New HDS Unit, ARCO Carson, CA Refinery

I. Introducción

¿Por qué el estudio de los reactores?

1. El Diseño de un reactor para procesos catalíticos requiere de conocimiento de diseño, la optimización y la selección de operación

2. El progreso en la mejora de nuestro nivel de vida depende de la capacidad para diseñar reactores

3. Nuestra existencia personal depende del control de reacciones celulares de nuestro cuerpo, mientras que el de la raza humana depende del resultado de las enormes reacciones globales.

B. Definicion y Clasificacion de Reactores

1. ¿Qué es un reactor?a. Un dispositivo que encierra el espacio de reacción, y que alberga el

catalizador y medios que reaccionan.

b. Un contenedor para que los reactivos se alimentan y eliminado productos, que proporciona para el control de las condiciones de reacción.

2. Clasificación de los Reactoresa. Tamaño

b. Métodos de carga / descarga: batch o flujo en estado estacionario

c. Movimiento de las partículas con respecto a la otra

d. Tipo de flujo de fluido: tubular o mixta

Table 12.1 Clasificacion de Reactores Cataliticos 

Bases para Clasificacion Clases Ejemplos

Tamaño LaboratorioEscala PilotoEscala Planta

0.5 cm diam. tubular microreactor (0.1-1 g catalizador)7.5 cm diam x 6-10 m largo reactor tubular (20-100 kg catalyst)1-6 m diam x 20-50 m largo reactor tubular (20-100 ton. Metricas de cat.)

Metodos de carga y descara

BatchFlujo, en estado estacionario

Líquido agitado y sólidosa. cama tubular, de catalizador fijob. suspensión, fluido mezclado, sólidos mezclados

Movimiento de partículas de catalizador respecto a la otra

Fijomovimiento relativo

Tubulares sólidos fijos (lecho fijo)a. lecho fluidizadob. suspensión de columna de burbujas

El flujo de fluido Tubular, flujo pistónFlujo de fluido mezclado

Gas turbulento en lecho fijo tubular  Suspensión del reactor con agitación mecánica

D. Selección de los reactores en el laboratorio y planta

Reactors son usados para muchos diferentes propositos: 1. para estudiar los mecanismos y la cinética de las reacciones químicas

2. para proporcionar datos para la validación de las simulaciones de procesos

3. para investigar el rendimiento del proceso a través de una gama de variables de proceso

4. para obtener los datos de diseño

5. para producir energía, materiales y productos

La elección del reactor adecuado es fundamental para el proceso de ingeniería y es dictada por muchas variables diferentes, tales como

reacción de tipo

velocidad de desactivación

ciencias económicas

otros requisitos del proceso

Fig. 12.2 Features of representative laboratory reactors [Levenspiel, 1979].

Figure 12.4 Berty internal recycle reactor.

Gas-Liquid CSTR (UCSB) Batch Reactor (UCSB)

Bench scale reactor (courtesy of Shell Corp.)

II. Laboratory and Bench Scale Reactors

B. Criteria for selection of lab and bench-scale reactors; applications

1. Satisfying intended application

2. Avoiding deactivation

3. Avoiding inter- and intra- particle heat and mass transport limitations

4. Minimizing temperature and concentration gradients

5. Maintaining ideal flow patterns

6. Maximizing the accuracy of concentration and temperature measurements

7. Minimizing construction and operating costs

Table 12.3 Seven Criteria for Selection of Laboratory and Bench-Scale Catalytic Reactors

Criterion Issues Involved/Measures of/Methods to Meet Criterion

1. Satisfy purpose of measurement (i.e., application)

Measure: (1) intrinsic activity/selectivity, (2) kinetics of reaction and deactivationObtain mechanistic understandingSimulate process

2. Avoid catalyst deactivation where possible; where not, decide if fast or slow

See Chap. 5 (B&F) on avoiding different kinds of catalyst deactivationFast decay causes activity and selectivity disguises and requires use of transient or transport reactorSlow decay best studied using CSTR or differential reactor

