System for energy recovery in a vacuum pressure swing adsorption apparatus

A VPSA apparatus includes a first adsorbent bed and a second adsorbent bed, a feed blower for providing a flow of a gas mixture at about atmospheric pressure to the beds, and a vacuum blower for removing a flow of gas therefrom and venting the gas to a space at atmospheric pressure. The VPSA process causes the first adsorbent bed to be poised for evacuation by the vacuum blower and concurrently, the second adsorbent bed is under vacuum conditions and is poised for pressurization by the feed blower. A single motor is coupled by a common shaft to both the feed blower and the vacuum blower and operates both. A conduit/valve arrangement is operative during at least a portion of a process time when the adsorbent beds are in pressurizing/evacuation states, respectively, to couple the feed blower to the second adsorbent bed when at vacuum and for concurrently coupling the vacuum blower to the first adsorbent bed which is to be evacuated. The feed blower is thereby caused to operate in a gas expansion mode and imparts expansion energy, via the common shaft, to the vacuum blower. During idling conditions, a valve-conduit system is controlled to enable significant reductions in pressure rise across the feed and vacuum blowers.

FIELD OF THE INVENTION 
This invention relates to apparatus for separation of a preferred gas, such 
as oxygen, from a mixture of the preferred gas and other gases and, more 
particularly, to gas separation apparatus which employs a vacuum pressure 
swing adsorption (VPSA) process and recovers energy from blowers employed 
by apparatus that performs the VPSA process. 
BACKGROUND OF THE INVENTION 
VPSA processes and systems are known in the art for separating components 
of a feed gas mixture. Such a gas mixture contains a more readily 
adsorbable component (i.e., a "more preferred" gas) and a less readily 
adsorbable component (i.e., a "less preferred" gas), and is passed through 
an adsorbent bed capable of selectively adsorbing the more readily 
adsorbable component at an upper adsorption pressure. The bed is 
thereafter depressurized to a lower desorption pressure (e.g. a vacuum) 
for desorption of the more readily adsorbable component and its removal 
from the bed prior to introduction of additional quantities of the 
feed-gas mixture. In a multiple bed VPSA system, the beds are cyclically 
operated through the same series of process steps, but the step sequence 
in one bed is offset from the same step sequence applied to another bed. 
The step sequence offset is accomplished to allow use of common feed and 
exhaust systems and to achieve process and energy savings. 
In conventional VPSA systems, multiple adsorber beds are commonly employed, 
with each bed subjected to a VPSA processing sequence on a cyclic basis so 
as to enable efficiencies to be achieved. VPSA systems are often used to 
separate oxygen from an input air stream. At certain times during 
operation of a VPSA system, either a feed blower or a vacuum blower, or 
both, are caused to operate in an "idle" mode, where they do not interact 
with associated adsorbent beds to actively move feed or exhaust gas 
through the system. Such operation is hereafter referred to as the unload 
state. The term turndown state will hereafter be used and will refer to 
the condition when: both the feed and vacuum blower are set into the 
unload state (idling) for an extended period of time; and the VPSA system 
is not producing product. 
During the VPSA process, gas streams are frequently expanded when pressure 
transferred during the process. Such gas transfer takes place at both the 
product and feed ends of the adsorber bed. Energy recovery from expanding 
gas streams in VPSA processes has long been a goal in systems design. 
Most present VPSA systems and, in particular, two-bed systems incorporate 
process steps which throttle gas streams for the purpose of pressure 
transferring the gas. The throttling results in lost power and added 
inefficiencies. Energy recovery in the prior art has also employed a 
natural aspiration of the feed air during vacuum conditions, at the 
beginning of the VPSA cycle. The natural aspiration method requires an 
additional air inlet regulation system and results in only a modest 
reduction in the feed gas compression requirement. Nor does the aspiration 
system recover energy from the expanding stream, but rather merely 
provides an air inlet without additional power consumption. 
