Medium residence time

GL 19] ]R 9] ]P 20] A micro-reactor test imit allows operation at short and medium residence times (1-11 min), whereas a mini batch has to be used for reactions longer than this period [70],... 

Concentrations of Some Elements with Medium Residence Times... 

The concentration pulses can be smoothened out in remixing devices. To do this, a vessel with a residence characteristic of a perfectly mixed flow vessel and a medium residence time of five to ten times the dosing interval is necessary. In the case of a total standard volumetric flow rate of 2 SLM and a pulsing interval of one minute, one needs a device that can hold approximately 10-20 L, which conforms to a delay in the system response of 5-10 min. 

If the liquid transport is not continuous, the vaporizer can smoothen the concentration by remixing. That is why the vaporizer should show the residence time behavior of an perfectly mixed flow vessel in relation to the carrier gas, and the medium residence time of the gas is about five to ten times longer than the conveying pulse interval. 

Continuous fluid-bed dryers, steam tube rotary dryers, and rotary dryers can be designed to provide medium residence time (of the order of minutes). 

Catalytic cracking is a key refining process along with catalytic reforming and alkylation for the production of gasoline. Operating at low pressure and in the gas phase, it uses the catalyst as a solid heat transfer medium. The reaction temperature is 500-540°C and residence time is on the order of one second. 

The problems of monomer recovery, reaction medium viscosity, and control of reaction heat are effectively dealt with by the process design of Montedison Fibre (53). This process produces polymer of exceptionally high density, so although the polymer is stiU swollen with monomer, the medium viscosity remains low because the amount of monomer absorbed in the porous areas of the polymer particles is greatly reduced. The process is carried out in a CSTR with a residence time, such that the product k jd x. Q is greater than or equal to 1. is the initiator decomposition rate constant. This condition controls the autocatalytic nature of the reaction because the catalyst and residence time combination assures that the catalyst is almost totally expended in the reactor. 

Another hydrogenation process utilizes internally generated hydrogen for hydroconversion in a single-stage, noncatalytic, fluidized-bed reactor (41). Biomass is converted in the reactor, which is operated at about 2.1 kPa, 800°C, and residence times of a few minutes with steam-oxygen injection. About 95% carbon conversion is anticipated to produce a medium heat value (MHV) gas which is subjected to the shift reaction, scmbbing, and methanation to form SNG. The cold gas thermal efficiencies are estimated to be about 60%. 

An independent development of a high pressure polymerization technology has led to the use of molten polymer as a medium for catalytic ethylene polymerization. Some reactors previously used for free-radical ethylene polymerization at a high pressure (see Olefin polymers, low density polyethylene) have been converted to accommodate catalytic polymerization, both stirred-tank and tubular autoclaves operating at 30—200 MPa (4,500—30,000 psig) and 170—350°C (57,83,84). CdF Chimie uses a three-zone high pressure autoclave at zone temperatures of 215, 250, and 260°C (85). Residence times in all these reactors are short, typically less than one minute. 

In 1973, a multistage surface-fermentation process was patented in Japan for the production of acetic acid (42) eight surface fermenters were connected in series and arranged in such a way that the mash passed slowly through the series without disturbing the film of yAcetobacter on the surface of the medium. This equipment is reported to produce vinegar of 5% acidity and 0.22% alcohol with a mean residency time in the tanks of 22 h. 

The volume of the manifold and the sampling flow rate determine the time required for the gas to move from the inlet to the collection medium. This residence time can be minimized to decrease the loss of reactive species in the manifold by keeping the manifold as short as possible. 

Residence time in heated zone Low Medium Low Medium... 

Flooded-bundle (kettle) Easy maintenance and cleaning. Convenient when heating medium is dirty. Equivalent to theoretical plate. Contains vapor disengaging space. Lower heat transfer rates. Extra piping and space required. High residence time in heated zone. Easily fouled. 

Schiesser and Lapidus (S3), in later studies, measured the liquid residencetime distribution for a column of 4-in. diameter and 4-ft height packed with spherical particles of varying porosity and nominal diameters of in. and in. The liquid medium was water, and as tracers sodium chloride or methyl orange were employed. The specific purposes of this study were to determine radial variations in liquid flow rate and to demonstrate how pore diffusivity and pore structure may be estimated and characterized on the basis of tracer experiments. Significant radial variations in flow rate were observed methods are discussed for separating the hydrodynamic and diffusional contributions to the residence-time curves. 

To run the residence time distribution experiments under conditions which would simulate the conditions occurring during chemical reaction, solutions of 15 weight percent and 30 percent polystyrene in benzene as well as pure benzene were used as the fluid medium. The polystyrene used in the RTD experiment was prepared in a batch reactor and had a number average degree of polymerization of 320 and a polydispersity index, DI, of 1.17. 

P 29] A set-up comprising a steel caterpillar mini mixer and four steel tubes attached was used, being dipped into a cylinder completely filled with a cooling medium (scale-up set-up) [48,108]. By means of a 5/2-way valve, it was possible to switch the reactants to either of the tubes acting as delay loops, differing in inner diameter and hence residence time. 

Large stirred tank reactors are generally not suited for use at high pressures because of mechanical strength limitations. They are used mainly for liquid phase reaction systems at low or medium pressures when appreciable residence times are required. 

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