Methanol—continued equilibrium

The points R have to be on a straight line terminating in the composition of the methanol hydroquinone clathrate (A) in equilibrium with a-hydroquinone at 25°C. The point B roughly corresponds to the composition of the clathrate obtained by Palin and Powell24 when crystallizing hydroquinone from methanol points between A and B form a continuous range of solid solutions in equilibrium with liquid phases whose compositions lie on the curve CE. It is found that the equilibrium clathrate has a composition corresponding to y — 0.474 at 25°C. 

Ardizzone et al. used the esterification of benzoic acid with methanol to test the catalytic performance of different SZ catalysts. " Water had to be continuously removed from the reaction medium to shift the reaction equilibrium to product formation and to avoid catalyst deactivation by sulfate leaching. According to these authors, catalysts with a higher density of acid sites with KdL values in the range —14.2 to -5.6 performed better. Acid sites with pKa. of... 

The fact that silanol persistence can be favored by equilibrium conditions rather than control of condensation kinetics by steric or electronic factors is usually not considered. The phase separation which results from highly condensed systems continuously removes material from deposition solutions, depleting soluble silane species. While condensed silanols or siloxanes are typically not regarded as participating in a reversible reaction with water or alcohol, they do indeed participate in an equilibrium reaction. Iler [16] has shown that even hydrated amorphous silicon dioxide has an equilibrium solubility in methanol, which implies the formation of soluble low molecular... 

An interest in anomalous CD properties of 4,4-dimethyl-3-keto steroids and 4,4,8/3-trimethyl-3-keto steroids117118 continued in the studies of Tsuda and coworkers on onoceranediones119,120. Analysis of the CD spectra of 31-35 in methanol and dioxane led to the conclusion that the A-ring conformation in solution is in equilibrium between chair... 

The first reaction step is most probably hydrolysis of the aryl methyl ether by hydrobromic and acetic acid. Such a mixture is very efficient because one of the products, methanol, is continuously removed from the reaction mixture by esterification with acetic acid, and thus the reaction equilibrium between the starting material and the product is shifted toward the product. 

Systems that have the most potential for reactive distillation are those where the reaction is reversible, heat of reaction is not excessively large, and the products have the correct volatilities in relation to the reactants. Those systems reach chemical equilibrium (i.e., reaction stops) unless the reactants are in large excess or the products are continuously removed. An example system has been reported in the literature by Eastman Chemical (Agreda et al., 1990) for the production of methyl acetate from methanol and acetic acid. The discussion about process operation and the control strategy shown in the paper certainlv adhere to the plantwide control principles we have outlined in this book. 

Another obvious requirement of a nonaqueous solvent is chemical stability under a variety of conditions. Thus, methanol, especially after standing in the presence of air, may contain small amounts of formaldehyde which can react with groups on proteins and nucleic acids. Forma-mide, A, A-dimethylformamide, and related compounds, are slowly decomposed by acid or base in the solvent, and the possibility exists that such decomposition may be catalyzed to some extent by a protein dissolved in the solvent. Thus Rees and Singer (1956) found that the apparent osmotic pressure of a solution of insulin in lV,A -dimethylformamide continually increased over a period of a week at 25°C but reached equilibrium at 13.8°C, which might have been due to the slow decomposition of the solvent on the solution side of the osmotic membrane at the higher temperature. 

Fortier and Fritz [13] separated water by lEC and devised a unique equilibrium system for in-line spectrophotometric detection. This method has been refined and its capabilities expanded by continuing research by Chen and Fritz [14-17]. Water is separated chro-matographically from the other sample components on a short column packed with cation-exchange resin in the form using dry methanol as the eluent. Detection of the water peak is made possible by addition of a low concentration of cinnamaldehyde to the methanol eluent. In the presence of an acid catalyst, such as a -cation exchanger, cinnamaldehyde reacts with methanol to form the dimethylacetal. 

Esterification is finally an equilibrium reaction (35 per cent methyl methacrylate), which can be continued to completion by removing one or both of the products obtained as soon as they are formed. It takes place preferably in the liquid phase, in the presence of sulfuric acid or cation exchange resins as a catalyst, with a slight excess of methanol (1.2/1 in mol), at temperatures (110 to 115°Q apd pressures (30 to 50 kPa absolute) designed to limit polymerization reactions. The addition of an inhibitor (such as hydro-quinone) is also practised. With residence time of about 1 h. once-through conversion is total and the molar yield is close to 99 per cent. 

