Section 200 – Back End

The Back End of the process incorporates two main parts: (1) product recovery and (2) solvent and nonsolvent recovery with recycle. For safety reasons, as toluene and isopropanol are flammable (See Section 25.4), inert nitrogen will be run through the process to occupy space. To vent the vessels in the back end process, flare disposal systems will be added to the surge tanks, filter, and dryer to burn off exhaust. Because operating pressures are low, seals will be placed between the components and flares to prevent flashback. Costs of these safety measures are included in the overall installation costs.

13.4.1. Product Recovery

The stream leaving the precipitation vessel contains solid LDPE granules in a slurry with toluene and isopropanol. These LDPE particles are considered micro-granules with diameters of approximately 300 microns in size. The recovery of LDPE product with 99.99% purity requires two additional steps following the precipitation vessel: filtration and drying.

For the filtration step, a rotary-drum vacuum filter is employed to separate the solid LDPE from the solvent and nonsolvent. The rotary-drum vacuum filter was selected because it allows for continuous filtration and can easily be modified to control particle cake- thickness. The filter will be enclosed and pumped with inert nitrogen gas to prevent exposure to atmospheric air conditions thereby minimizing the danger of flammability. The enclosed filter will be held at a pressure of 2 atmospheres (29.4 psia) and the vacuum will pull the solvent/nonsolvent liquid mixture through the rotary drum using a pressure of 14.7 psia to a surge tank. The solid LDPE particles remaining on the filter area of the drum will be scraped off using a scraper blade and fall by gravity into a star valve. It is assumed that the wet particles contain a 10% impurity of the toluene-isopropanol mixture.

The star valve discharges the wet solids by gravity into the second step of the recovery process: the drying step. A star valve is used because it allows for the pressure drop between the filter and dryer to be controlled. An indirect-heat steam tube dryer pulling vacuum at 7.4 psia (0.5 atmospheres) dries the incoming wet solid stream at 194°F. An indirect dryer was selected because like the filter, the exposure to atmospheric air must be zero to prevent the risk of flammability. The pressure is reduced to less than atmospheric to decrease the boiling points of the toluene and isopropanol (See Section 25.8 for T-xy diagrams) thereby minimizing the total heat duty requirement of the dryer. Low pressure steam (50 psig) is run through 14 tubes in the dryer to heat up the LDPE and evaporate the toluene and isopropanol (See Section 15.7 for design of dryer). The vaporized solvent/nonsolvent mixture is sent to a condenser before entering the same surge tank from the filtration step. The solid LDPE is dropped by gravity into another star valve before dropping into a storage bin. The process produces 4125.3 lb/hr of 99.99% LDPE product – the 0.01% being impurities in the form of pigments and dyes.

13.4.2. Solvent/Nonsolvent Recovery

To maintain an economical process, it was decided early on to recycle the solvent and nonsolvent back into the process to minimize overall raw material costs. From the rotary- drum vacuum filter, the solvent/nonsolvent mixture with 1% of the non-precipitated LDPE is sent to a surge tank. It is combined with the condensed vapor from the drying process. This surge tank is maintained at atmospheric pressure and a temperature of 176°F. This surge tank is designed to maintain continuous operation of the process in case of temporary downtime on either side of the tank (See Section 15.4 for surge tank specifications).

The liquid in the surge tank is then pumped to a distillation column at 29.4 psia (2 atmospheres). This distillation column separates the toluene from the isopropanol with 99.97% recovery of isopropanol in the distillate (nonsolvent recycle stream) and a 99.8% mass composition of toluene in the bottoms (solvent recycle stream). The remaining composition of the bottoms solvent recycle stream is 0.17% LDPE and 0.03% isopropanol. If the previous assumption of only 99% polymer precipitation in the precipitation vessel is reevaluated to be a higher percentage precipitation, the mass composition in the bottoms stream will also increase as less polymer will make up that stream. See Section 15.5 for the design of the distillation column.

The distillate vapor stream at the top of the column passes through a total condenser and is separated between the reflux reentering the distillation column and the nonsolvent recycle stream using the reflux accumulator. It is held at 29.4 psia and 203.9°F. The recycle stream is purged 0.1% to remove any impurity build-up before entering a heat exchanger where it is cooled using cooling water to 174.0°F. This cooled nonsolvent stream is combined with fresh isopropanol nonsolvent at 77°F in the nonsolvent feed tank. The tank is well- insulated, pressurized to 29.4 psia and remains at 173.8°F (See Section 14.1 for Energy Integration). Like the earlier surge tank, the nonsolvent feed tank is modeled to facilitate continuous operation in the case that part of the process if forced to temporarily shut down. The hot nonsolvent recycle is then pumped to the precipitation vessel.

The bottoms liquid stream passes through a partial reboiler where part of the stream is sent back to the distillation column and the other to the solvent recycle loop. The recycle stream is purges 0.1% to remove any impurity build-up before entering a heat exchanger.

solvent feed tank. Like the nonsolvent feed tank, the solvent feed tank combines fresh solvent with the recycle loop, is pressurized to 29.4 psia and is maintained at a temperature of 208.4°F due to good insulation and energy integration as described in Section 14.1. It is similarly designed to maintain continuous operation in the case of temporary shutdown. The solvent recycle stream is then pumped to the dissolution vessel.