8/17/2019 Dr. Sultan Al-Salem
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Presented by :
Dr. Sultan Al-Salem
Petroleum Research CenterKuwait Institute for Scientific Research
Modelling Pressure Swing Adsorption
to Recover Hydrogen from RefineryOff-Gases/Steam Reformer Feeds in
Comparison to Cryogenic Technique(CT)
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Agenda• Introductory Note:
• Program Development within the Petroleum Research Center
• Motivation & Objectives
• Overall Plan & Implementation
• Problem Statement: How to devise a mathematical solution!!
• Methods & Implementation
• Overview & System Studied
• Model Implementation
• PSA System ‘first and second trials’ • Cryogenic separation
• Past experiences with MT separation
• Recommendations
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Motivation: Message of PRC-KISR
Within the Petroleum Research Center (PRC) of KISR, the RCEF program is set to meet thefollowing challenge in the petroleum industry:
“To supp ort the oi l indu stry in developing new pro duc ts, new technolog ies and extending
the throughp ut and f lex ib i l i ty of petro leum ref in ing activities”
Solution Areas
Improving the operability of selected refineryprocesses
Database of the molecular composition of refinerystreams
Utilization of refinery waste streams
1. Utilizing ROGs
2. Hydrogen Management3. MT Implementation
4. CCS
5. Energy Efficiency Solns.
6. LESP
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Background
• Refinery off-gases (ROGs) are by-product gases produced in an oil refinery typicallyused as a fuel gas or as a stand-alone product.
• ROGs encompass somewhat high HVs.
• Hydrogen gas enrichment and separation is
Typically undertaken via three main techniques:
1) Pressure Swing Adsorption (PSA)
2) Membrane Separation; &
3) Cryogenic (cryo-system) separation.
Table 1: Main Components in the ROGs
Component Mole% Component Mole%
Hydrogen 10 – 50 Ethane 5 – 20
Hydrogen Sulfide 0.1 – 3 Propane 1 – 6
Nitrogen 2 – 10 Butane 0.5 - 1
Methane 30 – 55 Pentane + 0.2 - 2
Hydrogen is extensively used in HDT processes for increasing the hydrogen to carbon (H/C)
ratio of products (hydrocracking) and for the hydropurification treatments, amongst other
processes crucial in Kuwait.
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Problem Statement
• Hydrogen gas is the main utility of refineries considered heart and sole, hence utilizing every
stream of it is essential for refining flexibility and expansion.
• ROGs contain a hefty proportion of hydrogen, hence trying to separate this fraction is of
paramount importance.
• Reliance on heavier crudes nowadays means optimizing the carbon emissions from
process, whilst using more hydrogen.
• Hydrogen production processes intensify the carbon emissions. HENCE, need for optimal
refinery studies that will utilize every fraction of H2 gas available.
Aim of the Work• In this work, a ROG stream is mathematically modeled with the aim of recovering hydrogen
gas using a PSA unit and CT system.
• Sensitivity analysis shows the strong points.
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The Studied Case:
Focus on the H2 recovery unitUnit objective:
Recover high purity H2 from the desulfurized high pressure H2 rich gases obtained from the
refinery.
The unit operates an H2S absorber designed to handle 92 MMSFCD of high pressure sour
gas with a pressure swing adsorption (PSA) unit capable to handle 89.4 MMSFCD of
desulfurized high pressure H2 rich gas.
H2 rich gas from a number of units in the refinery are passed through a ADIP solution
absorber, in order to, remove H2S. After which, the gas is sent to the PSA unit.
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The Studied Case:
Focus on the H2 recovery unit Tail gas was modeled via the separation techniques.
Components of the Modeled gaseous stream.
Composition Mole%
Hydrogen
Ammonia
Methane
Ethane
Propane
Butane
Nitrogen
29.13
9.48
50.2
5.46
3.84
1.57
0.32Schematic flow diagram of hydrogen recovery unit (i.e.
PSA) in MAA showing tail gas processing in the
refinery’s configuration.
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H2S
Absorption
(ADIP)
H2 Rich Sour Gas
Sweet Gas
Compression
P
S
A
Product
Hydrogen
Tail GasCompression
To Fuel Gas
H2 To Users
Refinery H2 Recovery Unit
Configuration.
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Technique #1
- Pressure Swing Adsorption -
• PSA is a cyclic process based on the effect of
pressure changes in fixed beds for selective
adsorption.
• PSA beds are loaded with an adsorbent to separate
the desired gas
• Zeolite was considered in this work.
• The separation consists of four consecutive steps:
recompression, adsorption, desorption and blow
down.
i1i2
i3
i4
i5
i6
i7
i8
i9i10
L (m)
F e e d
D (m)
yi (mol)
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The following assumptions were made to develop mathematical expression:
Due to high concentration of H2 gas and methane (CH4) gas, the ROG was assumed to be a binary mixture
of H2 and CH4.
The mixture behaves as an ideal gas.
Axial pressure gradients and dispersion are negligible.
