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Coal Tar and its Distillation Processes


Coal Tar and its Distillation Processes

Coal tar, also known as crude tar, is the by-product generated during the high temperature carbonizing of coking coal for the production of the metallurgical coke in the by-product coke ovens. It is a black, viscous, sometimes semi-solid, fluid of peculiar smell, which is condensed together with aqueous ‘gas-liquor’ (ammoniacal liquor), when the volatile products of the carbonization of coking coal are cooled down. It is acidic in nature and is water insoluble. It is composed primarily of a complex mixture of condensed-ring aromatic hydrocarbons. It can contain phenolic compounds, aromatic nitrogen (N2) bases and their alkyl derivatives, and paraffinic and olefinic hydrocarbons.

In the process of coal carbonization the constituents of the tar escape from the coke ovens in the form of vapour, with a little solid free carbon (C) in an extremely finely divided state. The tar is precipitated in the hydraulic main, in the condensers, and scrubbers etc., in a liquid state, at the same time as the ammoniacal liquor is formed. The tar formed in the hydraulic main is, of course, poorer in the more volatile products than that formed in the condensers and scrubbers, and is consequently much thicker than the latter.

The normal yield of coal tar during the coal carbonizing process is around 4 %. Coal tar has a specific gravity normally in the range of 1.12 to 1.20, but exceptionally it can go upto 1.25. It depends on the temperature of carbonization. The lower specific gravity tars are generally produced when low carbonization temperatures are used. Viscosity of tar affected similarly. The heavier tars contain lesser benzol than the lighter tars, and more fixed carbon. The nature of the raw material and the temperature of carbonization affect the chemical composition, and therefore, the quality of the tar.

Coal tar contains more than 348 types of chemical compounds, which are very valuable. They are aromatic compounds (benzene, toluene, xylene, naphthalene, and anthracene etc.), phenolic compounds (phenol, cresol, xylenol, cathecol, and resorcinol, etc.), heterocyclic nitrogen compounds (pyridine, quinoline, isoquinoline, and indole, etc.), and oxygen heterocyclic compound (dibenzofuran, etc.), which all have been used as raw materials or intermediates materials in various chemical industries (as anti-oxidant, anti-septic, resin, softener ingredient in plastic industry, paint, perfume, and medicine etc.).

With reference to the effect of the temperature of carbonization on the constitution of tars, it is found that those produced at low temperatures yield on distillation, in addition to phenols of the carbolic acid series, phenols of a different series rather less acid in behaviour and probably of the creosol and guaiacol type. Also, there is a smaller yield of naphthalene and of the benzene hydrocarbons, and a large percentage of hydrocarbons of the paraffin and olefin series. Instead of most of the N2 occurring in the form of pyridine bases it appears in the form of aniline and its homologues. The amount of free C is also small. On the other hand, high temperature tars, i.e., those produced at high heats of carbonization of coal, yield on distillation only traces of paraffinic hydrocarbons, the predominating hydrocarbons being those of the benzene, naphthalene and anthracene series. The N2 occurs principally in the form of pyridine bases, and the phenols consist mainly of carbolic acid and its homologues. The percentage of free C is generally high.

Coal tar which is normally distillable at atmospheric pressure boils at upto around 400 deg C and contains principally aromatic hydrocarbons. These include (in order of the distillation fraction) (i) benzene, toluene and the xylene isomers, tri- and tetra-methylbenzenes, indene, hydrindene (indane), and coumarone, (ii) polar compounds, including tar acids (phenol and cresols) and tar bases (pyridine, picolines (methyl-pyridines) and lutidines (di-methyl-pyridines), (iii) naphthalene, contaminated with small but significant amounts of thio-naphthene, indene and other compounds, (iv) methyl-naphthalene isomers, (v) biphenyl, acenaphthene and fluorene, (vi) anthracene and phenanthrene, and (vii) pyrene and fluoranthene.

Tar also mechanically retain a certain quantity of water (around 4 % on the average), which is extremely  unpleasant during the process of distillation as leads to ‘bumping’ and hence, required to be removed before hand by prolonged settling in separators (tar decanters), preferably at slightly higher temperature, which makes the tar more fluid. The water which rises to the top is removed in a normal manner.

