Reactions in Steam Cracking
The reactions involved in thermal cracking of hydrocarbons are quite complex and involve many radical steps. The thermal cracking reaction proceeds via a free radical mechanism. Two types of reactions are involved in the thermal cracking (i) primary cracking where the initial formation of paraffin and olefin takes place (ii) secondary cracking reaction where light products rich in olefins are formed. The total cracking reactions can be grouped as follows:
Operating Variables of Steam Cracking
The main operating variables in the pyrolysis of hydrocarbon are composition of feed stock, reaction temperature, residence time, hydrocarbon partial pressure and severity.
Composition of Feed Stock
Naphtha are mixture of alkane, cycloalkanes, and aromatic hydrocarbons depending on the type of oil from which the naphtha was derived. The group properties of these components greatly influence the yield pattern of the pyrolysis products. A full range naphtha boiling range approximately 20 to 200oC would contain compound, with from 4-12 carbon atms. Short naphtha boiling point range from 100-140oC and long chain naphtha boiling point lies around 200-220oC. The steam cracking of the naphtha yields wide variety of products, ranging from hydrogen to highly aromatic heavy liquid fractions. The thermal stability of hydrocarbons increases in the following order: parafins, naphthenes, aromatics. Yield of ethylene as well as that of propylene is higher if the naphtha feed stock is rich in paraffins. Effect of feed stock on the yield of various gases is given in Table-5. It may be seen that relative production of ethylene decreases as the feed stock becomes heavier. The percentage of pyrolysis gasoline C5-200oC cut increases. Simultaneously butadiene yield varies slight with feed stock in the treatment of liquid petroleum fractions.
Furnace Run Length
Furnace run length can be calculated from the equation
Run len gth = = maximum allowable tube skin temperature
Average rise in tube skin temperature per day
Tmc = maximum metal skin temperature in the clean, uncoked condition For any feedstock the heater section run length depends on the pyrolysis coil selectivity, cracking severity and transfer line exchanger design. Run length varied between 21 - 60 days for gasbased furnaces and 21-40 days for liquid feed based furnace .
Pyrolysis temperature and Residence Time
The effluent exit temperature is generally considered a significant indicator of the operation of a furnace. As the furnace exit temperature rises, the yield also rises, while the yields of propylene and pyrolyis gasoline (C5-200oC at) decrease. With respect of ethylene yield, each furnace exit temperature, correspond to an optimum. The highest ethylene are achieved by operating at high severely, namely, around 850oC with residence time ranging from 0.2 to 0.4s However, operating at high temperature results in high coke formation.
Partial Pressure of Hydrocarbon and Steam to Naphtha Ratio
Pyrolysis reaction producing light olefins are more advanced at lower pressure. Decrease into the partial pressure of hydrocarbons by dilution with steam, reduces the overall rate reaction rate, but also help to enhance the selectivity of pyrolysis substantially in favour of the light olefins desired. Other role of steam during pyrolsis is (1) to increase the temperature of feed stock (2) reduction in the quantity of heat to be furnished per linear meter of tube in the reaction section (3) to remove partially coke deposits in furnace tubes.
The ethylene yield decreases as the partial pressure of hydrocarbon increases. The effect of H2O/naphtha on ethylene yield is given in Fig.4 for economic reason a value of 0.5 to 0.64 of steam per tonne of naphtha is generally adopted as the upper limit.
Severity and Selectivity Concept
Severity is often used to describe the depth of cracking or extent of conversion. The definition of severity varies with the different manufactures and may differ accordingly to the type of hydrocarbon treated. In the case of steam cracking of the ethane and propane, it is convenient to express the severity of the operating conditions in terms of feed conversion. At very high severities, the methane and ethylene yield level off, while those of propylene and C4 cut reach a peak and then decline consequently. The ratio of ethylene and propylene yield increases with severity, which hence favours the formation of ethyelene. The relative production of C5+ cut passes through a minimum and at the very high severity tends to increase. Modern ethylene plants are normally designed for near maximum cracking severity because of economic considerations.
