Chemistry and technology of petroleum

Chemistry and technology of petroleum By Dr. Dang Saebea

Hydroconversion

Introduction • Hydroconversion is a term used to describe all different processes in which hydrocarbon reacts with hydrogen. To describe the process of the removal of sulphur, nitrogen and metal impurities in the feedstock by hydrogen in the presence of a catalyst. Hydrotreating The process of catalytic cracking of feedstock to products with lower boiling points by reacting them with hydrogen. Hydrocracking aromatics are saturated by hydrogen to the corresponding naphthenes. Hydrogenation

Hydrotreating Objectives of Hydrotreating 1. Removing impurities, such as sulphur, nitrogen and oxygen for the control of a final product specification or for the preparation of feed for further processing. 2. Removal of metals, usually in a separate guard catalytic reactor when the organo-metallic compounds are hydrogenated and decomposed, resulting in metal deposition on the catalyst pores. 3. Saturation of olefins and their unstable compounds.

Role of Hydrotreating • HT are located before the reformer, hydrocracker and FCC • They are also needed to adjust the final product specification for various streams, such as light naphtha, kerosene and low sulphur fuel oils (LSFOs). Role of hydrotreating (HT) in the refinery

Main role of hydrotreating 1. Meeting finished product specification. • Kerosene, gas oil and lube oil desulphurization. • Olefin saturation for stability improvement. • Nitrogen removal. • De-aromatization for kerosene to improve cetane number. Cetane number is the percentage of pure cetane in a blend of cetane and alpha-methyl-naphthalene. The latter matches the ignition quality of kerosene sample.

Main role of hydrotreating 2. Feed preparation for downstream units: • Naphtha is hydrotreated for removal of metal and sulphur. • Sulphur, metal, polyaromatics and Conradson carbon removal from vacuum gas oil (VGO) to be used as FCC feed. • Pretreatment of hydrocracking feed to reduce sulphur, nitrogen and aromatics.

Hydrotreating reactions 1. Desulphurization a. Mercaptanes: b. Sulphides: c. Disulphides: d. Thiophenes:

2. Denitrogenation a. Pyrrole: b. Pyridine:

3. Deoxidation a. Phenol: b. Peroxides:

4. Hydrogenation of chlorides 5. Hydrogenation of olefins 6. Hydrogenation of aromatics

7. Hydrogenation of organo-metallic compounds and deposition of metals Vanadium deposited as vanadium sulphide (V2S3)

8. Coke formation by the chemical condensation of polynuclear radicals

Hydrotreating Processes • The liquid feed is mixed with hydrogen and fed into a heater and then fed into a fixed bed catalytic reactor. • The effluent is cooled and hydrogen-rich gas is separated using a high pressure separator. The main elements of a hydrotreating process

Hydrotreating Processes 3. Before the hydrogen is recycled, hydrogen sulphide can be removed using an amine scrubber. 4. Some of the recycle gas is also purged - To prevent the accumulation of light hydrocarbons (C1–C4) - To control hydrogen partial pressure.

Hydrotreating Processes 5. The liquid effluent for the reactor is introduced to a fractionator for product separation.

1. Naphtha Hydrotreating • Toremove the impurities so that the hydrotreated naphtha can be introduced to the catalytic reformer. • The expensive platinum based catalyst used in the reformer is sensitive to poisoning by such impurities.

1. Naphtha Hydrotreating recycle compressor reactor high and low pressure separators treated naphtha fractionator. a feed heater

1. Naphtha Hydrotreating H2S scrubber A catalyst of Co–Mo on alumina is used.

2. Middle Distillates Hydrotreating • Middle distillate is mainly composed of saturated paraffins and also some aromatics which include simple compounds with up to three aromatic rings. • Kerosene, jet fuel oil and diesel fuel are all derived from middle distillate fractions.

2. Middle Distillates Hydrotreating A hydrogen sulphide scrubber and a gas purging are usually used to improve the quality of recycled hydrogen.

3. Atmospheric Residue Desulphurization • atmospheric residue has a sulphur content and metals (Ni + V). • The purpose of this process is to remove most of the metals and reduce sulphur content in the product to less than 0.5 wt%.

3. Atmospheric Residue Desulphurization 1. The feed is introduced into the heater where steam is injected (to prevent coking) to a temperature below 371 ˚C. T<371 ˚C

3. Atmospheric Residue Desulphurization 2. The heated recycled hydrogen is mixed with feed and together, they are introduced into a guard reactor. 3. The stream leaving the guard reactor is quenched with cold recycle hydrogen and introduced to the first of the three fixed bed reactors. The main reactions of hydrodemetallization, hydrodesulphurization, denitrogenation and aromatic hydrogenation take place in the reactors. contains a hydrogenation catalyst similar to that in the main reactor but usually cheaper.