Dehydration of coal tar

One of the greatest difficulties faced during the tar distillation is the elimination of water, which is present in varying proportions. The tar is generally allowed to settle in large tanks, from the bottom of which it is pumped into the tar distillation plant. Its water content is thus reduced to a level of below 5 %. Some tars cannot be even partially freed from water in this way, as an emulsion forms, and no separation takes place even after long standing. These high water tars generally contain a large percentage of free C, and the difficulty experienced with any individual tar in separating water by mere settling may almost be taken as a measure of its free C content. The cost of distilling out water is usually very considerable.

Dehydration of tar can also be carried out by mechanical means but the mechanical separation is of little value except where small quantities have to be handled. Centrifuging can also be employed for separating tar and liquor, and the difference in specific gravity makes this process very feasible. The mixture is fed into the machine at around 50 deg C, and the tar, being heavier, rapidly goes to the periphery of the machine, the liquor remaining in the interior. The two liquids are drawn off by pipes inserted into the revolving mass at suitable depths. Tar so dehydrated contains less than 1 % of water. This method can be very effectively employed effectively with the emulsified water tar mixtures.

Chemical methods of separation have been tried, but none of them are of any practical importance. In one such method the watery tar is treated with chromic and sulphuric acids, when the heat evolved during the oxidation distils off without frothing the water and naphtha.

The universal practice is to heat the aqueous tar either by means of live steam, or steam coils, or fire. Live steam has in the past been used fairly largely for the primary distillation of tar, but is now rarely employed. In these cases, of course, super-heated steam is used, and together with the water some of the more volatile naphtha is distilled off, leaving a thick tar useful for road work, varnish, roofing felt, etc.

A plan normally adopted in the earlier method of tar distillation for the separation of water is to charge the tar still up to a given height, and warm the contents to nearly 100 deg C, when the water commences to boil. At this stage the firing is discontinued, and, if necessary, the liquid is allowed to settle for a short time. It is found that nearly all the water now has separated, forming a layer above the tar. A small cock is fitted in the side of the still at the correct height, so that most of the water can be drain off. Better results are obtained if a swing pipe is fitted with a raising and lowering rod connected to it and projecting out of the top of the still through a gland. By this means, the majority of the water can be separated so that the amount which is needed to be distilled is small.

The process of distillation

Fractional distillation process is used for the distillation of coal tar. Fractional distillation of tar refers to the process by which components in a chemical mixture are separated by taking advantage of the difference in their boiling points. Distillation of coal tar is carried out mainly to produce benzols, naphtha, creosotes, naphthalene, anthracene, carbolic and cresylic acids, pyridine and pitch.

The purpose of tar distillation is to (i) dehydrate the tar in the dehydration column, (ii) remove the pitch from dehydrated tar in pitch column and (iii) separate tar oils in fractionating column. Since the quality of the coal tar is dependent on the coal carbonizing process and since there are large numbers of chemical compounds available in coal tar, the design and composition of the tar distillation plant varies with the type of tar and the compounds which are required to be distilled. Hence, it is very rare that the two tar distillation plants are exactly similar.

Different fractions of coal tar can be recovered by distillation. The process of distillation gives a variety of valuable chemical products. The residue of distillation is coal tar pitch, which is further processed into coal tar pitch of desired chemical and physical properties. The primary objective of coal tar distillation process is to produce a number of tar acid products from the crude tar.

The number of fractions, and size of fractions etc., which are to be taken off when tar is distilled is dependent on so many factors. The quality of tar plays a big part. Also, the kind of plant available for distillation is an important factor. The market value of the products is also important.

Normally, difference in the boiling point of different fractions is used for their extraction. When the tar is heated for extracting various tar components then the extractions which take place are described below.

Generally the first fraction to be extracted contains ammoniacal liquor, and naphtha, which is the mixture of benzene, toluene, xylenes, and pyridine. The boiling-point range is from 80 deg C to around 140 deg C, and the specific gravity range is 0.87 to 0.95. The quantity of water is dependent upon the amount in the original tar, and whether it has been partially taken out before distillation. It separates easily from the naphtha, and is drawn off from the bottom, and sent direct to the ammonia plant.

