What is the fuel oil production process?

Crude oil is often distilled under reduced pressure (primary processing) to obtain about 40% light oil products, the rest are heavy fractions and residual oil. Without secondary processing, the heavy fractions and residual oils can only be used as raw materials for lubricant base oils and heavy fuel oils. At present, straight-run light fuel oil from domestic crude oil cannot meet the market demand, therefore, how to convert heavy fractions and even residual oil into light fuel by chemical methods is an important issue in fuel production. In addition, the primary processing (straight-run) gasoline octane number is low (generally in 40 ~ 60), directly used in automobile engines, there will be a burst phenomenon, easy to damage the parts of the automobile engine, reducing the service life, so the straight-run gasoline also needs a secondary processing, in order to improve its quality.

There are many secondary processes, such as catalytic cracking, catalytic reforming, catalytic hydrogenation, coking, viscosity-reducing cracking, alkylation and so on. This section only introduces the catalytic cracking and catalytic reforming processes which are widely used in refineries at present.

I. Catalytic Cracking

(I) Principle of Catalytic Cracking

The so-called catalytic cracking refers to the cracking process that employs a catalyst in the cracking reaction. FCC generally uses heavy fuel oils (e.g., reduced pressure distillate, coker wax oil, etc.) as feedstock. The reaction products are typically about 10% to 20% gas; about 30% to 60% gasoline yield; about 20% to 40% diesel yield; and about 5% to 7% coke yield. The heavy oil at the bottom of atmospheric pressure tower and residue oil at the bottom of reduced pressure tower contain more colloid and asphaltene, which are easy to produce coke in catalytic cracking, and also contain Fe, Ni and other heavy metals, which are easy to contaminate the catalyst and reduce its activity. If used as cracking raw material, the problem of heavy metal pollution and coke generation must be solved.

When catalytic cracking, the feed oil is carried out at about 500℃ and 0.2-0.4MPa. Under the condition of catalytic cracking, the reaction of hydrocarbons is not only one kind of reaction of cracking, not only the large molecules are cracked to become small molecules, but also the reaction of small molecules condensed to large molecules (even condensed to coke). At the same time, isomerization, aromatization, hydrogen transfer and other reactions are also carried out. Among these reactions, the cracking reaction is the most dominant one.

(II) Industrial type of catalytic cracking

Catalytic cracking is carried out in the presence of a catalyst in the raw oil, on the one hand, through cracking and other reactions to produce smaller molecules of products - gas, gasoline, diesel, etc.; on the other hand, condensed to coke. These coke deposits on the catalyst surface reduce the activity of the catalyst, so it is necessary to burn off the coke accumulated on the surface of the catalyst (carbon deposits) to restore the activity of the catalyst, and this process of burning the coke with air is called regeneration of the catalyst. The constant reaction and regeneration of the catalyst in a FCC unit is a feature of the FCC process.

Cracking reactions are heat-absorbing and regeneration reactions are exothermic. In order to maintain certain temperature conditions, it is necessary to solve the problems of periodic reaction and regeneration, heat supply and heat extraction, i.e., to supply heat to the device during reaction and to take heat away from the device during regeneration. The basic ways of solving this contradiction between reaction and regeneration are different, and industrial catalytic cracking units are divided into four types, namely, fixed-bed, fluidized-bed, moving-bed, and lifting-tube, which are shown in Fig. 8-4.

Figure 8-4 Industrial Types of Catalytic Cracking

1. Fixed-bed

Fixed-bed catalytic cracking units are the earliest catalytic cracking units in use. The preheated feed enters the reactor for reaction, and usually after only a few minutes to ten minutes, the activity of the catalyst decreases due to surface carbon buildup. At this point, the feed is stopped, and after being purged with water vapor, air is introduced for regeneration. Thus, the reaction and regeneration are carried out alternately and intermittently in the same reactor. In order to make the production continuous, several reactors can be grouped together to carry out the reaction and regeneration in turn. Fixed-bed catalytic cracking has a complex equipment structure, consumes a lot of steel and has poor production continuity, therefore, it has long been eliminated in industrial production.