3. Avoid inter- and intra-particle heat and mass transport limitations

Thiele modulus less than 0.5; small particles or thin catalyst layerMinimize film thickness with high flow rates, turbulenceOperate at low conversionsUse CSTR or differential reactor

4. Minimize temperature and concentration gradients

Gradients cause activity and selectivity disguisesMaximize mixing in batch reactor and CSTR; use inerts Use CSTR or differential reactor where possible

5. Maintain ideal flow patterns

Minimize mixing and laminar flow in tubular reactors;Maximize mixing and minimize gradients in CSTRAvoid gas or liquid holdup in multi-phase reaction systems

6. Maximize accuracy of concentration and temperature measurements

Sensitive analytical methods and well-placed, sensitive probesSufficiently high product concentrations

7. Minimize construction and operating costs

Select the least expensive reactor that will satisfy the other criteriaConsider ways of minimizing size of catalyst and volume of reactant gas

Table 12.4 Applications of Lab/Bench Test ReactorsReactor Type Catalyst Selection

Activity/SelectivityReactor/Design Fundamental

MechanismProcess

Simulation

Life Kinetics

Integral

Adiabatic X (overall avg. conv.) X X

Isothermal X (overall conv. at T) X X

Differential

Single Pass X (intrinsic) X (intrinsic) X (eliminate)

Recycle X (intrinsic) X (intrinsic) X (eliminate)

Stirred gas X (intrinsic) X (kinetics) X (intrinsic) X (eliminate) X (model)

Fluid bed/ Transport

X (fast deact.) X (fast deact.) X (fast deact.)

X

Micro-pulse X (comparative, initial) X

Transient X (elem. steps)

X X (model)

Common Types of Catalytic Plant Reactors

1. Fixed-bed Reactorsa. Packed beds of pellet or monolithsb. Multi-tubular reactors with coolingc. Slow-moving pellet bedsd. Three-phase trickle bed reactors

2. Fluid-bed and Slurry Reactorsa. “Stationary” gas-phaseb. Gas-phasec. Liquid-phase

i. Slurryii. Bubble Columniii. Ebulating bed

Table 12.5 Characteristics of Plant-Scale Fixed Bed Reactors

Advantages1. Ideal plug (or mixed) flow2. Simple analysis3. Low cost, low maintenance4. Little loss or attrition5. Greater variation in operating conditions and contact

times is possible6. Usually a high ratio of catalyst to reactants

long residence time complete reaction7 Little wear on catalyst and equipment8. Only practical, economical reactor at very high

pressures

Disadvantages1. Poor heat transfer in a large fixed bed. a. Temp. control and measurement difficult b. Thermal catalyst degradation c. Non uniform rates.2. Non uniform flow patterns e.g. channeling3. Swelling of the catalyst; deformation of the reactor4. Regeneration or replacement of the catalyst is difficult - shut down is required.5. Plugging, high pressure drop for small beads or pellets - ∆P is very expensive.6. Pore diffusional problems intrude in large pellets

Overcoming the Disadvantages1. Monolithic supports overcome disadvantages 2, 5 & 62. Temperature control problems are overcome with: a. Recycle b. Internal and external heat exchanges c. Staged reactors d. Cold shot cooling e. Multiple tray reactor - fluid redistributed & cooled between stages. Catalyst is easily removed - varied from tray to tray.f. Use of diluentsg. Temperature self regulation with competing reactions, one endo and one exothermic.h. Temp control by selectivity and temporarily poisoning the catalyst

B. Fixed-bed reactors: characteristics, advantages, limitations

Advantages:• Flexible- large variation in operating conditions and contact times

is possible• Efficient- long residence time enables a near complete reaction• Generally low-cost, low-maintenance reactors

Disadvantages:• Poor heat transfer with attendant poor temperature control• Difficulty in regenerating or replacing spent catalyst

Product

Feed

Inert balls

Catalyst

Reactants Inlet

Liquid OrGaseous

Bath

ReactorTube

Gas OrLiquidFlow

OutletProduct

ReactorTube

a..

