Reactant feed system

A commonly made mistake in these two-reactant feed systems is to assume that a control structure with one feed ratioed to the other will provide effective control. This scheme does not work because of inaccuracies in flow measurements and changes in feed compositions. Remember in neat operation the reactants must be balanced down to the last molecule. This can only be achieved by using some sort of feedback information from the process that indicates a buildup or depletion of reactant. 

The experimental system is outlined in Fig. 1 and consists of three main parts - a reactants feed system, the fixed-bed catalytic reactor, and the gas analysis system. 

The comparison of catalytic properties was made under identical reaction conditions, among three important candidate catalysts, namely, the Pt/y-Al203, Au/a-Fe203, and Cu Ce, x02 y systems [50], The catalytic tests were performed in the reactant feed containing CO, H2, C02, and HzO — the so-called reformate fuel. The effects of the presence of both C02 and H20 in the reactant feed on the catalytic performance (activity and selectivity) of these catalysts as well as their stability with time under reaction conditions have been studied. The composition of the prepared samples and their BET specific surface areas are presented in Table 7.6. The results obtained with the three catalysts in the presence of 15 vol% COz and of both 15 vol% COz and 10 vol% H20 in the reactant feed (with contact time wcat/v = 0.144 g sec/cm3 and X = 2.5) are shown in Figure 7.12. For comparison, the corresponding curves obtained under the same conditions but without water vapor in the feed are also shown in Figure 7.12. 

Dehydrogenation reaction of ethylbenzene was chosen as a test reaction for V205/AIP04-5. The reaction was carried out on a flow reactor equipped syringe pump, and gas feeding system. The reactant was diluted with nitrogen. The products were analyzed by on-lined gaschromatograph (HP 5890) with 10% Carbowax 20M, 3m X 1.8" SS column. 

The condition for the practical implementation of such a feed control is the availability of a computer controlled feed system and of an on-line measurement of the accumulation. The later condition can be achieved either by an on-line measurement of the reactant concentration, using analytical methods or indirectly, by using a heat balance of the reactor. The amount of reactant fed to the reactor corresponds to a certain energy of reaction and can be compared to the heat removed from the reaction mass by the heat exchange system. For such a measurement, the required data are the mass flow rate of the cooling medium, its inlet temperature, and its outlet temperature. The feed profile can also be simplified into three constant feed rates, which approximate the ideal profile. This kind of semi-batch process shortens the time-cycle of the process and maintains safe conditions during the whole process time. This procedure was shown to work with different reaction schemes [16, 19, 20], as long as the fed compound B does not enter parallel reactions. 

The process was performed for many months yielding 700 g of monofluorinated product with a nine-channel microstructured reactor [60]. A continuous 150 h operation was performed without decline of yield or conversion. Even in the scale-out to a 30-channel reador (see Figure 5.27), no loss in performance was noticed. A single feed system distributed the reactants and reagents to the various microchannels. 

Manual or automatic emergency shutdown systems to shut off one or more reactant feeds and then vent the contents to reduce reactor pressure ... 

The first inequality characterizes recycle systems with reactant inventory control based on self-regulation. It occurs because the separation section does not allow the reactant to leave the process. Consequently, for given reactant feed flow rate F0, large reactor volume V or fast kinetics k are necessary to consume the whole amount of reactant fed into the process, thus avoiding reactant accumulation. The above variables are grouped in the Damkohler number, which must exceed a critical value. Note that the factor z3 accounts for the degradation of the reactor s performance due to impure reactant recycle, while the factor (zo — z4) accounts for the reactant leaving the plant with the product stream. 

In the first control structure (Fig. 2.11a), both fresh reactant feeds are flow-controlled into the system, with one of the reactants ratioed to the other. This type of control structure is seen quite frequently because we want to set production rate with a reactant feed flow and we know that a stoichiometric ratio of reactants is needed. Unfortunately this strategy does not work It is not possible to feed exactly the stoichiometric amounts of the two reactants. Inaccuracies in flow measurement prevent this from occurring in practice with real instru-... 