2. Mobile bed

Mobile bed is different from fixed bed, the cracking reaction and regeneration reaction of mobile bed are carried out in the reactor and regenerator respectively. The reactor relies on the catalyst cycle to supply heat, no heater; regenerator catalyst cycle to take away part of the heat, but the regeneration reactor heat, still need to install some alloy steel pipe, through the high-pressure water to produce high-pressure steam, to take away the excess heat. Moving bed due to the complexity of the equipment structure, the problem of steel consumption, has been eliminated.

3. Fluidized bed

Fluidized bed catalytic cracking is similar to the moving bed, the reaction and regeneration are carried out in the reactor and regenerator respectively, the difference is that the catalyst is made into a microsphere of 20-100μm, so as to make the catalyst and the oil and gas or the air to form a fluidized state similar to that of the boiling liquid. This fluidized state, so that the temperature distribution in the two devices is uniform, the catalyst circulates a large amount of heat that can be carried, do not have to set up heating or heat facilities, so the equipment has a simple structure, easy to operate; raw materials, oil and gas and the catalyst to fully contact the catalyst to accelerate the reaction, improve the equipment's processing capacity, suitable for continuous production.

4. Lift pipe

The 1960s saw the emergence of a molecular sieve catalyst, which has a high catalytic activity, the cracking reaction is completed in a very short period of time (a few seconds), and it is necessary to quickly separate the reactants from the catalyst, otherwise it will cause a secondary reaction, generating more gases and coke, and lowering the yield of light oil, so the fluidized bed reactor can not give full play to the strengths of the molecular sieve catalyst, which prompted the development of the fluidized bed catalyst. Therefore, the fluidized bed reactor can not give full play to the strengths of the molecular sieve catalyst, prompting the improvement of the fluidized bed, the development of the lifting tube reactor.

The lifting tube reactor is an upright circular tube (i.e., lifting tube) in which the raw oil and catalyst enter the lifting tube reactor from the bottom and flow upward at high speed at the same time, and leave the reactor from the top after a few seconds of reaction, and then the reaction products are separated from the catalyst. The lift tube method greatly reduces the secondary reaction and improves the yield of light oil.

(C) Catalytic Cracking Process Flow

Figure 8-5 shows the process flow of a high-low juxtaposed lift tube catalytic cracking unit. It consists of three parts: a reaction-regeneration system, a fractionation system, and an absorption-stabilization system.

1. Reaction-Regeneration System

Fresh feed oil is mixed with refinery oil after heat exchange and heated up to 200-400°C by a furnace to the nozzle in the lower part of the lift tube reactor. Raw oil is atomized with steam and sprayed into the lift tube, and comes into contact with the high temperature catalyst from the regenerator (about 600~750℃), the oil mist is rapidly vaporized and reacted, and the reaction product carries the catalyst up and is fluidized in the reactor. The residence time of oil and gas in the reactor is very short (1~4s), which reduces the secondary reaction. After the catalyst entrained by the reaction product oil and gas through the settler, the diameter of the settler increases, so that the flow rate of oil and gas decreases, and the catalyst entrained in it scatters, and the oil and gas is separated from the catalyst entrained by the cyclone separator, and leaves the reactor to go to the fractionation tower.

The catalyst with carbon deposits (catalyst to be born) falls into the stripping section from the settler. The vaporizing section is equipped with several layers of herringbone baffles, at the bottom of which superheated water vapor can be introduced to replace the oil and gas on the catalyst to be born and return to the upper part, and the catalyst enters into the regenerator from the inclined pipe of the catalyst to be born after the vaporizing.

The main function of the regenerator is to burn off the carbon on the catalyst with air, i.e. to restore its activity. The air is supplied by the main fan. The regeneration process is also carried out in fluidized state, and the regenerated catalyst (regenerated catalyst) is sent back to the reactor through the regeneration slant tube for recycling.

Figure 8-5 Catalytic Cracking Process Flow Diagram

The regenerated flue gas is separated from the entrained catalyst by a cyclone separator, and then enters into the flue gas energy recovery system, which makes full use of the heat and pressure energy of the flue gas to do work. For some incomplete regeneration device regeneration flue gas contains 5% to 10% CO, sometimes with CO boiler, the use of regeneration flue gas to produce water vapor to recover energy.

