Transcript pressure
Pressure Swing Adsorption
Group 6 Ryan Hummel, Mike DeGuzman, David Vuong, and Miyoko Ohashi
What is Pressure Swing Adsorption?
• Requires a selective adsorbent • Usually binary separation • Not a continuous process
Skarstrom Cycle
• Published in 1959 • Two beds, four steps • Difference in purity • Improvements on the cycle
General Equations
• Assumptions -process is considered an adiabatic system -gas flow rate is affected by bed height and time -pressure drop can be ignored -negligible mass transfer between gas phase and solid phase
-Mass balances
C i
t
u
C i
z
C
u
z
bulk
q i
t
0 -Energy balances (
g C pg
bulk C s
)
T
t
(
g C pg
)
u
T
z
bulk j n
1
q i
t
H j
0 -Linear driving force model is adopted as the mass transfer equation
q i
t
k i
(
q i
*
q i
)
q i
*
q mi b i P i
1
b n j
1
P j j
• Four cycle process
• PSA unit on the steam reformer supplied to Hickson & Welch, Castleford, England • Hydrogen purification system
• Linde AG, Linde Gas Division, Germany • Hydrogen purification system • Process • capacity
Competing Methods
to PSA
• Other methods used for gas separation include: –
Cryogenics
–
Noncryogenics
Cryogenics
• Cryogenics: – Operated under
low temperature
. (T = 100K – 150K) – This involves liquid to separate gas • Both
temperature
and
pressure controlled
.
are • Difference between
boiling points
is utilized. (Extremely low temperature is necessary for the separation of gases such as air.
Cryogenics
•
Low temperature
achieved by: –
evaporation
of highly volatile liquids or –
expansion
of gases under low pressure
Noncryogenics (Swing Adsorption)
•
Vacuum Swing Adsorption
(
VSA
) • Recovery of methane from coal mine methane gas.
– Lower pressure than PSA (to the vacuum level) – Nitrogen and oxygen were removed 96-98% from the methane-rich gas. – Vacuum is used to evacuate after the product gas is adsorbed. • High purification level • Quick – Regeneration cycle takes seconds.
Noncryogenics (Swing Adsorption)
•
Temperature Swing Adsorption
(
TSA
) – High temperature let the gas to escape. – One cycle takes hours – lots of adsorbents are necessary. – If combined with PSA Pressure Temperature Swing Adsorption (PTSA)
Other Noncryogenics
Product not very pure not good for most industrial uses • Solvent Absorption: – Liquid absorbs the desired gas selectively.
– common in soda manufacturing. • Membrane Separation: – membrane diffusion uses different velocity of gas molecules. – inexpensive method
PSA – gas separation process
• Methods contribute to: •
air separation
– collection of potentially useful gases (
carbon dioxide
and
hydrogen
)
.
•
Removing harmful gases
(
VOCs
(volatile organic compounds),
CO 2
,
NO X
, or SO 2 ).