Diesel Hydro-Desulphurisation Process Plant with Capacity 1MMTPA

Various process options used for desulphurisation technology:

1. SYN/SAT technology
2. Unisol
3. Hydro Desulphurization (HDS)

Diesel Hydro desulphurization (HDS) Process:

The purpose of HDS unit is :

1. To perform desulfurization of the reactor feed. The specification to meet is 500ppm wt.
2. The hydrogenation of eventual olefin structures (achieved easily using a special catalyst )

Main chemical reaction mechanisms that influence the process:
1. Refining reactions.
2. Hydrogenation reactions.

(a). Refining reactions

Desulfurisation: Mercaptans, sulfides and disulphide easily react, leading to the corresponding saturated or aromatic compounds. Sulphur combined into cycles of aromatic structure, like thiophene is more difficult to eliminate. These reactions lead to the H2S formation and hydrogen consumption.

Denitrification: The denitrification reaction rate is lower than that of desulphurization. it occurs mainly in case of heterocyclic compounds having an aromatic structure(pyridine for example). These reactions lead to NH3formation and H2 consumption.

(b). Hydrogenation reactions :

• These reactions affect the Diolefins, olefins and aromatics and are highly exothermic. Diolefins and olefins are converted into saturated compounds.

• The hydrogenation rate of aromatics is limited.

Process Descriptive Flow sheet of Diesel Hydrodesulphurisation Plant

• The feed to the DHDS (Diesel Hydro Desulphurisation section) unit is a blend of straight run and cracked gas oils. The feed blend is filtered through feed filter package and sent to feed surge drum. The pressure in the feed surge drum is maintained by fuel gas blanketing.

• The liquid phase feed is pumped under flow control by a feed pump, mixed with the hydrogen recycle compressor delivery stream and let in the heat exchanger train. This mixing of recycle hydrogen with feed ensures an adequate hydrogen partial pressure at the inlet of the reactor train. Polymerization inhibitor is injected.

• The hydrogen make-up gas coming from B/L is routed through chlorine adsorbent and is then compressed by the compressor. The part of the gas is sent as quench gas to the reactor. The mixed stream (feed + recycle hydrogen) is heated in first feed/effluent heat exchanger then in the second feed/effluent exchanger and finally in the reactor heater to the required reactor inlet temperature.

• The reactor inlet temperature is maintained by the controlling the fuel gas/fuel oil to the heater burners. The stream is then let in the reactor which includes a catalyst in three beds.

• Cold quenches of hydrogen coming from recycling compressor are added at the inlet of each new bed to control the bed inlet temperature.

• The reactor effluent is cooled in the heat exchangers. To avoid ammonium salt deposits and risk of corrosion, water is injected at the inlet of the effluent cooled.This mixture is collected in the cold separator where three phases are separated.

• The sour water containing ammonium salts is partly recycled to the washing.

• The gas phase from cold separator goes to amine absorber where H2S is removed. In the amine absorber, the gas is washed by a 25%wt DEA solution. The lean DEA solution is pumped by the DEA booster pump to the top of the absorber. The rich DEA solution is withdrawn from the bottom of the absorber to be fed to the regeneration section. The gas from the absorber contains hydrogen which is recycled back to the reactor.

• The hydrocarbon liquid is withdrawn from the separator and sent to storage which is our product.