[Cracking Question Bank] What are the ways to represent cracking depth?

What are the ways to represent the cracking depth? It can be expressed by the outlet temperature conversion rate, the kinetic depth function KSF, or by the proportions of certain components in the pyrolysis products, such as ethylene/propylene, methane/propylene, and C/H ratio in the liquid products; among these, KSF is considered the most scientific approach.

It can be expressed by the outlet temperature conversion rate, the kinetic depth function KSF, or by the proportions of certain components in the pyrolysis products, such as ethylene/propylene, methane/propylene, and C/H ratio in the liquid products; among these, KSF is considered the most scientific approach.

1. Raw material conversion rate. 2. Methane yield. 3. Ethylene to propylene yield ratio. 4. Yield ratio of methane to ethylene or propylene. 5. Hydrogen content and hydrogen-to-carbon atom ratio of the liquid product. 6. Outlet temperature of the pyrolyzer. 7. Pyrolysis depth function. 8. Kinetic cracking depth function.

What are the ways to represent the cracking depth? The cleavage depth refers to the extent of the cleavage reaction. Due to the complexity of the pyrolysis reaction, it is difficult to describe it accurately using a single parameter. In engineering, depending on the specific circumstances, various parameters are often used to measure the degree of cracking. (1) Raw material conversion rate × The raw material conversion rate reflects the degree of conversion of the cracking raw materials during the cracking reaction. Although it does not reflect the extent of the secondary reaction, it still reflects to some extent the progress of the cleavage reaction. Therefore, the conversion rate of raw materials is often used to measure the degree of pyrolysis. When a single hydrocarbon is used as the feedstock (such as ethane or propane), the conversion rate x of the pyrolysis feedstock can be calculated using the equilibrium mole numbers N0 and NV before and after the reaction: X = (N0 – NV) / N0 = 1 – NV / N0. When mixed light hydrocarbons are used for pyrolysis, the conversion rate of each component can be calculated separately. During the pyrolysis of fractionated oil, the conversion rate can be calculated based on a certain equivalent component, and this is used to indicate the degree of pyrolysis. For distillate oil cracking feedstocks, the conversion rate of the feedstock is sometimes approximated by the weight percentage of light components such as C4 and below in the resulting cracking gas (the gas yield), expressed as a percentage of the weight of the feedstock (referred to as the gas production rate). Considering that the amount of gas generated from aromatic hydrocarbons in the feedstock during the cracking reaction is very small, the gas generation rate X(P+N) can be expressed as: X(P+N) = X_gas / (P+N), where X_gas represents the amount of gas generated ； P, N — Amount of alkanes and cycloalkanes in the raw material. When the raw material contains a high amount of cycloalkanes, the above formula will not apply. (2) Methane yield: The methane yield obtained from C10 pyrolysis increases as the degree of pyrolysis increases. Since methane is relatively stable, it essentially does not disappear due to secondary reactions. Therefore, the methane yield in the pyrolysis products can, to a certain extent, indicate the degree of pyrolysis of the reaction. Under tubular furnace pyrolysis conditions, aromatic hydrocarbons essentially do not produce gaseous products upon pyrolysis. To eliminate the influence of varying aromatic content, the aromatics in the feedstock can be deducted, and the weight of alkanes and cycloalkanes is used as the basis for calculating the methane yield, which is referred to as the aromatic-free methane yield C1(P+N)0. C1(P+N)0 = C10/(P+N) (3) The yield ratio of ethylene to propylene (C2/C3=). Within a certain range of cracking depth, as the cracking depth increases, the yield of ethylene rises, while the yield of propylene increases more slowly. After reaching a certain degree of cracking, the ethylene yield continues to increase as the cracking depth increases, while the propylene yield begins to decline from its maximum value. Therefore, within a certain range of cracking depth, the ratio of ethylene to propylene yields, C2/C3, can be used as an indicator to measure the cracking depth. However, after the cracking depth reaches a certain level, the ethylene yield will also decrease as the cracking depth increases. At this point, the yield ratio of C2/C3 can no longer accurately reflect the degree of cracking. (4) The yield ratio of methane to ethylene or propylene (C10/C2= or C10/C3=). Since the methane yield always increases as the reaction progresses, the yield of ethylene or propylene starts to decline after reaching a maximum value as the degree of cracking increases. Therefore, during high-depth pyrolysis, it is reasonable to measure the degree of pyrolysis using