Removal of pollutants from oil refinery can be classified in three types of treatments:
The classification can also be made on the type of treatment that is:
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1. Physical,
2. Chemical and
3. Biological.
1. Physical Treatment:
Wastewater may contain coarse, suspended and floating solids, grease etc. These need to be removed before wastewater is subjected either to chemical or biological treatment.
Common unit operations of physical treatments are bar screens, grinders, grit chambers, grease traps, preparation, flocculation, sedimentation, floatation, chemical precipitation, sludge pumping. This treatment basically removes inert material which may hinder the subsequent treatments.
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i. Bar Screens:
The purpose of bar screen is to remove large floating matter as well as to avoid clogging of filter media and obstructing aeration in activated sludge. These can be hand cleaned or mechanically cleaned. For the hand cleaned screen the velocity of approach should be limited to about 0.456 m/s.
ii. Grit Chambers:
Grit chambers removes grit i.e., sand, gravel etc., which have specific gravities greater than the suspended organic matter. Purpose of providing grit is to protect mechanical equipment and avoid deposition in pipelines. Two types of grit chamber generally provided are horizontal flow grit chamber and aerated grit chamber.
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iii. Primary Sedimentation:
Purpose of this process is to reduce settleable suspended solid contents of wastewater. Here the waste-water is left undisturbed, the particles of higher specific gravity than wastewater will settle and those with lower specific gravity will float.
About 50-65% removal of suspended solids and 20-40% of BOD5 (at 20°C) can be achieved in a properly designed sedimentation tank. Common detention period is 90-150 minutes based on average rate of flow. Surface loading rates for untreated wastes are 20-40 m3 pd/m2 with a peak flow of 40 m3 per day.
iv. Oil Separation:
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Oil can be present as free or emulsified form or both, in the wastewater. It is necessary to remove free oil before breaking the oil water emulsion. Attempts are made to float the free oil in suitably designed tank and then it is skimmed by a rotating arm or disc. The rise velocity (v) of the oil globules is governed by Stake’s law.
API separator is generally used for such type of oil separations. These separators are normally operated without using any chemicals or coagulant or aids. Oil and grease separated from these units as stop oil are recovered and reused in the refinery.
Recently titled plate separators (TPS) or corrugated plate Separators are used to separate, the free oil from the waste-water. TPS gives better performance over gravity separators as it reduces the rising distance of the oil droplet before reaching the surface where it coalesce to form oil layer which rises to top from where it is skimmed off.
The gravity separators are essentially rectangular chambers equipped with oil skimming and sludge scrapping device and design of the separators is based on removal of oil globules of 0.015 cm in diameter and above. The efficiency of oil removal is around 98%.
An emulsion is defined as a mixture of two immiscible liquids, one of the liquids being dispersed throughout the other in the shape of very fine droplets. Both the oil-in-water and water-in-oil emulsions are present in a refinery wastewater.
The de-emulsifying agents which are normally adsorbed on the surface of the emulsified particles rendering the emulsion stable are soaps, sulphates, sulphonic and naphthenic acids, quartenary ammonium compounds, organic ethens and esters.
To remove emulsified oil Dissolved Air Floatation (DAF) technique is used. This technique can be used alone or in combination with flocculation. This combination can remove 97% of the oil, 75% of the suspended solids, and reduce both biochemical oxygen demand (BOD) and chemical oxygen demand (COD) by 80%. This reduction varies with the characteristics of the waste stream.
The basic mechanism by which an oil-in-water emulsion is broken involves neutralisation of the charges carried by the oil droplets by forming a floc that is initially precipitated with a charge opposite of that on the oil.
Adsorption of the oil by a flocs which initially has a high adsorptive capacity for the oil and entrapment of the oil as the floc forms and grown around the oil droplets.
The above objective can be achieved by adjusting the pH of the wastewater between 5 and 6, subsequently precipitating the emulsion stabilising agents like sulphonic and naphthenic acids as water insoluble calcium salts by the addition of hydrated lime until a pH of 7.5 to 8.5 reached.
Cationic polyelectrolytes are available for de-emulsification of oil. This has remarkable advantage over the conventional methods. Also polypropylene adsorbent can be utilised for de-emulsification.
