Note: Descriptions are shown in the official language in which they were submitted.
CA 02210340 1997-07-15
SEPARATION OF GAS~S BY
SPRESSURE SWING ADSORPTION
This invention relates to a process for increasing the content of
primary gases in a primary gas stream containing other gases. More
particularly, this invention relates to a process for the purification of
15 a primary gas stream by the selective adsorption of primary gases,
the rejection of secondary gases, and the subsequent desorption of
primary gases to produce an enriched primary gas stream. In a
particular embodiment, this invention relates to such a separation
using pressure swing adsorption techniques.
There are various ways to separate gases. An effective way to
separate many gases is thr~ugh pressure swing adsorption. In
25 pressure swing adsorption, a component of a gas stream is selectively
adsorbed onto an adsorbent. Other components that are less highly
~dsor~ed are conceritrated and pass through tne adsorbent bed and
can be collected as one of the products. After an adsorbent bed has
become loaded with the highly selectively adsorbed component, the
CA 02210340 1997-07-1~ .
adsorbent bed can undergo regeneration by reducing the pressure
and recovering the selectively adsorbed component. An
intermediate step can involve flowing a concentrated stream of the
highly adsorbed component through the bed. Such a step
5 concentrates the highly adsorbed component in the bed while
producing an exit stream which is about at feed~ gas composition. The
order in which the process steps are performed can influence the
cost effectiveness of the process.
The cost effectiveness of a pressure swing adsorption process is
dependent on the capital cost of the pressure swing adsorption
system and the operating cost. The present process is an
improvement over prior processes with regard to both costs. There
is a reduced capital cost since fewer adsorbent beds are needed. This
15 reduces the amount of tanks, valves and piping that is needed. On
the operating side, there is greater cost effectiveness since the
process is conducted at a relatively low pressure and fewer streams
need ~o be increased in pressure and fuel gas would be available.
For instance, a recycle feed gas does not have to be compressed and
20 pressurized as in U.S. Application Serial No. 08/326,916 filed October
21, 1994.
The processes in this application for patent in preferred
embodiments are ver,v effective for the separation of n~trogen from
25 natural gas and natural gas from carbon dioxide. rn the separation of
nitrogen from natural gas, the natural gas is preferentially adsorbed
CA 02210340 1997-07-1~
with the nitrogen passing through the adsorbent bed. In the
separation of natural gas from carbon dioxide, the carbon dioxide is
preferentially adsorbed with the natural gas passing through the
adsorbent bed. All of these gases can be recovered and used.
S Commercially valuable products are natural gas and carbon dioxide.
As used in this application the term natural gas denotes a gas stream
that primarily is methane with small amounts of other lower alkanes
and some alkenes.
It is known to enrich natural gas using pressure swing
adso~ption techniques. ~n U.S. Patent 5,171,333 there is disclosed a
technique using four adsorbent beds, each of which contains a
faujasite adsorbent. Each bed in sequence goes through an
adsorption step, a desorption step by lowering the pressure and then
15 a repressurization step to bring a bed back up to adsorption
pressure. The adsorption step is conducted at about 100 to 500 psia
and consists of passing a feed gas into an adsorbent bed. The
desorpltion step consists of cocurrently depressurizing an adsorbent
bed and passing the gas to a bed undergoing repressurization,
20 further cocurrently depressurizing the adsorbent bed and passing
the gas as a purge gas to a bed-undergoing purging, countercurrently
depressurizing and collecting a methane/ethane stream and
countercurrently purging the adsorption zone with a purge gas from
another adsorbent bed and recQvering e~han The repressurization
25 step consists of repressurizing the adsorbent bed by cocurrently
CA 02210340 1997-07-1~
passing a repressurization gas into the adsorbent bed and further
repressurizing the adsorbent bed by passing a portion of the
adsorption effluent from another adsorbent bed to this adsorbent
bed.
