Recovery of Benzol from Coke Oven Gas
Recovery of Benzol from Coke Oven Gas
Benzol (also called as benzole) is the name normally applied in the chemical industry to a mixture of hydrocarbons of the benzene series, in which benzene itself predominates, in association with certain of its homologues and various impurities. The term is not applied to any particular mixture or quality of the liquids. The recovery of benzol from the coke oven gas implies the removal of vapours of the benzene series, and their subsequent conversion by condensation into different liquid products.
The benzene series is the most important group of substances in the class of aromatic hydrocarbons. It is the series of carbon-hydrogen compounds based on the benzene ring, with the general formula CnH2n-6, where ‘n’ is 6 or more. Examples are benzene (C6H6), toluene (C7H8), and xylene (C8H10). The members of particular interest of this series are the first three namely benzene, toluene and xylene, which at normal temperatures are clear colourless liquids having very similar properties. There are, in fact, three isomeric xylenes, or substances of the composition and molecular weight corresponding to C8H10. The first, in which the methyl groups assume the 1.2 position, is known as ortho-xylene, the 1.3 placing gives meta-xylene, and the 1.4 compound is para-xylene. These three isomers differ slightly in properties, and xylene, as commonly produced, is a mixture of all three, meta-xylene usually predominating.
Benzol also known as crude benzol is a product which is produced during carbonization of coking coal. The quantity of benzol in coke oven gas varies considerably with the treatment to which the gas has been subjected during carbonization and its processing, as well as with the type of coal, and the temperature of its carbonization. If a high yield of benzol is desired, it is obviously unwise to keep the gas in contact with coal tar for a prolonged period. Thus the available yield of benzol is dependent, to a certain extent, on the design of hydraulic main, and condensers etc. On the other hand, fairly drastic condensation is considered essential for the elimination of naphthalene. The type of coal carbonized has, however, a much greater influence on the recovery of benzol.
The conditions of carbonization are of fundamental importance as regards both the quality and quantity of benzol produced per ton of coal. It has been seen that the primary products of carbonization are largely composed of paraffins and olefins (alkenes), which, on heating, are decomposed (to a greater or lesser extent, depending on temperature, time-contact, and atmosphere etc.) to give members of the benzene series and other aromatics. On further cracking, there is a tendency for the higher benzenes to break down, giving benzene itself, and also two-ring or three-ring hydrocarbons, such as naphthalene and anthracene. Thus, from a given coal, carbonized at a moderately high temperature gives a benzol rich in benzene and almost free from paraffins, whilst the coke oven gas contains also a considerable amount of naphthalene. The same coal carbonized at low-temperature yields a smaller quantity of benzol per ton, in which paraffin appears and the proportion of benzene to its higher homologues are lower.
The effects of recovery of benzol from the coke oven gas are (i) reduction in the volume of coke oven gas, (ii) reduction in the calorific value of the gas, (iii) effect on flame temperature and flame volume, and (iv) alteration in density and composition of the gas.
Benzol fraction produced during the high temperature carbonization of the coal is around 0.7 % to 1.1 % of dry coal. It is present in the coke oven gas in the range of 25 grams per normal cubic meters (g/N cum) to 40 g/N cum of coke oven gas. It is also known as light oil. It is a complex mixture of the volatile aromatic hydrocarbons which includes mainly benzene, toluene, and xylene. It contains benzene (around 65 % to 70 %), toluene (around 10 % to 13 %), and xylene (around 4 % to 6 %) and several other minor constituent such as di methyl benzene, tri methyl benzene, ethyl toluene, and unsaturated compounds (cyclopentadiene, styrene, and indene). Sulphur compounds (carbon bi-sulphide, and thiophene), saturated hydrocarbons, phenol, pyridine are also part of the composition of crude benzol. Benzene, toluene, and xylene along with di methyl benzene, tri-methyl benzene, and ethyl toluene components of crude benzol is also called BTX fraction.
The method now universally adopted for the extraction of benzol from gas is the oil-washing process. This depends on the fact that a heavy hydrocarbon, or mixture of hydrocarbons, such as creosote or heavy petroleum oils, readily dissolves benzene and its homologues, which may be obtained out of solution by simple distillation, both solvent and solute being practically unchanged in the process. Other constituents of coke oven gas also dissolve in the oil and are similarly removed, to appear as impurities in the crude benzol. In practice, the gas is washed or scrubbed in a suitable form of equipment (known as scrubber) with such a quantity of oil which gives effective absorption, the scrubber being situated at a point where the coke oven gas is as cool, and as completely purified as possible. The efficiency of washing is dependent upon several conditions. The majors amongst these conditions are (i) intimacy of contact between oil and gas, (ii) time of contact of gas with the oil, and (iii) extent to which the operation approaches counter-current conditions. Simplified flow sheet for the recovery of benzol from coke oven gas is given in Fig 1.
