02 BOPs / Woods D.R 2008 rules-of-thumb-in-Engineering-practice (epdf.tips)

6.3 How the Type of Reaction Affects the Size of the Reactor 221

Figure 6.7 Residence time versus temperature for liquid, liquid–liquid, gas–liquid and gas–liquid–solid reactions. The factor-of-ten lines are drawn.

acid. (+) via disproportionation; terephthalic acid (+) via oxidation; toluene diisocyanate (+) via carbonylation; xylene, (+) via disproportionation.

Option 2: identify the type of reaction and details are given for each. Data are missing for some types of reactions. For many, the data are limited to a few data points and option 1 may be the only possible choice because no data are given for the system of interest. These are organized according to the type of reaction:

Acetoxylation: L, catalyzed, CSTR, 80 hC, 7200 s (1 species, 1 datum). Acetylation, LS, catalyzed, batch STR [–220 MJ/kmol]; 35 hC; 2000 s (2 species, 4 data) rate doubles in 15 hC for the range 20–60 hC.

Addition, G, solid catalyst, multitube catalyst bed/fluidized bed [–110 MJ/kmol] 180 hC, 10 s (2 species, 6 data) rate doubles in 30 hC for the range 100–225 hC. Aldol condensation, G, solid catalyst, 300 hC, 25 s (1 species; 2 data) rate doubles 100 hC.

222 6 Reactors

L, homogeneous catalyst, loop reactor; 20 hC, 5500 s (1 species, 1 datum) Alkylation, G, solid catalyst, series of adiabatic beds, fluidized bed; 425 hC, 100 s (3 species, 9 data) rate doubles in 15 hC for the range 300–475 hC.

GL, aluminum chloride catalyst, bubble column [–115 MJ/kmol]; relatively independent of temperature for the range 50–180 hC, 1500 s (3 species, 5 data)

L, solid catalyst, multistage adiabatic, reactive distillation [–114 MJ/kmol] 200 hC, 2000 s, (cumene, phenol, 6 data) trajectory doubles in 25 hC for the range 60–260 hC.

LL, sulfuric acid catalyst, multistage CSTR [–80 to –112 MJ/kmol] 4 hC, range 300–40 000 s (alkylate, 9 data); dimethylaniline, 200 hC, 18 000–22 000 s (2 data); with HF, 25–40 hC, 300–2500 s (alkylate, 2 data).

Amination by ammonolysis, G, solid catalyst, fluidized bed [–650 MJ/kmol] 440 hC, 4 s (acrylontrile, 5 data) rate doubles 30 hC for the range 400–550 hC; multitube fixed catalyst bed [–650 MJ/kmol] 230 hC, 4 s (acrylonitrile, ethyl amine, 4 data) rate doubles in 25 hC for the range 150–250 hC.

Amination by reduction, G, fixed catalyst bed, 110 hC, 3300–6500 s. (1 species, 2 data).

Carbonylation (oxo, hydroformylation) react synthesis gas with olefin, GL, catalyst, fixed bed, CSTR or bubble reactor [–140 MJ/kmol] wide range; temperatures 100–200 hC, 3–40 000 s. (5 species, 15 data) no apparent trends.

Fischer Tropsch from synthesis gas to make alkanes or alcohols, G, catalyst, series of fixed beds, for alkanes, [–165 MJ/kmol] 350 hC, 40 s (1 species, 4 data), negligible temperature effect; for alcohols [–245 MJ/kmol] 250 hC, 20 s (1 species, 4 data), negligible temperature effect.

Chlorination, G, no catalyst, some catalyst, tower, fluidized bed, PFTR, multibed, multitube [–82 to –220 MJ/kmol] 400 hC, 2 s (4 species, 8 data) trajectory doubles in 60 hC for the range 150–520 hC.

GL, no catalyst, CSTR 20–90 hC, 500 000 s (chloral, 1 data);

GL, solid catalyst, STR, bubble [–100 to –185 MJ/kmol] 40–50 hC, 15–90 000 s; (2 species, 3 data) no apparent trends.