~ n U.S. Patent 5,174,796 there is disclosed a pressure swing
adsorption process for enriching a natural gas stream which contains
nitrogen. A carbon adsorbent is used and the natural gas is
preferentially adsorbed. The process steps consist of cocurrent
10 adsorption, a first cocurrent depressuAzation and the use of a part of
this gas to repressurize another adsorbent bed, cocurrently
depressurizing the adsorbent bed to a yet lower pressure and
withdrawing a fuel gas stream, countercurrently depressurizing the
adsorbent bed and recovering a product gas, countercurrently
15 purging the adsorbent bed and recovering additional product gas,
then countercu~Tently repressurizing the adsorbent bed in two
repressurization steps with nitrogen gas from another adsorbent bed.
This re~ressurization brings the adsorbent bed up to about the feed
gas pressure.
These are interesting processes but they are not highly
efficient in the enrichment of natural gas. These pressure swing
adsorption processes to date have not optimized the recovery of the
r~aturql aas nroduct The oh~ective in increasing efficlency is to
25 desorb essentially all of the adsorbed gas in as high a purity as
possible as quickly as possible, utilizing the value of any off gases,
and then put the adsorbent bed back into production as quickly as
possible. This is what is accomplished in the processes of the present
CA 02210340 1997-07-1~
invention. In addition, the pressure swing adsorption processes
should have a low capital cost. The present pressure swing
adsorption processes produce a high purity separation requiring only
three adsorbent beds and their associated valving. This is an
5 improvement over prior art processes.
The present invention is directed to the selective separation of
a primary gas from secondary gases by means of pressure swing
adsorption. In one preferred mode it is directed to the removal of
15 nitrogen from a natural gas stream to upgrade the natural gas stream
to pipeline quality. ~n this mode, the hydrocarbons are adsorbed and
the nitrogen passes through the adsorbent bed. ln another preferred
mode, c?rbon dioxide is separated from a gas stream containing
natural gas. In this mode the carbon dioxide is preferentially
20 adsorbed with the hydrocarbon gases passing through the adsorbent
bed. The carbon dioxide and natural gas are each valuable products.
The pressure swing adsorption process comprises the use of
three adsorbent beds, each se~uentially going through the phases of
25 (a) adsorption; (b) first depressurization, (c) recycle; (d) second
depressurization; (e) evacuation; (f) secondary gas pressurization;
and (g) main pressurization.
CA 02210340 1997-07-1~
The adsorption phase consists of flowing the feed gas
cocurrently through an adsorbent bed until the secondary gas front
in the adsorbent bed approaches the exit of the adsorbent bed. At
this point the feed gas is stopped and the adsorbent bed is put onto
the first depressurization phase. The secondary gas is the gas that is
weakly adsorbed by the adsorbent bed.
The first depressurization phase consists of reducing the
pressure in the ~dsorbent bed to remove some of the non-adsorbed
10 gases that are present in and between the adsorbent particles. This
gas stream will have a gas composition similar to that of the feed gas.
This depressurization gas is used to pressurize the adsorbent bed
that is to enter an adsorption phase or can be used as a fuel source
when it is a hydrocarbon cont~inin~ gas. Preferably, this is
15 conducted countercurrently.
The recycle phase consists of passing a second depressurization
gas, preferably countercurrently into the present adsorbent bed in
order to further remove the non-adsorbed gas in the void space
20 between the adsorbent particles and to desorb some adsorbed
secondary product gas. As a recycle feed gas exits the adsorbent bed
in a recycle phase it is stored or fed to an adsorbent bed that now is
on a main pressurization phase. At the completion of the recycle
r~h~-3e the ~oid space and the adsor~ent wi~l contain mor~ than gO
25 percent primary product, and preferably more than 9S percent
primary product. At this point, the adsorbent bed undergoes a
second depressurization phase.
-
CA 02210340 1997-07-15
The second depressurization phase comprises reducing the
pressure of the adsorbent bed and flowing the released gas,
preferably countercurrently, from the adsorbent bed to a bed which
has just completed a first depressurization phase. This is a gas
S stream which contains more than 90 percent primary product, and
preferably more than 95 percent primary product. This adsorbent
bed then undergoes an evacuation phase.
The evacuation phase consists of reducing the pressure on the
10 adsorbent bed and flowing the gas countercurrently from the
adsorbent bed. The evacuation is conducted at a much reduced
pressure, and preferably under a vacuum of àt least 20 inches of
mercury and preferably Z8 inches of mercury or more. The
evacuation gas is a primary product which can be compressed for
15 transport or use. The adsorbent bed then undergoes a secondary
product gas pressurization phase.
