The chart below shows the main operational principles utilized in water treatment equipment. The typical equipment types are also listed within each working principle.
API separator resembles a rectangular basin where main driving force of the oil separation is gravity. Long retention time is required to allow the oil droplets separating as a floating scum. The main purpose of this device is to separate gross of oils and suspended solids from the effluent water. The lowest limit for the droplet size removal capacity of the API separator is around 100-150 μm. API separator has very simple design, but considerable space requirements necessary for sufficient retention time often turn it to an unattractive option.
Generally, skim vessels are very close in design and performance to API separators. The main addition to the design of the skim vessels is a presence of the devices that help promoting the coalescence, enhancing oil droplet separation from the main flow, and reducing the short-circuiting. Inlet spreaders, baffle plates, and outlet collectors are the examples of such additional devices. The main utilization of the skim vessels is the primary treatment of low-pressure effluent water with high oil concentration and solid contaminants. These vessels are not commonly used in offshore applications due to size and weight limitations, and the negative effect of platform movement on the separation.
This type of equipment uses gravity separation similar to the skim vessels, but in addition it promotes the coalescence of oil droplets. Bigger droplets flow faster to the phase interface. These devices resemble skim vessels retrofitted with the plate interceptors. Corrugated plate interceptors (CPI) and cross-flow devices are the most effective plate coalescers that are able to separate oil droplets down to sizes of 30-50 μm. The main difference between CPI and cross-flow devices is that the plate axes of the corrugations are parallel to the direction of flow in CPI and are perpendicular in the cross-flow devices.
Plate packs are usually oriented at certain angle to the surface. The flow direction of produced water through the inclined plate interceptors can be either upflow or downflow. Downflow CPI may experience issues with sand plugging if sediment production is anticipated. To negate the effect of sand production upflow CPI configuration can be used. Cross-flow devices are preferred option in pressurized vessels in addition to effective sand removal.
The plate coalescers are simple devices that have no moving parts and do not require power. Steady flow rates with oil concentrations up to 3000 ppm and limited amount of solids are the favorable operation conditions. Emulsified flow streams and oil droplets below 30 μm significantly reduce the efficiency of this equipment type.
A hydrocyclone is a static liquid-liquid separator that uses the centrifugal force to separate the liquid phases. Typically, hydrocyclones are assembled as a set of liners contained in the pressure vessel. A liner can have slightly different configuration, but mainly consist of inlet swirl chamber, tapered section, and tail section. Produced water enters the swirl chamber, develops the rotational motion throughout the tapered section, and leaves through the tale outlet. The centrifugal forces developed by rotational motion cause the lighter phase (oil droplets) to move toward low-pressure central core. The axial flow reversal occurs where inner oil phase volume starts to move upwards and leaves the hydrocyclone through the overflow.
Due to very low residence time, hydrocyclones provide highest throughput-to-size ratio among the water treatment equipment. They efficiently remove oil droplet sizes down to 10-30 μm. The proper use of hydrocyclones requires certain inlet pressure. Preferably, they should be installed as close as possible to the outlet of the three-phase separators or FWKD.
The advantages of this type of equipment are low weight and space requirement, no power consumption, no moving parts, and insensitivity to the motion. Negatively, presence of sand and scale will cause erosion to the liners and may cause the plugging of the outlet ports. If the system pressure is low, hydrocyclones may require a low shear pump to increase the operating pressure. Due to the number of liners in use being relatively static, hydrocyclone efficiency is also affected by changes in the water flow rate. This can cause changing differential pressure which affects the separation efficiency.
Centrifuge separators utilize the principle of enhanced gravity separation. The process takes place inside the rotating bowl, which is driven by the motor. The produced water enters in the middle of the bowl. At high rotational speed, heavier water phase is forced to the outer zone of the bowl where it is removed. The lighter oil phase flows towards the center and leaves the centrifuge through the overflow. It is common to use a disk-stack insert, which reduces the settling distance of the oil droplets. Centrifuges can effectively remove dispersed droplets down to sizes of 1-2 μm.