Combustion, G, no catalyst, plasma, burner [600 MJ/kmol] 1000–2700 hC, 0.01–0.002 s (acetylene, 10 data) no apparent trends.

Condensation, G, catalyst, fixed bed [–50 MJ/kmol] 400–550 hC, 45 s (ammonia, 9 data); fluidized bed [very exo] 300 hC, 0.6 s (acrolein, 1 data).

L, catalyst, pipe loop, CSTR (single and multistage), fixed bed [–45 to –100 MJ/ kmol] 100 hC, 6000 s (12 species, 15 data) trajectory doubles in 20 hC for the range 15–200 hC.

Cracking thermal (Pyrolysis), G, no catalyst, fired tube [60 to 150 MJ/kmol] 700 hC, 0.7 s (10 species, 21 data) trajectory doubles in 50 hC for the range 400–900 hC.

Dehydration, G, catalyst, tubular fixed bed [–290 MJ/kmol] 340 hC, 100 s (3 species, 6 data) rate doubles in 10 hC. L, batch STR 20 hC, 20 000 s (1 species, 2 data) rate doubles in 50 hC.

Dehyrochlorination, L, no catalyst, series of CSTR, 110 hC, 15 000 s (chloroprene, 2 data).

6.3 How the Type of Reaction Affects the Size of the Reactor 223

Dehydrogenation, G, metal or metal oxide catalyst, adiabatic fixed bed or multitube [51 MJ/kmol] 250 hC, 5 s (5 species, 8 data) trajectory doubles in 65 hC for the range 225–400 hC; [130 MJ/kmol] 650 hC, 10 s (3 species, 17 data) rate doubles in 25 hC for the range 550–800 hC.

L, catalyst, CSTR 115 hC, 2000 s (2 species 6 data) trajectory doubles in 25 hC for the range 100–130 C.

Disproportionation, isomerization, G, catalytic, adiabatic fixed bed, moving bed [–10 MJ/kmol] 310–500 hC, 0.1–150 s (2 species, 15 data) no apparent trends. L, catalytic, 200 hC, 1300 s (xylene, 13 data) trajectory doubles in 80 hC for the range 30–350 hC.

Epoxidation, L, catalytic, series of CSTR, 75 hC, 10 000 s (2 species, 5 data) trajectory doubles in 40 hC for the range 50–110 hC.

Esterification, L, sulfuric acid or IX resin catalyst, CSTR, reactive distillation, [–30 MJ/kmol] 100 hC, 6000 s (7 species, 20 data) rate doubles in 15 hC for the range 70–160 hC.

Ethynylation, GL, homogeneous catalyst, PFTR tube, [–230 MJ/kmol] 30 hC, 200 s (methyl butynol, 2 data).

Fermentation, L, anaerobic: 20–30 hC, 20 000–280 000 s (for tropophase and idiophase but excluding time in the seed tank).

aerobic, GL, sparged STR, airlift, 20–65 hC, 1200–1 600 000 s. Figure 6.4 illustrates the residence times for different types of products.

Hydration, G, catalyst, adiabatic PFTR [–45 MJ/kmol] 300 hC, 2500–126 000 s (ethanol, 2 data). L, no catalyst, [–75 MJ/kmol] 175 hC, 3000 s (ethylene glycol, 2 data); acid catalyzed, 55 hC, 1800 s (1 species, 1 datum).

Hydrodealkylation, G, no catalyst, fire tube [–50 MJ/kmol] 675 hC, 30 s (2 species, 4 data) rate doubles in 30 hC for the range 625–725 hC. G, catalyst, 675 hC, 0.025 s (3 species, 5 data) rate double in 25 hC for the range 400–700 hC.