The secondary product gas pressurization phase consists of
countercurrently flowing a secondary product gas from an adsorbent
20 bed on an adsorption phase into this adsorbent bed. The absolute
pressure of this adsorbent bed ~ill rise to about half of the operating
adsorption phase pressure. The adsorbent bed then enters a main
pressurization phase which consists of flowing the gas from the
~Asorbent bed on a first depressuri7ation phase and a recycle phase
2 5 into this adsorbent bed along with the feed gas stream. The gas flow
into this adsorbent bed preferably is cocurrent. The adsorbent bed
then repeats the cycle by undergoing an adsorption step by the flow
of feed gas into the adsorbent bed.
CA 02210340 1997-07-1~
Each adsorbent bed in turn undergoes the same process phases
in sequence. While one adsorbent bed is undergoing one phase, the
other adsorbent beds are undergoing other phases. The timing of the
S phases can vary. However, in a preferred embodiment, the
combined first depressurization, recycle, and second depressurization
phases will have~about the same timing as the combined secondary
gas pressurization, main pressurization and adsorption phases. The
evacuation phase alone in this preferred embodiment will have the
10 timing equivalent to these combined phases. Upon exiting the
pressure swing adsorption process the primary product gas will be
ready for transport or use.
The present pressure swing adsorption process conserves the
15 more highly adsorbed product gas. This is accomplished by taking
the gas from an adsorbent bed that is being depressurized and
flowing this gas into an adsorbent bed that has completed an
adsorption phase. This resulting recycle gas removes gas of about
feed gas composition from the adsorbent bed void space and
20 removes a secondary gas which has been adsorbed in the pores of
the adsorbent. This recycle feed gas then is fed to an adsorbent bed
that will be undergoing an adsorption phase. In this way, the
content of the more strongly adsorbed component is maintained in
the pressure swing systern. The ~nly gases exi~ing the pressure
25 swing system are a high purity primary product gas and a high
purity less strongly adsorbed secondary product gas. By maintaining
the more strongly adsorbed component in the pressure swing system
CA 02210340 1997-07-15
until separated from the other gases the efficiency of the system is
increased.
This pressure swing adsorption process is particularly adapted
S for use in separating nitrogen from natural gas and natural gas from
carbon dioxide. rn the separation of nitrogen from natural gas the
natural gas is preferentially adsorbed and is the primary product
gas. Nitrogen is the secondary product gas. In the separation of
carbon dioxide f-rom natural gas the carbon dioxide is preferentially
10 adsorbed and is the primary product gas and the natural gas is the
secondary product gas. All of these gases can be collected for use. -
All are commercially valuable gases.
Figure 1 sets out in a schematic form the seven phases used in
the adsorbent bed pressure swing adsorption system.
Figure 2 is a schematic of a three adsorbent bed system for the
20 separation of pAmary product gas from a gas cont~ining a primary
product gas and a secondary product gas.
Figure 3 sets out in tabular form the cycle times for a three
~dsorbe.~t bed pressure swing a~sorption system.
CA 02210340 1997-07-1~
The present process is directed to the separation of a primary
product gas and a secondary product gas. The primary product gas is
the component adsorbed by the adsorbent bed. This process is a
pressure swing adsorption process wherein the primary product gas
S such as natural gas or carbon dioxide is preferentially adsorbed by
the adsorbent. Preferential adsorption is the technique where one or
more substances are more strongly adsorbed than other substances.
~n this phenomenon essentially all of the substances are adsorbed by
the adsorbent to.some degree. As the adsorption progresses, the
10 more strongly adsorbed gases become concentrated on the adsorbent
and feed gases occupy the space within the adsorbent. The net result
is that at the time of breakthrough of feed gas from the bed the more
strongly adsorbed primary product gas components to a large extent
have saturated the adsorbent bed while the less strongly adsorbed
15 secondary product gas components to a large extent have exited the
adsorbent bed.
