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A technique known as pressure swing adsorption (PSA) has been employed in a novel way to concentrate a lean amount of helium present in natural gas through selective physical elimination of N2, CO2, CH4 and C+2 (heavier hydrocarbons) in a stepwise cycle sequence at preset time intervals. The PSA-based helium pilot plant consists of four stages, with each stage composed of three parallel adsorber beds (vessels packed with adsorbents). The plant has been designed and operated for purifying helium to a level of better than 99.0 vol% from a feed natural gas containing helium to the tune of 0.06 vol%. The normal feed pressure range is 4–5 bar (abs). The overall recovery of the pilot plant is around 61%. The features of the PSA system are described here with a detailed description of the operating parameters of the helium pilot plant.
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Purification of helium from natural gas by pressure swing adsorption
Nisith K. Das, Hirok Chaudhuri, Rakesh K. Bhandari, Debasis Ghose, Prasanta Sen and Bikash Sinha
A technique known as pressure swing adsorption (PSA) has been employed in a novel way to concentrate a
lean amount of helium present in natural gas through selective physical elimination of N2, CO2, CH4 and C+
(heavier hydrocarbons) in a stepwise cycle sequence at preset time intervals. The PSA-based helium pilot
plant consists of four stages, with each stage composed of three parallel adsorber beds (vessels packed with
adsorbents). The plant has been designed and operated for purifying helium to a level of better than
99.0 vol% from a feed natural gas containing helium to the tune of 0.06 vol%. The normal feed pressure
range is 4–5 bar (abs). The overall recovery of the pilot plant is around 61%. The features of the PSA system
are described here with a detailed description of the operating parameters of the helium pilot plant.
Large-scale extraction of helium is con-
ventionally realized from helium-bearing
natural gas existing in selected geographi-
cal locations across the world. Natural
gas is made up of a composite mixture of
diverse gaseous components, including a
substantial level of heavy hydrocarbons
apart from methane, nitrogen and helium
in variable amounts. A two-stage pres-
sure swing adsorption (PSA) process was
earlier developed by Knaebel and Rein-
hold1, and D’Amico et al.2. Their feed
gas contained about 2–4 vol% helium and
70 vol% nitrogen and concentrated helium
to a level of greater than 98 vol% from
the feed natural gas. The process involved
two stages of PSA used in series that
sequentially undergo a seven-step cyclic
separation process.
Helium stands out to be indispensable
in frontier technologies involving space,
atomic energy, defence, power, medicine,
welding and in many advanced research
activities, including fusion and behaviour
of materials at very low temperatures. In
view of any unforeseen scarcity in this
strategic element (even today India im-
ports almost 100% pure helium for its
domestic consumption), a stand-by meas-
ure for indigenous sources of helium is
deemed worthwhile in the country.
In India, effort towards exploration of
helium from different sources has led to
discovering helium, in small amounts, in
natural gas discharged through the natu-
ral gas wells in GCS Kuthalum, Nagapat-
tinam District, Tamil Nadu. Besides the
conventional way of helium recovery utili-
zing cryogenic separation which is an en-
ergy-expensive process, a non-cryogenic
helium purification system has been deve-
loped, with the help of Adsorption
Research Inc., Dublin, Ohio, USA, to
separate out helium from natural gas
stream by means of the PSA process. The
helium plant based on PSA produces a
constant stream of helium having a con-
centration of around 99.0 vol% from
natural gas feed stream bearing helium
content to the tune of 0.06 vol%, with an
overall recovery rate around 61%.
The developed PSA system for helium
purification involves three distinct com-
ponents: (a) pretreatment of feed gas, (b)
recovery of methane (CH4) and (c) sepa-
ration of helium. The multi-component
gas available from wellheads of the natu-
ral gas field at GCS Kuthalam, predomi-
nantly contains methane besides heavy
hydrocarbons, CO2, nitrogen and helium.
The raw natural gas is collected in the
gas storage vessel at a pressure of 16 bar.