This adsorbent is in a continuous belt form which is oleophillic (attracts oil) and also hydrophobic (water repellent). This when passed in the wastewater adsorbs oil on its surface.
Various coagulating agents like ferrous sulphate, ferric sulphate (chlorinated copper as) calcium chloride, calcium carbonate and hydrated lime have been used to de-emulsify an oil separator effluent.
One particular type of coagulant may be effective for the wastewater of a particular refinery but may, however be ineffective for the wastewater of another refinery.
The type and behaviour of the emulsifying agents would dictate the application of a particular coagulant best suited for the purpose. The flocs that are formed by such treatment adsorbs most of the oil and depending on the initial oil concentration, an effluent may be produced with an oil content of 10 mg/L or even less.
2. Chemical Methods:
After removal of grit and floating matter, suspended and dissolved organic matter are removed.
Important unit operations and processes involved in the chemical treatment of wastewater are:
i. Chemical Coagulation, Flocculation and Sedimentation:
Dissolved air floatation, both with and without flocculation, has been successfully used to obtain a low oil content in a refinery wastewater. Under favourable conditions it is possible to obtain an oil content of as low as 10 mg/l from a flotation cell using a coagulant and coagulant aid.
However, dissolved air flotation units require more skilled and regular supervision and power than gravity separators, hence their installations have been restricted to a few refineries in India.
Aluminium sulphate, lime, ferrous sulphate when used in the way they are used as in normal water clarification, on wastewaters, result in producing better effluent than plain sedimentation. With favourable conditions suspended matter can be reduced by about 90%, if the plant is operated skillfully.
Interest in chemical treatment of wastewaters has been revived because there is:
(i) Decrease in cost of chemicals.
(ii) Better understanding of mechanisms involved in coagulation.
(iii) Improvement in sludge handling processes.
Setting velocities of finely divided and colloidal particles are very low and hence require very long detention period. Also special efforts or mechanisms have to be provided so that these small particles aggregate together to settle under gravity at accelerated rates.
Aggregation or building up of flocs is induced by addition of chemical coagulants to decrease the zeta potential which stabilises the colloids. To encourage collisions between destabilised particles, agitation is necessary. Also coagulants have to be used between optimum pH ranges to achieve optimum efficiencies.
ii. Sulphide Precipitation:
Sodium hydroxide, when used to scrub cracked hydrocarbons for the removal of sulphur and phenol compounds, forms sodium sulphide and other products as per the following equation –
If the spent caustic is derived from caustic scrubbing cracked gasoline which has been debutanised, then it would contain very little sodium sulphide and the sulphur compounds will be mostly mercaptans and thiophenols.
In some refineries which discharge significant amount of spent caustic liquor containing very high sulphide concentration, chemical precipitation of sulphides by iron salts has been found to be effective and more economical than by neutralisation followed by steam or air stripping.
Sulphide precipitation is particularly to wastewater containing sulphides and mercaptans as the latter is also precipitated as iron sulphides. By using chlorine with ferrous sulphate, chlorinated copperas is formed which becomes more effective in the precipitation of the sodium sulphide as per the following equation –
The above reactions are carried out optimally in alkaline pH range.
iii. Filtration:
This removes finely divided suspended material. This may or may not be preceded by chemical coagulation.
iv. Air Stripping:
For removal of ammonia and hydrogen sulphide.
v. Ion-Exchange:
For the removal of phosphates, nitrogen and total dissolved solids.
vi. Reverse Osmosis:
For removal of organic and inorganic substances.
vii. Carbon Adsorption:
For reducing organic matter.
3. Biological Methods:
Biological treatment unit is primary meant for removal of pollutants like phenol, residual sulphide and BOD and also the non-recoverable oil present in the secondary effluent. Bacterial seeding and fertilisation of the oily waste with appropriate bacterial species will accelerate biological degradation provided that dissolved oxygen and sufficient time are available.
Approximately 66% of the hydrocarbon oxidising species are Pseudomonas species. Next in order of abundance are species of Mycobacteria, Proactinomyces, Zctinomyces, Yeast and moulds.
The BOD of mineral oil is relatively high as about 3-4 mg of oxygen are required for the oxidation of 1 mg of various kinds of hydrocarbons. Depending upon their chemical composition, the oxygen demand of various kinds of oils ranges between 3.1 to 3.5 mg/mg of oil.
