On the Deposition of Paraffin Wax in a Batch Oscillatory Baffled Column

Mr Lukman Ismail, Dr Robin E. Westacott and Professor Xiong-Wei Ni

School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh, EH14 4AS

www.COBRA.hw.ac.uk

Centre for Oscillatory Baffled Reactor Applications

(C.O.B.R.A)

Chemical Engineering, School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh

10th Meeting of Process Intensification Network, June 2004.

Outlines of presentation:

1. Introduction

2. Experimental set-up and procedure

3. Result and discussion

4. Conclusion and future work

To investigate the effect of several vital

parameters (e.g. T, T, cooling rates, frequency

& amplitude of oscillation) on paraffin wax

deposition in a batch oscillatory baffled column

(OBC) in order to understand the mechanisms of

wax deposition and subsequent prevention and

removal in oil pipelines.

Introduction

Research objective

Can oscillatory motion break up and suspend

deposition in pipelines?

Introduction

Wax deposition problem

• Paraffin wax continues to be a leading problem

area in the production and transportation of

crude oil.

• Severe plugging from the deposition.

• Production loss and replacement of pipelines.

Oscillatory Baffled Column

a) Oscillating the fluid at the base b) Oscillating the baffles at the top

Introduction

Oscillatory baffled column/reactor is a relatively new mixing technology and offers more uniform mixing and particle suspension than traditional reactors.

3-D CFD simulation

Reo = 1250 (xo = 4 mm, f = 1 Hz)

Real system

Parameters of importanceParameters of importance

Three geometrical --- tube diameter (D), Three geometrical --- tube diameter (D), baffle diameter (Dbaffle diameter (Doo) and baffle spacing ) and baffle spacing (L)(L)

Three operational --- oscillation Three operational --- oscillation frequency (f), oscillation amplitude (frequency (f), oscillation amplitude (xxoo) ) and fluid velocity (u)and fluid velocity (u)

Two physical --- fluid viscosity (Two physical --- fluid viscosity () and ) and fluid density (fluid density ())

Two Dimensionless GroupsTwo Dimensionless Groups

Oscillatory Reynolds number Oscillatory Reynolds number

Strouhal number Strouhal number

o

o

DxRe

ox

DSt

4

Research Strategy

Wax deposition without oscillation

Wax deposition with baffle oscillation

• Effect of time

• Effect of temp., T etc.

•Effect of oscillatory parameters in OBC eg. oscillating frequency, amplitude, baffle design, etc.

Experimental set-up

Computer

Specifications of the rig:• Diameter 25 mm• Length 130 mm• Baffle spacing 35 mm• Baffle free area 30 %• No. of baffles 2

Cold water reservoir (at final temp.)

Motor

Thermocouple

Oscillation

Baffles

Hot water reservoir (at initial temp.)

Peristaltic pump

Figure 1: Experimental set-up

Material selection

Wax

Oil

Diesel

Paraffin wax

- Aldrich Chemical, m.p 52-58 oC.

- National Oil Company.

- most common problem in crude oil compared to other substances eg. asphaltenes and resins.

- Less hazardous, less volatile, less error in wax deposit measurement.

- Cheap and easily obtained.

Procedure

Wax deposit measurement

•Stock of wax-oil solution prepared with certain wax-diesel ratio.

•Wax-oil was heated to desired initial temperature using hot water in

the reservoir.

•The column was cooled down for certain designated time.

• Uncrystallised wax and oil were then collected by putting the column

upside down.

• Wax deposit then measured as weight percentage of wax deposited

to the total wax-oil weight.

Result and Discussion (w/o osc.)

Effect of deposition time

0.00

10.00

20.00

30.00

40.00

50.00

60.00

70.00

80.00

90.00

100.00

0 1 2 3 4 5 6 7 8

Time (min)

Wa

x d

ep

osi

t (w

t.%)

Figure 2: Effect of time on wax deposition for 50 mL of wax solution, 10% initial wax content, initial temperature of 40oC and final temperature of 10oC.

-Wax deposition increased steadily with time reaching asymptotic value of 100% .- No liquid flowed out of the column after 7 min.- The wax deposit was in the form of an oil gel.

Effect of varying the volume of wax-oil solution in the column

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60

Wax/oil volume (mL)

Wa

x d

ep

osi

t (w

t.%)

Fig. 3: Effect of volume on wax deposition for initial wax of 15.5% with Tinitial =40 oC, Tfinal=15 oC, deposition time of 4 min, cooling water pump speed of 100 rpm

-Using different volume of wax-oil solution may affect the result thus fixed volumes were used in every experimental study.- The smaller the volume the more likely the error.

Result and Discussion (w/o osc.)

Effect of initial temperature

0.00

10.00

20.00

30.00

40.00

50.00

60.00

70.00

80.00

90.00

100.00

0 10 20 30 40 50 60 70 80

Initial temp. (oC)

Wa

x d

ep

osi

t (w

t.%)

Fig. 4: Effect of initial temperature with constant final temperature of 10 oC on wax deposition. Wax-oil volume is 50mL, deposition time was 2 min., and cooling water pump speed was 100rpm.

-The higher the initial temperature, the less wax deposited.- The higher the initial temperature, the more heat load in the solution thus less crystallisation.

Result and Discussion (w/o osc.)

