What are the quality requirements for naphtha as a cracking feedstock in the ethylene industry?
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This post was last edited by dongliangsir on 2011-1-23 20:15. What are the quality requirements for naphtha as a cracking feedstock in the ethylene industry?
What is the effect of high dryness on pyrolysis?
The impact of the properties of pyrolysis feedstocks on ethylene production I. Indicators for evaluating the pyrolysis performance of feedstocks It can be seen from the sources of pyrolysis feedstocks that their range is quite wide. The properties of the raw materials have a decisive impact on the pyrolysis results; therefore, it is very important to study and characterize the characteristics of raw material pyrolysis. There are many indicators used to characterize the cracking performance of raw materials. For hydrocarbon mixtures with known compositions, the properties of each component can be utilized for characterization. In the case of petroleum fractions, due to their complex composition and the difficulties associated with analysis, indicators such as hydrocarbon composition, group composition, boiling range, density, hydrogen content, average molecular weight, characteristic factors, correlation indices (such as M2J), residue carbon, and bromine value are commonly used to assess their cracking performance. Here are a few of the main indicators introduced. (1) Group composition The feedstock for cracking is composed of various hydrocarbons, which can be divided into four major groups based on their structure: alkanes, olefins, cycloalkanes, and aromatics. 1. Representation of the composition of the groups The composition of these four major groups is expressed using the Pd9Jr4 value, whose meaning is as follows: P – Paraffins (abbreviated as alkanes), which are relatively easy to crack to produce ethylene and propylene. Among them, the ethylene yield of n-normal alkanes (n–P) is higher than that of isomeric alkanes (i–P), whereas the yields of methane, propylene, butylene, and aromatics for n-normal alkanes are lower than those for isomeric alkanes. O — Olefins: difficult to crack and prone to coking. N — Naphthene; the cracking of cyclohexane produces ethylene, butadiene, and aromatics, while the cracking of cyclopentane produces ethylene and propylene. However, the yields of ethylene, propylene, and C4 compounds from naphthenes cracking are lower than those from alkanes, and aromatics are prone to be formed. A — Aromatics: Difficult to crack, tend to produce heavy aromatics and cause coking. 2. Relationship between the PONA value of pyrolysis feedstock and its pyrolysis performance. By measuring the PONA value of the pyrolysis feedstock, it is possible to understand to some extent the performance of its pyrolysis reaction. In the pyrolysis feedstock, the higher the content of alkanes, particularly n-alkanes, the higher the yield of trienes. (II) Hydrogen content: The hydrogen content of the pyrolysis feedstock refers to the mass percentage of hydrogen in the hydrocarbon molecules. The hydrogen content of the feedstock is an important parameter for assessing its cracking performance and the potential ethylene content. The higher the hydrogen content in the feedstock, the better the cracking performance. In terms of elemental composition, alkanes have the highest hydrogen content, followed by cycloalkanes, with aromatics having the lowest. In terms of the molecular weight of the raw materials, from ethane to diesel, as the molecular weight increases, the hydrogen content decreases accordingly, and the yield of ethylene also decreases accordingly. (III) Characteristic factor: The characteristic factor is a factor that reflects the hydrocarbon composition characteristics of crude oil and its fractions, denoted by the symbol φ. The value is highest for alkanes, next for cycloalkanes, and lowest for aromatics. The characteristic factors of hydrocarbons: Methane, Ethane, Propane, Butane, Cyclopentane, Cyclohexane, Toluene, Ethylbenzene, Benzene. Ki values: 19.54, 18.38, 14.71, 13.51, 11.12, 10.99, 10.15, 10.37, 9.73. As can be seen from the table above, the higher the K value, the higher the content of alkanes in the hydrocarbons, which indicates a greater waxy nature