Catalyst in the reaction and regeneration process there will be a loss or reduction, need to periodically replenish or replace a certain amount of catalyst to the reactor. For this reason, there should be at least 2 catalyst storage tanks in the plant for loading and unloading catalyst.

2. Fractionation System

The reactive oil and gas from the reactor enters the bottom of the fractionation tower, and is fractionated into several products in the fractionation tower: the top of the tower is rich gas (cracked gas) and crude gasoline, the side line has light diesel, heavy diesel oil, and back to the refinery, and the bottom of the tower is a slurry of products. Light diesel oil and heavy diesel oil are separately vaporized and then cooled by heat exchange before leaving the unit. The re-refined oil enters the re-refining tank and then goes into the reactor to be cracked again. The slurry at the bottom of the tower has catalyst powder (>2g/L), in order to reduce catalyst loss and improve light oil yield, part of the slurry is sent back to the reactor to crack again, and part of it is cooled and used in the lower part of the fractionating tower for recirculation to cool the superheated hydrocarbons (above 460℃) entering the fractionating tower into the saturated state, so as to avoid the catalyst powder from clogging the tower plate and to facilitate fractionation. Cracked rich gas and crude gasoline are sent to the absorption-stabilization system.

The typical FCC fractionator has 4 recirculation returns to take the residual heat from the tower: 1 top recirculation return, 2 middle recirculation returns, and 1 slurry recirculation return. The latter 3 refluxes take a large proportion of heat (80%), causing a large lower load and a small upper load on the tower. Therefore, the fractionation tower is generally reduced in size.

3. Absorption - stabilization system

From the top of the fractionating tower oil and gas separator separated from the rich gas with gasoline components, while the crude gasoline is dissolved in gaseous hydrocarbons. The function of the absorption-stabilization system is to use the absorption and distillation methods to separate the rich gas into dry gas (C2 and below) and liquefied gas (C3, C4), as well as a small amount of gas mixed in the crude gasoline to separate, and produce stable gasoline with a qualified vapor pressure.

Two, catalytic reforming

Catalytic reforming is a gasoline fraction (straight-run gasoline, coker gasoline, etc.) as raw material, in the catalyst (in the past is to use platinum, platinum rhenium bimetallic catalysts appeared in the 1960s or other metal catalysts), the molecular structure of the raw material to be re-adjusted, "the adjustment of the" process. Process. Catalytic reforming can produce high-octane reformed gasoline, as a high-quality engine fuel; can also produce aromatic hydrocarbons (benzene, toluene, xylene), as an important chemical raw material; at the same time, the by-production of hydrogen with high purity (75% ~ 95%), is an important source of refineries to obtain cheap hydrogen. Therefore, catalytic reforming is as important as the catalytic cracking process.

(I) The basic principle of catalytic reforming

In the process of catalytic reforming, there are five main chemical reactions occurring in the feedstock: dehydrogenation of six-membered cycloalkanes, dehydrogenation of five-membered cycloalkanes, dehydrogenation of cycloalkanes, isomerization, and hydrocracking. A large amount of H2 exists in the reforming reaction, and when a large molecule hydrocarbon is cracked into a small molecule olefin, the olefin is hydrogenated to a saturated hydrocarbon, resulting in a product with good stability. Reforming also generates coke on the catalyst surface, but compared with FCC, the reforming catalyst promotes the hydrogenation reaction and inhibits coke generation. Generally, platinum catalysts are used for one year and then coked and regenerated, while platinum rhenium or multi-metal catalysts can be used for two to three years and then coked and regenerated.

(B) catalytic reforming process

Production of different products at the same time, the use of the process is not the same. When the production of high-octane gasoline as the main purpose, catalytic reforming process mainly includes raw material pretreatment and reforming reaction. When the main purpose is to produce light aromatics, the process should also have an aromatics separation part. This part includes several unit processes such as post-hydrogenation of the reaction products to saturate the olefins, solvent extraction of aromatics, and distillation and separation of mixed aromatics. The following describes the production of high-octane gasoline for the purpose of platinum rhenium reforming process principle flow, see Figure 8-6.

