Level measurement column base

Both of the control structures discussed in Sec. 2.7.2 (CSl and CS4j work because they detect the inventories of the reactant components A and B in the system and bring in fresh feed streams to balance the consumption of the two components. Structure CSl does this by using the liquid level in the reflux drum of the second column as an indicator of the amount of A in the system and the liquid level in the base of the first column as an indicator of the amount of B in the system. Structure CS4 uses a composition analyzer to measure directly the concentration of one of the reactants in the reactor. But both of these structures lack a direct handle on production rate. 

A bottom temperature indicator measuring liquid temperature may read vapor temperature (which may be considerably lower) when the level drops. The consequences are similar to those of a fouled thermocouple. A malfunctioning level indicator can thus lead to a dangerously misleading, but apparently consistent, indication of both level and bottom temperature (Fig. 13.8). In one case (275), this led to overheating and an exothermic reaction at the column base, which in turn caused residue discharge from a column vent. 

Measure the response time for a hquid level process variable to reach 95% of the step change in setpoint when tuning a liquid level control loop for a reflux drum or distillation column base... 

There are a number of different methods used to express a pressure measurement. Some of them are natural units, i.e., based on a force per unit area, as with pound (force) per square inch (abbreviated Ibf/in or psi) or dyne per square centimeter (dyn/cm ). Others are based on a fluid height, such as inches of water (in H2O) or millimeters of mercury (mm Hg) units such as these are convenient when the pressure is indicated by a difference between two levels of a liquid as in a manometer or barometer. This measurement is based on the pressure at the base of a column (height) of fluid. Barometric pressure and atmospheric pressure are synonymous and measure the ambient air pressure. Standard barometric pressure is the average atmospheric pressure at sea level, 45° north latitude at 32°F. It is used to define another unit of pressure called the atmosphere (atm). Standard barometric pressure is 1 atm and is equivalent to 14.696 psi and 29.921 in Hg. As one might expect, barometric pressure varies with weather and altitude. 

Many problems have been encountered in making head measurements in and around distillation columns, that is, column AP, liquid level, and specific gravity. Of these column-base level is probably the one where difficulties are... 

With the basic mathematics of level measurement in hand, let us next turn our attention to the problem of noise. Distillation-column base-level measurements tend to be extremely noisy. This is mostly attributable to turbulence, but may also be due to the type of transmitter used. Regardless of its source, however, it is best to filter noise out at the transmitter input rather than at its output. If this is done properly, such filtering or damping greatly reduces the probability of saturation in the transmitter and minimizes output signal errors due to transient, nonlinear transmitter operation. In the case of pneumatics, such filtering minimizes demands on the air supply and therefore minimizes the probability of gulping. ... 

Occasionally we find enough diflference between column-feed specific gravity and normal bottom-level spe c gravity that there is a serious base-level measurement error at startup. Also, at times, a given column may have a different bottom specific gravity as a result of changes in bottom-product specifications. 

Heat-Flow Computations Column-Base Level Measurements Control Valves Column AP Measurement Temperature-Measurement Dynamics Flow and Flow-Ratio Conventions Control-Valve Split Ranging... 

Several authors observed CL emission based on reduction reactions. Lu et al. [59] developed a method by applying a Jones reductor for producing unstable reductants. A column (100 X 3 mm i.d.) filled with Zn-Hg particles was inserted into the flow stream of a flow injection system. CL was measured using a homemade CL analyzer. Although the Jones reductor was more effective for the species studied in 0.5-5 mol/L H2S04 solution, the authors found that a lower acid concentration improved the CL emission. Hie optimal pH was 6.5 for V(II), 2.5 for Mo(III), 3.5 for U(III), 3.0 for W(III), 3.0 for Cr(II), 2.5 for Ti(III), and 2.5 for Fe(II). The methods allowed determination of the above-mentioned species at pg/mL to ng/mL levels. It was assumed that the CL reactions were related to the production of superoxide radicals by dissolved oxygen in the solutions. The proposed methods could be successfully applied to the determination of V [60], Mo [61], and U [62] in water or steel samples. 

A distillalion column is used to separate two close-boiling components that have a relative volatility close to one. The reflux ratio is quite high (IS) and many trays are required (150). To control the compositions of both products the flow rates of the product streams (distillate D and bottoms B) an manipulated. Gas chromatographs are used to measure the product compositions. Base level is controlled by steam flow rate to the icboiler and reflux drum level is controlled by reflux flow rate. 

The amount of P supplied by resuspension was relatively small compared with water-column standing pools and major flux vectors. Thus, resuspension of bottom sediments may not be a major mode of phosphorus resupply. The pool of resuspendable P is finite. The deposition-resuspension cycle will not increase the amount of P in this pool unless P is added from another source (e.g., by diffusion of P from lower sediment levels). However, the diffusive flux would be relatively small. The resuspendable particulate P can be recycled during spring mixing by repeated deposition and resuspension, but this cycle does not increase the amount of P in the resuspendable pool. Eadie et al. (24) reported a resuspended P flux (sediment-trap-based) of3200 mg of P/m2, 66 times our estimate here. However, this large P flux would require the resuspension of over 2.0 cm of surface sediment and much higher suspended Al levels than were measured in the water column. 

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