Hydrogenation, G, no catalyst, tank [–44 MJ/kmol] 700 hC, 7.5 s (benzene, 1 datum); catalyst, fluidized bed, fixed bed [–500 MJ/kmol] 300 hC, 1 s (aniline, 2 data) rate doubles in 30 hC for the range 250–380 hC. catalyst, fixed bed, multitube [–200 MJ/kmol] 200 hC, 0.5 s (cyclohexane, 4 data) rate doubles in 35 hC for the range 150–250 hC.

GL, Ni, Cu, Cr catalyst, sparge STR, slurry, trickle bed, fixed bed [–156 and varies MJ/kmol] 20–250 hC, 200–40 000 s (8 species, 10 data) relatively temperature independent, no apparent trends.

Hydrolysis, L, acid catalyst, [–15 to –55 MJ/kmol] 100 hC, 3000 s (7 species, 10 data), trajectory doubles in 20 hC for the range 10–160 hC.

Neutralization, L, no catalyst, STR or slurry pipe 60 hC, 220 s (sodium benzoate, 1 datum).

Nitration, L, catalyst, CSTR [–100 to 145 MJ/kmol] 100 hC, 5000 s (4 species, 9 data) trajectory doubles in 30 hC for the range 30–160 hC.

Oxidation, G, catalyzed, multitube, shallow adiabatic [–150 MJ/kmol] 400 hC, 0.08 s (3 species, 5 data) trajectory doubles in 30 hC for the range 130–350 hC. catalyzed, multitube, shallow adiabatic, [–250 MJ/kmol] 400 hC, 0.7 s (8 species, 18 data) trajectory doubles in 30 hC for the range 180–750 hC. Catalyzed, fixed bed, fluidized

224 6 Reactors

bed [–900 MJ/kmol] 400 hC, 10 s (3 species, 5 data) trajectory doubles in 30 hC for the range 300–900 hC.

GL and L, homogeneous catalyst, bubble column, sparged STR, CSTR [–100 to –250 MJ/kmol] 150 hC, 1800 s (14 species, 21 data) trajectory doubles in 20 hC for the range 10–200 hC.

GLS, catalyst and GLL, columns with air agitation, data above are reasonable (3 species, 6 data).

Polymerization (thanks to A.E. Hamielec and R. Hutchinson for their data) L, catalyzed, STR [– 55 to –95 MJ/kmol monomer] 70 hC, 18 000 s (7 species, 15 data) trajectory doubles in 25 hC for the range 50–270 hC. Batch RIM, 70 hC, 45 s (polyurethane, 1 data); high pressure, continuous tubular, 200 hC, 45 s (LDPE, 3 data). Acrylates {E = 15–18 kJ/mol}, methyl acrylate kp at 30 hC = 14 000 L/mol s; dodecyl acrylate kp at 30 hC = 21 000 L/mol s; Methacrylates {E = 21–23 kJ/mol}, methyl methacrylate kp at 30 hC = 380 L/mol s; dodecyl methacrylate kp at 30 hC = 600 L/mol s; Styrene{E = 32–33 kJ/mol}, kp at 30 hC = 110 L/mol s; vinyl acetate {E = 20–21 kJ/mol} kp at 30 hC = 4000 L/mol s.

Reforming, cat, G, catalyst, fired tube, 550 hC, 0.2 s (refinery, 2 data). Sulfonation, G, catalyst, furnace, fixed bed, 600 hC, 4 s,(2 species, 3 data) trajectory doubles in 25 hC for the range 370–700 hC. L, catalyst, CSTR, 100 hC, 7000 s, (2 species, 2 data) trajectory doubles in 25 hC for the range 30–150 hC.

Example, the target is to produce ethylene from ethane by pyrolysis at 800 hC; the heat of reaction is 144 MJ/kmol and no catalyst is used. Estimate the residence time. Option 1, at 800 hC the residence time might be between 0.5 and 5 s. Option 2, Consult cracking, thermal (pyrolysis) gas. The data are for no catalyst, a heat of reaction in the correct range and suggest 700 hC, 0.7 s with the trajectory doubling in 50 hC. Hence, since the temperature of 800 is 100 hC higher and since the rate doubles every 50 hC the residence time would be reduced by 100/50 = 2 or 22 = 4 times faster or 0.7/4 s = 0.17 s or 0.2 s. Comment: published data report a residence time of 1.5 s or about 10 times more residence time required than predicted with Option 2. This illustrates the method and the error involved in this order-of-magnitude method.

