An article on Produced Water Treatment: Part 2 of 2

Continued from part-1 of this article…Click here to go the part-1….

API SEPARATOR (Fig. 5):

  • Principle: Gravity Separation
  • Description and Operation:
  • Fluid Enters at one end and separates along the length as per density.
  • Most of the suspended solids will settle to the bottom of the separator as a sediment layer, the oil will rise to top of the separator, and the wastewater will be the middle layer between the oil on top and the solids on the bottom.
  • Typically, the oil layer is skimmed off and re-processed or disposed.
  • The bottom sediment layer is removed by a chain and flight scraper (or similar device) and a sludge pump.
  • The water separated is sent for further processing.

Advantage and Disadvantage:

  • Simple in design
  • Large residence time also ensures solids separation
  • Is a very old design and has been replaced by CPI separator
  • Solids removal from the bottom is difficult and separated solids affect the separator efficiency
  • Large footprint area
  • Atmospheric Design-Cannot be used for PWRI
  • Need a degasser to be installed upstream

CORRUGATED PLATE INTERCEPTOR (CPI-Fig. 5):

  • Principle: Gravity Separation
  • Description and Operation:
  • Fluid Enters at one end and separates along the length as per density just like an API Separator
  • CPI includes corrugated plates arranged in a plate pack which installed at an angle of 45°
  • The plate provides more surface for suspended oil droplets to coalesce into larger globules.
  • Separated solids would slide down and separated oil drops move upwards due to its lesser density than water.
Typical Sketch of API Separator and Corrugated Plate Interceptor
Fig. 5: Typical Sketch of API Separator and Corrugated Plate Interceptor

Advantage and Disadvantage:

  • Corrugated plates enhance the degree of oil-water separation and therefore it requires significantly less space than a conventional API separator
  • Atmospheric Design-Cannot be used for PWRI
  • Need a degasser to be installed upstream

HYDRO-CYCLONES (Fig. 6):

  • Principle: Centrifugal Force
  • Description and Operation:
  • Fluids enter the hydro-cyclone through the tangential inlet at top which causes the fluids to spin and attain High Centrifugal forces
  • The centrifugal forces in a hydro-cyclone are of the order of several hundred times normal gravity force. This promotes rapid separation.
  • The clean water exits the Hydro-cyclone through the open end of the Tube at bottom
  • A vortex finder in the outlet port reverses the direction of the hydrocarbon core and it is discharged under differential pressure control, from the reject port at the centre of the Inlet End
  • In order to maintain hydro-cyclone separation efficiency, the Pressure Differential Ratio (PDR) must be maintained within certain limits. The PDR is defined as follows: PDR=DPir/Dpio Where, DPir = Pressure at inlet – Pressure at reject outlet ;DPio = Pressure at inlet – Pressure at water outlet ; Oil removal efficiency will decline <1.5=1.7-2.0=<Volume of liquid routed to the reject increases

Advantage and Disadvantage:

  • Compact in design
  • High Efficiency @ particle size less than 10microns
  • Modular Design gives flexibility for capacity enhancement
  • Energy Requirement to pressurise Inlet is high
  • High cost due to the Metallurgy-Erosion Issues

INDUCED GAS FLOATATION (IGF-Fig. 6):

  • Principle: Froth Floatation

Description and Operation:

  • IGF units operate by generating small gas bubbles and releasing these in the lower section of a vessel through which the water is flowing.
  • These bubbles attach themselves to the dispersed hydrocarbon droplets, reducing their density and floating them to the liquid surface.
  • The separated hydrocarbon is then removed and the treated water exits the vessel under level control.
  • There are two methods of Inducing bubbles viz. Mechanical and Hydraulic
  • The Mechanical method generated the gas bubbles by means of motor driven diffusers/impellers.
  • The unwanted side effect of the shear forces required to generate the micron sized gas bubbles was reduction in size of the oil droplets which adverse effect on the overall efficiency of these units.
  • The Hydraulic method uses a stream of clean water from the outlet and mixes it with the gas from the vessel top in an eductor. The mix stream is then injected in the floatation cells through nozzles.
  • Hydraulically inducing the bubbles is results in lower shear forces in the flotation cell

Advantage and Disadvantage:

  • Low Capex
  • High Inlet concentration can be acceptable
  • Relatively insensitive to changes in oil droplet size
  • Require steady flow for effective operation
  • Normally requires de-oiling chemical to be dosed upstream to optimize performance –High OPEX
Hydro-Cyclones and Induced Gas Floatation
Fig. 6: Hydro-Cyclones and Induced Gas Floatation