Irr the present pressure swing adsorption process the
adsorbent preferably is a porous carbon such as activated carbons
20 and carbon molecular sieves. The activated carbon can be derived
from wood, coal, coconut or petroleum sources. The requirement is
that the activated carbon have a selectivit,v for the primary product
gas greater than that for the secondary product gas and other gases.
The adsor~ent characteristics that deterr,lne seleftivity for
25 hydrocarbons include pore structure, pore size and treatment
conditions. Useful activated carbons have a carbon tetrachloride
number of about 60, a surface square of more than about 1~150 sq.
meters/g., a density of about 0.45 g./cc. and an average particle
CA 02210340 1997-07-1~
diameter of about 3mm to 5mm, and preferably about 4mm.
However, any adsorbent can be used as long as it has greater
selectivity for one component for another.
S Pressure swing adsorption systems are usually comprised of
tWO or more adsorbent beds. The number of adsorbent beds used is
the number that provides the better overall economic benefit. That
is, the pressure swing adsorption system that is selected must give
the lowest overall cost considering the capital cost of the equipment
and the operating cost. ~n this regard most pressure swing
adsorption systems are cost effective when three to five adsorbent
beds are used. In the present pressure swing adsorption process
three adsorbent beds are used. The economics are very favorable
using three adsorbent beds.
Figure 1 sets out in a schematic diagram the preferred three
adsorbent bed pressure swing adsorption system. For illustration
purposers, this process will be described for the enrichment of the
hydrocarbon content of a natural gas stream. If this natural gas
stream contains significant amounts of hydrogen sulfide, it will be
pretreated by scrubbing with monoethanoloamine. The feed gas can
be dried by treatment with diethylene glycol or triethylene glycol or
by passage through a bed of al~-min~, silica or aluminosilicate
zeoii.es. The feed to the pressure swing adsorption system in Figure
1 will be a gas containing primarily hydrocarbons and nitrogen.
There will be minor amounts of other gasses present.
CA 02210340 1997-07-1~ .
As schematically described in Figure 1 the process consists of
seven phases. These are an adsorption phase, a first
depressurization phase, a recycle phase, a second depressurization
phase, an evacuation phase, a nitrogen secondary gas pressurization
S phase and a main pressurization phase. ~n the Phase ~ adsorption
phase, feed gas is fed to an adsorbent bed until the feed gas is about
to exit the adsorbent bed. This is breakthrough when the feed gas
will exit the adsorbent bed. At this point the input of the feed gas
ceases and the adsorbent bed undergoes a Phase ~I first
10 depressurization phase. During the adsorption phase, an adsorption
front moves toward the exit end of the adsorbent bed. Before the
adsorption front the gases in the adsorbent bed consist of the more
lightly adsorbed or non-adsorbed secondary product gases. These
are the gases that have not been adsorbed and the gases in the void
15 space. Behind the adsorption front the gases are primarily the feed
gas and the adsorbed gases on the adsorbent.
In a first step of regeneration following an adsorption phase
this adsorbent bed undergoes the Phase II first depressurization
20 phase. The first depressurization comprises the reducing of the
pressure in the adsorbent bed from the adsorption pressure to a first
lower pressure. During this first depressurization gases primarily
are removed from the spaces between the adsorbent particles and
~lowed, preferaDly countercurrently, from the adsorbent bed. These
2 5 gases will have a composition about that of the feed gas. These first
depressurization gasses can be flowed along with recycle feed gas to
the adsorbent bed that will enter an adsorption phase to ~ressurize
CA 02210340 1997-07-1~
this adsorbent bed or they can be used for their fuel value in the
operation of the pressure swing adsorption system.
The adsorbent bed then enters a Phase III recycle phase. This
5 phase removes additional feed gas from the void space of the
adsorbent bed and secondary product gas from the adsorbent. ~n
this recycle phase, a second depressurization gas from an adsorbent
bed undergoing a second depressurization is flowed, preferably
countercurrently, through this adsorbent bed with the effluent
10 recycle feed gas flowed to an adsorbent bed on a Phase VII main
pressurization phase. At the completion of the recycle phase the
adsorbent bed will contain 90 percent or more, and preferably 9S
percent or more, of the more strongly adsorbed primar,v product gas
components. This adsorbent bed then undergoes a Phase IV second
15 depressurizing phase.
