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CHAPTER 6

CRACKING OF NAPHTHA TO LIGHT OLEFINS

6.1 INTRODUCTION

Light olefins have five or lower number of carbons. Ethylene and

propylene are the monomers for polyethylene and polypropylene respectively.

Butene and pentene are the feed stocks for high octane gasoline components.

They are also important feedstocks for the production of oxygenates used as

octane boosting additives. Currently they are manufactured mainly form

naphtha by thermal cracking also called steam cracking. The current steam

cracking process uses as much as 40% of the energy consumed by the entire

petrochemical industry. Therefore global environmental issues have

stimulated the development of process that maximise energy, resource savings

and minimise CO2 emissions. Besides, it is difficult to control the composition

of olefins formed. Hence, there is an increase in demand for processes capable

of controlling the composition of olefins. Naphtha catalytic cracking has been

started to achieve better olefin yield and lower energy consumption.

In addition use of alternative feedstocks such as methane, LPG, gas oil, crude

oil and ethanol have also been presumed. However, the conventional naphtha

is the most attractive feedstocks for this process as it can take advantage of

the existing facilities that are used for steam cracking.

Zeolites are the mostly employed catalysts for cracking of heavy

oil. The fluidised bed catalytic cracking units in petroleum refineries are the

major source of light olefins which are the important petrochemical feed

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stocks (Biswas and Maxwell 1990). ZSM-5 and other pentasil-ring zeolites

are added as additives to the cracking units in order to enhance the yield of

light olefins. Recently, applications of unidimensional 10-membered ring pore

zeolite and zeotype structures such as ZSM-22, ZSM-48, ZSM-23 and

SAPO-11, 17, 34 and uni dimensional 12-membered ring pore ZSM-12 were

reported. Low concentration of strongly acidic sites results in high cracking

activity to ethylene and propylene and suppression of hydrogen transfer

reactions (hydrogen transfer index-HTI) leading to paraffins, cyclolefins,

aromatics and coke like polyaromatic molecules. The HTI can reflect the

difference in hydrogen transfer activities on zeolites with different structures.

The hydrogen transfer reactions, which involve the formation of bulky

bimolecular reaction intermediates, are mainly controlled by steric constrains

due to space available inside the micropore of zeolites (Zhu et al 2005).

Of course they can also occur on the outer surface of the particles.

The smaller the pore size of zeolite, the greater the extent of suppression of

the hydrogen transfer reactions of alkenes. In other words the HTI decreases

with the pore size of the zeolite. For example, the decrease of order is

Y > beta > MCM-22 > ZSM-5 > other zeolites (Zhu et al 2005).

Cracking of naphtha to light olefins is also an additional process in

refineries that is carried out mainly over ZSM-5. The advantage of using

ZSM-5 zeolite is its medium pore which can discourage coke formation due

to absence of large cavities in the pore structure and low concentration of acid

sites. Based on the products of naphtha cracking two mechanisms have been

proposed. Haag and co-workers have proposed that the acid catalysed

cracking of simple paraffins occurs by both unimolecular and bimolecular

mechanisms (Haag et al 1981, Haag et al 1991). In the unimolecular

mechanism feed olefins are chemisorbed on the Brönsted acid sites to form

carbenium ions which then cracked by beta scission to yield olefins. In the

cracking of paraffins, protons of ZSM-5 are transferred to them to form a

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penta-coordinated carbonium ions as transition state as shown in Scheme 6.1.

The carbonium ion expels methane to form a carbenium ion. The latter

undergoes beta scission to form olefins.

H4C+

CH4

+

+

+

SiO

Al-

H+

+ +

CH2 CH2CH2 CH2HC CH2

H

+CH2 CH2

++Si

OAl

-H

+

Scheme 6.1 Cracking of paraffins

Similarly n-olefins are protonated by zeolite to form secondary

carbonium ion which is then cracked by beta scission to form ethylene and

propylene as shown in Scheme 6.2.

+Si

OAl

-H

+

+

SiO

Al-

+

H

+

+SiO

Al-

H+

H

++

+

+

SiO

Al-

H+

SiO

Al-

+

CH2 CH2 +SiO

Al-

H+

+

Scheme 6.2 Cracking of n-olefins

The bimolecular mechanism is composed of the following steps:

ethylene or propylene is chemisorbed on the Brönsted acid sites to form the

respective carbenium ion. Such a carbenium ion converts the feed alkane into

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a carbenium ion by a hydride ion abstraction. The resulting carbenium ion

cyclises to form naphthene which is converted to an aromatic by a sequence

of hydrogen transfer reactions (Scheme 6.3).

+

+

+ +SiO

Al-

H+

SiO

Al-

High HTI

Cyclisation

+ SiO

Al-

H+

Scheme 6.3 Bimolecular mechanism for cracking

Sequential hydrogen abstraction from naphthenes by this

mechanism results aromatics as shown in Scheme 6.4. It depends on HTI

which inturn depends on the strength and density of acid sites. Zeolites with

high HTI promote formation of naphthenes and aromatics. As the pore size of

ZSM-5 is about 5.5 , large space demanding bimolecular cracking

mechanism is largely suppressed and the interaction between the channel

surface and the reactants is enhanced. The latter factor is important as it could

result in high conversion.

CH4

+

High HTI

+ SiO

Al-

Cyclisation

+ SiO

Al-

H+

H+

Scheme 6.4 Formation of naphthenes and aromatics

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6.2 CRACKING ACTIVITY OF THE CATALYST

HZSM-5(Si/Al = 25, 50 and 75) synthesised in fluoride medium

containing phosphoric acid were tested for cracking of naphtha to light

olefins. The cracking of naphtha was carried at 675 ºC in 6 ml/min He flow,

3 ml/min N2 flow, 0.58 g/h water flow and naphtha at a flow rate of 1.15 g/h.

