DE69506565T2 - METHOD FOR THERMALLY CLEAVING RESIDUAL HYDROCARBON OIL - Google Patents

Description

The present invention relates to a process for thermal cracking a hydrocarbon residual oil. More specifically, the present invention relates to a process for thermal cracking a hydrocarbon residual oil in which the thermal cracking is integrated with a gasification treatment of an asphaltene-rich bottoms product derived from said thermal cracking.

Residual hydrocarbon oils can be obtained as the bottoms product of the distillation of crude oil at atmospheric pressure (“atmospheric” or “long” residue) or at reduced pressure (“vacuum” or “short” residue).

The conversion of hydrocarbon residue oils by thermal cracking has long been known. Basically, thermal cracking is an endothermic, non-catalytic process in which larger hydrocarbon molecules from residue oil fractions are broken down into smaller molecules. The energy required to break the larger molecules into smaller molecules is provided by heating the hydrocarbon residue oil feedstock to a sufficiently high temperature. However, a widely recognized problem with thermal cracking processes is the formation of coke, particularly under more severe cracking conditions. Several ways are known in the art to suppress this coke formation. For example, if the degree of conversion of the heavy hydrocarbons, i.e. those hydrocarbons with a boiling point of 520ºC and above (520ºC+ hydrocarbons), is kept sufficiently low, then coke formation can be largely avoided. Depending on the nature of the feedstock and the severity of the thermal cracking, this conversion rate (hereinafter referred to as 520ºC+conversion, i.e. the weight percentage of hydrocarbons boiling at 520ºC and above present in the feedstock that are converted to lower boiling components) should be kept below about 30 wt.%. Another way to substantially prevent coke formation is to deasphalt the residual hydrocarbon oil prior to thermal cracking, in which case 520ºC+ conversions of 30 wt% or higher are achievable. A disadvantage, however, is that the separated asphaltenes can no longer contribute to the production of distillates without further upgrading, independent of the deasphalted oil.

The furnace-soaker configuration is well known in the field of thermal cracking. Inside the furnace, the heating of the residual hydrocarbon oil feedstock takes place and a significant portion of the hydrocarbon oil feedstock is already cracked to lower boiling components. The heated oil is then fed into the soaker or "reaction chamber". In this soaker, the cracking reactions continue. Since furnace cracking is fairly inexpensive and simple, a common goal is to convert as much high boiling material as possible in the furnace and use the soaker to further increase the conversion level. However, the achievable conversion level in the furnace is limited by coke formation. Therefore, in the conventional furnace-soaker configuration, where a 520ºC+ conversion of about 30 wt% can be achieved, about half of the total conversion (i.e. about 15 wt%) occurs in the furnace and the other half in the soaker. If the degree of conversion in the furnace were higher, meaning more severe cracking conditions, then rapid coke formation and deposition on the furnace interior walls would occur, leading to a rapid reduction in the heating efficiency of the furnace and therefore a decrease in the total conversion.

On the other hand, coke formation and the subsequent coke deposition on the metal parts in the soaker are also a generally recognized problem in conventional thermal cracking processes. For this reason, the 520ºC⁺ conversion in the soaker is also tied to a maximum. It is understood that too rapid coke formation in the soaker also negatively affects the operating time of the process after each cleaning operation of the plant.

Moreover, since no heat is normally applied to the soaker, the temperature of the heated and partially converted oil decreases by about 15-30ºC as it passes through the soaker This temperature decrease is mainly caused by the endothermic nature of the cracking reactions, the evaporation of the light distillates and the loss of heat to the environment via the soaker walls. As a result of this temperature drop in the soaker, there is a decrease in the cracking reactions in the direction of the oil flow. Accordingly, the cracking efficiency within the soaker is not at an optimal level.

EP-A-0 328 216 discloses a process for thermal cracking of hydrocarbon residue oils in which no furnace is used and in which the hydrocarbon residue oil feedstock is fed directly into a soaker together with hot synthesis gas. This synthesis gas originates from the gasification of an asphaltene-rich heavy hydrocarbon oil derived from the cracked residue of thermal cracking. Accordingly, the heating of the hydrocarbon residue oil feedstock is carried out by direct heat exchange with the hot synthesis gas. Although this process works well and provides a very high degree of integration between thermal cracking and gasification, it cannot be easily carried out in an existing refinery with a thermal cracking unit and a gasification plant, since it would require fundamental changes in both the refinery plant and in particular in the thermal cracking plant. Such equipment would therefore be very expensive and would call into question its economic feasibility.

