Text 24

Catalytic processes

A typical feature of catalytic processes is Text 23

Thermal processes. Thermal cracking

Thermal processes

At high temperatures, the bonds between atoms in molecules of hydrocarbons are weakened and can break to form new compounds. In any homologous series, lighter [low-boiling] hydrocarbons split less easily than high-boiling ones. Along with splitting into lighter hydrocarbons, other transformations can take place, in particular, packing of molecules in which heavier fractions from preliminary petroleum processing are decomposed at elevated temperatures are call thermal processes. In petroleum processing industry, the most common processes of this type are thermal cracking, coking, and pyrolysis.

Thermal cracking, usually carried out at pressures up to 5 MPa and temperatures of 420-550 0C, is a process in which the starting material is changed qualitatively with the formation of new compounds having different physicochemical properties. Depending on the composition of the starting material and the process conditions, the yield of gasoline cracking is 7-30 % of the mass of the starting material; the process also gives some other products: gaseous, liquid and solid [coke].

Coking of residue is done at temperatures of 445-5600C [still coking] or 485-5400C. Depending on the quality of the starting material and the type and conditions of the process, it may yield 15-18 % of commercial coke, 49-77.5 % of liquid products [including 7-17 % of gasoline fractions] and 5-12 % of gases [up to C4].

Pylolysis of distillates and light hydrocarbons [from ethane to butane] is usually effected at 650-8500C. The main object of pyrolysis is to produce ethylene and propylene; earlier, it was aimed at producing aromatic [benzene] hydrocarbons.

In 1930-1950's, pyrolysis played an important part as a method for increasing the manufacture of gasolines for carburettor engines. At a later time, the quality of gasolines produced in thermal cracking plants could no more satisfy the rising requirements of consumers. Upon development of catalytic processes, thermal cracking still retains its role mainly for the manufacture of low-viscous fuel oils from residue products of preliminary petroleum processing , and also of gas oils intermediate products for making carbon black. The processes of coking are being developed further, mainly to satisfy the demands for coke, especially electrode coke. Liquid products of coking are utilized for increasing production of white petroleum products. Pyrolysis is being developed rapidly in association with increasing demands for olefin materials for the chemical and petrochemical industries.

Thermal Cracking

In 1890, V.G. Shukhov, a famous Russian scientist, designed the first cracking plant for producing light petroleum products from fuel oil. Later, as the need for automobile gasoline increased, a system with reaction chambers was developed, in which the starting material, preheated to the reaction temperature in the furnace coil, was retained and subjected to cracking up to the formation of coke. The time of filling of the reactor with coke determined the length of the whole working cycle of the plant. At a later time, the reaction chamber was replaced by the reaction volume formed in radiant pipes of a furnace. To prevent the clogging of the apparatus with coke, the reaction products were chilled at the exit from the furnace by the cold starting material [quench] which stopped the cracking process [in particular, Winker-Koch plants operated by this principle]. In later years, further improvements have been made in thermal cracking in foreign countries and in the USSA where the process was implemented in 1927-28.

As has been given earlier, the principal reaction of thermal cracking is the decomposition [or cracking ] reaction. Among various hydrocarbons, paraffins can be cracked most easily. Then follow naphthenic hydrocarbons. Benzene hydrocarbons are most stable against cracking. In any homologous series, hydrocarbons of a higher molecular mass are cracked more readily. Thus heavier fraction of petroleum products are less stable and can be cracked more easily than lighter ones. Brief data on the chemistry and mechanisms of cracking of the principal classes of hydrocarbons will be given below.

Paraffin hydrocarbons. Cracking of commercial paraffins which consist mainly of C24H50, C25H52 and C26H54 hydrocarbons forms paraffin hydrocarbons and olefins composed of 12, 13, or 14 carbon atoms, i.e. roughly one-half of the carbon atoms in the original paraffin. This is an indication of that the breakdown of C-C bonds in cracking of paraffins of high molecular mass occurs in the middle of a molecule. The new paraffin hydrocarbons formed by cracking can in turn break down into simpler molecules say a molecule of a paraffin hydrocarbon and that of an olefin, for instance:

4250C

C12H26  C6H14 + C6H12

dodecan hexane hexene

[paraffinic] [paraffinic] [olefinic]

At higher temperatures of cracking of paraffinic hydrocarbons, reactions in which the breakdown of molecules occurs at the end portion of the chain begin to prevail over those in which molecules break in the middle. The larger fragment of a broken molecule is an olefin and the smaller one is the paraffinic hydrocarbon [gaseous] or hydrogen. Isoparaffinic hydrocarbons are thermally less stable than those of the normal structure. The rate of the reaction at a given temperature increases almost linearly with the molecular mass. This is true of all groups of hydrocarbons.

