A plant originally built for Hydro Chemicals at Immingham UK to process weak Nitric acid at 60% by weight up to 98.5% by weight. Built in 1992 it was then sold to Nobel Enterprises and moved to Scotland, refurbished and rebuilt. Since then the plant has seen limited use and is in excellent condition. Plant capacity dependent on feed stocks concentration and make up.
The Process
Nitric Acid Concentration Unit
The design is based on the well known process of ex¬tractive distillation of nitric acid with concentrated sulphuric acid which follows the laws of the ternary system
H2S04 - HNO3 - H2O.
In this system the three components remain in the liquid state while in the vapour phase the two compo¬nents HNO3 and H2O are received under working condi¬tions.
In the concentration column the following process steps take place simultaneously:
Nitric acid evaporation while at the same time part of the uprising HNO3/ H2O vapours get condensed.
down passing liquid is heated up by counter-current flow of partly condensing live steam
live steam also strips dissolved NO/N02 out of the down passing sulphuric acid
condensing live steam dilutes the sulphuric acid stream liberating heat of dilution
The quantity of live steam has to be balanced with the liquid fed at the top of the concentration column to obtain the desired vapour composition resp. the liquid concentration required at the bottom.
Vapours leaving the column are condensed and bleached~ The product is delivered to the storage tank after cooling, while the non condensable elements are exhausted into absorption unit.
Sulphuric Acid Re-concentration unit
The design uses a forced circulation evaporator system under vacuum. This process developed by SCHOTT ENGINEERING offers superior technical and economical operation conditions.
Compared to competitive design the SCHOTT ENGINEERING devel¬oped concentration units feature the following advantages:
no vaporization on the heating surface, thus little of any fouling or incrustation at the heater
high heat transfer coefficient by high liquid velocity, thus small heating area even at relatively small tem¬perature difference between acid and steam
high evaporation performance is maintained at any vacuum
the relatively small hold up of acid ensures safe oper¬ation and quick adaption to changing demand in through¬put ranging from 0 - 100 '% the plant can be shut down and readily restarted in short time
use of most corrosion resistant and proved materials ensures long service life and lowest maintenance cost.
NOx-Absorption
The nitrous gases, coming from the bleaching system have to be absorbed.
The following reactions• take place
Nitric acid is obtained by absorption of nitrogen dioxide in water or in dilute nitric acid:
3 NO2 + H2O = 2 HNO3 + NO
The nitrogen monoxide formed in this reaction has to be oxidized in the same way as the nitrogen monoxide which is already present in the exhaust gas:
2 NO + O2 = 2 NO2
The flow of both reactions is exothermic and depends on pressure and temperature conditions. By using water a 50% nitric acid will be obtained under pressure.
The existing exhaust gas contains 200 ppm NOx
Process Description acc. to Flow Sheet J 875-110 and P+I-Diagram J 875-120, 121
The plant is designed to produce high concentrated nitric acid out of weak nitric acid and can be divided into three sections
- Nitric Acid Concentration
- Sulphuric Acid Re-concentration
- Absorption Unit
Nitric Acid Concentration unit
Weak nitric acid of about 60% , is fed into a scrubbing column 32-SB-01 where nitrous fumes coming from the bleaching column will be washed and liberated from residual nitric vapours.
The feed stream of weak nitric acid is preheated in heat exchangers 32-HE-02.1/2 before entering the con¬centration tower 32-CL-01. To achieve equilibrium in the mass balance sulphuric acid of 86% , strength is fed at the column head. Heat of reaction, live steam and indirect heating cause the nitric acid to evaporate.
Live steam injected at the bottom and steam caused by vaporisation in reboiler 32-HE-04 strips off all vola¬tile components from the down passing sulphuric acid which leaves the tower diluted, is pre-concentrated in 32-HE-04 and is collected in intermediate storage 32- TA-08~
The nitric acid enriched vapours leaving at the top of column 32-CL-01 are condensed in 32-CO-01 and 32-WE-09. From here the condensate flows into bleacher 32-CL-02 where the dissolved oxides of nitrogen are stripped out by nitric fumes and air. By that the level of NOx can be reduced to 0,1%
The overhead fumes from the bleacher containing resi¬dual nitric vapours are cooled in 32-HE-09 and pass to a NOx absorber via scrubber 32-5B-01.