In the Phase IV second depressurization, the pressure in the
adsorb~nt bed is reduced and an effluent gas flowed, preferably
countercurrently from the adsorbent bed. This depressurization gas
20 is fed to storage and ultimately to the adsorbent bed that is entering
a Phase III recycle phase. The sècond depressurization gas will
contain more than 90 percent by volume, and preferably more than
9S percent by volume, of the more strongly adsorbed hydrocarbon
components. The adsorbent bed then undergoes a Phase V
2 5 evacuation phase.
In the Phase V evacuation phase a vacuum is drawn on the
adsorbent bed to a vacuum of more than about 20 inches of Hg and
13
CA 02210340 1997-07-1~ ,
preferably to more than about 28 inches of Hg. The evacuation gas
flows countercurrently from the adsorbent bed. This gas is
comprised of the more strongly adsorbed components which in the
present process is the primary product gas which has a purity of 98
5 percent or more by volume. This is recovered as the primary
product gas. The adsorbent bed then undergoes repressurization to
feed gas pressure.
The Phase Vl secondary product gas pressurization phase
10 consists of flowing the less strongly adsorbed effluent secondary
product gas from an adsorbent bed on a Phase 1 adsorption phase
into the adsorbent bed being repressurized. Preferably this flow is
countercurrent. This secondary product gas pressurization plus the
phase VlI main pressurization by the cocurrent flow of gases into
15 this adsorbent bed brings this adsorbent bed up to the feed gas
pressure. The main pressurization phase consists of flowing the
recycle feed gas, feed gas and optionally some or all of the first
depres'surization gas into this adsorbent bed that will enter into an
adsorption phase.
Table I sets out the valve operati~g sequence for the schematic
chart of Figure 2. The valves listed in the table are open for the
note~ ~L'.le sequence. ~i ~ther valves are closed. The a, b and c
25 designation are the phase times for each time sequence. The
evacuation phase is the only phase that functions over a full a, b, and
c time sequence. For the other phases, secondary product~
pressurization, main pressurization and adsorption take up one time
14
CA 02210340 1997-07-15
sequence. First depressurization, recycle and second
depressurization phases take up another time sequence.
TABLE I
- OPEN VALVE
Time SeqJence I Tlme Secuence ll Tine Secuence lll D
,~.dsorbent Bed o
Misc. Valves a b c a b c a b O
A ~ 14 10 10; 13 11 11; 14 11 12 12 12 r
B 21 21,24 21 22 22 22 24 20 20,23
C 32 32 32 34 30 30,33 31 31,34 31
Misc. Valves 50; 80 50; 70 60 50; 80 50; 70 60 50; 80 50; 70 60
CA 02210340 1997-07-1~
Prior to adsorbent bed A entering into an adsorption Phase ~,
this adsorbent bed must be pressurized. This description of the gas
flows and valve operation will be discussed with reference to
adsorbent bed A entering into a nitrogen (secondary product)
S pressurization Phase VI. During this phase, valve 14 is open as is
valve 80. Nitrogen gas passes from reservoir 40 through conduits
38, 68, 42 and 36 and into adsorbent bed A. During this same time
valve 32 of adsorbent bed C is open with an evacuation phase
primary product gas passing from adsorbent bed C through conduits
10 58 and 61 to vacuum pump 90. This gas is the primary product,
natural gas. Concurrently, valves 21 and 50 are open with adsorbent
bed B undergoing a first depressurization phase ~.
Adsorbent bed A then undergoes a main pressurization phase
15 VII. Valve 10 is open with recycle feed gas flowing through conduit
16 from reservoir 88. Adsorbent bed C continues in an evacuation
phase V. Adsorbent bed B is a recycle phase III and has valves 21
and 24 open. Gas flows from reser~oir 74 through conduits 72, 42,
S~ and 51 and into adsorbent bed B. A recycle feed gas flows from
20 this adsorbent bed through conduits 46, 84, and 86 to reservoir 88.
This recycle feed gas is used fo'r main pressurization and can also be
used as a feed gas.