of those hydrocarbons ； The lower the K value, the stronger the aromaticity of the hydrocarbon. Therefore, the higher the K value, the better the cracking performance of hydrocarbons. (IV) Correlation index BMCI: In naphtha, most of the naphthenes N and aromatics A are monocyclic, whereas in diesel, a considerable portion of the naphthenes N and aromatics A are bicyclic and polycyclic; this is not reflected in the PONA value. The correlation index BMCI can represent this characteristic. Since the BMCI of n-hexane is 0 and that of benzene is 100, BMCI is an indicator of aromaticity; the higher its value, the greater the degree of aromaticity. Therefore, BMCI can also be called the aromatic index. The BMCI value of straight-chain alkanes is close to 0, while that of alkanes with more branches is around 10–15. The stronger the aromaticity of a hydrocarbon compound, the higher its BMCI value. The relationship between the BMCI value of hydrocarbons and their pyrolysis performance: the lower the BMCI value, the higher the ethylene yield; conversely, the higher the BMCI value, the lower the ethylene yield. Therefore, distillate oils with lower BMCI values are better feedstocks for cracking. II. The relationship between cracking feedstocks and production
Numerous factors, such as the characteristics of cracking feedstocks, cracking reaction conditions, and the type and structure of cracking reactors, influence the effectiveness of the cracking process. The various factors are interrelated and mutually restrictive. Different cracking feedstocks have the following effects on ethylene production: (1) Effect on ethylene yield. The table below shows the yields of the cracking products from various feedstocks. Typical yields of cracking feedstocks with different boiling ranges, in % (by mass):
Products: Ethane, Propene, Butane, Naphtha, Atmospheric Diesel, Vacuum Diesel, Hydrogen
Yields: 8.82, 2.27, 1.57, 1.56, 0.94, 0.78
Methane: 6.27, 27.43, 22.12, 17.20, 11.19, 8.75
Ethylene: 77.73, 42.01, 40.00, 33.62, 25.92, 20.49
Propylene: 2.76, 16.82, 17.27, 15.53, 16.15, 14.07
Butadiene: 1.81, 3.01, 3.50, 4.56, 4.56, 5.38
Butane and Butenes: 0.82, 1.29, 6.72, 4.21, 4.84, 6.28
Benzene: 0.87, 2.47, 3.02, 6.74, 6.03, 3.73
Toluene: 0.12, 0.53, 0.83, 3.34, 2.90, 2.90
Cyclooctane: –, 0.35, 1.76, 2.17, 1.87
Residues: 0.80, 3.62, 2.92, 6.75, 7.31, 0.77
Heavy oils: 0.55, 1.70, 4.70, 18.00, 25.00
As can be seen from the table, as the feedstock becomes heavier, the yield of ethylene decreases, while more feedstock is required to produce each ton of ethylene. The amount of raw material used when diesel is the feedstock is three times that when ethane is the feedstock. It can also be seen that as the raw material becomes heavier, the pyrolysis by-products increase.
Numerous factors, such as the characteristics of cracking feedstocks, cracking reaction conditions, and the type and structure of cracking reactors, influence the effectiveness of the cracking process. The various factors are interrelated and mutually restrictive. Different cracking feedstocks have the following effects on ethylene production: (1) Effect on ethylene yield. The table below shows the yields of the cracking products from various feedstocks. Typical yields of cracking feedstocks with different boiling ranges, in % (by mass):
Products: Ethane, Propene, Butane, Naphtha, Atmospheric Diesel, Vacuum Diesel, Hydrogen
Yields: 8.82, 2.27, 1.57, 1.56, 0.94, 0.78
Methane: 6.27, 27.43, 22.12, 17.20, 11.19, 8.75
Ethylene: 77.73, 42.01, 40.00, 33.62, 25.92, 20.49
Propylene: 2.76, 16.82, 17.27, 15.53, 16.15, 14.07
Butadiene: 1.81, 3.01, 3.50, 4.56, 4.56, 5.38
Butane and Butenes: 0.82, 1.29, 6.72, 4.21, 4.84, 6.28
Benzene: 0.87, 2.47, 3.02, 6.74, 6.03, 3.73
Toluene: 0.12, 0.53, 0.83, 3.34, 2.90, 2.90
Cyclooctane: –, 0.35, 1.76, 2.17, 1.87
Residues: 0.80, 3.62, 2.92, 6.75, 7.31, 0.77
Heavy oils: 0.55, 1.70, 4.70, 18.00, 25.00
As can be seen from the table, as the feedstock becomes heavier, the yield of ethylene decreases, while more feedstock is required to produce each ton of ethylene. The amount of raw material used when diesel is the feedstock is three times that when ethane is the feedstock. It can also be seen that as the raw material becomes heavier, the pyrolysis by-products increase.