Fig 1 Simplified flow sheet for the recovery of benzol from coke oven gas
The benzol vapour components in the coke oven gas are normally removed from the gas by absorption at atmospheric pressure by scrubbing with wash oil which is either coal tar oil or a petroleum based oil. Petroleum based wash oil is also known as solar oil. Solar oil is a petroleum fraction with a boiling point ranging from 270 deg C to 370 deg C. The benzol absorption process also removes a substantial proportion of the naphthalene vapour still present in the gas in addition to some of the carbon di-sulphide. Some of the compounds of benzol initially present in the gas leaving the coke ovens are absorbed in the early tar condensing and primary gas cooling sections of the plant.
The coke oven gas after ammonia saturator at around 60 deg C is taken to the final gas cooler where it is typically cooled to around 25 deg C. At the final gas cooler condensed materials such as water etc. are removed. Besides final gas cooler, benzol recovery unit has benzol scrubbers connected in series. After it is cooled in final gas cooler, the gas is passed into the benzol scrubber, over which is circulated the wash oil, which serves as the scrubbing medium to absorb benzol. The wash oil is sprayed in the top of the packed absorption tower while the coke oven gas flows up through the tower. The counter current flow facilitates the absorption of benzol by the wash oil. The wash oil absorbs around 2 % to 3 % of its weight of benzol, with a removal efficiency of around 95 % of the benzol vapour in the gas. The benzol rich wash oil is also known as benzolized oil (BO).
BO after final absorption in the benzol recovery unit is pumped to the storage tanks of the benzol distillation unit where the BO is treated with steam to produce crude benzol and de-benzolized oil (DBO). In the benzol distillation unit tripping and fractionating of BO is carried out to produce crude benzol as CB-I, and as CB-II (heavy crude benzol). The DBO is again pumped to the benzol recovery section. Makeup wash oil is added continuously to compensate for the losses in the process.
The BO is initially pre-heated in three pre-heaters, which are shell and tube heat exchangers. Pre heating is first done in oil dephelegmators, then oil-oil heat exchanger and finally in steam pre heaters. The temperature of BO is increased slowly to a level of around 120 deg C to 130 deg C in order to prevent its chemical decomposition.
Crude benzol in the BO is recovered by steam distillation. During the distillation process, the partial pressure of the benzol decreases and it easily get vaporized. Stripping of crude benzol with steam from BO is a continuous process. It is carried out in stripping column to produce crude benzol vapour. Pre-heated BO from the exchangers is fed to the top tray of the stripping column. Steam to the column is fed in the bottom. Hence, the benzol rich wash oil is passed to a countercurrent steam in the stripping column. The crude Benzol vapours along with steam pass upward from the column and are removed from the top of the stripping column through a heat exchanger to a condenser and water separator for recovery. The lean (stripped) wash oil leaves the bottom of the stripping column to the associated decanter and is recycled to the benzol scrubber. The column consists of bubble cap trays. Low pressure steam at a temperature of around 180 deg C and around 4 kg/sq cm pressure is injected at the bottom of the column. Low pressure steam injected at the bottom not only maintains temperature of the column but also decrease the partial pressure of the crude Benzol.
The vapours are passed through oil and water dephlegmators for separating phlegma condensate. The crude benzol vapours are partially condensed in the oil dephlegmators. Three sets of dephlegmators are provided, two sets for oil and one for water. The vapours are cooled to temperature range of 92 deg C to 95 deg C in oil dephlegmators by pre-heating the feed to the stripping column and further cooled to 84 deg C in water dephlegmator. The partial condensation removes any higher fractions present in the vapours which further increases the purity of the vapours.
The condensate collected in the heat exchanger is called phlegma. The phlegma from the exchangers, which contains water, is collected in a separator. Water is separated and phlegma over flows to the phlegma collecting tank from this tank phlegma is sent to stripping column as reflux.
The stripped oil is called de-benzolized oil (DBO). DBO is cooled in oil-oil and oil-water heat exchangers and sent to decanter where the oil is separated from emulsion. DBO is then pumped back to the benzol recovery unit for use in benzol scrubber for absorption of benzol from the coke oven gas.
The continuous circulation of DBO forms some polymer due to heating and cooling. This polymer must be removed from the DBO by regeneration. Regeneration is a hollow tank in which steam coils are arranged. Medium pressure steam is circulated through these coils. The regeneration of oil is done by feeding a small portion of hot BO to regenerator. The bottom content from the regenerator containing polymers called high boiling fraction is collected in high boiling fraction tank.
Top temperature in stripping column is typically in the range of deg C to 115 deg C. Bottom temperature in stripping column is typically in the range of 120 deg C to 130 deg C. Typically there are 23 number of trays in the stripping column.
