Lloyd L. Handbook of Industrial Catalysts

Подождите немного. Документ загружается.

8 Chapter 1

1.2.4. Strength

Plant operation will be adversely affected as pressure drop increases through the

catalyst bed. This may be due to:

• Dust formed from catalyst disintegration during changing and operation.

• Entrained liquid cementing the catalyst.

• Entrained solids blocking the bed.

• Collapsed beds following mechanical damage to bed supports.

• Carbon formation from organic feedstocks.

These problems generally affect the temperature proﬁles in the bed and,

possibly, the overall reaction. The catalyst must be strong enough to resist vari-

ous forms of regeneration or reactivation.

1.3. CATALYST PRODUCTION

Catalysts are produced in different ways depending on the chemical formulation

and the severity of the chemical process in which they will be operated. The

usual methods are listed in Table 1.5. Figures 1.2 and 1.3 show typical catalyst

production facilities (See also Table 1.9 where some unit operations used in cat-

alyst manufacturing are listed).

TABLE 1.5. Preparation and Application of Industrial Catalysts.

Preparation Application Catalyst

Impregnation Suitable supports impregnated with soluble

salts of the catalytic metals which are de-

composed to oxides and reduced before use.

Catalysts containing small

amounts of precious metals or

easily impregnated amounts of

base metals.

Precipitation Carbonates/hydroxides of catalytic metals

precipitated, decomposed and pelletted, ex-

truded or granulated before reduction.

Catalysts containing high con-

centrations of base metals,

which are required in a particu-

lar physical form.

Fusion Metal oxides fused and chill cooled before

crushing and sieving to required size.

Only used in relatively few

catalysts.

Metals Metal catalysts alloyed with a metal soluble

in alkali. Catalyst active after removing sol-

uble material. Alternatively, thin wires of

catalytic metals can be woven into ﬁne mesh

and used directly. Metals occasionally used

directly in granule form.

Only used in special applica-

tions.

Figure

up fro

Ed. by

sion of

Fig

1.2. Semi-tech

laboratory to f

. V. Twigg,

. Twigg.

re 1.3. Typica

ical unit for c

ll-scale manuf

olfe Publishin

production lin

talyst preparati

cture. Reprinte

, LTD., Londo

for manufactu

Industrial

n, an intermed

from Catalyst

, England, 198

e of catalysts b

Catalysts

ate stage in sc

andboo

, 2

y kind per

precipitation.

le-

d.,

is-

10 Chapter 1

Many of the earliest catalysts were based on natural products or porous re-

fractory materials that were available commercially, but improved catalysts were

especially made as large-scale processes developed. Table

1.6 shows some of

the materials that have been used as support.

The principal objective in catalyst production, however, has usually been to

make a suitable catalyst for any large-scale process by the most reasonable eco-

nomical procedure. Catalysts with a speciﬁc chemical composition have been

established and appropriate standards for such physical properties as particle

size and strength have been developed for most commercial processes. Target

speciﬁcations provide for reasonably predictable operation at an acceptable

pressure drop when used in standard plant equipment. After the initial period of

deactivation, provided that the decline of catalyst performance is very slow and

predictable, a process can be designed that is economic over the life span of the

catalyst. A typical catalyst speciﬁcation is shown in Table 1.7.

The most frequently used methods to produce catalysts are precipitation and

impregnation. In both processes the catalyst precursors are usually converted to

oxides by heating and the powders converted to solid granules or pellets. Cata-

lysts often contain promoters that are added during the preparation stages. If

TABLE 1.6. Catalyst Supports.

Material Function

Essential properties of support:

Inert Should not react with the active catalyst or take part in reaction.

Strong Should not disintegrate during handling or use; low attrition loss.

Porous Disperses the active catalyst to increase the activity and reduce

the cost of expensive material.

Early supports

Asbestos Contact process.

Pumice Deacon process, hydrogenation catalyst supports.

Kieselguhr (infusorial earth) Fat hardening, hydrogenation catalyst support.

