Reactors fast response

In this paper we will first describe a fast-response infrared reactor system which is capable of operating at high temperatures and pressures. We will discuss the reactor cell, the feed system which allows concentration step changes or cycling, and the modifications necessary for converting a commercial infrared spectrophotometer to a high-speed instrument. This modified infrared spectroscopic reactor system was then used to study the dynamics of CO adsorption and desorption over a Pt-alumina catalyst at 723 K (450°C). The measured step responses were analyzed using a transient model which accounts for the kinetics of CO adsorption and desorption, extra- and intrapellet diffusion resistances, surface accumulation of CO, and the dynamics of the infrared cell. Finally, we will briefly discuss some of the transient response (i.e., step and cycled) characteristics of the catalyst under reaction conditions (i.e.,... 

The gas-phase composition before or after the reactor was monitored by mass spectroscopy although no such data are shown here, experience indicated the necessity of a fast-response inlet system. 

A fast-response infrared spectroscopic reactor system has been described which is capable of operating at high temperatures (e.g., 450-500°C). The infrared reactor system was successfully used to monitor the response of the surface concentration of CO to step changes or oscillations in the feedstream composition, under both reactive and nonreactive conditions. 

As an excellent, simple example of how fluctuating parameters can affect a reacting system, one can examine how the mean rate of a reaction would differ from the rate evaluated at the mean properties when there are no correlations among these properties. In flow reactors, time-averaged concentrations and temperatures are usually measured, and then rates are determined from these quantities. Only by optical techniques or very fast response thermocouples could the proper instantaneous rate values be measured, and these would fluctuate with time. 

Screening in stationary mode will only give information about the activity of a single catalyst or a catalyst mixture. When a proper catalyst for a certain reaction is found, the next important information is the reaction kinetics. To obtain this information, several methods and reactors are recommended in the literature [66-73]. Most of them apply transient reactor operations to find detailed kinetic information. Microreactors are particularly suited for such an operation since their low internal reaction volumes enable a fast response to process parameter changes, e.g., concentration or temperature changes. This feature was already applied by some authors to increase the product yield in microreactors [70, 74, 75]. De Belle-fon [76] reported a dynamic sequential method to screen liquid-liquid and liquid-... 

Fig. 8.12 shows the setup of a laser beam and microphone for ethylene quantification with a PAS detector [24]. In the array, the eight reaction tubes are arranged linearly. A pulsed laser is passed through each effluent from the reactors to excite ethylene molecules. The pulsed laser used emitted at 943-950 cm-1 (where ethylene has a strong absorption) - a 10 or 100 Hz modulated 25 W laser with a pulse length of 35 or 25 ps. A microphone with a fast response time and decay was used. The ethylene concentration of each effluent was determined by the volume and response time. The signal from the most distant tube is weak so that the signals were accumulated for 2.5 s. Data presented in the reference are shown in Fig. 8.13. Ethylene concentrations were determined for the effluent from the mixed-oxide catalyst consisting of La, Ba, Pb, Th, Mn, Ni and Cu.

An attractive property of monolithic reactors is their flexibility of application in multiphase reactions. These can be classified according to operation in (semi)batch or continuous mode and as plug-flow or stirred-tank reactor or, according to the contacting mode, as co-, counter-, and crosscurrent. In view of the relatively high flow rates and fast responses in the monolith, transient operations also are among the possibilities. 

Response time. When a reactor is kept operating at high power output and the plant electricity is used for operating an electrolysis system, the electricity can be switched in a fraction of a second from the electrolysis plant to the electrical grid. The fast response times enables this system to be used to regulate the electrical grid (see section entitled Economics). 

Polymer reactor control then boils down to controlling all dominant variables to setpoint using manipulators with a fast response and then adjusting the setpoints of the controlled variables to achieve the desired economic objectives. The trick is to determine the dominant variables and manipulators in addition to their relationships. Some key manipulators are heat removal (for externally cooled systems) or conversion... 

Most importantly, we introduced the ideas of dominance, effective degrees of freedom, and partial control for chemical reactors. In essence, dominant variables are controlled by manipulators with a fast response and the setpoints are adjusted to achieve the economic objectives. These notions are useful in this context, but they can be utilized more widely for other unit operations. 

Rupture disks are often used upstream of relief valves to protect the relief valve from corrosion or to reduce losses due to relief valve leakage. Large rupture disks are also used in situations that require very fast response time or high relieving load (for example, reactor runaway and external fire cases). They are also used in situations in which pressure is intentionally reduced below the operating pressure for safety reasons. 

