Hydrogenation catalyst and hydrogenation reactor knowledge sharing

Overview

Hydrofinishing catalysts are composed of active components, additives and carriers. Its role is to hydrogenate and remove sulfur, nitrogen, oxygen and heavy metals as well as polycyclic aromatic hydrosaturation. The molecular structure of the feedstock does not change much and, depending on various needs, is accompanied by hydrocracking reactions, but the conversion depth is not deep and the conversion rate is generally around 10%. The hydrofinishing catalyst requires dual functions of hydrogenation and hydrogenolysis, while the acidity required for hydrogenolysis is not high.

Working Principle
The mechanism of catalytic hydrogenation (changing the reaction pathway and reducing the activation energy): hydrogen molecules adsorbed on the catalyst generate active hydrogen atoms to adduct with alkenes and alkynes whose bonds are weakened by the catalyst.

(1) The more alkyl groups on the double bond carbon atoms, the lower the heat of hydrogenation and the more stable the alkene: R2C=CR2 > R2C=CHR > R2C=CH2 > RCH=CH2 > CH2=CH2

(2) trans isomers are more stable than cis

(3) The heat of hydrogenation of acetylene is -313.8 kJ-mol-1, which is twice as large as that of ethylene (-274.4 kJ-mol-1), so acetylene is less stable than ethylene.

Applications
In the presence of Pt, Pd, Ni and other catalysts, olefins and acetylenes undergo addition reactions with hydrogen to produce the corresponding alkanes and give off heat, which is called heat of hydrogenation (heat of hydrogenation, heat given off when 1 mol of unsaturated hydrocarbon is hydrogenated). The mechanism of catalytic hydrogenation (changing the reaction pathway and reducing the activation energy): the hydrogen molecules adsorbed on the catalyst generate active hydrogen atoms to add to alkenes and alkynes whose bonds are weakened by the catalyst.

Classification
1、Hydrocracking catalyst

Hydrocracking catalysts are catalysts used in the hydrocracking process of petroleum refining, where heavy oil is hydrocracked at high temperature of 360~450℃ and high pressure of 15~18MPa to convert into gas, gasoline, jet fuel, diesel and other products. The hydrocracking process is a secondary process in the petroleum refining process, and the raw material for processing is heavy distillate, which can also be normal-pressure residue and reduced-pressure residue. The main feature of the hydrocracking process is that the production is flexible and the distribution of the products can be controlled by the operating conditions, which can produce gasoline, jet fuel with low freezing point and diesel fuel, and can also produce a large amount of tail oil for cracking raw material or producing lubricating oil. The resulting product is stable, but the octane number of gasoline is not high,. It is not as widely used as catalytic cracking due to harsh operating conditions and high equipment investment and operating costs. However, the hydrocracking process can handle feedstocks containing impurities such as sulfur and high aromatic content, and the feedstock is strictly refined in the hydrofinishing reactor before entering the cracking reactor. The cracking reactor feed can be fed with or without circulating oil depending on the intended product. The range of circulating oil is wide, and can be all distillates below gasoline, or heavy distillates below diesel (>350°C). Most feedstocks can use fixed bed reactors, but residue hydrocracking must use boiling bed reactors, and the hydrocracking catalyst must be adapted to the corresponding feedstock and reaction conditions and equipment.

2、Hydrofinishing catalyst

Hydrofinishing catalysts are used to remove sulfur and nitrogen compounds from oil and to convert aromatic hydrocarbons into cycloalkanes. The active components are compound oxides or sulfides of metals such as key, drill, tungsten and phosphorus.

Selection Guide
The following factors need to be considered.

(1) Activity

(2) Selectivity

(3) Stability

(4) Mechanical strength

(5) regeneration performance

(6) Safety

(7) Cost effectiveness

In addition, it is necessary to consider the type and composition of the API, the quality and distribution requirements of the intended product, the hydrogenation process, the pressure level, the hydrogen-oil volume ratio, and the volume air velocity.

To determine the suitable catalyst, the best process conditions, to meet the product quality and distribution requirements, while maximizing control and reducing the occurrence of side reactions, reducing the consumption of hydrogen and improving economic efficiency.

Precautions
Do not roll the catalyst on the ground to prevent it from crushing

Open the catalyst barrel, leave 10ml of sample in each barrel and mix it for catalyst analysis.

Decide whether to pass the sieve after opening the drum according to the catalyst condition.

The catalyst must be filled to the designed height.

