Reactor and Catalyst Design

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

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GBH Enterprises, Ltd.

Process Engineering Guide: GBHE-PEG-RXT-807

Reactor and Catalyst Design 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.



Process Engineering Guide: Reactor and Catalyst Design CONTENTS SECTION 0 INTRODUCTION/PURPOSE 2 1 SCOPE 2 2 FIELD OF APPLICATION 2 3 DEFINITIONS 2 4 CATALYST DESIGN 2 4.1 Equivalent Pellet Diameter 3 4.2 Voidage 6 4.3 Pellet Density 8 5 REACTOR DESIGN 8 6 CATALYST SUPPORT 10

6.1 Choice of Support 10



TABLES 1 CATALYST SUPPORT SHAPES 12 2 SECONDARY REFORMER SPREADSHEET 13 FIGURES 1 GRAPH OF EFFECTIVENESS v THIELE MODULUS 4 2 VARIATION OF COSTS WITH CATALYST SIZE 6 3 VARIATION OF COSTS WITH CATALYST BED VOIDAGE 8 4 VARIATION OF COSTS WITH VESSEL DIAMETER 9



0 INTRODUCTION/PURPOSE When the catalyst chemistry of a new fixed feed chemical reaction has been developed, decisions need to be made about the catalyst design. The issues that need to be decided are: (a) The catalyst particle size. (b) The catalyst shape to give a reasonable optimum pressure drop in the

catalyst bed. (c) The catalyst particle density to make the catalyst particles reasonably

effective. These issues are closely related to the reactor shape and the cost of pressure drop. This Process Engineering Guide provides some explanation of these issues and equations by which the key parameters can be determined. 1 SCOPE This Process Engineering Guide deals with the design of the catalyst, particularly its size and shape, and the reactor geometry as well as catalyst support types. It does not cover the chemical selection of the catalyst. 2 FIELD OF APPLICATION This Guide applies to process engineers and technologists in GBH Enterprises worldwide, who may be involved in the design of reactors and catalysts. 3 DEFINITIONS For the purposes of this Guide no specific definitions apply.



4 CATALYST DESIGN The design of catalyst particles can be characterized by three independent variables: (a) Equivalent pellet diameter de (b) Voidage e (c) Density ρ These can all be optimized. 4.1 Equivalent Pellet Diameter Larger catalyst size leads to:

(a) Lower pressure drop in reactor:

(1) Lower compression power.

(b) Lower catalyst effectiveness:

(1) Larger catalyst volume (2) Higher vessel cost (3) Higher catalyst cost.

Thiele modulus F = b x de .......................................... (1)

where: b is assumed to be a constant (probably related to the pore structure) de is the equivalent sphere diameter of the particle.

de = 6 x particle volume / particle surface area .......................... (2)



For a spherical catalyst particle:

Effectiveness E = 3/F x (1/tanh(F) - 1/F)) ................. (3) The constant b may be calculated from measurements of the effective catalyst activity at two different particle sizes. The intrinsic activity is defined as:

Intrinsic Activity = Apparent Activity / Effectiveness ....... (4) 4.1.1 Example: Calculation of Intrinsic Activity Results from pellet testing give: Test Pellet size Apparent Activity 1 2 4.2 2 4 3 Figure 1 shows a graph of Effectiveness v Thiele Modulus based on Equation 3.

FIGURE 1 GRAPH OF EFFECTIVENESS v THIELE MODULUS



What are the Thiele Moduli for the different pellet sizes and the intrinsic activity? Using Figure 1, it can be seen that to get an apparent activity increase of 40% for a halving of the pellet diameter, the only points that will fit are:

F = 2, E = 0.8

F = 4, E = 0.57

Thus, from Equation 4, the Intrinsic activity is 4.2 / 0.8 = 5.25. 4.1.2 Pressure Drop For turbulent flow:

Pressure drop ΔP = 2 x Velocity head x 1.75 x (1 - e) x L / (e3 x de) (5) where:

e is the bed voidage L is vessel length or height.

