Methanol Recovery Column

The column base and the decanter arc sized to provide 5 min of holdup when 50% full under steady-state conditions. The column diameter is 1.74 m. 

The feed is introduced on stage 12 of a 32-stage column operating at atmospheric pressure. The number of trays in this column was optimized by determining the TAC of columns over a range of tray numbers. Reboiler heat input and condenser heat removal are 8.89 and 9.53 MW, respectively. The reflux ratio is 2.1. High product purities are achieved in both the distillate (99.9 mol% methanol) and the bottoms (99.9 mol% water). Both of these streams are recycled. 

In preparation for exporting the steady-state flowsheet from Aspen Plus into Aspen Dynamics, all equipment is sized. Column diameters are calculated by Aspen Tray Sizing. Pumps heads and control valve pressure drops are specified to give adequate dynamic rangeability. Typical valve pressure drops are 2 atm. The prereactor heat removal is specified to use a constant coolant temperature. 

The fresh C5 stream containing the reactive isoamylenes and the chemically inert other C5 components is fed into the reactor on flow control. The methanol fed to the prereactor is ratioed to the fresh feed flowrate. The exit temperamre of the reactor is controlled using a ternperamre/temperamre cascade structure. The reactor effluent temperamre controller changes the setpoint of the circulating cooling water temperamre controller, which manipulates the cooling water makeup valve (see Fig. 14.7). 

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The oxidation reactor effluent and methanol ate sent to the esterification reactor, which operates at up to 250°C and a pressure sufficient to maintain the Hquid phase. This latter is about 2500 kPa (25 atm). The oxidation products are converted to methyl -toluate and dimethyl terephthalate without a catalyst. Excess methanol is suppHed, and steam and vaporized methanol ate removed and enter a methanol recovery column. The esterification products flow to a cmde ester column, which separates the toluate from the terephthalate. The overhead stream of methyl -toluate is returned to the oxidation reactor, and the bottoms stream of dimethyl terephthalate goes to a primary distillation. The distillate is dissolved in methanol, crystallized, and sohd dimethyl terephthalate is recovered. The dimethyl terephthalate can then be either recrystallized or distilled to yield the highly pure material needed for the polyesterification reaction. 

Purge from the water recycle stream, to waste treatment, 10 per cent of the flow leaving the methanol recovery column. 

Methanol recovery (8) improves efficiency and extends catalyst life by allowing greater methanol slip exit from the converter. Addition of a methanol-recovery column to existing plants can help to increase production rates. 

DMA and TMA. Product ratios can be varied to maximize MMA, DMA, or TMA production. The correct selection of the N/C ratio and recycling of amines produces the desired product mix. Most of the exothermic reaction heat is recovered in feed preheating (3). The reactor products are sent to a separation system where firstly ammonia (4) is separated and recycled to the reaction system. Water from the dehydration column (6) is used in extractive distillation (5) to break the TMA azeotropes and produce pure anhydrous TMA. The product column (7) separates the water-free amines into pure anhydrous MMA and DMA. Methanol recovery (8) improves efficiency and extends catalyst life by allowing greater methanol slip exit from the converter. Addition of a methanol-recovery column to existing plants can help to increase production rates. 

Figure 3.33 Flowsheet of the CDTech MTBE/ETBE/TAME process. (1) boiling point reactor, (2) CD column, (3) extraction column, (4) methanol recovery column. Source [61].

The methanol-recovery column C2 operates at a pressure of 2 bar so that the reflux-dmm temperature (318 K) is high enough for the use of cooling water in the condenser. The separation is a fairly easy one, so using only 10 stages and a reflux ratio of 0.4 yield a... 

Methanol - Boiling Boiling — — — N2 atmosphere, methanol recovery column stoch solutions sent to lab for test... 

Distillate Di has a composition of methanol (28 mol% methanol) that is near the azeotrope at 4 bar. It is fed at a rate of 1122 kmol/h to the methanol recovery columns section. Table 8.2 gives information about important streams associated with the prereactor and reactive distillation column. 

The control structure for the prereactor and reactive distillation column Cl is shown in Figure 14.7, The control structure for the extractive distillation column C2 and the methanol recovery column C3 is shown in Figure 14.8. 

