Process Instrumentation Two

After studying this chapter, the student will be able to  

Automatic/manual control—term describing two modes in which controllers can be operated. During plant start-up, the controller is typically placed in the manual position. In this mode, only manual control affects the position of the control valve it does not respond to process load changes. After the process is stable, the operator places the controller in automatic mode, which allows the controller to supervise the control loop function. At this point, the controller will automatically open and close the control valve to maintain the setpoint. 

Cascade control—a term describing how one control loop controls or overrides the instructions of another control loop to achieve a desired setpoint. 

Control loop—a collection of instruments that work together to automatically control a process. A loop includes a primary element or sensor, a transmitter, a controller, a transducer, and a final control element. 

Controller—device the primary purpose of which is to receive a signal from a transmitter, compare this signal to a setpoint, and adjust the (final control element) process to stay within the range of the setpoint. Controllers come in three basic designs pneumatic, electronic, and electric. 

---

Process Instrumentation and Control Systems. Investment for instmmentation and control systems and their installation typically range between 3 to 10% of the total installed cost for a grassroots continuous process faciUty. Instmmentation and control systems also represent a substantial percentage of the overall faciUty maintenance (qv) costs. Investment costs may be placed in one of two categories, ie, nondiscretionary and discretionary. 

In the process industries, flow measurement devices are the largest market in the process instrumentation field. Two web sites for process equipment and instrumentation, www.globalspec.com, and www.thomasnet.com, both list more than 800 companies that offer flow measurement products. There are more than one hundred types of flowmeters commercially available. The aforementioned web sites not only facilitate selection and specification of commercial flowmeters, but also provide electronic access to manufacturers technical literature. 

Figure 20.2. Schematic outline of typical pump-probe-detect experiments with femtosecond pulses, a molecular beam source, and mass spectrometric detection of transient species. Computer control and data processing instruments, as well as various optical components, are not shown. The time separation Af between pump and probe pulses is dictated by the difference in optical path lengths. Ad, traversed by the two components of the original pulse.

The description of the different types of protection in Chapter 6 indicates that there are two very different ways to solve this problem - if an electrical transmission is required at all. One way is to use intrinsically safe circuits, the other one applies industrial equipment as usual, additionally explosion protected by an enclosure as appropriate, e.g. flameproof housings for smallsized devices. In the history of process instrumentation, the appearance of semiconductors and integrated circuits has drastically reduced the power consumption of field devices. So, intrinsically safe circuits dominate this field today. 

SEC System. A Spectra Physics IsoChrom pump controlled the THE flow in the SEC part of the 2D instrument. Two SDV 5-jLtm SEC columns with 1000 and 10 A porosity (PSS) were used for size separation of the HPLC fractions. UV (SP 8450, Spectro Physics) and RI (Shodex SE 61, Diisseldorf, Germany) detection allowed lor conventional or multiple-detection data processing of detector traces. Eor the polyester analysis the SEC columns (50 and 100 A) were operated in acetone as eluent. 

A solid-phase microextraction process involves two steps, namely partitioning of the analytes between the coating and the sample, and desorption of the concentrated species into an analytical instrument. In the first step, the coated fibre is exposed and the target analytes are extracted from the sample matrix into the coating. In the second step, the fibre with the concentrated analytes is transferred to an instrument for desorption. A third, clean-up step can also be incorporated by using selective solvents, as in SPE. 

Other major shale constituents such as C, H, N, and S are determined by thermal decomposition and instrumental detection methods. Oxygen is determined by 14 MeV neutron activation analysis. Parr or Leco BTU bomb combustion and subsequent ion chromatographic determination is used for halogens, sulfate and nitrate. Ion chromatography is also suitable for anionic characterization of shale process waters. Two analytical procedures for oil shales should be used with caution. Kjeldahl nitrogen procedure has been found to give reproducible but considerably low results for certain oil... 

Fig. 11.8 are photographs of two instruments offered by Amherst Process Instruments (API). Fig. 11.8a is the API aerosizer, a high resolution particle size analyzer for fine powders (range 0.2 - 700 pm), which is based on aerodynamics. A gas containing the entrained particles expands through a nozzle at supersonic velocities into a partial vacuum which is contained within a barrel shock envelope. The exit velocity of a particle depends on its density and size. Two laser beams, separated by a defined distance, and two photomultipliers form the measurement zone. From the velocity and the known density of the particulate solid, the aerosizer determines size, one by one, with a speed of up to 100,000 particles per second and an accuracy of better than 1 % which make this instrument rather unique. The system uses interchangeable dispersers for different types of particles and, in total (refer to Fig. 11.8a), consists of particle dispersers (A), sensor unit (B), vacuum system (not shown), and computer (C). 

A central issue for enabling rapid response for any at process instrumentation is to ensure that the analyzer is promptly supplied with a sample that corresponds to the process being monitored. Two issues emerge elimination of sample discrimination and lag time. Materials... 

Since a control loop is a dynamic system, its efficiency is governed by the response time of the entire system [4], which includes that of the measuring instrument, the controller, the final operator, and the process itself. Two time-response characteristics of the process are important in selecting the proper control strategy— the dead-time and the resistance-capacitance RC) time-cons font. Dead-time was discussed above an example is the time required to initiate many polymerization reactions. The RC time-constant results from the ability of the process to absorb a change in input without an immediate proportionate change in output. 

Color labs are outfitted with laboratory size equipment that simulates the larger machines used for production internally and by their customers. Typical processing equipment found in the lab are small extruders, two-roll mills, ban-burry mills, and media mills. Small rotational, injection and blow molding machines are used to duplicate the customers process. Instruments and computers are required for testing physical properties and color. Most labs have a computer-controlled color measuring system and a light booth to evaluate color. The spectrophotometer with computer is initially used to assist in colorant formulation and later as a quality control (QC) tool to provide certification of the quahty of match to standard. The light booth provides a standardized set of conditions to visually observe color and appearance. 

---