For axial flow:

Capitalized cost = Cp x V / (de x D6) .............................. (6) where:

Cp is a constant for fixed voidage V is the catalyst volume D is the catalyst bed diameter.

4.1.3 Catalyst Volume

Catalyst volume (V):

V = V0 / E ................................................................ (7)



where:

V0 is the catalyst volume for unit effectiveness. Vessel cost:

Capital cost = Cv x (V + D3) ....................................... (8) where:

Cv is a constant. Catalyst cost:

Capitalized cost = Ccat x V ............................................ (9) where: Ccat is a constant that depends on catalyst cost and catalyst change frequency. Calculate the parameter (q):

q = 1.89 / (Cv + Ccat) x (Cv / V0)0.67 x (Cp x b)0.33 .............................(10)

For axial flow in an adiabatic pressure vessel optimum pellet size is given by:

q = (b x de)0.375 x (0.4 + 0.022 x (b x de)2) .................................... (11) If q < 20, calculate:

de = (2.5 x q)0.375 / b ................................................. (12) If q > 20, calculate:

d e = (q / 0.022)3/14 / b ................................................. (13)



If optimum pellet diameter is greater than 3mm - use pellets or rings. If optimum diameter is less than 3mm - examine other supports (see later). The necessary data to determine the Thiele Modulus and hence the optimum pellet diameter of most of the catalysts that GBHE uses is not available. Typical optimum particle sizes: Ammonia plant:

HT Shift 3.8 mm Methanator 1.8 mm Secondary Reformer 0.1 mm Methanol synthesis 4.7 mm

Figure 2 shows the variation of capitalized costs (pressure drop cost, vessel cost, catalyst cost and total cost) and effectiveness with catalyst size.

FIGURE 2 VARIATION OF COSTS WITH CATALYST SIZE



4.2 Voidage

Higher voidage leads to:

(a) Lower pressure drop.

(b) Larger catalyst vessel. It is possible to increase voidage by moving to more eccentric particles, i.e. length / diameter L / D ratio greater than 1.3, or by using rings instead of pellets. 4.2.1 Pressure Drop Cost

Pressure drop cost:

Capitalized cost = Cp1 x Vs / D6 / e3 ................. (14)

where:

Cp1 is a constant for constant particle diameter, etc.

Vs is the solid volume of catalyst D is the vessel diameter e is the bed voidage.

Vs = V x (1 - e) ............................................................... (15) where:

V is the catalyst volume of catalyst. 4.2.2 Vessel Cost Vessel cost:

Capital cost = Cv x (V + D3) ............................................. (16) where:

Cv is a constant



Calculate the parameter (w):

w = 1.89 x (Cp1 / Cv / Vs

2 )0.33 ...................................... (17) For axial flow in an adiabatic pressure vessel:

Optimum voidage = w0.5 / (1 + w 0.5) ........................... (18) If optimum voidage is less than 0.4 - use pellets or beads. If optimum voidage is greater than 0.4 - use rings or maybe eccentric Typical optimum voidages:

Ammonia plant secondary reformer 0.5 Ammonia plant methanator 0.36 Methanol plant converter 0.5

The catalyst needs to be strong to maintain a voidage above 0.4, so GBHE still uses pellets for methanol synthesis catalyst. Figure 3 shows the variation of costs (pressure drop cost, vessel cost and total cost) with catalyst bed voidage. FIGURE 3 VARIATION OF COSTS WITH CATALYST BED VOIDAGE



4.3 Pellet Density Higher density gives:

(a) More active catalyst component. (b) Lower pore volume:

(1) Lower effectiveness (2) Possibly lower selectivity.

There appear to be no established relationships to determine optimum pellet density. 5 REACTOR DESIGN Optimum length to diameter ratio of the reactor may be determined as follows: Pressure drop cost:

Capitalized cost = Cp x V / (de x D6) ................................ (6) where:

Cp is a constant for fixed voidage V is the catalyst volume de is the equivalent sphere diameter D is the catalyst bed diameter.

Vessel cost:

Capital cost = Cv x (V + D3) ............................................ (8) where:

Cv is a constant.