Bauxite/titanium dioxide Dehydration reactions, catalyst support and cracking catalyst.

Carbon Support for precious metals.

Metal salts, e.g., MgSO

MgCl

Contact process; oleﬁn polymerization.

Quartz lumps Used as an inert support for catalysts and also as a physical

support at the bottom of a catalyst bed.

Modern Supports

α-Alumina Gamma and

-alumina used as a catalyst support in many

reactions.

Silica Produced as a pure catalyst support in a number of forms.

Activated carbon Still used as a catalyst support in some processes.

Silica/alumina Only used in special applications now that it has been replaced

by zeolite in cracking reactions.

Cordierite Used as preformed monolith in automobile exhaust treatment

and other similar applications.

Industrial Catalysts 11

TABLE 1.7. Typical Catalyst Speciﬁcations.

Property Test

Chemical analysis

Major components: wt% (either maximum/minimum level or acceptable

range.)

Impurities: wt% or ppm (maximum permitted level.)

Loss on ignition: wt% (sample calcined to a loss-free basis at an appropriate

temperature.)

Physical properties Particle shape: pellets, granules, rings, extrusions.

Particle size: length, diameter.

Bulk density: kg.liter

-1

; lb.ft

-3

Crushing strength: lb or kg nominal.

Micromeritics Surface area: m

-2

-1

Mercury density: g.cm

-3

Helium density: g.cm

-3

Mean pore radius: Å

Testing is usually done on a bulked representative sample from the daily production.

necessary, a support may be included with the solutions used during

precipitation, as well as being impregnated directly with oxide precursors.

Many catalysts can be made by either of the two methods. Precipitation is

usually chosen if a support is not porous enough for sufﬁcient active metal to be

loaded by impregnation. On the other hand, low concentrations of expensive

precious metals are impregnated on to suitable supports particularly when they

can be deposited on the surface of the support for greater efficiency. Conditions

must be carefully controlled during catalyst production to give consistent

quality. The checks carried out during all stages of production are listed in Table 1.8.

TABLE 1.8. Routine Testing During the Production of an Industrial Catalyst.

Test Properties

Routine chemical analysis

(products and raw materials)

Major metal components, metal impurities, well-mixed phases.

Crushing strength Compression strength between faces (vertical) or across diameter

(horizontal).

Attrition/tumbling loss Dust formed during rotation in a tube under standard conditions.

Micromeritics Surface area, helium and mercury density, pore volume, mean

pore radius.

Particle size Average length of pellets, average diameter of spheres, average

length of extrusions.

Particle size distribution Proportion of required, undersize, and oversize particles.

Particle porosity Internal volume available during impregnation and operation.

Bulk density Indication of pelletting or granulation efﬁciency.

Catalyst activity and stability Determines preliminary reduction and operating efﬁciency.

12 Chapter 1

Porosity and surface area are probably the most important properties of a

catalyst as they control access to the active sites of the catalyst. Pore size and

surface areas can be moderated in a number of ways to control selectivity and

access of large molecules to the catalyst. It is interesting to remember that 500 g

of catalyst with a surface area of 100 m

−1

has a total area of about 12 acres. A

catalyst charge of 40 tonnes therefore has a total surface of almost 1 million

acres. An amazing ﬁgure!

1.3.1. Precipitation

Aqueous solutions of the metal salts, usually nitrates or sulfates, are precipitated

with an alkali such as sodium carbonate. Ammonium carbonate can be used to

avoid residual sodium impurity, but it is relatively expensive. Occasionally

precipitation conditions are controlled to form complex catalyst precursors and

higher-activity products. If necessary, any support material can be added as a

powder before the alkali is added.

