The following, specification particularly describes the nature and manner in which it is to be performed:-
The invention pertains to the field in house for Treatment of Vacuum Unit hot well off gases for preventing corrosion of Vacuum unit heater and Air Pre Heiater (APH), increased waste heat recovery from heater flue gases and environmental improvement.
The main object of this invention is Safe, Efficient and Environmental Friendly utilization of Vacuum unit Hot Well Gas. This invention has been set up for preventing corrosion, environment improvement and increased waste heat recovery.

DESCRIPTION
Vacuum distillation is empjoyed in the oil refinery for economically separating hydrocarbon streams having higher boiling points such as Vacuum Diesel, Vacuum Gas Oil from Reduced Crude Oil (RCO). Heating of RCO for vacuum distillation leads to mild thermal cracking, generating lighter hydrocarbons rich in Hydrogen Sulphide {H2S) and other pollutants. These hydrocarbon gases exit from the system at a pressure just above atmospheric pressure, since vacuum unit operates under vacuum.
The off gases generated from the vacuum unit have a high calorific value; hence it is being used as low-pressure fuel gas in the vacuum distillation unit heater. However, one of the major disadvantages of using these gases directly as fuel is its high content of Hydrogen Sulphide, {H2S), which produce Sulphur-Di-Oxide when burnt in the heater and leads to severe acid corrosion in heater convection section and air pre-heater modules and also cause severe environmental damage. To mitigate the corrosion problems' while firing this gas, flue gas temperature is maintained high for avoiding Sulpur-di-Oxide condensation by partly bypassing the air going through the Air Pre Heater (APH).
Quantity of H2S in the off gases was increasing due to increased processing of high sulphur crudes. An attempt to maintain higher flue gas temperatures to avoid acid corrosion has an impact on the energy efficiency, as the opportunity to recover waste heat from the flue gas is lost. Treatment of these off gases for the removal of H2S was a challenge as these gases were available at very low pressures and there was no proven operating technology in the Industry.
The Process
A system, the first of its kind in the country and not known to be in commercial operation in the world, was designed using in house expertise. The system developed was amine scrubbing of these gases over a packed bed. The process employs an absorption column in which a low pressure drop packing and de-

entertainment device is installed. The medium used for removal of Hydrogen Sulphide is a regenerable medium such as Di-Ethanol Amine (DEA).
The process employs Di-ethanol Amine as an absorption medium for absorbing the H2S in these gases.
Hotwell off gas from the vacuum unit are "routed to the bottom of an absorption coiumn containing a specially designed packing. This specially designed packing provides the maximum surface area for mass transfer at the minimum pressure drop. Design of the absorption column and the internals are such that the total drop in pressure across the column is minimal, thereby not affecting vacuum in the upstream section.
Lean Di-ethanol Amine (DEA) from the amine regenerator flows through specially designed distributors from the top of the absorber. The internal distributors are designed for proper distribution of liquid over the packing and at the same time minimizing the pressure drop in the vapour flow/ing inside the absorber. H2S in hotwell off gases are absorbed in the DEA solution over the packing and effluent hotwell off gases from the top of the absorber contains only traces of H2S. The height of packing provided is determined from the concentration of H2S at the inlet and the desired concentration at the outlet.
Rich DEA from the bottom of the absorption column is routed to an amine regenerator where the absorbed H2S is removed by providing steam reboiling. Lean DEA from the bottom of the regenerator is routed back to the absorber, thereby completing the process cycle.
These desulphurised gases are then routed to the furnace through flame arresters for preventing flame back up from the heater. The furnace is provided with low pressure burners for proper combustion. The above said process is explained with the pictorial representation marked as Diagram No. 1.

Adequate safety instrumentation and protection devices have been provided in order to prevent safety hazards due to various process upsets and also monitor the process.
End Products:
1. Sweetened low pressure fuel gas for combustion in a furnace.
2. Sulphur recovered in solid form from a Sulphur Recovery Unit.
Precautions Adopted:
1. Low pressure drop so that upstream vaccum system is not affected.
2. De-entertainment device to prevent carry over of liquid with the gas.
3. Providing a set-up to prevent back fire from the heater.
Specific advantages of the process :
1. No similar system is documented to have been implemented for such a process.
2. The pressure drop across the total system is very less and the system can be tailor made depending upon the flow rate of gas and the concentration of H2S in the gas.
3. The process designed employs a regenerable medium for absorption and does not use an alternative hydrocarbon stream which is being used in some applications. This reduces the need for recycling hydrocarbons and hence the system can be operated as a stand alone system, not integrated with any other processes.
4. The system virtually eliminates discharge of environmental pollutants such as Sulphur oxides to the atmosphere.

5. Corrosion in the heater and downstream sections such as the Air Preheater is drastically reduced.
6. Operating cost for the system is negligible.
7. Requires very low investment (Less than 1 Million USD )

CLAIM
1/ We claim the Invention as to the Treatment of Vacuum Unit hot well off gases for preventing corrosion of Vacuum unit heater and Air Pre Heater (APH), increased waste heat recovery from heater flue gases and environmental improvement vide desulphuhzation of low pressure vaccum unit hot well off gas.