Product yields with temperature residence time

This process yields satisfactory monomer, either as crystals or in solution, but it also produces unwanted sulfates and waste streams. The reaction was usually mn in glass-lined equipment at 90—100°C with a residence time of 1 h. Long residence time and high reaction temperatures increase the selectivity to impurities, especially polymers and acrylic acid, which controls the properties of subsequent polymer products. 

Table 9 presents typical results from an operation extending beyond 7200 sec in duration and compares them to the yield from the predictive model simulation. The conditions under which these results were obtained were flow rates of 0.57 g/sec (both for naphtha and for steam) and 17.8 g/sec (flue gas) at inlet temperatures of 723 K on the process side and 1298 K on the flue gas side. This design shows considerable promise for controlling product composition and conversion with short residence time. 

The later patent described the use of a tubular reactor in a more continuous process [21]. A suspension of 8 and two equivalents of NaOH in HzO was treated with one equivalent of aqueous NaOCl at temperatures no higher than 15°C. After about 20 minutes the resulting solution was passed through a tubular reactor heated to 80°C, with a residence time of about 1.5 minutes. The output from the tubular reactor was metered into a distillation column that contained refluxing HjO.The product 10 was recovered by distillation in 93% total yield for two fractions. The modifications offered in the second patent were claimed to increase productivity. 

A suitable microreactor system corresponding to the above mentioned requirements was developed by Worz et al. [89]. Their installation consisted of 32 stainless steel channels of 900 X 60 pm size separated by cooling channels (Figure 7.24). Reactant and the acid were mixed extremely fast in these microchannels and cooled simultaneously. As the product is sensitive for consecutive reactions, it is obvious that the absence of backmixing increases the product yield. At a temperature of 20 C, a maximum yield of 90-95% could be achieved with a residence time of 30 s. The reaction is quenched by diluting the concentrated sulfuric acid-reactant mixture with water. The dilution of concentrated sulfuric acid has an even higher exothermicity and must carried... 

When a solution containing sulfuryl chloride and 40 molar equivalents of cyclohexane was introduced to a microreactor with a residence time 19 min at room temperature, the reaction gave chlorocydohexane selectively in 22% yield (entry 1). While increasing molar ratio of cyclohexane to 80 equiv. did not affect the yield of the product (20%, entry 2), extending residence time (57 min, flow rate l.Oml/h) raised product yield to 35% (entry 3). 

The temperature profile is the most important aspect of operational control for pyrolysis processes. Material flow rates, both solid and gas phase, together with the reactor temperature control the key parameters of heating rate, highest process temperatures, residence time of solids and contact time between solid and gas phases. These factors affect the product distribution and the product properties. Solid residence time is another important factor in the bio-oil yields. A short residence time enhances biooil yields, while a longer residence time increases char production (Antal and Gronli, 2003). 

Pyrolysis of biomass is carried out under inert atmosphere and forms, depending on the residence time and temperature, char, oil, and gas. Pyrolysis with long residence time at low temperamre (400°C) produces a black solid (charcoal), while fast pyrolysis at high temperarnre (500°C) favors the formation of a black liquor (bio-oil). The short contact times (<2s at ca. 500" C) thus maximize the liquid yield. Fast pyrolysis is preferred by the chemical industry because of the relative ease of handling liquids. However, bio-oil produced by pyrolysis of bulk biomass contains more than 400 different components like carboxylic acids, ketones, aldehydes, sugars, furans, (substituted) phenols, aromatics, and tar (Table 1). Separation of useful chemicals from this complex pool is very difficult. As an alternative, pyrolysis can also be used as a first step for generating heat or electricity, followed by combusting the pyrolytic products. Excellent papers and reviews that describe fast pyrolysis in more detail are available [27-32]. 

Making a temperature-residence time map for such reactions is useful for controlling the reactions at optimum temperatures and with optimum residence times enabling selective synthesis of desired products with high yields. 

Thus, the optimal design is found at the limits of the constraints. A high flow rate yields a low residence time and thus a small concentration of C in the outlet stream (note that we maximize the product of this concentration with the volumetric flow rate). The A and B inlet concentrations are maximized to yield the fastest rate of the first reaction, and the temperature is at the upper limit. The loss of C by the third reaction is shght, as the short residence time and low C concentration make the rate of this reaction small compared to the first one. 

Figure 2 illustrates the three-step MIBK process employed by Hibernia Scholven (83). This process is designed to permit the intermediate recovery of refined diacetone alcohol and mesityl oxide. In the first step acetone and dilute sodium hydroxide are fed continuously to a reactor at low temperature and with a reactor residence time of approximately one hour. The product is then stabilized with phosphoric acid and stripped of unreacted acetone to yield a cmde diacetone alcohol stream. More phosphoric acid is then added, and the diacetone alcohol dehydrated to mesityl oxide in a distillation column. Mesityl oxide is recovered overhead in this column and fed to a further distillation column where residual acetone is removed and recycled to yield a tails stream containing 98—99% mesityl oxide. The mesityl oxide is then hydrogenated to MIBK in a reactive distillation conducted at atmospheric pressure and 110°C. Simultaneous hydrogenation and rectification are achieved in a column fitted with a palladium catalyst bed, and yields of mesityl oxide to MIBK exceeding 96% are obtained. 

When the first edition of Chemistry of Petrochemical Processes was written, the intention was to introduce to the users a simplified approach to a diversified subject dealing with the chemistry and technology of various petroleum and petrochemical process. It reviewed the mechanisms of many reactions as well as the operational parameters (temperature, pressure, residence times, etc.) that directly effect products yields and composition. To enable the readers to follow the flow of the reactants and products, the processes were illustrated with simplified flow diagrams. 

In this way, the operational range of the Kolbe-Schmitt synthesis using resorcinol with water as solvent to give 2,4-dihydroxy benzoic acid was extended by about 120°C to 220°C, as compared to a standard batch protocol under reflux conditions (100°C) [18], The yields were at best close to 40% (160°C 40 bar 500 ml h 56 s) at full conversion, which approaches good practice in a laboratory-scale flask. Compared to the latter, the 120°C-higher microreactor operation results in a 130-fold decrease in reaction time and a 440-fold increase in space-time yield. The use of still higher temperatures, however, is limited by the increasing decarboxylation of the product, which was monitored at various residence times (t). 

OS 40] [R 20] [P 28] The best yield obtained in the interdigital micro-mixer laboratory-scale set-up was 83% (22 °C 1000 ml h 8 s) ]48]. Compared with the performance of the industrial production process (65%, batch), this is an improvement of nearly 20%. Most experiments done at widely different residence time and reaction temperature did not differ from this best yield by more than 5-10%. 

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