Multiple flow control systems

Recent progress in microflow devices and systems is described in this chapter. Examples of passive and active flow control methods applicable in practical pTAS are described in Sect. 2. Multiple flow control systems, i.e., arrayed microvalves, for advanced high-throughput microflow systems are then introduced in Sect. 3. Examples of microflow devices and systems for chemical and biochemical applications are described in Sect. 4. 

Multiple Flow Control Systems Arrayed Microvalves... 

Recent advances in the fabrication of microflow devices using MEMS technology are described from the technological point of view. The flow control methods and multiple flow control systems reported here are applicable in efficient chemical microreactors as well as in chemical analysis systems. For high-performance flow device design, computational dynamics (CFD) simulation is indispensable. The... 

The two principal elements of evaporator control are evaporation rate a.ndproduct concentration. Evaporation rate in single- and multiple-effect evaporators is usually achieved by steam-flow control. Conventional-control instrumentation is used (see Sec. 22), with the added precaution that pressure drop across meter and control valve, which reduces temperature difference available for heat transfer, not be excessive when maximum capacity is desired. Capacity control of thermocompression evaporators depends on the type of compressor positive-displacement compressors can utilize speed control or variations in operating pressure level. Centrifugal machines normally utihze adjustable inlet-guide vanes. Steam jets may have an adjustable spindle in the high-pressure orifice or be arranged as multiple jets that can individually be cut out of the system. 

Sedimentation Tanks These tanks are an integral part of any activated-sludge system. It is essential to separate the suspended solids from the treated liquid if a high-quality effluent is to be produced. Circular sedimentation tanks with various types of hydraulic sludge collectors have become the standard secondary sedimentation system. Square tanks have been used with common-wall construc tion for compact design with multiple tanks. Most secondary sedimentation tanks use center-feed inlets and peripheral-weir outlets. Recently, efforts have been made to employ peripheral inlets with submerged-orifice flow controllers and either center-weir outlets or peripheral-weir outlets adjacent to the peripheral-inlet channel. 

Exhauster and Motor Air movers for LVHV systems are not conventional fans. The low static pressures needed to operate LVHV systems can be generated by multiple-stage centrifugal (turbine-type) exhausters. These utilize high-precision rotating blades that can be damaged by dust. Consequently, it is always necessary to have an air cleaner in an LVHV dust control system to protect the fan. The low static pressures also require air volume flow rates to be corrected to standard conditions for exhauster selection. 

A novel 24-channel HPLC by Nanostream called Veloce was introduced at PITTCON 04. The column cassette contains 24 parallel microbore columns. The eluted samples are detected by a 24-channel UV filter photometer. The advantage of such a system is that it allows one to work with multiple samples simultaneously. Other interesting systems for parallel HPLC were those introduced by Eksigent, based on microfluidic flow control, and Sepiatec GmbH, which allows the processing of 75 multiple-well plates. 

The progress of a given reaction depends on the temperature, pressure, flow rates, and residence times. Usually these variables are controlled directly, but since the major feature of a chemical reaction is composition change, the analysis of composition and the resetting of the other variables by its means is an often used means of control. The possible occurrence of multiple steady states and the onset of instabilities also are factors in deciding on the nature and precision of a control system. 

Most biofuel separations involve multiple components and produce two or more liquid or vapor products. When there is a sidestream product in addition to the overhead and bottom products, an additional degree of freedom is available for the control system, because the overall material balance becomes F = D + C + B, where C is the sidestream flow rate. Therefore, two... 

An alarm should sound any time redundant inputs disagree. In most cases, the operating personnel will have to decide what to do. In some cases the computer control system will have to decide by itself what to do if redundant inputs disagree. The more hazardous the process, the more it is necessary to use multiple sensors for flow, temperature, pressure, and other variables. Since it must be assumed that all measuring devices will fail, they should fail to an alarm state. If a device fails to a nonalarm condition, there can be serious problems. It is also serious if a device fails to an alarm condition, and there is really not an alarm condition. This is generally not as serious as the first case, but it can provide a false sense of security. Usually it is assumed that two... 

We noted earlier in this chapter that many reactions in the chemical industries are exothermic and require heat removal. A simple way of meeting this objective is to design an adiabatic reactor. The reaction heat is then automatically exported with the hot exit stream. No control system is required, making this a preferred way of designing the process. However, adiabatic operation may not always be feasible. In plug-flow systems the exit temperature may be too hot due to a minimum inlet temperature and the adiabatic temperature rise. Systems with baekmixing suffer from other problems in that they face the awkward possibilities of multiplicity and open-loop instability. The net result is that we need external cooling on many industrial reactors. This also carries with it a control system to ensure that the correct amount of heat is removed at all times. 

The MS instrumentation is the most expensive part of the LC-MS system, hence efforts to improve the throughput of the LC-MS analysis often involve the use of parallel multiple columns that feed into a single mass spectrometer. Zeng and Kassel [99] developed an automated parallel analytical/preparative LC-MS workstation to increase the throughput for the characterization and purification of combinatorial libraries. The system incorporates two columns operated in parallel for both LC-MS analytical and preparative LC-MS purifications. A multiple-sprayer ESI interface was designed to support flows from multiple columns. The system is under complete software control and delivers the crude samples to the two HPLC columns from a single autosampler. The authors demonstrated characterization of more than 200 compounds per instrument per day, and purification of more than 200 compounds per instrument per night. De Biasi et al. [100] described a four-channel multiplexed... 

What manipulated variables should be used A multiple-input, multiple-output system possesses several manipulated variables which can be used for the design of a control system. The selection of the most appropriate manipulations is a very critical problem and should be approached with care. Some manipulations have a direct, fast, and strong effect on the controlled outputs others do not. Furthermore, some variables are easy to manipulate in real life (e.g., liquid flows) others are not (e.g., flow of solids, slurries, etc.). 

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