The following article will guide you about how to dispose and control wastes from refinery.
Disposal of Solid Wastes from Refinery:
The solid wastes in the refinery are naturally obtained in the form of sludges containing solids, water or oil which pose problems in its treatment and disposal. The treatment disposal method depends on the type of sludge and its constituents.
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Disposal of water treatment sludge – This type of sludge is not allowed to flow in oily water drain. It is usually dewatered by gravity in sludge thickeners or ponds. Ultimate disposal is on land.
The possibilities of its reuse are as follows:
(i) Sludge may be acid treated to reduce volume requiring final disposal and to recover the flocculants.
(ii) Sludge may be utilised to neutralise certain wastewaters and in some cases can be dissolved in the water supply to combat corrosion. They are sometimes used as weighing agent in wastewater flocculation.
(iii) Sludges may be used in cement manufacture.
Sludges Containing Oil and Solids:
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The most oily sludges that are produced in refinery treatment process are:
(i) Spent contact clays
(ii) Percolation clays from treatment of lubricating oil.
Dewatering and de-oiling of sludge can be carried out by gravity settling, centrifugation and Alteration. The concentrate so obtained is disposed off by land filling, incineration or other suitable methods.
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1. Biological Decomposition of Solid Wastes:
This is used when sufficient land area is available where waste can be spread over. Bacteria decompose organic matter. The area of waste depends on the quantity of waste and rate of decomposition. The decomposition rate depends on composition of waste and atmospheric temperature.
Digestion of the sludge was normally done in the absence of air (anaerobic), the aerobic digestion of biological sludge has recently been adopted. Sludge can be treated in mechanical aeration tank and the extent of treatment possible is related to biodegradable fraction of biological sludge present.
Incineration – Incineration is generally applicable where sufficient land for disposal of raw waste is not available. The main advantage is reduction of waste containing organic material to inert material. It has high initial as well as operating cost.
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Tank Bottom Sludges – Sludges from the bottom of crude oil storage tanks are having very high contents of wax, water and oil. A process has recently been developed to extract the useful micro-crystalline wax which has a very high melting point.
The acidic sludges which are generated in this process is neutralised and used for grease making or incineration and the spent clay can be regenerated and reused.
2. Solvents and Effluents:
Refineries generally use solvents like Toluene, Methyl Ethyl Ketone, Benzene for removing waxes and furfural for the removal of aromatics during the manufacture of Lubricating Oil. During the operation with these solvents starting from receiving in the refineries to final operation there is every possibility for the loss of these through spillage, solvent extraction and maintaining the proper ratio.
The solvents lost by these processes ultimately accumulate in the effluent water system through surface drains with washout waters. Removal of these solvents contamination from the effluent water is a typical specialised job.
Solvent contaminated water streams from the units in the refineries get diluted in the total influent water which undergoes the various treatments. The physical treatment removes free oils and suspended solids and chemical treatment removes emulsified oils, sulphides, etc. while the biological treatment removes the dissolved degradable hydrocarbons. So there is every possibility that the solvents remain unremoved in the final effluent water unless special treatment procedures are followed.
On the contrary the chemicals like furfural are harmful for the biological treatment through trickling filter and activated sludge because the micro-organisms responsible for biodegradation get damaged through furfural. Quality of effluent water from petroleum refineries is given in Table 19.13.
Control of Effluents in Refinery:
1. Air Pollution Control:
For the recovery of SO2 and H2S, etc. refineries have Sulphur Recovery Unit (SRU) and monoethanol amine treating unit (MEA unit).
i. Sulphur Recovery Unit (SRU):
This unit is designed to recover elemental sulphur from H2S. The process involved in this recovery is ‘Clause process’, and ARI Process. The clause unit consists of thermal stage where hydrogen sulphide (H2S) in the feed gas is partially burnt with air to sulphur dioxide (SO2).
The resulting mixture of H2S and SO2 reacts to form elemental sulphur and water vapour. There are two catalytic stages to achieve the recovery of 94% of sulphur in the feed gas.
The general reaction can be given as:
H2S + 1/2 O2 → H2O + 1/n Sn
Where n is number of sulphur molecules.
In ARI process H2S is oxidised to elemental sulphur in presence of circulating catalyst, which is regenerated using air. This process has sulphur removal efficiency over 99.9% and can operate at ambient temperature. Typical stack emission data from each unit of refinery is shown in Table 19.14.
ii. Mono-Ethanol Amine Treating Unit:
This unit can be a part of the sulphur recovery unit or it may be a separate unit. This unit is also designed to remove H2S from the sour gases obtained from hydro finer or other units, using an aqueous solution of Mono-ethanol Amine (MEA) as absorbing medium.
The unit consists of two sections namely – the absorption section and the regenerating sections. The sour gases are scrubbed of H2S with MEA solution in the absorption section.
The regeneration section strips off H2S gas absorbed in MEA solution and this regenerated solution is re-circulated to the absorber in the absorption section. The H2S rich gas is then sent to sulphur recovery unit. Provisions are made available in the refineries to divert these gases to the flare also.
iii. Flare Stack:
The function of the flare stack is to burn the accidental hydrocarbon release from different units. This system has a knockout drum along with the water seal drum. The liquid hydrocarbons released from the units are collected in the knock out drum and is pumped back. Vapours that escape are burnt at the top of the flare stack.
iv. Gas Bottling Plant:
The LPG cylinders which are used for the domestic as well industrial purpose are filled in the gas bottling plant in the refineries. The general emissions in this place are mercaptans and hydrocarbons.
