Patent Grant
7550063
The invention relates to both a method and apparatus for thermal cracking hydrocarbons. The apparatus includes a rotating conical drum assembly within a vessel containing a hydrocarbon feedstock. The conical drum assembly is internally heated to cause cracking of hydrocarbons adjacent the conical drum surface and the formation of coke on its external surface as the conical drum rotates. Coke is removed from the drum surface by a coke removal system and cracked hydrocarbon product is collected as a vapor.
As is known hydrocracking is a process by which large/heavy hydrocarbons are broken down into smaller and more useful hydrocarbons including paraffins and olefins. There are generally two methods of cracking namely thermal cracking, in which the heavy hydrocarbons are subjected to high temperatures; and catalytic cracking, in which a catalyst is introduced into a reaction mixture to enable cracking to occur at lower temperatures.
While the cracking of hydrocarbons is known, it is desirable to provide improved thermal cracking processes in which the formation of lighter olefins and paraffins can be effectively controlled in an efficient and continuous process. More specifically, it is desirable to provide a process in which the smaller hydrocarbons are efficiently produced at reaction temperatures between the cracking temperature and the carbonization temperature of a heavy hydrocarbon mixture while also continuously removing coke and sulphur from the reaction mixture.
A review of the prior art reveals that such a system has not been previously described. For example, U.S. Pat. No. 6,005,149 describes a method and apparatus for processing organic materials to produce chemical gases and carbon char; U.S. Pat. No. 5,356,530 describes a method for upgrading petroleum residuum and heavy crude oil; U.S. Pat. No. 1,677,758 describes the treatment of carbonaceous and other materials; U.S. Pat. No. 1,622,573 describes a coking still; U.S. Pat. No. 1,541,140 describes a process and apparatus for distilling and cracking hydrocarbon oils; U.S. Pat. No. 1,183,457 describes an oil distillation process; U.S. Pat. No. 1,231,695 describes an apparatus for refining petroleum; U.S. Pat. No. 1,418,414 describes a process of making unsaturated hydrocarbon material; U.S. Pat. No. 148,806 describes oil-stills; PCT/EP 01/11016 describes a process and apparatus for the fractional distillation of crude oil; EP 1 067 171 describes a process for removing contaminants from oil; EP 0 667 799 describes a method for selective and/or unselective vaporization and/or decomposition of, particularly, hydrocarbon compounds and an apparatus for carrying out such a method; PCT/RU 00/00097 describes a method and device for resonance excitation of fluids and method and device for fractionating hydrocarbon liquids; and DE 41 07 294 describes a cracking system.
In accordance with the invention, there is provided a system for cracking hydrocarbons comprising: a vessel for operatively receiving and containing a volume of liquid hydrocarbon feedstock; a rotary drum assembly operably and rotatably contained with the vessel and in heating contact with the hydrocarbon feedstock, the rotary drum assembly including a heating system for heating the internal surfaces of the drum assembly during rotation of the drum assembly within the hydrocarbon feedstock wherein simultaneous rotation and heating of the drum assembly causes cracking of hydrocarbons; a vapor product collection system operatively connected to the vessel for receiving cracked hydrocarbon vapors; and a coke removal system operatively contained within the drum for removing coke from the drum assembly and vessel.
In one embodiment, the drum assembly includes two frustoconical end sections and a central cylindrical section operatively mounted between front and back pipe sections.
In another embodiment, the drum assembly and vessel includes a bearing system and the bearing system includes a cooling system for cooling the bearings during operation. In a preferred embodiment, the bearing cooling system includes a spiral oil path within each of the front and back pipe sections and the bearing system.
In one embodiment, the burner includes a secondary air injection system for injecting secondary air into an inside position of a conical flame.
In yet another embodiment, the vapor product collection system includes a quenching system for quenching vapor product to prevent coking within the vapor product collection system.
In yet still another embodiment, the coke removal system includes a plurality of scrapers in operative contact with the exterior surface of the drum assembly, the coke removal system including at least one coke pit for allowing coke to be removed from the drum.
In another aspect of the invention, a process for cracking hydrocarbons is provided, comprising the steps of: heating a rotating surface to a given temperature; exposing the rotating surface to a volume of oil to be cracked thereby causing the volatization of light hydrocarbons and the formation of a thixotrophic suspension of cracked heavy oil and coke on the rotating surface; collecting evaporated cracked heavy oil; and scraping and removing coke from the rotating surface. Preferably, the rotation speed and rotating surface temperature are balanced to produce dry coke before coke is scraped from the rotating surface.
In further embodiments, the rotating surface is operatively contained within a vessel and the volume of oil to be cracked is maintained at a fixed height relative to the rotating surface and the rotation speed of the rotating surface is controlled to produce a thin-film thixotropic mixture of olefins and paraffins on the rotating surface for a given temperature.
