Determination of the suitability of a crude oil for use in crude oil
washing
This section contains guidance in determining the suitability of a crude oil
for use in crude oil washing. In lieu of a list of crude oils unsuitable for
carriage and crude oil washing to be inserted in the Crude Oil Washing and
Equipment Manual the following standard text maybe used:
- For a Tanker not fitted with heating coils.
This tanker is not fitted with heating coils in the cargo
tanks and should not carry cargoes which will require to be heated either to
obtain pumpability criteria or to avoid excessive sludging of both the
designated clean ballast tanks and the vessel's tanks to be washed for
sludge control.
Notes and
Definitions:
- Pumpability Criterion is determined by the oil's
Kinematic Viscosity at the observed temperature of the cargo prior to
its discharge. In order to attain optimum efficiency for discharge this
viscosity should not exceed 250 centistokes and never in excess of 600
centistokes.
- Excessive sludging criterion is determined primarily by the crude
oil's temperature throughout its transportation and storage. If the
cargo's temperature is likely to drop below the crude oil cargo's cloud
point temperature then sludging of cargo tanks is to be expected. The
cloud point temperature is the temperature at which the crude oil's wax
and associated oil phase separates from the bulk liquid phase of the
cargo.
- For a Tanker fitted with heating coils in Slop Tank(s)
This tanker is fitted with heating coils only in her Slop
Tank(s) and should not carry cargoes which will require to be heated for
pumpability.
Notes:
If sludge deposition is
suspected or determined in the main cargo tanks then an alternative crude
oil washing programme should be utilised. Recommendations regarding this
programme and procedure are to be found in Section 11 of this manual.
- For a Tanker fitted with heating coils
This tanker is fitted with heating coils in all the cargo
tanks and, subject to the limitations of the cargo heating system, can carry
cargoes which require heating for either pumpability or sludge deposition/
control.
General Guidance
Attention is drawn to the
difficulties which may be encountered with certain crude oils. During the
discharge of crude oils that exhibit the necessary criteria that would create
either pumpability problems or sludge deposition, crude oil washing of each
tank scheduled for such an operation should be carried out concurrently with
the discharging of the particular tank in order to minimise the affect on the
crude oil residues for cooling. Cooling will increase both the Kinematic and
Dynamic Viscosities1 of the tank residues and
therefore affect the efficiency of the Crude Oil Washing programme.
As a general guidance to the suitability of an oil for crude oil washing on
board this tanker, the following criteria should be used:
For Aromatic crude oils whose Kinematic viscosity is the temperature
controlling characteristic, the Kinematic viscosity of the oil used for crude
oil washing should not exceed 60 centistokes at the oil wash medium
temperature;
OR
For Paraffinic crude oils whose pour point temperature is the controlling
characteristic, the temperature of the cargo to be used for crude oil washing
should exceed its Cloud Point temperature by at least 10°C if excessive sludging is present and should only be used once in
a "closed cycle" washing programme.
The approximate Cloud Point temperature of an oil may be calculated by use of
the following formula where the Pour Point temperature(x) of the crude oil is
known
The Cloud Point Temperature °C = 20.2 (100.00708 x -
0.1157714) + 8
Worked example:
Assume the Pour Point
Temperature is +1°C.
Then:
OR
Consult the accompanying graph, (Fig 1) where if the Pour Point is known then
the Cloud Point can be extracted.
OR
If the Pour Point exceeds Approximately 25°C then the Bondi test procedure
could be used to determine this temperature.
The Bondi Test Procedure (See Fig 2)
This
method will obtain the cloud point temperature of high pour point type crude
oils.
Equipment and Procedure
- Obtain a representative sample of the cargo. Do not allow the Cargo sample
to cool before commencing the test procedure.
- Obtain a glass container (preferably nearly spherical) that is made of
heat/ temperature resistant glass and fill the container with the sample.
- Insert a thermometer into the centre of the sample volume and heat the
sample to a temperature of at least 30°C above the pour point temperature of
the crude oil in a hot water bath. Do not tightly stopper the
container during this process.
- Prepare an alternative bath with water whose temperature is at least 15°C
below the sample's pour point temperature. Have a clock or watch available
with a second hand.
- Immerse the heated sample contained into the cold bath and record at very
regular intervals (at least every 30 seconds) the temperature of the
sample.
- Graphically plot the sample temperature against time elapse from the start
of the test. Determine the temperatures at which there was no loss of
temperature with time elapse. The first such instance as detected will be
the approximate Cloud Point temperature and the second instance, if testing
time is extended to the required period, will be the approximate pour point
temperature of the crude oil.
1Definitions: "Kinematic viscosity" is the measure of resistance to flow
with gravitation under its own mass force. This is normally reported in the
units of centistokes (cst) or mm2/sec. "Dynamic viscosity" is the measure of resistance to flow with an induced
shear stress or at a known rate of shear. This is determined from the
equation of Shear Stress (Pascals - Pa) divided by Rate of Shear (inverse
seconds - s-1). The unit of Dynamic viscosity is either Centipose (cps) or
mPa s.