When the precipitate has formed and settled, it is ﬁltered and carefully

washed before drying. Dried mud is then calcined at a temperature in the range

300

– 450

C to decompose hydrates and carbonates. The calcined mud can then

be grounded to powder and densiﬁed with a suitable lubricant before being

pelletted. Many important practical details are involved in precipitating

catalysts. The following are some points to remember:

• It is necessary to precipitate rapidly and maintain a uniform pH. This pro-

duces small active crystallites and well-mixed oxides in the ﬁnished

catalyst. Under these conditions speciﬁc compounds form such as those

described by Feitknecht, with the composition (M

)

)2 (OH)16 (CO3)

4H2O. The Adkins catalyst, copper/ammonium chromate, is another

example of applying a speciﬁc precipitation procedure.

• Slow precipitation of metals from solution is possible by the slow

hydrolysis of urea or nitrite at about 90

C. This procedure is quite slow

and using it may be expensive for many catalysts.

• When preparing a nickel oxide/kieselguhr catalyst, reaction between the com-

ponents leads to the formation of nickel silicates, which are difﬁcult to

reduce. This can affect operation and the catalyst may require prereduction

before use. The addition of a promoter such as copper oxide allows reduction

at a lower temperature.

• When calcining chromium catalysts in air any Cr

is oxidized to Cr

temperatures exceeding 200

C. Further calcination up to about 450

reduces the Cr

content to a practical level and avoids an exothermic

reduction reaction in the plant.

Industrial Catalysts 13

1.3.2. Impregnation

Preformed, absorbent supports are uniformly saturated with a solution of the

catalytic metals. Several impregnations may be needed to obtain the required

metal loading. Supports must be strong enough to be immersed directly into the

solution and any dust forming should not contaminate the catalyst surface. To

avoid these difﬁculties, the supports can be sprayed with just enough solution to

completely ﬁll the pores. At this point the support suddenly appears to be wet

and the procedure is known as incipient wetness. In some cases, when the

support has been impregnated, the metals may be precipitated by immersion in a

second solution.

After impregnation catalysts are carefully dried and most are calcined

before use to decompose the metal salts to oxides. Care is necessary to avoid

high concentrations of metals forming at the support surface as the solution

evaporates. Most types of supports can be used to produce impregnated

catalysts, although alumina or silica, in various forms, is usually chosen. The

support should not, of course, react with the metal solution. For example, if the

support is soluble in acid, there is a possibility of re-precipitation in an

undesirable form.

1.3.3. Other Production Methods

Many other procedures have been used to produce catalysts:

• Platinum/rhodium alloy gauze is used as a catalyst in the selective

oxidation of ammonia during nitric acid production and in the production

of hydrogen cyanide. The wire in the gauze is only a few thousandths of

an inch in diameter woven at 80 wires per inch. Several layers of gauze,

up to about 8 ft in diameter, are used.

• Silver granules are used to oxidize methanol to formaldehyde. Raney

nickel is produced by leaching aluminum from a nickel/aluminum alloy

with alkali solution. Not all of the alumina is removed and the catalyst

may be regenerated a number of times by alkali treatment.

• Mixtures of natural magnetite and various promoters are melted in a

furnace at about 1600

C and chill cooled by casting on a ﬂat surface. The

catalyst can be crushed and separated into appropriate size ranges before

use in ammonia synthesis or Fischer–Tropsch processes.

• In large modern ammonia plants the synthesis catalyst is often used in a

pre-reduced form. The catalyst is carefully reduced in a mixture of

hydrogen and nitrogen and then re-oxidized with a mixture of oxygen and

nitrogen. Less than 20% of the iron is re-oxidized, making plant start-up

14 Chapter 1

much easier and ensuring that the catalyst is not pyrophoric before use.

Some nickel oxide catalysts supported on silica are also pre-reduced.

1.4. CATALYST TESTING

Industrial catalysts must conform to a strict speciﬁcation and physical and chem-

ical properties are measured at all stages of production. The tests most often

included in catalyst speciﬁcations are listed in Table 1.7.

1.4.

1. Physical Tests

It is most important that the catalyst be strong enough to resist breakage and

attrition. Fixed bed and tubular reactors are carefully ﬁlled with catalyst to

ensure that the pellets or granules are not damaged and pack with a uniform

density.