Their laboratory PVMR consisted of a reservoir in which the reactants were placed together with Nafion pellets, which acted as the catalyst. The liquid in the reservoir was continuously recirculated through the membrane tube, which was placed externally to the reactor. The membrane, itself, was also shown to be catalytic. A flow of inert gas (rather than vacuum) was used to remove the vapors and water from the membrane permeate. For the methanol esterification reaction the improvement in yield was modest (final conversion 77 % vs. 73 % corresponding to equilibrium), because the membrane was not very permselective towards the reaction products. Significant improvements, on the other hand, were observed with the butanol reaction (final conversion 95 % vs. 70 % corresponding to equilibrium), as the membrane is more permselective towards the products of this reaction. Exchanging the acidic protons in the Nafion membranes with cesium ions significantly improved the permselectivity, but also reduced membrane permeance. 

An open kettle contains 50 kmol of a dilute aqueous solution of methanol (2 mol% of methanol), at the bubble point, into which steam is continuously sparged. The entering steam agitates the kettle contents so that they are always of uniform composition, and the vapor produced, always in equilibrium with the liquid, is led away. Operation is adiabatic. For the concentrations encountered it may be assumed that the enthalpy of the steam and evolved vapor are the same, the enthalpy of of the liquid in the kettle is essentially constant, and the relative volatility is constant at 7.6. 

This procedure resulted in an efficient and selective substrate conversion and it was established by spectrophotometry that there was no catalyst leaching to the apolar phase. However, a marked decrease in the catalytic activity was observed after the third cycle. This was probably caused by a continuous loss of free triphenylphosphine ligand present in equilibrium with the rhodium complex, ultimately generating inactive species. To overcome this problem the cationic rhodium complex (Rh(cod)(dppe)]PFg was tested. In the ternary mixture containing CH2CI2 this complex showed poor catalytic activity. Using methanol instead clearly increased the activity. This effect of increased activity in methanol is well known for rhodium complexes. 

Figures 10.7 and 10.8 show the liquid-phase compositions for the reboiler and condenser as functions of time. After column start-up, the concentration of methanol decreases continuously whereas the distillate mole fraction of methyl acetate reaches about 90%. A comparison of the rate-based simulation using the Max-well-Stefan diffusion equations (Eq. (10.1)) and experimental results for the liquid-phase composition at the column top and in the reboiler demonstrates their satisfactory agreement. Figure 10.9 shows the simulation results obtained after an operation time of 10000 s with different modeling approaches the model including the Maxwell-Stefan diffusion description, the model with effective diffusion coefficients, and the equilibrium-stage model. Both the Maxwell-Stefan...

The experiment is performed on a rotary evaporator in order to drive the equilibrium process to completion (by continual removal of liberated methanol). Because the product is somewhat unstable on silica gel, a small quantity of triethylamine is added to neutralise the slurry and minimise decomposition during column chromatography. 

D8. We plan to batch distill a mixture of methanol and water in a batch distillation system at 1.0 atm pressure. The distillation system consists of a large still pot that is an equilibrium contact, a distillation column that acts as two equilibrium contacts (total 3 equilibrium contacts), and a total condenser. The feed to the system is F = 10.0 kmol and Xp = 0.4 (mole fraction methanol). We operate with a constant Xq = 0.8 as we increase L/D. The batch operation is continued until L/D = 4.0. CMO is valid. Equilibrium data are in Table 2-7. Find ... 

Another situation we can explore by analogy occurs when you want to go in one direction, but a number of people are headed in another direction. The contact or friction with these other bodies will tend to carry you in the direction they are going, and if there are sufficient numbers of them, you may be swept along with them The molecular equivalent can occur in a flow situatiom Assume we have a water stream with a modest amount of methanol and a small amount of ethanol on the left, and another water stream with a small amount of methanol and a fairly large amount of ethanol on the right. If we allowed the system to come to equilibrium, we would have equal methanol concentrations everywhere. In the nonequilibrium flow situation, we can continually transfer ethanol from the right to the left. We (the people—not the molecules, which just move randomly) would expect random fluctuations to move methanol from the left to the right, but if there is sufficient ethanol movement, we may observe the reverse transfer direction of methanol. This case is discussed in Section 15.7. 

Semicontinuous rectification processes are of practical importance for vapor mixtures generated in a discontinuous chemical reaction. These then have to be separated into a component continuously fed back to the reactor and a discharging fraction. For example, the main task during the transesterification of dimethyl teraphthalate with ethylene glycol is to continuously separate methanol from the reaction mixture. The reaction equilibrium is then shifted toward the product. Methanol is the top product of the column which is connected directly to the transesterification reactor. The high-boiling, or heavy, bottom product. 

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