3. Atmospheric Residue Desulphurization 4. The flow diagram also contains high and low pressure separators, recycled hydrogen stream with online amine treatment and purge. 5. The liquid stream from the separators are send to a fractionator to produce naphtha, diesel and low sulphur fuel oil (LSFO).

Reactor • The catalyst should have wide pores to avoid plugging due to metal deposition. • Due to the fast deactivation of this catalyst, usually two reactors are used and the catalyst is changed in one of them while the other reactor is still online. • Three to four reactors are usually used with different combinations of catalysts to achieve desired objectives.

Hydrogen requirements for hydrotreating are classified into: • Chemical requirement: • This is the amount of hydrogen required to remove impurities such as sulphur, oxygen, nitrogen, olefins and organometalic compounds, according to the stoichiometry of these reactions. • Sometimes, it might be required to convert aromatics and naphthenes to corresponding paraffins.

Hydrogen requirements for hydrotreating are classified into: (2) Hydrogen lost due to the dissolution of hydrogen in the hydrocarbons treated. (3) Amount of hydrogen lost with the purging of light hydrocarbons (C1–C4) and hydrogen sulphide (if not removed by amine treatment).

Make-up Hydrogen • A certain hydrogen partial pressure should be maintained in the reactors by recycling un-reacted hydrogen and adding a make-up hydrogen to compensate for the amount consumed. • The make-up hydrogen can be calculated by the following expression

Operating Conditions The operating conditions of the hydrotreating processes • pressure • temperature • catalyst loading • feed flow rate • hydrogen partial pressure

Operating Conditions • Increasing hydrogen partial pressure improves the removal of sulphur and nitrogen compounds and reduces coke formation. • Higher temperatures will increase the reaction rate constant and improve the kinetics. However, excessive temperatures will lead to thermal cracking and coke formation. • The space velocity is the reverse of reactor residence time (y). High space velocity results in low conversion, low hydrogen consumption and low coke formation.

The range of operating conditions for hydrotreating of different feed fractions

Hydrocracking • Hydrocracking is a catalytic hydrogenation process in which high molecular weight feedstocks are converted and hydrogenated to lower molecular weight products. • The catalyst used in hydrocracking is a bifunctional one. It is composed of a metallic part, which promotes hydrogenation, and an acid part, which promotes cracking. • Hydrogenation removes impurities in the feed such as sulphur, nitrogen and metals. • Cracking will break bonds, and the resulting unsaturated products are consequently hydrogenated into stable compounds.

Role of Hydrocracking in the Refinery • It is mainly used to produce middle distillates of low sulphur content such as kerosene and diesel. • If mild hydrocracking is used, a LSFO can be produced. • It has been used to remove wax by catalytic dewaxing and for aromatic removal by hydrogen saturation. • This has been applied to the lube oil plants and is gradually replacing the old solvent dewaxing and aromatic solvent extraction.

Feeds and Products • VGO is the main feed for hydrocrakers

Hydrocracking Chemistry 1. Alkanehydrocracking 2. Hydrodealkylation 3. Ring opening

4. Hydroisomerization 5. Polynuclear aromatics hydrocracking

Hydrocracking Catalysts • The cracking function is provided by an acidic support, whereas the hydrogenation–dehydrogenation function is provided by active metals.

Hydrocracking Processes The following factors can affect operation (product quality), yield (quantity), and the total economics of the process: 1. Process configuration: one stage (once-through or recycle) or two stages 2. Catalyst type 3. Operating condition (depends on process objective) - Conversion level - Maximization of certain product - Product quality - Catalyst cycle - Partial hydrogen pressure - Liquid hourly space velocity - Feed/hydrogen recycle ratio

Process Configuration Simplified flow diagram of one-stage hydrocracking process with and without recycle • In commercial hydrocrackers, a conversion of 40–80% of the feed can be achieved. • However if high conversion is required the product from the bottom of the distillation tower is recycled back to the reactor for complete conversion. • This configuration can be used to maximize a diesel product, and it employs an amorphous catalyst.

Process Configuration The catalyst in the first stage has a high hydrogenation/acidity ratio, causing sulphur and nitrogen removal In the second reactor, the catalyst used is of a low hydrogenation/acidity ratio in which naphtha production is maximized Conventional two-stage hydrocracker • The effluent from the first stage reactor is sent to a separator and fractionator. • The fractionator bottoms are sent to the second reactor.

Two-stage hydrocracking • A hydrotreatment reactor may be added before the first hydrocracker to help in removing sulphur and nitrogen compounds from the feed. • Since H2S and NH3are separated before entering the second hydrocracker, this allows the selection of special catalysts in the second reactor without the poisoning effect of sour gases. • The two-stage configuration offers more flexibility than the single stage scheme. It is better suited for heavy feedstocks

The End

Chemistry and technology of petroleum