Ethylene Furnace Design
Pyrolysis furnace design during the last three decades made significant development. Prior to 1960, the ethylene pyrolysis furnaces were box type with horizontal radiant tubes. The capacity of these furnaces were small capacity (40 MM lb/y) today standards (250 MM/lb/y). High thermal efficiency furnace design can contribute greatly to minimum overall plant utility costs. Higher efficiency can be achieved by (i) Upgrading of pyrolysis furnace capacity (ii) increasing cracking severity (iii) improving ethylene selectivity (iv) improving thermal efficiency (v) reducing downtime for decoking (vi) reducing maintenance cost. This can be achieved by radiant coil with shorter residence time and lower pressure drop, combustion air preheating and short residence time. Small diameter coils coupled with increased dilution steam, with use of booster compressors to reduce furnace outlet pressure can increase efficiency ethylene selectivity. Radiant coils with a short residence time and low hydrocarbon partial pressure give higher ethylene selectivity.
Coke Formation during Pyrolysis and Decoking Measures
Pyrolysis of any hydrocarbon feedstock is always accompanied by coke formation, which deposits on the walls of the tubular reactor. Under typical operating conditions, the coke formation in naphtha pyrolysis is about 0.01 wt.% of the feed . The coke deposits on the walls of reactor reducing the overall heat transfer coefficient and increasing the pressure drop across the reactor. This results in gradual decrease with run time of both the reactor tube metal temperature and the pressure drop across the reactor necessitating periodic shut down. The coke formation inside the tube will depend upon (i) characteristics of feedstock and the coking precursor (ii) hydrocarbon partial pressure (iii) thermal condition of coil (iv) mass velocity which controls the dynamics of gas film close to the wall. Controlling coking rate permits increasing the severity of the furnace to increase conversion rate, reducing the cycle rate and unloading downstream limiting equipment which increases throughput .
Frequent decoking operating result in loss of production effect the coil life and increase fuel and utility costs. Run length between two successive decoking varies according to the installation and the type of feed stock, but can be estimated at a few weeks on the average. In ideal conditions, a furnace operating on naphtha can run for go days without decoking. However, run length is always shorter due to the inevitable fouling of the quench boiler. In practice run length is as long as 90 days on ethane feed stock, 65 days naphtha and 40 with gas oil .
Various approaches for coke mitigation are based on reduced coke production in the coils and increased rates of coke removal or removal of coke precursors during pyrolysis . By using improved metallurgy and innovative coating systems, ethylene producers seek to improve unit reliability, increase carburization resistance, extend processing run length, reduce downtime and plant shut downs, increase yields and throughput, extend furnace tube life . Mechanical decoking, steam air decoking are the method used for decoking.
Mechanical decoking processes takes 4-7 times longer than steam air decoking. The principal method of decoking with steam air are spalling and burning. The coke is burnt in the presence of steam and air at temperature from (600-800oC).
Trends in Technological Developments of Steam Crackers for Production of Ethylene
From the late 1960s through the 1970, the petrochemical industry built a generation of new steam crackers with an ethylene capacity of several million tonnes capacity. Older plant consists of typically 10-17 small furnaces with radiant coils having residence time 0.4-0.6 sec, thermal efficiency below 90percent, central waste heat recovery system and nitrogen oxide (NOX) emissions 75-100 ppm. Present day olefin plants have capacity more than 1,000,000 tonnes per year ethylene produced with 5-7 modern cracking furnaces using twin-cell designs. Short residence time and radiant coil smaller diameters increase yields. The higher selectivity of modern coils reduces specific energy consumption. The modern olefin plants have better ethylene selectivity and improved health, safety and environment standards by incorporating current emission and safety standards.
Major energy improvements by revamp or by replacing the existing furnace sections can be achieved by:
Performance of the steam cracking furnace can be upgraded by: (i) increasing furnace capacity (ii) increasing cracking severity (iii) improving ethylene selectivity (iv) improving thermal efficiency (v) reducing downtime for decoking and reducing maintenance