TITLE OF THE INVENTION

A method and an apparatus for the recovery of cyclohexane from the grease stream

FIELD OF INVENTION

This invention relates to a method and an apparatus for the recovery of cyclohexane from the grease stream.

PRIOR ART DESCRIPTION

The grease stream is obtained as a waste stream during the manufacture of polyethylene by solution polymerization, especially linear low density polyethylene (LLDPE). During treatment of the waste products and by products formed in the manufacture of polyethylene, the grease column is obtained at the bottom stream of the recovery section of the manufacturing plant. The grease column essentially comprises low molecular weight polyethylene, unreacted cyclohexane, higher boilers or heavier components, acids and ketones. The grease stream is generally burnt in vapourizers to recover the heat value thereof.

OBJECTS OF THE INVENTION

An object of the invention is to provide a method for the recovery of cyclohexane from the grease stream which method is simple and easy to carry out.

Another object of the invention is to provide an apparatus for the recovery of cyclohexane from the grease stream which apparatus is simple in construction and easy and convenient to operate.

DETAILED DESCRIPTION OF THE INVENTION

According to the invention there is provided a method for the recovery of cyclohexane from the grease stream, the method comprising the steps of preheating the grease stream to 160 to 165°C in a preheater, flashing the preheated grease stream in a flasher at 115 to 125°C, heating and discharging the higher boilers from the flasher into a vapourizer and distilling the vapours from the flasher in a distiller at 70 to 88°C , condensing the vapours from the distiller in a condenser and collecting the condensate (cyclohexane) from the condenser in a reflux drum and recycling part of the cyclohexane in the reflux drum into the distiller and recovering the remaining part of the cyclohexane in the reflux drum.

According to the invention there is also provided an apparatus for the recovery of cyclohexane from the grease stream, the apparatus comprising a flasher connected to a grease feeder line provided with a feed control valve and preheater, the flasher being connected at the bottom thereof to a vapouriser through a pump and a heater, the flasher being further connected at the top thereof to the bottom of a distiller, the distiller being connected at the top thereof to a condenser which in turn is connected to a reflux drum, the reflux drum being further connected to the top of the distiller through a reflux pump and a flow control valve, the reflux pump also being connected to a recovery valve, the distiller being connected at the bottom thereof to the bottom of the flasher through a recirculation pump and a flow control valve and a heater unit for the distiller and the flasher.

The following is a detailed description of the invention with reference to the accompanying drawings in which the sole Fig 1 is a flow diagram of the apparatus for recovery of cyclohexane from the grease stream according to an embodiment of the invention. The apparatus 1 as illustrated in the drawings comprises a flasher 2 connected to a grease feeder line 3 provided with a feed control valve 4 and a preheater 5. The flasher is connected at the bottom thereof to a vapouriser (not shown) through a pump 6 and a heater 7. The flasher is further connected at the top thereof to the bottom of a distiller 8. The distiller is connected at the top thereof to a condenser 9 which in turn is connected to a reflux drum 10. The reflux drum is further connected to the top of the distiller through a reflux pump 11 and a flow control valve 12. The reflux pump is also connected to a recovery valve 13. 14 and 15 are the pressure control valves of the reflux drum. 16 is a reboiler connected to a low pressure stream source (not shown) through a flow control valve 17. The distiller is connected at the bottom thereof to the bottom of the flasher through a recirculation pump 18 and a flow control valve 19. The grease stream is fed into the flasher after preheating it to 160 to 165°C in the preheater 5. The inflow of the grease stream into the flasher is controlled by valve 4. The grease stream is flashed in the flasher at 115 to l25°C. The lower boilers in the grease in the flasher are vapourised and travel into the distiller. The higher boilers in the grease stream in the flasher are pumped into the heater 7 by pump 6. The higher boilers are heated in the heater 7 and discharged into the vapouriser to be burnt in the vapouriser. The vapours in the distiller are distilled at 70 to 88°C and allowed to flow into the condenser 9 where the vapours are condensed using chilled water. The condensate ( cyclohexane ) is collected in the reflux drum. Part of the cyclohexane is pumped into the top of the distiller by reflux pump 11 for maintaining the cyclohexane purity. The flow of cyclohexane into the distiller is controlled by the control valve 12. The remaining part of the cyclohexane is recovered by

opening valve 13. Valve 19 controls the flow of heavier grease steam into the flasher from the distiller bottom. The recirculation pump 19 recirculates the heavier grease steam collected at the bottom of the distiller. Nitrogen is used to maintain the pressure of the reflux drum. The heater unit of the apparatus as illustrated in the drawings can be different. Instead of low pressure steam, other heating fluids like thermic fluid also can be used for heating the grease stream in the flasher and the cyclohexane in the distiller.

According to a typical experimental example of the invention grease stream was fed to the preheater at the rate of 260 kgs/hr. The grease stream was preheated in the preheater at 160 to 165°C and flashed in the flasher at 115 to 125°C. The cyclohexane vapours were allowed to vapourise at the top of the distiller at 84°C. Recovery of cyclohexane at the recovery valve was 174 kg per hour. The cyclohexane as obtained through the recovery valve was analysed by gas chromatography and its purity was found to be 99.5%.

According to the invention a substantial quantity of the cyclohexane in the grease stream which would have been otherwise burnt in the vapouriser is recovered and reused. The higher boilers in the grease stream are burnt in the vapouriser to recover the heat value thereof. Thus the grease stream is gainfully utilized into two ways namely to recover and reuse the cyclohexane and to recover the heat value. Therefore, wastage of valuable materials from the grease stream is minimized. The method of the invention is simple and easy and convenient to carry out. The apparatus of the invention is also simple in construction and operation.