Interface levels

An extraction plant should operate at steady state in accordance with the flow-sheet design for the process. However, fluctuation in feed streams can cause changes in product quaUty unless a sophisticated system of feed-forward control is used (103). Upsets of operation caused by flooding in the column always force shutdowns. Therefore, interface control could be of utmost importance. The plant design should be based on (/) process control (qv) decisions made by trained technical personnel, (2) off-line analysis or limited on-line automatic analysis, and (J) control panels equipped with manual and automatic control for motor speed, flow, interface level, pressure, temperature, etc. 

Operation When operated correctly, thickeners require a minimum of attention and, if the feed characteristics do not change radically, can be expected to maintain design performance consistently. In this regard, it is usually desirable to monitor feed and underflow rates and sonds concentrations, flocculant dosage rate, and pulp interface level, preferably with dependable instrumentation systems. Process variations are then easily handled by changing the principal operating controls—underflow rate and floccirlant dose—to maintain stability. 

Liquid Residence Rules of Thumb for Proper Automatic Control of Interface Level (References 3 and 4)... 

Assume 20% cross-sectional area is occupied by an emulsion and is recognized as a dead volume. This is actually the height over which the interface level wdll vary during normal operations [26]. 

Interface Level Assume Hold interface one foot below top of vessel to prevent interface from reaching the top oil outlet. 

In the LPG contactor the amine is normally the continuous phase with the amine-hydrocarbon interface at the top of the contactor. This interface level controls the amine flow out of the contactor. (Some liquid/liquid contactors are operated with the hydrocarbon as the continuous phase. In this case, the interface is controlled at the bottom of the contactor.) The treated C3/C4 stream leaves the top of the contactor. A final coalescer is often installed to recover the carry-over amine. 

Specify the diameter, height and weir dimensions and sketch the method of interface level control which is proposed. 

The recovery most often used at sites with significant quantities of recoverable LNAPL is the two-pump system. A submersible water pump installed below the lowest possible probable interface level is used to create a drawdown cone of depression, while a second pump is suspended with its intake port located at the oil-water interface. A typical two-pump system installation is shown in Figure 7.15. 

Fig- 2—Typical configuration of a three-phase horizontal separator. Interface level control maintains water level, weir maintains oil level. Vessels may be equipped with sand jets If sand production is a problem. jets are designed for 20 fps velocity, and produced water normally is used for backwashing. 

Produced water flows from a nozzle in the vessel located upstream of the oil weir. An interface level controller senses height of rhe oil/water interface The controller actuates the water dump valve, allowing the correct amount of water to leave the vessel so that the oil/water interface is maintained at design height. 

The second method shown uses a weir to control the gasoil interface level at a constant position. This results in better oil/water separation as ail the oil must rise to the height of the oil weir before exiting the vessel. Disadvantages arc that the oil box takes up vessel volume and costs money to fabricate. Sediment and solids can collect in the oil box and be difficult to drain, and a separate low-level shut-down may be required to guard against the oil dump valve failing open. 

The third method uses two weirs, eliminating need for an interface float. Interface level is controlled by height of the external water weir relative to the oil weir or oudet height This is similar to the bucket and weir design of horizontal separators. Advantage of this system is that it eliminates the interface level control. The disadvantage is that it requires additional external piping and space. 

Interface level control problems exist. and barge pitch motions are usually less than roll motions. In such cases the pitch... 

Poorly operating interface-level controllers can upset the interface level and cause a loss of water resulting in oil being dumped out. There is also the problem of improperly positioned inlet splash baffles within the vessel and this can cause problems regardless of the chemical. This, in most cases, can only be eliminated by removing or repositioning the baffle... 

Horizontal treaters. Multiwell installations normally require horizontal treaters. Fig 2 shows a typical design. Flow enters the front section of the tteatei where gas is flushed. Liq uid falls to the vicinity of the oil-water interface where it is water washed and freewater separated Oil and emulsion rise past the lire tubes and are. skimmed into the oil surge i Irani -her. "Fhe oil-water interface in the inlet section is controlled by an interface level controller that operates a dump valve for the freewater... 

The wet crude tar.k operates at approximately 50t level and gives a period of several hours retention time, allowing formation water to settle out of suspension. Separated free water settles at bottom of tank and Is transferred to waste water conditioning unit through an interface level controlle . 

The oilfwater interface level in the separation chamber is governed by the elevation of water-weir crest and the head of flow ing water over it. The oil weir has been adjusted 0.2 in. [5 nun] above the oil water interface level This ensures a constant oil pad over the oil water interface and continuous skimming of oU into the J-type oil tray. 

The operation of the VTC. though very simple with no moving parts inside the main unit, posed some unique problems during operations These problems basically relate to surge in flow through the units, which is caused by process upsets upstream of the system. It should be recognized that VTC and ail other similar equipment operates satisfactorily if the head over the effluent weir ts stable. Any surge in flow caused by process upset upstream of the unit would destabilize the oil/waier interface level. This may cause water to enter the J-type oil-collection chamber. 

Tests conducted on the Hutton produced-water system used a two-stage mono pump installed downstream of the produced-water flash lank, which collects water from first, second, and test separators under interface-level control. 

Varying the steam-to-condensate interface level to control the reboiler duty will promote steam leaks in the channel head-to-shell flanged closure. This is caused by the thermal cycling and stresses that result from constantly varying the level of condensate in the channel head. However, when low-pressure steam (<60 psig) is used, this becomes a minor problem, which may be safely ignored. 

Water is drained off the boot to maintain an interface level between water and hydrocarbon in the boot. These interface level controllers, especially on reflux drum water draw-off boots, are typically set locally. The operator finds the visible interface level in the boot s gauge glass, and then sets the local interface level controller, to hold the level half way down the boot. Level taps in the lower portion of any vessel, tend to plug. This is especially true with the level taps in water draw-off boots. You need to blow out these taps on some frequent basis, because a high boot interface level can push water into the tower s reflux pump. 

When detecting the interface between two liquids, electrical conductivity, thermal conductivity, opacity, or sonic transmittance of the liquids can be used. Interface-level switches are usually of the sonic, optical, capacitance, displacer, conductivity, thermal, microwave, or radiation types. Differential pressure transmitters can continuously detect the interface, but, if their density differential is small relative to the span, the error will be high. On clean services, float- and displacer-type sensors can also be used as interface-level detectors. In specialized cases, such as the continuous detection of the interface between the ash and coal layers in fluidized bed combustion chambers, the best choice is to use the nuclear radiation sensors. 

When measuring the level of the interface between two liquids, the span of the cell is the product of the density difference between the two liquids (SG2-SG1) and the distance between the maximum and minimum interface levels (X). An example of determining the required range depression is given in Figure 3.115. 

The aqueous feed pump to the last battery in the cycle (down the organic stream) is started and adjusted to the prescribed flow rate. A start at this end is necessary to guarantee that stripping and clean-up batteries are operating properly before the organic phase becomes loaded. When the interface level starts to rise in... 

Operation. "Active" operation is started by turning a three-way valve in the feed-line to the "high active" feed pump. When the new feed solution enters the system minor adjustments on motor speeds and interface levels are usually necessary to compensate for small changes in density and viscosity. 

Interface levels are monitored continously during operation. The first sign of pump or mixer/motor malfunction or of crud accumulation is usually seen on the interface levels. 

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