In the second control structure (Fig. 2.11b), which does work, the fresh feed makeup of the limiting reactant (.F0B) is flow-controlled. The other fresh feed makeup stream (FCvl) is brought into the system to control the liquid level in the reflux drum of the distillation column. The inventory in this drum reflects the amount of A inside the system. If more A is being consumed by reaction than is being fed into the process, the level in the reflux drum will go down. Thus this control structure employs knowledge about the amount of component A in the system to regulate this fresh reactant feed makeup to balance exactly the amount of B fed into the process. 

We might be tempted to control reflux drum level with one of the fresh reactant feeds, as done above. The problem with this is that the material in the drum can contain a little of component C mixed with either A or B, Simply looking at the level doesn t tell us anything about component inventories within the process and which might be in excess. The sj stem can fill up with either. Some measure of the composition of at least one of the reactants is required to make this system work. Compositions in the reactor or the recycle stream indicate an imbalance in the amounts of reactants being fed and being consumed. If direct composition measurement is not possible, inferential methods using multiple trays temperatures in the column are sometimes feasible (Yu and Luyben, 1984). 

A fresh reactant feed stream cannot be flow-controlled unless there is essentially complete one-pass conversion of one of the reactants. This law applies to systems with reaction types such as A + B -> products and was discussed in Chap. 2, In systems with consecutive reactions such as A - B -> M - C and M + B -> D - C, the fresh feeds can be flow-controlled into the system because any imbalance in the ratios of reactants is accommodated by7 a shift in the amounts of the two products (M and D) that are generated. An excess of A will result in the production of more M and less D. An excess of B results in the production of more D and less M. 

Note that setting the production rate with variables at the reactor or within the process specifies the amount of fresh reactant feed flow required at steady state. The choices for the control system made in Steps 6 and 7 must recognize this relationship between production rate and fresh reactant feed flowrate. 

Figure 1 shows the dramatic effect that the CO purge exerts on the decomposition of ethylene over carbon nanofiber supported Rh at 100°C. At each designated point 10% CO was added to the reactant feed for a period of 30 minutes and then removed from the system in order to determine the impact of this treatment on the performance of the catalyst. Upon introduction of CO to the reactant feed an immediate effect was realized, seen in the the decomposition of ethylene, which exhibited a sharp decrease. This result is not entirely unexpected since CO is predicted to strongly adsorb on the catalyst surface displacing any hydrocarbon species that are present. As a consequence, the decomposition of ethylene will... 

Active A reaction capable of generating 22 kPa, realized in a reactor with a 1 kPa high-pressure interlock to stop reactant feeds and a properly sized 3 kPa rupture disc discharging to an effluent treatment system The interlock could fail to stop the reaction in time, and the rupture disk could be plugged or improperly installed, resulting in reactor failure in case of a runaway reaction. The effluent treatment system could fail to prevent a hazardous release... 

Thus, the optimization of PEMFC performance in terms of efficiency and reliability requires a proper design and management of reactant feeding sections, as well as of cooling and humidification sub-systems [4]. 

To illustrate the concept of combining analytics to improve process understanding an example chemical reaction was run using a Cellular Process Chemistry Systems (CPC) continuous feed micro reactor. This microreactor is configured to operate as a small-scale chemical production plant. It has two reactant input lines and two solvent/wash lines. The thermally controlled microreactor block of the continuous feed reactor has a total internal volume of 50 pL. The reactor system contains active control for both temperature and feed rate of the two reactants. The system flows product from the microreactor block to a residence time module (12 mL volume) and then out of the reactor for product collection and work-up. 

The issue of reactant feed policy in recycle systems has been raised by Bill Luyben. Complete references and ampler presentation can be found in his recent book (1999). Here we will examine two typical situations involving a bimolecular reaction. In the first the reference reactant is completely converted, while the other one is recycled. The second case considers that both reactants are incompletely converted and recycled. 

This section analyses the second order reaction A + B P taking place in an isothermal CSTR-Separator-Recycle system. When the reactants are completely recycled, feasible operation is possible only if the ratio of reactants in the feed matches exactly the stoichiometry. For this reason, only one reactant feed may be on flow control (/a,o=1), while the feed flow rate of the second reactant (/b,o) must be used to control its inventory. Two possible control structures are discussed (Fig.13.22) flow control of the recycle stream of one reactant, or of the reactor effluent, respectively. 

Principles - The idea of a distributed-reactant feed is applied to systems with two competing reactions. A typical example is a partial oxidation of a hydrocarbon. The general reactions have the form,... 

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