These values may be compared with oxygen demand values of 1.07 for glucose, 1.18 for starch and cellulose, 1.5-1.8 for proteins and 2.5 to 2.9 for vegetable oils. It is found that the bacteria in biofilters and activated sludge to be upto 84% effective in removing oil from refinery waste.
Grease and oils are particularly resistant to anaerobic digestion and when present in sludge they cause excessive scum accumulation in digesters, clog the pores of filters and deter the use of filters and deter the use of sludge fertilisers. When these substances are discharged in wastewaters or treated effluent they often cause surface films and shoreline deposits.
A knowledge of the quantity of grease and oil present in a waste is helpful in overcoming difficulties in plant operation, in determining plant efficiencies and in controlling the subsequent discharge of these materials to receiving stream.
A knowledge of the amount of grease present in sludge can aid in the diagnosis of digestion and de-watering problems and indicate the suitability of a particular sludge for use as fertiliser.
Depending on the type of oxidising agent used, the biological methods which employ micro-organisms, is grouped under aerobic (when free molecular oxygen is present) or anaerobic (absence of free oxygen) methods of treatment.
The rate of decomposition by microbes is importantly a function of the oil water interface. The greater the oil water interfaces the faster the microbial decomposition rate. It is reported that microbial disposal of oil appeared to be feasible with specifically adopted cultures (about 51 to 61% of crude oil was utilised by a species of Pseudomonas over a period of 21 days).
Now-a-days a concept of engineering the species to utilise a particular waste has been developed. Also mixed cultures can be used to degrade a particular types of waste from oil refinery.
Aerobic Method of Treatment of Industrial Wastes:
The methods of biological treatment used in industrial waste treatment are essentially the same as those used in domestic sewage treatment.
The common methods used are:
(i) Trickling filters.
(ii) Activated sludge.
(iii) Oxidation ditch.
(iv) Aerated lagoons.
(v) Oxidation ponds.
(i) Trickling Filters:
A trickling filter is a device for bringing wastewater into contact with biological growths. These filters decrease the oil and phenol contents of the refinery waste.
(ii) Activated Sludge Process:
This technique is used for the large scale processes. Table 19.15 gives data of three refineries which have employed activated sludge process.
The advantages of activated sludge plant over trickling filters include:
(a) Small area requirement
(b) Freedom from odour and fly nuisance
(c) Small head requirement
(d) Lower cost of construction.
(iii) Oxidation Ponds:
This is a simple method for the microbiological disposal of oily wastes. In 30 days retention period the sulphides decreased from 15 to 0 mg/lit and phenols dropped from 20 to 7 mg/lit and reduction of oil was from 150 to 0 mg/lit. The retention period is particularly important for the effectiveness of oxidation ponds.
(iv) Aerated Lagoons:
It is generally an earthen basin having a depth of 2.4 to 3.0 metres of wastewater which is aerated either by diffused air or mechanical surface aerators. It acts as a settling cum aeration basin.
Anaerobic Lagoons:
Anaerobic lagoons for the treatment of wastes are designed on the principle of anaerobic digestion with the exception of not providing gas collection devices and are operated as open digesters.
The main advantages of anaerobic lagoons are:
(i) High organic loading rates can be applied on relatively small land area because of larger depth.
(ii) Low capital cost because only earthwork is needed.
(iii) Low running costs because of nutrient requirement is much less compared to aerobic treatment.
(iv) The problems of excess sludge disposal is also minimum.
Wet Air Oxidation of Waste Streams:
Wet air oxidation is a liquid phase oxidation of waste at high temperature and pressure. Wet air oxidation offers an opportunity to destruct toxic pollutants in wastewater from chemical, petrochemical, refinery dyestuff and other industries. This water then can be further treated conventionally and recycled back into the process.
The capital cost of wet air oxidation system depends on several factors such as capacity of the system, severity of the oxidation conditions to meet the treatment objectives and presence or absence of corrosive constituents in the waste. Hence, judicious choice needs to be made while selecting wet air oxidation in the overall water recycling scheme.
The search for new sources of water is never ending, particularly in developing countries, not only due to increasing population but also due to increasing demand of the industry.
