Light liquid level control

Apfc = pressure drop in the heavy-liquid exit pipe Api = pressure drop in the light-liquid exit pipe For satisfactory operation without liquid-level control, the Ap terms must be kept either very small or substantially equal, whence the relative height c for any desired interface position can be computed by... 

Reactor control in PHWR-500 is primarily achieved by the light water level control in 14 liquid zonal control compartments. Flux tilt control is achieved by individual control of levels in these compartments. The control functions, including reactor set-back and step-back, are assisted by four mechanical control rods. 17 Adjuster rods provide poison over-ride capability. 

Under a large slug condition, the condensate level can go very high. This situation causes severe liquid entrainment into the gas phase. It is a practice to reduce the inlet flow rate when the light liquid level increases substantially at high liquid level, the level control overrides the pressure control (through a low select UY) and starts reducing the inlet flow. 

Figure 3.16. Extraction tower control, (a) Operation with heavy solvent, interface in the upper section, top liquid level on LC. (b) Same as part (a) but with overflow weir for the light phase, (c) Same as part (a) but with completely full tower and light phase out at the top. (d) Operation with interface on ILC in the lower section, removal of the light phase from the upper section by any of the methods of (a), (b), or (c).

The process considered in this chapter is very simple. The reaction involves only one reactant and one product. Two inert components, one light and one heavy, are also present. These inerts must be purged from the system, The major plantwide control consideration is how to adjust the fresh feed of the reactant to balance exactly its rate of consumption by reaction. This is achieved by using the liquid level that is a good indication of the amount of reactant in the system (the base level in the DIB column). 

AUXILIARY EQUIPMENT. The dispersed phase in an extraction tower is allowed to coalesce at some point into a continuous layer from which one product stream is withdrawn. The interface between this layer and the predominant continuous phase is set in an open section at the top or bottom of a packed tower in a sieve-plate tower it is set in an open section near the top of the tower when the light phase is dispersed. If the heavy phase is dispersed, the interface is kept near the bottom of the tower. The interface level may be automatically controlled by a vented overflow leg for the heavy phase, as in a continuous gravity decanter. In large columns the interface is often held at the desired point by a level controller actuating a valve in the heavy-liquid discharge line. 

In an MB control scheme, product composition is controlled by manipulating the flow of material into and out of the column. This concept can be illustrated by examining the action of one of the common MB control schemes (Fig. 16.4a). Suppose the concentration of lights rises in the column feed. This will be sensed by a temperature drop, and the temperature controller will increase boilup. This will raise column pressure, and the pressure controller will step up condensation. Accumulator level will rise, and the level controller will increase distillate rate. Meanwhile, the increased boilup mentioned above will reduce the amount of liquid reaching the bottom sump, and the level controller will lower bottom product rate. 

The incoming flow is separated in the vessel into two streams a light vapour phase, which exits the top of the vessel, and a liquid phase, which exits the bottom of the vessel. The liquid level In the vessel Is maintained by the level controller (LICA245) that adjusts the liquid flow out of the vessel. The pressure In the vessel is maintained by a pressure controller (PIC214) in the vapour line. Over-pressure protection is provided by a pressure relief valve on the top outlet from the vessel. 

Air contaminants in solid or liquid state (aerosols), e.g., wood dust, welding smoke, or oil mist, are all in principle directly visible. The dispersion of those contaminants and the airflow patterns around the source may therefore be studied without any special tools. It is, however, not always possible to see the contaminant if, for example, the concentration in the air is low, the size of the particles is small, or the lighting is poor. The fact that the contaminant can t be seen may stem from the acceptable low level of the concentration but that can of course not be used to conclude that the control is acceptable. That conclusion depends not only on the contaminant s toxicological qualities but on how visible it is iit air. The ability to see the particles directly is also, as said above, a function of their size. Small particles, able to be transported deep into the thinner airways of the lungs, are many times also difficult to see directly. 

These four main types of apparatus being defined, (scientiste and manufacturers have let their imagination go in order to create apparatus). There are now about ten models, which differ by the volume of liquid used (from 2 cm to about 70 cm, the metal used for the cup (brass, aluminium), the heating mode (water bath, Bunsen burner, electrical), the type of gas used by the pilot light (natural gas, butane), the level of complexity of automatic controls some apparatus equipped with several cups can actually be programmed in order to make measurements automatically without the help of the operator. The liquid can be shaken manually or, thanks to an electrical motor, the ignition can be manual or automatic. 

To replace the old-fashioned eye-control method of detecting the level of the liquid, appliance manufacturers now use reed sensors to show when the container for Jet-Dry needs refilling. The reed sensor is mounted underneath, or next to, the liquid container. A floater with an internal permanent magnet is placed in the container and floats on the liquid. If its level falls to a minimum, the floater activates the reed sensor which, in turn, activates a light on the front control panel of the dishwasher, signaling the need to refill the Jet-Dry container. 

The test can be carried out on cultured cells or on cells from animals treated in vivo. In the former case the test chemical is usually evaluated in the presence and absence of the S-9 activation system from rat liver. Typically cells from a Chinese hamster ovary cell line are incubated in a liquid medium and exposed to several concentration of the test chemical, either with or without the S-9 fraction, for about 2 hours. Positive controls, such as ethyl methane sulfonate (a direct-acting compound) or dimethylni-trosamine (one that requires activation), as well as negative controls are also included. Test concentrations are based on cell toxicity levels determined by prior experiment and are selected in such a way that even at the highest dose excess growth does not occur. At the end of the treatment period the cells are washed, bromodeoxyuridine is added, and the cells are incubated for 24 hours or more. The cells are then fixed, stained with a fluorescent dye, and irradiated with UV light. Second division cells are scored under the microscope for SCEs (Figure 21.7). 

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