Fixed Bed Adsorber Design Guidelines

Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries

Web Site: www.GBHEnterprises.com

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)

Ads

orba

te L

oadi

ngC

apac

ity o

n A

dsor

bent

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.


Ads

orba

te L

oadi

ngC

apac

ity o

n A

dsor

bent

T1

T2

T3

T4

T5

A

B

C

WorkingEquilibriumCapacities

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.


Ads

orba

te L

oadi

ngC

apac

ity o

n A

dsor

bent

T

WorkingEquilibriumCapacity

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


Ads

orba

te L

oadi

ngC

apac

ity o

n A

dsor

bent

Adsorbent X - Greaterloading capacity at higheradsorbate concentrationsbut poor MTZcharacteristics

Adsorbent Y - Lower loadingcapacity at higher adsorbateconcentrations but superiorMTZ 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 A2ST-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 MODEL0

INLET GAS DETAILS Absorbent Dimen Molecular Weight 21.18 kg/kgmol Flowrate 5500 Nmü/hTemperature 20 øC 5197.19818 kg/hPressure 65 bara MinDensity 69.38 kg/mü kg S/day 9.422682252 MaxViscosity 0.0000126 Ns/mý

1.009047932 AvgH2S Concentratio 50 ppmvBED DETAILS OUTPUTSTotal Volume 1 mü Bed Diameter 1.13 m I.DBed Diameter 1.13 m Bed Height 0.997133327 mor L/D L/D Ratio 0.882418873 L/DSlice Depth 0.01 m

No of Slices 100Vol per Slice 0.01 mü

Stream Composition/Diffusion Rates (not used yet)Ev Dab molfrac

Methane 16 24.42 3.13E-03 0.8Ethane 30 44.88 2.08E-03 0.1Propane 44 65.34 1.65E-03 0.05Butane 58 85.8 1.40E-03 0.03Pentane 72 106.26 1.23E-03 0.02C6+ 86 126.72 1.11E-03CO2 44 26.9 2.28E-03H2O 18 12.7 3.71E-03H2S 34 20.96

Deff 2.69E-03 cmý/s2.69E-07 mý/s

Mwt 21.18Ev values calculated from Perry, Table 3-342, pp 3-285

000

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

Technology

Transcript of Fixed Bed Adsorber Design Guidelines