Process gas specification and operating condition of the drier bed :-
C) Catalyst Heating
2.8.8 STEAM GENERATION SYSTEM:
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PART-C
Flue gases of the reformer furnaces 15F-1, 15F-2 & 15F-3 exit at a temp of 615 Considering the immense heat recovery potential of these flue gases, a steam generation facility has been laid. Steam is generated at M.P. level. Originally HP steam was generated from the system to cater to the needs of the Ebara compressor that consumes HP steam in its turbine and exhausts LP steam. Later the steam generation system was modified to generate MP steam and the HP steam requirement of the RGC compressor is met from the better quality HP steam from TPS header.
The BFW is supplied to CRU from TPS .The BFW supplied & MP steam generated shall meet following specifications:
STREAM B.F.W. STEAM
TDS ,ppm <1
Total hardness (max) Not detectable
Silica as SiO2 < 0.02 mg/l 0.02 ppm (max) Dissolved Oxygen < 0.005 mg/l
Dryness >99.9%
Na + K 0.01 ppm (max)
Fe 0.02 ppm (max)
Fe + Cu < 0.01 mg/l
Cu < 0.005 mg/l
Cl ,ppm (max) nil
Conductivity, umho/cm 0.2 @ 20 C <1
pH 7-10 >8
Oil , ppm (max) Not detectable
The BFW is made available the CRU B/L from TPS at 106 kg/cm2 pr. & 125 C.
The steam will exit the desuperheater at 16 kg/cm2 g and 2500 C.
The BFW is dosed with TSP @ 1 ppm in steam drum. Blow down rate is kept at 3% of the steam generation rate, which is 16740 kg/hr normal.
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to the steam drum (15V-16) through 15 EE-191 (coil submerged in the steam drum) & then BFW preheater furnace coils 15 F-8.The steam drum level control is through a separate 2" line bypassing the above flow circuit & BFW comes directly through LIC-1901. BFW temp at B/L is 1200 C, at 15 EE-191 O/L is 1350 C & at 15 F-8 O/L is 2330 C. The steam drum temp is 2820 C.
BFW circulating pump 15 P-15 A/B takes suction from steam drum (15-V-16) &
its discharge is passed through two steam generation coils 15 F-5, 15 F-7 through FIC 1902 & FIC 1903 respectively. Water vaporisation occurs in the coils as a result of the heat pick-up by the circulating liquid, which then flashes in the steam drum. The generated steam leaves the drum under pressure control via PIC-1901 to the super heater coil 15 F-6.
The superheated steam temp control is done in the de-superheater 15-X-191 by BFW injection in the steam via TIC-1909.
The BFW coil, steam generation coils & the superheater coil cannot withstand dry run conditions. To avoid any chance of the dry running of the coils, the stand-by BFW circulation pump will take start in case of low flow detection (below 50 m3 /hr) through any of the two steam generation coils. The fuel gas to the reformer furnace will be cut-off in case the low flow condition persists for more than 30 seconds.
Trisodium Phosphate is continuously injected in the steam drum directly. TSP solution is stored in the plant in 15-V-17 as 10 % by wt. solution .15-V-17 tank is equipped with mixer 15-MX-1 for preparing homogeneous solutions. TSP is dosed @ 1ppm in steam drum by reciprocating pumps 15-P-16 A/B. Normal dosing rate is 0.2 LPH.
Steam drum blow down rate is kept at 3% of the steam generation rate. The blow down is made to the blow down vessel 15-V-15 from which it is let out after flashing out & cooling by mixing with service water.
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PART-C
2.8.9 PROCESS VARIABLES:
The main process variables in the catalytic reforming unit, which can be played upon to achieve desired results, are:
1)Pressure: All the hydrogen producing reactions i.e. dehydrogenation, dehydrocyclisation are enhanced by low pressure. The lower the pressure, the higher the yield of both reformate and hydrogen for a given octane no.
This is the reason for minimizing the unit pressure drop for having lower average reactor pressure and operating at lowest possible pressure. Low pressure however increases coke make. As the operating pressure has to be set within the equipment design pressure and the recycle gas compressor design power and design intake volume, there is little flexibility for varying the operating pressure.
2)Temperature: The reforming reaction rates are directly related to reactor temperature. Just by raising or lowering the temperatures, the reformate octane number can be raised or lowered. Thus Reactors inlet temperature is the single most direct variable available for adjusting the unit performance.
The most representative parameter used for the reactor inlet temperatures is Weighted Average Inlet Temperature (WAIT). An increase in WAIT results in increased dehydrocyclisation of paraffins to aromatics, increased cracking and coking, consequently causing increased octane no., decreased reformate yield, decreased H2 purity, increased coke deposit.
3) Space Velocity: Weight hourly Space Velocity (WHSV) is the amount of liquid (expressed in weight) which is processed per hour divided by the amount of catalyst (in weight). The inverse of the space velocity is directly related to the residence time in the reactors. The lower the Space velocity, the higher is the contact time, higher is the severity. The lowering of the space velocity has the same effects as increasing the reactor inlet
4) Hydrogen to Hydrocarbon ratio: The H2/HC ratio is the ratio of pure H2 in
reactors at a faster rate and supplies a greater heat sink for the endothermic heat of reaction. Also the hydrogen partial pressure is linked to the H2/HC ratio.
5) A lower H2/Hc ratio decreases the H2 partial pressure and increases coke formation. The H2 partial pr. is mainly adjusted through recycle flow because there is very little flexibility in varying total pressure. Within typical operating range, the H2/HC ratio has little influence on product quality or yields. For a given unit, it is set by design based on economic considerations.
6) Quality of feed: The quality of reformer feed, once pretreated, is mainly