Other prior art teachings related to energy recovery in gas separation 
systems are as follows. U.S. Pat. No. 5,429,666 to Agrawal et al. 
describes a vacuum swing adsorption (VSA) process which employs two beds 
that operate with product pressurization and pressure equalization between 
the beds. Simultaneous operation of the process steps, for the purpose of 
continuous utilization of feed and vacuum blowers, is described. The 
Agrawal et al. process employs a natural aspiration of feed air as an 
energy recovery process. The system attempts to lower feed power by 
utilizing the low adsorber bed pressure at the beginning of a cycle to 
allow for some fraction of the feed air to be drawn directly into the bed, 
without need for an air compressor. Such an ambient feed does nothing to 
recover energy that is available from the expansion of the feed air. 
U. S. Pat. No. 4,995,889 to Abel et al. describes a method for regulating 
product flow of an adsorption air separation system, especially under 
conditions of discontinuous product flow that result from variable 
customer demand. A control valve is connected to the product line of the 
separation apparatus and controls flow of the product through a variable 
or fixed orifice device that is upstream of the control valve. A 
differential pressure controller, which senses pressure upstream and 
downstream of the orifice device, is used to operate the control valve. 
U.S. Pat. No. 5,096,469 to Keefer details an adsorption air separation 
process which utilizes oscillations of a liquid column to change the 
volume of variable displacement chambers in order to create cyclic 
pressure changes that are required for the pressure swing process. In 
effect, the inertia of the oscillating fluid provides an energy exchange 
between air separation chambers. 
U.S. Pat. No. 5,183,483 to Servido et al. describes a pneumatic control 
process for a pressure swing adsorption (PSA) process. Adsorption, 
desorption and equalization phases are connected through use of two 3-way 
valves and a single compressor. By controlling the operation of the 3-way 
valves, the compressor can be used for adsorption and desorption or can be 
allowed to operate unloaded as well. 
U.S. Pat. No. 5,518,526 to Baksh et al. describes a PSA process which 
overlaps various steps to reduce total cycle time and to achieve improved 
efficiency and productivity. A unique step is described as being the 
simultaneous evacuation of a bed undergoing an equalization rising step, 
while the other bed is undergoing an equalization falling step. The next 
step in the cycle is simultaneous product and feed pressurization at 
opposite ends of the bed, followed by feed pressurization to the desired 
adsorption pressure. 
U.S. Pat. No. 5,042,994 to Smolarek (Applicant herein) describes a method 
for controlling a PSA system by the monitoring of a variable volume 
storage vessel during nitrogen production applications. The process cycle 
contains two steps where the feed blower and vacuum blower are idle. The 
first step is a counter-current oxygen repressurization step of a 
previously desorbed bed, while an adsorbed bed undergoes a blow-down of 
product nitrogen. The second step when the process machines are idled and 
not utilized occurs during a turndown step when the level of the variable 
volume product storage vessel is monitored in order to determine 
variations in customer demand. Thus, Smolarek teaches that the measure of 
idle time is proportional to some measure of customer demand. Smolarek 
further mentions that power reduction and energy savings can be achieved 
under turndown conditions by idling the machines proportionally with 
customer demand, while maintaining product purity. 
Notwithstanding the substantial development efforts that have been directed 
at improvements of pressure swing adsorption (PSA) and VPSA processes and 
systems, there is a continuing need for efficiency improvements therein. 
Accordingly, it is an object of this invention to provide a pressure swing 
adsorption system which exhibits energy usage efficiencies. 
SUMMARY OF THE INVENTION 
A VPSA apparatus includes a first adsorbent bed and a second adsorbent bed, 
a feed blower for providing a flow of a gas mixture at about atmospheric 
pressure to the beds, and a vacuum blower for removing a flow of gas 
therefrom and venting the gas to an area of atmospheric pressure. The VPSA 
process causes the first adsorbent bed to be poised for evacuation by the 
vacuum blower and concurrently, the second adsorbent bed is under vacuum 
conditions and is poised for pressurization by the feed blower. A single 
motor is coupled by a common shaft to both the feed blower and the vacuum 
blower and operates both. A conduit/valve arrangement is operative during 
at least a portion of a process time when the adsorbent beds are in 
pressurizing/evacuation states, respectively, to couple the feed blower to 
the second adsorbent bed when at vacuum and for concurrently coupling the 
vacuum blower to the first adsorbent bed which is to be evacuated. The 
feed blower is thereby caused to operate in a gas expansion mode and 
imparts expansion energy, via the common shaft, to the vacuum blower. 