In the first case, the reactions of interest are those which are intrinsically fast and exothermic, but which are currently limited by the poor heat and mass transfer for rates achievable in a stirred pot. Existing technology routinely entails substantial hazardous process inventories, possible reactor runaway and indifferent product selectivity. Fast response reactors open up the possibility of switching to more severe process conditions which would be prohibited in conventional reactors in view of the tendency to degrade the product. It may therefore be possible to exploit a virtuous circle - short residence time -higher temperature - faster kinetics - smaller reactor - shorter residence time. 

All activity measurements were conducted in an in-situ infrared reactor cell placed in the sample compartment of a DIGILAB 15C Fourier Transform Infrared (FTIR) Spectrometer. The reactor, described in detail elsewhere [11], consisted of two aluminum flanges with CaF2 IR transparent windows, a gas inlet and outlet, and two foil fast response thermocouples which were placed in direct contat with the catalyst. The reactor temperature was maintained constant by external heaters controlled by a temperature programmed controller. A Teflon coated recycle pump permitted to maintain near isothermal conditions and improve the mixing in the reactor. The reactor and associated lines were tested for activity at the highest temperature used, and it was found to have negligible activity. 

The temporal analysis of products reactor system (TAP) developed recently by J. Cleaves is another technique to study fast responses. 

Experience with sodium leaks at our domestic reactors indicates that all leaks without exception, were timely detected. All leaks that occurred on the electrically heated sodium system sections were registered by heaters control systems. In addition, primary sodium leaks were sensed by the radioactive sodium aerosol detection systems. Calculation and experimental analysis of the radioactive sodium aerosol detection system has revealed its high sensitivity and fast response. 

Fast heart transfer in microflow reactors is responsible for the control of such highly exothermic reactions. 

M. J. Castaldi, R. S. Boorse, S. Roychoudhury, P. V. Menacherry, W. C. PfefFerle, A compact lightweight, fast-response preferential oxidation reactor for PEM automotive fuel cell applications, in Proceedings of the National Science Foundation s Annual SBIR/STTR Meeting, San Juan, Puerto Rico, 2002, www.predsion-combustion.com/ proxpaper.pdf (accessed 10 September 2003). 

Cooling of the reactor core when the plant is in the shutdown condition or following the occurrence of abnormal events is reliably assured by making use of the natural force of gravity. Provision of a large water inventory inside the reactor pressure vessel as well as of a large source of water inside the containment makes active, fast-response safety equipment, pumps and electric power unnecessary in the event of disturbances in the reactor coolant system. 

A fast response, modulating-type valve, controlled by the steam bypass pressure regulator system, is used to perform three basic functions. The primary function is to reduce the rate of rise of reactor pressure when the turbine admission valves are moved rapidly in the closing direction. To perform this function, the bypass valve needs about the same speed of response as the turbine admission valves to prevent a pressure-induced reactor scram from high neutron flirx when the turbine load is suddenly reduced by partial or complete closure of the turbine admission valves. 

Modern high-speed GC systems are able to separate some light hydrocarbons within a second [117, 118]. This approach requires special sampling valves, narrow-bore columns (diameter = 0.005 cm), and detectors with a fast response. However, in EGA these fast separations are not necessary. Taking samples on-line from the reactor and separating them within 1 minute is frequently satisfactory in order to better understand degradation kinetics. The apparatus for this approach can be constructed of commercially available parts. 

Use cascade control from reactor temperature to coolant temperature for fast response. 

Until the ADS is initiated, the reactor behavior is analyzed with SPRAT-DOWN. After that, the blowdown is analyzed with SPRAT-DOWN-DP. Since SPRAT-DOWN-DP does not distinguish between the hot and average channels, only the hot channel parameters are transferred. After initiating the ADS, the reactor behavior for all the ATWS events is similar to the behavior described in Fig. 6.7 [1] because the depressurization is an intense phenomenon that is not influenced by the condition before it. The ATWS events having relatively fast responses before initiating the ADS are important here. Representative results are shown below. 

The characterization is performed by means of residence time distribution (RTD) investigation [23]. Typically, holdup is low, and therefore the mean residence time is expected to be relatively short Consequently, it is required to shorten the distance between the pulse injection and the reactor inlet. Besides, it is necessary to use specific experimental techniques with fast time response. Since it is rather difficult, in practice, to perfectly perform a Dirac pulse, a signal deconvolution between inlet and outlet signals is always required. 

The key to obtaining pore size information from the NMR response is to have the response dominated by the surface relaxation rate [19-26]. Two steps are involved in surface relaxation. The first is the relaxation of the spin while in the proximity of the pore wall and the other is the diffusional exchange of molecules between the pore wall and the interior of the pore. These two processes are in series and when the latter dominates, the kinetics of the relaxation process is analogous to that of a stirred-tank reactor with first-order surface and bulk reactions. This condition is called the fast-diffusion limit [19] and the kinetics of relaxation are described by Eq. (3.6.3) ... 

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