Hydrogen Generation from Naphtha
The hydrogen generation unit of the plant consists of 4 sections,
1. Pre-Desulphurization Section
2. Final-Desulphurization Section
3. Reforming Section
4. HT Shift Reaction and PSA Unit
Naphtha Pre-Desulphurization:
The sulfur content of the sour naphtha at battery limits may be as high as 1000ppm wt. This at 4.5 kg/cm2 is fed to the sour naphtha surge drum from where it is pumped by the sour naphtha pump with discharge pressure of 32.6kg and mixed with hydrogen recycle gas from the recycle compressor.
Make up hydrogen from the PSA Unit at 22.2 kg is fed to the suction of the recycle gas compressor to supply hydrogen required for the reaction. Naphtha and hydrogen mixture is vaporized in the sour naphtha vaporize and then superheated in the naphtha super heater to 340˚C (max).
The superheated recycle mixture is then fed to Hydro-Desulphuriser-I at 24.5 kg and 340˚C, where hydrogenation of sulphur compounds takes place. On leaving, the reactor effluent is cooled in sour naphtha vaporizer against reactor feed and against sea cooling water in the naphtha condenser. The condensed naphtha is separated and a portion of gas from the separator is purged and sends to the battery limits as sour gas.
The liquid naphtha containing dissolved H2S is from naphtha separator through the sour or sweet naphtha exchanger to the naphtha stripper, where it is stripped of dissolved H2S
Naphtha Vaporization and Final Desulphurization:
Sweet naphtha from the naphtha pre-desulphurization section or from the storage is fed to the sweet naphtha surge drum.
Naphtha is pumped with discharge pressure of 32.9 kg/cm2 by sweet naphtha feed pump mixed with recycle hydrogen from DHDS unit MGC discharge, and vaporized in the naphtha vaporizer by HT shift reactor process gas effluent. The mixture is then superheated to 390˚C in the convection coil of reformer.
The superheated mixture is then sent to Hydro-Desulphuriser-II at 30.8 kg/cm2 where sulfur compounds are converted to H2S over a Cobalt-Molybdenum oxide catalyst.
Steam-Reforming Naphtha:
The feed Desulphuriser gas leaves the FDS at 377˚C is combined with super heated steam at 379˚C in the appropriate ratio, and further heated in the mixed feed super heater to 520˚C. The superheated feed is distributed over the catalyst tubes in the radiant section of the reformer where the hydrocarbons are converted to hydrogen, carbon-monoxides and carbon-dioxide in the presence of steam, over a nickel based catalyst.
The reformed gas leaves the reformer at about 860˚C and 24.6 kg/cm2. The reformer is a top fired balance draft furnace, consisting of two sections the radiant section where the feed stock process steam reaction takes place and the convection section where mixed feed is pre-heated, steam generated and combustion air pre-heated by the flue gases.
As the reforming reaction is strongly endothermic, the heat requirement is partly satisfied by the combustion of purge gas from the PSA unit. The balance of the fuel requirements is made-up by firing naphtha. Combustion air is supplied by the combustion air fan and preheated in the blow down cooler and combustion air pre-heater to 410˚C. Hot flue gas, the product of combustion leaves the bottom of the radiant section at 999˚C generation coils, feed preheating coils, steam super heating coil and combustion air preheater coil.
HT Shift Conversion:
In the HT shift reactor, most of the carbon monoxide reacts with steam and is converted to Hydrogen and Carbon Dioxide over an iron oxide/chromium oxide catalyst in the so called ‘water-gas shift reaction’.
CO + H2O CO2 + H2
The shift reaction is moderately exothermic and is unaffected by pressure. The catalyst should be operated at the minimum practical temperature giving the desired conversion of carbon monoxide. This results in the increased life of the catalyst by limiting sintering and loss of low temperature activity. As the catalyst ages it is necessary to increase the converter inlet temperature in order to maintain a satisfactory exit carbon monoxide level.
The usual lifetime of a charge of HT shift catalyst is from 4-6 years, depending upon how it has been treated. In the plant the reformer catalysts are much more susceptible to poisons than the iron or chrome HT shift catalyst so care is taken to ensure that feed stocks are virtually poison-free.
Pressure Swing Adsorption:
In this unit hydrogen is recovered from the process gas by adsorption of methane, carbon monoxide, carbon dioxide and water vapor on molecular sieves and active carbon based adsorbents. This unit works on the principle that the adsorbent attracts and retains the impurities at higher pressure and releases them at lower pressure, thus the expression “Pressure Swing Adsorption”.
The pressure vessels containing the adsorbents are alternatively connected with the feed line, which is at the highest pressure in the cycle, and with the off gas-line, which is at the lowest pressure. Under the high pressure of the feed, the absorbent attracts the impurities and as a consequence a high purity product stream exits from the adsorber. This is the “Adsorption Step” and the hydrogen product stream is practically at the same pressure level as the feed.
Subsequently the adsorbent is regenerated by bringing the adsorber down to the lowest pressure level in the cycle. The waste gases (purge gas), which are being released during the regeneration is collected in the purge gas drum, from where they flow to the reformer burners.
The hydrogen product from the PSA is sent to a candle filter (designed to filter the particle size > 5 microns). Part of the hydrogen from the PSA unit is recycled back to the pre desulphurization section and is after compression mixed with sour naphtha before sour naphtha vaporizes. The remaining hydrogen is sent to DHDS as product at 22kg/cm2.
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