The gas leading to the pilot plant is taken
at a reduced pressure of 4–5 bar and is
routed through a refrigerated gas dryer to
remove the bulk moisture content present
in the feed stream, and fed into the sys-
tem at about a feed pressure of 4.5 bar. A
preliminary conceptual process of the
plant was presented in an international
seminar3. Unlike in the previous works1,2,
where the feed gas contains about 2–
4 vol% helium, the present feed stream
contains helium approximately 0.06 vol%,
which makes the system more challeng-
ing and involves four stages instead of
Process description
The pilot plant has four stages, with each
stage consisting of three adsorber beds.
The complete system comprises of a net-
work of piping, valves with associated
instrumentation and control equipment
that are integrated with the plant. The
number of adsorber beds used is an eco-
nomic balance between the capital cost
of the pilot plant and the desired product
quality. Stage I is adopted to remove the
heavier hydrocarbons and carbon dioxide
from the feed stream allowing methane,
nitrogen and helium to flow and continue
onto stage II. In stage I, the heavy com-
ponents include carbon dioxide (CO2),
ethane (C2H6), propane (C3H8) and butane
(C4H10) and are preferentially retained on
the adsorbent, while the less adsorbable
effluent gas comes out through the exit
end. Stage II adsorbs and rejects methane
as the heavy product, allowing nitrogen
and helium to flow and move onto stage
III. The methane-rich heavy product from
stage II is partially recycled for use in
stage I as purge gas for the purpose of
regeneration. Methane of high purity
from the waste gas is recovered as the
secondary product in this stage. Stage III
adsorbs and removes most of the nitro-
gen and allows a helium-rich mixture to
pass through and advance to stage IV.
The nitrogen-rich heavy product repre-
sents the third and final waste stream.
Stage IV operates similar to the stage III;
it adsorbs and cast-offs residual nitrogen,
allowing purified helium to pass through
as the final light product. The heavy
product (which is a mixture of nitrogen
and a little helium) from stage IV is
compressed and optionally recycled to
the inlet of stage III for the purpose of
boosting the overall helium recovery.
The entire process is controlled by
microprocessor based logic controllers
(PLCs). Figures 1–3 show some of the
components of the helium pilot plant. In
the present case, the absorbents used are
silica gel in stage I, activated carbon in
stage II, and zeolites in stages III and IV.
The adsorption system operates by a
sequence of steps, called a PSA cycle.
Each stage in the purifying system has an
associated cycle. The adsorber beds in
each stage sequentially undergo the fol-
CURRENT SCIENCE, VOL. 95, NO. 12, 25 DECEMBER 2008 1685
Figure 1. Network of adsorber vessels in stages I and II.
Figure 2. Partial view of stage II showing the methane compressor, vacuum pumps,
flow controller, etc.
Figure 3. Stages III and IV.
lowing seven steps: I, Adsorption (Fd);
II, Co-current depressurization for pres-
sure equalization (PEd); III, Blow down
to atmospheric pressure (Bd); IV, Eva-
cuation to the lowest pressure (Ev); V,
Purge with product at the lowest pressure
(Pu); VI, Counter-current pressurization
by pressure equalization (PEu), and VII,
Re-pressurization with product to a feed
pressure (Rp).
Adsorption primarily occurs during the
feed step. When the more strongly ad-
sorbed components reach the top of the
adsorber bed, and are about to contami-
nate the product, gas feed flow is discon-
tinued and switched to the second adsorber
bed. The first adsorber bed then under-
goes regeneration by depressurization
(called the blow down and evacuation
step) followed by the purge step. During
the purge step, the enriched heavy com-
ponent(s) are desorbed from the adsorber
bed at low pressure by admitting the
light component(s), instead of relying on
evacuation (vacuum) alone. These steps
induce desorption and release of the
‘heavy’ product. The heavy product will
contain both of the more adsorbable
components, which are enriched (relative
to the feed) by the adsorber bed, and
some of the less strongly adsorbed com-
ponents released from the voids. The
pressurization step, in which light prod-
uct is used to repressurize and adsorb,
can be thought of as regeneration, since
it tends to displace the more strongly ad-
sorbed constituents as it enters the
adsorber bed. It can also be viewed as an
adsorption step, since as pressure in-
creases, the light components also get
adsorbed to a greater extent. Pressuriza-
tion with feed causes some of it to be ad-
sorbed, while some feed remains in the
gas-phase to increase the column pres-
sure. The process flow diagram showing
the schematic flowsheet of P & I diagram
of each stage is given in Figure 4. Stages
I and III include equalization steps, during
which one adsorber bed depressurizes
into a parallel adsorber bed, partially pre-
ssurizing it. There are two equalization
steps per adsorber per cycle, because
each adsorber bed releases gas to one
adsorber bed and receives gas from an-
other adsorber bed. For example, stage I
delivers gas to stage III, but stage I also
receives gas from stage II.
