Method of thermal cracking of heavy-oil products

Description Field of the Invention

The invention refers to oil refining, in particular, to production of hydrocarbon fuel in oil-refining industry.

Prior Art

Under the effect of high temperature, molecules of oil stock are cracked, which is referred to as thermal cracking (Dekhterman A.Sh. Oil refining by a fuel variant. M., Khimiya, 1988. p.47). In today's oil refining, a few types of thermal processes are used. As a result of thermal (liquid-phase) cracking, or high temperature cracking (e.g., in the two-coil processing solution), high-boiling distillate or residual stock is processed into gas, gasoline, unsaturated hydrocarbons (ethylene, butenes, propylene), gas oil and cracked residue. Obtained cracked gasolines differ considerably from straight-run gasolines, as the former have higher contents of unsaturated and aromatic hydrocarbons, as well as paraffin hydrocarbons of isometric structure. Due to presence of the aforesaid hydrocarbons, cracked gasolines are characterized with higher antiknock properties, compared to straight-run gasolines.

Gas oil, in terms of its breakup, corresponds to the kerosene-gas oil cut and is used as a component of fuel oil. Cracked residue contains resinous substances, asphaltenes and carboides.

In spite of development of catalytic processes, thermal cracking is still highly significant. Thermal cracking in the visbreaking mode is widely used.

Heavy-oil products — fuel oil, tar — are most easily destroyed, when heated. The most important parameters, which determine direction and rate of cracking, are temperature, duration of process and pressure.

The process already becomes visible at 300 í 350 °C and is described with the first-order kinetic equation. The cracking reaction rate is increased along with temperature; in other words, thermodynamic probability of decomposition reactions would grow. Thus, pyrolysis of gaseous light or middle-distillate stock may have the process temperature reaching 750 í 800 °C. At lower temperatures, due to a low destruction reaction rate, the thermolysis products would mostly be constituted by naphtheno-aromatic structures, which would hinder further consolidation reactions.

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With temperatures exceeding the optimum one, the rates of destruction and polycondensation reactions would rise sharply, due to instantaneous formation of a large number of crystallization nuclei. Therefore, the process temperature is a significant control parameter.

5 Extending the process duration results in higher output of crackln'g products.

However, with the temperature growing, the process duration is generally required to be reduced. Otherwise, the process of coke formation on the heater walls would intensify. Nevertheless, temperature and duration of the process are not totally interchangeable. (Akhmetov S. A. Technology of deep oil and gas refining. Ufa,

10 Gilem, 2002. p.379). The interchangeability is only applied to a narrow temperature range. The process time dependence at given values of temperature and pressure may be presented as shown in Figures 1 and 2 (ibid. p. 370). The graph of the aforesaid dependence shows change in concentration of bases of distillate cracked residue of Krasnovodsk Oil Refinery as related to thermolysis duration at the pressure of 0.1

15 MPa and the temperature of 420°C (Fig.1) and 490°C (Fig.2).

At the beginning of the process, chain-radical reactions of decomposition and polycondensation result in accumulation of polycyclic aromatic hydrocarbons, resins and asphaltenes in the liquid phase (which corresponds to successive chemical '"evolution" of bases). A generally accepted criterion of succession of complex 0 reactions in chemical kinetics is availability of extremes in kinetic curves for concentration of intermediate products. Accumulation of consolidation intermediate products in the system is accompanied by two phase transitions in the liquid phase. At first, with a threshold concentration limit reached, the phase of asphaltenes is separated, which is followed by generation of the phase of anisotropic crystalline 5 liquid (mesophase) in this medium. Subsequent processing of asphaltenes of certain duration contributes to obtaining carbenes and further on, carboids.

Therefore, the process of thermolysis requires certain duration. Pressure affects composition of the process products (e.g. output of residual cuts and coke) due to changing rates and nature of secondary reactions of 0 polymerization and condensation, as well as volume of reacting mixture). Polymerization and condensation run faster at higher pressure. With growth of pressure, gas formation is reduced, and more gasoline is obtained, with a smaller

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3 content of unsaturated hydrocarbons in it. In industrial conditions, cracking of kerosene-straw cuts is held at a pressure up to 5 MPa, and cracking of heavy residual stock - up to 2 MPa (Dekhterman A. Sh. Oil refining by a fuel variant. M., Khimiya, 1988. p.48). Along with pressure growth, the content of paraffm-naphthene 5 hydrocarbons, which make asphaltenes separate, is going up. At the same time, the threshold concentration (and, accordingly, equilibrium concentration) of asphaltenes is going down, and they precipitate faster. As a result, the output of carboids would increase almost in proportion to pressure.

