1) Ash Slurry treatment Scheme
The ash slurry from steam generation plant is pumped by the existing sat of pumps to the proposed ash ponds. The ash is settled in the ponds by gravity settling. The clear overflow from the ponds is discharged into sump from where the clear water is pumped to steam generation plant for de-ashing purpose or can be drained to the river sump directly.
2) Oily Effluent Treatment Scheme
Two nos. of oil separators are provided to receive oil effluent. Each separator has about 75 m3 capacity and both put together can receive 150m3 of effluent per day. Oil
separators are to be operated alternately. They are building with stilling, inlet chamber, and deoiler pipe, baffle for retaining the floating oil and overflow weir. Oil separates and floats to the surface by gravity separation.
3) Reuse of Process condensate as boiler feed water make up
In ammonia plant, the main source of liquid effluents is process condensate. It is well known that steam is used in excess of stoichioimetric requirements for reforming and shift conversion in ammonia plant. This excess steam is subsequently removed as process condensate which contains impurities like carbon dioxide, ammonia, methanol etc. This condensate cannot be directly used anywhere due to the presence of impurities.
4) Non-Chromate based cooling water treatment
Cooling water treatment is of vital importance for all the chemical process industry. When the original plant was commissioned in 1974, chromate based cooling water treatment was adopted. About 15-20 ppm chromate was maintained in the circulating cooling water. The chromate is toxic and harmful to the environment. This hexavalent chromate was converted into trivalent chromate and separated out as sludge.
To overcome chromate sludge storage problem and to adopt environmental friendly system, non-chromate based cooling water treatment was adopted in Apirl,1998.
Non-chromate treatment programme are mainly based on o-phosphate and zinc with different combinations. o- phosphate concentration of 6 ppm minimum and Zincconcentration of 1 ppm maximum is maintained as corrosion inhibitor. Polymer based bio-dispersant and boicides are also added.
Three different biocides are shock dosed alternatively in addition to chlorination for micro biological growth control as per requirement. Dispersants are added to keep the salts of circulating water in dispersion condition. All the chemicals used for non chromate treatment are biodegradable chemicals.
Overhead vapours about 6000 kg/hr from condensate stripper were vented to atmosphere. the overhead vapours contain ammonia, methanol, amines and CO2. Ammonia present in the overhead vapours is about 5000 ppm. The limit specified by Gujarat Pollution Control Board (GPCB) for ammonia is 100 ppm. To avoid the venting of ammonia to atmosphere,
the overhead vapours from the top of the condensate stripper are diverted to the vent condenser (172-C). All the vapours are condensed and non-condensables mainly CO2 is vented to atmosphere. The condensate from the vent condenser can either be sent to Ammonia recovery and stripping system in ammonia plant or to hydrolyser stripper system in urea plant by condensate transfer pump. The system is shown as annexure - VI. The system has reduced the 720 kg/d ammonia to atmosphere and pollution norms has been also met.
6) Biological Treatment of Effluent
After giving treatment, the effluent is further bio-degraded in the effluent ponds having a holding capacity of 1, 00,000 m3 before final discharge to the river Damodar.
Factory effluent mixed with sanitary waste methanol water and is treated with the help of bacteria. The bacterial actions consist of hydrolysis, nitrification and denitrification. HYDROLYSIS
In the process of metabolism through heterotrophic bacteria.complex carbohydrates, proteins and insoluble fats present in sanitary waste are hydrolyzed into soluble sugar, amino acids and fatty acids.
Hydrolysis of urea takes place and converts organic nitrogen to ammonium carbonate. (NH2)2C03 +2H2O (NH4)2 C03
Biochemical oxygen demand (BOD) is reduced by hydrolysis under aerobic condition in presence of baceotrophic bacteria.
NITRIFICATION
Ammonical nitrogen is converted to nitrites and then to nitrate form in a presence of autotrophic bacteria under aerobic condition.
N02- + 1/2 O2 - N03-
NH4+ +2O2 - NO3H + H2O
DENTRIFICATION:
Heterotrophic bacteria in anaerobic condition reduce nitrite (N02) and Nitrate (N03) to
gaseous N2. An organic carbon, as methanol, acetic acid, acetone, ethanol or sugar is
reduced to act as a hydrogen donor to supply carbon for biological synthesis. The required BOD is supplied by waste methanol (6000 ppm) which serves as an energy source for survival and growth of bacteria.
First methanol reduces:
3O2 + 2 CH3OH - 2 CO2 + H2O
Then bacterial reduction of N02 & N03 takes place.
6 NO3- + 5 CH3OH - 3 N2 + 5CO2 + 7 H2O + 6OH-
2 NO2- + CH3OH – N2 + CO + H 2O+ 2 OH-
Aeration is helpful for the removal of gaseous product and increase dissolved oxygen in the effluent. In the step of hydrolysis urea (500 ppm) degrades to 50 ppm level.
Instrumentation
The even operation of a process is dependant upon the control of process variables. These are defined as the conditions in the process material and equipments which are
subjected to change. There may be several materials and several pertinent operating factors which may change in the simplest of process; the maintenance of control over an entire process is an important aspect of process design. In Ammonia manufacture temperature and pressure are two process variables on which operation of whole plant
depends upon. Instrumentation of each equipment is done separately. 1. REACTOR
Feed to the converter is at 750C which is made to pass through the internal heat
exchangers so that it may gain heat from outgoing gases to attain reaction initiation temp. This temperature of gas at the inlet of catalyst bed is taken into account by TIC (temp. indicator controller) put in line with the start up heater.
In case temperature goes below the reaction initiation temperature.
Then automatically start up heater works and temperature of product gas is increased which in turn heats the incoming gases to the required reaction initiation temperature. But if the temperature goes high then a portion of by pass feed controls the temperature. Temperature control is necessary because in first case, reaction would not start till the required temperature is attained. And in the later case, temperature goes too high then catalyst may get spoiled. Pressure inside the reactor is 300 atm. and any change in pressure will effect the conversion therefore pressure control is also necessary so we will have to employ pressure gauges and pressure controller valves.
Temperature indicator devices are put on both inlet and outlet streams and if temperature of outgoing gases goes below 162°C the water flow in the heat exchanger (waste heat boiler) is reduced with the help of a hand controller valve.
2. AMMONIA COOLED CONDENSER
A level indicator controller is installed in the ammonia refrigeration loop so as to maintain a constant level which is necessary to get required refrigeration to bring down the temperature of gas to -S °C where required condensation of ammonia is acting.
3. AMMONIA SEPARATOR
Here also a level indicator controller is required to maintain a particular level of liquid ammonia in it so no gas but only liquid product is obtained.
4. AMMONIA STORAGE
Ammonia is stored at a pressure of 40 psig and temperature 25.80 F. However, efficient
are formed. Pressure inside the tank increases so there must be some arrangement to reduce this pressure. A safety valve may be provided at the top of 'Horton Sphere'. Whenever pressure inside the vessel rises some of the vapors are vented. These vented gases either released at. 6 ft. high stack or can be compressed and condensed in a close loop to reduce atmospheric pollution. In a closed loop system vent are stored in a small vessel from where those gases are compressed, condensed and then fed to the storage tank.