Since the centrifuges are dynamic units, they require lower inlet pressure for the effective operation compared to the static hydrocyclones. The main drawback of this type of equipment is the high maintenance load. In addition, it consumes significant power and, therefore, have poor cost-benefit ratio.
Dissolved gas unit
Dissolved gas unit separates the oil droplets from the produced water using gas bubbles as a driving force. Before entering the unit, the water is saturated with the gas in a high-pressure vessel. When saturated water flows into the low-pressure unit, gas is flashed out from the liquid. Gas bubbles rise through the liquid and attach small oil droplets and/or solid particles to themselves. The effective specific gravity of gas bubbles with attached oil droplets is much smaller than oil droplets alone. Thus, the rising velocity is significantly enhanced. Eventually oil gathers at the surface of the vapor-liquid interface and overflows to the oil collector. Average sizes of generated gas bubbles are in range of 10-100 μm.
This type of unit is more effective in refinery operations where air can be used as the gas, and more space is available. The important aspect of using air is the presence of oxygen. In refinery operations fresh water is often used, which is already oxygenated. In offshore facilities, though, use of fuel gas or nitrogen is preferred. Elimination of oxygen prevents scaling, corrosion, and bacteria growth in offshore operations.
Dispersed gas unit
The principal difference between dispersed and dissolved gas units is the way the gas is introduced to the produced water. In the dispersed gas unit, gas bubbles are dispersed by inductor device or by mechanical rotors in a vortex set up. Dispersed gas unit typically contains three to five flotation chambers. Each chamber has the regions of gas-liquid mixing, flotation, and skimming. The produced water moves from one chamber to another by underflow baffles. The advantage of the dispersed gas flotation unit is that it operates on constant percent removal basis. The unit can remove oil droplet down to sizes of 10-20 μm. Within the normal operating range for this type of equipment removal efficiency is independent of oil concentration and droplet size distribution. Theoretical efficiency of each chamber is around 50%. Therefore, the four-chamber unit should remove around 94% of dispersed oil. The real field operating efficiency is slightly lower.
Since the gas creates natural gas blanket, it can be reused for induction constantly with no need for venting. Low retention time required for the effective separation together with relatively compact design makes it an attractive choice for the offshore applications. The disadvantage of the gas flotation units is the motion-sensitivity. Sloshing in the skimming area can degrade the separation performance.
Typical application area of the media filters is the removal of fine solids from the water. Certain media, such as walnut shells, can successfully remove dispersed oil droplets as well. One of the disadvantages of standard sand media filters is the requirement for periodic cleaning of the media via a backwash cycle that generates significant waste volumes. The walnut shell filters mitigate that by using mechanical agitation or walnut shell recirculation during the backwash cycles. These measures reduce the waste volume substantially. This type of filter can efficiently remove 95-99% of suspended solids and 90-99% of dispersed oil.
The efficiency of this type of filter is reduced when cleaning heavy crude oils from produced water. Chemicals and/or heat are often necessary in these situations.
This type of equipment filtrates the produced water through the membranes of various pore sizes. The produced water is injected into the membrane module, where it separates into a permeate (cleaned water) and a retentate (reject flow). The membrane filtration can handle very small droplet size and reduce the oil concentration in produced water down to 5ppm or less. It effectively deals with emulsions without additional need for chemicals. The drawback of the membranes is a very large reject stream compared to the other technologies. The membrane retentate may still contain over 90% of water, which must be disposed or recirculated. Some membrane materials are sensitive to certain chemicals and all membranes may suffer fouling. Membranes are made in many materials, but ceramic membranes are most compatible for produced water applications due to their high chemical and temperature resistance. Downside of ceramic membrane systems for offshore application is that they are expensive and require relative complex process systems to operate as they normally operate in a cross flow configuration. Additionally membranes are dependent on a good backwash and chemical clean in place system to operate. Typically, membranes have a lifetime of 3-5 years. Polymer type membranes are commonly used in onshore produced/waste water treatment facilities in bacterial treatment stages.
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