After the stripping column, crude benzol vapours are fed into a fractionating column to produce CB-I and CB-II. Emulsion from decanter is periodically transferred to emulsion tank for the separation of oil and tar acid waste by heating. Tar acid waste is semi solid mass, generated during the circulating of oil in the process of benzol recovery due to the absorption of tar fog. Tar acid waste is transferred to waste beds for disposal.
Steam condensate generated during the stream stripping of BO is separated and collected in a settling tank. The separated steam condensate is then directed to the separated water tank. Water from separated water tank is pumped to final gas cooling condensed water circuits. Water used in the decanters for the separation of sludge and other contaminated water is also pumped to final gas cooling condensed water circuit.
Crude benzol mainly consists of light crude benzol, heavy crude benzol, and polymer. The vapours containing crude benzol from the top of the stripping column is fed to the crude benzol column number 1 through water dephlegmator where the crude benzol is separated to heavy crude benzol and light crude benzol. Lighter fractions like benzene, toluene, and xylene are present in the light crude benzol and heavy crude benzol is similar to that of heavy polymer, which is used as furnace oil. The column typically consists of 16 numbers bubble cap trays. Simple distillation is carried out in this column. A reboiler provided at the bottom of the column supplies the necessary heat. Medium pressure steam is used as heating media. A crude benzol vapour at a temperature range of 80 deg C to 85 deg C is fed to the 6th tray of the column. The lighter components are vapourized and these are collected at top of the column, which are then condensed in a condenser by water. The condensed vapours are then fed to the separator where the moisture present in the vapours is separated and the light cooled benzol obtained is stored in CB-I tanks. Part of the light crude benzol is fed as reflux to the CB-I column. The bottom product obtained from CB-I column is fed to the CB-II column. Top temperature is in the range of 70 deg C to 80 deg C while the bottom temperature is in the range of 115 deg C to 120 deg C.
Crude benzol column number 2 typically consists of 6 bubble cap trays. The bottom product of the crude benzol column number 1 which mainly contains heavy crude benzol with small amount of light crude benzol is fed to the crude benzol column number 2.The liquid is distilled for the recovery of the light crude benzol in this column. The vapours of the light crude benzol obtained from the top of the column are fed to the crude benzol column number 1 as reflux. The bottom product obtained from the crude benzol column number 2 is called heavy crude benzol (HCB). The top temperature in the crude benzol column number 2 is around 120 deg C while the bottom temperature is around 140 deg C.
The DBO from the bottom of the stripping column is pumped through oil-oil heat exchanger to DBO cooler. In DBO cooler it is cooled to a temperature range of 45 deg C to 50 deg C. Due to high temperature exposure, part of the wash oil may get decomposed. This decreases the absorption efficiency of the wash oil. For the removal the decomposed particles, DBO is fed to the decanter, which is a horizontal cylindrical tank unlike mechanical decanters. Since DBO is fed to the decanter at the temperature range of around 45 deg C to 50 deg C, small amount of water is fed to the decanter which provides better removal of the sludge form of the oil. Water settles at the bottom carrying sludge with it. Water is drained from the decanter continuously. The cleaned oil being lighter floats on the water-sludge layer. The residence time in the decanter is in the range of 3 hours to 4 hours. Oil after its retention of 3 hours to 4 hours is fed to the DBO tank. Due to contact of oil, water and the sludge, oil-water and sludge-water emulsions are formed. These emulsions float on the surface of the water, which is fed to the de-emulsifier along with sludge.
De-emulsifier is a horizontal insulated cylindrical vessel. Medium pressure steam is fed through a coil into the de-emulsifier. Residence time for setting the oil, sludge, and water in the de-emulsifier is around 2 hours. Due to heating of emulsion with the medium pressure steam, oil and water get separated. This separation process is called as de-emulsification. In this process emulsion is broken which causes sludge to float on the water. The sludge is fed to the sludge tank and the water is drained. The temperature inside the de-emulsifier is in the range of 80 deg C to 90 deg C.
The degree of absorption of the crude benzol from the direct coke oven gas is a function of the oil temperature and the gas and oil flow rates. The yield and composition of crude benzol obtainable at different coke oven and by-product plants is almost identical, which is driven by uniformity of instrumentation of the coking process and high temperature carbonization of coking coal. Changes in the composition and yield of crude benzol are due to the type of coking coals, the temperature range of coking and the design of coke oven batteries.
Crude benzol is a transparent yellow brown liquid with its colour ranging from light colour to dark brown. It does not contain suspended particles and has the smell of hydrocarbons. The boiling point is 150 deg C to 200 deg C, and the freezing point is from -20 deg C to -25 deg C. Its density at 20 deg C is in the range of 0.871 grams/per cubic centimeters to 0.9 grams/cubic centimetres.