Due to diversity of feed stock and various requirements to obtained products

10 of thermal cracking, the process conditions - temperature, pressure and duration of the process - are specified individually for each particular case. However, the depth of stock conversion in a single run of the heater is limited by its coking. Next, the flow chart and equipment for further process are developed, accordingly.

There is a known method and device for its implementation: "Method of

15 deepening thermal cracking of heavy-oil products and device for its implementation", under RF Patent No. 2246523. The essence of the method is that oil products are heated in a pipe furnace, with decomposition products additionally exposed in a remote soaking chamber. Heating of the reacting mixture in the pipe furnace is carried out, with temperature and pressure corresponding to parameters of short-

20 stopping reaction of decomposition of oil products, which, after pressure relief at the furnace outlet, ensure transition of the reacting mixture to the state of attainable overheating, to be further converted to the non-associated state of molecules of the oil products bases in the course of decomposition in the soaking chamber. The patent also describes a device for implementation of the aforesaid method. It comprises a

25 heating furnace with a coil and a remote soaking chamber. Between them, there is a throttling reactor in the form of a flow resistor, which provides for pressure relief at the outlet of the heating furnace and delivery of homogenous additives to the reaction zone of the remote soaking chamber for intensification of the process of deepening thermal cracking of heavy-oil products.

30 Shortcomings of the aforesaid known method are determined by the fact that, downstream the throttling reactor, the moment of pressure relief is accompanied by release of light cuts in the gaseous form, which may be present in the feed stock or

formed in the course of thermal cracking. The evaporation process is proceeding along with lowering of temperature. Therefore, the temperature in the remote soaking chamber will be settled at a lower level, compared to the temperature of the stock flow at the outlet of the furnace, with the reaction rate in the chamber being lower. Besides, there is a rigid relation between temperature and pressure of stock in the furnace. If it is necessary to raise the process temperature, the pressure also has to be raised. Therefore, possibilities to adjust parameters of the thermal cracking process are limited. Adjustment of the process duration (stock exposure in the furnace coil) is related to the stock consumption, or its flow rate. Reduction of the rate of stock flow in the coil raises the probability of coke formation on the coil walls. Changing the length of the heating zone (coil length) requires the furnace design to be modified.

There is another known method of thermal cracking of heavy-oil products implemented according to the chart of a visbreaking unit integrated in a GK-3/2 combined industrial plant and shown in Fig. 3 (Dekhterman A. Sh. Oil refining by a fuel variant. M., Khimiya, 1988. p.52 - prototype model). Tar from the bottom of an atmospheric-vacuum tubular unit (AVT) is used as stock. This tar visbreaking circuit comprises: P-I - furnace; K-I - stripping column; K-2 — evaporator; K-3 — rectifying column; S-I — gas separator: T-3 - cooler.

The known method is implemented as follows. The stock comes to furnace P- 1 and passes through it in two streams up to a specified conversion level, that is, with a certain number of cracking products to be obtained. The vapors of cracking products are fed to evaporator K-2, from the bottom of which cracked residue is output, with the vapors going to rectifying column K-3. Gasoline vapors and gas are driven out from the top of the column. Upon being condensed in cooler T-3, the "rich" gas is separated from unstable gasoline in gas separator S-I. The diesel cut is removed from the middle section of column K-3 through stripping column K-I. The rectified residue is returned from column K-3 to furnace P-I to be recirculated. The amount of recirculated product does not exceed 25 % of the feed stock consumption. The remaining products of the process are removed from the bottom of column K-2. The temperature at the outlet of furnace P-I is 480í485 °C. To stop the cracking reaction, a cooling stream of oil product is introduced into the line of the product

outgoing from furnace P-I. Through the method implemented under this chart, a specific processing problem of heavy stock visbreaking is solved.

As the principal objective of visbreaking is to lower the stock viscosity down to a specified value, the level of its conversion is limited to prevent formation of carbenes and carboids. This is achieved through reducing the reaction duration. The processing chart docs not provide for installation of an additional soaking chamber and the second furnace (deep cracking furnace). At the same time, the output of products is not high.

Shortcomings of this method are limited possibilities for adjustment of the process parameters, namely, its temperature and duration. These shortcomings are determined by the fact that normal operation requires the aforesaid parameters to lie in a narrow range. It is only in this mode that a visbreaking unit can operate for a few months without having the machines cleaned. As a result, it is desirable to maintain higher product consumption in furnace P-I for its heating. In this case, the rate of the product flow along the coil is increased, with coke formation on its walls reduced.

In thermal cracking units, the amount of output products should be higher, as these are target products. To this effect, it is necessary to extend the period of stock exposure at the process temperature, or directly in the furnace, by means of increasing the coil length, or exposing the products in an additional soaking chamber to be installed downstream the furnace, according to the oil product flow. Operation of thermal cracking units is characterized by a short trouble-free period, which sometimes does not exceed 20 days (ibid. p. 51).