The present invention aims to improve the overall 520°C⁺ conversion of thermal cracking processes carried out in furnace-soaker configurations to a level of at least 35 wt.%. Furthermore, the present invention aims to provide a thermal cracking process that can be implemented relatively easily and relatively inexpensively in an existing refinery having at least one thermal cracking and optionally one gasification unit. More specifically, the present invention aims to suppress coke formation and deposition in both the furnace and the soaker, while at the same time improving the cracking efficiency within the soaker, thereby improving both the 520°C⁺ conversion Both the operating time and the operating time increase, which is obviously advantageous from an economic point of view.

All the above-mentioned objects are met by the present invention, which relates to a process for thermally cracking a residual hydrocarbon oil in which a 520°C+ total conversion of at least 35% by weight is achieved, i.e. wherein at least 35% by weight of the hydrocarbons present in the residual hydrocarbon oil having a boiling point of 520°C and higher are converted to lower boiling components, said process comprising the following steps:

(a) heating the hydrocarbon residual oil feedstock in a furnace to a temperature in the range of 400 to 510°C for a time sufficient to achieve 30 to 45% of the 520°C+ total conversion;

(b) feeding the partially converted hot hydrocarbon residue oil formed in step (a) and a hot hydrogen-containing gas into a soaker, said hydrogen-containing gas being of a sufficiently high temperature to maintain the temperature of the hydrocarbon oil in the soaker by direct heat exchange at a value in the range of 420 to 650°C, in which soaker the remainder to 100% of the 520°C+ total conversion takes place; and

(c) recovering a gaseous fraction containing the hydrogen-containing gas and a cracked residue from the soaker.

Suitable hydrocarbon residual oil feedstocks which can be used in step (a) are heavy hydrocarbon containing feedstocks which constitute at least 25 wt.% of the 520°C+ hydrocarbons, preferably more than 37.5 wt.% of the 520°C+ hydrocarbons and preferably even more than 75 wt.% of the 520°C+ hydrocarbons. Feedstocks containing more than 90 wt.% of 520°C+ hydrocarbons are most advantageously used. Suitable feedstocks thus include atmospheric residuals and vacuum residuals. If desired, the coke can The residual hydrocarbon oil may be mixed with a heavy distillate fraction, such as a cycle oil obtained by catalytic cracking of a hydrocarbon oil fraction or with a heavy hydrocarbon oil obtained by extraction from a residual hydrocarbon oil.

In step (a) of the process according to the present invention, the hydrocarbon residue is heated in the furnace to a temperature in the range of 400 to 510°C for a sufficient time to achieve 30 to 45% of the 520°C+ conversion. The exact combination of temperature and residence time in the furnace must in any event be such that 30-40% of the total 520°C+ conversion takes place in this furnace. Since in normal thermal cracking operations involving a furnace followed by a soaker, about 50% of the total conversion takes place in the furnace, this necessitates the use of relatively mild thermal cracking conditions, for example those conditions usually used in visbreaking operations. The result of using relatively mild conditions in the furnace is that less coke formation occurs in the furnace, thereby allowing longer operating times. It goes without saying that this is highly attractive economically.

In order to compensate for the relatively low degree of conversion in the furnace, the degree of conversion in the soaker must be higher than usual, i.e. higher than 50% of the 520°C+ total conversion. In the process of the present invention this is achieved by introducing hot hydrogen-containing gas into the soaker. In this way the previously mentioned temperature drop across the soaker in the direction of the oil flow is avoided and the thermal cracking reactions can thus proceed at a similar rate along the entire length of the soaker. The hot hydrogen-containing gas can be introduced into the soaker at one or more of its internal inlets and/or at the bottom of the soaker. In case the heated hydrocarbon oil feedstock from the furnace is fed at the bottom of the soaker, the hot gas is preferably introduced at one or more inlet openings of the soaker in order to guarantee efficient heating. On the other hand, if this oil feed at the top of the soaker, the hot gases can conveniently be discharged into the sump of the soaker, since in this mode of operation the hot gas and the oil stream move in countercurrent through the soaker, allowing effective heat exchange between the hot gas and the oil.