Optimum diameter D = ( 2 x Cp x V / (Cv x de))1/9 ...... (19)

If optimum L / D ratio is greater than 1 - use axial flow in vertical vessel. If optimum L / D ratio is between 0.2 and 1 - consider horizontal vessel. If optimum L / D ratio is less than 0.2 - consider radial flow vessel.



Typical optimum L / D ratios:

Ammonia plant secondary reformer 0.8 Ammonia plant desulfurizer 2.8 Ammonia plant LT shift 1.1

Figure 4 shows the variation of costs (vessel cost, pressure drop cost and total cost) with vessel diameter). FIGURE 4 VARIATION OF COSTS WITH VESSEL DIAMETER

6 CATALYST SUPPORT TYPES

The following catalyst support types are available:

(a) Pellets, beads, extrudates (b) Rings. (c) Monoliths (honeycombs). (d) Ceramic foams (macroporous catalyst):

(1) Sheet (2) Pellets.

(e) Knitted wire mesh. (f) Multi-holed extrudates



6.1 Choice Of Support 6.1.1 General Guidance

(a) Use pellets, rings or extrudates when optimum particle diameter is greater than 2 mm.

(b) Use rings if optimum voidage is greater than 0.4. (c) Use monoliths if optimum particle diameter is less than 2 mm and

cost of pressure drop is high. (d) Use ceramic foams or knitted wire mesh if optimum particle

diameter is less than 1 mm. (e) Use multi-holed extrudates when optimum particle diameter is less

than 2 mm and using a tubular reactor. Other factors that come into play are: (1) Length / diameter limitations. (2) Fouling. (3) Heat transfer limit in tubular reactors. (4) Vessel length / particle diameter ratio. (5) Degree of conversion required. (6) Want to be film diffusion limited. 6.1.2 Quantitative Evaluation The best method to evaluate alternative catalysts is on a spreadsheet model. In order to do this the supports need to be characterized. Since most novel supports are only generally used when the catalyst is severely pore diffusion limited (in order to get high surface areas), they can be characterized most easily on the basis of their geometric surface area.



The types of support can be characterized by:

Total bed voidage e = 1 - (1- eb ) x (1- ep) ................................ (20) where:

eb is the Interparticle voidage ep is the Intraparticle voidage.

Specific geometric surface area per unit volume of bed (As):

As = 6 x (1- e) / de ........................................... (21)

where:

de is the equivalent sphere diameter.

Specific pressure drop (Ps) is the pressure drop as velocity heads per unit geometric surface area. For turbulent flow through pellets, spheres, rings, gauze or ceramic foams:

Ps = 0.58 / eb3 …………………................................ (22)

For multi-holed extrudates of typical dimensions (particle size / de = 4):

Ps = 0.18 / eb

3 ............................................................. (23)

For monoliths with turbulent flow:

Ps = 0.015 / ep

3 ......................................................... (24)



For monoliths with laminar flow (Re < 3500 ):

Reynolds number Re = actual velocity x hole diameter / viscosity.. (25)

Hole diameter = 2/3 x de x ep / (1 – ep) ................................. (26) Ps = 16 / Re / ep

3 ……………...................................................... (27)

Pressure drop:

Pressure drop = Ps x surface area x superficial velocity head ... (28)

6.1.3 Cost of Catalyst

For some types of support it is most appropriate to measure the cost of the catalyst per unit volume, whereas for others, it is appropriate to measure it per unit geometric surface area. The vessel cost can be determined as before. Table 1 details typical manufacturing costs for each catalyst support shape.

TABLE 1 CATALYST SUPPORT SHAPES



6.1.4 Example: Secondary Reformer What is the optimum support and surface area / unit volume for the following example:

Surface area required A = 6000 m2 Cost of vessel Cd = 3600 (V + D3) Capitalized pressure drop cost Cp = £2 / pascal Mass flow M = 38 kg / s Density = 5 kg / m3 Catalyst life = 2 yrs Viscosity * 1000 = 0.4 Factor for optimum diameter b = 1990

The optimum support and surface area / unit volume can be determined from Table 2.



TABLE 2 SECONDARY REFORMER SPREADSHEET



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