Abstract
The research was aimed at providing water for cooling recycled cyclohexane in polyethylene plant to 15°C. This research could be redounding to the benefit of polyethylene plants that are using solution based polymerization technique. A chiller unit of 630 T, which has compressor power input of 378.33 kW and can provide chilled water capable of cooling recycle cyclohexane to 15°C, was designed using Aspen Hysys version 7.1. Four different refrigerants were tested to know the best fit refrigerant for the design and the best among them was R134a. The designed chiller has a coefficient of performance of 6.3 and a capacity greater than that of the defective chiller (550 TR). Unlike the defective chiller, with the increased cooling capacity and corresponding increase in compressor power from 296 kW to 378.6 kW, it could discharge chilled water capable of cooling recycle cyclohexane from 38°C to 15°C without tripping of the unit. With this design, ethylene absorption rate could increase to 40 T/h. Evaporator and condenser were designed and duties were 2166.39 KJ/sec and 2544.72 KJ/sec respectively. A thermostatic expansion valve with flow coefficient of 46.17 gpm was designed. The designed suction and discharge pressure were 414 kpa and 1053 kpa respectively while condenser temperature of 40°C was used for the design. The cooling of recycle cyclohexane from 38°C to 15°C using the chilled water supplied by the designed chiller was simulated using Aspen Hysys.
Highlights
Polyethylene production can be achieved using solution based polymerization technology [1], slurry method [2] or gas phase technology [3]
The following assumptions were made: 1) Potential and kinetic energy effect were negligible 2) Evaporator and condenser were at constant pressure 3) Evaporator outlet was saturated vapour while condenser outlet was saturated liquid 4) The vapour compression cycle was at steady state
The capacity of the designed chiller is 630 Refrigeration Tons. 7,799,004 kJ/h of heat could be removed when the chiller water was cooled from 31.65 ̊C to 15 ̊C at the evaporator using R134a
Summary
Polyethylene production can be achieved using solution based polymerization technology [1], slurry method [2] or gas phase technology [3]. A solvent is used to absorb ethylene and this solvent does not take part. In Eleme Petrochemicals, recycle cyclohexane at 7 Kg/cm is cooled from 180 ̊C to about 35 ̊C using series of heat exchangers. Some of the heat exchangers use cooling water supplied at 32 ̊C. In a quest to further cool the cyclohexane so as to improve ethylene absorption, a chiller was introduced and this only helped to drop the recycle cyclohexane temperature to 20 ̊C. Efforts to cool further to 17 ̊C resulted in tripping of the chiller and the unit eventually broke down
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