~n the next period of tirr~e, valv~s ,0 and 13 are open for
25 adsorbent bed A which is on an adsorption phase with a secondary
product exiting at conduits 36 and flowing to secondary product
reservoir 40 through conduit 38. During this time adsorb~nt bed C
continues in evacuation phase V. Adsorbent bed B is in a second
17
CA 02210340 1997-07-15
depressurization phase IV. Valve 21 is open in adsorbent bed B with
a second depressurization gas flowing through conduits 46, 84, 78
and 76 and valve 60 to reservoir 74. This gas will be used as recycle
gas in the recycle phase. This completes a first time sequence of
- 5 valve operation.
In the second time sequence of operation adsorbent bed A will
undergo a depressurization phase ~I, recycle phase III and a
depressurization phase ~V; adsorbent bed C, a nitrogen gas
10 pressurization phase Vl, main pressurization phase VII and
adsorption phase ~; and adsorbent bed B evacuation phase V. ~n the
first part of this second time period valve 11 of adsorbent bed A is
open as is valve 50. A first depressurization gas flows from this
adsorbent bed through conduits 26 and 84 to vacuum pump 82 and
15 then via conduit 86 to reservoir 88 or flowed through conduit 85
(appropriately valved) for use as a fuel gas. Valve 34 is open in
adsorbent bed C as is valve 80 with a nitrogen pressurization gas
flowingfrom reservoir 40 through conduits 68, 42, 66, and 62 and
into adsorbent bed C. Adsorbent bed B is being evacuated with valve
20 22 open and gas flowing through conduits 48 and 92 to compressor
90. This is a primary product natural gas.
Adsorbent bed A then enters into a recycle phase with valves
11 and 14 open. 5as flows ~rorn reservoir 74 through conduits, 2,
25 42 and 36 and exits through conduits 26, 84 and 86 to reservoir 88.
Adsorbent bed C undergoes a main pressurization phase VlI with
valve 30 open and gas flowing from reservoir 88 and through
conduit 16. Adsorbent bed B continues on an evacuation phase V.
18
CA 02210340 1997-07-15
~ n the next part of this time sequence adsorbent bed A
undergoes a second depressurization phase IV with valve 11 open
and gas flowing through conduits 26, 84, 78 and 76 to reservoir 74 to
5 be used in recycle phase III. Adsorbent bed C undergoes an
adsorption phase I with feed gas flowing into this bed through
conduit 56 and exiting at conduit 64. This gas flows by conduit 38 to
reservoir 40. DuAng this time ads'orbent bed B continues on the
evacuation phase V.
~ n the third sequence of time adsorbent bed A undergoes an
evacuation phase V; adsorbent bed C a first depressurization phase
II, recycle phase III and a second depressurization phase IV; and
adsorbent bed B a nitrogen (secondary product) pressurization phase
15 Vl, a main pressurization phase VII and an adsorption phase L
Valve 12 is open for adsorbent bed A with a natural gas primary
product flowing through conduits 26, 28 and 92 to vacuum pump 90.
During this time valve 31 is open in adsorbent bed C with a first
depressurization gas flowing through conduits 58, 84 and 86 to
20 reservoir 88. During the period of time valve 24 is open on
adsorbent bed B with a nitrogen secondary product gas flowing from
reservoir 40 through conduits 68, 54 and 51 to this adsorbent bed B.
rn the ni~xt part of thls period of time a~sorberlt bed A
25 continues in an evacuation phase V. Adsorbent bed B undergoes a
recycle phase III with a depressurization gas flowing from reservoir
74 through conduit 72, 42, 66 and 62 and into adsorbent bed C. A
recycle feed gas exits at 31 and passes by conduits 84 and 86 to
19
CA 02210340 1997-07-15
reservoir 88. DuAng this time adsorbent bed C is undergoing a main
pressurization phase VII. Gas flows from reservoir 88 through
conduits 16 and 44 and into adsorbent bed B. In the next part of this
time period adsorbent bed A continues in an evacuation phase V.
