industry by means of molecular-sieve zeolites.
Here the preferential adsorption of nitrogen over oxygen is utilised, that is, nitrogen
in the air is adsorbed onto the zeolite; the less strongly adsorbed components such
as oxygen and argon are collected as products at the outlet of the zeolite bed during
the passage of air through this bed. The adsorbed nitrogen can be desorbed, for
example, by evacuating the bed. In this case one refers to the VSA (Vacuum SwingAdsorption) process as opposed to the likewise known PSA (Pressure Swing
Adsorption) process. A continuous operation is achieved in the VSA process by the
following procedure:
20 a) passage of air through a zeolite bed into the inlet of an adsorber at, for
example, ambient pressure; withdrawal at the outlet of the adsorber is by
means of O2-enriched gas;
b) evacuation of the bed at the inlet by means of a vacuum pump to a pressure
of about 100 to 400 hPa in countercurrent to the stream of air, with possible
simultaneous flushing with part of the product;
c) filling of the bed with 07 product to approximately ambient pressure in
countercurrent to the stream of air. In the PSA process, step a) is carried out
at a pressure of 200 to 600 kPa and step b) is carried out at about 100 kPa
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with flushing with part of the ~2 product. (The pressures are invariably
based on absolute values.)
Because there are these three steps, the process is generally operated with three
5 separate zeolite beds, for brevity termed adsorbers, which are operated alternately
in cycles.
Separating processes with vacuum regeneration in which two adsorbers and a
reservoir for the product are operated cyclically have also been described (US 397
10 696) or even only one adsorber in alternation with a reservoir for the product.
The economic efficiency of these plants is affected by the input provided, for
example, quantity of adsorbents, size of vacuum pump and in particular by the
operating costs, such as the current consumption of the vacuum pump. The aim of
15 every development is therefore the optimal relationship between the quantity of
molecular sieves, the size of the vacuum pump and the energy consumption of the
vacuum pump. Vacuum pumps used hitherto in vacuum desorption are two-stage or
three-stage rotary piston compressors having a positive-displacement function (see
EPO 158 262) or water ring pumps, which are also based on a positive-displacement
20 action.
Other compressors which may also be employed are centrifugal compressors, which
are used as vacuum pumps (see, for example, EP 575 591). These compressors,
known as radial compressors, have the feature that they can be operated up to a
25 compression ratio of counterpressure to suction pressure of about 2.6, but for their
optimal use, that is, for the achievement of as low an energy requirement as
possible, they require a certain ratio of suction pressure to release pressure. This is
also referred to as optimal compression ratio Il. This compression ratlo Il ranges
from about 1.6 to 1.7 in conventional radial compressors. If, therefore, a radial
30 compressor is to be optimally used as a vacuum pump and the counterpressure
inclusive of the pressure loss in the sound absorber arranged in tandem is equal to
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1000 hPa, then a constant pressure of 625 or 588 hPa must be established at the
suction side. However, since in VSA plants the evacuation pressure falls within
about one minute from a maximum level (PDes ,), typically 950 hPa, to a minimum
value (PDes mjn), for example, 300 hPa, the use of only a radial compressor as a5 single stage is impossible in view of the optimal low energy input.
One possibility is to produce a pressure difference artificially by means of a choke
in front of the radial compressor. At a II of 1.6, however, the minim~l optimal
evacuation pressure is only 625 hPa. But during the evacuation time there are then
10 considerable energy losses through the choke owing to the decreased suction power.
If one therefore wishes to achieve a lower evacuation pressure than that prescribed
by the compression ratio II by means of a radial compressor, the radial compressor
must be installed in the suction line, and a vacuum pump having an approximatelyconstant suction capacity must be connected behind the radial compressor. Examples
15 of vacuum pumps having a virtually constant suction capacity are pumps havingpositive-displacement action, such as water ring pumps or oil-filled rotary slide-
valve pumps. The rotary piston compressor is another positive-displacement pump.
The object of the present invention was to find an energetically favourable process
20 for the O2-enrichment of air by means of a vacuum pump arrangement having as
low a current consumption as possible.
It has been found that in the case of the O2-enrichment of air by means of the
VSA/PVSA process, a combination of a radial compressor and a positive-
25 displacement pump in an arrangement which functions in parallel operation andsubsequently serial operation or which functions only in serial operation during the
entire evacuation time affords considerable advantages over a wide pressure range
compared with conventional two-stage rotary piston compressors.
