GB2050589A

GB2050589A - Drying carbon black

Info

Publication number: GB2050589A
Application number: GB8015320A
Authority: GB
Original assignee: Sevalco Ltd
Current assignee: Sevalco Ltd
Priority date: 1979-05-11
Filing date: 1980-05-08
Publication date: 1981-01-07

Abstract

Carbon black pellets (22) from a wet pelleter (14) are passed through a rotating drum (20) surrounded by a heating enclosure (32), through which heating gases are passed, and are discharged from the outlet end (24) of the drum through a discharge passage (26). Air for drying the pellets is drawn in through the discharge passage, and from thence through the drum in counter-flow to the pellets. The air is exhausted from the inlet end (15) of the drum, and can be taken to a filter (50) where carbon black particles are separated. Thus, the drying air is kept separate from the heating gases, and the exhaust gas is relatively uncontaminated and free from condensate other than steam. In addition, the drying air, being preheated by the discharged pellets, cools the pellets ready for banging. <IMAGE>

Description

SPECIFICATION Drying carbon black This invention relates to methods and apparatus for drying carbon black pellets.

Carbon black produced from a reactor or other prime generator is separated from the by-product gases, and is then usually agglomerated into pellets. A commonly used method of doing this is the wet pelleting process wherein the carbon black powder is mixed with water and subjected to the influence of pins carried on a rotating shaft thereby forming pellets. The wet pellets are then fed to a dryer which generally takes the form of a drum rotating about a substantially horizontal axis and having a sealed feed mechanism at one end and a sealed discharge mechanism at the other end. Wet pellets fed to the drum progress from the feed end to the discharge end under the influence of gravity and rotation.The discharge mechanism may include lifting means inside the drum, and is connected to means, for example a bucket elevator, for conveying the dried product to a suitable discharge point, for example a bulk storage tank or bagging machine or the like. The major portion of the rotating drum is surrounded by a refractory lined enclosure, generally referred to as the firebox. The space between the enclosure and the drum is swept by hot combustion products which heat the shell of the rotating drum, and hence indirectly heat the carbon black for removal of the water by evaporation. In conventional drying apparatus of this kind, the drum has inlets near the discharge end to allow part of the hot combustion gases to enter the drum, the remaining combustion gases passing from the firebox to a stack.The hot gases which have entered the drum pass along it to the feed end, and in doing so sweep up the generated steam and entrain some carbon black powder. From there the gases are led through an exhaust duct to a filter, known as a purge or effluent filter, from which recovered carbon black dust is fed back to wet pelleter. The hot combustion gases for the firebox are typically provided from a cheap fuel such as the by-product gas or off-gas derived from the prime carbon black generator. Commonly the feedstock used for carbon black production contains from 1% to 4% of sulphur, and the resulting combustion gases therefore contain substantial amounts of sulphur oxides. This leads to very high corrosion of the filter plant and also hence to contamination of the filtered carbon black dust which is being recycled, sometimes to the extent that it has to be discarded.The corrosion is particularly severe during start-up and shut-down operations, when condensation occurs most readily, and since the corrosion can itself give rise to breakdowns requiring repeated shutdown and start-up, the problem can be selfaggravating.

The present invention provides a method of drying carbon black pellets wherein wet pellets are introduced into a feed end of a rotating drum along which they pass during rotation of the drum and are discharged from the other end, the dried pellets being conveyed from the discharge end of the drum through a discharge passage, the drum being surrounded by a heating enclosure through which hot gases are passed to heat the drum; the evaporated moisture and unpelleted carbon black dust being removed from the drum by air drawn through the discharge passage in counterflow to the pellets, into the discharge end of the drum, through the drum to the feed end thereof and from thence through an exhaust passage.The exhausted air can, as previously, be taken to a filter from which carbon black particles are recovered and can be recysied. Thus, hot, dry and relatively dean air is provided to assist the drying process of the. drum, so that essentially only air, steam and entrained carbon black particles are admitted to the filter. The exhaust gases from the firebox are kept separate from the interior of the drum. The air being drawn in through the discharge passages is preheated by the hot carbon black pellets before it enters the drum. This has the additional advantage of cooling the pellets as they travel through the discharge passage.

