Feed-temperature

The feed temperature is one of the most influential variables on the dynamic behavior of the reactor. A common guideline for plant start-up states that the reactor must be started at the lowest possible temperature for safety concerns (Yan, 1980). This rule is based on the fact that the fresh catalyst is more prone to hot spot formation. 

FIGURE 8.30 Effect of feed temperature on bed outlet temperature under fresh catalyst conditions. 

Straightforward the heat release increases with feed temperature. The results show that when feed temperature is increased to 365°C-370°C, the outlet temperatures surpass 400°C but then gradually decrease as the catalyst is deactivated. If the feed temperature is increased even more, the reactor is exposed to overheating ( 420 C). Therefore, the best option is to start the reactor somewhere between 355 C and 360°C in order to keep the hottest regions at appropriate levels. The simulations also capture the transient disturbances caused by quenching. Bed 2 outlet profiles reflect the effect of the first quench, whereas bed 3 outlet profiles reflect the effect of the first and the second quenches. 

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Fig. 12. Correlatioa of AT. The three lines represeat the best fit of a mathematical expressioa obtaiaed by multidimensional nonlinear regressioa techniques for 99, 95, and 90% recovery the poiats are for 99% recovery. = mean molar heat capacity of Hquid mixture, average over tower AY = VA2 slope of equiHbrium line for solute, to be taken at Hquid feed temperature mg = slope of equilibrium line for solvent.

Among continuous reactors, the dominant system used to produce parasubstituted alkylphenols is a fixed-bed reactor holding a soHd acid catalyst. Figure 3 shows an example of this type of reactor. The phenol and alkene are premixed and heated or cooled to the desired feed temperature. This mix is fed to the reactor where it contacts the porous soHd, acid-impregnated catalyst. A key design consideration for this type of reactor is the removal of the heat of reaction. 

Both control schemes react in a similar manner to disturbances in process fluid feed rate, feed temperature, feed composition, fuel gas heating value, etc. In fact, if the secondary controller is not properly tuned, the cascade control strategy can actually worsen control performance. Therefore, the key to an effective cascade control strategy is the proper selection of the secondary controlled variable considering the source and impact of particular disturbances and the associated process dynamics. 

Two variables of primary importance, which are interdependent, are reaction temperature and ch1orine propy1ene ratio. Propylene is typically used ia excess to act as a diluent and heat sink, thus minimising by-products (eqs.2 and 3). Since higher temperatures favor the desired reaction, standard practice generally involves preheat of the reactor feeds to at least 200°C prior to combination. The heat of reaction is then responsible for further increases in the reaction temperature toward 510°C. The chlorine propylene ratio is adjusted so that, for given preheat temperatures, the desired ultimate reaction temperature is maintained. For example, at a chlorine propylene molar ratio of 0.315, feed temperatures of 200°C (propylene) and 50°C (chlorine) produce an ultimate reaction temperature of approximately 500°C (10). Increases in preheat temperature toward the ultimate reactor temperature, eg, in attempts to decrease yield of equation 1, must be compensated for in reduced chlorine propylene ratio, which reduces the fraction of propylene converted and, thus aHyl chloride quantity produced. A suitable economic optimum combination of preheat temperature and chlorine propylene ratio can be readily deterrnined for individual cases. 

One such approach is called cascade control, which is routinely used in most modern computer control systems. Consider a chemical reactor, where reac tor temperature is to be controlled by coolant flow to the jacket of the reac tor (Fig. 8-34). The reac tor temperature can be influenced by changes in disturbance variables such as feed rate or feed temperature a feedback controller could be employed to compensate for such disturbances by adjusting a valve on me coolant flow to the reac tor jacket. However, suppose an increase occurs in the... 

From assumed feed temperature (forward feed) or feed flow (backward feed) to the first effect and assumed steam flow, calculate evaporation in the first effect. Repeat for each succeeding effect, checking intermediate assumptions as the calculation proceeds. Heat input from condensate flash can be incorporated easily since the condensate flow from the preceding effects will have already been determined. 

The type of evaporator to be used and the materials of construc tion are generally selected on the basis of past experience with the material to be concentrated. The method of feeding can usually be decided on the basis of known feed temperature and the properties of feed and produc t. However, few of the listed variables are completely independent. For instance, if a large number of effects is to be used, with a consequent low temperature drop per effect, it is impractical to use a natural-circiilation evaporator. It expensive materials of construction are desirable, it may be found that the forced-circulation evaporator is the cheapest and that only a few effec ts are justifiable. 

The simplest continuous-distillation process is the adiabatic single-stage equihbrium-flash process pictured in Fig. 13-25. Feed temperature and the pressure drop across the valve are adjusted to vaporize the feed to the desired extent, while the drum provides disengaging space to allow the vapor to separate from the liquid. The expansion across the valve is at constant enthalpy, and this facd can be used to calculate To (or T to give a desired To). 

For specified feed temperature and pressure, an isothermal flash of the feed gave 13..35 percent vaporization. 

Nonisothermal hquid-phase processes may be driven by changes in feed temperature or heat addition or withdrawal through a column wall. For these, heats of adsorption and pressure effects are generally of less concern. For this case a suitable energy balance is... 

T pe of particle charging Feed Separation T pe of separator Feed temperature, °C Feed. size, mm Feed rate, metric tons per hour per start d No. of stages of separation... 

R = reflux ratio, g reflux/g product Tp = product temperature, °C Tp = saturated-feed temperature, °C Cp = specific heat of solid ciys-tals, cal/(g °C) and X = heat of fusion, cal/g. 