The usual norms followed in a refinery to avoid pollution in these areas are good housekeeping and ventilation. Hydrocarbon detectors are also placed in this area to detect any leaks as they may lead to Vapour Cloud Explosion (VCE) if an ignition source is present. The bottling area has all over sprinkler which can act as a water curtain in case of disastrous leak. All the filled cylinders are passed through a water bath for detecting leaks.
v. Efficient Absorption of SO2:
The need of the hour is to devise efficient methods for SO2 pollution abatement. So far many processes have been developed, particularly in the west and a few of them have been commercialised.
In SO2 removal processes, flue gases from stacks are subjected to physical and chemical treatments. Depending upon the nature of the techniques used, the processes may be broadly classified into wet and dry processes.
The wet methods make use of slurry or a solution of absorbent in water for absorption of SO2, while the dry systems make use of dry particles of absorbents in fluidised bed, fixed bed or moving bed type reactors.
In both these processes a further classification has been made depending on the reuse of the absorbent on regeneration or simply it’s throwing away.
In the throwaway process cheap absorbent is used so that it can be disposed of as a less harmful waste. Limestone, because of its low cost and easy availability in each continent, has gained popularity as the absorbent of choice.
In the regeneration/recovery process a separate step is introduced to convert the spent absorbent back into usable form. The sulphur recovered in this regeneration process adds to the economics of the process. In the present article we have considered dry recovery processes only.
The conventional dry absorption processes have been popular because of their apparent advantage of stack-gas treatment at elevated temperatures thereby sustaining thermal buoyancy of the flue gases.
So far only a few inorganic oxides like copper oxides, manganese oxide have found application in dry recovery process. In the Shell Flue Gas Desulphurisation (SFGD) process, copper oxide deposited on alumina support is used as the absorbent.
CuO + SO2 + 1/2 O2 → CuSO4.
Regeneration is carried out using reducing gases like H2, CO, or methane and results in sulphur rich gas from which either sulphur or H2SO4 can be produced. The need for a large amount of reducing gas for sulphur recovery has made the process uneconomical.
Another dry absorption process developed by Mitsubishi Heavy Industries, Japan, employs precipitated manganese dioxide for SO2 capture.
MnO2 + SO2 → MnSO4
High cost of sorbent and consequent complex regeneration steps are the major drawbacks of the process.
2. Novel System for SO2 Absorption and Regeneration:
The need of the hour is an efficient solid system for SO2 capture. Realising this necessity we have studied SO2 capture by two solid systems, which incorporate the advantages of inert solids.
In industrial practice solid pellets need to be prepared using inerts along with active sorbents and suitable binder followed by processing involving special techniques.
To overcome this drawback we have picked up red mud which is a solid waste from Bayer’s Alumina process. The red mud contains considerable amounts of iron oxide (41% Fe2O3) and rests are inert solid components, mainly consisting of Al2O3, TiO2 and SiO2.
Thus the red mud serves as an easily available cheap sorbent for SO2, with advantages of avoiding incorporation of inerts in active Fe2O3. Iron oxide belongs to the class of regenerable solid sorbents.
It captures SO2 at 300-550°C according to the following reaction –
Initially 12 mm dia cylindrical pellets made up of pure iron oxide with porosity of 0.44 were subjected to SO2 capture in a single pellet reactor. The maximum conversion reached was less than 10%.
On the other hand with the same porosity pellets consisting of red mud, the conversion of Fe2O3 was more than 50% in the same time and temperature ranges.
In order to examine the practical suitability of red mud in SO2 capture and regeneration, five sets of SO2 capture and regeneration runs were carried out—capture runs at 440°C and regeneration runs at 700°C.
SO2 capture and regeneration cycles were of 60 minutes and 45 minutes duration, respectively. The SO2 capture rate expressed in terms of fractional conversion of the initial oxide present. It is observed that SO2 capture efficiency is quite attractive even after fifth cycle and regeneration efficiency is quite high in all the cycles.
The fact that a good efficiency was maintained for 5 cycles and more is further confirmed by the marginal decrease of effective diffusivity De from 2.56 × 10–2 cm2/second for the first cycle to 2.35 × 10–2 cm2/second for the fifth cycle. This is a sure sign of effective use of red mud for a few cycles more beyond the five cycles studied here.
Special Behaviour of an Efficient and Cheap Sorbent for SO2:
An interesting solid system for SO2 capture discovered in the laboratory is the mineral red ochre. Here the active component is iron oxide (≅ 14%) and rest are inerts like oxides of Si, Al and Ti. The mineral in the original form contains iron in the goethite structure (FeOOH).
It can be calcined easily to change to Fe2O3 form, by removal of water and subsequent oxidation. The mineral red ochre was soft and quite porous in nature. It could be turned into cylindrical pellets easily using a lathe. Cylindrical pellets of red ochre of 12 mm dia were prepared by turning a lump of ore.
It was calcined at 650°C to attain a constant weight and then subjected to SO2 capture at low temperatures of 420-560°C. The conversion-time plots of these pellets showed a sigmoidal behaviour characterised by slow initiation accompanied by accelerated rate and terminating into slow rate i.e., deceleration in rate.
The porosity of the calcined product used for SO2 capture was 0.44. The pellet prepared from powders of red ochre of similar porosity also showed similar sigmoidal behaviour and it was possible to attain quite high utilisation of solid (above 70%) in two hours reaction time.