Because of the steepness of the demand curves and the tendency of the steepness to increase, engineers everywhere have sought ways to recycle the water as much as possible. Thus, the need to improve the wastewater treatment and enhance the removal of priority pollutants has focused attention on the application of new processes like wet air oxidation.
Once the toxic pollutants from the chemical and petrochemical industry wastewater are oxidised and made amenable to biological treatment, normal water recycling treatments such as chlorination/dechlorination, clariflocculation, and/or membrane processes can be used, depending on the effluent characteristics.
Wet air oxidation involves oxidation of organics and some inorganics in the aqueous phase at high temperatures and pressures. The oxidation takes place in the temperature range of 100 to 350°C and pressures ranging from 10 to 300 atm.
The dissolved oxygen in the water reacts with the oxidisable substance at these high temperatures, to produce a wide range of oxygenated products and generating heat. The water moderates the oxidation reaction by acting as a diluent and by supplying oxidant at low concentration. Higher pressure is required to keep the water in liquid phase, at the high operating temperatures.
Few other names for this process are flameless combustion and oxidation, liquid phase oxidation and oxidative detoxification.
Process Description:
In this process, the wastewater which contains the oxidisable constituents is brought up to the system pressure using a high pressure pump. Compressed air or oxygen gas is introduced into the pressurised wastewater stream using a compressor.
The mixture of air and wastewater is heated in process heat exchanger by heat exchange with the oxidised hot effluent leaving the reactor. An external source of heat is used to initiate the wet air oxidation process or to sustain the oxidation temperature if insufficient heat of reaction is released in the wet air oxidation reaction.
After heating, the mixture of gas and wastewater flows into the reactor, where it is detained for a period of time, sufficient to complete the desired degree of oxidation. The reactor is a vertical bubble column pressure vessel, which is sized to provide the desired hydraulic residence time.
Wet air oxidation reactions are exothermic and raise the temperature. The hot oxidised effluent that flows from the reactor is used to recover heat by preheating the incoming mixture. An optional water cooler may be used to further cool the oxidised effluent.
After cooling, the oxidised effluent passes through a Pressure Control Valve (PCV) and is directed into an atmospheric separator where the non-condensable gases separate from the oxidised liquid phase.
Basic Principle and Mechanism:
Wet air oxidation involves hydrolysis (solubilisation of solids and break-down of long chain hydrocarbons) mass transfer of oxygen into solution and chemical oxidation.
The various steps involved in WAO process are discussed below:
(i) Hydrolysis:
Hydrolysis is a free radical reaction. It results in solubilisation of solids and splitting of long chain compounds. Typical reactions are –
The degree of hydrolysis depends on the residence time in the WAO reactor. This mechanism is controlled by the pH and temperature conditions prevailing in the reactor.
(ii) Mass Transfer:
This involves dissolution of oxygen from air into the aqueous phase and mass transfer of oxygen through the gas-liquid interface. This is controlled by the pressure and liquid-gas interface characteristics.
In determining the mass-transfer characteristics in a WAO reactor, it is assumed that:
(i) The transport of oxygen in the gas phase is much more rapid than in the liquid.
(ii) The liquid is well-mixed and the interface between the liquid and gas consists of a film through which the oxygen diffuses.
(ii) The reaction between oxygen and organics occurs only in the liquid phase.
The WAO process involves diffusional mass transfer accompanied by chemical reaction.
Depending on the relative rates of diffusion and reaction, four regimes are possible:
(a) Very slow reaction.
(b) Slow reaction.
(c) Fast reaction.
(d) Very fast reaction.
The first two regimes are of main importance to WAO. The mass flux of oxygen per unit volume through the interface is given by –
(iii) Chemical Kinetics:
The kinetics of the oxidation reactions occurring in the WAO process are controlled by the temperature, oxygen concentration and the presence or absence of a catalyst. The WAO reaction is first order with respect to both reactants i.e. the reaction rate is proportional to the concentration of the component to be oxidised and the oxygen concentration in the liquid.