the yield ratio of methane to ethylene or to propylene (C10/C2= or C10/C3=). (5) The hydrogen content and the hydrogen-to-carbon atom ratio (H/C) of the liquid product increase as the degree of pyrolysis increases; consequently, the yield of gaseous products with a high hydrogen content, such as C4 or lower hydrocarbons, also increases. Based on the hydrogen balance, it can be seen that the hydrogen content and the hydrogen-to-carbon ratio (H/C)L of the liquid products of C5 or higher obtained from pyrolysis decrease as the degree of pyrolysis increases. Experience from production practice shows that when cracking distillate oils, the degree of cracking should be such that the hydrogen-to-carbon ratio (H/C) of the resulting C5+S liquid product is not lower than 0.96 (or the hydrogen content is not lower than 8%). When the cracking depth is too high, resulting in a hydrogen-to-carbon ratio (H/C)L of the C5+S liquid products obtained from cracking below 0.96 (or a hydrogen content below 8%), the coking removal cycle may be **shortened due to severe coking in the furnace tubes or waste heat boilers. Therefore, the hydrogen content or hydrogen-to-carbon ratio of the liquid product obtained from pyrolysis is also an important indicator to measure the degree of pyrolysis. (6) Outlet temperature of the pyrolyzer (Tout): In the 1950s, the outlet temperature Tout of the pyrolyzer was used to indicate the degree of pyrolysis; temperatures ranging from 690–720°C were considered to correspond to shallow pyrolysis, those from 720–750°C to moderate-depth pyrolysis, and temperatures above 750°C to deep pyrolysis. Since this method does not take into account the distribution of reaction temperatures or residence time, using the outlet temperature to reflect the degree of pyrolysis is quite one-sided. With the improvements in the design of modern pyrolysis furnaces, the residence time has been significantly reduced, which has led to a substantial increase in the pyrolysis temperature required to achieve the same degree of pyrolysis. With the furnace type determined, the arrangement of the furnace tubes and its geometric parameters have been fixed. At this point, for a given pyrolysis feed and load, the furnace outlet temperature can to some extent characterize the degree of pyrolysis. Therefore, in production operations, the furnace exit temperature is still often used to indicate the degree of pyrolysis. The outlet temperature ranges used to distinguish between shallow, medium-concentration, and deep pyrolysis will also vary. (7) The pyrolysis depth function (S) takes into account the influence of temperature and residence time on the pyrolysis depth; some people relate the pyrolysis temperature T to the residence time θ using the following relationship: S = Tθm, where m can be 0.06 or 0.027. S is called the cleavage depth function. (8) Kinetic cracking depth function: If the cracking reaction of the feedstock is treated as a first-order reaction, then there is a relationship between the feedstock conversion rate a, the reaction rate constant k, and the residence time θ: 1/(1-a)∫kdθ = ln∫kdθ. This expression can indicate the influence of the temperature distribution and residence time distribution on the degree of conversion or cracking depth of the feedstock, and to some extent, it allows for the quantitative determination of the cracking depth. However, ∫kdθ is not only a function of the temperature and residence time distributions, but also a function of the properties of the pyrolysis feedstock. To avoid the influence of the properties of the pyrolysis feedstock and to evaluate the pyrolysis performance of different reaction coils in a tubular pyrolyzer, the same feedstock can be used for pyrolysis in various reaction coils; thereby, the reaction coils can be evaluated based on the different ∫kdθ values obtained from each coil. Based on this, S&W Company proposed using n-pentane as the “standard material,” and defined the value obtained from the cracking of n-pentane, ∫kdθ, as the kinetic cracking depth function (KSF): KSF=∫kc5dθ=∫Ac5exp(-Ec5/RT ), where Kc5 is the reaction rate constant for the cracking of n-pentane ； Ac5——Frequency factor for the pyrolysis reaction of n-pentane ； Ec5 — the activation energy for the pyrolysis of n-pentane. Clearly, the kinetic cracking depth function KSF is a parameter independent of the properties of the feedstock; it reflects the influence of the cracking temperature distribution and residence time on the cracking depth. Recently, some researchers have defined ∫kdθ as the cleavage depth index CSI, where k is the reaction kinetic constant of the specific feedstock. Therefore, the pyrolysis depth index CBS=∫kdθ is related not only to temperature and residence time, but also to the properties of the pyrolysis feedstock.

These include: 1) raw material conversion rate α, 2) methane yield, 3) ethylene yield, 4) H/C ratio in pyrolysis gasoline, 5) pyrolysis depth function (S), 6) kinetic pyrolysis depth function (KSF), etc.