Develop and implement operating instructions to control feed temperature and shut off feed when temperature rises above a certain level... 

Hot feed Provide and maintain an automated inerting (increases system—oxygen concentration or pressure fire/explosion risk controlled with flammable. Eliminate leakage sources (ftimes/air) solvents). Use alternative solvents (nonflammable or less flammable) Reduce feed temperature and/or monitor temperature of feed and interlock with feed shutdown NFPA 69... 

The unit was built in a loop because the needed 85 standard m /hour gas exceeded the laboratory capabilities. In addition, by controlling the recycle loop-to-makeup ratio, various quantities of product could be fed for the experiments. The adiabatic reactor was a 1.8 m long, 7.5 cm diameter stainless steel pipe (3 sch. 40 pipe) with thermocouples at every 5 centimeter distance. After a SS was reached at the desired condition, the bypass valve around the preheater was suddenly closed, forcing all the gas through the preheater. This generated a step change increase in the feed temperature that started the runaway. The 20 thermocouples were displayed on an oscilloscope to see the transient changes. This was also recorded on a videotape to play back later for detailed observation. 

After the 20K step increase in feed temperature, not much change could be observed for two minutes. Then the last thermocouple started to increase from 560 to 1200 K level, and the hot zone widened. The forward migration rate of the hot zone was about 5 cm/min. After about six minutes, the oxygen content of the cycle gas became very low and temperature slowly started to decline. With this the experiment terminated. 

The original van Heerden diagram, as presented in his paper of 1953, was constructed for an adiabatic reactor case. In that case, at fixed feed temperature, there was a different slope (representing heat removal rate) for each feed rate. There was also a different heat generation versus temperature... 

A graphical explanation for the constant flow and variable feed temperature controlled adiabatic CSTR is given in Figure 9.6.6. 

Several other changes that are supposed to slow down the reaction can cause runaway. In the case of ethylene oxidation, chlorinated hydrocarbons are used as inhibitors. These slow down both the total and the epoxidation, although the latter somewhat less. When the reaction is running too high and the inhibitor feed is suddenly increased in an attempt to control it, a runaway may occur. The reason is similar to that for the feed temperature cut situation. Here the inhibitor that is in the ppm region reacts with the front of the catalytic bed and slowly moves down stream. The unconverted reactants reach the hotter zone before the increased inhibitor concentration does. 

Changing feed temperature can be approximated by adding one half of the change in feed temperature to the top and bottom effective temperatures. Eor a large feed temperature change, the feed flash and the column heat balance will have to be redone. 

The plate to plate type calculation is a fundamental procedure wherein the tower is assumed to be composed of theoretical equilibrium plates. The actual plates required are determined from the number of theoretical plates using a predicted overall tower efficiency. The starting point for a tower calculation is usually a specified feed composition, feed temperature, and tower operating pressure. The procedure involves defining the compositions and temperamres on each plate in the tower and subsequently the resultant compositions and temperatures of the product streams. The actual computations, which involve trial... 

Calculate what the feed temperature must be if the reactor is to be operated adiabatically at 27°C. 

Given the following design data, determine (a) under what conditions adiabatic operation is feasible, and (b) what cooling area is required if the feed temperature is 30°C ... 

For a known reactor and kinetics, and at a given feed temperature Tq, the intersection of the energy balance line with the S-shaped mass... 

For the minimum volume in a CFSTR under adiabatie eondition, Equation 6-192 requires only that the operating temperature within the reaetor be T p. In this ease, there is nothing about the mode of operation, sueh as the feed temperature Tg or heat transfer, either within the reaetor or upstream of it. However, if the CFSTR is operated adiabatieally for speeified eonversion (X ) and molar flowrate (F q = uC o) without internal heat transfer, Tq must be adjusted to a value obtained from the energy balanee as determined by... 

The feed temperature (Tq) for the adiabatie operation at the optimal temperature T p. ... 

A column is to be designed to separate the feed given below into an overhead of 99.9 mol % trichloroethylene. The top of the column will operate at 10 psig. Feed temperature is 158°F. 

Enthalpy, Btu/unit flow 2,901.076 lb = 31.48 Feed temperature 90°F, liquid at stage 5 from top, Equimolal overflow not assumed Column Pressure 0.39 (top) to 0.86 (bottom) psia, distributed uniformly to each tray... 

Determine feed temperature from ethylene tower. Bottoms 25°F, Boiling point feed Bubble point of bottoms At 445 psia... 

Adiabatic Reaction Temperature (T ). The concept of adiabatic or theoretical reaction temperature (T j) plays an important role in the design of chemical reactors, gas furnaces, and other process equipment to handle highly exothermic reactions such as combustion. T is defined as the final temperature attained by the reaction mixture at the completion of a chemical reaction carried out under adiabatic conditions in a closed system at constant pressure. Theoretically, this is the maximum temperature achieved by the products when stoichiometric quantities of reactants are completely converted into products in an adiabatic reactor. In general, T is a function of the initial temperature (T) of the reactants and their relative amounts as well as the presence of any nonreactive (inert) materials. T is also dependent on the extent of completion of the reaction. In actual experiments, it is very unlikely that the theoretical maximum values of T can be realized, but the calculated results do provide an idealized basis for comparison of the thermal effects resulting from exothermic reactions. Lower feed temperatures (T), presence of inerts and excess reactants, and incomplete conversion tend to reduce the value of T. The term theoretical or adiabatic flame temperature (T,, ) is preferred over T in dealing exclusively with the combustion of fuels. 

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