Long residence time

Achieving complete conversion of FEED to PRODUCT in the reactor usually requires an extremely long residence time, which is normally uneconomic (at least in continuous processes). Thus, if there is no byproduct formation, the initial reactor conversion is set to be around 95 percent, as discussed in Chap. 2. The reactor effluent thus contains unreacted FEED and PRODUCT (Fig. 4.1a). 

Many optical studies have employed a quasi-static cell, through which the photolytic precursor of one of the reagents and the stable molecular reagent are slowly flowed. The reaction is then initiated by laser photolysis of the precursor, and the products are detected a short time after the photolysis event. To avoid collisional relaxation of the internal degrees of freedom of the product, the products must be detected in a shorter time when compared to the time between gas-kinetic collisions, that depends inversely upon the total pressure in the cell. In some cases, for example in case of the stable NO product from the H + NO2 reaction discussed in section B2.3.3.2. the products are not removed by collisions with the walls and may have long residence times in the apparatus. Study of such reactions are better carried out with pulsed introduction of the reagents into the cell or under crossed-beam conditions. 

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. 

The feed is normally introduced to the top hearth where the rabble arms and teeth attached to the central shaft rotate and spiral soflds across the hearth to the center, where an opening is provided and the soflds drop to the next hearth. The teeth of the rabble arms on the hearth spiral the soflds toward the outside to ports that let the soflds drop down to the next hearth. Soflds continue downward, traversing each hearth until they reach the bottom and the ash is discharged. The primary advantage of this system is the long residence time in the furnace controlled by the speed of the central shaft and pitch of the teeth. 

Opera.tlon, Because of the long residence time of the materials (8—10 h), the blast furnace process can exhibit considerable inertia, and control is usually appHed where the goal is maintaining smooth, stable input conditions. One of the most important aspects of blast furnace control is supply of consistent quaUty raw materials, which is why there is a strong emphasis on quaUty control at coke plants, peUeti2ing plants, and sinter plants (see Quality ASSURANCE/QUALITY control). 

Anionic polymerization offers fast polymerization rates on account of the long life-time of polystyryl carbanions. Early studies have focused on this attribute, most of which were conducted at short reactor residence times (< 1 h), at relatively low temperatures (10—50°C), and in low chain-transfer solvents (typically benzene) to ensure that premature termination did not take place. Also, relatively low degrees of polymerization (DP) were typically studied. Continuous commercial free-radical solution polymerization processes to make PS, on the other hand, operate at relatively high temperatures (>100° C), at long residence times (>1.5 h), utilize a chain-transfer solvent (ethylbenzene), and produce polymer in the range of 1000—1500 DP. 

Open-loop systems have inherently long residence times which may be detrimental if the retentate is susceptible to degradation by shear or microbiological contamination. A feed-bleed or closed-loop configuration is a one-stage continuous membrane system. At steady state, the upstream... 

Both types of coalescence can be important in the foam separations characterized by low gas flow rate, such as batchwise ion flotation producing a scum-bearing froth of comparatively long residence time. On the other hand, with the relatively higher gas flow rate of foam fractionation, the residence time may be too short for the first type to be important, and if the foam is sufficiently stable, even the second type of coalescence may be unimportant. 

The high temperature pyrolysis of sulfonyl fluonde results in the elimination of sulfur dioxide, although secondary reactions also occur, depending on the residence tune With perfluorooctanesulfonyl fluonde, long residence times result in perfluoro(Cg-Cig) compounds, and shorter residence times lead to perfluoro-hexadecane [98] (equation 65)... 

A low reactor temperature may not fully vaporize the feed unvaporized feed droplets will aggregate to form coke around the feed nozzles on the reactor walls and/or the transfer line. A long residence time in the reactor and transfer line also accelerate coke buildup. 

Low Reactor temperature Long Residence Time in the Reactor and Main Column High Bottoms temperature Low Bottoms Pumparound Rate Cold exchanger tube wail V temperature ... 

They claim that in substantially all of the prior art processes, the operation is really a modified batch process in that high hold-up vessel-type nitrators are employed. Moreover, extensive circulation, recirculation, and a relatively long residence time of the reaction mixt in the nitrating zone are characteristics of a majority of the previously proposed methods, and such features are inherently undesirable because they favor degradative side reactions, which occur at all stages of the nitration of toluene to trinitrotoluene, and particularly in the final stage. ... 

Croup II (Ca, Na). This group includes the remaining cations with relatively long residence times. One important constraint is the charge balance of seawater, re-arranged in the following format ... 

This function is shown in Figure 15.9. It has a sharp first appearance time at tflrst = tj2. and a slowly decreasing tail. When t > 4.3f, the washout function for parabohc flow decreases more slowly than that for an exponential distribution. Long residence times are associated with material near the tube wall rjR = 0.94 for t = 4.3t. This material is relatively stagnant and causes a very broad distribution of residence times. In fact, the second moment and thus the variance of the residence time distribution would be infinite in the complete absence of diffusion. 

A Near isothermal Slow >10 min Long residence time. Plug flow or CSTR depending on kinetics and reaction network. Improved QC, lower inventory... 

All conventional reactors, tested before using the micro reactor (simply since micro reactors were hardly available at that time), only fulfilled the demands of one measure, at the expense of the other measures. For instance, a single-tube reactor can be operated nearly isothermally, but the performance of the oxidative dehydrogenation suffers from a too long residence time. A short shell-and-tube reactor provides much shorter residence times at improved heat transfer, which however is still not as good as in the micro reactor. 

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