The invention is described with reference to the drawings in which:
In accordance with the invention and with reference to the figures, a process and apparatus for cracking hydrocarbons using a rotating heated surface is described.
Overview of Apparatus and Process
More specifically, the process includes, as described in relation to one rotation cycle, the steps of heating a rotating surface to a given temperature; exposing the rotating surface to a volume of oil to be cracked thereby causing the volatization of light hydrocarbons and the formation of a thixotrophic suspension of cracked heavy oil and solid coke on the rotating surface; collecting evaporated cracked product; continuing to heat the suspension until coking occurs; and, scraping and collecting coke from the rotating surface.
Furthermore, in accordance with the invention, the preferred apparatus to carry out the process is a rotary disc thermal cracker (RDTC) 10 as shown in
More specifically, in operation, as the drum assembly 12 rotates within the external vessel 14, a portion of the outer surface of the drum assembly 12 is always partly submerged within the feedstock oil. As the outer surface of the drum assembly 12 rotates in the feedstock oil, the hot surface will rapidly heat the oil, resulting in flash evaporation of light hydrocarbons and cracking of the heavy hydrocarbons as the drum assembly rotates. A high temperature gradient is created within the oil in proximity to the drum assembly, which causes the formation of a thixotropic suspension of solid coke and cracked products that adhere to the outside of the cylinder and cone surfaces.
As the coated surface of the drum assembly 12 rotates out of the oil, adhered hydrocarbons continue to be heated, evaporating the cracked products and forming dry coke against the heated surface. Scrapers 30a (
The RDTC also includes two vapor quenchers 40, 42 (
Rotating Drum Assembly 12
With reference to
The rotating drum assembly is designed to contain the heat of a burner operably connected to the input end H of the drum assembly 12. Preferably, the burner flame produces a flame temperature in the range of 3000° F. within the drum assembly and fills the internal volume 12d of the assembly thereby providing an exterior surface temperature of the rotating drum in the range of 1200° F. The resulting flue gas will have a temperature in the range of 1500° F. that leaves the fired chamber through back shaft 12f at the exhaust end E. The exhaust end E is preferably connected to a heat recovery furnace (
As noted above, the design temperature of the exterior of the drum assembly is in the range of 1300° F. As the torque forces and material stresses are highest in the area of pipe section welding to the cone, materials such as 18Cr-8Ni—Ti type steels (ASTM A312 TP-321H) having creep limit of 4.5 kG/mm2 @ 1292° F. are preferably utilized in the construction of the drum assembly 12.
As described in greater detail below, the external surfaces and bearing systems of the front and back shafts 12e, 12f are preferably cooled by a bearings cooling system 22 including vacuum oil in a spiral labyrinth system. The front shaft 12e and back shaft 12f are protected against cooling oil leakage by spring rings 12g at the vessel side and packing gland 12h at the burner or exhaust side, respectively.
External Vessel 14
The external vessel 14 of the RDTC is a horizontal vessel as shown in
The vessel includes a feedstock inlet nozzle 15a located on the underside of the vessel to introduce feedstock into the system and appropriate outlets 15b, 15c for vapor products and the vapor quenching system 40, 42. The vapor product nozzles 15b, 15c are preferably located on the opposite side of the casing to the scraping system (described below) to avoid contact between baked coke and quencher return oil.
The operating temperature of the feedstock within the drum is approximately 700° F. with vapor temperatures in the range of 750° F. As the system is normally operated with high sulphur concentrations within the typical feedstock, it is preferred that the drum casing is manufactured from type SS 304 steel, ASTM A240 TP304 or equivalent.
Scraping System 30
The vessel casing 14 also operatively retains a scraping system 30 (
The adjustment system 34 includes a scraper strap 34a fastened on a scraper base 34d. Whilst
Coke leaving the drum through chutes 30f and 30g is quenched with stagnant water as will be described below.
Bearings Cooling System 22
In a preferred embodiment, the system also includes a bearing cooling system 22 (
As noted above, the typical design temperature of the rotating drum connections between the frustoconical surfaces and the shafts 12e, 12f is approximately 1300° F. In addition, the design temperature of the vessel casing 14 at the connection to the external base pipes 22a and 22b is 840° F. The typical design temperature of the bearings is less than 300° F. As a result, it is necessary that both the shafts 12e and 12f and base pipes 22a and 22b be cooled down against the heat flux by metal conduction and heat radiation from the rotating drum 12 to the bearing races through the shafts.
In a preferred embodiment, the bearing cooling system includes a series of spiral channels 22c/d and 22e/g surrounding the shafts that enable the flow of cooling oil around the front and back shafts. The spiral channel is preferably a trapezoidal thread fixed on the internal surface of the supporting pipes 22a, 22b. The distance between the crest of each thread to the shaft 22e, 22f is minimized to scrape any coke forming on hot shaft surfaces caused by the partial thermal cracking of the cooling oil during operation.