I II III IV

1 2 3 4

Fresh feed

Recycle gas

b.

Fig. 12.6 Commercial fixed-bed reactor designs for controlling temperature: (a) multi-tubular heat-exchange reactor, (b) series of fixed-bed, adiabatic reactors with interstage heating or cooling.

Figure 12.5 Commercial fixed-bed, adiabatic catalytic reactor.

Advantages1. Frequent regeneration of the catalyst

possible.

2. Rapid mixing of solids in fluid beds means uniform gas composition.

3. Isothermal operation and efficient temperature control is practical.

4. Small-diameter particles in fluid minimize pore diffusional resistance.

5. Improved thermal efficiency because of high heat transfer rates.

6. In the case of highly exothermic, liquid phase reactions, slurry reactors are less complex and less expensive than heat-exchange-tubular systems.

Disadvantages

1. Fluidized beds are complicated systems involving multiple reactors, heat exchangers, extensive valving and piping to provide continuous system.

2. $$ Extensive investment. Maintenance is high.

3. Fluid flow is complex in fluidized and slurry bubble columns - less than ideal contacting. Product distribution is changed - less intermediate formed in a series reaction.

4. Only a small variation in residence time possible. Low residence times. Conversion may be limited.

5. Attrition & loss of Catalyst.

Table 12.6 Characteristics of Plant-Scale Fluidized and Slurry Bed Reactors

Deentrainedvapor

Product

Spentcatalyst

Hydro-clone

Freshcatalyst

Feed

a.

Continuousphase Product

Dispersedphase

b.

Figure 12.7 Liquid-phase slurry reactors: (a) forced-circulation, slurry-bed reactor, (b) bubble-column, slurry-bed reactor.

H2

reactants + H 2

products

shaft heater

liquid

P

productwithdrawal

suspendedcatalystparticles

rotor

pressurevessel

to heat source

H2 from reservoir (consumption measured)

Figure 12.8 Batch-slurry reactor for hydrogenation of specialty chemicals.

Product

Steam stripping

Flue gas

Fluid-bedregenerator

Air

Transfer line

Feed

Transfer-line reactor

Fig. 12.9 Design of typical FCC transfer-line (riser) reactor with fluidized-bed regenerator.

Products

Riserreactor

Catalyststripper

Steam

Reactorfeed

SteamAir

Overflowwell

Regenerator

Fluegas

Cyclone

a. Products

Cyclone

Reactor

Catalyst stripper

Steam

Air

Reactor

Regenerator

Flue gas

feed

b.

Figure 12.10 Commercial FCC riser reaction designs (a) Exxon, (b) UOP.

Fluid Cat Cracker (Chevron) Stacked Fluid Cat Cracker (UOP)

Shell Cat-Cracker All-riser Cracking FCC Unit

Criteria for Selection of Plant Reactors1. General Criteria.

a. deactivation rate and regeneration policyb.reaction conditionsc. catalyst strength and attrition resistanced.process economics

2. Role of Cp/(-DHr)

slope =

T

XA

Cp

-HR

1rA

XA

1

100

10

1000

T

XA 1rA

XA

c

d

a

b

Fig. 12.11 (a) Operating line for a highly exothermic reaction in an ideal tubular reactor with pure reactant and (b) corresponding reciprocal rate versus conversion curve and area V/FAo for a CSTR. (c) Operating line for a highly exothermic reaction in an ideal tubular reactor with dilute reactant and (d) corresponding reciprocal rate versus conversion curve and area V/FAo for a PFR.

3. Optimal Temperature Progression

Figure 12.12 Optimum temperature progression (and locus of maximum rates) of (a) reversible endothermic reaction and (b) reversible exothermic reactions.

