5 replies to this topic

[jatinder]

Dear All,

 

I have asked question before regarding electrostatic coalescer but I would like to make the question more clear for better understanding.

 

My question is:

 

I have crude oil stream entering 3 phase separator. The water coming in is indeed 5%. In outlet stream, I want to have my water content below 0.5%. 

 

Therefore questions are:

 

- Is 3 phase separator is able to bring down the water content below 0.5%? (maybe using second stage separator as well)

 

- Or is it better to use Electrostatic coalescer below the 3 phase separator stage? If yes, can the coalescer perform the task successfully when the inlet water content is only 5% because I heard it should have a minimum water loading of 10%?
 

Thanks

 

Jatinder 

 

[ankur2061]

Jatinder,

 

Some information on electrostatic coalescers I am reproducing from oil & gas manual of a reputed company:

 

Electrostatic coalescers are pressure vessels fitted with electric grid internals and are suitable for deep dehydration of crude oil. The water content of the crude oil at outlet is typically in the range 0.1 to 0.5% vol. The units are relatively compact and therefore suitable for use offshore.

Bulk oil water separation occurs in the lower half of the vessel and relatively dry crude moves up into the grid section. Deep dehydration occurs in the grid section due to the enhanced coalescence conditions, which are created by the action of the electrostatic field. Conventional electrostatic coalescers feature an AC field whereas some of the later designs use an AC-DC field combination which have higher throughput capabilities. Both types can handle feed water cuts in excess of 20% vol.

A relatively recent development in electrostatic coalescer design is the bi-electric unit. In this case the feedstream is introduced directly into two grid sections, effectively doubling the throughput potential. In practice, however, throughputs 50% higher than the conventional design can be achieved. In this case the water cut of the feed stream should be limited to 10% vol. otherwise electrical short-circuiting will occur.

 

The same company's oil & gas manual also gives information about dehydration of crude oils using separators and has the following to say:

 

A separator designed for ‘dehydration service' will dehydrate crudes down to low water content levels, typically 1 to 3% vol. for a liquid-liquid separator and 1 to 5% vol. for a three-phase separator.

 

It is obvious that a 3-phase separator design to reduce the water content in the oil to below 0.5% would be very difficult.

 

Hope this helps.

 

Regards,

Ankur.

[jatinder]

Thanks a lot Ankur Sir for explanation!

So, according to your experience, what would you suggest to bring the water content below 0.5% in crude oil outlet stream?
I mean my inlet water content is 5% and it should be below 0.5% at outlet. As you suggest that a 3 phase separator can not perform this task so, what would you suggest to bring the water content below 0.5%?? If you are in this situation, How would you deal with this problem as a process engineer?

Thank you so much in advance for your reply Sir !

[Ibnu84]

Hi Ankur,

 

as per your experience, have you ever seen any Electrostatic Colaescer combined with a degasser in one vessel?

 

my client has given me a FEED process flow diagram with a simple processing as follow:

 

Crude Oil from Production Separator (50 psig) is directed to an Electrostatic Coalescer operates at 15 psig. The gas then flashed out from the Electrostatic Coalescer. Oil is sent to pipeline, and water to a closed drain system. The BSW required is 0.5 %.

 

 

Due to my knowledge limitation, my understanding is, Electrostatic Colaescer shall be gas free to avoid gas contact with electrical charges. So, my opinion, an atmospheric separator (degasser) is required before entering the Electrostatic Colaescer.

 

 

Appreciate your feedback..

 

 

Thanks

Ibnu84

[Argoen]

Hi Jatinder,

 

As mentionned by Ankur, the 3-phase separator will not allow you to bring you BSW down to 0,5% vol.

 

You need to install an electrostatic coalescer downstream the 3-phase or somewhere in your separation train in order to reach this BSW.

 

Most of the time, the design of such equipment is performed by an EC package supplier taking into account the performence to be done as well as the crude oil characteristic at the inlet.

[Neelakantan]

a simple gravity separation CAN and will separate oil and water normally to 1% bs&w; but the issue is how much time it takes for the separation? depending on the oil API, viscosity and emulsification quality one must design/decide the need of the separation time. however. since oil can change over time of operation (change of production zone, wells in operation, life of the field etc) the water content and the oil quality would change and hence oil industry has decided, based on experience, certain norms.

 

oil-water separation is essentially breaking the emulsion and moving the bulk of water to the heavier phase. it can be achieved thermally alone if the emulsion is easily breakable; at times additional demulsification methods (using demulsifier chemicals,., electric field etc ) are used.

 

typically a 3-phase gravity (thermal) separator moves the feed to 4-12% of bs&W; typically a second stage may get 3~6%; after that we use what is called electrostatic dehydration (where the oil-water emulsion is forced through an electric field)

 

[ Aside:. I have designed an oil train for a russian pipe line where the oil is stabilized and dehydrated to 1% without any extra effort. (the information about the oil-water emulsion came from russian design institute which conducted extensive tests.)

 

i have also been involved in an emulsion breaking train where oil is washed with fresh-water to achieve salt content of 10 ptb; ironically, there is something called emulsion inversion or reversal where excess water is added so that the trains separate oil from oil+water emulsion (bulk phase would be water!); i have been party to design of such a system where a huge amount of water is recycled to achieve the inversion,....]

 

coming to electrostatic dehydration, typically it can be a third stage where some degassing is also done using a piggy (double vessel) degasser.; it can also be a separate dehydrator; the dehydrator may also function as desalter if the salt content of the residual water is controlled; to achieve this de-salted water may be mixed with oil to reduce the salt content of the water and the mixed oil-water emulsion is dehydrated. There have been advances in this field and retrofitting of v-type electrostatic coalescing is carried out nowadays in the first and 2nd stage also;

 

regards

neelakantan