The results of naphtha cracking activity are presented in Table 6.1. The data

obtained for commercial HZSM-5(25) are also presented in the same table for

comparative purpose. The data obtained at 625 and 650 °C were not given as

the conversion and yield of olefins were very much low.

Table 6.1 Naphtha cracking activity over HZSM-5 zeolites

Catalyst

Gas fraction Commercial

HZSM-5(25)

HZSM-5

(25)

HZSM-5

(50)

HZSM-5

(75)

CO 0.9 0.4 0.4 0.5

CH4 9.6 6.6 8.1 6.8

CO2 0.4 0.2 0.0 0.2

C2= (Ethylene) 19.6 18.4 21.6 19.0

C2 (Ethane) 8.5 5.6 7.4 6.0

C3= (Propylene) 15.7 18.8 17.7 19.4

C3 (Propane) 7.2 4.0 5.3 4.0

C4 (Butane) 7.0 8.3 7.4 8.7

C5 (Pentane) 5.9 5.3 2.7 5.2

Unknown 0.7 1.8 0.9 2.0

C2= + C3= 35.3 37.2 39.3 38.4

C2=/C3= 1.3 1.0 1.2 1.0

Gas yield 75.3 69.4 71.7 71.9

The data in Table 6.1 indicate higher yield of olefins for the

synthesised samples than for the commercial sample. As the synthesised

samples without phosphoric acid possess low bulk density than the reference

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samples, 0.3 g of them was used for catalytic studies compared to the

reference sample of about 0.5 g. But the low density of synthesised samples in

fluoride medium without phosphoric acid is not a problem, as our latter

experiments with phosphoric acid gave solids of high density. It is known that

zeolite crystals of large dimensions yield less olefins than those of small

dimensions due to promoted hydrogen transfer reactions that facilitate

formation of aromatics and oligomeric products. Though the synthesised

samples possess larger crystal dimension than that of commercial one, they

produced high olefin yield. Based on the products yield and composition, it

could be suggested that there may be minimum bimolecular hydrogen transfer

reactions occurred in all the synthesised samples. There may be possibly

minimum or no external acid sites, and very scattered distribution of acid sites

inside the channel instead of assembled them closer. Such close acid sites can

be suggested to be the main cause for the enhanced consumption of olefins to

yield aromatics and oligomers (Wang et al 1994). Hence, fluoride with its

high mineralising and complexing properties is indirectly proved to exhibit

the property of planting acid sites internally and also keeping them well

scattered inside the channel. Although these properties were not verified

experimentally at this stage, they are to be established in the future work.

HZSM-5(50) showed higher yield of olefins than HZSM-5(25). The acid sites

are certainly better isolated in the former than the latter as the Si/Al is high.

Hence, it could be expected that by avoiding close acid sites hydrogen

transfer activity could be largely suppressed and this is in accordance with the

previous report (Wang et al 1994). Moreover, the acid sites of the present

catalysts are weak and medium type and so hydrogen transfer reactions of

naphtha cracking are still minimised. Therefore, the olefins can freely diffuse

through the pores of such catalysts without being rapidly converted to side

products.

Since all the catalysts showed same yield of olefins irrespective of

Si/Al ratios, high density of acid sites is not a prerequisite for high naphtha

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cracking activity. Generally ZSM-5 with high density of acid sites could

exhibit high naphtha cracking activity as well as high hydrogen transfer

activity and hence those with low density of acid sites (optimum) could be

adequate.

Generally cracking of naphtha to light olefins resulted total gas

yield higher than the liquid yield. In this study the gas yield was about 3 times

higher than the liquid yield. The liquid product was mainly composed of

aromatics, namely, benzene, toluene and xylene. This result, therefore,

suggested existence of still hydrogen transfer reactions, though the catalysts

carry mainly weak and medium acid sites. So hydrogen transfer reactions may

not be exclusively controlled by strong acid sites. Even weak acid sites at high

temperature may be capable of effecting such reactions. Further, HZSM-5

synthesised in fluoride medium gave less yield of olefins than the commercial

catalyst.

Another important observation is the ratio of ethylene to propylene,

which is close to one in all cases. This ratio is important because of high

propylene demand. Commercial as well as ZSM-5 catalysts synthesised in

alkaline medium generally showed high ratio of ethylene to propylene.

But the present HZSM-5 catalysts synthesised in fluoride medium gave

ethylene to propylene ratio close to one. This is an additional advantage of

HZSM-5 for cracking of naphtha to light olefins.

6.3 CONCLUSION

The study concluded that fluoride mediated synthesis of HZSM-5 is

advantageous for application in the catalytic cracking of naphtha. Though the

medium planted only weak and medium acid sites in the catalysts, the yield of

olefins is same as that of a commercial catalyst with strong acid sites.

Hence, presence of strong acid sites is not a necessity for naphtha cracking.

The hydrothermal stability of ZSM-5 is mandatory as the cracking is carried

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out above 650 °C in steam. Though this study was not undertaken right now,

it becomes a main part of our study in the future. Phosphoric acid

modification of ZSM-5 has been very elaborately examined for hydrothermal

stability and hence this will also become part of our future investigation.

The influence of rare earths has also been investigated on the hydrothermal

stability of ZSM-5 for naphtha cracking. This will also form yet another study

in the future.