A further embodiment of the invention includes additional conduits and 
valves which couple the feed conduit from the feed blower to the exhaust 
conduit input to the vacuum blower. When the system is in a turndown 
state, the adsorbent beds are isolated from the vacuum blower and both the 
feed blower and vacuum blower are idling and in the unload state. In such 
condition, the valving is operated to enable the feed blower to exhaust 
its air flow via the exhaust conduit, thereby resulting in a lower 
pressure drop across both the feed and vacuum blowers. 
A further, less preferred embodiment, uses independent motors to power the 
feed and vacuum blowers but, during turndown, couples the feed blower to 
the second adsorbent bed (which is at vacuum), causing the feed blower to 
be operated in a gas expansion mode. A generator is coupled to the feed 
blower motor and generates electrical energy into the main, thereby 
creating a credit for energy which is either later or concurrently used to 
power the vacuum blower.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION 
As will be hereafter understood, the preferred embodiment of the invention 
shown in FIG. 1 is configured to enable a VPSA system 10 to recover energy 
that is associated with expanding air streams and to deliver that energy 
directly to a vacuum blower. VPSA system 10 comprises a pair of adsorbent 
beds 12 and 14 which are coupled to an output product tank 16 via control 
valves 18 and 20, respectively. 
It is to be understood that while the description hereafter will only 
consider adsorbent beds 12 and 14, the system can be configured with 
additional beds, as is known in the prior art. Further, the system will be 
described in the context of an air separation process, however, it is 
known that pressure swing gas separation systems can be applied to other 
separations where a more preferred gas and/or a less preferred gas is 
separated, and provided as product, from a mixture of the more preferred 
gas and a less preferred gas. In the example to be described below, it is 
the less preferred gas (oxygen) that is output as product (with the 
adsorbent beds being selective for nitrogen). Accordingly, the invention 
is to be considered as applicable to all processes wherein appropriate 
gases are separated. 
A feed blower 22 is coupled via a feed conduit 24 and feed valves 26 and 28 
to beds 12 and 14, respectively. A vacuum blower 30 is coupled via an 
exhaust conduit 32 and exhaust valves 34 and 36 to adsorbent beds 12 and 
14, respectively. A controller 38 enables operation of each of the 
aforesaid components in the known manner to enable a separation of an 
inlet air feed 39 into an oxygen output stream (via conduit 40) feed to 
product tank 16 for storage. 
Feed blower 22 and vacuum blower 30 are both operated by a motor 50, which 
is coupled to both thereof by common shaft 52. By this arrangement, as 
will be described in further detail below, when feed blower 22 operates in 
an air expansion mode, the expansion energy which results is transferred 
via shaft 52 to vacuum blower 30, thereby enabling the electrical power 
input into motor 50 to be reduced, while enabling vacuum blower 30 to 
maintain its level of operation, albeit at a lower energy cost. 
A first unload valve 54 is coupled between feed conduit 24 and exhaust 
conduit 32 and a second unload valve 56 further couples exhaust conduit 32 
to a vacuum silencer 58. Vacuum silencer 58 is further provided with a gas 
flow from vacuum blower 30 via vent conduit 60. Vacuum silencer 58 
provides a vent action via vent conduit 60 and enables venting of both 
vacuum blower 30 and feed blower 22, when a control valve 62 is opened and 
couples feed conduit 24 to vacuum silencer 58. 
As will be hereinafter understood, the provision of motor 50 and common 
shaft 52 to connect feed blower 22 to vacuum blower 30 essentially creates 
an integral machine which enables vacuum blower 30 to be operated at 
reduced electrical power input as a result of gas expansion energy 
imparted to feed blower 22 during a portion of a VPSA cycle. The 
electrical power supplied to motor 50 during such gas expansion time is 
lowered in direct proportion by controller 38. 
Secondarily, the piping arrangement that includes unload valves 54, 56 and 
62 and their interconnection to vacuum silencer 58 allows feed blower 22 
to be discharged to the suction created by vacuum blower 30 during unload 
periods. This action results in a decrease of the pressure rise across 
vacuum blower 30 and feed blower 22 during times in the cycle when the 
blowers are not loaded and reduces their resultant power draw. 
