Interface-level control

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. 

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 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 . 

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. 

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. 

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. 

First, obtain a sample of the reflux naphtha. Does it contain free water If so, manually drain the water from the reflux drum until hydrocarbons blow out of the drain. About 30 minutes of water-free refluxing should clear up the dirty naphtha. A malfunctioning reflux drum water-naphtha interface level control valve is often the culprit. Perhaps the steam stripping rate has been increased, and the water drain line cannot handle the increased flow. 

If the interface level is unusually high, your problem is solved. The interface level controller is broken or set incorrectly. Perhaps the level control valve has jammed in a closed position. Maybe a capricious sprite has blocked it in. 

Using amine as the continuous phase also increases the LPG treater amine residence time and provides mote time for the operator to intervene if the interface level controller fails. This minimizes the chance of hydrocarbon breakthrough causing a major sulfur plant upset... 

The LPG feed is distributed evenly and formed into droplets by injection into the continuous amine phase at the bottom of the colunm. The lean amine is distributed across the top of the pacldng where it joins the continuous amine phase. The densi differmce betweoi the two phases causes the dispersed LPG ro flow upward through the contiimous amine phase. The LPG/amine interfece is maintained above the top bed and the aniine distributor by an interface level controller which controls the rate of discharge of rich amine fiorn the bottom of the contactor. 

The oil level is maintained by pneumatic or lever-operated dump valves. The oil-water interface is controlled by an interface level controller or an adjustable external water leg. 

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