1. Raw material conversion rate. 2. Methane yield. 3. Ethylene to propylene yield ratio. 4. Yield ratio of methane to ethylene or propylene. 5. Hydrogen content and hydrogen-to-carbon atom ratio of the liquid product. 6. Outlet temperature of the pyrolyzer. 7. Pyrolysis depth function. 8. Kinetic cracking depth function.

The cleavage depth refers to the extent of the cleavage reaction. Due to the complexity of the pyrolysis reaction, it is difficult to describe it accurately using a single parameter. In engineering, depending on the specific circumstances, various parameters are often used to measure the degree of cracking. (1) Raw material conversion rate × The raw material conversion rate reflects the degree of conversion of the cracking raw materials during the cracking reaction. Although it does not reflect the extent of the secondary reaction, it still reflects to some extent the progress of the cleavage reaction. Therefore, the conversion rate of raw materials is often used to measure the degree of pyrolysis. When a single hydrocarbon is used as the feedstock (such as ethane or propane), the conversion rate x of the pyrolysis feedstock can be calculated using the equilibrium mole numbers N0 and NV before and after the reaction: X = (N0 – NV) / N0 = 1 – NV / N0. When mixed light hydrocarbons are used for pyrolysis, the conversion rate of each component can be calculated separately. During the pyrolysis of fractionated oil, the conversion rate can be calculated based on a certain equivalent component, and this is used to indicate the degree of pyrolysis. For distillate oil cracking feedstocks, the conversion rate of the feedstock is sometimes approximated by the weight percentage of light components such as C4 and below in the resulting cracking gas (the gas yield), expressed as a percentage of the weight of the feedstock (referred to as the gas production rate). Considering that the amount of gas generated from aromatic hydrocarbons in the feedstock during the cracking reaction is very small, the gas generation rate X(P+N) can be expressed as: X(P+N) = X_gas / (P+N), where X_gas represents the amount of gas generated ； P, N — Amount of alkanes and cycloalkanes in the raw material. When the raw material contains a high amount of cycloalkanes, the above formula will not apply. (2) Methane yield: The methane yield obtained from C10 pyrolysis increases as the degree of pyrolysis increases. Since methane is relatively stable, it essentially does not disappear due to secondary reactions. Therefore, the methane yield in the pyrolysis products can, to a certain extent, indicate the degree of pyrolysis of the reaction. Under tubular furnace pyrolysis conditions, aromatic hydrocarbons essentially do not produce gaseous products upon pyrolysis. To eliminate the influence of varying aromatic content, the aromatics in the feedstock can be deducted, and the weight of alkanes and cycloalkanes is used as the basis for calculating the methane yield, which is referred to as the aromatic-free methane yield C1(P+N)0. C1(P+N)0 = C10/(P+N) (3) The yield ratio of ethylene to propylene (C2/C3=). Within a certain range of cracking depth, as the cracking depth increases, the yield of ethylene rises, while the yield of propylene increases more slowly. After reaching a certain degree of cracking, the ethylene yield continues to increase as the cracking depth increases, while the propylene yield begins to decline from its maximum value. Therefore, within a certain range of cracking depth, the ratio of ethylene to propylene yields, C2/C3, can be used as an indicator to measure the cracking depth. However, after the cracking depth reaches a certain level, the ethylene yield will also decrease as the cracking depth increases. At this point, the yield ratio of C2/C3 can no longer accurately reflect the degree of cracking. (4) The yield ratio of methane to ethylene or propylene (C10/C2= or C10/C3=). Since the methane yield always increases as the reaction progresses, the yield of ethylene or propylene starts to decline after reaching a maximum value as the degree of cracking increases. Therefore, during high-depth pyrolysis, it is reasonable to measure the degree of pyrolysis using the yield ratio of methane to ethylene or to propylene (C10/C2= or C10/C3=). (5) The hydrogen content and the hydrogen-to-carbon atom ratio (H/C) of the liquid product increase as the degree of pyrolysis increases; consequently, the yield of gaseous products with a high hydrogen content, such as C4 or lower hydrocarbons, also increases. Based on the hydrogen balance, it can be seen that the hydrogen content and the hydrogen-to-carbon ratio (H/C)L of the liquid products of C5 or higher obtained from pyrolysis decrease as the degree of pyrolysis increases. Experience from production practice shows that when cracking distillate