The catalyst is poured into the filling hopper and loaded into the reactor, which must be in a canvas bag, and the catalyst must be handled gently without rolling the barrel.

Participants must not bring other items into the reactor, such as keys, knives, wires and lead wires.

Personnel entering the reactor must be certified and wear a dust mask.

Before entering the reactor, the gas in the reactor should be sampled and analyzed and allowed to enter the reactor. The reactor must be isolated from the system and blinded.

Basic principle and structure of hydrogenation reactor
The hydrogenation reactor is operated under high temperature and high pressure in the hydrogen environment, and the materials entering the reactor often contain impurities such as sulfur and nitrogen, which react with hydrogen to produce corrosive hydrogen sulfide and ammonia. In addition, the hydrogenation reaction is exothermic, which will increase the bed temperature, but without local overheating.

Classification of hydrogenation reactors

Depending on the nature of the feed oil for the catalytic hydrogenation process, the process flow and catalyst used are different, and the form of the reaction is also different, there are generally three types: fixed-bed reactor, moving-bed reactor and fluidized-bed reactor.

There are three types of reactors: fixed bed reactor, moving bed reactor and fluidized bed reactor.

Cold-wall reactor
The cold wall reactor is equipped with non-metallic heat insulation layer on the inner wall of the equipment, and some of them are also lined with stainless steel sleeve inside the heat insulation layer, so that the design wall temperature of the reactor is reduced to below 300℃, thus it is possible to choose 15CrMoR or carbon steel, and the inner wall does not need to weld stainless steel, which greatly reduces the manufacturing difficulty.
However, because the cold wall reactor insulation layer occupies the inner shell space, reducing the utilization rate of the reactor volume, wasting materials, and the non-metallic insulation layer inside the cold wall reactor is easily damaged under the washout of the medium, or in the change of temperature, and may need to be repaired or replaced after a period of operation, and the construction and repair costs are high. If the lining comes off during operation, the reactor wall near where the lining comes off will exceed the design temperature and the paint will be discolored when viewed from the outside. Therefore, the unsafe potential of the reactor is greatly increased, and in serious cases, it even causes forced stoppage of the plant.
Hot-wall reactor
Hot-wall reactors are called hot-wall reactors because the wall is in direct contact with the medium and the temperature of the wall is basically the same as the operating temperature. Although hot-wall reactor is more difficult to manufacture and the one-time investment is higher, it can ensure long-cycle safe operation and is now commonly used internationally.
Pictures
Internal components of a hydrogenation reactor
Picture
Due to the exothermic reaction of gas, liquid and solid phases in the hydrogenation process, in order to make the reaction feed (gas and liquid phases) and catalyst (solid phase) fully, uniformly and effectively contact, the hydrogenation reactor is designed with multiple catalyst beds, with a distribution tray on the top of each bed and a temperature control structure (cold hydrogen box) between two beds to ensure the safe and smooth production of the hydrogenation unit and to extend the The catalyst is equipped with an inlet diffuser.
The reactor is equipped with inlet diffuser, distribution tray, scale accumulation basket, catalyst support tray, catalyst discharge pipe, hydrogen cooling pipe, hydrogen cooling tank, outlet collector, thermocouple and other reactor components.
Inlet diffuser

Picture

Dispense tray

The top distributor plate consists of a tower plate and a distributor evenly distributed on the plate. The top distributor plate is on top of the catalyst bed, the purpose is to evenly distribute the reaction medium, improve its flow condition, achieve good contact with the catalyst, and then achieve a uniform distribution in radial and axial directions. There are more types of distributors, and most of our self-designed and manufactured hydrogenation reactors use bubble cap type distributors.

In order to better break the liquid into droplets entering the descending tube and change the flow direction of liquid from vertical to oblique downward, causing further diffusion, a crusher can also be added under the bubble cap.

