Free-water knockout

Determine the weight for the following free-water knockout. It is butt weld fabricated with spot x-ray and to be built to Division 1. A conical head (bottom of the vessel) is desired for ease in sand removal. Compare this weight to that of a vessel without the conical section and that to ti ves.sel with a 4-m. plate internal cone. 

Free-water knockouts, as the name implies, must remove excessive volumes of free water ahead of the treating systems. To best accomplish this, the chemical should be Injected prior to the knockout to break out the maximum amount of water. Because of this, the demulsifier cannot be sensitive to agitation. Interface conditions are also extremely important, as oily effluents will result if the interface is not smooth and dear. 

In a two-part series. Zeme discusses the importance of good separator hydraulics. A poor hydraulic design can make a good separation scheme ineffective. Zemel provides the methods and procedures to run a tracer test to identify short-circuiting, stagnant-flow regions, and shear forces. Analysis of the residence-time distribution curve that results is presented. Actual tests run on separators indicate that the most successful separator was the sequential dispersed-gas flotation cell, which closely followed the tanks-in-serie< model. This is contrasted with the poor performance of a conventional 2, 006-hbl [3 0-ms] wash tank The tracer responses of a pressurized flotation cell, a 15j000-bbl [2400 mJj wash tank, and a horizontal free-water knockout with and without baffles are also discussed. 

Trie previous installment in this senes, wtuch appeared In World Oil last March, inventoried waste water treating equipment installed downstream of the free water knockout. This installment adds another piece of equipment to the list—SP packs that can substitute for plate coalescers in some applications—and details how equipment is selected and sized to meet project requirements. Authors divide the design process into simple, easy-to-folow steps and demonstrate the procedure with an example problem. 

Various attempts have been made to develop procedures to determine oil concentration in water outlets from propcrlv designed free water knockouts and treaters. A conservative assumption would be that the water contains less than 1.500 to 2,000 mg/1 of dispersed oil. 

It is possible to theoretically trace particle size distribution up the tubing, through the choke, flowlines, manifolds and production equipment into the free water knockout using equations presented in previous installments However, many parameters needed to solve these equations, especially those involving coalescence, are unknown... 

Because of dispersion through the water dump valve, oil size distribution at the outlet of a free water knockout or heater treater is not a significant design parameter. From dispersion theory, it can be shown that after passing through the dump valve, a maximum droplet diametet on the order of 10 to V) microns will exist, no matter what the droplet vi/e distribution was upstream of this valve. 

It is desirable to bring information included in earlier installments of this series into a format that can be used in selecting and sizing individual pieces of equipment needed for a complete water treating system. Federal regulations require that produced water from the free water knockout receive at least some form of primary treatment before being sent to a disposal or skim pile. Deck drainage may be routed to a properly sized disposal pile that will remove free oil. 

Every water treating system design must begin with sizing the free water knockout, heater treater or ihree-phaac separator. These vessels should be sized in accordance with procedures discussed previously. 

Free-water knockout with removable baffles... 

Horizoatal free-water knockout. This horizontal free-water knockout (Fig. 5) is provided with a sc of removable baffles by the manufacturer. 

The most significant things are, first, that this horizontal free-water knockout, with or without baffles, gives a surprisingly narrow distribution about its mean. 

The produced water is a combination of water from a free-water knockout and from heater treaters. An aeration tower is used to oxidize the iron and hydrogen sulfide, and chlorine is also added to the tower. The chemical treatment utilizes 45 to 60 ppm alum, 75 ppm chlorine, and a corrosion inhibitor. The finished water quality is as follows ... 

It is important to note that you should consider removing all free water before attempting to size the electrostatic water separation section. The dL factor calculated is only for this electrostatic section. You should therefore make a good estimate of how much water will pass into the electrostatic section and input this value in the Prod + Desalting Water, BPD input block. Inputting all of your production water will seriously err sizing results. Free water should be removed in free water knockout (KO) tanks or vessels upstream. True and needed electrostatic treater sizing may thereby be determined. 

Oil-Water Interface Control. In any petroleum processing unit in which emulsions are resolved, an interface between oil and water must occur. The quality of this interface is directly related to the efficiency of demulsification in either a refinery desalter or an oil-field free-water knockout or treater. The sharper the transition between clean water and clean oil (or the tightness of the interface), the better the ability to control oil and water retention times and quality and operate the vessel. 

Free-Water Knockout. By definition, free water is any water associated with the crude-oil emulsion that settles out within 5 min while the produced fluids are stationary in a settling space within a vessel. Free-water knockouts (FWKO) are simply three-phase separation vessels that separate... 

