Interfacial tension between crude oil and paraffin wax deposit 1

[aali94]

Hi,

I am trying to model the sloughing of paraffin wax deposits from the walls of oil pipelines due to the flow of crude oil. One of the parameters I require is the interfacial tension between crude oil and the wax deposit. I have varied the interfacial tension in my CFD simulations from 10 to 1000 mN/m. The results show that low surface tension values result in a wavy surface, while high surface tension does not produce a wavy surface (see attachment). However, I have not been able to find any values in literature for the interfacial tension between oil and solid wax. I have found that the interfacial tension between oil and water can only be as high as 50 dyn/cm. Could you please let me know what values of surface tension I should be using.

Also, I am modelling the wax as a highly viscous fluid with a viscosity of 100,000 cP. I am not sure if this is the best way to model wax sloughing. I am modelling the crude oil as a Newtonian fluid above the Wax Appearance Temperature (WAT) with two viscosities: 4.3 cP and 29.5 cP. Below the WAT I am modelling the crude oil as a shear thinning fluid with a behaviour index of 0.6.

I would be grateful if you could provide any insight?

 

[BigInch]

Isn't 100KcP behaving more like solid. I think that cP would be more representative of wax at pour point, rather than waxes sloughing off at temperatures near WAT. The 4 cP seems pretty low for 25C. Not much total wax content?

 

[aali94]

I am modelling light crude oils. I found the 4 cP viscosity at a temperature of 100 deg F in a paper by Jessen and Howell (1958). I think they used WTI oil. Also Arabian Light has a viscosity of 9 cP at 20 deg C.

I am using 100 kcP because according to Burger et al. (1981), the wax deposit can be considered to be a "brushy network of solid wax crytals". Is it right to treat it as solid like at WAT? What range of viscosities should I be looking at?

Do you think an interfacial tension of 10 mN/m would be reasonable given that the wax and crude oil have a similar molecular structure. Or can interfacial tension be neglected?

 

[aali94]

Sour cream has a viscosity of around 100 kcP

 

[BigInch]

I have seen shear strengths of wax reported at 400-800 Pa, 0.05-0.1 psi. Dependent on cooling rate. Fast cooling of 10F/min gets 400 Pa. Slow cooling rate 0.1F/min reaches the higher strengths.

My feeling is that interfacial tension can be ignored. Sloughing is shear stress dominated. Passing oil flow shears off growing fingers and the softest inner layer.

 

[aali94]

Thanks for your reply.

It seems that my results show that the wax is driven by the difference in pressure between the wax free region and the wax deposit region, rather than shear stress. It seems that the shear rate at the oil-wax interface is zero. The wax is only displaced at the beginning of the deposit lengths, with no influence throughout the length of the deposit. At the end of the deposit length the wax layer is displaced backwards. Could this be due to the positive pressure gradient at the end of the deposit length. Could you please have a look at the attached document?

 

[BigInch]

I don't think wax is initially so uniformly deposited. With such a smooth deposit, there will be little turbulence or protruding undulations, or fingers, for the oil to catch and dislodge.

 

[aali94]

I've carried out all my simulations this way and my dissertation is due in a few weeks. I don't think I can make changes now, considering that the simulations take more than 14 hours. But I think I can put that down as future work.

Do you think my explanation of the wax being driven by pressure rather than shear is plausible. I've modelled the flow in a 50 mm ID and 100 mm ID pipe at the same Reynolds numbers. The results show that the wax is displaced by a much smaller amount in the larger pipe. I think this is because of the smaller pressure drop.

Thanks again!!