Fixed Bed Adsorber Design Guidelines

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GBH Enterprises, Ltd.
Fixed Bed Adsorber Design Guidelines
Gerard B. Hawkins
Managing Director
Information contained in this publication or as otherwise supplied to Users is believed to be
accurate and correct at time of going to press, and is given in good faith, but it is for the User to
satisfy itself of the suitability of the information for its own particular purpose. GBHE gives no
warranty as to the fitness of this information for any particular purpose and any implied warranty
or condition (statutory or otherwise) is excluded except to the extent that exclusion is prevented
by law. GBHE accepts no liability resulting from reliance on this information. Freedom under
Patent, Copyright and Designs cannot be assumed.

OVERVIEW - FIXED BED ADSORBER DESIGN GUIDELINES
Fixed-bed adsorber design is based upon the following considerations:
• Adsorbent bed profile and media loading capacity characteristics for the
specific application and adsorbent material used.
• Pressure drop characteristics across the adsorbent bed.
• Reaction kinetics.
Typically, adsorber design entails use of the following methodology:
• Adsorbent selection based upon performance and application information.
• Bed sizing based upon adsorbent loading data and service life requirements.
• Bed sizing adjustment based upon pressure drop criteria.
• Bed sizing adjustment based upon reaction kinetics criteria.
A discussion of each design consideration follows.

ADSORPTION BED PROFILE
The diagram below depicts the three basic regions of an adsorption bed: the
Equilibrium Zone, the Mass Transfer Zone, and the Active Zone.
Adsorption Bed Profile

The Equilibrium Zone defines how much adsorbate the adsorbent media can
hold under the process conditions (temperature, pressure and adsorbate
concentration in the feed). This adsorbate loading limit is referred to as the
Equilibrium Capacity, reported as a weight percentage (ex: X lbs adsorbate /
100 lbs adsorbent), and used as the theoretical maximum assuming 100% of the
bed volume is used to capacity. The Equilibrium Zone is also a measure of the
adsorbent volume (as a percentage of total bed volume) that can be used to full
capacity before unacceptable adsorbate breakthrough is suffered.
The Mass Transfer Zone defines the adsorbent volume (as a percentage of total
bed volume) that is required to reduce adsorbate inlet concentration to that
required by effluent specifications in the portion of the bed where active
adsorption is taking place.
Equilibrium capacities and mass transfer characteristics for a given adsorbate on
a given adsorbent are reflected in that adsorbent material's isotherm data and
are usually graphically represented. An isotherm relates adsorbate loading
capacity to adsorbate concentration in the process stream at a given
temperature. Concentration is typically represented by partial pressure or
relative humidity in vapor phase systems and weight per volume for liquid
applications. Isotherms for vapor phase applications are much more readily
available than those for liquid phase applications due to wide variance in diffusive
characteristics in liquids having significant effects on adsorbent performance.
Many adsorbents will have isotherm data available at several different
temperatures.

Typical Adsorbent Isotherm
The Equilibrium Zone, Equilibrium Capacity and Mass Transfer Zone, taken
together, define the performance characteristics of a given system and are
essential to the successful design of a fixed bed adsorber. Bed sizing is typically
determined by the size of the Equilibrium Zone and Equilibrium Capacity
value. VULCAN Series adsorbents are designed to be application-specific and
therefore the equilibrium capacity will be optimized for each case. In non-
regenerative systems, maximum Equilibrium Capacity can be read directly off
the isotherm at inlet adsorbate concentration. In regenerative systems,
Equilibrium Capacity is determined by the differential between process and
regeneration conditions:
Adsorbate Concentration In Process Fluid
(ex: partial pressure, %RH, ppm)
AdsorbateLoading
CapacityonAdsorbent
T1
T2
T3
T4
T5

Equilibrium Capacity Prediction from an Adsorbent Isotherm
(Thermal Regeneration)
A ---> C represents Working Equilibrium Capacity obtained from the
temperature differential between process and regeneration conditions.
A ---> B represents Working Equilibrium Capacity obtained from the
concentration differential between process and regeneration conditions.
A ---> D represents Working Equilibrium Capacity obtained from the
combined temperature/concentration differential between process and
regeneration conditions.
AdsorbateLoading
CapacityonAdsorbent T1
T2
T3
T4
T5
A
B
C
Working
Equilibrium
Capacities
D

