HDS Hydrogen Sulphur
Hydrodesulphurisation: hydrodesulphurisation or HDS is a key step in the purification of feed gases on hydrocarbon plants since it converts organic hydrocarbons to hydrogen sulphide. These organic sulphur compounds will not be absorbed in the zinc oxide bed and therefore must be hydrogenated.
The key hydrogenation that occurs as
Generally we can say
CnH (2n+1)+H2→ CnH(2n+1)H+H2S
If there any COS present in feed gas when there is water present, it could be from hydrogen recycle.
The choice HDS catalyst is dependent on the synthesis gas plant, on methanol plants where the purge gas from the synthesis loop is returned to the desulphuriser the CO and CO2 levels are small. However on the event of a feed trip high levels of CO2 and CO can enter the HDS catalyst. There is the potential of methanation on the HDS catalyst, NiMo does not promote methanation catalyst therefore would be the preferred choice in methanol plants.
At temperatures of less than 3000C, the hydrogen activity of the standard range of hydrodesulphurisation catalyst is low. There fore a higher metal loading catalyst i.e.:
Higher levels of CoMo have to be used for temperatures between200-3000C. However, these catalysts must be sulphided before commissioning in order to have sufficient activity at lower temperatures. This can be achieved by pre- sulphiding either before installation or post installation.
OPTIMAL TEMPERATURE OF ZINC OXIDE:
For the zinc oxide, the higher the operating temperature, the higher the adsorption capacity. The below graph shows the Zinc oxide performance against temperature.
INSERT FIG 1
However, there is an upper limit to the purification operating temperature; at temperatures in excess of 4000C, there is potential for hydrocarbon cracking in the feed pre heat coil. As the temperature inlet the feed pre heat coils increases, so does the potential for hydrocarbon cracking. It is therefore typical purification feed is preheated to 4000C.
There are number of purification schemes are available to use for purify the feed gas.
location of the purification method:
The below figure shows the conventional purification system for sulphur.
INSERT FIG 2.
Zinc oxide saturation profile: Zinc oxide is different from the other catalyst in hydrocarbon purification plant like absorption of hydrogen sulphide, in that it not a catalyst but an absorbent. The below figure shows the adsorption profile with in the Zinc oxide bed.
INSERT FIG 3.
As we seen the above figure , it says there is fully saturated zone at the leading edge of the bed; a partially saturated zone below this, with the sulphur saturation level dropping from fully saturated to unsaturated. Underneath this zone and bottom of the bed, there is an unsaturated zone.
The below figure shows Zinc oxide adsorption profile variation with time.
INSERT FIG 4.
Single vessel scheme for HDS and ZnO:
This type of single vessel for purifying hydrocarbons in many modern plants where the sulphur content of the feed gas is very low.
INSERT FIG 5
The advantage of this type of single vessel is use which minimises the capital expenditure of the purification section of the plant. Unless the vessel is supplied with specially designed internals, the disadvantage of this scheme is that the HDS catalyst has to be discharged at the same time as the zinc oxide and this increases the operational expenditure. If the sulphur content of the feed gas is higher than the expected then the zinc oxide will be fully saturated before the design life is achieved and therefore the HDS catalyst will be discharged before it has reached the end of life. A further disadvantage again occurs if the sulphur content of the feed gas is higher than design in that the plant will have to be shut down prematurely which will add additional turn a rounds. A further disadvantage of this scheme is that the zinc oxide starts to slip sulphur well before the whole bed is fully saturated. Since the bed has to be discharged once sulphur starts to slip to prevent long term damage to the downstream catalysts, but this increases the operating expenditure.
This figure shows the single bed saturation profile
INSERT FIG 6.
We can see there is a portion of the zinc oxide bed that is not fully saturated at the bottom of the bed.
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