Exothermic reversible Exothermic irreversible Endothermic

Qin Q

Optimum

Tmax

T T T

Tmax

Coldfeed

Furnace

Adiabaticreactors

Coldproduct

A B C

D E

F G

Q Q Q

XA

a

b

Fig. 12.13 (a) Use of staged adiabatic tubular fixed-bed reactors with interstage cooling to achieve optimum temperature progression in the cases of exothermic reversible, exothermic irreversible and endothermic reactions. (b) Design schematic for staged adiabatic fixed-bed reactors with interstage furnace heating for a strongly endothermic reaction such as reforming of methane.

Collecting, Analyzing and Reporting Data from Laboratory Reactors

Different purposes:1. activity/selectivity and life data for catalyst selection

2. chemical reaction mechanistic and kinetic data for understanding the reaction at a fundamental level, modeling the reaction process, and/or designing reactors

3. process variable data over a wide range of conditions for purposes of designing large-scale reactors, experimentally validating models and optimizing the catalytic process.

Data collection typically involves three major steps (Fig. 12.14):

4. selection of a reaction and catalyst

5. selection of a reactor type

6. analysis of the data

SelectReaction

andCatalyst

Criteria Data

Select reactorand conditions Data Analysis

IntegralDifferentialInitial Rates

BatchFlow CSTR Plug - integral or differential

mechanismrate expressionprocess optimization

new experiments

Figure 12.14 Process of obtaining rate and kinetic data; note that statistical methods are used in Steps 2 and 3 and in the recycle process.

Table 12.7 Proposed Guidelines for Choosing Catalyst Form, Pretreatment, and Reaction Conditions and for Reporting of Data [Ribiero et al., 1996].

1. Catalyst Properties and Characterizationa. Catalysts/surfaces should be carefully prepared and pretreated so as to be free of

solid contaminants such as sulfur, chlorides, and carbon that might affect activity.

b. Support effects should be avoided by studying reactions on single crystals of the active catalytic phase, e.g., metal, metal films, and/or relatively highly-concentrated, poorly-dispersed supported metals. Preparation methods should be used which minimize decoration of the metal surface, e.g., decomposition of metal carbonyls on supports. Supported base metal catalysts need to be well-reduced to avoid complications due to unreduced metal oxides.

c. In the case of structure-sensitive reactions, effects of surface structure and/or dispersion need to be taken into account.

d. Metal dispersion/surface area should be measured using proven and/or standard (ASTM) methods, e.g., hydrogen chemisorption or titration rather than CO chemisorption for metals.

e. Methods of preparation and characterization should be reported in detail. Methods for calculating surface area and dispersion should also be carefully reported. Reporting these methods and the surface area or dispersion of catalyst samples should be a requirement for publication of specific activity data.

Table 12.7 Continued

2. Reaction Conditionsa. TOF and kinetic data must be measured in the absence of pore diffusional

restrictions, film mass transfer limitations, and heat transfer limitations (generally at low temperature and low conversion). Experimental evidence and calculations based on well-known criteria (e.g., the Thiele modulus for pore diffusional resistance) should be provided in publications to demonstrate that the data were obtained in the absence of these effects.

b. TOF and kinetic data must be measured in the absence of deactivation effects, e.g., poisoning, coking, and sintering. The burden of proof that such effects are absent should be on the authors of a publication.

c. TOF data should be collected over wide ranges of temperature and reactant concentrations to facilitate valid comparison with data from other laboratories and to provide meaningful data for determining temperature and concentration dependencies.

d. TOF data should be reported at specified conditions of temperature, reactant concentrations, and conversion. These specifications should be used by reviewers and editors as a minimum reporting requirement for publication in a journal.