Strength is particularly important in processes in which catalysts are

circulated continuously between the reactor and a regenerator. In the ﬂuid

catalytic cracking process signiﬁcant daily additions of catalyst must be made to

compensate for losses through attrition as well as catalyst deactivation.

1.4.2. Chemical Composition

Careful checks on the chemical composition of both raw materials and products

are required to ensure that the speciﬁcation is achieved. It is important to

conﬁrm that the desired chemical compounds are formed with the required

crystalline form and particle size. It is very easy for the pH of the solution to

change during precipitation reactions and inﬂuence the composition of the

precipitate.

Elemental analysis of the bulk components in a catalyst is measured by X-

ray ﬂuorescence (XRF), which has now replaced traditional wet analysis. If

required, electron probe analysis can also provide information on the

distribution of elements in a catalyst particle. Bulk phases in a catalyst and

crystallite size are determined by X-ray diffraction (XRD), as either a routine

check or a diagnostic procedure to examine catalysts damaged during operation.

Thermogravimetric analysis (TGA) and differential thermal analysis (DTA)

with either air or inert atmospheres show weight changes or heat evolution as

catalyst samples are heated and intermediate chemical compounds decompose.

The tests are therefore useful in providing information on temperature-related

phase changes at different stages of catalyst preparation. Tests also show how

discharged catalysts react during regeneration or oxidation. Temperature

Industrial Catalysts 15

programmed reduction (TPR) in hydrogen is used to investigate similar changes

during catalyst reduction over the appropriate temperature range. These

procedures are widely used in catalyst development and routine testing.

Many other tests are used to measure the physical and chemical properties

of industrial catalysts during development and routine examination. These are

fully described in other publications but are summarized here in Tables 1.9 and

1.10.

4.3. Activity Testing

Activity measurements have been important ever since industrial catalysts were

ﬁrst introduced. BASF carried out some 20,000 tests during the production of a

successful ammonia synthesis catalyst. Until the 1960s, however, when many

new catalytic processes were developed, laboratory activity tests were fairly

crude. It was only possible to screen different samples and obtain relative

activities before new catalysts reached the semitechnical plant stage and

commercial trials were considered possible. More recently, new testing

equipment has been able to simulate plant conditions and provide accurate

kinetic information for reliable design calculations.

TABLE 1.9. Physical Testing of Industrial Catalysts.

Test Procedure

Chemical analysis X-ray ﬂuorescence (XRF) Samples emit secondary X-rays

following bombardment with hard X-rays which allow complete

elemental analysis.

Particle size Simple measurement of solid particles or size grading of powders.

Crushing strength Determined by compressing catalyst particles between anvils or by

bulk crushing the catalyst in a standard container.

Bulk density Packing weight per unit volume of loose or dense packed catalyst

particles.

Attrition loss Dust formed after the rotation of catalyst for a ﬁxed period in a

standard cylinder.

Crystalline phases X-ray diffraction (XRD) identiﬁes crystalline compounds by

reference to standard tables. The proportion of each phase present

in the sample can be calculated.

Crystallite size Estimated from X-ray diffraction line broadening as crystallite size

decreases.

Surface area/pore size Calculated from the volume of a gas monolayer adsorbed by the

catalyst and the known area covered by a gas molecule. Pore

volume can be calculated from the helium density (helium ﬁlls the

pores) and the mercury density (mercury does not ﬁll the pores).

The average pore radius assuming cylindrical pores is calculated

as twice the pore volume divided by the surface area.

16 Chapter 1

Thermogravimetric analysis

(TGA)

Measures the weight loss as catalyst composition changes at

increasing temperature with oxidizing or inert atmospheres.

Differential thermal analysis

(DTA)

Measures the exo-or endothermal temperature changes taking place

as catalysts are heated in oxidizing or inert atmospheres.

Temperature programmed

reduction

Measures the reducibility of oxides in catalyst samples in reducing

atmospheres.