As indicated above, VPSA systems commonly cause adsorbent beds 12 and 14 to 
be respectively pressurized and at a vacuum, during several process steps 
of the separation procedure. For example, VPSA systems employ a purge and 
overlap equalization cycle wherein continuous waste removal from one bed 
results in an expanding feed stream which produces energy simultaneously 
with a vacuum level waste stream requiring energy. During such action, 
adsorbent bed 12, for instance, remains at vacuum conditions, rising from 
9-13 psi, while feed air is supplied to adsorbent bed 12 by feed blower 
22. This action results in vacuum conditions being present in feed conduit 
24, thereby creating expansion of the feed air during the entire step. The 
feed air rate must also be limited during this period, hence extraction of 
work by limiting the air feed flow by expansion through feed blower 22 is 
advantageous from a process standpoint. 
Concurrently, adsorbent bed 14 must be evacuated by the operation of vacuum 
blower 30 through exhaust conduit 32 and vacuum silencer 58 to vent pipe 
70. At such time, waste nitrogen is removed from adsorbent bed 14 and is 
vented via vent pipe 70 to the atmosphere. During such time, adsorbent bed 
14 experiences a pressure fall into a vacuum condition (e.g., from about 
16 to 13 psi). 
Thus, when feed air is fed from feed air inlet 39 by feed blower 22 into 
feed conduit 24, the feed air is expanded and the expansion energy is 
imparted to feed blower 22 which, in turn, supplies mechanical power via 
shaft 52 to vacuum blower 30. At such time, controller 38 reduces the 
electrical power that is input to motor 50 in accordance with the 
expansion energy input from feed blower 22. Thus, motor 50 and common 
shaft 52 enable the expansion gas to directly transfer energy, via feed 
blower 22, to vacuum blower 30 which is concurrently operating in a 
compression mode to extract gas via exhaust conduit 32 from adsorbent bed 
14. 
In addition to the energy savings achieved by the aforementioned 
arrangement, a more compact plant layout is achieved, while at the same 
time reducing capital costs of the system through elimination of another 
drive motor and a motor starter. The single motor arrangement also 
simplifies the start-up controls. A common drive motor starts both blowers 
simultaneously, eliminating any possibility of not starting both blowers 
at the same time which could result in some undue loading on the machines, 
causing unwanted wear. 
At certain times during the VPSA cycle, plant unload may be achieved by 
interrupting the cycling of adsorbent beds 12 and 14, by isolating the 
beds and venting feed blower 22 and vacuum blower 30. During loaded 
operation, feed blower 22 and vacuum blower 30 are loaded by transfer of 
gas to and from adsorbent beds 12 and 14, respectively. Feed valves 26 or 
28 are either open or closed in dependence on which bed is being adsorbed. 
The same is true for exhaust valves 34 or 36, depending on which bed is 
being desorbed. During loaded operation, unload valves 54, 56 and 62 may 
be closed. Depending on the VPSA cycle, either or both of the feed and 
vacuum blowers can be unloaded during portions of the cycle. 
When the VPSA cycle reaches a point where feed blower 22 and vacuum blower 
30 are to be unloaded, unload valves 54 and 56 are opened and feed valves 
26, 28 and exhaust valves 34, 36 are closed. When this occurs, vacuum 
blower 30 operates in a recirculation mode, while feed blower 22 
discharges its gas (during unload), via unload valve 54, into the 
recirculation loop employed by vacuum blower 30 (i.e. vacuum blower 30, 
conduit 60, vacuum silencer 58, unload valve 56 and exhaust conduit 32). 
The system of FIG. 1, when in the unload state, is typically operated with 
unload valves 54 and 56 open and third unload valve 62 closed. Under 
certain plant design conditions related to conduit sizing, it may be 
beneficial to also open third unload valve 62 in the unload state if a 
further pressure drop in the unload conduits can be achieved. The 
additional reduction in overall pressure drop will result in additional 
power savings. 
