L ~ IN HEATAhDMASS TRANSF~ 0094-4548/82/050371-05503.00/0 Vol. 9, pp. 371-375, 1982 ©Pergamon Press Inc. Printed in theUnitedStates

EFFECTS OF CONCENTRATION AND SEEDING ON THE SOLIDIFICATION

FOULING OF PARAFFIN WAX FROM HYDROCARBONS

A. Sharma and J.P. Gupta

Chemical Engineering Department Indian Institute of Technology

KANPUR-208016, INDIA

(Comm~%icatedbyJ.H. Whitelaw)

Introduction

In an earlier paper [i] we had described the effects of

rotation speed and wall temperature on the solidification foul-

ing of paraffin wax from a solution in kerosene in a jacketed

vessel which was stirred at different speeds while a refrigerant

was circulated through the jacket. This set up was used to

measure the effect of varying shear stress on the deposit thick-

ness. It was hypothesized that the initial deposition is diff-

usion controlled instead of particulate migration controlled.

In this paper, further evidence is provided about the diffusion

controlled mechanism by studying the effects of wax concentra-

tion. The effects of seeding and very high shear rates are also

discussed.

Results and Discussion

The apparatus and the experimental procedure have been

described in [i]. The results are plotted in Figs. (i to 3).

In Fig. (i), the asymptotic thickness is found to increase with

371

372 A .

100

7£

E u E 60 o

5-O

b

~o

~3 3,0

,%arma and J.P. Gupta

9-0

B.O

c~ \ O N0ppm Qluminium particle dia.<100p

\ ~ a 3000ppm sond l:)Qrticle~ \ \ dio < Ioo~ \ , ~ , Nose=~sp~e.nt

1.0 I , I

Oo o., oL o% o% Asymptotic wax deposit thickness.era

FIG. 1

Vol. 9, No. 5

Effect of Wax Concentration on Asymptotic Deposit Thickness

10.0 Stirrer r pm =160 Both temp. = -10"C Ambient letup =35"C 9 / I , o

9.0 Se~.d ~rtlcieS(Sand) / /

8.C I : ~ X ) ~ ~ . ~ 7,0

~O Wax cancentrollon : N Ld

3< 2~

I , 0 0 0!1 012 01.3 01.& 01.5 0-6 01'7 0,8 Asymptotic wax deposit thickness,era

FIG. 2

Effect of Seeding on Asymptotic Deposit Thickness

Vol. 9, No. 5 SOLIDIFICATION FOULING OF PARAFFIN ~X 373

the increase in the wax concentration. Though in agreement with

the results of Bott and Gudmundsson ~2], it is surprising never-

theless since the concentration should affect only the rate of

deposit and not the asymptotic deposit thickness for a particu-

lar shear rate since the equilibrium thickness is governed by

the shear rate. The only feasible explanation appears to be

that the initial deposit is diffusion controlled and the initial

rate of diffusion increases with the increasing concentration

giving a thicker layer with stronger bonding. After the initial

induction period, the solution became turbid (as viewed from the

top) indicating that some nuclei have grown to 'particulate'

size and the deposit thereafter is due to particulate migration

to the wall. The particulate deposit has lower strength than

the cLiffusion-controlled deposit. Thus, the difference in depo-

sit thickness is due primarily to the difference in the thickness

of the strongly bonded, diffusion-controlled layer.

Effect of seeding

Seeding is commonly used in crystallizers. It provides

nuclei £or the growth of crystals. We used minute particles of

aluminum and sand (Fig. 2) and found that the fouling deposit

on the wall decreased in thickness. It is due to the fact that

the 'particulates' formed on foreign nuclei are rather big and

have very low cohesive strength. These stay in suspension, thus

reducing the effective concentration in the solution available

for the deposit. Hence the equilibrium thickness is reduced.

The difference between the sand and aluminum particles is due to

the fact that the sand particles tend to agglomerate and hence

present lesser number of nucleation sites. The net reduction of

374 A. Sharma and J.P. Gupta Vol. 9, No. 5

the effective wax concentration is less compared to the case of

aluminum and hence the equilibrium deposit thickness is higher

than in the case of aluminum seed particles.

Effect of very high shear rate

* . N -2 In 1 , Fig. (7) gives a plot of xf vs . Studies

done at very high shear rates reveal that there is a sudden

reduction in the deposit thickness after a certain shear rate or

rpm, indicating that at still higher shear rates, no deposit

would be formed (Fig. 3).

(i)

(ii)

(iii)

Conclusions

The deposit thickness increases with increasing bulk

concentration at a constant shear rate due to higher

initial diffusion rates.

The effect of seeding is to reduce the deposit thick-

ness. Further studies need to be done regarding the

effects of the type of seeding used, its concentration,

particle size and density. It may then be used in

crude pipelines during cold weather. Arrangements to

remove these 'seeds' at the end of the pipeline before

processing the crude will have to be made. It may also

be used in heaters/coolers handling waxy hydrocarbons.

At very high shear rate, the deposit thickness is

significantly low.

Acknowledgement

This work was partially supported by Grant No. 12(36)/

78-SERC provided by the Department of Science ana Technology,

Government of India.

Vol. 9, No. 5 SOLIDIFICATION FOULING OF PARAFFIN WAX 375

xf

N

Nomenclature

asymptotic thickness of wax deposit

stirrer speed (rpm)

I.

2.

References

A. Sharma, D. Garg and J.P. Gupta, 'Solidification fouling of paraffin wax from hydrocarbons', Letters Heat Mass Transfer, 9(3), 209-219 (1982).

T.R. Bott and J.S. Oudmundson, 'Deposition of paraffin wax from kerosene in cooled heat exchanger tubes', Can° J. Chem. Eng., 5_~5, 381 (1977).

1.1

0.9 E u

~" o.8

C

.u_ 0-7 c

8 0.6

05

,u__ O.Z, o

~0.3 <

0 .2

0.1

0 I,

0

Concent ra t ion of wax in ke rosene(wt °/o)=2.23

Ear l ier work [1]

- - Present work

I I I I I I 0-5 1.0 1-5 2.0 Z.5 3.0

1/N2x 10 *4

Distance frorr the bot tom

/ _ -- 6 cm(impeller

locat ion)

FIG. 3 Effect of Shear Rate on Asymptotic Deposit Thickness