30 This invention provides a process for the separation of oxygen or nitrogen from air
using an adsorption plant having one or more adsorbers containing adsorbents for
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nitrogen or oxygen, preferably for nitrogen, to which adsorption plant is connected
a vacuum pump arrangement consisting of a radial compressor and a vacuum pump
operating according to the positive-displacement principle, in particular a rotary
piston compressor, the air being passed into the adsorber, for example, through the
S inlet of the adsorber, in an adsorption phase at ambient pressure or at a slightly
lowered pressure of up to -100 hPa compared with ambient pressure, or at an excess
pressure of up to 500 hPa, and being withdrawn at the outlet of the adsorber by gas
enriched with oxygen or with nitrogen, the pressure in the adsorber being brought,
after a certain adsorption time, preferably after a time of from 20 to 120 seconds,
10 in an expansion phase, to a pressure PDes l corresponding to ambient pressure or to
a pressure PDes l up to at least 0.6 times ambient pressure, then in a desorption
phase, the adsorber cont~ining the adsorbent enriched with nitrogen or oxygen,
within a certain desorption time, in particular of from 20 to 120 seconds, in order
to desorb the adsorbed nitrogen or oxygen, then being brought by means of the
15 vacuum pump arrangement from the higher pressure PDes l to a lower pressure PDes
min corresponding to at least 0.05 times ambient pressure, and in a compression
phase again being brought to the pressure of the adsorption phase, characterised in
that, at the commencement of the desorption phase, the optionally previously choked
radial compressor and the positive-displacement pump of the vacuum pump
20 arrangement, being connected parallel or in series, in particular connected in
parallel, pump the adsorber and at a lower desorption pressure the radial compressor
and the positive-displacement pump, being connected in series, pump the adsorber,
the positive-displacement pump being connected to the delivery side of the radial
compressor, and that, during the serial mode of operation of the radial compressor
25 and of the positive-displacement pump, the positive-displacement pump operating
on the delivery side is so adjusted or dimensioned that the radial compressor attains
on average its optimal compression ratio II during the evacuation phase.
The possible rearrangement of the pump arrangement from parallel operation to
30 serial operation is carried out preferably at an evacuation pressure PDes o in front of
the radial compressor in particular when the evacuation pressure PDes o has attained
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at least the value obtained from the pressure P0 at the outlet of the positive-
displacement pump on the delivery side divided by 0.65 * II.
Preferably the radial compressor and the positive-displacement pump are operated5 already connected in series at the commencement of the evacuation phase.
A preferred variant of the process according to the invention is characterised in that
the pressure PDes o at which the change from parallel operation to serial operation
is effected is equal to at least the pressure P0 at the outlet of the positive-displace-
ment pump on the delivery side divided by 1.15 times the compression ratio 11 of the radial compressor.
A particularly advantageous variant of the invention is characterised in that at agiven initial evacuation pressure PDes l at the comrnencement of the desorption phase
15 the minim;~l evacuation pressure PDes mjn is within a pressure range obtained from
PDes min = Po/1030 hPa (0.25 * PDes l - 100 hPa)
and
PDes mjn = P0/1030 hPa * (0 5 * PD 1)
The rearrangement of the pump arrangement from parallel operation to serial
operation can be controlled, for example, according to a time setting or a pressure
25 setting via a control system of the adsorption plant.
In the special case that the tandem-arranged vacuum pump operating according to
the positive-displacement principle has to operate at an evacuation pressure of less
than 0.25 of ambient pressure, it may consist of two or three positive-displacement
30 pumps connected one behind the other.
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- 6 -
Further special embodiments of the invention are to be found in the dependent
clalms .
The VSA process according to the invention is explained in more detail below by
means of Examples with the aid of the Figures.
In the Figures:-
Fig. 1 shows the characteristic curve for the suction power of a known one-stage rotary piston compressor
Fig. 2 shows the characteristic curve for the suction power and the shaft power of
a known two-stage rotary piston compressor
Fig. 3 shows the characteristic curve and the pressure-dependent shaft power of a
pump arrangement comprising a radial compressor and a rotary piston
compressor connected in series
Fig. 4 shows the characteristic curve and the pressure-dependent shaft power of a
pump arrangement comprising a radial compressor and a rotary piston
compressor where there is a change from parallel operation to serial
operation
Fig. 5 shows a diagram of a VSA plant for carrying out the process according to
the invention
Fig. 6 shows the measured progressive change in pressure at the adsorber inlet
during the evacuation phase
a) a combination of radial compressor and rotary piston compressor in
serial operation
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b) a two-stage rotary piston compressor
c) a combination of radial compressor and rotary piston compressor
with a change from parallel operation to serial operation
at an initial pressure of 950 hPa
Fig. 7 shows curves of the progressive change in pressure as in Fig. 6 at an initial
pressure of 800 hPa.