A linear driving force model was assumed for the fixed bed.
The process as a whole was assumed to be isothermal due to low adsorption heat.
A total cycle time (tend) of 600 s was assumed, and a bed length (L) and diameter of 9 m and 0.5 m were
selected, respectively, simulating industrial conditions.
Technique #1
- Pressure Swing Adsorption - Freundlich isotherm
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Technique #1 - Pressure Swing Adsorption - Freundlich isotherm
Starting with the equilibrium equation for fixed beds:
The superficial gas velocity (u, m/s) and isotherm equation are as follows:
C avg A
F
y = 3.4509x0.2404 R² = 0.9883
y = 3.1238x0.2594 R² = 0.9838
y = 3.5962x0.2391 R² = 0.9698
1
1.5
2
2.5
3
3.5
4
0 0.2 0.4 0.6 0.8 1 1.2 A d s o r b e d H y d r o g e n
o n Z e o l i t e ( w t % )
H2 mol fraction
CHA 298K CHA 350K CHA 273K
Where qi is amount adsorbed in the solid phase (kg H2/kg
adsorbent), t is dimensionless time (ti/tend), z is the
dimensionless axial distance (z j/L) and L is the length of
bed (m).
A, B and C are constants and are defined as follows:
m
i
y K q
Two boundary conditions were defined to solve the model for
time (i) and position (j).
Dt is the difference between the two dimensionless times
corresponding to each isotherm and is equal to 0.1;
Dz is the difference in the axial distance corresponding to each of thetwenty isotherms and is equal to 0.5.
The successive concentration values are evaluated using explicit
method at new position as function of all known concentrations at
previous time step according to the following expression.
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• Recovery issues w/model
• Laplace transform solution.
• Design parameters effect.
Second trial0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0 0 . 5 1
1 . 5 2
2 . 5 3
3 . 5 4
4 . 5 5
5 . 5 6
6 . 5 7
7 . 5 8
8 . 5 9
tow = 0
tow = 0.1tow = 0.2
tow = 0.3
tow = 0.4
tow = 0.5
tow = 0.6
tow = 0.7
tow = 0.8tow = 0.9
tow = 1
-0.05
0.05
0.15
0.25
0.35
0 0 . 5 1
1 . 5 2
2 . 5 3
3 . 5 4
4 . 5 5
5 . 5 6
6 . 5 7
7 . 5 8
8 . 5 9
H 2 C o n c . ( m o l )
No. Transfer Unit
0.25-0.35
0.15-
0.25
0.05-0.15
-0.05-0.05
y = 0.0045x - 0.0015R² = 0.9999
0
0.005
0.01
0.015
0.02
0.025
0.03
0.035
0.04
0.045
0.05
4 9 14 N o n - r e c o v e r e d H 2
f r a c t i o n ( m o l )
Adsorption Bed Length (m)
Technique #1 - Pressure Swing Adsorption - Freundlich isotherm
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Technique #1
- Pressure Swing Adsorption – Langmuir isotherm
Where Dt is the cycle time used at
position z, and Dz is the difference in
the axial distance corresponding toeach of the ten theoretical equivalent
height trays and is equal to 0.9.
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0
10
20
30
40
50
60
70
80
90
100
0 20 40 60
H y d r o g e n R e c o v e r y ( % )
Inlet Feed Pressure (bar)
Technique #1 - Pressure Swing Adsorption – Langmuir isotherm
Hydrogen recovery as a function of inlet ROG feedpressure [D=0.5 m, F=5800 kg/hr].
55
60
65
70
7580
85
90
95
100
0 200 400 600 800 1000 H y d r o g e n R e c o v e
r y ( % )
ROG Stream Temp. (K)
50 bar
10 bar
Gas temperature (K) effect on
hydrogen recovery (%) for
ROG flow rate of 5800 kg/hr:
Feed pressure 50-10 bar
0
20
40
60
80
100
0 1 2 3 4 5 6 7 8 9 10
H y d r o g e n R e c o v e r y ( %
)
No. trays
1 bar 5 bar
10 bar 20 bar
30 bar 40 bar
50 bar
Effect of number of trays and feed pressure on
hydrogen recovery.
• Increase in Pf
results in increase
with %R.
• The rate of flow
entering the columnwas found to be of no
significance
• Highest recovery is obtained for a column length of 9
meters at 50 bar.
• Adsorption temperature immensely affects the hydrogen
recovery due to isotherm dependency on the temperature at
which this surface phenomena occurs.
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0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0
100
200
300
400
500
600
700
0 20 40 60 80 100 F i n a l h y d r o g e n c o n c e n t r a t i o n
( m o l / L )
A d s o r p t i o n c y c l e t i m
e ( s )
Hydrogen recovery (%)
10 bar 30 bar 50 bar
• As the number of trays
increase in PSA, recovery
of hydrogen gas is
increasing due to increase
in residence time with the
zeolite bed.
• Increasing the number of
theoretical trays above ten
doesn’t affect the model
results significantly.