(II) Impact on energy consumption 1. Utility consumption The table below shows the utility consumption for producing ethylene from different raw materials. List of main utility consumption items for producing ethylene from different raw materials
Unit: per ton of raw material
Ethylene propylene/butane, naphtha, diesel – electricity/kilowatt-hour per ton of ethylene: 35, 40, 49, 68
Cooling water – tons per ton of ethylene: 32, 63, 50, 43, 45, 66
Fuel – GJ per ton of ethylene: 16.8, 72, 126, 32.3
As can be seen from the table above, as the raw materials become heavier, the utility consumption increases. 2. Dilution steam ratio: During hydrocarbon cracking, steam must be injected as a diluent in order to reduce the hydrocarbon partial pressure and prevent coking on the walls of the furnace tubes. The amount of dilution steam is generally expressed as the dilution steam ratio (steam volume/hydrocarbon volume). The table below lists the dilution steam ratios for different raw materials. The dilution steam ratio for different feedstocks:
Feedstock: Ethane, Propane, Butane, Naphtha, Diesel
Dilution steam ratio: 0.3–0.35, 0.35–0.4, 0.4, 0.5–0.65, 0.6–0.8
As can be seen from the table above, as the feedstocks become heavier in nature, the dilution steam ratio gradually increases. 3. Energy consumption As can be seen from the above analysis, the amounts of raw materials, water, electricity, steam, and fuel required for ethylene production increase as the weight of the raw materials increases; consequently, the energy consumption of the plant also increases. The table below shows the energy consumption for different types of cracking. Energy consumption for different types of cracking (water, electricity, steam, fuel): unit – gigajoules per ton of ethylene. Years: Ethane, Propylene, Naphtha, Diesel. 1970s: 26.75, 28.42, 39.29, 44.3; 1980s: 21.32, 3.42, 5.91, 33.02; 1990s: 13.82, 15.91, 20.94, 24.28. (III) Impact on plant investment: As the feedstock becomes heavier, the process becomes more complex, and the number of processing equipment increases, the investment required for such plants also increases. The trends in investment changes for large-scale ethylene plants of the same scale due to different raw materials are listed in the table below. Relationship between raw materials and investment ratio: Raw materials – Butane, Propylene, Naphtha, Diesel; Investment ratio: 0.8, 1.0, 1.1, 1.5. (IV) Impact on production costs: Differences in raw materials affect the amount of material used in cracking, energy consumption, and the scale of investment required for the facilities, all of which ultimately influence the cost of producing ethylene. The table below shows the impact of raw materials on production costs. Impact of different raw materials on production costs Unit: USD/ton of ethylene Item Butane Propene Naphtha Diesel Raw material cost 126.75 321.63 627.92 754.84 By-product revenue 25.59 237.69 471.66 579.81 Net raw material cost 101.16 83.94 156.26 175.03 Utility costs 45.51 51.45 68.89 80.11 Variable costs 146.68 135.39 220.15 255.14 Total production cost 264.17 267.33 370.23 426.37 As can be seen from the table above, as the density of the raw materials increases, the production cost of the facility rises accordingly.
Unit: per ton of raw material
Ethylene propylene/butane, naphtha, diesel – electricity/kilowatt-hour per ton of ethylene: 35, 40, 49, 68
Cooling water – tons per ton of ethylene: 32, 63, 50, 43, 45, 66
Fuel – GJ per ton of ethylene: 16.8, 72, 126, 32.3
As can be seen from the table above, as the raw materials become heavier, the utility consumption increases. 2. Dilution steam ratio: During hydrocarbon cracking, steam must be injected as a diluent in order to reduce the hydrocarbon partial pressure and prevent coking on the walls of the furnace tubes. The amount of dilution steam is generally expressed as the dilution steam ratio (steam volume/hydrocarbon volume). The table below lists the dilution steam ratios for different raw materials. The dilution steam ratio for different feedstocks:
Feedstock: Ethane, Propane, Butane, Naphtha, Diesel
Dilution steam ratio: 0.3–0.35, 0.35–0.4, 0.4, 0.5–0.65, 0.6–0.8
As can be seen from the table above, as the feedstocks become heavier in nature, the dilution steam ratio gradually increases. 3. Energy consumption As can be seen from the above analysis, the amounts of raw materials, water, electricity, steam, and fuel required for ethylene production increase as the weight of the raw materials increases; consequently, the energy consumption of the plant also increases. The table below shows the energy consumption for different types of cracking. Energy consumption for different types of cracking (water, electricity, steam, fuel): unit – gigajoules per ton of ethylene. Years: Ethane, Propylene, Naphtha, Diesel. 1970s: 26.75, 28.42, 39.29, 44.3; 1980s: 21.32, 3.42, 5.91, 33.02; 1990s: 13.82, 15.91, 20.94, 24.28. (III) Impact on plant investment: As the feedstock becomes heavier, the process becomes more complex, and the number of processing equipment increases, the investment required for such plants also increases. The trends in investment changes for large-scale ethylene plants of the same scale due to different raw materials are listed in the table below. Relationship between raw materials and investment ratio: Raw materials – Butane, Propylene, Naphtha, Diesel; Investment ratio: 0.8, 1.0, 1.1, 1.5. (IV) Impact on production costs: Differences in raw materials affect the amount of material used in cracking, energy consumption, and the scale of investment required for the facilities, all of which ultimately influence the cost of producing ethylene. The table below shows the impact of raw materials on production costs. Impact of different raw materials on production costs Unit: USD/ton of ethylene Item Butane Propene Naphtha Diesel Raw material cost 126.75 321.63 627.92 754.84 By-product revenue 25.59 237.69 471.66 579.81 Net raw material cost 101.16 83.94 156.26 175.03 Utility costs 45.51 51.45 68.89 80.11 Variable costs 146.68 135.39 220.15 255.14 Total production cost 264.17 267.33 370.23 426.37 As can be seen from the table above, as the density of the raw materials increases, the production cost of the facility rises accordingly.