A good amount of care is needed in getting off the first fraction, as frothing is very prevalent, particularly in a tar with high free C content. The point when this danger is passed can be easily noticed by the noise which is heard inside the still, known as the ‘rattles’. When nearly all the water is off, globules of water condense on the inside of the top of the still and occasionally fall back into the hot liquid below, to be immediately turned into vapour again with almost explosive force, with the resulting rattling noise.

The second fraction is known as the light oil fraction which boils from around 140 deg C to 200 deg C. It has a specific gravity range of around 0.95 to 1. It contains the higher hydrocarbons of the benzene series such as mesitylene, cumenes, some naphthalene, also phenol, and higher homologues of pyridine. In many distillation plants, this fraction is not separated, but mix of first and second fraction is removed together.

The third fraction is collected purely to obtain the phenol in as concentrated a state as possible, and is consequently called the carbolic oil or middle oil fraction. It boils between 200 deg C and 240 deg C and has a specific gravity of 1 to 1.025, and contains phenol, cresols and higher hydroxyl acids, much naphthalene and creosote hydrocarbons. In the distillation of this fraction, great care is required to be taken to see that the condenser water is quite hot, so that crystallization of the naphthalene does not take place in the coils. The cold water is to be turned off in the middle of second fraction, and if the cooling water does not get warm quickly enough, steam is to be turned into the condenser.

This carbolic oil fraction is not separated, when the tar contains too small a quantity. It is sometimes found more economical to re-distil the creosote fraction.

The fourth fraction is known as the creosote oil fraction. It is the largest of all fractions and contains naphthalene and heavy oils, which are aromatic hydrocarbons with a high C and hydrogen (H2) content, and cresols and other phenol homologues. The boiling point is in the range of around 240 deg C to 280 deg C, and specific gravity in the range of 1.025 to 1.065.

The fifth fraction is marked by its distinctive colour, and is consequently called the green oil, yellow oil, or anthracene oil fraction. Its specific gravity is 1.065 to 1.1, and boiling point ranges from 280 deg C upwards to the end of the distillation. It contains still higher aromatic hydrocarbons, anthracene, phenanthrene, also carbazole etc.

Numerous attempts have been made to largely increase the number of fractions taken off the tar with the idea of better isolating the products. All these have failed, as the distillates obtained are no purer, so many complex azeotropic mixtures being formed. Again, nothing is saved, as many of the fractions have to be mixed together again for treatment in subsequent processes.

Tar distillation plant

As stated earlier, the design and composition of a tar distillation plant is dependent on the type of the tar to be distilled and the compounds which are to be extracted. Hence, the design and composition of a tar distillation plant differs from location to location. A present day, typical tar distillation plant is described below.

It consists of different sections namely (i) tar distillation section, (ii) caustic washing section, (iii) de-oiling section, (iv) springing section, and (v) recasting section. The block diagram of this tar distillation plant is given in Fig 1.

Fig 1 Block flow diagram of a typical tar distillation plant

The crude tar stored at elevated temperature in the storage tank is drawn through crude tar filter and mixed with caustic soda (NaOH) pumped from caustic tank by dosing pump. The mixture is pumped through tar vapour exchanger and steam-heated preheater to the bottom of the dehydration column. In the column the crude tar is contacted with a relatively large stream of hot dehydrated tar. The azeotropic water and oil mixture is vapourized and goes up to the top of the column and condensed in a light oil condenser. A portion of the azeotropic light oil is sent back to the column as reflux and the remaining portion is sent to an azeotropic distillation column. The bottom fraction of the dehydrator column is pumped at a high rate through pipe-still economizer and heated. This bottom fraction is dehydrated tar, a part of which is sent back to the lower part of the column.

In pitch column the dehydrated tar is mixed with a relatively large stream of hot circulating pitch. The more volatile oils in the tar are vapourized and rise up through the column. Stripping stream is injected in the column to run the operation. Crude pitch is drawn from the bottom of the column by pitch circulating pump and heated by a pipe-still heater. A part of this pitch is put into the top of the column for contacting with the dehydrated tar.