• Inlet feed pressures between 40 and 50 bar show that a plateau of recovery is reached.
• However, final concentration of hydrogen existing the column is inter-related with both the
pressure (due to dependency on isotherm type) and the length of the column (see Fig).
Effect on hydrogen recovery (%) by cycle time (s) and final hydrogen concentration (Cf ).
Note: Same symbols indicate similar pressure (bar) for both axis.
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• The ROG feed pressure, which one might argue as the main influencing parameter
in such units, showed highest recovery of hydrogen (95-96%) at pressure between
40 to 50 bar.
• This agreeable with the conditions in this work since the ROG stream originates
from a compression stage to a fired heater that is of similar conditions.
• Nonetheless, results obtained by a past study were validated with high accuracy in
this work.
Technique #1
- Pressure Swing Adsorption – Remarks
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Technique #2
- Cryogenic Separation -First stage ‘compression’
02
2
S EC
ou t ou t
c
z
c
in
in
c
z
c
W W Qm g
g
g
u H
m g
g
g
u H
H W D dT C dP V P dT C ndP P nRT
P
)(2/))(2/())(()/ln(
/)/ln(
2
1
2
2
2
1
2
21212
3
12
2
T T DT T C T T BT T A
dT DT CT BT A P P R
LHS T T DT T C T T BT T A y Min F O N
i
i
)(2/))(2/())(()/ln(100..
2
1
2
2
2
1
2
21212
1
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Technique #2 - Cryogenic Separation -
Second stage ‘Heat Exchanger’
Applying the Antoine equation
ii T C
B
A P
)log(
i pi
T C
B A Min P OF
Third stage ‘Separation Valve’
V y L x F z Objective function of (1 - )
N
i
N
i y x /
A three cycle process was estimated that operate with the following temperatures after solving
the problems at hand (oC): 70 (10 bar), 25 (8 bar) and -30 (5 bar).
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Comparative assessment
10 bar 8 bar 5 bar
L V L V L V
CH4 8.35 41.84 1.94 39.90 0.86 39.03
C2H6 1.51 3.94 0.38 3.55 0.21 3.34C3H8 1.49 2.34 0.38 1.95 0.23 1.72
C4H10 0.80 0.76 0.20 0.56 0.13 0.43
NH3 3.57 5.90 1.00 4.89 0.71 4.18
N2 0.04 0.27 0 0.27 0 0.26
H2 4.15 24.97 0.84 24.12 0.29 23.83
Individual recovery of each component by the three cycles in the cryogenic technique (100 mole basis).
• For PSA: 5800 kg/hr, U = 14 m/s, P = 1 bar: %R = 12%
• Recovery in PSA stretches to 96% at normal OC.
• Optimizing CT delivers a 24% recovery of hydrogen gas.
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Past experience with membrane separation
• Membranes work on the principle of selective separation of the polymer material.
• Smaller molecules (hydrogen gas) are separated from lager ones, due to their high permeability.
• The partial differential pressure across the membrane module is the driving force that such
operation work on.
• The following assumption are made:
1. The polymer of choice is polyvinyl (tri-methyl ) silane.
2. A perfect mixing model is assumed.
3. A multi-component mixture is assumed.
Feed (Lf )
Permeate side (Lp)
Φ)(lPPΦl
Φ
AQ
IΦL
PZ
pF
ii
f
Fi
PF is the feed pressure (Pa),
Pp is the permeate pressure (Pa),
I is the membrane thickness (m),
Ai is the membrane surface area (m2),
Qi is the permeability coefficient of the ith component (kmol (m/hr) m2 Pa),
Lf is the feed flow rate (kmol/h) on Feed (Lf )
Permeate side (Lp); and
Φ is the dimensionless stage cut (Lp/LF).
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y = 0.0045x + 0.0097R² = 0.9961
0
0.05
0.1
0.15
0.2
0.25
0.3
0 20 40 60 H 2 m o l F r a c t i o n i n P e r m e a t e
Perm. Coff. (Barrer)
y = 0.0892x + 0.0006
R² = 0.9999
0
0.10.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 5 10 15
H 2 m o l F r a c t i o n
i n P e r m e a t e
Feed Press. (Bar)
• Membrane model applied resulted in a 67% recovery [1 bar,
5800 kg/h].
• By comparison it is much more economical and better in
performance than other two.
• But in practice can it perform under nominal conditions.
• Continuation of the work: Validate the membrane model
for the refinery case at hand; SA on CT.
Past experience with membrane
separation
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Concluding remarks:
• As far as work objectives, these were met to the full understanding and representation of the
case at hand.
• PSA was modeled, as well as, CT to estimate amount of hydrogen recovery from a ROG
stream.
• PSA presented a better choice for recovery, that could be manipulated with varying OCs.
• Past experiences show that MT show strong point?!
• The economics of all systems must be considered at later stages, incorporating the
economy of scale to understand fully benefits of each technique.
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8/17/2019 Dr. Sultan Al-Salem
23/23
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