Results and discussions
Figure 5 presents the chromatogram of
the composition of the feed natural gas at
Table 1. Composition of feed and product gases at respective stages of the pressure swing adsorp-
tion (PSA) helium pilot plant
Product of Product of Product of
Feed of stage I/feed of stage II/feed of stage III/feed of Product of
Composition stage I vol% stage II vol% stage III vol% stage IV vol% stage IV vol%
He 0.06 0.08 1.40 15.65 99.00
N2 1.18 1.27 16.10 72.03 1.00
CH4 88.50 98.42 82.50 12.32 0.00
CO2 0.40 0.00 0.00 0.00 0.00
2 9.86 0.23 0.00 0.00 0.00
Table 2. Operating parameters of four stages of PSA helium pilot plant
Parameter Stage I Stage II Stage III Stage IV
Feed pressure (bar) 4.50 4.50 2.25 2.00
Feed flow rate (slpm) 820.00 600.00 60.00 4.90
Product pressure (bar) 4.50 2.25 2.00 1.50
Product flow rate (slpm) 600.00 60.00 4.90 0.90
Regeneration pressure (bar) 1.3 (blowdown) 1.25 (blowdown) 1.1 (blowdown) 1.1 (blowdown)
75 m bar (evacuation) 75 m bar (evacuation) 60 m bar (evacuation) 60 m bar (evacuation)
Purge amount (slpm) 81 425 (rinse flow rate) 0.6 0.5
Helium in feed (vol%) 0.06 0.08 1.40 15.65
Helium in product (vol%) 0.08 1.40 15.65 99.00
PSA cycle time (min) (three-bed cycle) 24.00 4.50 6.75 36.00
Bed composition (kg/bed) Silica gel (~198) Activated carbon (~38) Zeolite 13 X (~3) Zeolite 13 X (~3)
Column dimensions (H, Height (cm); H = 129.54 H = 104.14 H = 152.40 H = 152.40
OD, Outer diameter (cm)) OD = 50.80 OD = 30.48 OD = 6.35 OD = 6.35
the GCS Kuthalam site. It is observed
that the major constituent of the feed
stream is methane, while helium content
is as low as 0.06 vol%. The composition
of the feed and product gases in four
stages of the PSA are shown in Table 1.
Figure 4. Schematic flowsheet of P&I diagram of each stage.
Figure 5. Feed gas composition.
Figure 6. Composition of the stage I
product gas as also feed of stage II.
Figure 7. Composition of the stage II
product gas as also feed of stage III.
CURRENT SCIENCE, VOL. 95, NO. 12, 25 DECEMBER 2008 1687
Figure 6 shows the product gas composi-
tion of stage I after removal of majority
of the heavy hydrocarbons and carbon
dioxide. This product is primarily meth-
ane with marginal increase in helium and
nitrogen concentrations, and it is also the
feed gas of the stage II. Figure 7 displays
the product gas composition of the stage
II as also the input gas of stage III. Fig-
ure 8 presents the product gas of stage
III, which is also the feed gas of stage
IV. The output gas of this stage III con-
sists of nitrogen and helium, accompany-
ing a small quantity of methane. Figure 9
shows the final product quality of stage
IV. Any residual methane is completely
removed from the gas stream and almost
all the nitrogen is eliminated, essentially
leaving purified helium as the yield of
stage IV. Table 2 provides the operating
parameters of the four stages of the PSA
helium pilot plant. The plant intake is a
constant feed of 50 NM3/h of natural gas
containing 0.06 vol% helium. The output
is a constant stream of helium (>99.0 vol%)
at approximately (18 l/h). Practical im-
plementation of this new system brings
out the fact that loss of helium in stages
II and III needs to be minimized through
more precise cycle times and further fine
tuning to optimize the pressure ratios.
