Separation processes, sensor systems

Research has been done showing that rapid pressnre-driven LC analysis can be done with little solvent consumption, demonstrating this as a viable process analytical tool. Using electrokinetic nanoflow pumps LC can be miniaturized to the point of being a sensor system. Developments in terms of sampling to enable sampling directly from a process stream, to the separation channel on a chip are critical for the application of miniaturized process LC. The components (valves and pumps) required for hydrodynamic flow systems appear to be a current limitation to the fnll miniatnrization of LC separations. Detection systems have also evolved with electrochemical detection and refractive index detection systems providing increased sensitivity in miniaturized systems when compared to standard UV-vis detection or fluorescence, which may require precolumn derivatization. 

On the other hand, its should be emphasized that such basic analytical properties as precision, sensitivity and selectivity are influenced by the kinetic connotations of the sensor. Measurement repeatability and reproducibility depend largely on constancy of the hydrodynamic properties of the continuous system used and on whether or not the chemical and separation processes involved reach complete equilibrium (otherwise, measurements made under unstable conditions may result in substantial errors). Reaction rate measurements boost selectivity as they provide differential (incremental) rather than absolute values, so any interferences from the sample matrix are considerably reduced. Because flow-through sensors enable simultaneous concentration and detection, they can be used to develop kinetic methodologies based on the slope of the initial portion of the transient signal, thereby indirectly increasing the sensitivity without the need for the large sample volumes typically used by classical preconcentration methods. 

To apply control to a process, one measures the controlled variable and compares it to the setpoint and, based on this comparison, typically uses the actuator to make adjustments to the flow rate of the manipulated variable. The industrial practice of process control is highly dependent upon the performance of the actuator system (final control element) and the sensor system as well as the controller. If either the final control element or the sensor is not performing satisfactorily, it can drastically affect control performance regardless of controller action. Each of these systems (i.e., the actuator, sensor, and controller) is made up of several separate components therefore, the improper design or application of these components, or an electrical or mechanical failure of one of them, can seriously affect the resulting performance of the entire control loop. The present description of these devices focuses on their control-relevant aspects. Later, troubleshooting approaches and control loop component failure modes are discussed. 

The key to effective troubleshooting is expressed in the old adage, divide and conquer. It is important to locate the portion of the control loop hardware that is causing the poor performance the hnal control element, the sensor system, the controller, or the process. The place to start is to test each system separately to determine whether that portion of the control loop is operating properly. The hnal control element can be evalnated by applying a series of input step tests. That is, the input to the hnal control element, which is normally set by the controller, can be manually adjusted. The test allows the determination of the dynamic response and deadband of the actuator system. If the performance in these two areas is satisfactory, there is no need to evaluate the actuator system further. 

Figure 15.6 A schematic of a flashlamp-excited oxygen sensor system. Items are 10, optical fiber 12, fiber input/output end 14, fiber-sensor interface 16, sensor 54, 56, reference and signal detectors 42,46,52, 55, lenses 44, short-wavelength pass excitation filter 48, beam splitter 50, excitation wavelength rejecting and emission wavelength separation means 40, 58, 60, electronic signal processing means. (From the patent of Hauenstein et al [12].)...

The term pTAS is sometimes interchanged with lab-on-a-chip (LOG), more often when the manipulation of fluids is involved. pTAS and LOG range in size from a few microns to a few millimetres. The technique of pTAS is interdisciplinary it combines the advantages of chemical sensors and the resolving power of modem benchtop analytical systems and is constantly evolving. The main advantage of pTAS is integration of the entire separation process onto one analytical microdevice, so early efforts focused on micropumps and valves to manipulate fluids inside a microfabricated structure. In such a fluid-based pTAS,... 

Nanostructured adsorbents could play an important role in sample concentrators. The high surface are of nanostructures would provide ample opportunity for improved adsorption. Equally important, properly designed porosity could minimize the transport time intervals of desorbed entities. Nanostructures have been demonstrated to penetrate a highly concentrated plug of a sample agent in a short time span. Nanoscale porosity might also play a role in separation processes in sensor systems. 

Process modeling and simulation ate nevertheless extremely important tools in the design and evaluation of process control strategies for separation processes. There is a strong need, however, for better process mo ls for a variety of separations as well as process data with which to confirm tiiese models. Confidence in complex process models, especially those that can be used to study process dynamics, can come only from experimental verification of these models. This will require more sophisticated process sensors than those commonly available for temperature, pressure, pH, and differential pressure. Direct, reliable measurement of stream composition, viscosity, turbidity, conductivity, and so on is important not only for process model verification but also for actual process control applications. Other probes, which could be used to provide a better estimate of the state of the system, are needed to contribute to the understanding of the process in the same time frame as that of changes occurring in the process. 

The LAHH is an independant aiarm driven by a separate ievei sensor etc. it wiii warn of a faiiure of some eiement of a primary (process) contnoi system, it shouid be set at or beiow the tank rated capacity to aiiow adequate time to terminate the transfer by aiternative means before ioss of containment/damage occurs,... 

HIPS are critical safety systems, essentially replacing pressure relief and/ or flare systems. These systems are used to provide overpressure protection and/or flare load mitigation for process equipment, pipelines, wellhead flowlines, gas manifolds, or other special purpose applications. Technically HIPS is a safety instrumented function that consists of a set of components, such as sensors, logic solvers, and final control elements (e.g., valves), arranged for the purpose of taking the process to a safe state when predetermined conditions are violated. The HIPS shall operate independently and be completely separate from the basic process control system (BPCS). 

Plastics identification by spectroscopic techniques has increasingly focused on the use of near-infrared and Raman spectroscopic techniques. LLA Instruments, in conjunction with Daimler-Chrysler [78] have developed a superfast near-infrared (NIR) sensor system that has been used to separate mixed plastics by type from shredded automotive parts. NIR spectroscopy uses the near infrared region of the electromagnetic spectrum (from about 800 to 2500 nm). Their two-phase process initially separates bright and colored polymers and black polypropylene from the mix. A second long-wavelength NIR sensor is employed to then separate black plastics such as PC, PMMA, ABS, PC/ABS blends, and others. 

It is often desirable to automate a process, peirticularly when a process is repeated for a number of cycles. This can be achieved with either microprocessor-based controllers or via computer programs to operate valves [26]. The process control system can also be used to monitor, record and change the separation conditions such as pressure, pH, conductivity, flow rate and temperature. Use of sensors allows feedback control of the process. These can be used to control flow rate and pressure and to detect the presence of air in the system. The emergence of protein from the column can be monitored by measuring the absorbance of the eluate and this can be used to initiate isolation of the product [26]. 

Assemble the components into the process system and apply FMEA techniques to determine if protection devices on some components provide redundant protection to other components. For example, if there are two separators in series, and they are both designed for the same pressure, the devices protecting one from overpressure will also protect the other. Therefore, there may be no need for two sets of high pressure sensors. 

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