Vapour pressure toluene separation

Mujtaba and Macchietto (1988) and Mujtaba (1989) used two typical binary mixtures and a variety of separation specifications to demonstrate how optimal recycle policies can be obtained and to assess the validity of the degree of difficulty measure. The mixtures were 1) Benzene-Toluene 2) Butane-Pentane. For simplicity ideal equilibrium and Antoine s vapour pressure equations were used. The light component was always the first one. 

After standing in reactor 12, the mixture is cooled there down to 30 °C and filtered in nutsch filter 16 from diethylaminochloride. The filtrate is sent into tank 17 for distillation, and the filter cake is washed with toluene to eliminate amidation products as completely as possible. After the filtrate has been loaded, cooler 18 is filled with water, and the tank agitator is switched on. A residual pressure of 40-55 GPa is created in the system and the tank jacket is filled with a heat carrier or vapour. First, receptacle 20 receives toluene (below 60-65 °C) after separating toluene, amidation products are distilled into fractions. Receptacle 21 receives the intermediate fraction (below 106 °C) the distillation is monitored by the refraction index. At no20 = 1.4210+1.4230 the target fraction, diethylaminomethyl-triethoxysilane, is separated into receptacle 19. The distillation is continued up to 140 °C. As it accumulates, the intermediate fraction from receptacle 21 is sent into apparatus 12 for repeated amidation, and the ready product, diethylaminomethyltriethoxysilane, is sent after additional filtering (in case there is a filter cake) from receptacle 19 into collector 22. 

Toluene vapours are sent into cooler 13 to condense. The condensate is collected in receptacle 14. First, toluene is distilled at atmospheric pressure and 110-120 °C, then, vacuum is gradually created in the system. The final distillation is carried out at 110-120 °C and the residual pressure of 200 GPa, until the toluene content in the product is not more than 1 %. When the analysis is positive, the distillation of toluene is stopped and the product is cooled to 50-60 °C by sending water into the tank jacket. The distilled toluene from receptacle 14 self-flows into settling box 15, where it is separated from water and mechanical impurities. Purified toluene can be re-used in the production of oligomethylsiloxanes. 

In tank 25 the products of hydrolytic condensation are distilled from toluene. Cooler 26 is filled with water, and the tank jacket is filled with water vapour. The contents of the tank are heated to 80-90 °C and held at this temperature for 1 hour. The separated water and the intermediate layer are poured off into the intermediate container (not shown in the diagram) then toluene is distilled. First, the temperature in the tank at atmospheric pressure reaches 130 °C then, the tank is cooled to 70-90 °C and a residual pressure of 0.04-0.06 MPa is created in the system. Further distillation is conducted in the tank to 150 °C. The toluene vapours condensed in cooler 26 are collected in receptacle 27 and sent by compressed nitrogen flow (0.07 MPa) into flusher 28 as they accumulate. The flusher is filled with water, and the mixture is agitated for 10 minutes after that the agitator is switched off and the mixture is settled for 2 hours. The bottom layer, aqueous-alcoholic solution, is poured into neutraliser 13, and the top layer, washed toluene, is sampled for moisture content. If moisture content does not exceed 0.06%, toluene is poured into receptacle 30, sent to azeotropic drying (until the moisture content does not exceed 0.02%) and re-used in reactive mixtures. 

The neutral product in the hydrolyser is sampled to determine the solid residue content (10-15%), filtered through portable filter 12 with a metal mesh and poured into vacuum distillation tank 13 to distil toluene. The distillation is carried out in two stages the first takes place under atmospheric pressure to 50-55% concentration of the product the second, at a residual pressure of 860-870 GPa and 80-100 °C until the solid residue content is not less than 97%. Toluene vapours are sent into water cooler 14 to condense. Toluene is poured into collector 15. Raw toluene with impurities of volatile products and water self-flows from receptacle into settling box 16, where it is separated from them. Then it is sent to regeneration and re-used in manufacture. 

Toluene is distilled at 70-120 °C (liquid) and a residual pressure of 145 70 GPa until the solid residue content is 87-97% (depending on the varnish type). Toluene vapours from the distillation tank are sent into water cooler 15 to condense. Toluene is poured into collector 16 from there raw toluene enters settling box 17, where it settles, is separated from water and sent to regeneration. Regenerated toluene can be re-used in manufacture. After the distillation of toluene the heating of the tank is stopped and the jacket is filled with water. The obtained polydimethylphenylsilazane varnish is cooled down to 60 °C and filtered through cotton and metal mesh into collector 18. 

The solvent is distilled from polyalumophenylsiloxane in the same apparatus, 8. Before the distillation the product is clarified at 45-50 °C. The settled water is poured into collector 18 the clarified product (after switching inverse cooler 9 into the direct mode) is distilled to separate the toluene and butanol or ethanol mixture at a residual pressure of 800 65 GPa. The distillation temperature gradually rises to 90 °C. The distillation is considered finished when the resin concentration in the varnish is 40-65%. The vapours of the distilled solvent enter water cooler 9. There they condense and flow into receptacle 14. 

The analysis of the separation system should determine the appropriate simulation models. The simulation of the train of distillation columns may be studied in a separate flowsheet (Fig. 3.4). After pressure reduction through the valve VI, the liquid mixture enters the stabiliser (Stab) where dissolved gases are removed. An appropriate model is Rigorous Distillation with vapour distillate. After a second pressure reduction through the valve V2, the separation of benzene, toluene and Heavies takes place in a second column (Dist), for which the same rigorous distillation model is used. 

Fig. 13.9 presents the flowsheet for the separation section of a HDA process. The first column is a stabiliser. The vapour distillate - a mixture of H2, CH4 with traces of benzene and toluene - can be used to hold constant the pressure. The quality of the bottom product is ensured by controlling a sensitive temperature in the top zone (inferential control) with the reboiler duty. Reflux and bottoms flow rates control the levels in reflux drum and sump, respectively. 

The second stage of the styrene process involves the dehydrogenation of ethylbenzene. The reaction is carried out in the vapour phase at temperatures of 600—650°C over catalysts based on either ferric or zinc oxides with lesser amounts of other metallic oxides such as chromic, cupric and potassium oxides. The reaction is favoured by low pressure and in order to reduce the partial pressure of the ethylbenzene the feed is mixed with superheated steam before passage over the catalyst. Normally, a conversion of 35—40% per pass is achieved. The product is cooled and allowed to separate into two layers the aqueous layer is discarded. The organic layer consists of styrene (about 37%), ethylbenzene (about 61%) and benzene, toluene and tar (about 2%). The separation of styrene by distillation is difficult because of the susceptibility of the monomer to polymerization at quite moderate temperatures and because the boiling point of styrene (145°C) is rather close to that of ethylbenzene (136°C). It is necessary therefore to use specially designed columns and to add a polymerization inhibitor (commonly sulphur) before distillation and to distil under reduced pressure. In a typical process, a four-column distillation train is used. In the first column benzene and toluene are removed at atmospheric pressure in the second and third columns ethylbenzene is removed at about 35 mm Hg in the fourth column styrene is separated from sulphur and tar, also at about 35 mm Hg. Finally, an inhibitor is added to the styrene t-butyl catechol is preferred for this purpose rather than sulphur which leads to discoloration of the final polymer. Styrene is a colourless liquid with a characteristic odour. 

---