Hydrogen gas is used to purify the light crude benzol. It is recovered from coke oven gas and Light Crude Benzol (LCB) from the benzol distillation plant. LCB consists of benzene, toluene, xylene, solvent naphtha, non-aromatics, and residue. Initially, the LCB is purified from sulfur, non-aromatics, and other compounds. This consists of the following sections. They are,
- De-fronting section
- Reaction section
- Purification section
DE FRONTING SECTION:
In this section, carbon disulfide is removed from the crude benzol and this is called de-fronted crude benzol. LCB from the storage tank is pumped to a surge tank, which is meant for intermediate storage. The LCB from the surge tank is pumped to the distillation column through the feed preheater. The feed enters the column at a rate of 3 T/hr and at 70°c. Pressure in the column will be 0.5 kg/cm2. The sulfur content in the feed is 2000-1800 ppm. This is decreased to about 1200 ppm in the column.
The distillation column consists of 30 bubble cap trays of which the 17th tray is the feed tray. Steam is fed into the reboiler, which heats the bottom product recycled to the column. The remaining bottom called de-fronted crude benzol is fed to the reaction section through feed pre-heater. The sulfur is removed in the form of CS2. Simple distillation is carried out and due to heating CS2 vapors rise to the top and these are condensed in a water condenser. Condensed CS2 is collected in a CS2 vaporizer. Part of it is fed to the column as reflux and the other part is stored. The DCB obtained is at 70°c and this is fed to the intermediate storage.
The feed rate to the column 3T/hr
Pressure in the column 0.5 kg/cm2
Sulfur content in the feed 2000-1800 ppm
Sulfur content in DCB 1200 ppm
No. of bubble cap trays 30
The boiling point of CS2 45
The temperature at the top of column 55-65°C
Column bottom temperature 105°C
This section consists of reactors and evaporators. Here the hydro-refining takes place in the reactors provided which remove the oxygen, nitrogen, and sulfur content in crude benzol.
The crude benzol (CB) is pumped into the storage tank through a filter. The filter is provided to remove the solid particles and polymers, which may be present in the crude benzol. The benzol filter is an edge-type filter and consists of a slotted tube inside a shell with a specified filter fineness, which is determined by the slots and scrappers. This is agitated by a hand crank.
The particles are retained at the edges of the slots and must be scraped off. If the pressure difference between the inlet and the outlet streams is too high the concerned filter must be opened and cleaned. The filtered CB is stored in the surge drum. The drum is set to approximately two bars split range controlled by feeding N2 and venting gases. From the surge drum, the CB is fed to a pre-vaporizer at a pressure of 30 bars using 32-stage centrifugal pumps.
It is nothing but a vertically mounted shell and tube heat exchanger. The feed is mixed with a part of recycled gas (containing H2 approximately 15% of the total gas) before it is fed to the vaporizer. This feed is pre-vaporized to about 160-165°C by means of the main reactor effluent passing through the shell side. The feed at a temperature of 160-165°C is fed to the third mixing nozzle of the stage evaporator.
This vertical heat exchanger is provided with turbulence promoters on the tube side to achieve high turbulence so that more heat exchange will occur and no scale formation is attained. This arrangement is provided as the feedstock in the partial vapor stage (gas-liquid stage) and so fouling of the tubes will occur rapidly. This arrangement also provides easy cleaning of tubes by simply pulling the turbulence promoters.
The stage evaporator is a long cylindrical vessel provided with three stages, which are separated by two plates. Demister pads are provided at the top of the evaporator. Each stage is provided with a mixing nozzle. Two re-boilers are provided for the second and first stages respectively. A gas pre-heater is also provided in which the recycled gas (85% of the total gas) is pre-heated to 210°C from the main reactor effluent. The two re-boilers are heated by hot oil through tubes at a temperature of 250°C. Recycle gas mixed with feed is passed through the shell side.
Downcomers are placed so that the liquid in the third stage will enter the second and from second to first. The pressure inside is about 20 Kg/mm.
The Crude Benzol mixed with 15% of recycled gas is fed at the third mixing nozzle of the evaporator. The vapors coming from the second stage and the feed are mixed thoroughly and fed to the third stage. Lighter vapors are passed through the demister pads and to the pre-reactor. The liquid containing lighter and heavier substances is passed through downcomers to the second stage. Here the fed is mixed with the vapors from the first stage in the mixing nozzle II and heated in a re-boiler. This is fed to the top of the second stage.
Similarly liquid from the second stage flows to the first stage. This liquid is pre-heated in a re-boiler and mixed with 85% of the recycle gas in the first mixing nozzle and again fed to the first stage. The temperature at the bottom of the evaporator is 210°C. Due to the heating of the feed, the vapors are sent to the top and any residue or polymers in the feed are collected at the bottom. Part of the liquid from the first stage is fed to the residue flash drum, from where they are recycled to the benzol distillation plant. The lighter vapors from the flash drum are fed to the surge drum nearly this residue would be 3-4% of the total feed.
The vaporization of feed (CB) in the evaporator is done by the reduction of the partial pressure of CB, which is manipulated by the addition of the recycled gas. This results in lower operating temperatures even at higher pressures. Vaporization of feed in the heat exchanger should be avoided to reduce the fouling of surfaces.