The rest of the rules of thumb are organized based on reactor type.

6.4 Burner

x Area of Application

Suitable for gas reactions that are homogeneous and fast and are either highly exothermic or endothermic. Usually have a high reaction temperature. Short residence time.

6.5 PFTR: Pipe/Tube, Empty Pipe for Fluid Systems 225

x Guidelines

Burner designed to introduce reactants into the “combustion” zone. Be careful to ensure that the system is not operating within the explosive limits. Examples include the fluorination of UF4 to UF6 at 350–600 hC with solids feed at rate of 1 Mg/d; uranyl nitrate dehydrated and denitrified to uranium dioxide at rate of 1 Mg/d. Air oxidation of phosphorous to P2O5 at 2500 hC at 1.5 Mg/h. This reactor is 1.8 m diameter, 10 m high with gas flow of 1 m/s and molten flow of 0.4 kg/s. Most reactors have to be custom designed.

6.5

PFTR: Pipe/Tube, Empty Pipe for Fluid Systems

The empty tube can be coiled, straight, or hairpins; horizontal or vertical; in a cooling bath, in the atmosphere or can be placed in a furnace for temperatures i 500 hC. Can operate under pressure or vacuum. Synonyms include “fire tube” (for high temperature applications); “tube loop” (for gas liquid applications) where liquid is pumped into the bottom of a vertical coil, exits and recycles to the pump; gas is injected into the bottom of the loop. Another GL option is the “tubular reactor with injector” to create very fine bubbles and large surface area. For GL, the velocity and method of introduction of the second phase varies. Use external surface area if temperature control is important. Characteristics of these are given in Section 1.6.1.

The characteristics of this configuration are Bd = T. Pe i 100.

x Area of Application

Usually controlled by reaction kinetics or heat transfer or both. Provides a welldefined residence time and is capable of good temperature control.

Phases: Gas, liquid, gas–liquid, liquid–liquid. Use if the order of the reaction is positive and i 95 % conversion is the target, and for consecutive reactions with an intermediate as the target product. For homogeneous reactions.

Gas (or gas with homogeneous catalyst): heat of reaction: endothermic; reaction rate, fast; capacity: 0.001–200 L/s; good selectivity for: consecutive reactions and irreversible first order; volume of reactor 1–10 000 L; OK for high pressures or vacuum. For temperatures I 500 hC. For temperatures i 500 hC use fire tube. For example, used for such homogeneous reactions as acetic acid cracked to ketene. Liquid (or liquid with homogeneous catalyst): heat of reaction: endothermic; reaction rate, fast or slow; capacity: 0.001–200 L/s; good selectivity for consecutive reactions; volume of reactor 1–10 000 L; OK for high pressures. For temperatures i 500 hC use fire tube. For example, used for visbreaking and delayed coking.

Gas–liquid and GL + microorganisms (bio): Residence time: short; heat of reaction: primarily for endothermic reactions. Beware of highly exothermic reactions because of inability to control temperature; good selectivity for consecutive reactions in which the product formed can react further. see bubble reactors, Section 6.13. Use with irreversible reactions and pure gas feed. Area per unit volume 50

226 6 Reactors

to 2000 m2/m3; high pressure drop. For tube loop: modest OTR, ease in injection along the tube. For injector into tube: extremely large surface area at the expense of power input. Bubbles 1 to 30 mm diameter.

Liquid–liquid: see also bubble reactors, Section 6.13.

x Guidelines

Gas: Residence time 0.5–1.3 s; gas velocity 3–10 m/s; Re i 104, L/D i 100. To eliminate backmixing Pe i 100. For temperatures i 500 hC place in a furnace. Fire tube: or tubes in furnace.