Equilibrium Capacity Prediction from an Adsorbent Isotherm
(Reduced Pressure Regeneration)
A ---> B represents Working Equilibrium Capacity obtained from the
concentration differential between process and regeneration conditions.
In general, the most efficient regenerative adsorption systems take advantage of
both thermal and concentration effects in order to maximize Working Equilibrium
Capacity and minimize frequency of regeneration. An exception to this; are
applications where highly volatile adsorbate species with only moderate affinity
for the adsorbent(s) are being removed from an essentially adsorbent-inert
process stream (ex: light hydrocarbons from hydrogen or nitrogen). In these
applications, adsorption units utilizing only adsorbate pressure and concentration
differentials have proven economical. These are known as Pressure Swing
Adsorption, or PSA, Units.
AdsorbateLoading
CapacityonAdsorbent
T
Working
Equilibrium
Capacity
A
B

It is also important to realize the role the Mass Transfer Zone plays in overall bed
capacity. As adsorbate concentration in the process fluid decreases through the
MTZ, adsorptive capacity also falls. Useful adsorption bed volume thus becomes
a function of the compactness of the MTZ and adsorbate concentration
specification in the effluent.
Example: Comparison between Adsorbent X and Adsorbent Y for a given
application may show X affording greater loading capacity than Y. However,
evaluation of the mass transfer characteristics would reveal the Mass Transfer
Zone for X to be nearly twice that required for Y to achieve the same degree of
adsorbate removal in the effluent. If a very low adsorbate specification is
required in the product, the Adsorbent Y would be the material of choice for the
application. The use of Adsorbent X would mandate a greater amount of
adsorbent material (and larger vessel) and/or a shorter length of time between
adsorbent bed regenerations.
Isotherm profiles can indicate MTZ characteristics as indicated below:

Mass Transfer Zone as Indicated by Adsorbent Isotherm
AdsorbateLoading
CapacityonAdsorbent
Adsorbent X - Greater
loading capacity at higher
adsorbate concentrations
but poor MTZ
characteristics
Adsorbent Y - Lower loading
capacity at higher adsorbate
concentrations but superior
MTZ characteristics

PRESSURE DROP CHARACTERISTICS
The second major consideration in fixed-bed adsorber design is pressure drop
characteristics, which set the permissible operational limits for the adsorption
bed. Insufficient pressure drop will result in uneven distribution and channeling of
the process fluid, resulting in poor performance. Excessive pressure drop will
result in bed compaction or lifting. Pressure drop guidelines for nominal 1/16"
thru 1/4" spherical, granular or extruded adsorbent media are as follows:
VAPOR PHASE SERVICE
<0.01 psi / ft Un-even distribution and channeling
0.01 - 0.20 psi / ft Upflow or downflow operation
0.20 - 100 psi / ft Downflow operation only (upflow will result in
bed lifting)
>100 psi / ft Bed compaction
LIQUID PHASE SERVICE
<0.001 psi / ft Un-even distribution and channeling
0.001 - 0.20 psi / ft Upflow or downflow operation
0.20 - 10 psi / ft Downflow operation only (upflow will result in
bed lifting)
>10 psi / ft Bed compaction

FIXED BED PRESSURE DROP CALCULATION
Pressure drop is a function of process fluid properties, adsorbent characteristics,
and vessel dimensions, and is calculated by use of the modified Ergun Equation:
ΔP / L = kftCtG2
/ ρDp
where: ΔP= pressure drop (psi, kg/cm2
)
L = length (depth) of adsorbent bed (feet, meters)
ft = friction factor (dimensionless)
Ct = pressure drop coefficient (ft-hr2
/in2
) = 3.6 x 10-10
ft-hr2
/in2
for "ξ" of 0.37
G = superficial mass velocity (lbs/hr-ft2
, kg/hr-m2
)
ρ = process fluid density (lbs/ft3
, kg/m3
)
Dp = average adsorbent particle diameter (inches, mm)
k = units conversion constant
"ΔP" is calculated. "L", "G2
", "ρ", and "Dp" are process values. Ct and ft can be
obtained from standard plots.
Pressure drop across fixed beds is also reflected in bed operating parameters
such as:
• Linear Velocity - volumetric process flow per cross-sectional area of
adsorbent bed (ex: ft/min, m/hr)
• Superficial Mass Velocity - gravimetric process flow per cross-sectional
area of adsorbent bed (ex: lbs/hr-ft2, kg/hr-m2)
• Modified Reynold's Number - dimensionless value relating gravimetric
process flow to fluid viscosity and adsorbent particle size
For further discussion concerning pressure drop and its implications in fixed bed
adsorber design and operation, see www.gbhenterprises.com (Fixed Bed
Pressure Drop Calculations.)

