Simulation of Extractive Distillation for Recycling Tetrahydrofuran from Pharmaceutical Wastewater with Chem CAD Software

The functions and application of ChemCAD simulation software was introduced. Mathematical model of extraction distillation process was established. The extractive distillation process for preparation of tetrahydrofuran (THF) was simulated by SCDS rectification model in ChemCAD software, methanol-THF-solution and ethanedollithium chloride solution as extractant. Influence of extraction agent on vapor-liquid equilibrium curve of methanolTHF system and effects of theoretical plate number, feed and extractant input positions, extraction agent ratio (m(chlorinated lithium): v(ethanediol)), extractant ratio (m (extractant): m(feed)) and reflux ratio on tower top mass fraction of THF were investigated. The results of simulation were compared with experimental data. Under optimum extraction distillation conditions: theoretical plate 30, extractant plate 6, feed plate 18, extraction agent ratio 0.24g/mL, extractant ratio 2.7 and reflux ratio 5.0, mass fraction of THF on tower top can reach 97.2%. The distribution characteristic parameters of the column were simulated. The results of simulation and experimental data agree satisfactorily.


INTRODUCTION
At present famous chemical process simulation software has AspenPlus, Pro/II and so on, but they are large chemical simulation software, higher cost of buying.ChemCAD chemical process simulation software was developed by Chemstations company, it with the graphical user interface way, basically have input Aspen Plus and Pro/II software all sorts of functions, but the price is cheap, suitable for normal small chemical enterprise.In ChemCAD software internal, there are four kinds of distillation modules, including TOWR Column, TOWR PLUS Column, SCDS Column and SHORTCUT Column.TOWR is strictly calculation model, TOWR PLUS applicable to with heat exchanger, circulating pump lateral-line system recovery.SCDS module can timely correct.SCDS module is mainly used for simulating nonideal K value.It can be used for Newton-Raphon mathematical convergence and calculation and strict derivation of equation, including the DK/DX (Derivation main mixture of K value main mixture of K value) aspects, that is, effective chemical simulation system.
THF is a good solvent and an important fine chemical raw materials.In industrial production, as solvent used often will bring impurities such as water (Liu, 2005), because of THF containing impurities and can not be reused, directly abandoned, waste and pollution of environment, do not accord with requirement of energy conservation and emissions reduction.In recent years, Along with expansion of market demand of high purity THF, at home and abroad, THF purification, dehydration is more and more attention (Ju et al., 2006).People have done much about extraction distillation recovery tetrahydrofuran work (Xu and Chen, 2002), especially in the vapor liquid equilibrium (Xu et al., 2001), the selection of extraction agent (Xu et al., 1999), simulation calculation (Wang and Yang, 2011), etc. Extraction agent and salt types, solvent ratio, extraction solvent feeding position, raw material feeding position and reflux ratio has obvious influence on the results.The simulation calculation of THF-methanol-water-composite extraction agent system and the study of adding salt extraction distillation recovery THF experimental less limited its application in industry.It is necessary to simulate the process on influence of the quality of factors, using advanced computer simulation technology, in order to attain process optimization.

Simulation:
Mathematical model: Extraction rectification column includs stripping section, extraction section and rectifying section 3 parts.The equilibrium stage model of rectifying column to carry on the calculation, the whole tower has N equilibrium level, overhead condenser for the first, bottom reboiler for N level.Hypothesis: • Adding salt extraction distillation was a steady-state operation • Each component had the same molar latent heat of vaporization • Vapor and liquid were constant molar flow • All levels of vapor and liquid no temperature gradient and the concentration gradient • Leave plate, vapor and liquid mass transfer and heat transfer balance • Lgnored heat loss.Mathematical model was established such as formula (1)-( 4) shows (Wang et al., 2008) Material balance equation: Vapor-liquid equilibrium equation: (2) Normalization equation: (3) Heat balance equation: For methanol-THF-water-composite extraction agent content system, NRTL equation than Wilson equation of higher calculation accuracy.Therefore, NRTL equation was used to calculate each composition activity coefficient.