The diagram of the VSA plant used for carrying out the following Examples is
shown in Fig. 5.
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Examples
The VSA plant has the following components:
Inlet valves: 11A, 12A, 11B, 12B, 11C, 12C
Outlet valves: 13A, 14A, 15A, 13B, 14B, 15B, 13C, 14C, 15C
Control valves: 17ABC, 18ABC
Valve 16ABC
Air compressor C10
15 Heating H10
Product compressor G10
Vacuum pump arrangement V10
In the description of the process below, the following abbreviations are used:
P0 = Pressure at the outlet of the pump arrangement A ambient pressure
plus dynamic pressure of the sound absorber at the end of the pump
arrangement
PDCS-I = Pressure in front of the pump arrangement at the commencement of
the evacuation phase
30 PDes o = Pressure in front of the pump arrangement at which the change from
parallel operation to serial operation is effected
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~Des-o min= Lowest pressure in front of the pump arrangement at which the
change from parallel operation to serial operation is effected
~Des-min = Lowest evacuation pressure in front of the pump arrangement
The adsorbers A, B and C are filled with Ca zeolite A granular material of grainsize 1 to 2.5 mm, produced as in Example 2 of EP-A 0 170 026. The nitrogen
adsorption onto this granular material at 1000 hPa and 25~C is 14 Nl/kg and the
oxygen adsorption is 4.3 Nl/kg.
The internal diameter of the adsorber was 1000 mm and the overall height of the
bed was 2200 mm. A 20 cm layer of silica gel was placed at the entry to the
adsorber. The height of the bed of zeolite granular material was 200 cm and the
weight of the zeolite was 1000 kg.
The adsorbers A, B and C are operated in cycles. The diagram of the course of the
process commences at the time t = 0, at which the adsorption in the adsorber A has
ended.
20 During the period up to t = 8 sec (also referred to as BFP time), the process is as
follows: -
In the adsorber A only the valve lSA is open. In the adsorber C only the valves12C and 13C are open. Drawn in by the pump arrangement VlO, O2-rich gas flows
25 from adsorber A via valve lSA, the open control valve 17ABC and valve 13C into
adsorber C. As a result of this, the pressure in adsorber A falls from the adsorption
pressure to a lower pressure PDes l (expansion phase). In adsorber C the evacuated
is ended here, with the pressure in adsorber C rising from the final pressure PDes m
to a higher pressure.
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Adsorber B commences with the air separation (adsorption phase), that is, ambient
air enters the adsorber B through valve 1 lB, O2-rich product gas leaves the adsorber
via valve 14B and is led away to the product reservoir (not shown) by compressorG10.
s
During the cycle time from 8 to 60 seconds, the process is as follows:
In adsorber A valve 15A is closed again and only valve 12A is open. In the
desorption phase, the adsorber A is partially evacuated from the pressure PDes 1 to
10 the pressure PDes mjn by means of the vacuum pump V10. The adsorber B is in the
adsorption phase, that is, the valves 11B and 15B are open. At the same time, the
adsorber C is filled with Ol-rich gas via the valves 18ABC, 16ABC and 13C. In the
adsorber C only valve 13C is open. The quantity of filling is calculated so that at
the end of this period the pressure in the adsorber C almost attains the adsorption
15 pressure (compression phase).
In the next phase of the cycle, adsorber C separates the air (adsorption phase), in
the third phase of the cycle adsorber A, that is, the two cycle times from 0 to 8
seconds and from 8 to 60 seconds, are each correspondingly repeated.
The factors used for the evaluation of the following experimental Examples were the
quantity of O, produced at 93% volume concentration and the progress with time
of the evacuation pressure in front of the pump arrangement, the quantity of gas
evacuated and the volumetric capacity of the pump arrangement at 300 hPa.
The maximal adsorption pressure was invariably 1100 hPa and the minimal
evacuation pressure PDes mjn was invariably 300 hPa. In addition to the type of pump
arrangement, the pressure PDes l at the commencement of the evacuation step was
compared, this initial pressure being 950 hpa in the first variant and 800 hPa by way
30 of comparison. The pressure at the outlet of the pump arrangement (P0 ~ ambient
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pressure inclusive of dynamic pressure of the sound adsorber installed behind the
pump arrangement) was on average 1050 hPa.
The mode of operation of the following pump arrangements was investigated with
regard to the separation process:-
D) A two-stage rotary piston compressor, for which in the experiment the
capacity at 300 hPa was about 1000 m3/h.
10 E) A combination consisting of a radial compressor and rotary piston com-
pressor, i.e. both compressors were always connected in series and for
which in the experiment the capacity at 300 hPa was about 1000 m3/h.
