April 2018

Special Focus: Petrochemical Technology

Modeling and optimization of pressure distillation to achieve pharma-grade THF

This case study details the production of tetrahydrofuran (THF), with a purity of 99.96 wt% and water moisture of less than 200 parts per million (ppm).

Shirpurkar, V., Saudi International Petrochemical Co. (Sipchem)

This case study details the production of tetrahydrofuran (THF), with a purity of 99.96 wt% and water moisture of less than 200 parts per million (ppm). In this work, a systematic study of the separation of the THF-water mixture, with a high-pressure, rigorous distillation method, was performed to break the binary azeotrope and achieve moisture of less than 150 ppm. The two packed beds were used with mellapack packing materials, which provide extensive purification at a lower pressure drop at a high mass transfer rate. THF and water formed a minimum boiling azeotrope at 64.2°C. The moisture was significantly reduced to a range of 35 ppm–200 ppm in the THF product. The process feasibility analysis study was performed by rigorous simulation, with a proprietary process simulation software.a The simulation results were then implemented at the distributed control system (DCS) operating window as revised operating conditions. Based on several samples tested in a laboratory, the analytical and simulation results matched. The column’s hydraulic performance was checked to obtain the design limitation. The steady-state model found no hydraulic separation issue in the column. The model results summary showed “zero ppm” moisture in the product; however, in reality, it reached between 35 ppm–200 ppm. The online analyzer setup was installed at various locations to find and verify the moisture content.

The THF refining section consists of a series of four packed towers designed with a low-pressure (LP) steam reboiler, direct-injection LLP steam and a condenser at subcooled temperatures. The purpose of the recovery unit is to purify crude oil from 35 wt% to 99.95 wt%. Crude THF is generated as a byproduct from the polyester reaction. The THF will be purified to break the azeotrope by the pressure swing technique. The aqueous solution contains more than 60 wt% water, and is readily concentrated by low-boiling azeotrope composition. The THF is further distilled in a high-pressure distillation column to shift the THF-water azeotrope composition by 12% water at 9 barg pressure.

FIG. 1. Schematic diagram of the pressure swing distillation process for the THF and water binary system.
FIG. 1. Schematic diagram of the pressure swing distillation process for the THF and water binary system.

A schematic diagram of the pressure swing distillation process for the THF and water binary system is shown in FIG. 1. The function of the T-1 column is to strip off THF from the THF/effluent stream received from the polyester process plant. Column T-2 is used to strip off water from the THF/effluent stream received from THF stripper column T-1, primarily to break the binary azeotrope. Water-free THF can be sent to the THF purification columns (T-3 and T-4) to further enrich the quality of the THF to the required grade. The THF/water stream is preheated at an elevated temperature by using the T-1 bottom wastewater stream as heating fluid. Preheated THF/water enters the top section of the column. The stripping media used is low-pressure steam that is directly injected at the bottom of column T-1.

The temperature at the bottom is the most critical to avoid the slippage of THF to the bottom. The total condenser serves as the THF stripper column condenser. The condensed THF-rich liquid is collected in the THF stripper reflux drum. The condensed liquid is then fed to column T-2 as feed, with part of the liquid recycled back to column T-1 as reflux by the THF stripper reflux pumps. Tower T-1 is operated slightly above atmospheric pressure. Pressure is controlled by the manipulating condensing area of the condenser.

Column T-3 removes lighter components and impurities, such as 2-methyl THF (MTHF), 3-MTHF and 2-3 dihydrofuran (DHF) from the top of the overhead reflux. The product stream passes to the final purification column (T-4). The heavier components, including THF, dimethyl succinate (DMS), n-butanol, gamma-butyrolactone (GBL) and 1-butanediol, are removed from the bottom of the column, and the refined product cut is removed from the top for storage.

Thermodynamics and separation. The non-random, two-liquid (NRTL) model was used to calculate the vapor phase non-idealities for the modeling of the high-pressure column, along with rigorous simulation to understand the vapor-liquid equilibrium relationship. Since this column operates around an atmospheric pressure, isobaric experimental data of the THF and water mixture at 1.013 bar is calculated by using the NRTL model. The theoretical stages of each distillation column were fixed according to height equivalent to theoretical plate (HETP), including a condenser and a reboiler.

FIG. 2. VLE diagram of the THF mole fraction and temperature of the azeotrope as a function of pressure.
FIG. 2. VLE diagram of the THF mole fraction and temperature of the azeotrope as a function of pressure.

THF and water form a minimum-boiling azeotrope, or positive azeotrope, at 95% water and 5% water composition (by weight). THF boils at 65°C (water boils at 100°C), but the azeotrope boils at 64.2°C, which is lower than either of its constituents. Indeed, 64.2°C is the minimum temperature at which any THF/water solution can boil at atmospheric pressure. In general, a positive azeotrope boils at a lower temperature than any other ratio of its constituents.

In the first column, which operates at a lower pressure of 1 bar, the high-boiling component water is removed as a bottoms stream. The composition of the overhead product is as close as possible to that of the azeotrope at this pressure. The pressure is increased to 9 barg in the second column. At this higher pressure, the azeotrope forms at a lower concentration of the low-boiling component (THF), which can then be removed as bottoms. The overhead product of the second column is returned to the first column in the vapor state after pressure reduction. THF mole fraction and temperature of the azeotrope, as a function of pressure, are explained in the vapor-liquid equilibrium (VLE) diagram (FIG. 2).