In addition to its function as a heating medium, the hot gas also serves as a stripping medium to remove the lighter fractions from the cracked oil, thereby increasing the stability of the remaining liquid, which in turn leads to a decrease in coke formation on the metal parts inside the soaker which are in direct contact with the hot gas. In this way, longer operating times and a high 520ºC⁺ total conversion can be achieved. The use of hydrogen-containing gas as a stripping medium in the thermal cracking process of the present invention also forms a separate aspect of the present invention. The presence of hydrogen during thermal cracking is also considered beneficial for the stability of the remaining liquid and therefore for suppressing coke formation. Thermal cracking in the presence of hydrogen is known to reduce the formation of carbon-like products during thermal cracking of heavy hydrocarbon oils and to be beneficial for the stability of the oils formed, as reported, for example, in JP-A-62-96589.

The hydrogen-containing gas used in step (b) may in principle be any gas which is stable at elevated temperatures and which contains hydrogen. It may, for example, be pure hydrogen or a hydrogen-rich gas. In particular, in a refinery in which hydrotreating units are present, the use of such gases may be useful. Hot synthesis gas may also be applied as the hot hydrogen-containing gas. This is a very feasible option if the refinery in question has a gasification unit in which hot synthesis gas is produced by gasification, i.e. by partial oxidation of heavy asphaltene-rich oil fractions. Furthermore, synthesis gas from a gasification unit may also contain soot. Without wishing to be bound to any particular theory, the presence of Soot can be very useful in the soaker as it provides a furnace surface for the deposition of coke and coke precursors, thereby preventing fouling of the metal parts in the soaker, and it can act catalytically by activating the hydrogen present in the synthesis gas due to the metals present in the soot. In this context, the presence of nickel (as nickel sulphide) is considered to be particularly important. It will be understood that due to the previously mentioned beneficial effects in the thermal cracking reactions, hydrogen can also be present in stage (a) of the process according to the present invention, i.e. in the furnace.

In any event, the hydrogen-containing gas must be stable at the high temperatures necessary to maintain the temperature of the hydrocarbon oil in the soaker at a value in the range of 420 to 650°C, preferably 450 to 600°C, by direct heat exchange. As previously mentioned, by introducing heat and hydrogen into the soaker in the form of hot hydrogen-containing gas, a temperature drop across the soaker is at least significantly reduced and coke formation on the metal parts inside the soaker which are in direct contact with the hydrogen-containing gas is suppressed, as a result of which 55% or more of the total conversion in the soaker can be accomplished without excessive coke formation. Since coke formation in the furnace is also significantly reduced due to the milder cracking conditions, the overall result is less coke deposition on the furnace and soaker inlets and therefore longer operating times can be achieved. In other words, by shifting the conversion from the furnace to the soaker to a certain extent and under certain conditions, coke formation is reduced and longer operating times are achieved.

The total pressure in the soaker can vary from 2 to 100 bar. For economic reasons, however, it is preferred to use total pressures in the range of 2 to 65 bar. At pressures above 65 bar, and especially at pressures above 100 bar, the high-pressure system required becomes so expensive that it becomes increasingly difficult to operate the process economically.

After thermal cracking has taken place, a gaseous fraction containing the hydrogen-containing gas and a cracked residue are recovered from the soaker in step (c). This gaseous fraction can then be further separated in a fractionator into a top fraction containing methane, ethane and the hydrogen-containing gas, into one or more gaseous lower hydrocarbons, namely propane, butane and the like, and into a bottom fraction. If desired, the hydrogen-containing gas can then be separated from this top fraction, for example by pressure swing adsorption. The cracked residue can have various purposes. For example, it can be partially or completely recycled and mixed with the furnace and/or soaker feedstock to be subjected to the thermal cracking conditions again. However, it is preferred that the cracked residue is further separated in a subsequent step (d) into one or more asphaltene-poor fractions and an asphaltene-rich bottom fraction. This separation can be conveniently carried out by vacuum flashing or vacuum distillation. In this mode of operation, the bottom fraction obtained from the fractionation of the gaseous fraction obtained from the soaker can optionally be vacuum flashed together with this cracked residue.