Radiant heat flux in furnaces 30–80 kW/m2. Reformers:

xgas oil: heat flux: 40–50 kW/m2; fluid velocity inside tubes 1.5–2.5 m/s

xlight oil: heat flux: 25–40 kW/m2; fluid velocity inside tubes 1.4–2.3 m/s

xheavy oil: heat flux: 25–35 kW/m2; fluid velocity inside tubes 1.7–2.1 m/s

Cracking: ethylene ex

xethane: heat flux: 23–28 kW/m2 at exit conditions with double values at inlet

xpropane: heat flux: 14–17 kW/m2 at exit conditions with double values at inlet

xbutane or naphtha: heat flux: 11–15 kW/m2 at exit conditions; at inlet the values are twice as large.

See thermal energy: furnaces, Section 3.2.

Liquid: Residence time: 0.4–2000 s with the usual values 1–6 s; liquid velocity 1–2 m/s; Re i 104, L/D i 100. PFTR is smaller and less expensive than CSTR. PFTR is more efficient/volume than CSTR if the reaction order is positive with simple kinetics.

For fast reactions: use small diameter empty tube in turbulent flow.

For slow reactions: use large diameter empty tubes in laminar flow. If reaction is complex and a spread in RTD is harmful, consider adding static mixer, Section 6.6.

Examples: hydrolysis of corn starch to dextrose; polymerization of styrene; hydrolysis of chlorobenzene to phenol; esterification of lactic acid.

Gas–liquid including bio: The gas is introduced into the liquid via a tube or by an ejector. Surface area varies dramatically depending on the power input and configuration. In general, superficial gas velocity, 0.01 to 0.4 m/s; holdup 0.05 to 0.95; energy 0.1 to 100 kW/m3. I 10 cm diameter tubes, kL a = 0.01 to 0.7 1/s. For tubes: 50–400 m2/m3; power 1–80 kW/m3 total volume; for vertical tube loop: 50–2000 m2/m3; power 0.1–100 kW/m3 total volume, OTR: 2.7 g/s m3; mixing time: 80 s; gas content: 30 %; maximum volume 3000 m3; for horizontal coil 50–700 m2/m3; for ejector into a tube: 40 000–500 000 m2/m3; power 100–

6.7 PFTR: Empty Pipe/Tube for Fluids and Solids 227

10 000 kW/m3 total volume. Non-coalescing bubbles kLa+ = 5 q 10–5–2 q 10–4 and increasing with Power/Volume. OTR data, Section 1.6.1. See also two-phase flow Section 2.4.

Liquid–liquid: see size reduction, Sections 8.3 and 1.6.2.

6.6

PFTR: Static Mixer in Tube

x Area of Application

Phases: gas with mixer as catalyst, gas–liquid, liquid, liquid–liquid.

Fast competing parallel or consecutive reactions that are highly exothermic. For gas–liquid: fast reactions in the liquid phase. Flexible, interstage addition possible. Large heat transfer area; intensive radial mixing with negligible backmixing; narrow RTD; suitable for processes where viscosity increases. Not for foaming systems.

x Guidelines

See Heat transfer Section 3.5 and Mixing Sections 7.1 and 7.3.

For gas reactions: Example: oxidation of ammonia to nitric acid, production of maleic anhydride, xylene, styrene, vinyl chloride monomer, ethylene dichloride. L/D for mass transfer mixers 6:1–20:1. Gas velocity for turbulent flow.

For gas–liquid reactions: cocurrent mass transfer in bubble flow: gas superficial velocity 0.6–2 m/s; liquid superficial velocity 0.3–3 m/s; volumetric flowrate ratio of gas to liquid = 1 at the nozzle. Holdup 0.5, energy 10 to 700 kW/m3; kL a = 0.1 to 3 1/s. spray flow: gas superficial velocity 3–25 m/s; area to volume 1000– 7000 m2/m3. See Section 1.6.1.