REACTION KINETICS
The third consideration involves rate of interaction and reaction of the adsorbate
with the adsorbent. While most physical adsorption interactions occur readily,
many chemical adsorption mechanisms do not. As a result, most adsorptive
processes based on physisorption can be designed based solely upon adsorbent
capacities and pressure drop criteria. With chemisorption, reaction time between
adsorbate and adsorbent can become an issue.
GBH Enterprises, offers a series of promoted adsorbents which take advantage
of both physisorption and chemisorption mechanisms.
These reaction kinetics criteria are defined and reflected in adsorption bed
operating parameters such as:
• Gas or Liquid Hourly Space Velocity (GHSV or LHSV)- volumetric process
flow per volume of adsorbent (ex: hr-1)
• Contact Time - the time in which a given volume of process flow remains in
contact with the adsorbent medium. Reciprocal of Space Velocity (ex: sec,
min)
Reaction kinetics criteria are typically established on a per application
basis.
For example, Defluorinator operation with VULCAN Series A2
ST-3000 alumina
(removal of organic fluoride and free HF from hydrocarbons using activated
alumina) targets a Contact Time of ~160 - 165 seconds.
Vapor phase adsorption of hydrogen sulfide on copper oxide targets a GHSV of
~1500 - 4000hr-1
while liquid phase H2S adsorption targets a LHSV of ~1.5 -
3.0hr-1
.
Recommendations for GHSV, LHSV, and Contact Time are usually supplied with
adsorbent and catalyst application data.

ABSORPTION MODELLING - SHRINKING CORE MODEL
0
INLET GAS DETAILS Absorbent Dimen
Molecular Weight 21.18 kg/kgmol Flowrate 5500 Nmü/h
Temperature 20 øC 5197.19818 kg/h
Pressure 65 bara Min
Density 69.38 kg/mü kg S/day 9.422682252 Max
Viscosity 0.0000126 Ns/mý
1.009047932 Avg
H2S Concentratio 50 ppmv
BED DETAILS OUTPUTS
Total Volume 1 mü Bed Diameter 1.13 m I.D
Bed Diameter 1.13 m Bed Height 0.997133327 m
or L/D L/D Ratio 0.882418873 L/D
Slice Depth 0.01 m
No of Slices 100
Vol per Slice 0.01 mü
Stream Composition/Diffusion Rates (not used yet)
Ev Dab molfrac
Methane 16 24.42 3.13E-03 0.8
Ethane 30 44.88 2.08E-03 0.1
Propane 44 65.34 1.65E-03 0.05
Butane 58 85.8 1.40E-03 0.03
Pentane 72 106.26 1.23E-03 0.02
C6+ 86 126.72 1.11E-03
CO2 44 26.9 2.28E-03
H2O 18 12.7 3.71E-03
H2S 34 20.96
Deff 2.69E-03 cmý/s
2.69E-07 mý/s
Mwt 21.18
Ev values calculated from Perry, Table 3-342, pp 3-285
0
0
0
Adsorption Modeling: Shrinking Core

FIXED BED ADSORBER TROUBLESHOOTING GUIDELINES
SYMPTOM POSSIBLE CAUSE
Pre-Mature Adsorbate Breakthrough Upstream Coalescer Failure / By-Pass
Excessive Flowrate
Bed Channeling
Bed Lifting (Upflow Only)
Media Fouling
Faulty Monitoring
Inefficient Regeneration
- Low Regen Flowrate
- Wet Regen Stream
- Insufficient Regen Temperature
Low Pressure Differential
Across Adsorber Bed Insufficient Flowrate
Bed Channeling
Bed Lifting (Upflow Only)
High Pressure Differential
Across Adsorber Bed Excessive Flowrate
Bed Compaction (Downflow Only)
Media Attrition or Fracture
Media Fouling

Fixed Bed Adsorber Design Guidelines

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