Residence times for

In this condenser, part of the stripper off-gases are condensed (the heat of condensation is used to generate low pressure steam). The carbamate formed and noncondensed NH and CO2 are put into the reactor bottom and conversion of the carbamate into urea takes place. The reactor is sized to allow enough residence time for the reaction to approach equiUbrium. The heat required for the urea reaction and for heating the solution is suppHed by additional condensation of NH and CO2. The reactor which is lined with 316 L stainless steel, contains sieve trays to provide good contact between the gas and Hquid phases and to prevent back-mixing. The stripper tubes are 25-22-2 stainless steel. Some strippers are still in service after almost 30 years of operation. 

Computer Models, The actual residence time for waste destmction can be quite different from the superficial value calculated by dividing the chamber volume by the volumetric flow rate. The large activation energies for chemical reaction, and the sensitivity of reaction rates to oxidant concentration, mean that the presence of cold spots or oxidant deficient zones render such subvolumes ineffective. Poor flow patterns, ie, dead zones and bypassing, can also contribute to loss of effective volume. The tools of computational fluid dynamics (qv) are useful in assessing the extent to which the actual profiles of velocity, temperature, and oxidant concentration deviate from the ideal (40). 

Because of the formation of nitrogen oxides, a steady-state ozone concentration cannot be obtained instead, due to the buHdup of nitrogen oxides, an increase in residence time in the discharge results in a decrease in ozone concentration beyond the maximum value. Thus, there is an optimum residence time for maximum ozone production. 

Residence Time. Eor cost efficiency, residence time in the reactor should be minimized, but long enough to achieve complete combustion. Typical residence times for various thermal processes are incineration (0.1 s to 1.5 h), catalytic incineration (1 s), pyrolysis (12—15 min), and wet air oxidation (10— 30 min) (15). 

Process development of the use of hydrogen as a radical quenching agent for the primary pyrolysis was conducted (37). This process was carried out in a fluidized-bed reactor at pressures from 3.7 to 6.9 MPa (540—1000 psi), and a temperature of 566°C. The pyrolysis reactor was designed to minimize vapor residence time in order to prevent cracking of coal volatiles, thus maximizing yield of tars. Average residence times for gas and soHds were quoted as 25 seconds and 5—10 rninutes. A typical yield stmcture for hydropyrolysis of a subbiturninous coal at 6.9 MPa (1000 psi) total pressure was char 38.4, oil... 

For systems following invariant growth the crystal population density in each size range decays exponentially with the inverse of the product of growth rate and residence time. For a continuous distribution, the population densities of the classified fines and the product crystals must be the same at size Accordingly, the population density for a crystallizer operating with classified-fines removal is given by... 

Batch readers—optimum residence time for series and complex reactions, minimum cost, optimal operating temperature, and maximum rate of reaction... 

In most cases, direct-heat rotary dryers are still sized on the basis of pilot-plant tests, because rarely is all the moisture to be removed truly Tree moisture, and residence time for diffusion is frequently needed. 

Operating holdup contributes effectively to mass-transfer rate, since it provides residence time for phase contact and surface regeneration via agglomeration and dispersion. Static holdup is hmited in its contribution to mass-transfer rates, as indicated by Thoenes and Kramers [Chem. Eng. ScL, 8, 271 (1958)]. In laminar regions holdup in general has a negative effecl on the efficiency of separation. 

Besides looking at just the mixing, it is important at this time to also consider the settling time of the phases after mixing since this will impact on the settler design. Higher intensity of mixing may decrease the residence time for mass transfer, but at the same time create fine dispersions which are difficult to settle. 

Typical Temperatures and Residence Times for Destruction of Common Organics ... 

Many other combinations exist but will not be described here. In small cabins, for storage or work, it is possible to supply and exhaust air in a controlled way to have a defined climate. There are also special sluices, where air is used to rinse the clothes from settled contaminants before a person proceeds to the next, cleaner room. In this case, very high air velocities are used, which could cause discomfort to the person. The residence time for the person usually is less than a couple of minutes and the main objective is to clean the clothes (and sometimes the skin) and therefore the high velocities do not matter. 

An ideal plug flow reactor, for example, has no spread in residence time because the fluid flows like a plug through the reactor (Westerterp etal., 1995). For an ideal continuously stirred reactor, however, the RTD function becomes a decaying exponential function with a wide spread of possible residence times for the fluid elements. 

A fines removal system is installed on the crystallizer designed in the first example. Assuming that the cut size for the fines removal system is 50 im and the ratio of mean residence times for product and fines, rp/rp( = 7), is 10, calculate the mean product residence time now required to produce the same dominant size of 600 pm at the same production rate and suspension density. 

One useful practical result of the study crystallizer dynamics is that it can take about eight residence times for the CSD to achieve steady state after start up or return to it following a disturbance, as illustrated in Figure 7.12. 

Chemical Factors. These involve mainly the kinetics of the reaction. The design must provide sufficient residence time for the desired reaction to proceed to the required degree of conversion. 

Pollutants have various atmospheric residence times, with reactive gases and large aerosols being rapidly removed from air. In the London air pollution episode of December 1952, the residence time for sulfur dioxide was estimated to be five hours daily emissions of an estimated 2,000 tons of sulfur dioxide were balanced by scavenging by fog droplets, which were rapidly deposited. Most relatively inert gases remain in the atmosphere for extended periods. Sulfur hexafluoride, used extensively in the electric power industiy as an insulator in power breakers because of its inertness, has an estimated atmospheric lifetime of 3,200 years. 

The residence time for an ideal plug flow system is stipulated as ... 

Gal-Or and Resnick (G2, G6, G8) recently proposed a theoretical model, based on the gas residence time, for total mass transfer in a gas-liquid agitated contactor. They assumed that the number of bubbles in the vessel... 

Frequently, stirred tanks are used with a continuous flow of material in on one side of the tank and with a continuous outflow from the other. A particular application is the use of the tank as a continuous stirred-tank reactor (CSTR). Inevitably, there will be a vety wide range of residence times for elements of fluid in the tank. Even if the mixing is so rapid that the contents of the tank are always virtually uniform in composition, some elements of fluid will almost immediately flow to the outlet point and others will continue circulating in the tank for a very long period before leaving. The mean residence time of fluid in the tank is given by ... 

The initial kinetic energy of 0 ions produced by dissociative attachment in 02 at an electron energy of 6.9 e.v. may be determined from Equation 4 to be 1.64 e.v. using values of 1.465 e.v. (1) for A(0) and 5.09 e.v. (7) for D(O—O). The residence time for 0 ions calculated from Equation 1 is 6.0 X 10 7 sec. at 10 volts repeller potential. Rate constants for Reaction 6 determined from data at varying Vr are shown in Table I and are seen to increase sharply with increasing repeller potential, as expected for an endothermic process. 

The total amount of tertiary ion formed is the integral of this expression from P = 0 to tp = tp, where rP is the residence time for an unreacting primary ion. 

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