Methanol recovery section for the A7 example simulation

Overview

In the methanol recovery section, we wash the overhead product of the MTBE reactive distillation column with water to extract methanol. We simulate this unit as a liquid-liquid extractor column. The raffinate, which contains less than 10 ppmw methanol, is suitable for use in other processes, such as recovering high purity C4 isomers or as a feed to an alkylation unit.

We warm and flash the extracted phase—which contains water, methanol, and small amounts of dissolved hydrocarbons—to remove the hydrocarbons. We then fractionate the resultant methanol-water mixture in a distillation column to recover methanol as the overhead product. We recycle the methanol (with a trace of water) to the MTBE reactor. We return the wash water stream from the column bottoms, along with a small amount of make-up water, to the liquid-liquid extractor column.

Process specifics

The P1 pump delivers the methanol-C4s azeotrope (S6) to the HX3 heat exchanger, which cools it to 38°C against cooling water. The HX3 heat exchanger also calculates the cooling water requirement given a cooling water delivery temperature of 16°C and a return temperature of 32°C. We then feed the cooled process stream to the bottom of the liquid-liquid extractor column (T2).

We simulate the T2 Extractor with 5 theoretical trays and a top pressure specification of 792 kPa. We feed recirculating wash water to the top of the Extractor. The Extractor uses the P2FluidVLLE Fluid for the VLLE calculations. We place Fluid Changers (FC1 and FC3) before the Extractor to switch to the P2FluidVLLE Fluid from the P2Fluid Fluid, the main Fluid for the process. We calculate the fresh water feed to the Extractor (WMa.W) to set the mass composition of methanol in the top product (C4s.zm[MeOH]) at 10 ppmw.

The raffinate leaves the top of the Extractor (S11_C4s) and contains the unreacted and non-reactive C4s. The extracted liquid phase (S12) exits at the bottom of the Extractor and goes through another Fluid Changer (FC2) to return the stream to the P2Fluid Fluid. The stream then enters the feed-bottoms heat exchanger (HX4), which warms it to 99°C against the recycled wash water (S21).

The XV1 Valve drops the pressure of the heated methanol-water stream (S13) to 241 kPa. This generates a mixed-phase stream (S14) which we adiabatically flash in the D1 Drum. We vent the vapor phase (S15) containing the dissolved hydrocarbons there were released during the flash as a flare gas; we use the P2 Pump to send the liquid phase (S16) to the methanol distillation column (T3) to recover the methanol.

We simulate the T3 Column with 20 theoretical trays, a top pressure of 138 kPa, and an overall pressure drop of 34.5 kPa. The feed (S17) enters on tray 9.

The column has a sub-cooled, fixed-temperature condenser that operates at 30°C and 103.5 kPa. The performance specifications are 99.5% recovery of methanol in the overhead product and 99.95% recovery of water in the bottom product. Because the Column does not calculate the recoveries, we add the MeOHRecovery and WaterRecovery flowsheet variables to the simulation as well as the flowsheet equations to calculate their values (Eqn2 and Eqn1). We calculate the RefluxRatio and BoilupRatio in the Column to specify the methanol recovery (MeOHRecovery) and the water recovery (WaterRecovery) at the indicated performance specifications.

We also perform Simple Tray calculations for this Column for first-pass estimation of the diameter by specifying the flooding factor at 80%.

We combine the recovered wash water from the column bottoms (S18_WaterRecycle) with additional make-up water (MKUP). The P4 Pump sends the combined water stream (S21) to the HX4 heat exchanger.

A trim cooler (HX5) further cools the wash water (S22) to the desired temperature of 38°C before it (S10) goes back to the liquid-liquid extractor column.

The P3 Pump recycles the overhead product (S19) from the top of the methanol distillation column (T3) to the reactor section.