For liquid–liquid reactions: dispersed phase drops diameter 100–2000 mm with diameter decreasing as the velocity increases, the surface tension decreases and the hydraulic radius of the mixing element decreases; surface area 100– 20 000 m2/m3 depending on the drop diameter and the concentration of dispersed phase. Turbulent flow. Example reactions as a PFTR: polymerizations of polystyrene, nylon, urethane; sulfonation reactions and caustic washing. See Size reduction Sections 8.3 and 1.6.2.

x Good Practice

Prefer because small volume provides a means of intensification (H).

6.7

PFTR: Empty Pipe/Tube for Fluids and Solids

Synonyms include “transported or slurry reactors” and “transfer line”. The velocity and method of introduction of the second phase vary. Related options include open multitube, Section 6.8. fluidized reactor, Section 6.30.

228 6 Reactors

x Area of Application

Phases: Gas–solid, gas–solid reactant, liquid–solid, gas–liquid–solid. Good for very fast reactions and for consecutive reactions. Large transfer area; temperature can be controlled by injection; little backmixing that gives a well-defined residence time.

Gas plus catalytic solid: Reaction rates very fast and very rapid deactivation of catalyst. Solid particle diameter 0.007–1.5 mm.

Gas plus solid reactant: solid particle diameter 0.007–1.5 mm.

Liquid plus solid catalyst: slurry reactors.

Gas–liquid plus solid catalyst: For fast hydrogenation reactions. Compared with trickle bed Section 6.17 or PFTR fixed bed with up flow, Section 6.9.

1.Catalyst particles are small so less chance of diffusional resistance to mass transfer.

2.Better control of temperature (because of better heat transfer efficiency and high heat capacity of slurries) attractive for exothermic reactions.

3.Don’t have to shut down for catalyst replacement of reactivation.

4.Partial wetting and need to maintain a coating film of liquid (as needed in the trickle bed) are not issues.

5.Usually the space time yield is better in slurry reactors (under comparable conditions).

Gas–liquid plus microorganisms (bio) see Section 6.5.

x Guidelines

Gas plus catalytic solid: Gas residence time in milliseconds; see pneumatic conveying, Section 2.6 and transported bed drying, Section 5.6.

Gas plus solid reactant: Solid residence time, 0.8–300 s; gas residence time, I 1 s; see pneumatic conveying, Section 2.6.

Liquid plus solid catalyst: see Section 2.5. Liquid plus solid reactant: see Section 2.5.

Gas liquid plus solid catalyst: Usually operate in the churn-slug and piston slug flow regimes with gas velocities i 0.05 m/s for water-like liquids. Flow regimes are given in Fig. 2.2, Section 2.4. Gas holdup is proportional to the (superficial gas velocity)n where n = 0.7–1.2 in the bubbling regime and n = 0.4–0.7 in the churn turbulent regime. Gas holdup is independent of diameter but very sensitive to trace contaminants and foaming. Used for some hydrogenations.

x Good Practice

Ensure the flow regime is maintained.

6.9 PFTR: Fixed Bed Catalyst in Tube or Vessel: Adiabatic 229

6.8

PFTR: Empty Multitube, Nonadiabatic

Related to empty single tube, Section 6.5 or transfer line, Section 6.7.

x Area of Application

Suitable for fast, gas or liquid homogeneous, exothermic reactions. This provides narrow residence time distributions, and a large heat transfer area. Multistaging is possible. Static mixers can be used if viscosity is high.

x Guidelines

Size on residence time and heat transfer area, Section 3.3. Shell and tube exchanger with reactants inside the tube, 250–400 m2/m3. Tube diameter I 50 mm. The smaller the diameter of the tubes, the larger the surface area from the tubes.

Gas. Use high mass gas velocity to improve heat transfer kg/s m2 i 1.35. To ensure good gas distribution and negligible backmixing, Pe i 2. Gas velocity 3– 10 m/s; residence time 0.6–2 s. Heat transfer coefficient: Gas at 0.1 kPa g vs. liquid: U = 0.05 kW/m2 K; Gas at 20 MPa vs. liquid: U = 0.5 kW/m2 K.