Olefinic Hydrocarbons. These are the principal ones among all unsaturated hydrocarbons produced by cracking. They prevail as gaseous compounds [from ethylene C2H4 to butylene C4H8] and liquid ones [from amylenes C5H10 to pentadecenes C15H30]. Cyclic olefins and diolefins form in relatively small quantities. In contrast to paraffinic hydrocarbons, olefins undergo appreciably more diverse primary reactions during cracking, the most important among them being polymerization reactions [i.e. combination of a few molecules into a single molecule] and depolymerization reactions, especially at an early stage of the process . Polymerization is the main reaction at moderately high and high pressures; it can occur not only between like molecules, but also between unlike molecules of olefins, for instance:

C2H4 + C3H6  C5H10

At later stages of the process, olefins are dehydrogenated partially and form diolefins, which typically have two double bonds, and hydrogen or split into diolefins and paraffinic hydrocarbons:

CH3- CH2- CH= CH2  CH2= CH- CH= CH2 + H2

butylene divinyl

[olefin] [diolefin]

Secondary reactions between olefins and diolefins may give cycloolefins which are present in cracking products in very small quantities. Olefins can transform into cyclic hydrocarbons [naththenes]:

n-hexene -1  cyclohexane

Naphthenic hydrocarbons. The main reaction in cracking of these hydrocarbons are dealkylation [splitting of paraffinic side chains] and dehydrogenation of hexacyclic naphthenic hydrocarbons into benzene hydrocarbons; the two reaction can occur simultaneously.

Dehydrogenation of hexacyclic naphthenes in thermal cracking with the formation of benzene hydrocarbons is of minor importance. Owing to the dealkylation reaction taking place in thermal cracking, naphthenic and benzene hydrocarbons loss most of their long side chains. Paraffinic side chains in turn break to form gaseous and low-boiling paraffinic hydrocarbons and olefins. In high-temperature processes, naphthenic rings can break; the result is that hydrocarbons lose their cyclic structure and that polycyclic structures are partially decycled [if they had several rings]. In that case, paraffinic, olefinic and naphthenic hydrocarbons form.

Benzene Hydrocarbons. These are obtained by dehydrogenation of the cycloolefins or naphthenes which were formed at earlier stages of the process. Benzene hydrocarbons are quite stable at high temperatures, especially benzene, toluene and xylenes. The main reaction in cracking of benzene hydrocarbons with alkyl chains are dealkylation and condensation. Condensation may occur between the molecules of benzene hydrocarbons [or some other unsaturated hydrocarbons]. This gives polycyclic benzene hydrocarbons which can condense further to asphaltenes and coke.

Sulphur compounds. They are decomposed in cracking and form hydrogen sulphide. Cyclic sulphur-organic compounds, such as thiophene and thiophane, have the greatest stability against decomposition. Hydrogen sulphide and elemental sulphur [as the product of oxidation of hydrogen sulphide] which form in cracking of sulphurous petroleum grades can cause strong corrosion of process equipment.

Inert tars and asphaltenes. These may contain various heterocyclic compounds [usually including oxygen, sulphur, nitrogen and some metals]. In cracking they form gases, liquid products and large amount of coke. The yield of coke in cracking of asphaltenes may reach 60% and that in cracking of tars 7-20% [depending on the molecular mass of tars].

Since the starting materials for industrial thermal cracking are usually mixtures of many hydrocarbons of complicated structure, many reaction can occur simultaneously and the mechanism of thermal cracking can not be explained in detail. It is assumed however, that most reaction of thermal cracking can be described by the theory of formation of free radicals.

Exercises

Answer the following question

1. What are thermal processes ?

2. What are the most common processes of thermal processes ?

3. What are the products of thermal cracking ?

4. What are the products of coking?

5. What are the products of pyrolysis?

6. Who designed the first cracking plant?

7. Which type of hydrocarbon can be cracked most easily?

8. Which C-C bonds are broken down in cracking of high paraffins at lower temperature ?

9. Which C-C bonds are broken down in cracking of high paraffins at higher temperature ?

10. What are the products of cracking of high molecular mass paraffins at higher temperature ?

11. Which relationship is there between the rate of a reaction and its temperature?

12. What are the primary reaction of olefins in thermal cracking condition?

13. What are the secondary reaction of olefins in thermal cracking condition?

14. Which reactions happen with naphthenic hydrocarbons in thermal cracking condition?

15. Why are gaseous, low-boiling parafinic hydrocarbons and olefins formed in thermal cracking of naphthenic hydrocarbons ?