The vapors from the top of the evaporator at 180°C are heated in a heat exchanger to 190-225°C by passing the main reactor effluent through the shell side. The reactor is provided with a bed of catalyst i.e. NICKEL MOLYBDENUM. In this pre-reactor such as di-olefins, styrene and CS2 are removed by hydrogenation. Feed enters from the bottom of the reactors through the catalyst bed. Hydrogenation of di-olefins, and styrene takes place in the presence of a catalyst.
The temperature at the inlet of the reactor is 190-225°C and this depends on the life cycle of the catalyst. Due to the exothermic reaction, the outlet temperatures increase to 200-235°C. Due to the continuous operation of the catalyst bed, coke-like polymerization products deposit on the catalyst bed resulting in lower efficiency. This can be overcome by increasing the inlet temperature of the reactor. Catalyst activity can be determined by the temperature difference between the inlet and outlet, which should be more than 10°C. The catalyst can be regenerated by heating the bed with steam and air. The reactions in the pre-reactor are
Di-olefins + H2→ Mono-Olefins
In the main reactor, treated pre-reactor effluent is hydrogenated on a special sulfide molybdenum catalyst. The main reactor consists of two beds of catalyst makeup gas i.e. pure H2 gas from the compressor at a pressure of 18 bar providing more hydrogenation and hence complete saturation of olefin hydrocarbons. The inlet temperature is about 270°C and the outlet temperature is 330°C due to exothermic reaction. Mainly desulphurization, densification, and olefin saturation feedstock occur in the main reactor. The hydrogen is fed through a distributor below the first bed of the catalyst oxygen content in H2 gas should be very low so that no polymerization occurs in the reactor. Hydrogenation of aromatics should be prevented by optimizing the temperature. Catalyst deactivation can be determined by the amount of thiophene content at the outlet of the reactor. If this increases, the hydrogenation of aromatics and coke formation increases. So the temperature of the reactor should be increased or other regenerations should be done.
Main reactions are:
Benzene + H2 →Cyclo hexane
Toluene + H2 →Methyl cyclo-hexane
Hence required to maintain a heater to which a part of the effluent is passed, heated, and fed to the main reactor supplies the temperature. Coke oven gas is used as fuel in the heater.
The effluent from the main reactor is collected at the bottom, which is at 330оC. This effluent is passed through several heat exchangers and finally cooled in the water cooler. This condenser effluent is fed to the separator. Before the water cooler, hot water is dosed into the effluent. This dissolves the deposits of salts such as NH4HS2 and NH4Cl. The cooled effluent at 50 оC is fed to the separator. A water leg provided separates the dosed water. The water-free effluent is fed to the stripping column. The gases i.e. unreacted hydrogen gas and other gasses are sucked by recycle gas compressor and are recycled. Part of the gas is purged out through a vent provided.
Hot oil system:
The heat demand of the process is supplied by a separate hot oil system. The hot oil is used as a heating medium for several heat exchangers in the hydro refining unit and extractive distillation unit. A horizontal furnace is used to heat the oil; the furnace is fired using coke oven gas. Hot oil is pumped into the coils in the furnace. The temperature of the oil increases to about 340-350 оC. The oil at a temperature of 340 оC is fed to the HR unit by using another pump.
Pressure swing adsorption unit:
The required hydrogen gas for HR units is supplied from this section. The clean coke oven gas after benzol recovery is fed to a filter at a pressure of 800mm. Moisture and carbon particles present in the gas are filtered and the filtered coke oven gas is fed to a reciprocating compressor, which compresses the gas to about 2.5 kg/cm2. The compressed gas is again fed to the other compressor where the pressure of the gas increases to 6.5 kg/cm2. The gas is then fed to another filter, which removes the moisture in the gas.
From the filter, the gas is fed to the pressure swing adsorption unit. It consists of 4 cylindrical vessels in a bed of molecular sieves is placed. The coke oven gas is passed from the bottom of the bed and the molecular sieves absorb the hydrogen present in the gas. The hydrogen thus collected is fed to the makeup gas compressor. The gas is passed through one catalyst bed only. At this time, the remaining beds are in regeneration. This is because catalyst life is for 180 seconds only. Then it has to be regenerated. This is done by using pure H2 gas. The regeneration of the bed is then done automatically.
The H2 gas is collected from the top of the bed and is fed to the makeup gas compressor. This is a vertical reciprocating compressor of the double stage. The H2 gas is compressed to about 30 bars. The recycled gas from the gas separator is fed to the recycle gas compressor, which is a horizontal single-stage compressor.
This section consists of a stripping column in which the sulfur content as H2S and any dissolved gases in the CB is removed.
The liquid part from the separator is fed to the stripping column through a pre-heater, which is heated by BTX solvent from the stripping column. The feed at a temperature of 135оC is fed to the column. The column consists of sieve trays. The top temperature is 125-135оC and the bottom temperature is 150оC. Pressure is about 4.3 kg/cm2. A reboiler is provided which supplies the required heat to the column. MP steam is fed to the shell side of the reboiler. The gas from the column contains H2S. These are condensed in the condenser where water is used. This condensate (70оC) is fed to the reflux drum. Part of the condensate is refluxed to the column. Moisture present in the gas is removed from the water leg and the off-gasses are fed to the off-gas mains.
The bottom product called BTX solvent raffinate is passed through the preheater where it is cooled and finally, the raffinate is cooled in the raffinate cooler which is cooled by water. This is stored in the intermediate storage.