The asphaltene-rich bottoms fraction can then be used in various ways. For example, it can be used in bitumen for road construction and roofing, in emulsion fuels or in solid fuels by pelletizing. However, in a preferred embodiment of the present invention, the asphaltene-rich bottoms fraction is partially oxidized (gasified) in an additional step (e) in the presence of oxygen and steam, usually high pressure steam, thereby producing hot synthesis gas. This synthesis gas can in turn be used as clean fuel gas in the refinery or for the simultaneous production of electricity and steam, for hydrogen production and for hydrocarbon synthesis processes. However, for the purpose of the present invention, it is preferred that at least part of the hot synthesis gas formed in step (e) is introduced into the soaker according to step (b) of the process of the present invention.

Figure 1 depicts an example of a refinery plant incorporating a preferred embodiment of the thermal cracking process according to the present invention, namely a furnace-soaker configuration integrated with a gasification unit.

The hydrocarbon residue oil feedstock (6) is fed into the furnace (2) where it is heated to a temperature of 400-510ºC and where 30-45% of the total conversion takes place. The hot partially converted hydrocarbon oil (7) exits the furnace and is fed into the soaker (3) together with the hot syngas (9) formed in the gasification unit (1) by the partial oxidation of the asphaltene-rich bottoms fraction (19) in the presence of oxygen/steam (8). The gaseous fraction (10) and the cracked residue (15) are recovered from the soaker (3). The gaseous fraction (10) is separated in the fractionator (4) into the head fraction (11) - which contains methane, ethane and synthesis gas - into light hydrocarbon fractions (12) and (13) and the bottom fraction (14). This bottom fraction (14) is fed into the vacuum flasher (5) together with the cracked residue (15) where the separation into the asphaltene-poor fractions (16), (17) and (18) and the asphaltene-rich bottom fraction (19) takes place. Part of the bottom fraction (19) is subsequently used as feed material for the gasification unit (1).

The invention is further explained by the following examples.

Example 1 and comparative example 1

When a short Middle East residue having the properties given in Table I is subjected to thermal cracking, a 520°C+ total conversion of 40 wt.% can be achieved when the process according to the present invention is applied as illustrated in Figure 1 (Example 1). Under similar conditions, a conventional thermal cracking process only gives a 520°C+ total conversion of 31 wt.% (Comparative Example 1). The pressure in the soaker in Example 1 is about 10 bar. The other conditions under which both thermal cracking processes, as well as the conversion rates in the furnace and soaker and the yields of the product streams are given in Table II. The numbers in the column "Stream/Unit No." refer to the reference numbers used in Fig. 1.

From the results presented in Table II, it can be concluded that, compared to the conventional furnace soaker thermal cracking process, the process of the present invention enables higher 520°C+ overall conversion, resulting in higher yields of useful product streams boiling below 520°C and less hydrocarbon residues.

Table I - Properties of the feedstock

350-520ºC fraction (wt%) 5.0

520ºC+ fraction (wt%) 95.0

Sulphur (wt.%) 5.4

Conradson carbon number (wt%) 20.3

C7 asphaltenes (wt.%) 10.2

Viscosity at 100ºC (mm²/s) 2350

Density 70/4 0.998 Table II - Thermal cracking tests

The abbreviations and expressions used in Table II have the following meaning:

t/day: tons/day

% by weight: percent by weight

Feed gas: Synthesis gas from the gasification unit

T Gas: Temperature of the feed gas

FOT: Furnace outlet temperature

SOT: Soaker outlet temperature.

The product streams are listed by their boiling point range and the 520ºC⁺ conversion is indicated by how much the total conversion is ("Total") and what portion is achieved in the furnace. The percentage in parentheses indicates the percentage of the 520ºC⁺ total conversion that occurs in the furnace.