F) A combination consisting of a radial compressor and rotary piston com-
pressor with performance data as for Fig. 4, pumping being parallel up to
an evacuation pressure of 650 hPa in front of the pump arrangement of
radial compressor and rotary piston compressor and pumping being in series
at a pressure of below 650 hPa, that is, the radial compressor on the suction
side and the rotary piston compressor on the delivery side; the delivery
volume at 300 hPa was approx. 1000 m3/h.
Figure 1 shows the characteristic curve of a one-stage rotary piston compressor. It
can be seen from this that at a suction pressure of less than 400 hPa, the suction
power is already considerably limited compared with the suction power at 1000 hPa.
Figure 2 shows the characteristic curve of a two-stage rotary piston compressor. The
second stage connected in series has at ambient pressure, compared with the first
stage of the suction side, according to the ratio of graduation, a suction power which
is lower by 40%. Between 1000 hPa and 200 hPa, the suction power of the
30 characteristic curve falls by about 10%.
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Figure 3 shows the characteristic curve of a pump arrangement with a radial
compressor on the suction side and a rotary piston compressor on the delivery side
(serial operation).
The same pump parts are used for the measurement of the characteristic curves inFigure 4 as are used for the determination of the characteristic curve in Figure 3.
However, in the range of 650 to 1000 hPa, the radial compressor and the rotary
piston compressor exert suction in parallel, with the radial compressor being
regulated at 650 hPa on the suction side via a choke. In the region below 650 hPa,
the radial compressor and the rotary piston compressor are connected in series, as -
for the measurement of the characteristic curve in Figure 3.
In another Example of the procedure, the evacuation pressure PDes, is attained by
the valve 12C being closed during the above-mentioned cycle time from "0 to 8
seconds", that is, there is a pressure compensation or a partial pressure compensa-
tion between adsorbers A and C. During this time, the vacuum pump V10 does not
evacuate adsorber C and functions in "idle running operation".
In another Example. the evacuation pressure PDes ~ is attained by only the valve 12C
being open in adsorber C during the above-mentioned cycle time from "0 to 8
seconds", by which means adsorber C is evacuated to its final pressure. In adsorber
B only valve llB is open, through which the air compressor C10 fills the adsorber
B with air. In adsorber A only the valve 14A is open, through which the product
compressor G10 withdraws O2-rich gas and in the adsorber A the pressure falls tothe required evacuation pressure PDes ,.
In a further Example of the process, on connection of the vacuum pump arrange-
ment, the optimal initial pressure PDes ~ for the evacuation is attained relatively
rapidly by only the valve 13C being open in adsorber C during the above-mentioned
cycle time from "0 to 8 seconds" . In adsorber B only valves 1 lB and 14B are open,
through which the air compressor C10 fills the adsorber B with air and produces
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already O -rich gas. In adsorber A, the valves 12A and 15A are open. O2-rich gasfrom adsorber A fills up the adsorber C via valves l5A, 17ABC and 13C. The
pressure in adsorber A is lowered relatively rapidly to the required optimal initial
pressure PDes l by means of the vacuum pump V10 connected at valve 12A.
s
By means of the calculation programme "ADSIM" from the firm ASPEN TECH/
Cambridge, for an oxygen concentration plant with 5000 Nm3/h oxygen in the
product and with an O, concentration of 93 vol. %, the course of the evacuation was
calculated for the use of the above three pump arrangements D), E), F) in the two
evacuation ranges 950 hPa (= PDes l) to 300 hPa (= PDes min) and 800 to 300 hPa
and, with the determination of the oxygen yields (ratio of quantity of oxygen in the
product to atmospheric oxygen), the pump sizes were calculated. Here the
characteristic data, the delivery volume and the energy requirement from Figures2, 3 and 4 were used, proportionally converted for other pump sizes.
Figure 6 shows the measured progressive change in the pressure at an initial
pressure PDes ~ of 950 hPa. According to this, the pump arrangement E) (serial
connection of radial compressor and rotary piston compressor) with its low
absorption capacity at higher pressures has a relatively high pressure level in the
20 course of exhaustion compared with the type D) (two-stage rotary piston com-
pressor).
The pump arrangement F) (starting with parallel operation of radial compressor and
rotary piston compressor) with its higher absorption capacity at higher pressures
25 has, compared with the type D (two-stage rotary piston compressor), a relatively
low pressure level in the course of exhaustion, which accordingly indicates an
unfavourable energy requirement.
Figure 7 shows the measured progressive change in pressure at an initial pressure
30 PDes l of 800 mbar. The characteristic curves of the evacuation are not as far apart
from one another as are those in Figure 6.