Summary of the Invention

The technical objective of the invention is creation of an. efficient method of thermal cracking of heavy-oil products, which could expand possibilities of application of thermal cracking in industry.

The technical result to ensure reaching the set problem lies in raising controllability of the thermal cracking process, extending the range of adjustment of the process parameters and, respectively, versatility and efficiency of the method. The essence of the invention is that the method of thermal cracking of heavy- oil products provides for that a stream of oil products outgoing from the heating furnace is supplied with a stream of the source oil product, whereupon they are jointly

fed to a throttling device used for maintaining required pressure in the heating furnace, whereupon the liquid-vapor mixture formed as a result of throttling is delivered to a low pressure separator, from the bottom of which the heavy cracked residue is removed in two directions: the smaller flow would go as the process product, and the bigger flow would go through an intermediate container to return to the heating furnace, while from the upper part of the separator the light cuts are delivered to a rectifying column, from which are removed the cuts of cracked products of required composition and the rectification residue, which is fed to the aforesaid intermediate container for return to the heating furnace jointly with the separation residue, with the aggregate consumption exceeding consumption of the stock oil product with a factor of at least two, where the level of conversion of this oil product in the heating furnace coil is set in the same proportion to its target value as the ratio of consumption of the stock oil product to consumption of the residue returned to be cracked in the furnace. Preferably from the top of the rectifying column is removed the liquid- vapor mixture, which, upon being cooled, comes to the gas separator, where it is divided into gas, water and gasoline cut, one part of which is used to reflux the top of the low pressure separator, and the other part is recovered for the process to follow — stabilization, with the light gas oil cut concurrently removed through the stripping column.

In specific cases of the method implementation, a heavy-oil product (fuel oil or tar) is used as the stock oil product, a pipe furnace being used as a heating furnace, where to prevent coke deposits from building up on the furnace coil walls, the coil is supplied with a baffle, e.g. water steam, the intermediate container is supplied with additives of the physical-chemical nature to activate the thermal cracking process, while the residue is returned from the intermediate .container to the furnace with a flow rate exceeding consumption of the stock oil product with a factor of 8 to 12.

Achieving the technical result is based on the fact that, according to the claimed method, the process of thermal cracking to a required level of conversion of the input stock is going on fractionally, as the same oil product is delivered successively to the cracking zone (heating furnace coil) and to the zone of thermolysis products separation. After that, it is again returned from the separation

zone to the input of the cracking zone. The duration of the oil product exposure in the cracking zone (furnace coil) in a single run may be reduced. Accordingly, the level of conversion of the feed stock in a single run makes a part of its required target value. The share of the conversion level as related to its target value is set in the same proportion as the ration of consumption of input stock to consumption of the oil product residue returned to the cracking zone. Here, consumption of the returned oil product residue is considerably higher than consumption of the feed stock (with a factor of at least two). By adjusting consumption of this oil product residue passing through the furnace, it is possible to change the aggregate (for several runs) duration of the cracking process. Therefore, two thermal cracking parameters - the process aggregate duration and temperature - are not rigidly interrelated. Pressure in the heating zone (furnace) may be maintained at a required level through a known method, e.g., by means of throttling the flow with a predetermined and adjustable value of pressure differential. The feed stock is delivered to the hot stream of oil products downstream the heating zone, but upstream the throttling z.one. The separation residue is removed from the process out of the separation zone. Light thermolysis products are delivered for rectification to be separated, e.g., into gas non- condensable in the cooler, gasoline cut, light gas oil and the residue of rectification process, which is returned to the heating zone input mixed with the separation residue.

Preferred embodiment Detailed Description of the Invention Fig. 1 presents curves of changes in concentration of bases of distillate cracked residue of Krasnovodsk Oil Refinery as related to thermolysis duration at the pressure of 0.1 MPa and the temperature of 420 ⁰ C .

Fig. 2 presents curves of changes in concentration of bases of distillate cracked residue of Krasnovodsk Oil Refinery as related to thermolysis duration at the pressure of 0.1 MPa and the temperature of 490 °C.

Fig. 3 presents a simplified diagram of the visbreaking unit for implementation of the prototype method integrated in the GK-3/1 combined industrial plant. (Dekhterman A.Sh. Oil refining by a fuel variant. M., Khimiya, 1988. p.52)..

Fig. 4 presents a simplified flow chart of installation for implementation of the claimed method of thermal cracking of heavy-oil products.

The installation according to Fig.4 comprises heating furnace 1 (e.g., pipe furnace), throttling device 2, low pressure separator 3, intermediate container 4, rectifying column 5, heat exchanger 6, gas separator 7 and stripping column 8.

Designation of fluid media streams on the diagram: I — feed stock; II - gasoline cut; III - kerosene-gas oil cut; IV — process residue; V - gas; VI — water; VII - water steam; VIII - additives activating the thermal process cracking.