The system of FIG. 1, when in the unload state, can be operated with the 
bed feed and vacuum valves in a closed condition. Under certain plant 
design conditions related to conduit sizing, it may be beneficial to open 
these valves if a further reduction in pressure drop in the unload 
conduits can be achieved. This additional reduction in pressure drop will 
result in additional power reductions. In such case, feed and vacuum 
valves 26,34 and/or 28,36 would be opened in addition to the opening of 
unload valves 54,56 and 62. Further, first unload valve 54 may be 
eliminated from the system if the feed and vacuum valves are opened as 
described. 
The opening of the bed valves requires a design of the control system that 
maintains the appropriate pressure levels in the beds during the unload 
period. Those pressure levels are controlled to be equal to the unload 
pressure to eliminate any flow into or out of the adsorbent beds. The 
opening of the feed and vacuum valves to augment pressure reduction in the 
unload conduits can be employed in single and multi-bed systems. 
As an example of how turndown/unload is achieved in a 60-ton per day oxygen 
VPSA system, during normal cycle operation at full production, first and 
second unload valves 54, 56 and third unload valve 62 are closed. During 
unload, third unload valve 62 is opened to unload feed blower 22, while 
unload valves 54 and 56 are kept closed. During turndown periods, unload 
valves 54 and 56 are opened, while unload valve 62 may or may not be 
opened as described above. The unload discharge pressure drop for feed 
blower 22 is 0.5 psi, while the vacuum unload suction pressure drop is 1.0 
psi for the less preferred system of FIG. 2. Through implementation of the 
invention as depicted in FIG. 1, the feed unload discharge pressure drop 
reduces to 0.2 psi, while the vacuum unload suction pressure drop is 0.3 
psi. If plant flow is reduced to 66% of full flow, the reduction in power 
consumption is calculated to be 5.0%. 
The improvement under turndown conditions results from the manner in which 
turndown control is implemented. When customer demand is low, the system 
reacts by unloading its machinery and idling the cycle for a period of 
time in inverse proportion to the customer demand rate. Significant 
reductions in flow, therefore, accentuate the benefits of the invention as 
the amount of time that the machines are unloaded constitutes a larger 
fraction of the entire cycle time. This improvement in power consumption, 
for example at 33% of full flow, is 10%. 
Energy recovery aspects of this invention that are achieved by utilization 
of the expanding feed stream from feed blower 22 can be practiced 
independently from the vent energy reduction aspects of the invention 
(achieved through the use of unload valves 54 and 56 and other valves, as 
described above). Such energy recovery system is applicable to any cycle 
which produces an expanding air stream through a feed blower. 
Turning now to FIG. 2, a less preferred embodiment of the invention is 
illustrated wherein feed blower 22 and vacuum blower 30 are powered by 
independent motors 71 and 72, respectively. In this instance, however, 
motor 71 includes a generator component 74 whose output is coupled to the 
electrical main via conductor 76. A silencer 78 is coupled to feed blower 
22 via feed unload valve 80. 
Vacuum blower 30 is coupled to a silencer 58 via a vacuum unload valve 82 
and exhaust conduit 32 is coupled to silencer 58 via exhaust valve 84. 
When pressure in one adsorbent bed, in a vacuum condition, is rising in 
pressure and in another bed, in a vacuum condition, is falling in 
pressure, controller 38 connects feed blower 22 to the adsorbent bed that 
is under vacuum condition and rising in pressure. At such time, the 
expansion energy experienced by feed blower 22 is transferred via motor 71 
to generator 74 which feeds power into the main, via conductor 76. 
Accordingly, an energy credit is accumulated. 
At the same time (or at some other time), vacuum blower 30 operates to 
remove a gas from the bed which is in a pressure falling state. Under such 
conditions, the input energy to motor 72 can be supplied partially from 
the energy generated by generator 74 or can be taken, in its entirety from 
the main, with the credit previously obtained being utilized to offset the 
costs of the input energy. In such manner, energy savings are achieved. It 
is to be understood, however, that this system is less efficient than the 
most preferred system depicted in FIG. 1 as a result of the 
mechanical-electrical-mechanical transformations which are required, with 
their inherent energy losses which reduce the overall energy savings. 
It should be understood that the foregoing description is only illustrative 
of the invention. Various alternatives and modifications can be devised by 
those skilled in the art without departing from the invention. 
Accordingly, the present invention is intended to embrace all such 
alternatives, modifications and variances which fall within the scope of 
the appended claims.