Organic pollutants can be broadly classified with respect to their oxidisability, as follows:
(a) High Oxidisability –phenols, aldehydes, aromatic amines, thioalcohols, thioethers, milder conditions i.e. < 200°C & pressures of 15 to 25 kg/cm2
(b) Medium Oxidisability – alcohols, alkyl-substituted aromatics, nitro-substituted aromatics, unsaturated alkyl groups, carbohydrates, aliphatic ketones, acids, esters, and amines, intermediate conditions i.e. 200°C & pressures of 30 to 40 kg/cm2
(c) Low Oxidisability – halogenated hydrocarbons, saturated aliphatics, and benzene, severe conditions i.e. 280 to 300°C & pressures of 80 to 100 kg/cm2.
Each individual constituent will have an individual rate reaction-
WAO Process Parameters:
The critical operating parameters affecting the performance of wet air oxidation system are:
(i) Temperature – The rate of oxidation of substrate compounds present in hazardous waste increases with increasing operating temperature and results in higher overall COD reduction. Typical range for WAO reactor temperature is 150-340°C.
(ii) Reactor Residence Time – Decreasing the reactor residence time results in lower destruction efficiencies of substrate compounds and lower overall COD reduction. Typical range for reactor residence time is 0.5-2 hrs.
(iii) Off-Gas Oxygen Concentration – Wet air oxidation systems are rarely oxygen-transfer limited in the temperature range used for processing hazardous wastes. However, if the oxygen concentration in the off-gas is depleted, the system will be starved for oxygen. Again, lower destruction efficiencies for substrate compounds and lower COD reduction will result. Typical range for off-gas oxygen concentration is 2-18%.
(iv) Reactor Pressure – The wet air oxidation system pressure is chosen and maintained to limit the degree of evaporation of liquid water at the corresponding operating temperature. A small variation in system pressure will not generally affect the oxidation efficiencies in WAO. Typical range for pressure is 20 to 200 atm.
Reaction and End Products:
The products of wet air oxidation reaction depend on the constituents of the waste and the operating conditions of the reaction. Most organic compounds are stoichiometrically oxidised in this process. The organic carbon gets oxidised to carbon dioxide, the hydrogen is converted into water, any halogen into inorganic halides, any sulphur to soluble inorganic sulphate.
Any phosphorus into phosphate and any nitrogen to ammonia, nitrate or elemental nitrogen. At higher operating temperatures (above 260°C), where higher COD reductions are achieved, the remaining organic compounds in the oxidised solution comprise of low-molecular weight oxygenated compounds, predominantly carboxylic acids.
Halogenated aromatic compounds such as chlorobenzene, PCB’s etc., are the most resistant to wet air oxidation. Even at temperatures of 320°C, typical destruction achieved is around 70%. Most other organic compounds are destroyed in excess of 99%.
Typical reactions occurring in wet air oxidation can be generally divided into following types:
Advantages of WAO Process:
(i) Energy Efficiency:
Wet air oxidation does not require auxiliary fuel i.e. it proceeds auto-thermally, if the COD levels are above 15000 mg/l. This is because, in wet air oxidation, the only energy required is the difference between the enthalpies of incoming and outgoing streams.
While in incineration, the sensible heat, heat of vapourisation of liquid, as well as heat required to heat the water vapours, combustion products and excess air to combustion products and excess air to combustion temperatures in the range of 800 – 1000°C need to be provided. Thus autogenous incineration requires COD levels as high as 300,000 to 400,000 mg/l.
(ii) Toxic Wastes:
Wet air oxidation easily detoxifies wastewaters which are too toxic for bio-treatment e.g. stream containing cyanides, phenols etc. or having BOD/COD ratio less than 0.5. It can handle shock loads unlike biological processes.
(iii) No Air Pollution:
Wet air oxidation does not cause any air pollution, as the gaseous products contain mainly spent air and carbon dioxide. In wet air oxidation, sulphur or nitrogen oxides are not formed.
(iv) Non-Toxic Products:
The wet air oxidation effluent is readily bio-treatable as it converts non-biodegradable, toxic. High molecular weight oxygenated compounds, predominantly carboxylic acids which are non-toxic and easily biodegradable in subsequent steps. With progressive oxidation, the BOD/ COD ratio increases, indicating the reduction of toxicity to bio-treatment.
The Applications of Wet Air Oxidation:
(i) Thermal Sludge Conditioning:
Sewage sludge disposal poses a unique and difficult problem because of the high proportion of waste activated sludge from the secondary treatment of industrial as well as municipal waste water.