Modeling and optimization of THF recovery.

The separation of THF and the water stream was carried out by using a rigorous, pressure-swing distillation method to measure hydraulic and thermal performance of the columns at constant liquid/vapor loading.

The distributions of liquid and vapor flows in packed towers T-1 and T-2 are the key components in mass transfer. The driving force for liquid is downflow, and pressure differential moves the vapor up through each section. The surface-wetted packing contact area required for mass transfer was found to be in an intact position and provided sufficient area for uniform mass transfer, as per the temperature profile. Vapor flow is adjusted on the higher side for efficient liquid-vapor contacting, and to ensure that weeping prevention loss does not assist entrainment column flooding. Limitations in the mass transfer area or packing geometry were not observed. Due to the high bottom temperatures of column T-2, a noticeable loss of THF product recovery was observed.

Before a simulation is run in the simulation software, it is necessary to establish the operating conditions. To do so, a sensitivity analysis should be completed. This analysis will determine which values of some parameters give the highest purity in the distillate, with moderate energy consumption (condenser and reboiler duty).

Sensitivity analysis

The sensitivity analysis was completed using proprietary software.a The analysis evaluated different variables over the THF distillate composition of 99.95 wt% and 200 ppm of water, as well as the condenser and reboiler duty. The variables tested were the number of theoretical stages, reflux ratio, the location of the feed stage plate, the reboiler duty and the condenser duty.

The purpose of this column is to concentrate the THF near an azeotrope point between THF and water, as well as to remove water as a column bottom stream. The complete miscibility of THF with water is one factor that distinguishes THF from the other ethers commonly used for organic reactions. If the aqueous solution contains more than 5 wt% water, it can be readily concentrated by distillation to the low-boiling azeotrope composition, which is 94.7 wt% THF at atmospheric pressure. The performance of each stage was tested at 100% load, and the pressure drop across the bed was found to be well within the design limit. The maximum allowable pressure drop is 12 mbar at 62.7% flooding. However, the actual results were well within the flooding range of 56%. The operating point, at constant V/L, was significantly below the ultimate capacity of the column. No loss of sensitivity at high vapor velocity was observed. The tower pressure drop was checked to discriminate between flooding and weeping across the packing. No indication of weeping or dumping was observed.

FIG. 3. Performance of column T-2’s water stripper.
FIG. 3. Performance of column T-2’s water stripper.

Column T-2 (water stripper) performance

The performance of column T-2’s water stripper is shown in FIG. 3. The pressure helps to produce a dry THF stream that is up to 30 ppm. The azeotrope overhead composition is recycled to the atmospheric distillation in column T-1. Column T-2’s bottom temperature is controlled by steam flow to the reboiler, according to the TXY diagram in FIG. 3 and high-water THF recycle to T-1 (FIG. 4). An increase of the water content in the bottom product of column T-2 is adjusted to 159.4°C.

The performance of the packed column is calculated by converting HETP into 20 stages. Each stage, at 100% load, is checked, with the pressure drop across the bed being well within the design limit. The maximum allowable pressure drop is 5.75 mbar at 62.1% flooding. However, the actual results were well within flooding range at maximum hydraulic load. The operating point at constant V/L is below the ultimate capacity of the column. However, all of the columns are designed for capacity factor at 80% flooding.

FIG. 4. Temperature vs. composition profile of column T-2 at each stage.
FIG. 4. Temperature vs. composition profile of column T-2 at each stage.

Column T-3 (THF purifier) performance

The purpose of this column is to purify THF from the low boilers, particularly the 2-3 DHF. The reflux stream in the top is enriched with the 2-3 DHF and THF water mixture. The low boiler fractions, especially water, are efficiently removed by increasing the draw amount from the top reflux. The rest of the column’s thermodynamics are kept unchanged.

Column T-4 (THF purifier) performance

The primary purpose of this column is to distill high boilers from the THF to obtain on-spec, dry pharmaceutical-grade THF at the top of the column. Column T-4 (final purification) receives the low-boiler free THF. The heavy, rich concentration of components in the column was hydrogenated into DMS. Further reactions produce GBL and BDO chemicals. Dehydrogenation of these components results in decomposition into THF and water in an acidic environment. The pH of the rich, organic concentration is measured to 2.5 acidity, which is highly acidic in nature.

The reaction kinetics under the acidic phase include:

  1. DMS + H-H r GBL + methanol
  2. GBL + H-H r BDO
  3. BDO r THF + water.

Therefore, the level was reduced in column T-4 to minimize the residence time. The temperature was reduced to 90°C to create unfavorable conditions for DMS decomposition.

Results, discussion and recommendations

The simulation and optimization work for the distillation process between a crude-THF mixture with water components was completed by using an NRTL model and proprietary simulation software. The results of this program were implemented at the actual plant level and achieved successful separation. THF is used as a solvent to manufacture paint, adhesives, impression ink, pharmaceutical products, etc. It is also an intermediate product and monomer. THF dehydration is a process of special economic concern since anhydrous THF demand is increasing. HP

NOTES

                                    Refers to Aspen Plus Radfrac modeling software

The Author

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