16. What are the main reactions of benzene hydrocarbons ?

17. Which compounds can be obtained in cracking of benzene hydrocarbons ?

18. Which sulphurous compounds are formed in cracking ?

19. What is the main product in cracking of tars and asphaltenes?

20. What is the main mechanism of thermal cracking ?

the use of catalysts, i.e. substances which can accelerate [or decelerate] the reactions and cause the formation of new hydrocarbons and other substances not present in the starting material. Catalytic processes occur under softer conditions [at lower temperatures and pressures] than thermal ones, but may involve the reactions which are impossible in purely thermal processes.

A catalyst usually consists of an active substance [which determines the course of desirable reactions] applied onto a carrier substance [mostly alumina] having a largely extended surface. In some cases, some other substances [promptors] are added to improve characteristics of catalytic process. The particles [granules] of catalytic process an enormous porosity and therefore a very large internal surface area. The activity of a catalyst is due mainly to the surface of pores rather than to their external surface. The name of a catalyst depends on the process where it is to be used, for instance, reforming catalysts, cracking catalysts, etc.

The technico-economical characteristics of a catalytic process are determined by the quality of the starting material and the process conditions, as well as by the properties of the catalyst used. The capability of a catalyst to accelerate the rate of desirable reactions and retain the rate of unwanted ones at a constant low level is called selectivity. Activity is another important characteristic of catalysts; it is estimated in terms of the yield of the end product relative to the use of the starting material. In particular, the catalyst activity in catalytic cracking is determined as the yield of gasoline [end product].

Catalysts can participate in process reactions in a stationary [fixed-bed] or moving [circulating] state. In both cases, they gradually lose their activity and selectivity owing to ageing. This may be call normal ageing, it is unavoidable and can only be accelerated under more rigid process conditions. Along with normal ageing of a catalyst may also take place. This occurs often when the process is run under abnormal conditions, say, at an excessively high temperature. Many catalysts can be affected by certain substances containing sulphur, nitrogen and heavy metals [V, Ni, and other] and by water in the starting material.

Catalysts can be regenerated to restore their activity and partially, the selectivity, which is usually done by removal [burning-off] of the coke deposits settled on catalyst particles during operation. By another method, the properties of catalysts [especially of fixed-bed type] are restored by gradually raising the temperature in the reactor. With circulating catalysts, a fresh catalyst is added in portions to compensate for the loss of the catalyst in the system.

Catalyst processes make it possible to remove unwanted impurities, for instance, sulphurous compounds, and to convert certain hydrocarbons into the products which cannot be obtained by preliminary distillation of petroleum or in thermal processes.

Brief Description of Catalyst processes

Catalytic cracking is the process of conversion of high-boiling petroleum fractions into high-octane base components of aviation and automobile gasolines and middle distillates.

Industrial processes of catalytic cracking are based on contacting the starting material with an active catalyst under appropriate conditions to convert a considerable portion of the material into gasoline and other light products. In cracking reactions, carbon deposits form on the particles of the catalyst and thus reduce sharply the activity, in particular, the cracking ability. The activity of the catalyst is restored by burning off the carbon precipitates [usually called coke] in air.

There exist many types and systems of catalyst cracking plants, those with circulating flow of the catalyst, especially in a fluidized bed, being most popular.

Catalytic reforming is employed widely to obtain high-octane gasolines from low-octane gasoline fractions. Reforming of gasoline or gasoline fractions in combination with various methods of separation of benzene hydrocarbons, for instance, with solvent extraction, make it possible to produce benzene hydrocarbons [benzene, toluene, xylenes and higher aromatics] for the petrochemical and chemical industries.

Catalytic reforming processes are based on contacting the starting material with an active catalyst usually containing platinum. The yield of reformate may vary within 63 to 85 % of the mass of the starting material. The catalyst is regenerated periodically to restore its activity. A feature of importance is that the catalytic reforming occurs in a medium of hydrogen-containing gas at high temperatures and pressures. The hydrogen formed in various reactions of reforming is removed from the system as an excess of hydrogen-containing gas. The high content of hydrogen is the gas mixture [up to 80% by volume] makes it possible to utilize it in hydrogenation processes, in particular, for hydrofining of diesel fuels.

Hydrogenation processes occur in the medium of hydrogen at elevated temperatures and pressures. They can be used for preparing high-quality products from sulphurous and high-sulphurous petroleum grades, the yields and quality of these products being varied depending on the degree of destruction and the prevailing reactions. Among the processes of this kind, hydrofining of various fractions and products is most important.