Analyzing and reporting data from laboratory reactors

1. Analysis of rate data: objectives a. finding a rate equation

b. determining catalyst activity, selectivity and activity stability

c. determining the effects of important process variables such as temperature, pressure, reactant concentrations, and space velocity on activity, selectivity and stability

2. Extracting rate constants and concentration dependenciesa. Integral analysis

b. Differential analysis

WF

dx(-rA)

x

xx

xx

x

o

Figure 12.15Test for integral analysis of rate data involving plot of W/FAo versus integrated reciprocal rate.

Integral analysis of rate data involves the following steps:

1. A series of runs are made in a packed bed at a fixed initial concentration CAo and a fixed temperature while varying the catalyst mass W and/or the initial molar flow rate FAo to generate a range of W/FAo values at different conversions (XA).

2. A candidate rate equation is selected for testing using the design equation for plug flow. First try simpler rate equations, e.g. zero-, first- and second-order irreversible and use differential analysis to scope out possible reaction orders.

3. The left-hand side (W/FAo) and right-hand side (the integral of 1/ rA) of the design equation are each evaluated numerically and plotted against each other, as shown in Figure 4.19 to test for linearity.

4. Linearity of the plot (e.g. Figure 12.15) is used as the criterion for judging if the candidate rate equation is a useful model of the data, i.e. consistent with the data.

5. This should be followed by a nonlinear regression with statistical analysis to determine kinetic parameters.

Integral Analysis of Rate Data

Differential Analysis of Rate Data

X

slope =

ln CA

b

ln (-rA)

W/FA0

XA

1.0

0.5 X

X XXX

XX

X

X0

slope = -rA'a

XX

X

Fig. 12.16(a) Differential analysis to obtain reaction rates. (b) Plot to obtain reaction orders.

Differential analysis of rate data involves the following steps:

1. The identical series of runs (as in integral analysis) are made in a packed bed at a fixed initial concentration CAo and fixed temperature while varying the catalyst mass W and/or the initial molar flow rate FAo to generate a range of W/FAo values at different conversions (XA).

2. A plot of conversion XA versus W/FAo data is made for each set of runs at a fixed temperature. A best fit of the data is made using a simple quadratic or cubic equation, and the corresponding curve is plotted on the same figure, as illustrated in Figure 4.20(a).

3. Tangents to the curve are drawn at regular intervals along the curve corresponding to the best fit, and the slopes of these tangents are evaluated (Figure 12.16a); the slopes on this plot correspond to the reaction rate, i.e. dXA = ( rA)[d(W/F)]. Tangents can be evaluated more accurately by differentiating the equation for the best fit of the data and evaluating the derivative at various intervals over the data set.

4. Values of rA versus XA are plotted on linear or log-log plots to determine reaction orders. For example, for an irreversible reaction in which data are fitted by a simple power rate law, rA = k CA

α, a plot of ln( rA) versus ln(CA) is linear with a slope (Figure 12.16b).

5. Differential analysis is a useful scoping tool to explore forms of the rate expression; however, in the end a nonlinear regression and rigorous statistical analysis is required to obtain kinetic parameters with acceptable accuracy.

References

1. O. Levenspiel, Chemical Reaction Engineering, 2nd and 3rd Eds., John Wiley and Sons, 1972, 1999.

2. O. Levenspiel, The Chemical Reactor Minibook, OSU Bookstores, 1979. 3. J.M. Smith, Chemical Engineering Kinetics, 3rd Ed., McGraw Hill,

1981.4. "Reactor Technology," Kirk-Othmer: Encyclopedia of Chemical

Technology, Vol. 19, 3rd Ed, John Wiley, 1982, pp. 880-914.5. G.F. Froment and K.B. Bischoff, Chemical Reactor Analysis and

Design, John Wiley, 1990.6. O. Levenspiel, The Chemical Reactor Omnibook, 1993, OSU

Bookstores, 1993. 7. C. H. Bartholomew and R. J. Farrauto, Fundamentals of Industrial

Catalytic Processes, Chapman and Hall, 2005, Chap. 4.