Charge residence time

The emulsion process can be modified for the continuous production of latex. One such process (68) uses two stirred-tank reactors in series, followed by insulated hold-tanks. During continuous operation, 60% of the monomers are continuously charged to the first reactor with the remainder going into the second reactor. Surfactant is added only to the first reactor. The residence time is 2.5 h for the first reactor where the temperature is maintained at 65°C for 92% conversion. The second reactor is held at 68°C for a residence time of 2 h and conversion of 95%. 

Some slurry processes use continuous stirred tank reactors and relatively heavy solvents (57) these ate employed by such companies as Hoechst, Montedison, Mitsubishi, Dow, and Nissan. In the Hoechst process (Eig. 4), hexane is used as the diluent. Reactors usually operate at 80—90°C and a total pressure of 1—3 MPa (10—30 psi). The solvent, ethylene, catalyst components, and hydrogen are all continuously fed into the reactor. The residence time of catalyst particles in the reactor is two to three hours. The polymer slurry may be transferred into a smaller reactor for post-polymerization. In most cases, molecular weight of polymer is controlled by the addition of hydrogen to both reactors. After the slurry exits the second reactor, the total charge is separated by a centrifuge into a Hquid stream and soHd polymer. The solvent is then steam-stripped from wet polymer, purified, and returned to the main reactor the wet polymer is dried and pelletized. Variations of this process are widely used throughout the world. 

Nonreacdive substances that can be used in small concentrations and that can easily be detected by analysis are the most useful tracers. When making a test, tracer is injected at the inlet of the vessel along with the normal charge of process or carrier fluid, according to some definite time sequence. The progress of both the inlet and outlet concentrations with time is noted. Those data are converted to a residence time distribution (RTD) that tells how much time each fracdion of the charge spends in the vessel. 

Plugflow reactor (PER), in which all portions of the charge have the same residence time. The concentration varies with time and position, according to the equation,... 

Equations (2-3.7) forQjand Eqs. (2-3.1), (2-3.2), and (2-3.8) forr are used extensively in static hazard analysis. Examples include selection and use of instrumentation (3-5.3) and residence time provisions for charged liquids (5-2.4). 

Various theoretical and empirical models have been derived expressing either charge density or charging current in terms of flow characteristics such as pipe diameter d (m) and flow velocity v (m/s). Liquid dielectric and physical properties appear in more complex models. The application of theoretical models is often limited by the nonavailability or inaccuracy of parameters needed to solve the equations. Empirical models are adequate in most cases. For turbulent flow of nonconductive liquid through a given pipe under conditions where the residence time is long compared with the relaxation time, it is found that the volumetric charge density Qy attains a steady-state value which is directly proportional to flow velocity... 

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 ... 

In a batch vessel, the question of good mixing will arise at the start of the batch and whenever an ingredient is added to the batch. The component balance, Equation (1.21), assumes that uniform mixing is achieved before any appreciable reaction occurs. This will be true if Equation (1.55) is satisfied. Consider the same vessel being used as a flow reactor. Now, the mixing time must be short compared with the mean residence time, else newly charged... 

Data of Badder and Brooks (24). Figure 7 shows the comparisons of EPM with the experimental data obtained by Badder and Brooks (2A) in a CSTR (run C-24). The reactor feed contained 22.8% styrene, 0.64% emulsifier (sodium dodecylsulfate), and 0.39% initiator (ammonium persulfate). The residence time was 114 min. The initial reactor charge was water and emulsifier. In this case the size of the primary precursors was varied slightly from its baseline value of 1 nm to 0.8 nm. Although the experimental data show some scatter, EPM reproduces very well both the transient and steady state... 

A fixed-bed reactor system was employed (Figure 32.2). Each of the two reactors was charged with 38 cc of Amberlyst BD20 catalyst. Sample ports located at the exit of each reactor enabled increased acquisition of residence time data. Pressure was maintained by a back pressure control valve to maintain methanol in the liquid phase. After charging, the 1st and then 2nd reactors were connected to the pumps and filled with the reaction mixture while vapor was released from each through the top vent valve. Once each reactor was filled with liquid and emptied of vapor, the pressure regulator was connected to the output and both reactors were immersed into the water bath. 

A feed containing Ca0 = 1.5 and Cb0 - 1.75 lbmol/cuft is charged at the rate of 100 cuft/hr to a CSTR followed by a plug flow reactor with half the residence time of the CSTR. The concentration leaving the system is to be Ca2 = 0.4. Find the composition C.1 leaving the CSTR and the sizes of the two reactors. Also, find the concentrations with the same sizes when the PFR is first. 

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