Residence Time and Recycling

The Tm occurs at a relatively sharp point for crystalline materials. Amorphous materials basically do not have Tm they simply start melting as soon as the heat cycle begins. In reality there is no single melt point, but rather a range, which is often taken as the peak of a differential scanning calorime- 

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The experimental DDO crystallizer was shown in Figures (2-3). A run was made for each set of conditions, e.g. DDO ratio, residence time and recycle ratio. The run was continued until a steady state CSD was obtained. (Size analysis was made using a PDI ELZONE 80 XY Particle Counter). Steady state was ascertained by analyzing particle size in sample intervals of one hour. 

Tsai et al. (2007) investigated flotation for CMP wastewater treatment at both the lab and pilot scale. The CMP wastewater had a pH of 9.4, total solids of 8200 mg/L, total Si of 4(X)0 mg/L, turbidity of550 NTU, zeta potential of —50 mV, and a mean particle size of 106 nm. Following screening of alternative coagulants and surfactants, they used a 2k factorial design to evaluate removal efficiency for total solids, dissolved silica, and mrbidity as a function of four operating variables PACl concentration, sodium oleate concentration, hydraulic residence time, and recycle ratio. Optimal... 

Oxidation of cumene to cumene hydroperoxide is usually achieved in three to four oxidizers in series, where the fractional conversion is about the same for each reactor. Fresh cumene and recycled cumene are fed to the first reactor. Air is bubbled in at the bottom of the reactor and leaves at the top of each reactor. The oxidizers are operated at low to moderate pressure. Due to the exothermic nature of the oxidation reaction, heat is generated and must be removed by external cooling. A portion of cumene reacts to form dimethylbenzyl alcohol and acetophenone. Methanol is formed in the acetophenone reaction and is further oxidized to formaldehyde and formic acid. A small amount of water is also formed by the various reactions. The selectivity of the oxidation reaction is a function of oxidation conditions temperature, conversion level, residence time, and oxygen partial pressure. Typical commercial yield of cumene hydroperoxide is about 95 mol % in the oxidizers. The reaction effluent is stripped off unreacted cumene which is then recycled as feedstock. Spent air from the oxidizers is treated to recover 99.99% of the cumene and other volatile organic compounds. 

P 62] The acid cleavage was carried out at 45-75 °C at a pressure of 1-5 bar. Water may be added at levels of 0.3-1 wt.-% [64]. This addition was made upstream of the micro reactor or directly inside. The residence time was set in the range 0.5-5 min. Sulfuric acid was used as catalyst. By changing residence time and acid addition, the residual cumene hydroperoxide content was favorably reduced to 0.1-0.3 wt.-%. For this, an acid concentration of 50-500 ppm is typically required. Part of the so cleaved product stream may be recycled. 

Continuous Operation Continuous operation (also called feed-and-bleed) involves the partial recycle of the retentate. The residence time and number of pump passes are in between those of single-pass and batch operation, depending on the fraction of retentate recycled. Several continuous units can be plumbed with the retentate flow feeding the next consecutive system. This configuration is commonly used for large-scale membrane systems. 

In FCC there are therefore four separate residence times, and the riser and regenerator are each described by two coupled mass-balance equations. The mass flow rates of catalyst between the reactors are the same since catalyst is recycled. 

Again, Wg is the quantity of catalyst and t is the average residence time. The recycle ratio, t] = vrec /1>0, which is determined by the volumetric stream, vrec, of the recycled reaction mixture and the feed, v0, should be large enough that the concentration- and temperature-gradient over the catalyst bed are only small. Criteria for selecting the recycle ratio are presented in References 17 and 18. Appropriate -values are in the range of 20 - 400,... 

The crude alcohol is distilled in falling film evaporators. These units are designed to provide very low residence time and also low absolute pressures. This assures low product and catalyst decomposition losses while minimizing alcohol recycle, thereby minimizing alcohol losses to the heavy ends bleed. 

Equation 5-354 determines the value of the recycle ratio for a given conversion, the residence time, and the rate constant in a plug flow reactor. Alternatively, an increase in R will lower the conversion since it produces backmixing in the reactor as it mixes with the feed entrance in the plug flow reactor. As R it yields... 

In AFBC units, heat is removed from the flue gas by a convection-pass tube bank. The particulates leaving the boiler with the flue gas consist of unreacted and spent sorbent, unburned carbon, and ash. Multiclones after the convection pass remove much of the particulate matter and recycle it to the combustor, increasing the in-furnace residence time and improving combustion efficiency and sulfur retention performance. Bubbling PFBC units do not have convection-pass tube banks and do not recycle solids to the boiler. 

The main goal of the CFB design is the determination of the reactor (riser) dimensions to firstly fulfil requirements of conversion (and thus of residence time) and secondly to achieve the heat exchange rate, if so required The recycle loop is of additional importance and its design can be based on correlations found in lito-ature (e.g. Smolders et al. [1]) and is not reported in this paper. 

The recycling of solids causes a spread in the residence time, and hence in the conversion of the particles as some rise only once through the riser while otha s are recycled several times. Partly unreacted feed can hence be discharged, causing insufficient conversion. It can be demonstrated (Smolders et al. [8]) that the conversion in the product stream (X a ), is equal to ... 

The Chemelec cell, Fig. 4(b), uses a fluidized bed of glass spheres as turbulence promoters to improve the mass transport to the electrodes consisting of a series of closely spaced gauze or expanded metal sheet. The residence time and the degree of conversion per pass are relatively low because the flow rate has to exceed the fluidization velocity of the bed. This cell is therefore suitable for pretreatment or recycling operations and is commonly used in the electroplating industry for maintaining a moderate metal ion concentration in a recirculated wash-water tank. 

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