Example 2

A 100 ml stirred autoclave was charged with about 25 g of the same short Middle East residue as used in Example 1. The filled autoclave was pressurized with synthesis gas to 50 bar. The reactor and its contents were then rapidly heated to 450ºC (within 2 minutes, starting from 350ºC) and held at this temperature for 20 minutes to allow the thermal cracking reactions to proceed. The reactor was then rapidly cooled to room temperature. The autoclave was then depressurized and the gas and liquid were collected and samples taken for analysis. The amount of coke was determined by extraction with tetrahydrofuran. It was found that only 4.7% by weight of the total amount of gas and liquid recovered was coke. In addition, the surfaces of the autoclave and the stirrer parts in contact with the synthesis gas remained free of coke deposits.

Comparison example 2

The procedure of Example 2 was repeated, only this time nitrogen was used to pressurize the filled autoclave to 50 bar instead of synthesis gas. It was found that the coke represented 7.8% by weight of the total amount of gas and liquid and that coke was formed on the surfaces of the autoclave and on the agitator parts in contact with the nitrogen.

When comparing the results of Example 2, in which the conditions in a soaker were imitated according to the process of the present invention, with those of Comparative Example 2, it is found that under thermal cracking conditions in the presence of a hot hydrogen-containing gas, coke formation is significantly reduced and deposition of coke on the metal part inside the soaker, which are in direct contact with this hot gas, is even completely avoided.

Example 3

A stirred autoclave of 100 ml capacity was charged with about 25 g of the same short Middle East residue as used in Example 1. The filled autoclave was pressurized to 10 bar with pure hydrogen gas. The autoclave was operated at constant pressure using a pressure regulator in the outlet line of the autoclave and with a continuous gas supply through the liquid residue via a hollow stirrer. The gas flow was kept constant at 200 Nl/kg.h. The residue was then preheated to a temperature of 340ºC with stirring and held there for 15 minutes. It was then heated to the desired reaction temperature (450ºC) at a rate of 45ºC/min and held there for 15 minutes to allow the thermal cracking reactions to proceed. The autoclave was then cooled to room temperature at a rate of 90ºC/min. The autoclave was then depressurized and the gas and liquid were collected and samples taken for analysis. The amount of coke was determined by extraction with tetrahydrofuran. It was found that only 3.5% by weight of the total amount of gas and liquid recovered was coke. In addition, the internal surfaces of the autoclave and the agitator parts in contact with the synthesis gas remained free of coke deposits.

The results of this example show that under thermal cracking conditions in the presence of hydrogen, even at a pressure as low as 10 bar, coke formation is significantly reduced, while coke deposition on the metal parts inside the soaker which are in direct contact with the hydrogen is completely avoided.

Claims (5)

1. A process for thermally cracking a hydrocarbon residual oil in which a 520ºC⁺ total conversion of at least 35 wt.% is achieved, which process comprises the following steps: (a) heating the hydrocarbon residual oil feedstock in a furnace to a temperature in the range of 400 to 510°C for a time sufficient to achieve 30 to 45% of the 520°C+ total conversion; (b) feeding the partially converted hot hydrocarbon residue oil formed in step (a) and a hot hydrogen-containing gas into a soaker, said hydrogen-containing gas being of a sufficiently high temperature to maintain the temperature of the hydrocarbon oil in the soaker by direct heat exchange at a value in the range of 420 to 650°C, in which soaker the remainder to 100% of the 520°C+ total conversion takes place; and (c) recovering a gaseous fraction containing the hydrogen-containing gas and a cracked residue from the soaker. 2. The process of claim 1, further comprising the steps of (d) separating the cracked residue obtained in step (c) into one or more asphaltene-poor fractions and an asphaltene-rich bottoms fraction. 3. The process of claim 2, which additionally comprises the step (e) of partially oxidizing the asphaltene-rich bottoms fraction formed in step (d) in the presence of oxygen and steam, thereby forming hot synthesis gas. 4. A process according to claim 3, wherein at least a portion of the hot synthesis gas formed in step (e) is used as the hot hydrogen-containing gas in step (b). 5. Use of a hydrogen-containing gas as a stripping medium in a process according to one of claims 1 to 4.