Instable operation

Figure 2.4 Semenov diagram the intersections S and I between the heat release rate of a reaction and the heat removal by a cooling system represent an equilibrated heat balance. Intersection S is a stable operating point, whereas I represent an instable operating point. Point C corresponds to the critical heat balance.

Thus, this equation is a stability condition for the adiabatic CSTR. If this condition is not fulfilled, such as in strongly exothermal reactions, there may also be a situation where there are multiple solutions (dashed line in Figure 8.4). In such a case, a small perturbation of one of the process parameters makes the reactor jump from low conversion to high conversion, or reversely, leading to an instable operation. The stability conditions of the CSTR were studied in detail by... 

To test for fouling, a 24 h run of a pilot-scale microreactor for azo pigment production was performed using a diazo suspension [65]. At the end of this period, the pressure loss of the microreactor increased exponentially. Special means were developed to prevent clogging and instable operation. By partial removal of the deposits, the pressure loss was brought back to normal. 

Feed gas composition spike is the main cause of instable operation of deethanizer column. 

A cold branch operating point. B instable operating point. 

Note 2. In view of the instability of hexatriyne, all operations during work-up must be carried out under nitrogen without any delay. 

To achieve sufficient vapor pressure for El and Cl, a nonvolatile liquid will have to be heated strongly, but this heating may lead to its thermal degradation. If thermal instability is a problem, then inlet/ionization systems need to be considered, since these do not require prevolatilization of the sample before mass spectrometric analysis. This problem has led to the development of inlet/ionization systems that can operate at atmospheric pressure and ambient temperatures. Successive developments have led to the introduction of techniques such as fast-atom bombardment (FAB), fast-ion bombardment (FIB), dynamic FAB, thermospray, plasmaspray, electrospray, and APCI. Only the last two techniques are in common use. Further aspects of liquids in their role as solvents for samples are considered below. 

The flame can become unstable if too large an amount of sample is introduced or if the sample contains substances that can interfere with the basic operation of the plasma. For example, water vapor, air, and hydrogen all lead to instability of the plasma flame if their concentrations are too high. 

Compressor Configuration To propeny design a centrifugal compressor, one must know the operating conditions—the type of gas, its pressure, temperature, and molecular weight. One must also know the corrosive properties of the gas so that proper metallurgical selection can be made. Gas fluctuations due to process instabilities must be pinpointed so that the compressor can operate without surging. 

The maximum attainable production was sought that did not cause thermal runaway. By gradually increasing the temperature of the water, boiling under pressure in the reactor jacket, the condition was found for the incipient onset of thermal instability. Runaway set in at 485.2 to 485.5 K for the 12 m reactor and at 435.0 to 435.5 K for the shorter, 1.2 m reactor. The smaller reactor reached its maximum operation limit at 50 K lower than the larger reactor. The large reactor produced 33 times more methanol, instead of the 10 times more expected from the sizes. This... 

Consider this eompressor operating in steady state at point A, with the reeyele valve elosed. If the resistanee in the eompressor diseharge system were to rise to point B, the eompressor would eneounter the surge region—essentially a region of flow instability. Catastrophie surge ineidents ean result in eomplete destruetion of the rotor. 

The PI controller, even when optimally tuned, is also unable to prevent surge. Furthermore, it is unable to stop surge once it occurs. In the above situation, the operator would correctly identify the problem as instability of the closed-loop PI controller. The only viable action would be to open the closed control loop by placing the controller in manual, thereby freezing the valve open. In this scenario, open-loop control will stop surge. 

Compressor surge is a phenomenon of considerable interest, yet it is not fully understood. It is a form of unstable operation and should be avoided in both design and operation. Surge has been traditionally defined as the lower limit of stable operation in a compressor and involves the reversal of flow. This reversal of flow occurs because of some kind of aerodynamic instability within the system. Usually a part of the compressor is the cause of the aerodynamic instability, although it is possible that the system arrangement could be capable of augmenting this instability. Figure 6-36 shows a typical... 

One of the most serious forms of instability encountered in journal bearing operation is known as half-frequency whirl. It is caused by self-excited vibration and characterized by the shaft center orbiting around the bearing center at a frequency of approximately half of the shaft rotational speed as shown in Figure 13-15. 

The frequencies of a spectrum can be divided into two parts subharmonic and harmonic (i.e., frequencies below and above the running speed). The subharmonic part of the spectrum may contain oil whirl in the journal bearings. Oil whirl is identifiable at about one-half the running speed (as are several components) due to structural resonances of the machine with the rest of the system in which it is operating and hydrodynamic instabilities in its journal bearings. Almost all subharmonic components are independent of the running speed. 

Maddox shows how the major process concerns of corrosion, erosion, and column instability must be met in the design and operation of a hot carbonate process. These items will impact the capital and operating/main-tenance costs. 

When the eore s flux is eompletely emptied prior to the next cyele, it is referred to as the discontinuous-mode of operation. This is seen in the induetor eurrent and voltage waveforms in Figure 3-4. When the eore does not eompletely empty itself, a residual amount of energy remains in the eore. This is ealled the continuous mode of operation and ean be seen in Figure 3-5. The majority of boost-mode eonverters operate in the diseontinuous mode sinee there are some intrinsie instability problems when operating in the eontinuous mode. 

---