Cobalt-Molybdenum Hydrodesulfurization Reaction

The hydrogen production process can be divided into several reaction steps: cobalt-molybdenum hydrodesulfurization reaction, dechlorination reaction, zinc oxide desulfurization reaction, hydrocarbon-steam conversion reaction, carbon monoxide shift reaction and hydrogen purification part (this device uses PSA). In order to separate the hydrogen in part of the hydrogenated dry gas, a membrane separation unit is added between the dry gas compressor and the hydrogenation heating furnace.

Reaction principle

The raw material oil and gas for hydrogen production contain various organic sulfur. Under certain temperature (generally 260-400°C) and the presence of H2, the cobalt-molybdenum hydrodesulfurization catalyst can convert organic sulfur into inorganic sulfur. The sulfur is then absorbed by other desulfurizers (such as ZnO), the olefins contained in the raw material can also be hydrogenated and saturated, and the organic chlorides are hydrogenated to generate HCL.

The reaction formula as follows:

(1) Mercaptan hydrogenation reaction formula: R-SH + H2 → RH + H2S

(2) Disulfide hydrogenation reaction formula: R-S-S-R'＋ 3H2 → RH ＋ 2H2S ＋ R'H

(3) Sulfur ether hydrogenation reaction formula: 2R-S-R'H ＋ 5H2 → 2RH ＋ 2R'H2 ＋ 2H2S

(4) Carbon disulfide hydrogenation reaction formula: CS2 + 4H2 → CH4 + 2H2S

(5) Hydrogenation reaction formula of carbon oxysulfide: COS ＋ H2 → H2S ＋ CO

(6) Olefin hydrogenation reaction formula: RCH=CHR' + H2 → RCH2-CH2R'

(7) Reaction formula for hydrogenation of organic chlorides: R-CL + H2 → R-H + HCL

(In the reaction formula: R, R'represents an alkyl group)

Influencing factors

(1) Temperature

When the cobalt-molybdenum catalyst is used for hydrodesulfurization, the operating temperature is usually controlled within the range of 260-400°C. When the temperature is lower than 220°C, the hydrogenation effect is significantly reduced, and the surface polymerizatio.But carbonization of the catalyst increase when the temperature is higher than 420°C.

(2) Pressure

Since the content of organic sulfides in light oil is not high, the pressure has little effect on the hydrogenolysis reaction.The pressure is determined by the requirements of the entire process, usually controlled at 3.0-4.0MPa.

(3) Airspeed

Unit time (h), the volume quantity of gas (converted to the standard state) passing through the unit catalyst volume (m3) is called space velocity.The unit is m3/h.m3, which can be abbreviated as h-1. In some reactions, water vapor participates in the reaction process, but the volume of water vapor is not calculated when calculating the space velocity. The space velocity at this time is called dry gas space velocity. In the conversion reaction of hydrocarbons, the hydrocarbons with more than 2 carbon atoms are converted into CH4.And then the space velocity is calculated. The space velocity at this time is called the carbon space velocity. When using a liquid raw material, the space velocity calculated according to the volume of the liquid is called the liquid space velocity. The hydrogenation reaction is controlled by internal diffusion. If the space velocity is increased, the flow rate of the raw material will be increased, so that the residence time of the raw material in the catalyst bed will be shortened.The reaction will not be complete. Therefore, the hydrogenation reaction must be carried out at a certain space velocity. However, in order to improve production capacity, the highest space velocity is usually adopted under the condition of ensuring that the sulfur content of the outlet meets the process requirements. Generally, the space velocity of light oil ranges from 1 to 6h-1.

(4) Hydrogen oil ratio

In the hydrodesulfurization reaction, increasing the ratio of hydrogen to oil, that is, increasing the partial pressure of hydrogen, can not only inhibit the carbon deposition of the catalyst, but also facilitate the hydrogenolysis process. On the contrary, if the partial pressure of hydrocarbons increases, the hydrocarbons will be adsorbed on the surface of the catalyst, thereby reducing the surface area of the catalyst and inhibiting the hydrogenolysis reaction. Therefore, a high hydrogen-to-oil ratio is beneficial to the hydrogenation reaction, and the general hydrogen-to-oil ratio is 80-100 (by volume). If the hydrogen-to-oil ratio is too low, the desulfurization can’t meet the requirements.