Decanters interface control

Decanters design drop diameter feed configuration horizontal inlet velocity limit interface control Dew point Diafiltration... 

The vapour mixture of steam and solvent leaving the first stripper is condensed and recovered in decanter vessels, where water and solvent separate. The solvent is recycled into the wet solvent storage tanks for re-use and Ihe water phase is recycled into the strippers. Decantation is controlled by an interface detector system based on the density difference between the water and the solvent. 

The aqueous interface in the decanter is controlled by manipulating the flowrate of the aqueous stream to the top tray of the first column. 

For a decanter that operates under gravity flow with no instrumentation flow control, tire height of the heavy phase liquid leg above the interface is balanced against the height of one light phase above the interface [23]. Figures 4-12 and 4-13 illustrate the density relationships and the key mechanical details of one style of decanter. 

In any equipment where an interface exists between two phases (e.g. liquid-vapour), some means of maintaining the interface at the required level must be provided. This may be incorporated in the design of the equipment, as is usually done for decanters, or by automatic control of the flow from the equipment. Figure 5.16 shows a typical arrangement for the level control at the base of a column. The control valve should be placed on the discharge line from the pump. 

Decanters are normally designed for continuous operation, but the same design principles will apply to batch operated units. A great variety of vessel shapes is used for decanters, but for most applications a cylindrical vessel will be suitable, and will be the cheapest shape. Typical designs are shown in Figures 10.38 and 10.39. The position of the interface can be controlled, with or without the use of instruments, by use of a syphon take-off for the heavy liquid, Figure 10.38. 

The height of the liquid interface should be measured accurately when the liquid densities are close, when one component is present only in small quantities, or when the throughput is very small. A typical scheme for the automatic control of the interface, using a level instrument that can detect the position of the interface, is shown in Figure 10.40. Where one phase is present only in small amounts it is often recycled to the decanter feed to give more stable operation. 

A mixture of two immiscible liquids is fed into a decanter. The heavier Hquid a settles to the bottom of the tank. The Hghter liquid P forms a layer on the top. The two interfaces are detected by floats and are controlled by manipulating the two flows andFp. 

Oil and water are mixed together and then decanted. Oil flow rate is ratioed to water flow rate Ff. Interface is controlled by oil flow Fj, from the decanter with a proportional level controller. Water flow F ) from the decanter, which is liquid full, is on pressure control (PI). Steadystate flow rates are ... 

Since there are two liquid phases in a decanter, the total liquid level and the interface level must both be controlled. Of course this requires that they both be measured. Measuring the interface between a liquid and a vapor is usually not difficult because the density difference between the two phases is large. However the density difference between two liquid phases is relatively small, and this can make liquid-liquid... 

Steady-state control of a continuously fed extraction column requires maintenance of the location of the liquid-liquid interface at one end of the column. The main interface will appear at the top of the column when the light phase is dispersed and at the bottom of the column when the heavy phase is dispersed. If needed, extraction columns can be designed with an expanded-diameter settling zone to facilitate liquid-liquid phase separation by reducing liquid velocities. If sufficient clarification of the phases cannot be achieved, then it may be necessary to add an external device such as a gravity decanter or centrifuge. (See Liquid-Liquid Phase Separation Equipment. ) Sometimes a column is built with expanded ends at... 

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. 

Two liquid phases are separated using the continuous decanter shown in Figure 21.40. The output variables, which must be controlled, are F, the volumetric feed rate, Pj, the operating pressure, and 7, the dispersion interface level in the decanter. The positions of the three control valves, m, m2, and m3 are the manipulated variables. A linear model is available to describe the proeess ... 

Entrained liquid in overhead gas sensor error/[entrainment GL]. Incomplete separation of oil from water faulty design of separator/residence time of liquid phases too short/liquid velocity in the decant phases too fast/Marangoni instabil-ities/liquid feed velocity too fast/poor distribution of liquid feeds/faulty location of exit nozzles for liquid phases/overflow baffle corroded and failure/interface level at the wrong location/faulty control of interface/no vortex breaker at water and heavy oil exit nozzles/liquid exit velocities too high/ emulsification]. ... 

Fluid holdup time in the decanter is not short. It may be 5 to 30 minutes — probably closer to the latter. The decanter is a simple tank, though likely with cove comers (to avoid fluid retention), and certainly without agitation (to avoid mixing the phases together). Since the decanter contains two phases, there will be an interface. An interface monitor, often optically artivated, controls pumps which keep the interface within a range of decanter heights by periodically (usually) pumping away a batch of aqueous (RA) material or soil-laden SA material as needed. ... 

The interaction can also easily be eliminated by a simple process change, as shown in Figure 4.8a. An overflow weir is installed at one end of the decanter. The hghter liquid flows over this weir into a surge volume whose level can easily be measured and controlled. Measurement of the interface level is still required with this system. 

Heavy distillate is manipulated to control interface level in the decanter. Because reflus flow is less than that of light distillate, it can be accurately manipulated for composition control decanter level is then controlled by manipulation of light distillate. Vapor and reflus flow interact in their effect on bottoms composition. They can be determined from feed composition by V = F Zc Zb), L = F 8zc 2zb), Two bottoms-composition controllers are necessary, their outputs ms and me taking the place of the unknown feed compositions in the previous equations. Then the decoupling control system manipulates heat input and reflus with a forward loop from feed rate V = F 9m.c — ms), L = F 8nic — 2m ). 

Modifled from McAvoy, 1983). A decanter shown in Fig. E18.14 is used to separate a feed that consists of two completely immiscible liquids, a light component and a heavy component. Because of the large difference in their densities, the two components form separate liquid phases very rapidly after the feed enters the decanter. The decanter is always full of liquid. The level of the interface I between the two liquid phases is measured by a dp cell. Each liquid flow rate can be adjusted by using a control valve, which is connected to a standard PI controller. The control valve equations relate flow rates, pressures, and controller output signals (mi, m2, m3) ... 

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