Sewage sludge is a complex mixture of waste solids forming a gelatinous mass which is nearly impossible to dewater without further treatment. The organic fraction of the sludge, largely of biological origin, consists of lipids, proteins and carbohydrates, all bound by physico-chemical forces in predominantly water gel-like structure. The ‘bound water’ content of sewage sludge is 90-98%.
When sludge is heated under pressure at temperatures of 175-200°C in a wet air oxidation system, the gel like structure of the sludge is destroyed, liberating the bound water. Dewatering by filtration, centrifugation or drainage beds without chemical conditioning is then possible. The residue is sterile and suitable for land-fill or incineration.
The mixing of air with sludge in WAO system provides oxidation compounds, thereby giving odour reduction. Thermally conditioned sludge cake possesses a heat value equivalent to low grade fuel – about 6600-8325 kcal/kg of volatile solids – and can be combusted without auxiliary fuel after start-up.
(ii) Activated Carbon Regeneration:
Generally, the treated effluent from the primary or secondary treatment plant cannot be recycled directly, due to the presence of organic contaminants. This water can be further purified by addition of chemicals or polyelectrolyte and powdered activated carbon is added.
Chemical treatment followed by settling provides simple and effective precipitation of phosphorus and removal of suspended and colloidal matter. The resulting dissolving organic material is adsorbed onto powdered activated carbon resulting in high quality water which can be recycled.
The use of powdered carbon has been limited in the past due to difficulties in economically and efficiently regenerating the spent carbon. Wet air oxidation can be used here to regenerate the carbon, since only the adsorbed organics get selectively oxidised without destroying the carbon. In this process, the spent carbon is withdrawn from the wastewater contact system and concentrated by gravity thickening.
Thickened carbon slurry is pressurised to system pressure, mixed with compressed air and heated to reaction temperature before introducing in WAO reactor where selective oxidation of organics takes place. Typically this regeneration is carried out at 200-245°C.
i. Materials of Construction:
Depending on the effluent nature, normal materials of construction like carbon steel, stainless steel etc. can be used. However, if the effluent contains corrosion inducing constituents like chlorides in high concentration, then costlier materials of construction like titanium, alloy 20 are used.
Thus the nature of inorganic constituents and the severity of oxidation conditions have a significant bearing on the capital cost involved. It is assumed that the wet oxidation unit would operate at maximum conditions of 280°C and 135 atm, and would accomplish a 50 g/l COD reduction.
ii. Operating Cost:
The major sources of energy consumption in wet air oxidation system are electrical energy consumption for air compression and high-pressure liquid pumping and preheating of feed waste water. The energy required for preheating depends on the flow-scheme chosen and the amount of oxidation occurring.
The electrical power for air compression corresponds to about 0.136-0.167 kW per kg of oxygen consumed per second. Wet air oxidation systems consume approximately the amount of oxygen equivalent to the oxygen demand reduction for the treated wastewater. The electrical power for high pressure liquid pumping is generally about ten per cent of that for the air compression.
The energy required for preheating is inversely proportional to the amount of oxidation occurring. The heat required for preheating is less if more heat is released in oxidation. No external energy is required if the oxidation occurring corresponds to 15-20 g/l of oxygen demand reduction. If the oxidation corresponds to less than 15-20 g/l oxygen demand reduction, auxiliary energy addition would be necessary.
When a process heat exchanger is used, maximum auxiliary energy consumption is in the 139-167 MJ/m3 range. When a process heat exchanger is not used i.e. all preheating is provided by external means, the energy requirement for preheating can range from nil for highly concentrated wastes to about 1115 MJ/m3 for very dilute wastewaters.
The operating costs are made on the basis that wet air oxidation unit would operate at maximum operating conditions of 280°C and 135 atm. and would accomplish a 50 g/l COD reduction. The utilities shown in this figure are limited to cooling water and electric power, since for most applications of wet air oxidation, operation will be auto-thermal.
In conclusion, wet air oxidation offers a bright possibility of treating toxic/primary pollutants, to make the effluent amenable for recycling. However, the overall cost is significantly affected by the presence of corrosive constituents and hence judicious choice will have to be made to see the overall impact on the project cost involved in recycling.