Individual coefficient on shell side: coolants: boiling water, h = 1–3 kW/m2 K; boiling organic, h = 0.2–1.5 kW/m2 K; molten salts, h = 0.5–1.5 kW/m2 K. Heating agents: steam, h = 2–5 kW/m2 K; combustion gas, h = 0.01–0.03 kW/m2 K.

6.9

PFTR: Fixed Bed Catalyst in Tube or Vessel: Adiabatic

x Area of Application

Phases: Gas, liquid, gas–liquid interacting with solid catalyst or inert solid. Use if the order of the reaction is positive and i 95 % conversion is the target, and for consecutive reactions with an intermediate as the target product. Caution use for highly exothermic reactions. Not suitable for Arr (DTad+) i 10; or DTad i 100 hC; usually keep DTad I 50 hC. Provides large gas throughput. Related topics for GL, trickle reactor, Section 6.17, or bubble reactor, Section 6.13.

Gas with fixed bed of solid catalyst: heat of reaction: endothermic or slightly exothermic; reaction rate, fast; good selectivity and activity for: consecutive reactions and for irreversible first order reactions; volume of reactor 1–10 000 L; OK for high pressures. High conversion efficiency, simple, flexible, gives high ratio of catalyst to reactants.

Liquids plus fixed bed of solid catalyst: capacity: 0.001–200 L/s. Same general expectations as for gas.

Gas–liquid–solid: similar to trickle bed, Section 6.17.

x Guidelines

Catalyst diameter 1–5 mm.

230 6 Reactors

Gas with fixed catalyst bed: Residence time I 1 s; favored if the life of the catalyst is i 3 months. If the catalyst deactivates rapidly select fluidized, Section 6.30 or slurry reactors, Section 6.7. Catalyst must have an axial crush strength i 50– 80 kg/cm2. To ensure good gas distribution and negligible backmixing, Pe i 2; Height/catalyst particle diameter H/Dp i 100 and Dp/DI 0.10. Usually bed volume porosity 0.42 which decreases to 0.38 as the bed ages. PFTR gives less volume than slurry or fluidized bed reactors. If the main reactant undergoes 90 % conversion within a reactor length of bed height/catalyst particle diameter = 100, then the reaction is not mass transfer controlled. Select a bed height such that the length of the tube/mass flow velocity (kg/s m2) i 0.5 m3 s/kg fluid. Dp is I 1 to 10 % of total pressure; if Dp too high then use larger catalyst or change catalyst.

For adiabatic operation with exothermic reactions, limit the height of the bed to keep temperature increase I 50 hC. Tube diameter I 50 mm to minimize extremes in radial temperature gradient. For fast reactions, catalyst pore diffusion mass transfer may control if the catalyst diameter i 1.5 mm.

Temperature gradients within the catalyst and in the external bulk phase:

1.Within a catalyst pellet the internal temperature gradient is rarely i 1–2 hC between the surface and the center. Assume temperature at the center of the catalyst = surface temperature.

2.The external temperature gradient is usually high with the catalyst surface temperature 10–30 hC hotter than the temperature in the gas phase.

Concentration gradients within the catalyst and in the bulk phase:

3.Within the catalyst pellet the internal concentration gradient is often very high (with 0 concentration at the center).

4.External concentration gradient is usually small except for very fast reactions.

Shallow beds, including catalytic gauze, are used for high reaction rates and unstable products. These provide very short residence times. OK for autothermal operation.

Related topic gas adsorption, Section 4.11.

Liquids with fixed catalyst bed: To minimize backmixing, Pe i 1; use H/Dp i 200 and Dp/DI 0.10. Temperature gradient within the catalyst and in the external bulk phase:

1.Within the catalyst, the internal temperature gradient is low. Assume the temperature at the center of the catalyst = surface temperature.

2.The external temperature gradient from the catalyst surface temperature to the bulk is low.