There are several steps during the catalytic reaction before a gas molecule is

available for reaction at the active surface. First, it must (a) pass through the gas

ﬁlm surrounding the catalyst particle, (b) diffuse through the catalyst pores and

reach an active site, and (c) adsorb on the active site to react with adjacent

molecules. Products then (d) desorb from the active site, (e) return through the

catalyst pores to the catalyst surface, and (f) re-enter the gas phase.

This process is dynamic and may depend on the reacting molecules moving

from site to site until reaction takes place and products can desorb. Under ideal

conditions, there would be no ﬁlm or pore diffusion limitations but, practically,

these are often encountered in catalytic reactions, particularly when large rings

or pellets operate at a relatively low linear velocity.

Simple screening tests can be developed for most reactions to compare the

activity of different catalysts. It is important, however, to standardize operating

conditions and to operate well away from equilibrium conversion, to obtain the

most useful results. To avoid all diffusion limitations, catalyst samples are

normally tested at a high linear velocity with small crushed particles. Test units

operate with pure gas mixtures and the effects of typical poisons must be

considered in separate tests. Until the 1960s, the screening tests operated at

atmospheric pressure and compared the performance of new catalysts with an

accepted standard at constant space velocity.

TABLE 1.10. Chemical and Structural Analyses of Industrial Catalysts.

Test Result

Electron microscopy Used to study the surface structure and composition of

catalysts and has extended the use of optical microscopy in

determining the characteristics of particles.

Scanning electron microscopy

(SEM)

Electrons scan the surface of the sample and give a magniﬁ-

cation of 20,000–50,000. It can focus on sizes down to 5

nm. This gives crystallite shape, size, and size distribution.

Transmission electron microscopy

(TEM)

Higher magniﬁcation and resolution possible to give three-

dimensional images of crystallites down to 0.5 nm and

changes during operation.

Electron spectroscopy for chemical

analysis (ESCA) better known as

X-ray photoelectron spectroscopy

(XPS)

Can analyze for all elements and atomic electron binding

energies to give structural data and compound types in

surface layers.

Industrial Catalysts 17

Auger spectroscopy (AES) Elemental analysis of surface layers.

Secondary ion mass spectroscopy

(SIMS)

Elemental analysis of surface layers.

High-resolution electron energy-

loss spectroscopy (HREELS)

Types of chemical bond present.

Atmospheric pressure test units cannot give the information required for

modern catalyst and process development and have been replaced with high

pressure micro-reactors. To avoid diffusion limitations, micro-reactors operate at

very low conversions, under isothermal conditions, using small quantities of

crushed catalyst. Experimental catalysts are usually tested in the same process

conditions over a range of gas rates to provide useful design information. The

ratio of space velocities measured at the same conversion gives the relative

activity of different catalysts and an indication of the catalyst volume needed to

give the same performance. It is also possible, when using pulsed gas ﬂow at a

constant space velocity and process conditions, to measure the catalyst

selectivity in a reaction over an appropriate temperature range.

Knowledge of reaction kinetics is required to enable engineers to design a

catalytic reactor. To obtain this information it is usual to use full-size catalyst

particles over the total range of plant operating conditions to determine an

accurate rate of reaction. This became possible by the introduction of continuous

stirred tank reactors. These give results directly for conditions that are almost

uniform throughout the reactor with none of the gradients found in micro-

reactors. Two popular types of high-pressure reactors are the Carberry reactor,

in which the catalyst is held in a cross-shaped basket and rotated, and the Berti

reactor, which recycles gas through a ﬁxed bed of catalyst. Several catalytic

reactors are listed in Table 1.11.

TABLE 1.11. Catalyst Activity.

Reactor type Comments

Micro-reactor Provides rapid automated screening of several catalysts.

Requires small samples of small catalyst particle.

Compares rate constants, i.e., activity directly.

Allows initial life testing.

Can be used for accelerated aging task.

Eliminates unsuitable formulations.

Continuous stirred tank reactors Use full size catalyst particles.

Carberry—rotating catalyst basket Operates over full range of plant conditions.