The above solution makes sense from the economic viewpoint, because the methyl ester is by far the most demanded, while 2-ethyl-hexanol is a cheap alcohol, in general a waste. The flowsheet in Figure 8.2 is still valid, in which the heavy alcohol is fed on the top stage, as an entrainer. Figure 8.15 shows the liquid concentration, reaction rate and temperature profiles. It can be seen that 2-ethyl-hexanol indeed plays its double role. The concentration of methanol in the top can be maintained at the minimum, such that there is no need for additional columns for methanol recovery and recycle. Note also that the operation takes place at low pressure (1.5 bar), in contrast with 6-12 bar and vacuum with individual light and heavy alcohols. 

In the DME column (4), DME is separated from the top and condensed. The DME is cooled in a chilling unit (5) and stored in a DME tank (6) as a product. Water and methanol are discharged from the bottom and fed to a methanol column (1) for methanol recovery. The purified methanol from this column is recycled to the reactor after mixing with feedstock methanol. 

In this instance distillation in a series of columns is necessary for methanol recovery [5]. 

In methanol synthesis purge recovery, a water scrubber is also used with a similar purpose, and it too pays for itself in recovered methanol. The meth-anol/water mixture is simply sent to the existing crude methanol distillation column. Hydrogen recovered from this purge can result in energy savings or if additional carbon oxide is available, it can be used to obtain increased methanol production. PRISM separators have operated on stoichiometric as well as non-... 

Most producers operate with a molar ratio of alcohol-to-oil of at least 6 1. This is 100% more than is consumed in the transesterification reaction so the excess must be removed and recycled. The excess methanol splits 60%/40% between the methyl esters and glycerin, so methanol must be removed from both streams (Ma et al., 1998a 1998b 1999). Methanol recovery is frequently accomplished by flash vaporization, which yields the methanol plus any water that may have been present in the reaction mixture. Excessive water is removed by a distillation column. 

Water and methanol are discharged from the bottom and fed to a methanol column (1) for methanol recovery. The purified methanol from this column is recycled to the reactor after mixing with feedstock methanol. 

Recovery column for unreacted methacrolein and methanol and removal of low-boiling-point products, (e) High-boiling-point products separation tower, (f) MMA purification tower. 

Figure 5.5 gives the Txy diagram for methanol/DMSO, which indicates an easy separation in the solvent recovery column. As shown in Figure 5.2, the column only requires 17 stages and runs with a low reflux ratio (RR = 0.5). The Uniquac physical property package in Aspen Plus is used. 

The bottoms of the extractive column has a very small impurity of acetone (0.01 mol%), so it is essentially a binary mixture of methanol and DMSO. The bottoms stream is fed to the 17-stage solvent-recovery column on Stage 8. The pressure is 1 atm, and the reflux ratio is 0.5. The distillate product is high-purity (99.95 mol%) methanol. The bottoms is recycled back to the extractive column as high-purity (99.99 mol%) DMSO solvent. 

The process flowsheet has a prereactor, a reactive distillation column, and a methanol-recovery section. A methanol-recovery section is required because the inert C5 components coming in with the reactive iso-amylenes in the C5 fresh feed form azeotropes with methanol. The result is that a significant amount of methanol is present in the distillate from the reactive column. 

Figure 5.12 gives the flowsheet for the entire process. The distillate from the reactive distillation colmnn Cl has a methanol concentration of 28.1 mol%. It is fed into the two-column pressiu e-swing methanol recovery system. Equipment sizes and economic data are given in Table 5.3. 

The distillate D2 is fed to the C5-recovery column C3. This column operates at a pressure of 10 bar, which shifts the azeotropic composition so that the distillate stream from this column D3 has a composition of 34.2 mol% methanol. Note that this is slightly above the composition of the iCs-methanol binary azeotrope, but remember that there is a mixture of a number of C5 components in this system. Higher and lower pressures were explored to see their effect on the economics. The 10 bar pressure seems to be about the optimum since going above this pressure does not shift the azeotrope significantly and raises the base temperature, which would require higher-temperamre energy input. 

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