9 replies to this topic

[cea]

Dear All-

I am in process of designing three phase separator for one of offshore platform project. The design calculation is with me. My query is to know how much maximum efficiency of three phase separator one can expect with respect to oil content in water. The desire value of oil content is 10 ppm in water.

Further, we have envisaged produced water system with hydrocyclon technology, to remove oil from water before it is being disposed off. Since, there is concern for plot area availability, I am looking for some innovative technology, there by reducing number of equipments & its sizes. The request is to intimate availability of any such technology.

Thanks & Regards,

[shan]

The term “maximum efficiency of three phase separator” is kind new to me. What is it the definition? Anyway, your desired value 10 ppm oil in water from a separator is really ambitious. The standard for oil contents in the discharged water in the Gulf of Mexico is 20 ppm monthly average and 42 ppm daily maximum, as what I remember. I don’t think any separator vendor will guarantee you 10 ppm oil in water. Otherwise, those hydrocyclone guys and flotation cell guys would be out of business now.

[Technocrat]

Dear cea,

Why don't you check with a coalescer vendor. Coalescer can drastically reduce size of the separator vessel. He can check the oil concentartion in the outlet water.

Regards.

[cea]

Dear Technocrat-

Please note that is 3 phase separator, in which one of the phase is gas. As I undesrtand, coalescer handles only liquids. Please confirm if coalescer, as suggested by you, can handle gas phase as well.

Regards

[Technocrat]

Dear cea,

Coalescer can separate only liquid-liquid phases. In a three phase separator there is a gas space above liquid level. The coalescer is installed only in the liquid phase.

Regards.

[daryon]

Hi,

I've been involed in the design of a number of floating production facilities. For a standard gravity separator with no fancy internals except a inlet cyclone/vane pack and calming baffles the best you usally design for is 1000 ppm(vol) OIW concentration at the produced water outlet. A more conservative value is 2000 ppm.

If, like shan says, you install coalescing internals (like a plate pack which essentially give a surface for rising oil droplets to coalesce on) then outlet OIW conentrations can be improved, maybe to 500 ppm, possibly lower. You really need to consult a specialist internals supplier like Zeta PDM or CDS.

Hydrocyclones can handle 2000 ppm OIW at the inlet and typically achieve 40 ppm at the outlet. Hydrocyclones perform better when the differential density between the oil in water is higher so increased temperature usally helps performance. The performance is also highly dependant on the inlet oil droplet size, so avoid control valves and restrictions in the inlet piping that cause turbulence and shear the oil droplets resulting in small droplet diameters. Specify full bore valves and place control valves downstream of the hydrocyclones. If you are pumping the produced water to the hydrocyclone specify a low shear pump, if a centrifugal pumps is used limit the speed to less than 1750 rpm (lower if possible) and use a closed impeller.

You may be able to achieve down to 30 ppm out of the hydrocylcone but the i doubt any supplier will give a process guarante for this low OIW concentraiton. So to achieve 10 ppm you will need water polishing, in the past i'v seen walnut filters on low flowrate applications but more typically an IGF unit (induced gas floatation) is used to achieve very low OIW content. IGF units can be vertical to minimise footprint.

New unproven Technolgy is risky business, i'd advise you stick with what has been proven offshore.

Hope this helps

[cea]

Thanks daryon for your valuable sharing.

Above info has given me crystal clear picture for water treatment facilities that are required on offshore platform. Concluding this discussion, I understand that by no way, one can achieve produced water with oil ingress to the extent of 10 ppm (25 ppm max), without having produced water treatment (& polishing system, case to case basis), so as to discharge produced water directly to sea. However, if someone would like to share more about his / her experience with any morden technology, he / she is most welcome.

Thanks & Regards,

[daryon]

Hi Cea,

You're welcome. Your conclusion is right there is no way you will get 10 ppm out of reasonably sized gravity separator, you need produced water treatment.

I was on a FSO (floating stoarge & offloading vessel) recently and they had converted one of the cargo tanks to a produced water holding tank, the tank volume was 20,000 m³ and the water residence time was days. When we sampled the lower layer of water the OIW content was 3 to 5 ppm. To achieve this water cleanliness they need a couple of days residence time, you just can't achieve this in a separator.

All the best
Daryon

[toine123]

Hi,

Daryon is right, 10 ppm out of a gravity separator is normally not possible in a reasonable size and produced water treatment is necessary. With properly designed gravity separator internals including coalescing plate packs you can achieve quite low oil in water values (exact OiW level dependent on oil and water properties, residence time etc). The internals selection for the separator is crucial to enable maximum performance, down to 50-100 ppm OiW is often possible with advanced separation internals and in some cases even lower than that.

The combination of a gravity separator followed by downstream hydrocyclones and a degassing vessel allows in most cases to reach the required OiW discharge quality. The max. OiW content is dependent on environmental regulations which may differ from location to location but most of them are in a 20-30 ppm range.

I am working with a separation company specialising in high end oil/gas separation equipment called Ascom Separation, for more info see Ascom Separation. If you would like to receive a more specific advise for your case we would be happy to advise based on your process system. If of interest please submit the process data you have and we'll review it without any cost or obligation from your side.

Best regards,

Toine

[ArthurC]

I was doing produced water treatment with coalescer for 3 years. I have a pending patent(coalescer) for oil/water separation. For any water, you have to analyze the properties of water first, for example how is your emulsion stability, then you can try to choose the right technology. A lot of chemicals are being used to enhance oil recovery in offshore platform but it enhances the difficulty of produced water treatment as well. That is two sides of chemicals. If you are not using a lot of surfactants in the process and the emulsion stability is not very high, coalescer can easily reach your target 10 ppm. we need to analyze your water to know if coalescer can work or not.
I myself like this topic very much. Last week I opened a topic for produced water treatment in http://futurewater100.blogspot.com/.

http://futurewater100.blogspot.com/