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- 14 -
The following pump sizes were determined:
Table 1
Evacuation pressure 950 to 300 hPa
Type of pump Pump size Pump size
at 1000 hPa at 300 hPa
m3/h m3/h
D) Comparison 136,024 A 100% 124,460 ~ 100%
E) (series) 78,452 ~ 58% 132,765 ~ 106.6%
F) (parallel-series) 143,920 G 106% 119,870 A 96%
Table 2
Evacuation pressure 800 to 300 hPa
Type of pump Pump size Pump size
at 1000 hPa at 300 hPa
m3/h m3/h
D) Comparison 136,425 ~ 100% 119,335 ~ 100%
E) (series) 72,160 ~ 55% 122,115 ~ 102%
F) (parallel-series) 141,713 A 108.6% 118,029 ~ 98.6%
As the evacuation pressure in the range of 600 to 700 hPa, within which a
rearrangement of the system comprising a radial compressor/rotary piston
compressor from parallel operalion to serial operation should be carried out, will
25 pass relatively rapidly, this rearrangement can be carried out via the control system
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of the O2-VSA/PVSA plant. This can be effected by changing appropriately on the
attainment of a certain evacuation pressure, or by setting a given evacuation time.
As Figure 1 shows, the suction power of a one-stage rotary piston compressor
5 already falls relatively greatly at a pressure of 400 hPa. To attain lower evacuation
pressures of below 25% or even below 15% of ambient pressure using a serial
combination of a radial compressor/rotary piston compressor, without the failure of
the radial compressor owing to the necessity of an excessively high compression
ratio, it is proposed that the rotary piston compressor be operated optionally two-
10 stage and multistage (serial operation).
The energy requirement of the three pump arrangements D), E), F) for the twopressure ranges 950 hPa to 300 hPa and 800 to 300 hPa was calculated for 5000
Nm3/h oxygen in the product and an ~2 concentration of 93 vol.% from the
15 progressive course of the evacuation and from the characteristic data of the vacuum
pump arrangements D), E) and F) and the calculated pump sizes. Here the
characteristic data for the shaft power shown in Figures 2, 3 and 4 were used,
proportionally converted for other pump sizes. The energy requirement here is based
on the quantity of oxygen produced.
The following specific shaft powers at an operating efficiency of 4 % were
determined:
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Table 3
Evacuation pressure 950 Evacuation pressure 800
to 300 mbar to 300 mbar
Type of pump Energy requirement Energy requirement
KWh/Nm3/02 KWh/Nm3/02
D) Comparison 0.405 e 100% 0.408 A 100%
E) (series) 0.382 ~ 94.38% 0.356 ~ 87.3%
F) (parallel-series) 0.371 ~ 91.6% 0.364 ~ 89.2%
It is surprising that the lowest energy requirement is displayed by the pump type E)
(see Table 3, second column, 800 hPa initial pressure), which has continuous serial
operation (of the radial compressor and the rotary piston compressor), despite the
limited delivery volume at higher pressures even at an initial evacuation pressure of,
15 for example, 800 hPa, hence far above the theoretically favourable initial pressure
(PDes o = Po/ll of about 1050/1.6 = 650 hPa) compared with the combination D)
( = two-stage rotary piston compressor) and even compared with the pump type F)
with initial parallel operation and serial operation commencing at 650 hPa.
20 It is surprising that a saving in energy is achieved with the pump type E) compared
with a two-stage rotary piston compressor (type D)) even at higher initial pressures,
for example, 950 hPa (Table 3, first column).
In an Ol VSA process using a vacuum pump arrangement consisting of a radial
25 compressor and rotary piston compressor, the combination of the radial compressor
and rotary piston compressor can therefore be rearranged to serial operation long
before attainment of the optimal initial evacuation pressure PDes o ( = ambient
pressure inclusive of dynamic pressure of the sound absorber to optimal compression
ratio lI of the radial compressor).
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In the normal case, at 1000 hPa ambient pressure and 50 hPa dynamic pressure of
the sound absorber and at a compression ratio ~I of 1.6, the favourable initial
pressure of the serial operation (PDes o) would be about 650 hPa. In the case of the
O2-enrichment of air using the VSA technique, however, at relatively high pressures
5 the evacuation can already be carried out optimally using a serial operation of the
radial compressor/rotary piston compressor. This means that at pressures far above
the optimal initial pressure PDes o the pump combination can already be rearranged
from parallel operation to serial operation, or that the serial operation of the pump
combination is already cornmenced with at the beginrling of the evacuation.