Designations on Figures 1 and 2: C _AI — light oils; C _A2 - polycyclic aromatic hydrocarbons; C _A3 — resins; CA ₄ - asphaltenes; CA ₅ - carbenes; C _A6 - carboids; C _A 7 — volatile matters. The method of thermal cracking of heavy-oil products is implemented as follows.

The feed stock (stream I) is delivered to the stream of the oil product outgoing from heating furnace 1 , with the stock oil product stream fed up to throttling device 2, which is used for maintaining a required pressure of the process in heating furnace 1. To prevent formation of coke deposits on the walls of the coil (shown as a sketch) of furnace 1 (if necessary), the coil is supplied with a baffle, e.g. water steam (VII). Further on, the liquid-vapor mixture is delivered to low pressure separator 3, from the bottom of which the heavy cracked residue is removed from the process (stream IV). From the lower part of separator 3, through intermediate container 4, the same residue is also removed to be returned to heating furnace 1 with a flow rate exceeding the feed stock consumption with a factor of at least two. Therefore, the same heavy residue, before being removed from the process, repeatedly undergoes thermal cracking. Light cuts are delivered from separator 3 to be rectified in column 5, from which light products of thermal cracking are removed. From the top of column 5 the vapor-liquid mixture is removed, which, upon being cooled (heat exchanger 6), comes to gas separator 7. In gas separator 7, the mixture is separated into non- condensable hydrocarbon gas (stream V), water (stream VI) and gasoline cut. A part

of the gasoline cut is used to reflux the top of column 5, with the remaining amount (stream II) removed out of the process and fed to stabilization. Through stripping column 8, the light gas oil cut is removed (stream III). Water steam (VII) is delivered to stripping column 8. The residue of rectification process (high-boiling part of light cuts) is delivered to intermediate container 4. If necessary, this container is also supplied with additives (stream VIII) of the physical-chemical nature to activate the thermal cracking process. The balance of material streams should be as follows. The total of the streams removed from the process - gasoline cut (stream II), light gas oil (stream III), heavy residue (stream IV), non-condensable hydrocarbon gases (stream V) - must be equal (by weight) to the feed stock flow (stream I).

Calculations of thermal conditions chosen as an example give the following results. With the ratio between the flow rate of separation cracked residue returned to heating furnace 1 from separator 3 and the consumption of feed stock equal to 8:1, the feed stock temperature being 200 °C, the temperature at the output of heating furnace 1 being 450 ⁰ C, the temperature in the volume of separator 3 is 390 °C. This is the temperature, at which the separation residue to be returned to furnace 1 is removed from separator 3. Heating in furnace 1 may be carried out with a lower thermal factor, which makes it possible to reduce coke deposits on the inner walls of the tubular coil. By increasing the ratio of flow rates, we reduce the time of oil product exposure in the coil of furnace 1 ; on the other hand, the residue returned to furnace 1 will have a higher temperature. In other words, the temperature of the thermal cracking process will remain virtually unchanged.

Therefore, control over technological parameters of the thermal cracking process subject to the claimed method is flexible in terms of setting the process parameters: its duration, temperature and pressure, as well as versatility in respect of properties of feed stock and specifying properties of products to be obtained. This allows working on the same plant with various feed stock, process it under optimum process conditions and obtain as a result a required conversion level and desired composition of thermal cracking products. The claimed method was tried on a laboratory pilot plant. Fuel oil of the M-

^' 100 brand from the West-Siberian oil was used as feed stock. As an example, the results of thermal treatment were obtained under the following specified

technological parameters of the process: the temperature at the outlet of the heating furnace (furnace 1) was 440 ⁰ C, with the absolute pressure in separator 3 maintained at the level of 0.12 MPa; the ratio of consumptions of the returned separation residue and the feed stock was set as 12:1. As a result, the aggregate conversion level was obtained for thermolysis products having the end boiling point lower than 360 ⁰ C, as calculated for feed stock in the amount of 43.5 % of weight: non-condensable gas - 3 % of weight, gasoline cut — 8.5 % of weight, kerosene-gas oil cut — 32 % of weight. The separation residue with density being 1050 kg/m ³ is suitable, by its composition, as a source material for obtaining bitumen. Meanwhile, the level of conversion of the oil product passing through the furnace coil was less than 3 % of weight. This makes it possible to raise considerably the trouble-free life of the furnace.

Therefore, an efficient method for thermal cracking of oil products has been created, which may expand possibilities for application of thermal cracking in industry. At the same time, controllability of the thermal cracking process has been raised, with the range for adjusting the process parameters expanded and the method efficiency increased.

Industrial Applications

The present invention is embodied with multipurpose equipment extensively employed by the industry.