Volatile portion along with the stripping steam is recovered from the pitch column and separated into the light oil and water fraction, a middle oil fraction, and a heavy oil fraction. The light oil and water fraction combines with the same stream from the overhead of dehydration column and are sent to light oil condenser and then to a decanter. Middle oil flows by gravity through middle oil cooler either to middle oil buffer tank or directly to the mixing vessel in the caustic washing section. Middle oil can be transferred from buffer tank to the caustic section as per requirement.

Middle oil from the tar distillation section is counter currently contacted with a flow of 10 % NaOH solution. The system consists of three mixing vessels and three separators, situated alternatively. Middle oil, stripped of its tar acids, flows by gravity from top of the separators to the middle tank. The caustic solution, which is sodium phenolate solution mainly, after contacting with oil flows by gravity from the bottom of the separator to phenolate tank.

The sodium phenolate solution contains small amount of middle oil, which is needed to be removed to get good quality of tar acids. Sodium phenolate solution in buffer tank is pumped via overhead exchanger into the top of the sodium phenolate stripping column. Stripping steam is introduced at the bottom of the column which strips out the middle oil from the sodium phenolate solution. The overhead vapour heats the incoming sodium phenolate solution and cools down. Clean sodium phenolate solution is recovered from the bottom of the stripping column and sent to the springing section via cooler.

The objective of springing section is to recover tar acids from sodium phenolate solution by springing with a carbon dioxide (CO2) rich gas in a series of two packed column in counter flow. Gas is passed in upward motion through the descending sodium phenolate solution in the first column, where sodium carbonate (Na2CO3) is formed. The bottom of the first column is introduced at the top of the second column where the stream is again contacted with CO2 counter currently. The Na2CO3 solution is sent to a separator from the bottom of the column. Crude tar acid collected and stored in the tar acid buffer tank. CO2 rich gas is continuously bubbled through the crude tar acid buffer tank to reduce the alkali and water content of tar acids.

In the recasting section, the Na2CO3 solution from the springing section is concentrated with hard burnt lime to produce NaOH.

Recovery of tar acids

Crude wet tar acids recovered from springing plant contains little amount of water and pitch. It is pumped to the top of the dehydration column which operates under vacuum, maintained by ejector system. Azeotropic mixture of water and phenol is stripped out from tar acids and removed as an overhead vapour. The dry tar acids obtained as bottom product is sent to a depitching still which operates under high vacuum. Crude tar acids are vapourized and condensed in a condenser. The tar acids are flown to a buffer tank which is fitted with a steam coil to prevent the solidification of tar acids. The phenolic pitch is collected at the bottom of the still, mixed with the heavy oil and sent to a storage tank, jacketed with steam to maintain the pitch in a free flowing state. The crude tar acids from the tank are pumped to the primary distillation unit operated under high vacuum. During distillation, the crude tar acids are separated into three fractions  namely (i) crude phenol as overhead product, (ii) crude cresol as side stream, and (iii) crude xylenols/high boiling tar acids (HBTA) as the bottom product.

The crude phenol collected in a tank from this column is pumped to a vacuum column after heating in a still. Pure phenol is collected at the top condenser. A portion of it is sent to the column as reflux. The other portion is pumped to a storage tank. The residue of this column is mixed with the crude cresol in the storage.

Crude cresol from the storage tank is pumped from the storage tank into a still to preheat and then vacuum distilled in a column. The top product from this column is phenol, which is sent to the crude phenol storage tank. The first side fraction is o-cresol, next one is a mixture of m- and p-cresol, and the bottom product is crude xylenol/HBTA mixture which is sent to xylenol/HBTA storage tank.

Another vacuum batch distillation is carried out to recover xylenol product and HBTA. Crude xylenols is pumped from the storage tank to a preheater still and sent to high vacuum distillation columns. Four cuts are distilled which require three different column arrangements. The first cut is a mixture of m- and p-cresol, the second cut is of mixed xylenols, the third cut is a mixture of xylenols and HBTA mixture, and the fourth and the last fraction or residue is HBTA.

 

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