DISSOLVED GAS FLOATATION (DGF-Fig. 7):

Principle: Froth Floatation

Description and Operation:

  • Bubbles are generated by saturating a liquid stream with gas, typically at a pressure of 44 to 87 psig (3 to 6 barg).
  • As the liquid enters the Flotation Chamber it is depressurized and the gas is released as fine bubbles.
  • The rising bubbles attach themselves to hydrocarbon and float them to the water surface from where they can be removed by a skimmer and weir arrangement.
  • The main advantage of DGF is that the method of producing bubbles is relatively gentle. The absence of High Shear Forces helps in better separation.

Advantage and Disadvantage:

  • Proven technology
  • Moving parts and associated maintenance requirements
  • De-oiling chemicals normally dosed upstream to optimize performance
  • Gas solubility decreases with increasing temperature which can make the technology less effective at higher operating temperatures

COMPACT FLOTATION UNIT (Fig. 7):

Principle: Froth Floatation and Cyclonic Effect

Description and Operation:

  • The CFU combines the swirling tangential flow effect of a hydrocyclone within a gas flotation unit.
  • The inlet fluid enters the vessel via one or more tangential inlets, establishing a mild rotation of the liquid in the vessel.
  • Nitrogen or fuel gas is introduced in to the vessel via a bottom distribution nozzle or “sparger” (there are some differences between vendors regarding how the gas is introduced in to the vessel).
  • The oil reject from the vessel is normally removed via a central weir pipe at the top of the vessel.

Advantage and Disadvantage:

  • The residence times in CFU’s are significantly lower than traditional IGF systems, with residence times of 1 minute being typical, compared to 4 minutes for IGF systems.
  • Significantly smaller and lighter than conventional IGF
  • Excellent turndown
  • Normally requires deoiling chemical to be dosed upstream to optimize performance
  • Sensitive to vessel motion

CRUSHED NUT FILTER (Fig. 7):

Principle: Hydrophillic Nature of Crushed Nut Shells

Description and Operation:

  • Nutshell filtration involves the removal of suspended hydrocarbon liquids and solid matter from waste water by passing the water through a bed of crushed nutshells(Typical-Wallnut)
  • The crushed nut shells have a affinity to hold the oil particles and the suspended particles.
  • As the impurities are absorbed the media gets choked resulting in a higher pressure drop.
  • This is an indication that the filter need to be taken for backwash
  • Once backwashed it is again ready for filtration. To keep the process un-interrupted the filters are with N+1 configuration to take care of the capacity reduction during backwash.

Advantage and Disadvantage:

  • High quality water effluent
  • Very efficient for IW
  • Removes TSS in addition to OiW
  • The backwash mechanism is high on Energy and Maintenance
  • Erosional issues due to abrasive nature of media
  • Large and heavy equipment’s
DGF, Compact Floatation Unit and Crushed Nut Filter
Fig. 7: DGF, Compact Floatation Unit and Crushed Nut Filter

DUAL MEDIA FILTER (Fig. 8):

Description and Operation:

  • In a multimedia filter, two or more medias with different grain Size and density are used.
  • Within each layer there will be size segregation with larger particles at top and smaller at the bottom.
  • The advantage is that this helps to minimise blockage and also maximum bed depth utilization.
  • As the impurities are absorbed the media gets choked resulting in a higher pressure drop.
  • This is an indication that the filter need to be taken for backwash
  • Once backwashed it is again ready for filtration. To keep the process un-interrupted the filters are with N+1 configuration to take care of the capacity reduction during backwash.

Advantage and Disadvantage:

  • High quality water effluent
  • Very efficient for IW
  • Removes TSS in addition to OiW
  • The backwash mechanism is high on Energy and Maintenance
  • Erosional issues due to abrasive nature of media
  • Large and heavy equipment’s

Future Development (Fig. 8):

  • MEMBRANE FILTERS
  • CENTRIFUGE
  • CARTRIDGE FILTERS
  • VENDOR SPECIFIC DESIGN
Dual Media Filter
Fig. 8: Dual Media Filter

 

Anup Kumar Dey

I am a Mechanical Engineer turned into a Piping Engineer. Currently, I work in a reputed MNC as a Senior Piping Stress Engineer. I am very much passionate about blogging and always tried to do unique things. This website is my first venture into the world of blogging with the aim of connecting with other piping engineers around the world.

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