S Adsorbent bed C undergoes a second depressurization phase ~V with
valve 31 open and gas flowing through conduits 58, 84, 78 and 76 to
reservoir 74. During this time adsorbent bed B is on an adsorption
phase I with feed gas flowing through conduit 16 to conduits 44 and
46 and into adsorbent bed B. A nitrogen secondary product gas
10 flows from this adsorbent bed through conduits 52 and 38 to
reservoir 40 and to secondary product. This completes a full cycle of
the operation of the process.
As noted, this process can be used to separate carbon dioxide
15 from natural gas. In that case, the carbon dioxide is the primary
product gas and natural gas the secondary product gas. Also, for this
separation the operating pressure will be higher. This will be about
35 to 600 psig while separation of nitrogen from natural gas, the
operating pressure is about 15 to 100 psig.
The phases of the process have been descAbed for a full cycle.
Upon the completion of a full cycle, the cycles are then repeated. In
Figure 3 there is a phase sequencing for a full cycle which consists of
360 seconds. This is a useful tiining. The timiIlg w~l be af~tected by
25 many factors including feed stream composition, adsorbent bed
geometry and adsorbent particle size. This phase sequence is for
three adsorbent beds. Consequently, the time periods are~set at 120
seconds each. In this regard one 120 seconds sequence for each
CA 02210340 1997-07-15
adsorbent bed consist of pressurization and adsorption with a
secondary gas pressurization phase, the main pressurization phase
and adsorption phase. Another for each adsorbent bed is the
depressurization which consists of the first depressurization phase,
5 recycle phase and the second depressurization phase. Yet another is
the evacuation phase which requires the full 120 seconds. The
adsorption part of the sequence can be up to 90 seconds, but usually
will be about 45 to 75 seconds depending on factors such as the more
strongly adsorbed hydrocarbon content of the feed gas. The time
10 allocated for both the secondary gas pressurization phase and the
main pressurization phase will be about 30 to 45 seconds. This will
be divided between these two phases. Adsorption should be
continued until just prior to breakthrough and then the first
depressurization phase of the sequence is initiated. The
15 depressurization segments will be at essentially time of 40 seconds
each.
The adsorbent beds sequentially go through these phase
sequences in the order as set out in Figure 3. ~n Table 1 there is set
20 out the position of the open valves for a full cycle of operation. All
valves not noted as open are closed. The valve numbers are with
reference to the schematic diagram of Figure 2. By reference to
Table 1 and Figure 2 the operation of a full cycle of 360 seconds (or
other ti~.ing) of the ,~r=~s~ure swir.g adsorp~l3n proc~ss can be
2 5 conducted.
The size of each adsorbent bed will depend on the pa~rticular
gas stream, gas flows, hydrocarbon content and other factors such as
21
CA 02210340 1997-07-15
capital cost. The adsorbent beds will range in height of from about 2
to 15 meters or more. The width or diameter of each adsorbent bed
will be from about 1.5 meters to 4 meters or more. E~ch bed will
contain from 1000 pounds to 40,000 pounds of adsorbent. The flow
5 rate of gas through the beds will be in the range of about 300 to
7000 scf/minute depending on the adsorbent bed size.
Standard piping, valves and controllers can be used. The
on/off valves will usually be butterfly valves. Throttle valves and
10 check valves are used where noted. In most instances the system
will be computer controlled, with built in safeguards.
The preferred embodiments of the present hydrocarbon
recovery processes have been disclosed in this specification.
15 However, various modifications can be made to the processes and yet
comprise the present concepts. Such modifications are considered to
be within the present discoveries.
r
Example
A natural gas stream hav'ing a hydrocarbon content of 70
percent by volume and a nitrogen content of 30 % by volume is fed
at a pressure of 50 psia to a pressure swing adsorption system as
shown in Fig. 2 where the adsorbent beas ~ach contain about 4250
25 pounds of activated carbon adsorbent. The cycle timing is as shown
in Fig. 3 with adsorption being 60 seconds, nitrogen pressurization 30
seconds and main pressurization with recycle feed gas and feed gas
is 30 seconds. The feed flow rate is 860 SCFM. The output product
22
CA 02210340 1997-07-15
natural gas is pipeline quality at a purity of 98 percent hydrocarbons
by volume. The product gas is produced at a rate of 590 SCFM. The
pressure swing system operates continuously until the system needs
maintenance.
s