The 98.5% nitric acid product is cooled down in 32-HE- 05 and 32-HE-06 heat exchangers and discharged to stor¬age.
Sulphuric Acid Reconcentration
The diluted hot sulphuric acid coming from tank 32-TA¬-08 is directly pumped into the concentration stage where re-concentrated acid is circulated by means of pump 32-PU-12.
In heater 32-HE-07 the acid is heated some degrees above its boiling temperature. Due to the static liquid head evaporation does not occur in the heater but when the liquid enters the flash chamber 32-TA-10.
The vapour piping is a special layout and incorporates several deflections to separate entrained droplets from the vapours. The deposited droplets run back into the circuit.
The vapours are condensed in 32-CD-02 and the distil¬late is discharged through a barometric leg 32-TA-12. A liquid ring pump 32-CO-01 maintains the required vacuum.
Some condensate is re-circulated by distillate pump 32- PU-13 to saturate the vapours before entering the condenser
The produced concentrated sulphuric acid is obtained through an overflow from separator 32-TA-11 and flows into intermediate storage 32-TA-09 after it has been pre-cooled in the cooler 32-HE-08.
From tank 32-TA-09 the concentrated acid is delivered back to the concentration column after it has been cooled to operating temperatures in pre-heater 32-HE- 02.1/2 and cooler 32-HE-11.
NOx-Absorber
The plant is intended to absorb NOx out of exhaust gas coming from the concentration plant.
Product obtained will be nitric acid at a strength of 50% by wt.
By means of compressor 32-CO-02 the off-gases are com¬pressed and in the absorption column. The NOx vapours are oxidized and absorbed. The required air is given to the bleaching column. Make up water is introduced at the top counter of the gas flow.
Nitric acid, as product, flows to the compressor 32-CO- 02 via the receiver 32-TA-14 and is used as liquid ring fluid there. The excess acid is given back to the scrubber 32-SB-01.
Heat developed during absorption is removed by cooling coils in the absorption tower.
The existing exhaust gas contains approx~ 200 ppm NOx
Process Data
Plant Capacity: Concentrated Nitric Acid 98.5% 75.0 t/d (as 100%)
Composition of feed: HNO3 - 60% wt
H2O - 40% wt
5221 kg/hr
Composition of product: HNO3 - 98.5% wt
H2O - 1.5% wt
3172 kg/hr
Concentration column
Feed: 5269 kg/hr HNO3 - 60%, 75ºC
10097 kg/hr H2SO4 - 86%, 60ºC
Live Steam: 175 kg/hr
Product: 3190 kg/hr HNO3 - vapour - 98.5%
11 kg/hr NOx
12324 kg/hr H2SO4 - 70.5%,
HNO3 - content 0.06%,
Bleaching Column
Feed: 3190 kg/hr HNO3 - vapour - 98.5%
11 kg/hr NOx
Bleaching Air: 40 Nm³/hr
Product: 3172 kg/hr HNO3 - 98.5%
HNO2 content 0.1%
Absorption Column
Feed: 11 kg/hr NOx - gas
17 kg/hr Process water
31 kg/hr air
Product: 31 kg/hr HNO3 - 50%
Exhaust Air: 29 kg/hr air + 200 ppm NOx
Re-concentration Unit
Feed; 12311 kg/hr H2SO4 - 70.5%
Product: 10085 kg/hr H2SO4 - 86%
Vapour: 2238 kg/hr H2O - 99%
HNO3 + H2SO4 content < 1%
Utilities
Saturated steam: 18 bar, 5438 kg/hr
Cooling water: 3 bar, 27ºC 275 m³/hr hardness as CaO: 100 ppm
Cold Water: 3 bar, 15⁰C 20 m³/hr.
Instrument Air: 6 bar, max. 45⁰C, oil and dust free, dry condition, dewpoint - 30ºC 12Nm³/hr.
Electric Power: 415 V, 50 Hz 48.8 kW
Equipment Sourcing
Capacity: 18000 ton/Yr [convert]
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