The helium recovery from the four stages,
as observed, are: stage I ~97%, stage-II
~87%, stage-III ~81% and stage IV ~87%.
The overall helium recovery from the pi-
lot plant is ~61%.
The helium pilot plant presented above
was installed at the GCS Kuthalum site
during 2007 and commissioned in early
2008. The plant was formally inaugu-
rated and dedicated to the nation on 11
May 2008.
To the best of our knowledge it has been
demonstrated for the first time that, al-
though on a pilot scale, employing the
non-cryogenic PSA technique, helium
has been purified to a level better than
99.0 vol% starting from a level as low as
0.06 vol% present in the feed natural gas.
The system is completely automated and
separation has been achieved at room
temperature. This enables considerable
savings in energy and manpower com-
pared to the low-temperature separation
process. We are of the opinion that ex-
ploration of existing natural gas reserves
in India, on a commercial scale, would
be able to meet the requirement for the
domestic consumption of pure helium
through the scaling-up of such pilot
1. Knaebel, K. S. and Reinhold, H., Proceed-
ings of Fundamental of Adsorption (ed.
Meunier, F.), Elsevier, 1998, pp. 763–767.
2. D’Amico, J. S., Reinhold, H. and Knaebel,
K. S., US Patent No. 5542966 dated 6 Au-
gust 1996.
3. Das, N. K., Bhandari, R. K., Sen, P. and
Sinha, B., Proceedings of International
Seminar on Gas Technology, Kolkata, 19–
20 November 2004, pp. 252–256.
ledge the financial and infrastructural support
of ONGC, GCS Kuthalam, Tamil Nadu and
thank the Department of Atomic Energy, and
the Department of Science and Technology,
Government of India for sponsoring the pro-
Received 11 June 2008; revised accepted 20
November 2008
Nisith K. Das, Rakesh K. Bhandari and
Bikash Sinha are in the Variable Energy
Cyclotron Centre, 1/AF, Bidhannagar,
Kolkata 700 064, India; Hirok Chaud-
huri, Debasis Ghose, Prasanta Sen* and
Bikash Sinha are in Saha Institute of Nu-
clear Physics, 1/AF, Bidhannagar, Kol-
kata 700 064, India.
Figure 8. Composition of the stage III product gas as also feed of stage IV.
Figure 9. Stage IV product gas composition.
... Helium's other industrial uses-as a pressurizing and purge gas, as a protective atmosphere for arc welding and in processes such as growing crystals to make silicon wafers-account for half of the gas produced. A well-known but minor use is as a lifting gas in balloons and airships [4] . On Earth it is relatively rare 5.2 ppm by volume in the atmosphere. ...
... In 2008, Nisith Kr. Das [4] [5] reported on the low temperature adsorption properties of He, H2, and N2, using three activated carbons with different pore size distributions. In 2012, R. ...
Full-text available
A two-bed pressure swing adsorption system on a commercial type of zeolite 13X adsorbent has been studied numerically over a wide range of operating conditions to helium separation from gaseous mixture. The model includes energy, mass and momentum balances. The coupled partial differential equations are solved using fully implicit forth order Rung-Kutta scheme in the simulation. The effects of adsorption step pressure, adsorption step time and feed flow rate on the helium purity and recovery were investigated. Results shown that as the adsorption step pressure increases the helium purity will be increased. In addition, the helium recovery increases, and the helium purity decreases when the feed flow rate increases. Finally, the simulation results indicated a very good agreement with some current literature experimentally work.
... However, this PVSA system required an adsorption/desorption pressure ratio of 34.5 to achieve such a separation performance. Das et al. [15] also reported direct helium recovery from raw natural gas entirely by PVSA. They used a pilot-scale, four-stage PVSA unit to produce a high-purity helium product from a 0.06 mol% He natural gas feed at 820 SLPM. ...
