Dr. Sultan Al-Salem

8/17/2019 Dr. Sultan Al-Salem

1/23

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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2/23

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


3/23

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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4/23

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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5/23

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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6/23

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.


7/23

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.


8/23

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.


9/23

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)


10/23

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


11/23

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.


12/23

• 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


13/23

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.


14/23

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.


15/23

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.


16/23

• 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


17/23

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


18/23

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).


19/23

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.


20/23

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).


21/23

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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22/23

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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23/23
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Dr. Sultan Al-Salem