Above the catalyst bed, the distributor plate is used to evenly distribute the reaction medium, improve its flow condition, achieve good contact with the catalyst, and thus achieve uniform distribution in radial and axial directions.
Scum accumulation basket
Basket frames made of different sizes of stainless steel mesh and skeleton are placed on top of the upper catalyst bed of the reactor to provide a larger flow area for the reaction flow and to pounce on the top of the upper catalyst bed to collect more deposits of mechanical impurities without causing the reactor pressure drop to grow too fast.
The scale accumulation basket frame is evenly arranged in an equilateral triangle in the reactor cross section, which is empty (not filled with catalyst or porcelain balls), and must be connected together with stainless steel chain after installation, and firmly tethered to the support beam of the upper distribution plate, the stainless steel metal chain should have sufficient length margin (considered by 5% sinking of the bed height) so that it can adapt to the sinking of the catalyst bed.
Catalyst support plate
The catalyst support tray consists of a T-shaped beam, grille and wire mesh. The two sides of the beam are placed on the tabs of the reactor wall, while the grille is placed on the beam and the tabs. The grille is covered with a layer of coarse stainless steel wire mesh and a layer of fine stainless steel wire mesh, which is then filled with magnetic balls and catalyst.
The catalyst support beam and grille should have sufficient high temperature strength and rigidity. In other words, the bending deformation at 420°C is small, and has certain corrosion resistance. Therefore, the beams, grids and screens are made of stainless steel. The design should consider the weight of the catalyst and magnetic spheres on the catalyst support tray, the weight of the catalyst support tray itself, the bed pressure drop and the operating fluid weight and other loads, and the structural dimensions of the support beam and grille should be calculated.
Picture
Picture
Catalyst discharge pipe
Several discharge pipes are installed in the lower part of each catalyst bed of the fixed-bed reactor, crossing the catalyst support tray, the material distribution tray and the cold hydrogen tank, leading to the next bed as a discharge channel for discharging catalyst during reactor shutdown.
Cold Hydrogen Tubes
Hydrocarbon hydrogenation is an exothermic reaction. For a multi-bed hydrogenation reactor, the temperature of oil and hydrogen will increase after the reaction in the upper bed, and in order to continue the effective reaction in the next bed, cold hydrogen gas must be introduced between the two beds to control the temperature. The tube that introduces cold hydrogen into the reactor interior and disperses it is called a cold hydrogen tube.
The role and requirements of the cold hydrogen addition system are
Adequate amount of cold hydrogen is supplied in a uniform and stable manner.
The cold hydrogen must be well mixed with the hot reactants and have a uniform temperature and material distribution when entering the next bed.
The cold hydrogen tubes are divided into inline, dendritic form and annular structure according to the form.
For small diameter reactors, the inline structure with simple structure and easy installation is sufficient.
For the reactor with larger diameter, the cold hydrogen injected into the cold hydrogen tube is not mixed well with the oil and gas after the upper reaction, which directly affects the remixing effect of the cold hydrogen tank. In this case, a dendritic or ring structure should be used.
Cold hydrogen tank
Picture
The cold hydrogen tank is actually a combination of mixing tank and pre-distribution tray. It is the place where the hot reactants in the hydrogenation reactor are mixed and heat exchanged with the cold hydrogen gas. Its function is to mix the reaction products flowing down from the upper layer with the cold hydrogen injected by the cold hydrogen tube in the box, so as to absorb the reaction heat, reduce the temperature of the reactants, meet the reaction requirements of the next catalyst bed and avoid the reactor over temperature.

The first layer of the cold hydrogen tank is a baffle plate with throttle holes. The cold hydrogen coming from the cold hydrogen tube and the oil and gas after the reaction in the previous bed are first pre-mixed on the baffle plate and then enter the cold hydrogen tank through the throttle holes. The cold hydrogen entering the cold hydrogen box and the hot oil and gas coming down from the upper layer are mixed by repeated folding, and then flow to the second layer of the cold hydrogen box – the sieve plate tray, the sieve plate tray, where the mixing effect is strengthened by folding again on the sieve plate tray, and then the distribution is made. There is sometimes a layer of bubble cap distribution plate under the sieve plate plate for the final distribution of the pre-distributed oil and gas.
Outlet collector
The outlet collector is a cap-shaped part with a round hole at the top and long holes on the side walls, covered with stainless steel mesh. Its function is mainly to stop the porcelain balls at the bottom of the reactor from leaking out of the outlet and to drain the fluid.
The outlet collector at the bottom of the reactor is used to support the lower catalyst bed, reduce the pressure drop in the bed and improve the distribution of the reaction material. The lower edge of the outlet collector in contact with the lower head has several notches for draining fluid during shutdown.
Thermocouple
Monitoring of the operating temperature to monitor the bed temperature increase caused by the hydrogenation exothermic reaction and the temperature distribution in the bed cross section.
Damage to hydrogenation reactors

High temperature hydrogen corrosion
Hydrogen embrittlement
High-temperature hydrogen sulfide corrosion
Stress corrosion cracking caused by even more sulfuric acid
Tempering embrittlement of Cr-Mo steels
Hydrogen stripping of austenitic stainless steel overlays