Global demand for helium has risen over the past few decades driven by a vast range of applications based on its unique properties. Helium is often produced industrially from the vent streams of nitrogen rejection units (NRU) in liquefied natural gas (LNG) plants, where it accumulates because of its low boiling point. Further deep cryogenic processing stages are then required to upgrade its concentration to saleable levels. We report here an experimental and simulation-based investigation of an alternative, non-cryogenic process based on dual reflux pressure swing adsorption (DR PSA) cycles to recover and purify helium from model binaries including a 1 mol% He + 99 mol% N2 mixture representative of NRU vent streams. Binderfree zeolite 13X was used as the adsorbent in an experimental campaign employing a laboratory-scale DR PSA apparatus. The experimental runs validated a non-isothermal numerical model with an average deviation of 1 mol% between the simulated and experimental product compositions. The single-stage DR PSA experiments produced helium product streams with purities ranging from (30 to 99) mol% from feeds with concentrations ranging from (1 to 50) mol% He. Two DR PSA stages were required to upgrade mixtures containing 1 mol% He to a target product purity of >99 mol%. A cascade arrangement of two DR PSA systems with a waste recycle enabled a 99.999 mol% He purity product at 95 % recovery, which is competitive with conventional cryogenic systems. This cascade DR PSA process used either 2 or 3 compressors depending on the feed gas pressure, with associated duty costs of 1.5 MJ∙(mol He produced)–1. This is lower than several membrane processes described in the literature and could be more cost-effective for smaller-scale applications than cryogenic processes with similar levels of separation performance.
... 14 However, none of the former research and patents have used natural gas feed composition for helium purification, as mentioned. Das et al. 15 published the only reference in open sources more similar to the real conditions, in which it was reported that, in a pilot scale plant, helium was purified from natural gas to a high concentration of 99 (vol %) through only the adsorption process. In their work, a pretreated demoisturized natural gas stream containing He:CH 4 :C 2 + :N 2 :CO 2 with a composition of 0.06:88.5:9.86:1.18:0.4 ...
The conventional method of helium production is a cryogenic process, which is installed downstream of a liquefied natural gas (LNG) plant, where a helium extraction unit (HeXU) would be usually developed as the byproduct unit, due to the high cost of the process. In this study, pressure swing adsorption method accompanied by a relative vacuum condition for the bed regeneration (PVSA) was investigated as an alternative method for recovery of helium from natural gas source. The PVSA design was studied for a continuous product, utilizing zeolite 13X as the adsorbent, through a formerly validated model, for a dilute feed of 1.5 vol % He, mainly comprised of methane in 82.5 vol % accompanied by nitrogen. The impacts of repressurizing medium and equalization steps of the design conditions on the He purity and recovery were investigated. It was revealed that, generally, the bed repressurizing step should be performed by the feed as compared to the product. However, at high space velocities, a combination of the feed and product would be preferable to achieve higher He purity. Adding an equalization step is not advantageous to the process design at medium to high space velocities, but can be exploited only for recovery improvement when the purity is in an acceptable high range at low space velocities. Feasibility of the adsorption method and its performance was then analyzed in a basic 4bed-4step PVSA cycle design for different effective parameters including dimensionless space velocity (), bed pressure in adsorption step, and vacuum pressure in regeneration step and their interactions. However, due to the complex trends of the effective parameters and their interactions, response surface methodology (RSM) was implemented for optimization of the cycle performance. At the optimal design point with a feed pressure of 5.75 (bara), regeneration pressure of 0.05 (bara), and () of 2.25, a helium purity of 99.9% and a recovery of 70% with productivity of 0.335 (mol/kg-adsorbent·h) would be achieved.
... −6 %) 36 in comparison to its heavier isotope, and its extraction from natural gas is usually done by means of expensive cryogenic distillation and pressure-swing adsorption methods. 37 An alternate and more energy-efficient process is to use the 2D porous membranes in isotopic gas separation since they usually do not involve costly liquefaction of the gases. 38 and references mentioned therein. ...
