Variable residence time reactors

Hydrocarbonization processes are characterized by three primary independent variables - temperature, hydrogen pressure, and coal type - and five other, important independent variables -solid residence time, gas residence time, reactor configuration, coal pretreatment, and catalyst impregnation. Control of these variables permits control, over a wide range, of (1) the relative yields of liquid, gaseous, and solid products, (2) the quality of one or more of these products, (3) hydrogen consumption, and, ultimately (4) product cost. 

Continuous Stirred Tank Reactors. Biesenberger (8) solved for the MWD with condensation polymerization in a CSTR, analogous to the treatment Denbigh (14) provided for the other two mechanisms. In this case, the variable residence time distribution leads to an extremely broad MWD with even the maximum weight fraction at the lowest molecular weight (monomer). The dispersion index approaches infinity as the condensation is driven to completion in a stirred tank reactor. A sequential analytical solution of the algebraic equations was obtained with a numerical evaluation of the consecutive equations. 

There are situations where reactor temperature is not a dominant variable or cannot be changed for safety or yield reasons. In these cases, we must find another dominant variable, such as the concentration of the limiting reactant, flowrate of initiator or catalyst to the reactor, reactor residence time, reactor pressure, or agitation rate. 

Flow conditions may cause a more or less broad distribution of residence times for individual fluid packets or molecules. Effects of variable residence times have to be taken into account in the design and operation of large industrial reactors with adequate precautions the chemical engineer can prevent the undesirable effects of a residence time distribution, or utilize them. 

Figure 6.34. Calculation of the expected yield in a normal catalytic process (a) Double plot of concentration/time for a discontinuous process (p/Pend)/( AendX and curve for a continuous reactor with variable residence time distribution, (b) Evaluation of yield. (Adapted from Reusser, 1961.)...

Thus, on its way out of the reactor, in the volume element, there are species with variable residence times. The species in the volume element that have similar residence times, however, must have been introduced into the reactor at the same time. This is why we can even derive Equations 4.4 and 4.5 on the basis of a volume element entering the reactor. In this case, function E(t) dt describes that portion of the elements in the volume element that attains residence times between t and t + dt. If the distribution pattern of the species is known in the volume element leaving the reactor (at the outlet), it is possible to predict the RTD in an incoming volume element. A precondition for this, however, is that the system should have reached its steady state and that the flow conditions should remain constant during the time the volume element passes the system. 

Conrad process—American Plastics Council Variable residence time and pyrolysis temperatures, rotary kiln reactor, CaO for HCl capture Feedstock for steam crackers giving higher amount of monomers, 86% naphta grade products 381a... 

At still higher temperatures, when sufficient oxygen is present, combustion and "hot" flames are observed the principal products are carbon oxides and water. Key variables that determine the reaction characteristics are fuel-to-oxidant ratio, pressure, reactor configuration and residence time, and the nature of the surface exposed to the reaction 2one. The chemistry of hot flames, which occur in the high temperature region, has been extensively discussed (60-62) (see Col ustion science and technology). 

The unit Kureha operated at Nakoso to process 120,000 metric tons per year of naphtha produces a mix of acetylene and ethylene at a 1 1 ratio. Kureha s development work was directed toward producing ethylene from cmde oil. Their work showed that at extreme operating conditions, 2000°C and short residence time, appreciable acetylene production was possible. In the process, cmde oil or naphtha is sprayed with superheated steam into the specially designed reactor. The steam is superheated to 2000°C in refractory lined, pebble bed regenerative-type heaters. A pair of the heaters are used with countercurrent flows of combustion gas and steam to alternately heat the refractory and produce the superheated steam. In addition to the acetylene and ethylene products, the process produces a variety of by-products including pitch, tars, and oils rich in naphthalene. One of the important attributes of this type of reactor is its abiUty to produce variable quantities of ethylene as a coproduct by dropping the reaction temperature (20—22). 

How closely a design approaches minimum energy is largely determined by the raw materials and catalyst system chosen. However, if reaction temperature, residence time, and diluent are the only variables, there is still a tremendous opportunity to influence energy use via the effect on yield. Even given none of these, there is stiU wide freedom to optimize the heat interchange system (see Reactor technology). 

The production rate is 2—4 t/h, depending on the feed rate, monomer concentration in the feed, and conversion. The conversion of isobutylene and isoprene typically ranges from 75—95% and 45—85%, respectively, depending on the grade of butyl mbber being produced. The composition and mol wt of the polymer formed depend on the concentration of the monomers in the reactor Hquid phase and the amount of chain transfer and terminating species present. The Hquid-phase composition is a function of the feed composition and the extent of monomer conversion. In practice, the principal operating variable is the flow rate of the initiator/coinitiator solution to the reactor residence time is normally 30—60 minutes. 

The important process variables are reactor temperature, residence time, and steam/hydrocarhon ratio. Feed characteristics are also considered, since they influence the process severity. 

Semibatch Model "GASPP". The kinetics for a semibatch reactor are the simpler to model, in spite of the experimental challenges of operating a semibatch gas phase polymerization. Monomer is added continuously as needed to maintain a constant operating pressure, but nothing is removed from the reactor. All catalyst particles have the same age. Equations 3-11 are solved algebraically to supply the variables in equation 5, at the desired operating conditions. The polymerization flux, N, is summed over three-minute intervals from the startup to the desired residence time, t, in hours ... 

