transportation of Waxy Crude Oils

8/19/2019 transportation of Waxy Crude Oils

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TRANSPORTATION OF WAXY CRUDE OILS INTRODUCTION

• Crude oil contains a mixture of light and heavy hydrocarbons. Thelighter components in the crude oil keep the heavier components insolution. This solubility depends very strongly on the temperature.

• At low temperatures separation of wax takes place, which is a problemin the transportation of the crude.

• Formation of wax crystals leads to following problems:1. Leads to higher viscosity with increased energy consumption for

pumping and a decreased capacity.

2. The wax crystals tend to deposit on the pipe walls duringtransportation. This leads to increased pipeline roughness, reduced

effective diameter, more frequent pigging requirement andpotential blockage.

3. In extreme cases, wax crystals may also cause oil to gel and lead toproblems of restarting the pipeline.

4. Safety hazard due to deposits interfering with the operation ofvalves and instruments.

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1/18/2016Transporation of waxy crude oil by Dr. V. K. Sangal 2

Typical problems caused by wax deposition include:

• Reduction or plugging of pipe work, blocking flow

• Increased pumping pressure

• Reduced operating efficiency and process upsets with

interruptions to production or shut downs

• Costly and technically challenging removal, especially indeepwater pipelines

•

Safety hazard due to deposits interfering with theoperation of valves and instruments

• Disposal problems associated with accumulated wax


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Paraffin wax


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Microcrystalline , Microcrystalline, and Crystal Deposit Network of Wax

The wax present in petroleum crudes primarilyconsists of paraffin hydrocarbons (C18 - C36)known as paraffin wax and naphthenichydrocarbons (C30 - C60). Hydrocarboncomponents of wax can exist in various statesof matter (gas, liquid or solid) depending ontheir temperature and pressure. When the waxfreezes it forms crystals. the crystals formed of

paraffin wax are known as macro crystalline wax. Those formed from naphthenes are knownas microcrystalline wax

http://www.phy.bris.ac.uk/research/polymers/paraffin_pic.htmlhttp://www.phy.bris.ac.uk/research/polymers/paraffin_pic.htmlhttp://www.phy.bris.ac.uk/research/polymers/paraffin_pic.htmlhttp://www.phy.bris.ac.uk/research/polymers/paraffin_pic.html


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Advantages of predicting wax deposits

The knowledge of the magnitude of wax deposition can leadto reduction of insulation requirements

Process heat loads can be reduced by increasing efficiency of

heat transfer

Problems related to start up and shutdown can be solved cost

effectively

The size of export pumps and flow lines can be reduced

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Pipeline Transportation:

For the transportation of large quantities ofcrude oils, pipelines are the most economicmeans. Under safety aspects, transportation by

pipeline guarantees the best protection for the

environment. Further, there is no handling ofother traffic and no disturbance by noise or air

pollution. A continuous supply to the refineries is

normally assured and this is not endangered by

weather conditions such as fog, icy roads, or

traffic conditions along inland waterways, such

as high or low water level, ice, etc.



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Shutdown of a pipeline is the most undesirable situation. Itmay occur due to the following reasons :

1. There is an inadequate stock of oil in the terminal. Thiscould be caused by distribution in tanker schedules, forinstance, due to weather conditions.

2. There are no delivery requirements by the refineries fed by

pipeline. This could be caused by disruption in operations of oneor more of the processing units of the refineries, forinstance, due to equipment breakdown.

A pressure test for leakage control of the pipeline has to beperformed.

Repair, maintenance work on the pipeline system including pumpsis required.

The pipeline might be shutdown automatically by exceeding theoperational safety limits.



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Rheological Parameters

• Shear Stress: Force per unit area to initiate flow

• Shear rate: Velocity gradient dU/dr• Viscosity: Ratio of shear stress to shear rate

• Yield Stress: The ability of fluid to restart its flow aftershutdown

As at low temperature wax formation in crude oil takesplace, it behaves as non-Newtonian fluid. Hence,knowledge of the rheological factors is important.



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Classification of Fluids



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PUMPABILITY CHARACTERISTICS OF WAXY CRUDE OILS

• Guidelines for design of pipeline :

1. Operating safety

2. Operating economy

• Factors affecting the pumpability of crude oil :

1. Temperature

2. Flow at restart

3. Effective Pipeline Viscosity



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METHODS FOR PIPELINE TRANSPORTATION OF WAXY CRUDE OILS

1. Select pumps to allow a parallel/series arrangement, which could transport at slowerrates and higher pressures when required. The piping could be manifold so that parallelarrangement would be accommodated by repositioning of valves to handle higher flow

rates.2. Use of separate low flow, high head pumps for restarting.