Hitherto it has frequently been found necessary to spray the pellets with water at the outlet of the dryer or in the conveying system in the discharge passage, to cool the pellets so as to prevent them melting the polyethylene bags at a subsequent bagging station. The present invention also provides venting of the system as a whole, whereas in previous systems separate venting systems have been installed.

The invention also includes apparatus for carrying out the method of the present invention.

In order that the invention may be more clearly understood, one embodiment will now be described with reference to the accompanying drawing which shows diagrammatically the layout of the carbon black drying apparatus.

Referring to the drawing, carbon black 10 and water 12 are introduced into a wet pelleter 14 from which the carbon black pellets are taken through a feed duct 16 into the feed end 18 of a drum 20 which rotates about a horizontal axis.

Carbon black pellets 22 pass along the drum as the drum rotates, and are led from the discharge end 24 of the drum into the discharge passage 26, which typically comprises one or more bucket elevators 23 which discharge the pellets at a higher level to a screw conveyor 30 which takes them to a suitable discharge station such as a bulk storage tank or bagging machine. The drum 20 is surrounded by a refractory lined firebox 32 into which hot combustion gases are introduced via a duct 34 from a combustion chamber 36 to which gas 38 and air 40 are introduced. The hot combustion gases pass through the firebox around the drum and are exhausted at 42 to a stack.

At the feed end of the drum is an exhaust duct 44 leading via a flow control regulator 46 and fan 48 to a filter unit 50. Air and steam from the drum 20, drawn through the exhaust duct 44 and delivered to the filter unit 50 are there separated from the entrained carbon black dust. The exhaust gases are taken from the filter at 52 to a stack, while the separated carbon black dust is led from the filter unit via a passage 54 to join the fresh carbon black entering the wet pelleter 14. The fan 48 draws air into the drum through the discharge end, the air passing through the discharge passage, as indicated by the arrows B, in counterflow to the carbon black pellets (arrows A), the air entering the discharge passage at a suitable inlet 56. The air passing through the discharge passage is preheated by contact with the counterflowing hot pellets and passes through the drum 20, where it mixes with steam produced by the heat from the firebox acting on the water in the pellets, the air and stream entraining carbon black dust and carrying it out through the exhaust passage 44 to the filter.

The present process is particularly important during warm-up and shut-down operation because it substantially avoids the passage of condensible gases (other than steam) and particularly condensible corrosive gases passing to the filter. On start-up the combustion chamber is initially operated at a low output, but sufficient to heat the drum which in turn heats the throughflowing air to preheat the filter.

Claims

1. A method of drying carbon black pellets wherein wet pellets are introduced into a feed end of a rotating drum along which they pass during rotation of the drum and are discharged from the other end, the dried pellets being conveyed from the discharge end of the drum through a discharge passage, the drum being surrounded by a heating enclosure through which hot gases are passed to heat the drum; the evaporated moisture and unpelleted carbon black dust being removed from the drum by air drawn through the discharge passage in counter-flow to the pellets, into the discharge end of the drum, through the drum to the feed end thereof and from thence through an exhaust passage.

2. A method according to claim 1 wherein the exhausted air is taken to a filter from which carbon black particles are recovered and recycled.

3. A method of drying carbon black pellets, substantially as described herein with reference to the drawing.

4. Apparatus for drying carbon black pellets comprising a rotatable drum having an inlet for wet pellets at one end and an outlet for dried pellets at the other end, the drum being surrounded by a heating enclosure, the dried pellet outlet leading to a discharge passage, and means for drawing air through the discharge passage in counter-flow to the pellets, into the outlet of the drum, through the drum to the inlet thereof, and from thence through an exhaust passage.

5. Apparatus according to claim 4 wherein the exhaust passage leads to a filter adapted to separate carbon black particles from the exhaust air.

6. Apparatus according to claim 5 wherein the separated carbon black particles are recycled to a pelleter which supplies wet pellets to the drying apparatus.

7. Apparatus according to any one of the preceding claims wherein the discharge passage comprises an elevator system for the dried pellets, said system being used as a heat exchanger to preheat the drying air drawn in.

8. Apparatus according to any one of the preceding claims wherein the air used for drying is additionally used to vent the pellet conveying systems downstream of the drum and to extract residual heat from the dried pellets.

9. Apparatus for drying carbon black pellets, substantially as described herein with reference to the drawing.