Full-text available
Microscopic-level understanding of the separation mechanism for two-dimensional (2D) membranes is an active area of research due to potential implications of this class of membranes for various technological processes. Helium (He) purification from the natural resources is of particular interest due to the shortfall in its production. In this work, we applied the ring polymer molecular dynamics (RPMD) method to graphdiyne (Gr2) and graphtriyne (Gr3) 2D membranes having variable pore sizes for the separation of He isotopes. We found that the transmission rate through Gr3 is many orders of magnitude greater than Gr2. The selectivity of either isotope at low temperatures is a consequence of a delicate balance between the zero-point energy effect and tunneling of 4He and 3He. RPMD provides an efficient approach for studying the separation of He isotopes, taking into account quantum effects of light nuclei motions at low temperatures, which classical methods fail to capture.
... Two of most crucial gas fields in USA are Hugoton Panhandle Field, in parts of Oklahoma, Texas and Kansas and LaBarge gas plant located in Wyoming [9]. The idea of helium separation from atmospheric air is only proposed under the intense conditions given the concentration is 5.2 ppm [10]. ...
Preponderance of membranes have Helium (He) selectivity which causes their gas separation has a potential for helium revival and purification. This review conveys the recent investigations and patent reports for membranes embarking on separation of He. This involves direct retrieval from natural gas which is an auxiliary stage in natural gas processing, alongside niche applications where helium recycling is capable. A summary of the available literatures on polymeric and inorganic membranes for helium separation is presented. In comparable gas industries, discussion on commercial gas separation membranes is with regard to their capacity for helium separation. This paper includes the assorted patented designs of helium recovery and purification by membranes from variety of sources, considering that these exhibit the viability of current available polymeric membranes to separate helium. The review particularly emphasizes processes in which membranes are utilized together with other available separation technology such as pressure swing adsorption (PSA). The combination of technology able to generate high-purity helium gas. This paper also aims to demonstrate the viability of membrane separation process for helium recovery and purification. Current process is focusing on reusing helium instead of separation from its raw sources.
Low-cost helium recovery processes optimised for unconventional sources e.g. small-scale natural gas (NG) reservoirs, offer a new approach to meeting growing demand. We investigated a two-stage dual reflux pressure swing adsorption (DR PSA) cascade for the recovery and purification of dilute helium from various natural gas source analogues comprising of CH4 + N2 + He. First, adsorption isotherms of a binderless zeolite 13X with high methane and nitrogen uptake were measured at 283 to 323 K and pressures up to 10 bar. This material was then used in a laboratory-scale DR PSA apparatus to identify key parameters affecting the separation performance, namely feed step time and location, as well as heavy product and light reflux flow rate. The experiments agreed well with the results of a non-isothermal numerical DR PSA model with an average deviation of 1 mol% between simulated and experimental product purities. Then, the model was extended to consider a two-stage DR PSA cascade with waste recycle. Axial composition profiles within the beds of each stage were obtained numerically and analysed to explain the impact of key process parameters on the separation performance. Tuning these parameters accordingly led to a helium-rich product with over 99 mol% He purity, which could be produced from various natural gas feeds containing from 0.1 to 1 mol% He at 90 to 98% recovery, respectively. The separation performance was compared to other helium recovery processes based on cryogenic distillation, membranes, pressure swing adsorption, and combinations thereof. Compared to these other systems, the DR PSA cascade process delivered a 99 mol% purity product from a 0.1 mol% helium in natural gas feed with 8% higher helium recovery.
Extraction of helium from natural, carbon dioxide and nitrogen rich gas streams is seemingly a simple process involving cryogenics and/or pressure swing adsorption methods. These facilities are typically very expensive. The extraction methods are the major economic cost for any helium project. The simplest gas composition for economic helium extraction is the element is associated with a pure nitrogen gas stream. The nitrogen gas waste from processing, can be released into the atmosphere with no environmental impact. Complex processing is required when carbon dioxide and methane are present requiring disposal or sale of one or both gas streams. Carbon dioxide and methane can no longer be vented into the atmosphere because of environmental concerns. Methane is typically sold into a pipeline but in some areas no pipeline maybe available complicating economics and waste disposal. Waste carbon dioxide in certain areas can be used for enhanced oil recovery but typically has to be disposed of properly. Carbon dioxide in some areas is the main product and helium is actually a waste or secondary product. Hydrogen sulfide, trace metals also present need to be removed and adds to the cost of extracting helium from any gas stream. The cost and potential complexity of any helium extraction facility, the uncertainty of the actual reserves of the helium deposit, prior to production, always present an major economic risk and requires careful evaluation.