Monomer concentration dynamics are presented in Figure 5. Additional observations for Run 5 are accurately correlated during the reactor startup and at final steady state. The observation at one residence time, Run 4, may be in error. The total cummu-lative, molar concentrations of macromolecules as a function of time are presented in Figure 6. The errors associated with this dependent variable are also evident during the steady state analysis of initiation... 

Solution It is easy to begin the solution. In piston flow, molecules that enter together leave together and have the same residence time in the reactor, t. When the kinetics are first order, the probabiUty that a molecule reacts depends only on its residence time. The probability that a particular molecule will leave the system without reacting is exp(— F). For the entire collection of molecules, the probability converts into a deterministic fraction. The fraction unreacted for a variable density flow system is... 

The fraction unreacted is /< > . Set z = L to obtain it at the reactor outlet. Suppose = 1 and that kai /Ui = 1 in some system of units. Then the variable-density case gives z = 0.3608 at = 0.5. The velocity at this point is 0.75m . The constant density case gives z = 0.5 at a = 0-5 and the velocity at the outlet is unchanged from The constant-density case fails to account for the reduction in u as the reaction proceeds and thus underestimates the residence time. 

Simulation studies are also conducted for a dispersed PFR and a recycle reactor at 260 °C, 500 psig and feed with DCPD=0.32 mol/min, CPD=0.96mol/min and ethylene=3.2mol/min. Peclet number (Pe) or the recycle ratio is selected as a variable parameter for the dispersed PFR or for the recycle reactor, respectively. Conversion approaches to that of PFR over Pe=50 as can be seen in Fig.4. It is also worth mentioning that the reactor performance is improved with recycle if the residence time is low. 

When the space time and the mean residence time differ, it is the space time that should be regarded as the independent process variable that is directly related to the constraints imposed on the system. We will see in Sections 8.2 and 8.3 that it is convenient to express the fundamental design relations for continuous flow reactors in terms of this parameter. We will also see that for these reactors the mean residence time cannot be considered as an independent variable, but that it is a parameter that can be determined only... 

ILLUSTRATION 8.5 DETERMINATION OF MEAN RESIDENCE TIME IN A PLUG FLOW REACTOR UNDER ISOTHERMAL OPERATING CONDITIONS—VARIABLE DENSITY CASE... 

As we stressed earlier, the reactor space time is the independent variable at the control of the reactor designer. This parameter is more meaningful than the mean residence time in the reactor. 

We note the use of t as a scaling factor for reactor size or capacity. In Example 15-2, neither V nor qa is specified. For a given r, if either V or q0 is specified, then the other is known. If either V or q0 is changed, the other changes accordingly, for the specified t and performance (cA or /A). This applies also to a CSTR, and to either constant- or variable-density situations. The residence time t may similarly be used for constant-density, but not variable-density cases. 

Flow reactors usually operate more nearly at constant pressure, thus at variable volume with gases. An apparent residence time is defined as the ratio of the reactor volume to the inlet volumetric rate,... 

Continuous Multicomponent Distillation Column 501 Gas Separation by Membrane Permeation 475 Transport of Heavy Metals in Water and Sediment 565 Residence Time Distribution Studies 381 Nitrification in a Fluidised Bed Reactor 547 Conversion of Nitrobenzene to Aniline 329 Non-Ideal Stirred-Tank Reactor 374 Oscillating Tank Reactor Behaviour 290 Oxidation Reaction in an Aerated Tank 250 Classic Streeter-Phelps Oxygen Sag Curves 569 Auto-Refrigerated Reactor 295 Batch Reactor of Luyben 253 Reversible Reaction with Temperature Effects 305 Reversible Reaction with Variable Heat Capacities 299 Reaction with Integrated Extraction of Inhibitory Product 280... 

In a CSTR, each reaction mixture component has an equal chance of being removed at any time regardless of the time it has been in the reactor. Thus, in a CSTR, unlike the tubular and bach systems, the residence time is variable and can take the exponential form... 

Kinetic data are frequently acquired in continuous reactors rather than batch reactors. These data permit one to determine whether a process has come to steady state and to examine activation and deactivation processes. These data are analyzed in a similar fashion to that discussed previously for the batch reactor, but now the process variables such as reactant flow rate (mean reactor residence time) are varied, and the composition will not be a function of time after the reactor has come to steady state. Steady-state reactors can be used to obtain rates in a differential mode by maintaining conversions small. In this configuration it is particularly straightforward to vary parameters individually to find rates. One must of course wait until the reactor has come to steady state after any changes in feed or process conditions. 

In a variable-density reactor the residence time depends on the conversion (and on the selectivity in a multiple-reaction system). Also, in ary reactor involving gases, the density is also a function of reactor pressure and temperature, even if there is no change in number of moles in the reaction. Therefore, we frequently base reactor performance on the number of moles or mass of reactants processed per unit time, based on the molar or mass flow rates of the feed into the reactor. These feed variables can be kept constant as reactor parameters such as conversion, T, and P are varied. 

Whenever the density of the fluid in the reactor varies as the reaction proceeds, the reactor residence time r is not a simple independent variable to describe reactor performance. Typically, we stiU know the inlet variables such as Uq, Tq, Fjo, and Co, and these are independent of conversion. 

Since the volumetric flow rate is a function of X, T, and P, the residence time V/v depends on these variables. Instead of using the reactor residence time T to describe performance, an analogous quantity called the space time ST, defined as... 

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