## Fluids

#### setting up model for solid deposition in oil pipelines

• RSharma
Subscriber

Dear Ansys Members,

I am a beginner of Fluent and have a very basic query regarding solid deposition in crude oil pipewalls. I am finding difficulty to setup an appropriate model for the following problem statement which is described in the file in the attachment below.

Kindly suggest which model combination would be suitable to model such a problem.

I am finding problem to implement the diffusion of wax molecules due to thermal gradient, may be due to convection process dominating the heat transfer process. Kindly provide some insight into how to model primarily molecular diffusion process in the flowing wax-oil mixture, which is very important to solve the above problem.

Every response would be highly appreciated.

Thanking You

Rohit Sharma

• Rob
Ansys Employee

Are you concerned about wax formation on the walls, or the build up of material as wax forms in the oil and drops out?

• RSharma
Subscriber

Yes "rwoolhou" the problem is surely concerned about wax formation on the walls, or the build up of material as wax forms in the oil and drops out? Do we have any previous discussion about this or any other lead about this type of problem.

I would like to mention the content of the attachment here as follows:

Dear Ansys Members,

I am a beginner of Fluent and have a very basic query regarding solid deposition in crude oil pipewalls. I am finding difficulty to setup an appropriate model for the following problem statement which is described in detail below.

Geometry

A 4 m length copper tube having diameter of 4 mm is used to transport a wax-oil mixture. The geometry is simply a straight pipe section (where one end could be velocity inlet and other end be pressure outlet or outflow). I am trying to replicate a closed flow loop setup (where there is circulation of fluid to and from between hot and cold water tanks) with this type of simplified geometry. Pipeline wall is exposed to ambient temperatures and the hot fluid is flowed in the pipe.

Problem Statement

To model wax (solid) deposition in a pipeline using a wax-oil mixture (representing a crude oil)

Theory

• Wax molecules are dissolved in wax-oil mixture upto certain temperature called as cloud point , after which if oil temperature drops, wax crystals starts to precipitate and the flow of oil completely freezes at pour point temperature. In our case, pour point- 4 degree Celsius and cloud point -13.9 degree Celsius.

• So, at temperatures between cloud and pour point , a wax-oil gel type would form (intermediate product) , and after temperature drops below 4 deg Celsius we would observe complete solid deposition at the pipewalls.

• Due to temperature difference of bulk mixture and ambient temperature, there generates a concentration gradient and where wax molecules transport from bulk to pipewalls owing to molecular diffusion phenomena.

• Wax solubility is dependent on temperature.

Operating conditions

1.  Laminar Flow (Reynolds No: 500 - 2100)

2.  Flow Rate  1, 2.5 and 4 Gallons per minute

3.  Oil temperature 22 degree Celsius

4.  Ambient Temperature 8.3 degree Celsius

Assumptions

1.         Molecular diffusion is the primary reason for wax deposition on pipewalls from the bulk fluid

2.     Incompressible fluid

3.    Gravity effect is not to be considered for wax deposition

4.    Shear force does not affect the wax deposition.

Quantitites to measure:

Simulate the wax/solid deposition with time and pipe length and measure the wax/solid deposition height and wax/solid content deposited on the pipelines from the bulk wax-oil mixture , with time.

Possible Models in Fluent  : (Could you suggest which models to start with so that I could achieve the wax deposition phenomena) - especially the models should incorporate the molecular diffusion phenomena

1.     Single phase, Viscous, Solidification, Energy (I have tried this model, but found little success)

2.      Single phase, species transport model, solidification, Energy model

3.      Multiphase (possibly where phase 1- wax oil mixture and phase 2- wax), Viscous , Energy model

4.    Is DPM model be applicable for this problem.

Kindly suggest which model combination would be suitable to model such a problem.

I am finding problem to implement the diffusion of wax molecules due to thermal gradient, may be due to convection process dominating the heat transfer process. Kindly provide some insight into how to model primarily molecular diffusion process in the flowing wax-oil mixture, which is very important to solve the above problem.

Every response would be highly appreciated.

Thanking You

Rohit Sharma

• RSharma
Subscriber

Dear rwoolhou,

Yes, the problem is concerned about both of the things you mentioned in the comment

1. wax formation of the walls.

2. As well as material buildup ( with decrease in temperature it precipitates out of oil , and re-dissolves as temperature increases)

•         Is there any method to incorporate the data of temperature solubility of wax in material properties.

•          Additionally, what type of flow models in Fluent could simulate this type of wax deposition in pipelines

•        How to simulate molecular diffusion as the wax deposition phenomena in flow of wax-oil mixture.

Thanks

Rohit Sharma

Assumptions

•       Molecular diffusion is the primary reason for wax deposition on pipewalls from the bulk fluid

•      Incompressible fluid

•    Gravity effect is not to be considered for wax deposition

•    Shear force does not affect the wax deposition.

Quantitites to measure:

Simulate the wax/solid deposition with time and pipe length and measure the wax/solid deposition height and wax/solid content deposited on the pipelines from the bulk wax-oil mixture , with time.