3. Side traps at frequent intervals to allow short sections to be started separately.

4. Reverse pumping to create back and forth pumping sequence which prohibits static cooldown.

5. Use of pour point depressants/flow improvers.6. Adding hydrocarbon diluents such as a less waxy crude or light distillate.

7. Injection of water to form a layer between pipe wall and crude.

8. Mixing water with crude to form an emulsion.

9. Displacement with water or light hydrocarbon liquid in case of shutdown of pipeline.

10. Separation at higher than normal pressure to allow as much gas and light hydrocarbonsas possible to remain in the crude.

11. Conditioning the crude before pipelining to change the wax crystal structure and reduce pour point and viscosity.

12. Further sub-division of pipeline into smaller segment or reducing batch length of waxycrude to increase maximum shear stress available.

13. Combination of the above methods. 1/18/2016 16Transporation of waxy crude oil by Dr. V. K. Sangal


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Use of Pour Point Depressants/Flow Improvers:hold the greatest promise of achieving the desired overall objectives of operational

safety and operating economy

Reduce the pour point, viscosity and yield stress under dynamicconditions; and

Restart the pumping after a shutdown with the available shear stress.

Chemically pour point depressants/flow improvers are ashless polymericadditives which when added into the crude oil at 200-600 ppm levelreduce the pour point and viscosity of the crude oil

Mechanism of Flow Improvement

Point of Additive Injection:

The additives should be injected into the crude above or around its cloud

point. The additive pour point could be depressed considerably bycutting (diluting) the basic component with kerosene, or aromaticsolvents.

No heating of the oil is required.

Subsequent external application of heat can be avoided or minimized.

The maximum benefit can be derived in the system downstream.1/18/2016 17Transporation of waxy crude oil by Dr. V. K. Sangal


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Pour Point Reduction by Additives:

1. Pour point measurements on crude oils have

been used to detect low temperature

handling problems

2. Pour point can at best be used for

preliminary screening of various additives

(a) alkyl acrylate polymers and copolymers,

(b) olefin alkyl maleate copolymers,

(c) vinyl ester polymers and copolymers, and(d) alkylated polystyrene



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Effect of Flow Improvers on Yield Stress and Viscosity

Doping level Yield stress @ 16° C Viscosity @ 16°C

ppm dynes/cm2 cP

Nil 330.0 45.0

250 62.5 45.0

300 62.5 42.4

400 45.8 41.0


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Dose of Pour point

Depressant in

Bombay High Crude

Yield stress at 16 oC

dynes/cm2

Viscosity at 16°C at flow

development, cP

Nil Test was abandoned as oil could not be transferred to

model pipeline due to congealing at 18°C.

400 21.2 435 at 45 dynes/cm2

750 20.2 589 at 45 dynes/cm2

1000 27.5 966 at 45 dynes/cm2


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Incorporation of Low Pour Point Crudes in Waxy

Crudes

Crude blendYield value at 16°C,

dynes/cm2

100% Bombay High 330.0

100% Bombay High with 250 ppm of pour point

depressant62.5

90% Bombay High + 10% Basrah with 250 ppm pour

point depressant 5.0


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Doping temperature is quite important as far as yield stresses are

concerned

Crude blend Yield stress at 16°C, dynes/cm2

50oC Doping 30°C Doping

90% Bombay High + 10% Basrah with 250 ppm

pour point depressant 5.0 25.0






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Crude Oil Conditioning

crude oil conditioning is a unique process is developed by Oil

India Limited.

The crude oil is first heated (100°C) to melt and dissolve thewax in it. Thereafter, on dynamic cooling (65°C) andworking through the pump the crude oil is subjected to

static cooling at a predetermined rate. The result is theconditioned crude oil which has much improved physicalproperties than the virgin crude oil.

The conditioned crude oil remains fluid at much lower

temperature (18-20o

C) and possesses satisfactory physicalproperties so far as transportation of crude oil through thepipeline to refineries during the winter months isconcerned. This has been employed for transporting waxycrude oils from Assam (India).



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Thanks

forlistening


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1/18/2016Transporation of waxy crude oil by Dr V K Sangal 25

ASSIGNMENT

1. Wax formation and its control during the transportation of crude oil (200 words.Max)

2. Methods for pipeline Transportation of crude oils.

3. Short Notes: (150 words. Max)

•Pour point depressants/Flow improvers

•Crude oil conditioning

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