AuthorWrite something about yourself. No need to be fancy, just an overview. Archives
June 2023
Categories |
Back to Blog
Dessiner molecule mestrenova1/2/2023 ![]() ![]() The tracer concentration at the outlet is measured and integrated with respect to time. All the tracer molecules enter together but leave at varying times. ![]() Suppose a small amount of tracer is instantaneously injected at time 1 = 0 into the inlet of a reactor. Impulse Response and the Differential Distribution. Most systems of chemical engineering importance are closed to a reasonable approximation. Systems with unidirectional inlet and outlet streams are closed in the sense of the axial dispersion model i.e., Di = D ut = 0- See Sections 9.3.1 and 15.2.2. The major theoretical requirement is that the inlet and outlet streams have unidirectional flows so that molecules that once enter the system stay in until they exit, never to return. It is usually possible to meet these requirements in practice. The flow properties of the tracer molecules must be similar to those of the reactant molecules. If the tracer has a background concentration, it is subtracted from the experimental measurements. The concentration of tracer molecules leaving the system must be accurately measured at the outlet. A good step change must be made at the reactor inlet. Washout experiments can be used to measure the residence time distribution in continuous-flow systems. Consider some time t > 0 when the fraction of molecules remaining in the system is W(t) = C ut(0ICo- These molecules must necessarily have entered the reactor before time t = 0 since no tracer was fed. The exponential form of Equation (15.1) is specific to a CSTR, but the concept of washout applies to any flow system. Tracer molecules originally in the system at time t = 0 gradually wash out. Perfect tracer Tracer molecules behave identically with the process fluid molecules within the process unit. Therefore, a large D indicates a rapid spreading of the tracer curve, a small D indicates slow spreading, and D = 0 means no spreading (hence, plug flow). The dispersion coefficient D (m /sec) represents this result in the tracer cloud. Additionally, a non-uniform velocity profile causes different portions of the tracer to move at different rates, and this results in a spreading of the measured response at the reactor outlet. The distribution of tracer molecule residence times in the reactor is the result of molecular diffusion and turbulent mixing if tlie Reynolds number exceeds a critical value. This phenomenon is likely to occur in high specific activity compounds stored at high radiochemical concentrations in the absence of free-radical scavengers. ![]() A related mechanism of degradation involves the direct interaction of the radioactive emission with other tracer molecules in the preparation. ![]()
0 Comments
Read More
Leave a Reply. |