oils, the degree of cracking should be such that the hydrogen-to-carbon ratio (H/C) of the resulting C5+S liquid product is not lower than 0.96 (or the hydrogen content is not lower than 8%). When the cracking depth is too high, resulting in a hydrogen-to-carbon ratio (H/C)L of the C5+S liquid products obtained from cracking below 0.96 (or a hydrogen content below 8%), the coking removal cycle may be **shortened due to severe coking in the furnace tubes or waste heat boilers. Therefore, the hydrogen content or hydrogen-to-carbon ratio of the liquid product obtained from pyrolysis is also an important indicator to measure the degree of pyrolysis. (6) Outlet temperature of the pyrolyzer (Tout): In the 1950s, the outlet temperature Tout of the pyrolyzer was used to indicate the degree of pyrolysis; temperatures ranging from 690–720°C were considered to correspond to shallow pyrolysis, those from 720–750°C to moderate-depth pyrolysis, and temperatures above 750°C to deep pyrolysis. Since this method does not take into account the distribution of reaction temperatures or residence time, using the outlet temperature to reflect the degree of pyrolysis is quite one-sided. With the improvements in the design of modern pyrolysis furnaces, the residence time has been significantly reduced, which has led to a substantial increase in the pyrolysis temperature required to achieve the same degree of pyrolysis. With the furnace type determined, the arrangement of the furnace tubes and its geometric parameters have been fixed. At this point, for a given pyrolysis feed and load, the furnace outlet temperature can to some extent characterize the degree of pyrolysis. Therefore, in production operations, the furnace exit temperature is still often used to indicate the degree of pyrolysis. The outlet temperature ranges used to distinguish between shallow, medium-concentration, and deep pyrolysis will also vary. (7) The pyrolysis depth function (S) takes into account the influence of temperature and residence time on the pyrolysis depth; some people relate the pyrolysis temperature T to the residence time θ using the following relationship: S = Tθm, where m can be 0.06 or 0.027. S is called the cleavage depth function. (8) Kinetic cracking depth function: If the cracking reaction of the feedstock is treated as a first-order reaction, then there is a relationship between the feedstock conversion rate a, the reaction rate constant k, and the residence time θ: 1/(1-a)∫kdθ = ln∫kdθ. This expression can indicate the influence of the temperature distribution and residence time distribution on the degree of conversion or cracking depth of the feedstock, and to some extent, it allows for the quantitative determination of the cracking depth. However, ∫kdθ is not only a function of the temperature and residence time distributions, but also a function of the properties of the pyrolysis feedstock. To avoid the influence of the properties of the pyrolysis feedstock and to evaluate the pyrolysis performance of different reaction coils in a tubular pyrolyzer, the same feedstock can be used for pyrolysis in various reaction coils; thereby, the reaction coils can be evaluated based on the different ∫kdθ values obtained from each coil. Based on this, S&W Company proposed using n-pentane as the “standard material,” and defined the value obtained from the cracking of n-pentane, ∫kdθ, as the kinetic cracking depth function (KSF): KSF=∫kc5dθ=∫Ac5exp(-Ec5/RT ), where Kc5 is the reaction rate constant for the cracking of n-pentane ； Ac5——Frequency factor for the pyrolysis reaction of n-pentane ； Ec5 — the activation energy for the pyrolysis of n-pentane. Clearly, the kinetic cracking depth function KSF is a parameter independent of the properties of the feedstock; it reflects the influence of the cracking temperature distribution and residence time on the cracking depth. Recently, some researchers have defined ∫kdθ as the cleavage depth index CSI, where k is the reaction kinetic constant of the specific feedstock. Therefore, the pyrolysis depth index CBS=∫kdθ is related not only to temperature and residence time, but also to the properties of the pyrolysis feedstock.

1) Raw material conversion rate α 2) Methane yield 3) Ethylene yield 4) H/C ratio in pyrolysis gasoline 5) Pyrolysis depth function (S) 6) Kinetic pyrolysis depth function (KSF), etc

1. Methane yield. 2. Ethylene to propylene yield ratio. 3. The yield ratio of methane to ethylene or propylene. 4. Hydrogen content and hydrogen-to-carbon atom ratio of the liquid product. 5. Outlet temperature of the pyrolyzer. 6. Pyrolysis depth function. 7. Kinetic cracking depth function.

1) Raw material conversion rate α 2) Methane yield 3) Ethylene yield 4) H/C ratio in pyrolysis gasoline 5) Pyrolysis depth function (S) 6) Kinetic pyrolysis depth function (KSF), etc.