In operation, cooling oil entering the spiral channels on the burner side is split into two streams as shown in
During passage through the spiral channels, the cooling oil may be heated to temperatures in the range of 390° F. thereby cooling both the base pipe 22a and shaft 12e. The cooling oil exits the spiral channels through nozzles 23a, 23b adjacent the spring rings 12g and the bearings base 22h, respectively.
Heated cooling oil is collected within an external heat radiator system (
The length of the spiral channels and nozzle diameters 23a, 23b are selected to control the pressure of the two cooling oil streams. Preferably, the back pressure of cooling oil through channel 22d is set to the cracker operation pressure, whereas the back pressure through channel 22c is set at atmospheric pressure.
The cooling system 22b on the exhaust side is comprised of three sections. Cooling oil enters an inlet chamber located at the midpoint of external pipe 22b. As with the burner side, cooling oil is directed through a first spiral channel 22e adjacent the vessel 14 where it will be heated to approximately 390° F. thereby cooling both base pipe 22b and shaft 12f. The cooling oil exits the spiral channel through an outlet nozzle 22q adjacent the spring ring 12g.
A second stream of cooling oil is directed through spiral channel 22f into an intermediate chamber 22m closed from the exhaust side bearing race by packing glands 12h. The intermediate chamber 22m is connected to a system of cooling channels 22n within the bearing support adjacent a third series of spiral grooves 22g. Cooling oil passes through and exits spiral grooves 22g through outlet channel 22o.
As with the burner side, the length of the spiral channels and outlet nozzle diameters are selected to control the pressure of the two cooling oil streams. Preferably, the back pressure of cooling oil through channel 22e and nozzle 22q is set to the cracker operation pressure, whereas the back pressure through channels 22f, 22g and nozzle 22o is set at atmospheric pressure.
In a preferred embodiment, the bearing system on the exhaust side is positioned at a greater radial distance than the bearing system on the burner side due to the higher temperatures on the exhaust side. The exhaust side also preferably includes a refractory liner 12m on the inside of tube 12f.
Quenching System 40
Hot hydrocarbon vapors leaving the cracker will typically have temperatures higher than the cracking temperature of particular hydrocarbons. As a result, and in order to avoid coke accumulation within the vapor line, the vapors are quenched by oily water injection into the quenching system 40. The quenching system includes a quenching drum 40a, a product vapor tangential outlet 40b, a hot vapor inlet 40c, an eccentric circular weir 40d, an oil/water spray nozzle 40e and cleaning device access 40f.
The injection of oily water with the resulting water evaporation is applied to decrease the temperature of HC vapor down to 600˜680° F. which is a safer temperature for its further handling and to prevent coke formation. Preferably, the vapor outlet nozzles 40b are tangential, thereby promoting a vortex flow of vapors through the quencher which promotes mixing of quenching liquids with the HC vapors. The quenching system will also preferably include cleaning nozzles located in a diametrically opposite position to the outlet nozzle enabling the cleaning of the oil/water spray nozzles during normal cracker operation.
The decrease in vapor temperature also causes the condensation of small quantity of HC liquid that is collected on the lower surfaces of the quencher and that will spill back to the drum 14 over weir 40d.
The internal surface of the hot vapor inlet nozzle 40c is wet by HC condensates at temperatures higher the cracking temperature of the HC condensates and, as a result, can be plugged by a coke layer. To avoid coking, the internal surface is preferably protected against coking by a silicate coating that is periodically washed by a small quantity of cooling oil.
Burner Assembly 50
With reference to
Combustion air is supplied from a burner fan (
Pumped fuel oil also flows through four channels into four dispersing chambers C4 via central axial inlet nozzles 50h. Inside of chamber C4, the oil liquid is dispersed into droplets by contact with dispersing fuel gas or a steam vortex created by tangential nozzles 50i. The vortex of chamber C4 is accelerated by central axial outlet nozzles 50h and injected into mixing chamber C2.
The jets from nozzles 50g, have a lower cone angle than the drum cones 12b and are ignited by a small pilot flame created by an ignition and flame watch system.
A small turn of the burner base 50b around fixing pipe 50a restricts swirlers 50e and thus changes the ratio of primary to secondary airs. Tightening the distribution tip 50d throttles the circular nozzle of secondary air and decreases its flow and impact. As a result of the secondary air conical shape, the flame angle can be controlled from a long cylindrical shape to full contact of fire to with the internal surface of rotating drum 12. As a result, control of the net heat power can be controlled in the range of 25% to 100%.
Process Thermodynamics
The thermal cracking process is simplified to the following steps;
The typical crack point temperature of heavy crude oil compound is usually 600˜715° F., the carbonization temperature of 95 Wt % carbon products is c.a. 1000° F.
