Settler interface level

Settler interface levels. The interface level in the end stage where the dense phase leaves the extractor is simply controlled by means of a dense phase overflow weir. This is set at a suitable height to maintain the interface below the level of the mixed phase port. 

The causes of carry-over can be related to high settler interface level, excessive mix-valve pressure drop, inadequate settler residence time, dirt, settler internal mechanical damage, or all of the above. 

Figure 4. Settler interface measuring device. The upper part of the figure is a stereoscopic drawing of the level transducer. Note that one electrode is covered with PTFE insulation, except for the lower tip. Electrode materials have been graphite or titanium. Lower part of the figure shows the level measuring circuit.

Figure 14-1 shows a typical feed cleanup system. The system s components are a settling drum, circulation pump, mix valve, interface level control valve, and wash liquid. The wash liquid, usually caustic or water, is contacted with a hydrocarbon stream. The mix valve provides the turbulence required for good contacting. The settling drum separates the two phases. The circulating pump returns the wash liquid to the mix valve. Finally, the interface level control valve sets the relative volumes of the wash liquid and hydrocarbon in the settler. 

Calculate the residence time of the hydrocarbon in the drum. Include only that part of the settler above the interface level. 

Determining the interface level in a water/light hydrocarbon settler can be extremely difficult. If both phases are clean, they will also be transparent. First, blow the gauge glass assembly taps out after draining down the gauge glass, allow it to slowly refill (see Chapter 12). The interface can then be seen by its upward motion. For many settlers the aqueous phase will occupy the lower third of the vessel. 

Amine entrainment in the LPG is minimized by providing adequate LPG residence time above the normal LPG/amine interface level. Tse and Santos (1993) cite a case where the LPG residence time, as measured from the interface to the top of the contactor, was 8 to 10 minutes. This is quite conservative. A more reasonable criterion is believed to be the greater of 8 feet or 5 minutes LPG residence time as measured from the LPG/amine interface to the LPG contactor top tangent line. This assumes that an external grasity settler or coalescer is provided for the LPG product stream as shown in Figure 2-96. 

The level of the heavy phase outlet within the settler controls the level of the interface (Fig. 8.8). At the interface, the static pressure due to light phase above the interface is determined by the density of the light phase, p , and the depth of the light phase, H. Similarly, the static pressure above the interface in the overflow leg is determined by the density of the heavy phase, p, and the height h of the weir above the interface. The pressure must be the same at the same elevation, so h-p = Hpo, or... 

The level of the interfaces in jthe other settlers may be calculated, Fig. 4.26, assuming negligible change of density from stage to stage, and... 

The type of stirrer used in a simple stirred mixer-settler is relatively unimportant provided it mixes the two phases adequately. It is clearly not necessary for it to have any pumping action. Neither is it essential for any individual stirrer to be operable at any particular time, since the effect of a breakdown in a single stage is merely to lose the extraction effect of that stage. An interface will form in the faulty mixing chamber at the level of the mixed phase port and both phases will take their normal paths, except without mixing. 

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