Module 2: Solid Fossil Fuel (Coal) Lecture 12: Combustion of coal and coke making

Transcription

1 1 P age Module 2: Solid Fossil Fuel (Coal) Lecture 12: Combustion of coal and coke making

2 2 P age Keywords: Coal tar, distillation, caustic washing, deoiling 2.4 Combustion of coal and coke making Coal tar distillation Coal tar is obtained as a by-product while processing coking coal to form metallurgical coke in a recovery-type coke oven plant. Coal tar accounts for around 3.5-4% of coke produced. Coal tar pitch is a complex chemical mixture of phenols, cresols and xylenols (which together termed as tar acids), polycyclic aromatic hydrocarbons (PAH), and heterocyclic compounds, which can be recovered by distillation to give 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. Tar distillation plant (TDP) consists of the following sections, i) Tar distillation section ii) iii) iv) Caustic washing section De-oiling section Springing section v) Recasting section The block diagram of TDP is shown in Figure 1.

3 3 P age Tar distillation section The purpose of tar distillation is to 1) dehydrate the tar in the dehydration column, 2) remove the pitch from dehydrated tar in pitch column and 3) separate tar oils in fractionating column. Fig 1. Block flow diagram of 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 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 vaporized 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

4 4 P age bottom fraction of the dehydrator column is pumped at a high rate through pipe-still economizer and heated. This bottom fraction is dehydrated tar, some 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 vaporized and rose 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. Some 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 are 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. Caustic washing section Middle oil from the tar distillation section is counter currently contacted with a flow of 10% caustic soda 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.

5 5 P age De-oiling section The sodium phenolate solution contains small amount of middle oil, which must 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. Springing section The objective of this section is to recover tar acids from sodium phenolate solution by springing with a carbon dioxide 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 is formed. The bottom of the first column is introduced at the top of the second column where the stream is again contacted with carbon dioxide counter currently. The sodium carbonate solution is sent to a separator from the bottom of the column. Crude tar acid collected and stored in the tar acid buffer tank. Carbon dioxide rich gas is continuously bubbled through the crude tar acid buffer tank to reduce the alkali and water content of tar acids. Recasting section In this section, the sodium carbonate solution from the springing section is concentrated with hard burnt lime to produce caustic soda.

6 6 P age 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 is nothing but a kettle reboiler operates under high vacuum. Crude tar acids are vaporized 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 kettle, 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: crude phenol as overhead product, crude cresol as side stream and 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 kettle. 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 kettle 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.

7 7 P age Another vacuum batch distillation is carried out to recover xylenol product and HBTA. Crude xylenols is pumped from the storage tank to a preheater kettle 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; second cut is of mixed xylenols. Next cut is a mixture of xylenols and HBTA mixture and the last fraction or residue is HBTA.

8 8 P age Reference 1. CFRI report, all.pdf 2. Coal Tar Distillation and Working Up of Tar Products, Arthur R. Warnes, Crastre Press, Distillation of Coal Tar, Wilsnack, G. C., NABU Press, 2010.