Objective
A critical examination of the processes on which the treatment equipment are based, shows that considerable savings could be made regarding weight, bulk and performances on offshore platforms, by using a new technique for treating fine oil/water/gas emulsions.
The results obtained within the scope of the project nr. TH/03165/84 make the application of a new settling process by cyclone flow a serious possibility for treatment units such as crude dehydration, and produced water deoiling.
The project consists in :
- developing a fast-flow electrostatic coalescer, capable of handling high water contents, coupled with a cyclone dehydrator.
- optimizing the deoiling cyclone design, in particular regarding oil recovery and energy consumption aspects.
a) Electrostatic dehydrator
The feasibility tests, run on the electrostatic coalescing mock-up show that careful attention has to be given in the choice of the insulating sheath of the central electrode in order to avoid short circuits. It is capable of converting at appropriate voltage and frequency, an emulsion with a given size distribution into a much coarser emulsion. The suitable residence time is very low : 1 to 10 seconds. At constant residence time, a flowrate increase could have a favourable effect on water droplet coalescence.
Further work has been done to find a modelisation of the coalescence phenomena but the model cannot explain the saturation effect which happens on the mock-up at high voltage. More fundamental research studies are needed to better understand and modelize the drop-drop coalescence phenomena.
A 2 to 5 m3/h dehydrator cyclone has been constructed and tested in serie with the coalescer. Good results have been obtained at 1 m3/h with the association coalescer-cyclone dehydrator, ie 99% separation of incoming water droplets. However the hydraulic capacity of the cyclone dehydrator is practically half that of the de-oiler at the same efficiency which gives a maximum capacity of 5000 BOPD per cyclone dehydrator.
A technical study has been done for the selection and realization of an industrial prototype of 5000 BOPD. A market study has been launched in order to evaluate the potential of the concept coalescer - dehydrator. The results of this study have shown that only a small specialized market presently exists for this type of equipment (re-vamping of aged or undersized dehydrators, submersible platforms) and that competition with existing improved equipment and technology trend towards multiphase pumping make the commercial viability of this equipment questionable.
Also it has been decided to stop this project phase and to continue the development of the DYNACLEAN Hydrocyclone with the construction and testing on an offshore platform of the biggest size hydrocyclone re size T4.
b) Cyclone de-oiler
Several solutions for reducing pressure losses were found. For each industrial application, it would now be possible to select the best efficiency-pressure losses arrangement.
The industrial cyclones will be sealed with mechanical packing seals. Previously, we wish to check them on the basis of a specification allowing for the special and severe offshore operating conditions.
The mechanical endurance tests were run with success on the 50 m3/h prototype installed for this purpose at an oilfield in the Paris region.
A paper presentation at the 7th GPA European Chapter in Biarritz on the 17t-18th May 1990 has been done. This paper presented the last comparative tests results of the DYNACLEAN with at the OWTC and also the performance of 2 field tests on gas producing platforms.
Another T2 (2500 BWPD) has been constructed and installed in September 1990 on a Total field near Paris for mechanical reliability tests.
a) ELECTROSTATIC DEHYDRATOR
The work carried out within the scope of the project nr TH 03.165/84 revealed that it was possible to reduce weights by a factor of five, by developing a "CONTACTOR"-type technique, based on electric field liquid-liquid extraction. The "contactor" is vertical, meaning a considerable reduction of the surface area required. The particular design of the electrodes makes it possible to treat water-in-oil emulsions, having water contents of as much as 50% (5 to 10% is the maximum for traditional electrostatic dehydrators).
Preliminary work showed that it should be possible to reduce weight and bulk yet further, by separating the "coalescence" and "setting" functions. The setting function being operated in a specialized unit (cyclone dehydrator type) placed downstream the contractor, the water droplets coalesced in the contractor can be driven with the oil; it is possible to circulate faster in the apparatus.
b) CYCLONE DEOILER
The results obtained within thescope of the EEC contract TH 03.165/84 led to the design of a new type of cyclone known as a "rotary" cyclone. Improvements both regarding separation efficiency and flow rate flexibility was proved in the field on a 50 m3/hour prototype.
The rotation of the walls of the unit eliminates friction between the fluid and the wall, and means that the cylone, has hydraulic properties very similar to those of a theoretical cyclone, which explains its level of performance regarding deoiling.
Further work will consist in optimizing the many geometrical characteristics of fluid inlets and outlets, which govern the hydraulic behaviour of the deoiling cyclone. In order to be able to operate under nearly any field conditions, the characteristics of the cyclone should be such, that it would not be necessary to install a pumping unit upstream the water treatment system.
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