Molecular flow rate

The molecular flow rate of B (FB) can also be expressed as a function of the stoichiometric ratio (M) and the feed time, tfd ... 

The batch reactor is characterized by its volume, Fr, and the holding time, t, that the fluid has spent in the reactor. Flow reactors are usually characterized by reactor volume and space time, r, with the latter defined as the reactor volume divided by the volumetric flow rate of feed to the reactor. The physical significance of r is the time required to process a volume of fluid corresponding to Rr. For catalytic reactions, the space time may be replace by the site time, xp, defined as the number of catalytic sites in the reactor, Sr, divided by the molecular flow rate of feed to the reactor, F. The physical interpretation of rp is the time required to process many molecules equal to the number of active sites in the reactor. 

For complex catalytic reactions requiring numerical analyses, it is useful to write the material balance equations for flow reactors in terms of molecular flow rates per active site (/ /, = Fi/Sr), which are denoted as molecular site velocities. For batch reactors, the number of gaseous molecules per active site (Ns,i = Ni /.SR) is used. (These normalized quantities are typically of the order of unity.) The batch reactor, CSTR, and PFR material balance equations become the following ... 

The sum of inlet molecular flow rates per site for a flow reactor, (FT), is equal to the reciprocal of the site time, l/rp, which may be defined as the overall inlet site velocity, //. ... 

The reaction scheme outlined previously leads to a kinetic model containing 226 unknowns (i.e., 151 gaseous molecular flow rates, 74 surface species, and the fraction of free sites). We assumed that the reactor operates as a PFR. Therefore, we solve 151 differential equations for the gaseous... 

N — molecular flow rate of water, moles/hr Nu = Nusselt number for heat transfer, hDlk p = partial pressure of water vapor in local atmospheres q = energy transfer rate, Btu/hr... 

The problem is two-fold firstly, the total molecular flow rate into the mass spectrometer must be sufficiently low that economically feasible vacuum systems (Section 6.6) can maintain a sufficiently good vacuum that the ion trajectories through the m/z selection stage are not significantly perturbed via ion-molecule collisions (this problem... 

The most common alternative to distillation for the separation of low-molecular-weight materials is absorption. In absorption, a gas mixture is contacted with a liquid solvent which preferentially dissolves one or more components of the gas. Absorption processes often require an extraneous material to be introduced into the process to act as liquid solvent. If it is possible to use the materials already in the process, this should be done in preference to introducing an extraneous material for reasons already discussed. Liquid flow rate, temperature, and pressure are important variables to be set. 

The most common alternative to distillation for the separation of low-molecular-weight materials is absorption. Liquid flow rate, temperature, and pressure are important variables to be set, but no attempts should be made to carry out any optimization at this stage. 

The efficiency of separation of solvent from solute varies with their nature and the rate of flow of liquid from the HPLC into the interface. Volatile solvents like hexane can be evaporated quickly and tend not to form large clusters, and therefore rates of flow of about 1 ml/min can be accepted from the HPLC apparatus. For less-volatile solvents like water, evaporation is slower, clusters are less easily broken down, and maximum flow rates are about 0.1-0.5 ml/min. Because separation of solvent from solute depends on relative volatilities and rates of diffusion, the greater the molecular mass difference between them, the better is the efficiency of separation. Generally, HPLC is used for substances that are nonvolatile or are thermally labile, as they would otherwise be analyzed by the practically simpler GC method the nonvolatile substances usually have molecular masses considerably larger than those of commonly used HPLC solvents, so separation is good. 

Adsorption systems employing molecular sieves are available for feed gases having low acid gas concentrations. Another option is based on the use of polymeric, semipermeable membranes which rely on the higher solubiHties and diffusion rates of carbon dioxide and hydrogen sulfide in the polymeric material relative to methane for membrane selectivity and separation of the various constituents. Membrane units have been designed that are effective at small and medium flow rates for the bulk removal of carbon dioxide. 

A sharp separation results in two high purity, high recovery product streams. No restrictions ate placed on the mole fractions of the components to be separated. A separation is considered to be sharp if the ratio of flow rates of a key component in the two products is >10. The separation methods that can potentially obtain a sharp separation in a single step ate physical absorption, molecular sieve adsorption, equiHbrium adsorption, and cryogenic distillation. Chemical absorption is often used to achieve sharp separations, but is generally limited to situations in which the components to be removed ate present in low concentrations. 

Chlorination of Hydrocarbons or Chlorinated Hydrocarbons. Chlorination at pyrolytic temperatures is often referred to as chlorinolysis because it involves a simultaneous breakdown of the organics and chlorination of the molecular fragments. A number of processes have been described for the production of carbon tetrachloride by the chlorinolysis of various hydrocarbon or chlorinated hydrocarbon waste streams (22—24), but most hterature reports the use of methane as the primary feed. The quantity of carbon tetrachloride produced depends somewhat on the nature of the hydrocarbon starting material but more on the conditions of chlorination. The principal by-product is perchloroethylene with small amounts of hexachloroethane, hexachlorobutadiene, and hexachloroben2ene. In the Hbls process, a 5 1 mixture by volume of chlorine and methane reacts at 650°C the temperature is maintained by control of the gas flow rate. A heat exchanger cools the exit gas to 450°C, and more methane is added to the gas stream in a second reactor. The use of a fluidi2ed-bed-type reactor is known (25,26). Carbon can be chlorinated to carbon tetrachloride in a fluidi2ed bed (27). 

W = vapor flow rate, Ib/h D = drum diameter, ft Pl = hquid density, Ib/fF p, = vapor density, Ib/fF M = molecular weight of vapor P = pressure in the drum, psia T = temperature of the vapor, °R... 

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