The cracking process can be simplified to a two-step reaction:
As crude oil is a complex and indefinite mixture, the value of “n” will vary from 6 up to a few hundred. As an example,
Taking the Le-Chatelier rule into consideration, the expected thermodynamic effects of oil cracking are:
The total reaction heat of long paraffin breakage into a mixture of lighter paraffin and olefins and free carbon is exothermic. Thus, during the thermal cracking, olefin product losses by evaporation will be limited and the content of olefins in the reaction end-product will be reduced.
Operational Parameters
It is important that the thickness of the boundary layer is controlled to ensure that the boundary layer is sufficiently thin in order to ensure an optimum dry coke layer and to prevent contact of a thick boundary layer (comprising a thixotropic mixture of olefins and paraffins) with the scrapers.
Thus, various operational parameters of the system may be controlled to ensure desired product formation based on feedstock composition and desired products. Flame temperature and rotational speed of the rotary drum are the primary parameters adjustable to optimize the operation of the system. The liquid feedstock level within the drum is preferably controlled to maintain a consistent time relationship between wetting time and coking time at about 1:2. That is, the liquid level within the drum casing is maintained such that approximately one third of the outer circumference of the drum is wetted at any given time. Preferably the speed of rotation of the drum assembly provides a coking time in the range of 3-8 seconds for a given flame temperature.
As indicated above, the process described above provides:
With reference to
Feedstock is unloaded into 3-day tank 20-T-01 equipped with a steam coil, where it is heated to 140° F. and subsequently pumped through pump 20-P-01 into heat recovery furnace 10-F-01, where it is further heated to 608° F. The resulting two-phase stream enters the 8-th tray of oil fractionation column 11-C-01. The liquid phase flows down column 11-C-01 over trays 9-12, where a light fraction is liberated. Tar bottoms flow by gravity from column 11-C-01 into a mixer 10-M-01 at a temperature of 644° F., where hydrated lime powder is added. The resulting suspension is pumped by centrifugal pump 10-P-01 to the RDTC 10-R-01 through vapor trap 10-V-02. The liquid level in 10-V-02 and, as a consequence, in 10-R-01 is controlled by pump 10-P-01.
The RDTC cracker 10-R-01 is operated within a temperature range of 779° F. (evaporation) and 1238° F. (end of coking). The HC vapors produced in the thermal cracking reaction flow to two parallel quenchers 10-V-01 A/B where they are cooled to 680° F. by oily water injection. Product vapors feed the bottom of oil wash columns 11-C-01, where they are contacted with fresh feedstock at trays 9-12 and with product oil at trays 1-8. A small withdraw of oil is taken from tray 7 to cover losses of the cooling oil circuit within the RDTC.
HC vapors from 11-C-01 at a temperature of 549° F. flow to the main condenser 11-A-01, where the resulting water/cracked oil/fuel gas streams are separated in three-phase separator 11-B-01. The gas phase feeds the FG header and any excess is burnt in bottom flare 11-X-01 equipped with flame arrestor 11-X-02.
The water phase flows by gravity to the oily water drum 11-B-02, also serving as a sump pump for the oily water header connected to quenchers 10-V-01 A/B and coke pit 10-S-01.
The cracked oil phase returns back to column 11-C-01 top as the reflux through pump 11-P-01 with net production being sent to an export facility.
Excess carbon and calcium sulphide resulting from the thermal cracking reaction is rejected from cracker 10-R-01 into the coke receiver 10-S-01, where it is cooled down to 212° F. in contact with oily water. Following cooling, it is removed by a bottom grate conveyor and transported in handle bins into a stacking yard, where contact with air causes intrinsic oxidation of calcium sulphide into water insoluble gypsum.
The RDTC cracker utilizes a cooling oil circuit, to keep bearing temperatures lower than 300° F. The return oil (typical temperature of 266° F. ) from the bearing cooling systems flows by gravity into cooling oil bin 10-B-02 after which it is pumped by 10-P-02 back to the cracker through the cooling oil cooler 10-A-01, where it is cooled down up to 140° F. If the product viscosity is out of range (for example due to an insufficient reflux), a portion of the cooling oil can be transferred to export facility using the pump 10-P-02.
Representative Data for Feedstock and Heavy and Light Cracked Products
Feedstock data:
At dry conditions:
ASTM D86 760 mmHg, by volume:
Cracked oil data:
ASTM D86 760 mmHg, by volume:
Heavy crack oil data:
At work conditions:
ASTM D86 760 mmHg, by volume:
The above-described embodiments of the present invention are intended to be examples only. Alterations, modifications and variations may be effected to the particular embodiments by those of skill in the art without departing from the scope of the invention, which is defined solely by the claims appended hereto.
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| Number | Date | Country | |
|---|---|---|---|
| 20070045098 A1 | Mar 2007 | US |