The unit operations in the miniplant employ proven emulsion-treatment principles free-water knockout, dual-polarity electrostatic treatment (DPET), heavy-oil evaporation (HOE) dehydration, and induced gas flotation (IGF). The overall process configuration provides maximum flexibility and allows for performance evaluation of units on either an individual basis or in various combinations. 

Free-Water Knockout (Figure B.3) V-1 FWKO vessel... 

Figure B.3. Schematic flow sheet of free-water knockout.

The DPET is designed to hold 1720 L (10.8 bbl) and operates safely at pressures up to 517 kPa (75 psi) and temperatures up to 150 C (300 T). The application of a high-voltage, dual-polarity electric potential to electrodes inside the vessel is used to coalesce and remove small droplets of water in the oil emulsion. The oil should be degassed and have a water content less than 15% before entering the vessel however, the treater does have the capability for free-water knockout. Preheated emulsion is pumped into the bottom portion of the vessel, below the electrodes, where free water generated by heating or chemical treatment may drop out. As more emulsion is... 

Free-Water Knockout (FWKO) A vessel designed to separate the readily separated (nonemulsified or Tree ) water from oil or an oil-containing emulsion. Further water and solids removal may be accomplished in a treater. 

The thermodynamics of this process are described in detail in references (67 —72, 80,81). Let us examine a typical methanol injection system. In a typical methanol injection and recovery system for a cold-oil absorption or turboexpander plant, feed gas passes through a free-water knockout drum and into a gas-gas exchanger with methanol being sprayed on exchanger tube-sheets. Methanol inhibits hydrate formation and aqueous methanol condenses in the exchanger (and the chiller following it) and is pumped to a primary separator. The methanol-water solution is then flashed in a flash drum and filtered into a methanol still to recover methanol. Normally, methanol dissolves in the hydrocarbon liquids and is distilled as a mixture of propane and methanol. Some of the methanol is recovered as the overhead product to recover the methanol dissolved in the heavier solution, the bottoms of the methanol still (propane product or hydrocarbon liquids from the demethanizer)... 

The free water knockout (FWKO) is a separation vessel used extensively in the oil industry to treat oilfield-produced emulsions. The FWKO separates the large volumes of free water associated with the produced fluid. Produced solids can collect in the FWKO. The use of water jets and drains is also an effective method for eliminating produced solids in the FWKO operation. 

Whether or not an emulsion is stabilized by solids will determine the nature of the demulsifier that will be most effective. In addition, the presence of multiple emulsions (water-in-oil-in-water-in-oU, etc.) is often symptomatic of demulsifier overtreatment. Figure 5 shows an oUfield emulsion formed when a free water knockout vessel was contaminated with viscosity reducers from an earlier well fracture. Similar multiple emulsions can result from overtreatment of the produced fluid by demulsifiers in the process. 

After passing through a free-water knockout separator, the gas is cooled in a gas-to-gas heat exchanger in which methanol is sprayed on the tubesheets to inhibit the formation of solid hydrates. A methanol-water solution condenses in the heat exchanger and subsequent chiller and is removed from the gas stream in the separator. The aqueous solution is flashed to remove dissolved gas. filtered, and distilled to recover methanol. A significant amount of methanol dissolves in the liquid hydrocarbon product and is recovered by washing either the total hydrocarbon stream or the propane fraction with water from the methanol still. 

Uses Cone, dispersant for removing paraffin deposits, water wetting soiids, and iron suifides demuisifier intermediate emuisifier in emuision breaker formulations paraffin dispersant for downhole, flow line, free-water knockout, or tank bottom applic. 

When hydrocarbons (crude oil, condensate, and natural gas) are produced, the wellstream typically contains water produced in association with these hydrocarbons. The produced water is usually brine, brackish, or salty in quality but in rare situations may be nearly "fresh" in quality. The water must be separated from the hydrocarbons and disposed of in a manner that does not violate established environmental regulations. Typically, the produced water is mechanically separated from the hydrocarbons by passing the wellstream through process equipment such as three-phase separators, heater-treaters, and/or a free-water knockout vessel. These mechanical separation devices do not achieve a full 100% separation of the hydrocarbons from the produced water. The produced water separated from the hydrocarbons in these mechanical separation devices will contain 0.1-10 vol.% of dispersed and dissolved hydrocarbons. Produced water treating facilities are used to further reduce the hydrocarbon content in the produced water prior to final disposal. 

---