Figure 8-6 catalytic reforming process principle flow chart

(a): 1 - pre-fractionation tower; 2 - pre-hydrogenation furnace; 3, 4 - pre-hydrogenation reactor; 5 - dehydration tower (b): 1 - pre-fractionation tower; 2 - pre-hydrogenation furnace; 3, 4 - pre-hydrogenation reactor; 5 - dewatering tower. -Dewatering tower (b): 1, 2, 3, 4-heating furnace; 5, 6, 7, 8-reforming reactor; 9-high pressure separator; 10-stabilizing tower

1. Raw material pretreatment section

Feedstock pretreatment includes pre-fractionation, pre-desiccation and pre-hydrogenation of the feedstock. Its purpose is to obtain the fraction range and impurity content are in line with the requirements of the reformed raw materials.

(1) pre-fractionation: straight-run gasoline fraction (≤ 180 ℃ fraction) into the pre-fractionation tower, from the top of the tower to remove raw materials below 80 ℃ fraction (≤ C6, because this part of the hydrocarbons are easy to crack into a non-gasoline fraction and reduce gasoline yields), as a gasoline blending component or chemical raw materials. The bottom of the tower to get 80 ~ 180 ℃ fraction can be used as raw materials for reforming.

(2) pre-hydrogenation: the purpose of pre-hydrogenation is to remove arsenic, lead, copper, iron, oxygen, sulfur, nitrogen and other catalysts in the raw material "poison", so that its content is reduced to within the permissible range, and at the same time can make the olefin saturated to reduce the carbon on the catalyst. Pre-hydrogenation reaction releases H2S, NH3, H2O, etc., as well as arsenic, lead and other metal compounds, arsenic, lead and other adsorption in the hydrogenation catalyst (nickel molybdate or cobalt molybdate) to remove. Pre-hydrogenation reactants after cooling into the high-pressure separator, after separating the hydrogen-rich gas, the liquid oil dissolved in a small amount of H2S, NH3, H2O, etc. need to be removed, so the liquid oil will be sent to the dewatering tower, desulfurizer, after treatment, can be used as a reforming reaction part of the feed.

Some refineries set up a separate pre-arsenic reactor in the pre-hydrogenation unit to remove arsenic by adsorption or chemical oxidation.

2. Reforming reaction and fractionation section

The pretreated raw oil is mixed with circulating hydrogen, heated by a furnace and then entered into the reforming reactor. The reforming reaction is a heat-absorbing reaction, and the temperature has to drop during the reaction. In order to maintain a high reaction temperature (480 to 520°C) in the reactor, industrial reforming reactors use three to four reactors in series, each with a heating furnace in front of it, which is heated to the desired temperature of each reactor.

During the catalytic reforming reaction, the reactor should be fed with a large amount of hydrogen for circulation in order to inhibit the coking reaction and protect the catalyst; at the same time, it plays the role of a heat carrier to reduce the temperature drop of the reaction bed and increase the average temperature inside the reactor; in addition, it can dilute the feedstock to make the feedstock distribution more uniform.

The reaction product from the last reactor enters the high pressure separator after heat exchange and cooling, and the gas (containing 85% to 95% hydrogen) is separated, and most of it is used as the circulating hydrogen in the reforming reactor after being pressurized by circulating hydrogen compressor, and a small part of it goes to the pretreatment part, and the separated reformed oil enters the stabilizing tower. Stabilizer tower is a fractional distillation tower, the top of the tower to separate out liquid hydrocarbons, the bottom of the tower for the vapor pressure to meet the requirements of the stabilized gasoline.

From the crude oil by decompression, catalytic cracking and other processing of light fuel, still contains a small amount of impurities (such as sulfur, oxygen, nitrogen and other compounds), these impurities on the use of oil has a great impact on the performance of the oil, will make the oil deepen the color and smell thicker, so that the oil corrosive, combustion gases, easy to deteriorate and so on, so it is necessary to remove these impurities. Thus, through the fuel product refining process will be semi-finished products processed into commodities to meet the product specifications. Sometimes, the refining alone still can not meet some of the performance requirements of the product, which can be added to the fuel oil additives (such as anti-explosive agents, antioxidants, depressants, etc.) to improve the quality of fuel. The blending of oil products has no certain norms, determined by the actual situation of each refinery. For example, the blending of automotive gasoline, the main components of straight-run gasoline, gasoline produced by the secondary process, in addition to adding anti-knock agents, antioxidants, metal passivators and so on.