Dehydration

1. Gas separation &
Dehydration

2. Desalting also takes place during
Dehydration
• First step in the produced gas separation
process is to separate crude and gas
• If the salt content of the crude is greater than
10 lb/1000 bbl (expressed as NaCl), the crude
requires desalting to minimize fouling and
corrosion caused by salt deposition on heat
transfer surfaces and acids formed by
decomposition of chloride salts.

3. Purpose of Dehydration
REMOVES
• Salt
• Solid metals
• Water
• Amines
• Suspended
Particles
NEGATIVE DOWNSTREAM EFFECT
• Fouling
• Scaling
• Catalyst Poisoning
• Unit Upset

4. Methods
• Gravity or mechanical Method
• Electrochemical Method
• Ultra-sonic Method
• Chemical Method
• Filtering hot crude using diatomaceous earth

5. Types
• Chemical and Electrostatic separation:
 Washing crude oil with water
 Water phases are separated
 Adding chemicals to assist in breaking up emulsion
 Application of electrostatic field to collapse the
droplets
• Chemical separator:
 Water and chemical surfactant (demulsifiers)
 Heated
• Electric separator:
 Under charge condition polar molecules get oriented
and get separated.

6. Process
• The crude oil feedstock is heated to 65-180°C
to reduce viscosity and surface tension for
easier mixing and separation of the water.
• In all methods chemicals may be added.
• Ammonia is often used to reduce corrosion.
• Caustic or acid may be added to adjust the pH
of the water wash

7. Design Calculation
• Maximum quantity of water requires for desalting
(Salt Specification)
• Cso = salt content of the oil, lbm/1,000 bbl
• Csw = concentration of salt in produced water, ppm
• γw = specific gravity of produced water
• fw = volume fraction of water in crude oil

8. Mixing efficiency
• The fraction of wash water that actually mixes
with the produced water
• Bypasses the desalting stag
• Depending Parameters
Mixing intensity and duration
Diffusion transport
Inter-drop-collision frequency

9. Water recycle
• Collision frequency, between drops of
dispersed brine and dilution water is
proportional to the drop populations
• Factors affecting are
Recycle ratio
Temperature
pH

10. Dilution water
• To satisfy mass-balance requirements for diluting
the dispersed brine enough that the salt
specification can be met
• It must have low enough salt content to achieve
the required equilibrium
• Water must not contain
Coke particles
Suspended solids
Iron sulfide
Emulsified oil

11. Analytical Methods
• Extraction
• Aqueous layer (Jar Test)
• Spectro-photometric

12. Single stage

13. Single stage Gravity Settler

14. Cross Section

15. Mechanism

16. Two-Stage

17. Hybrid

18. Working of Electrostatic Coalescer
• Distributor injects crude just below multiple
layers of electrostatic grids.
• Water droplets present in the crude are
exposed to the electrostatic field that
rearranges the (salt) ions within the droplets.
• Droplets will then attract each other and as a
result, they coalesce, grow in size, and then
fall out of the upward flowing crude.

20. AC Current
• Coalescers are designed using alternating
current (AC), which is a proven and reliable
technology.
• Other types of power supply are DC, AC+DC
• Significant drawbacks are tendency for arcing
(short-circuiting) and electrical complexity.

22. References
• Warren, K.W. 1993. Reduction of Corrosion through Improvements
in Desalting. Paper presented at the Benelux Refinery Symposium,
Lanaken, Belgium, 2–3 December.
• ASTM D-96-88, Standard Test Methods for Water and Sediment in
Crude Oil by Centrifuge Method (Field Procedure). 1988. West
Conshohocken, Pennsylvania: ASTM.
• ASTM D-4007-02, Standard Test Methods for Water and Sediment
in Crude Oil by Centrifuge Method (Laboratory Procedure). 2002.
West Conshohocken, Pennsylvania: ASTM.
• ASTM D-3230-05, Test Method for Salts in Crude Oil (Electrometric
Method). 2005. West Conshohocken, Pennsylvania: ASTM.
• API RP-45, Recommended Practice for Analysis of Oilfield Waters,
third edition. 1998. Washington, DC: API.