Produced water treatment

Introduction

 

Many industrial sites use water for different purposes. In some cases, water is the product of industrial activity. For example, in oil production fields, oil is often produced together with reservoir water. In mining industry, oily water often takes place as wastewater in workshops, refueling, lube bays and so on. In most cases, water has no value to the operator, and is therefore discharged. Produced water and wastewater usually contain variable amounts of dispersed and dissolved oil, chemicals, residuals and solids. Many countries have introduced environmental regulations for discharge limits of produced water/wastewater disposal. Therefore, before contaminated water can be discharged, it has to be routed to water treatment facilities in order to achieve the required level of purity.

 

 

Deoiling hydrocyclones

 

Deoiling hydrocyclones, or “deoilers,” provide the highest throughput-to-size ratio of any water-treating technology and are insensitive to motion or orientation. For a given water treatment capacity, deoilers have the smallest footprint and size compared to the alternative technologies. Deoilers use fluid energy to create rotational fluid motion. This rotational motion causes relative movement of substances with different densities, thus permitting separation of these substances one from another. In the case of produced water, deoilers can remove even very small oil droplets from the water stream.

 

Because hydrocyclones are pressure driven systems, they ideally should be located as close as possible to the water outlet of the three-phase separator. This placement results in the simplest and most cost-effective operation. If this arrangement is not feasible or insufficient feed pressure is available, a pump can be installed to increase the system pressure in the produced water stream. Pumps are generally viewed as damaging to the oil droplets dispersed in the produced water that requires the separation. Pumps introduce shear forces to the flow, which may break oil droplets. Suppliers classify pumps as low or high shear pumps regarding the ability to avoid the droplet break-up.

 

The separation efficiency of the hydrocyclones is strongly dependent on the properties of the produced water. Unfavorable composition of the feed stream may lead to sub-optimal performance.

 

 

Figure 1 - Hydrocyclone performance efficiency curve.

Figure 1. - Hydrocyclone performance efficiency curve.

 

Fig. 1 shows the hydrocyclone efficiency as a function of inlet oil droplet size. Pump selection and operation are crucial for the overall performance efficiency of the water treatment facility. The wrong pump type, or even the correct pump type operated incorrectly, can introduce considerable shear, and reduce the average size of the dispersed oil droplets.

 

 

Pump selection

 

There are many types of pumps, varying in principles of operation, design features etc. Depending on the specific industry standards, only certain types can be considered for these industries. Properties of contaminated water and process conditions have high importance for the choice of the correct pump. In various situations, certain types of pumps would have better performance than others. Following guidelines can be drawn for produced water treatment pumps:

 

  • Low-shear pumps provide the best performance in this application.
  • Progressive cavity pumps can be excellent low-shear pumps, but they may require more maintenance than what is acceptable.
  • Centrifugal pumps are not typically low-shear pumps, but some standard centrifugal pumps can be used very effectively for low-shear service. Because of their simplicity, reliability, and relatively low cost, centrifugal pumps should be considered.

 

Progressive cavity pump

 

The progressive cavity pump is a type of rotary positive displacement pump that has a single-threaded helically shaped rotor turning inside of a double-threaded helically shaped elastomer stator. This creates a set of cavities sealed tightly between the rotor and the stator. As the rotor rotates the liquid filled in cavities moves to the pump discharge.

 

An experimental study of shearing characteristics for this type of pump showed that with proper operation it causes minimal shear. These pumps can be viewed as low shear type. Table 1 summarizes the pros and cons of this pump type.

 

Table 1. – Advantages and disadvantages of progressive cavity pumps.

AdvantagesDisadvantages
- High hydraulic efficiency
- Can handle a wide variety of thin and thick liquids, corrosive liquids and liquids containing solids
- Relatively low power costs
- Low shearing of the fluids
- Relatively high weight and size
- Require pressure relief system
- Sensitivity to fluid environment; can lead to high maintenance load

 

 

Low shear coalescing pump

 

The low shear coalescing pump is a centrifugal pump with improved design suited for produced water treatment. The main feature of the pump is the use of several pumping stages in order to increase pressure, at the same time keeping the rotational speed at lower levels. This design allows controlling the level of the turbulence, thus ensuring a low shear operation. Tests show that in the typical produced water treatment system, this pump not only is capable to maintain droplet sizes but also may enlarge them, thus providing the coalescing effect.

 

In addition to providing low shear operation, it has the following economic benefits compared to progressive cavity pumps, summarized in Table 2.

 

Table 2. – Comparison of coalescing pump and progressive cavity pump in terms of CAPEX and OPEX.

Technical parameterCAPEX benefitOPEX benefit
Size & WeightLS lighter and smaller than PC for equal flow and pressure performance (except coalescing version which is similar in size and weight to PC)
Pressure relief systemsLS do not require pressure relief systems, no need for piping and valves to flare, no heat tracingNo periodic calibration of relief valves. No energy consumption for heat tracing
Noise requirementsLS do not require noise protection enclosure. Rated noise level 60-65dB (A).
MaintenanceLS is acc. to API 610, i.e. 3 years of uninterrupted service. PC require yearly service on gearboxes.
Erosion resistanceLS handle low to moderate particle content with minimal wear. PC have significant stator erosion when certain particles are present

(LS – Low shear coalescing pump; PC – progressive cavity pump)

 

 

Additional Reading

 

Walsh, J., 2016. The Effect of Shear on Produced Water Treatment. The Savvy Separator Series: Part 5. Oil and Gas Facilities.

 

Flanigan, D.A., Stolhand, J.E., Scribner, M.E. et. al. 1988. Droplet Size Analysis: A New Tool for Improving Oilfield Separations. Paper SPE 18204 presented at the SPE Annual Technical Conference and Exhibition, Houston, Texas, 2-5 October. http://dx.doi.org/10.2118/18204-MS

 

Walsh, J.M., Frankiewicz, T.C. 2010. Treating Produced Water on Deepwater Platforms: Developing Effective Practices Based Upon Lessons Learned. Paper SPE-134505-MS presented at the SPE Annual Technical Conference and Exhibition, Florence, Italy, 19-22 September. http://dx.doi.org/10.2118/134505-MS

 

Morales, R., Pereyra, E., Wang, S. et. al. 2013. Droplet Formation Through Centrifugal Pumps for Oil-in-Water Dispersions. SPE Journal 18 (01): 172-178. SPE-163055-PA. http://dx.doi.org/10.2118/163055-PA

 

http://petrowiki.org/Removing_hydrocarbons_from_water

Leave a Reply

Be the First to Comment!

Notify of
avatar
wpDiscuz