Hydrofining of petroleum distillates and products is one of the most popular catalytic processes, especially for treating sulphurous and high-sulphurous petroleum grades. The process is carried out in a hydrogen medium at a pressure of 3-5 MPa. The main object of hidrofining of petroleum distillates and products is to reduce the content of sulphur and other harmful compounds in them. These substances are destructed in the process, and the destruction products [hydrogen sulphide and ammonia] are removed from the system with gases.

Hydrofining processes are based on contacting petroleum distillates and products with a fixed-bed or circulating catalyst , usually alumina-cobalt-molybdena or alumina-nikel-molybdena. The process takes place in the medium of hydrogen at elevated temperatures and pressures so as to convert 95-99% of the starting material into the refined product or distillate [hydrogenate]. Minor quantities of gasoline, hydrogen sulphide and ammonia also form in the process.

Alkylation is a process by which isoparaffinic hydrocarbons are combined with olefins to form higher-boiling isoparaffinic hydrocarbons which can be used as high-octane components in aviation and automobile gasolines. Other kind of alkylation are also in use, in particular, alkylation of benzene hydrocarbons by olefins [for instance, alkylation of benzene by ethylene to make ethylbenzene or alkylation of benzene by propylene to make iospropylbenzene].

Up to quite recently, catalyst alkylation of isobutane was carried out by butylenes in the presence of sulphuric or hydrofluoric acid as a catalyst. In modern plants, alkylation of isobutane is done by using the materials containing ethylene, propylene and even amylenes, as well as butylenes.

Alkylation processes may differ in the starting material, catalysts, productivity, and especially in the design of catalyst plants. With the use of sulphuric acid as a catalyst, the alkylation process is characterized by a low temperature of the reaction and the necessity to maintain a high concentration of isobutane and olefins in the reaction zone. The total yield of alkylate from olefinic starting materials is 1.5-1.8 units per unit volume of the starting material, depending on the quality of the material and the process conditions. The significance and scope of alkylation increase with the rising production of high- octane automobile gasolines having a low content of TEL.

Isomerization is the process of conversion of relatively low-octane paraffinic hydrocarbons [mostly C5¬¬-C6 and their mixtures] into corresponding isoparaffinic hydrocarbons having a high octane number. In industrial isomerization plants using various catalysts, including alumo-platinum ones, the yield of isomerizates attains 97%. The process of isomerization takes place in a hydrogen atmosphere. As in other processes, the catalyst is regenerated peridically.

Isomerizates are used together with alkylates for preparation of high-quality gasolines, by compounding them with high-aromatic gasolines of catalytic cracking and reforming.

Novel catalyst processes, in particular disproportionation. are being paid much attention now. The process is based on converting two molecules of a hydrocarbon into two unlike molecules, one having by one carbon atom more and the other, by one atom less than the original molecules, for instance:

2C3H6  C2H4 + C4H8

The process is carried out at 66-2600C and a pressure of 1.4-4.1 MPa, with the starting material being supplied at a high rate [10 to 100 h -1]. Disproportional takes place with a high selectivity: the total yield of ethylene and butylenes attains 97% of the propylene converted and the degree of conversion of the latter, up to 45%. Disproportionation can be employed for making benzene from toluene [2C7H8  C6H6 + C8H8 ] to replace the less efficient process of toluene alkylation.

In industrial practice, a number of processes are often combined in a single plant [for instance, hydrogen cracking and catalytic reforming]. This make it possible to process low-octane starting materials into high- octane gasoline with a high concentration of benzene hydrocarbons [obtained by reforming] and isoparaffinic ones [obtained by hydrogen cracking]. In this combined technique, the process of hydrogen cracking occurs without hydrogen supply from the outside.

Exercises

Answer the following question

1. What is the typical feature of catalytic processes?

2. What can you say about the conditions of catalytic processes?

3. What is the composition of a catalyst?

4. What is the main characteristic of a catalyst?

5. What is the selectivity of a catalyst?

6. How can catalysts participate in process reaction?

7. When does the quick ageing happen?

8. What can affect the activity and selectivity of catalysts?

9. How can you regenerate catalysts?

10. Do you know what the main catalytic processes are?

11. Can you show the basis of catalytic cracking?

12. What is the purpose of catalytic reforming?

13. In which condition do catalytic reforming happen?

14. What is the purpose of hydrogenation processes?

15. Which process is the most important in hydrogenation processes?

16. What is the basis of hydrofining process?

17. Can you define the alkylation reaction?

18. Which alkylation processes are used in petroleum processing?

19. How can they carry out the catalytic alkylation of isobutane?

20. What do the alkylation processes depend on?

21. By what is the akylation processes with the use of sulphuric acid as a catalyst characterized?

22. What is the purpose of isomerization?

23. What is disproportionation?

24. How can you say about the characteristic of gasoline obtained by catalytic processes?