Simulation of distillation process:
The extractive distillation process for preparation of THF was simulated by SCDS rectification model in ChemCAD software, ethanedol-lithium chloride solution as extractant.Effects of theoretical plate, feeding and extractant input positions, extraction agent ratio (m(chlorinated lithium): v(ethylene glycol)), extractant ratio (m(extractant): m(feed)) and reflux ratio on Experimental: Extraction distillation was done to the mass fraction 38.31% THF, mass fraction 17.02% methanol aqueous solution.Extractive distillation tower diameter is 40 mm, the experiment device processes as shown in Fig. 2. Wang and Yang (2012) Demarcate rectifying column, determine the theoretical plate number 30.First full reflux operation, when stability, the partial backflow operation; when the tower top temperature and kettle temperature stability, distillate had been sampling analysised (Wang et al., 2008).

RESULTS
Table 1 and 2 give the results of simulation and experimental of extraction distillation.The temperature of tower, flow rate, and mass fraction distribution as shown in Fig. 3 to 6.

DISCUSSION
Influence of extraction agent on the vapor liquid equilibrium phase diagrams: Figure 7 to 9 along with molar fraction increases of lithium chloride and ethanediol in methanol-THF solution, equilibrium curve away from the diagonal line that the volatility of methanol relative THF also gradually increases.Ethanediol and lithium chloride eventually can all make methanol-THF solution from the bondage of azeotropic.
Influence of theoretical plate number on tower top content of THF: Figure 10 when the theoretical plate number increases to 27, the effect on separation is small.Therefore, the whole tower theory plate selected 30 is appropriate.Influence of extraction agent ratio on tower top content of THF: Figure 13 along with lithium chloride concentration increases in the composite extraction agent, salt effect enhancement.Due to its solubility reasons, concentration can not be too big, concentration is too high may appear crystallization in the tower, reducing plate efficiency.So optimum extraction agent ratio was 0.24 g/mL.Add 0.04 g/mL, 0.12 g/mL, 0.24g/mL composite extraction agent respectively, the results of experiment and simulation have some difference, may be due to rectifying column itself caused by the error.

Influence of extraction agent feed position on tower top content of THF:
Influence of extractant ratio on tower top content of THF: Figure 14 along with extractant ratio increases overhead THF content increased; When the extractant ratio more than 2.7, the overhead THF content increases tend to be gentle.When solvent ratio more than 2.7, composite extraction agent for THF-methanol-water system of dilution effect reaches balance, thus separation effect is not obvious.So optimum extractant ratio was 2.7.Extractant ratio respectively was 1.4, 2.7, 4.1, 5.4, the results of experiment and simulation have some difference, may be due to rectifying column itself caused by the error.

Influence of reflux ratio on tower top content of THF:
Figure 15 along with reflux ratio increase overhead THF content increases; When the reflux ratio is small, overhead THF content increases quickly; When the reflux ratio more than 4.0, overhead THF content increases little.Reflux ratio more than 4.0, already satisfies the separation requirements.So optimum reflux ratio was 5.0.Reflux ratio respectively was 3.0, 4.0, 5.0, the results of the experiment and simulation results of experiment and simulation have some difference, may be due to rectifying column itself caused by the error.

CONCLUSION
• NNRTL requation was used to calculate vaporliquid equilibrium data of methanol -THF-water system, salt extraction distillation mathematical model was established.• Vapor-liquid equilibrium data of methanol-THF system was determined; Compared vapor-liquid equilibrium of methanol-THF changes before and after adding salt or extraction agent.The results shows that the ethylene glycol, lithium chloride was suitable extracting agent for separation methanol-THF system.

Fig. 10 :
Fig. 10: Influence of theoretical plate number on tower top mass fraction of THF

Table 1 :
Simulation results of distillation

•
Optimum extraction distillation conditions: theoretical plate 30, extractant plate 6, raw material feed plate 18, extraction agent ratio 0.24 g/mL, extractant ratio 2.7 and reflux ratio 5.0.Under this condition, on tower top mass fraction of THF can reach 97.2%.Simulation results and experimental data agree satisfactorily.