Effect of final temperature

0

10

20

30

40

50

60

70

80

0.0 5.0 10.0 15.0 20.0 25.0 30.0

Final temperature (oC)

Wax

dep

osit

(wt.%

)

Fig. 5: Effect of final temperature on wax deposition with initial temperature of 40 oC, deposition time of 2 min. Cooling-water pump speed was 100 rpm, wax-oil volume was 50 mL and initial wax content was 10%.

-The higher the final temp., the less wax deposited.

- At temp. of 24 oC, no deposition occurred for 10% initial wax content.

Result and Discussion (w/o osc.)

Effect of temperature difference (T)

0

10

20

30

40

50

60

70

80

0 10 20 30 40 50 60 70

Temperature difference (oC)

Wax

dep

osit (

wt.%

)

Fig. 6: Effect of temperature difference on wax deposition. Comparison between constant T with constant Tinitial of 40oC and constant T with constant Tfinal of 10 oC. The initial paraffin wax content used was 10%, deposition time was 2 minutes and cooling water pump speed was 100 rpm

Tinitial const. (40oC)

Tfinal const. (10oC)

Const. Tinitial

– wax deposition increased rapidlyConst. Tfinal

– Wax deposition decreased gradually

Thus, final temperature has more influence on wax deposition

Result and Discussion (w/o osc.)

Effect of constant temperature difference with varied initial and final temperature

0.00

10.00

20.00

30.00

40.00

50.00

60.00

70.00

80.00

90.00

25-5 30-10 35-15 40-20

Initial & final temperature (oC)

Wax

dep

osit

(wt.%

)

Fig. 7: Effect of constant T (of 20oC) with varied initial and final temperatures. The initial paraffin wax content used was 10%, deposition time was 2 minutes and cooling water pump speed was at 100 rpm.

-The higher Tfinal and Tinitial, the less wax deposited.- Effect of Tfinal were more evident.

Result and Discussion (w/o osc.)

Effect of cooling rates

0.0010.00

20.0030.00

40.0050.00

60.0070.00

80.0090.00

100.00

0.00 0.50 1.00 1.50 2.00 2.50

Cooling rate (oC/min)

Wax

dep

osit

(wt.%

)

Fig. 8: Effect of cooling rates (oC/min) on wax deposition . Initial wax content was 40%. Tinitial = 40 oC,

Tfinal = 30 oC. Different cooling rates were achieved by

varying the cooling water pump speed (25, 50, 100, 150 & 200 rpm).

-The higher the cooling rates, the less wax deposited.

- The longer the crystallisation period, the bigger the crystal, thus more deposit.

Result and Discussion (w/o osc.)

Effect of initial wax percentage

0.00

10.00

20.00

30.00

40.00

50.00

60.00

70.00

80.00

0 10 20 30 40 50 60 70

Initial wax content (wt.%)

Wax

dep

osit

(wt.%

)

Fig. 9: Effect of initial paraffin wax content (wt..%) on wax deposition. The initial wax content was 10, 20, 40 and 60%. Deposition time was 2 minutes, with cooling water pump speed at 100 rpm, wax-oil volume of 60 mL, Tinitial = 50 oC

and Tfinal = 15 oC.

- Wax deposition increased with higher initial wax content with non-linear relation

Result and Discussion (w/o osc.)

Effect of oscillation frequency – preliminary results

0

10

20

30

40

50

60

70

80

0 0.5 1 1.5 2

Frequency (Hz)

Wax

dep

osit

(Wt.%

)

Total wax

Baffle

Column wall

Fig. 10: Effect of oscillation frequency on wax deposition. Initial paraffin wax content was 20%. Deposition time was 2 min., amplitude of oscillations = 30 mm, Tinitial = 40 oC, Tfinal

= 15 oC and wax-oil volume = 60 mL.

Wax on baffle – less wax collected with oscillation, agitation effect.

Wax on column wall – more wax deposited with oscillation, better heat transfer thus better cooling

Total wax deposition – generally reduced with oscillation but small increase for higher frequency due to better heat transfer and enhanced cooling.

Result and Discussion (with osc.)

Effect of oscillation amplitude – preliminary results

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8

Deposition time (min)

Wax

dep

osit

(wt.

%)

10 mm

5 mm

No osc.

30mm

Fig. 11: Effect of oscillation amplitudes on wax deposition. Initial paraffin wax content was 20%, frequency of oscillations = 0.86 Hz, Tinitial = 40 oC, Tfinal = 15 oC and wax-oil volume = 60 mL.

-higher amplitude cause earlier crystallisation.

-Total wax deposition was reduced over the time

Result and Discussion (with osc.)

Conclusion

- The effect of time, initial & final temperature, temperature difference, wax-oil volume, wax-oil ratio and cooling rates without baffle and oscillation had been investigated, and an optimal condition established.

- From the preliminary results, increasing in oscillation frequency decreases wax deposition, though increasing the magnitude of the frequencies causes small increase in the total wax deposition.

- From the preliminary results, increasing the oscillation amplitude generally decreases the amount of wax deposition.

Future work

More experiments with fluid oscillation to cover a wide range of

mixing intensity.

Different baffle designs.

Effect of using other oil such as Octane (C8) and Decane (C10).

Continuous flow system.