Full-text available
Increasing helium (He) demand in fundamental research, medical, and industrial processes necessitates efficient He purification from natural gas. However, most theoretically available membranes focus on the separation of two or three kinds of gas molecules with He and the underlying separation mechanism is not yet well understood. Using molecular dynamic (MD) and first-principle density function theory (DFT) simulations, we systematically demonstrated a novel porous carbon nitride membrane (g-C9N7) with superior performance for He separation from natural gas. The structure of g-C9N7 monolayer was optimized first, and the calculatedcohesive energy confirmed its structural stability. Increasing temperature from 200 to 500 K, the g-C9N7membrane revealed high He permeability, as high as 1.48×10⁷ GPU (gas permeation unit, 1 GPU = 3.35 × 10⁻¹⁰ mol∙s⁻¹∙Pa⁻¹∙m⁻²) at 298 K, and also exhibited high selectivity for He over other gases (Ar, N2, CO2, CH4, and H2S). Then, the selectivity of He over Ne was found to decrease with increasing the total number of He and Ne molecules, and to increase with increasing He to Ne ratio. More interestingly, a tunable He separation performance can be achieved by introducing strain during membrane separation. Under the condition of 7.5% compressive strain, the g-C9N7 membrane reached the highest He over Ne selectivity of 9.41×10². It can be attributed to the low energy barrier for He, but increased energy barrier for other gases passing through the membrane, which was subject to a compressive strain. These results offer important insights into He purification using g-C9N7 membrane and opened a promising avenue for the screening of industrial grade gas separation with strain engineering.
Porous two-dimensional materials are potential candidates for the realization of gas separation, which are crucial for developing clean energy sources. Here, we found that CrI3 monolayer which has uniform nanopores around 2.9Å in intrinsic structures can simultaneously fulfill the requirements of both high permeability and high selectivity. Our results show that the diffusion energy barriers of H2 and impurity gases (N2, CO, CO2, H2O and CH4) passing through the nanopores of CrI3 monolayer are 0.27eV, 1.49eV, 1.31eV, 1.54eV, 0.87eV and 3.88eV, respectively, which indicates that these impurity gases hardly pass through the CrI3 monolayer and H2 can be effectively separated from the gases mixture. Also, at the temperature of 300K, the permeability of H2 (1×10⁻⁵ mol·m⁻²·s⁻¹·pa⁻¹) is significantly larger than the minimum acceptable permeability for industry (6.7×10⁻⁹ mol·m⁻²·s⁻¹·pa⁻¹). Thus, the CrI3 monolayer is a promised candidate of the hydrogen purification for the future applications.
Vacuum pressure swing adsorption process was developed for light hydrocarbons recovery from natural gas in this study. Silica gel has been selected as adsorbents used in adsorption process because of its appropriate adsorption capacity and easier regeneration. Three kinds of cycle configurations were developed and evaluated for the extraction of light hydrocarbons from low pressure natural gas on the basis of six bed VPSA process, meanwhile effects of operating parameters on VPSA process performances were investigated. Results demonstrated that C-VPSA process with co-current blowdown step could be able to improve the enrichment ratio of light hydrocarbons in heavy product stream significantly in comparison with B-VPSA process only with counter-current blowdown step. Further enhancing in enrichment ratio of light hydrocarbons in heavy product stream was capable to be achieved by R-VPSA process with heavy product stream reflux, but its energy consumption for light hydrocarbons recovery was much higher than that of C-VPSA process and B-VPSA process. Therefore, the C-VPSA process has been considered to be a good option for light hydrocarbons recovery, in which the best case demonstrated that the enrichment ratio of light hydrocarbons in heavy product stream could achieve 5.06 without sacrifice of quality of light product stream. Moreover, the energy consumption of C-VPSA process was 4.93 kJ per mole light hydrocarbons, which was one-third of energy consumption of R-VPSA process.
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