Modelling for suspension of coalin multiphase reactor system.A statistical approach

Ashis Mukherjeea ,*, Sushanta Hazraa and Pranab Bandopadhyayb

aCentral Fuel Research Institute, Dhanbad, IndiabIndian School of Mines, Dhanbad, India(Accepted 3 April 1998)

Experiments have been conducted in a 40 mm diameter and 1.5 m length glass reactor treating different coals inthe size range 53-76mm using anthracene oil as the liquid medium to prepare the slurry to study the suspensioncharacteristics of coal in oil. The variables studied are coal to oil ratio, coal ash, slurry velocity, gas velocity andpressure drop. The effect of these variables on the minimum superficial velocity of gas required to keep the coalparticles in suspension have been studied qualitatively and models have been developed for prediction ofminimum superficial velocity and pressure drop. Utilizing graphical as well as statistical approach modelparameters have also been optimized.q 1998 Elsevier Science Ltd. All rights reserved

(Keywords: glass reactor; coal suspension; minimum superficial velocity)

INTRODUCTION

In any coal–oil based reactor normally slurry (coal–oilmixture) is passed from bottom to top of the reactor, at lowvelocity depending upon the residence time1–5 of thereactants. The solid particles in the slurry have tendencyto settle which can be controlled by injecting gas6 at suitablevelocity from bottom. The minimum superficial gas velocityrequired to keep all the coal particles in suspension isinflunced by the different operating variables. In the presentinvestigation attempt has been made to find out the relationbetween the minimum superficial velocity and the processvariables like coal to oil ratio,coal ash,slurry velocity andpressure drop. Finally the process variables have beenoptimized for obtaining minimum superficial velocity andminimum pressure drop to keep the coal particles insuspension.

EXPERIMENTAL

Tests were carried out in a glass reactor6 of inside diameter40 mm and length 1.5 m using anthracene oil and coal in thesize range -200þ 300mm. The analysis of the coals and theoil are given inTable 1.

The reactor was fed from a slurry tank and was equippedwith compressor for feeding gas into the reactor. Aperforated plate was used as gas distributor. Gas flow rateand the pressure drop were measured by a rotameter andmanometer, respectively. Slurry of specific coal to oil ratiowas prepared in the slurry tank which was fed to the reactorthrough a control valve and recycled to the slurry tank.

Slurry velocity was measured by collecting sample for aknown period from the reactor outlet.

RESULTS AND DISCUSSIONS

The minimum superficial gas velocity required to keep thecoal particles in suspension was measured. Differentprocess variables with respect to the minimum superficialvelocity are given inTable 2

Effect of slurry velocityFor a particular ash and coal to oil ratio if slurry velocity

(V1) is plotted against superficial air velocity, a straight linewith negative slope is observed given inFigures 1–3. For aparticular ash with different coal to oil ratio straight lines areobtained. These straight lines can be superimposed into asingle one plotting modified slurry velocity (V2) against airvelocity where modified slurry velocity may be representedas

V2 ¼V1

0:9746 ln11:167

R

(1)

Similarly if pressure drop (P1) is plotted against air velocityfor a particular ash and coal to oil ratio, polynomial curveswith a negative slope as given inFigures 4–6are observed.

Similarly for a particular ash with different coal to oilratio different curves are obtained which can also besuperimposed into a single curve by plotting modifiedpressure drop (P2) against air velocity.where modifiedpressure drop may be written as

(P2) ¼Pressure drop(P1)

1:924 ln6:73

R

(2)

Fuel Vol. 77, No. 14, pp. 1683–1689, 1998q 1998 Elsevier Science Ltd. All rights reserved

Printed in Great Britain0016-2361/98 $19.00+0.00PII: S0016-2361(98)00062-3

Fuel 1998 Volume 77 Number 14 1683

* Author to whom correspondence should be addressed. Tel: 00-91-326-202381; Fax: 00-91-326-203042

Coal suspension in a multiphase reactor: A. Mukherjee et al.

1684 Fuel 1998 Volume 77 Number 14

Table 1 Analyses of coal and oil

Sample Moisture Ash Volatile matter Fixed carbon Hydrogen Carbon Sulphur Nitrogen

Samla coal 8.5 10.6 31.8 49.1 4.4 64.96 0.44 1.89

Sirka coal 6.3 15.5 29.0 49.2 3.88 63.52 0.61 1.40

Golokdih 1.3 40.7 17.3 40.7 3.24 48.5 0.2 1.07

Anthracene oil 6.8 90.8 0.5 1.4

Specific gravity of anthracene oil is 1.09 and boiling range 3008C–3608C.

Table 2 Different process variables corresponding to minimum superficial velocity

Slurry velocity (cm s¹1) Coal/oil Coal ash Air velocity (minimum)(cm min¹1 3 103)

Pressure drop (cm of Hg)

1.88 2.0 12.4 0.490 18.5

1.65 2.0 12.4 0.510 16.0

1.63 3.0 12.4 0.470 13.0

2.4 3.5 12.4 0.390 18.5

2.0 2.5 12.4 0.460 17.0

2.65 3.0 12.4 0.390 19.5

1.65 2.5 12.4 0.460 22.3

1.9 3.5 12.4 0.430 14.0

1.8 3.5 12.4 0.440 13.1

0.85 3.5 12.4 0.520 9.6

2.88 4.0 12.4 0.330 19.6

2.75 4.0 12.4 0.350 17.0

2.2 4.0 12.4 0.370 15.0

1.86 2.5 14.5 0.549 12.5

2.08 3.5 14.5 0.477 11.5

1.65 3.5 14.5 0.517 9.0

2.2 4.0 14.5 0.437 11.0

1.95 4.0 14.5 0.460 9.0

2.67 3.5 40.7 0.537 13.8

2.35 4.0 40.7 0.517 11.4

2.2 4.0 40.7 0.532 10.3

Figure 1 Effect of slurry velocity on minimum air velocity for ash¼ 10.6 cm min¹1 3 103.

Effect of ash in coalIt has been observed that for different ash in coal, curves

may be obtained by plotting modified slurry velocity withair velocity. These different curves can be superimposedinto a single curve giving consistent characteristics whenthe re-modified slurry velocity (V3) is plotted againstair velocity where re-modified slurry velocity (V3) isgiven as

(V3) ¼Modified slurry velocity(V2)

0:7713 ln(0:3255A)(3)

If the above procedure is repeated for the curves plottedby modified pressure drop against air velocity with

varying ash content; the different curves can be put intoa single curve by plotting modified pressure drop (P3)against air velocity where modified pressure drop3 (P3) isgiven as

(P3) ¼Modified pressure drop(P2)

0:30443 A0:4823 (4)

Effect of coal to oil ratioVariation of minimum fluidization velocity with coal to

oil ratio is given inTable 2with a specific ash and slurryvelocity. It is observed that if coal to oil ratio increases

Coal suspension in a multiphase reactor: A. Mukherjee et al.

Fuel 1998 Volume 77 Number 14 1685

Figure 2 Effect of slurry velocity on minimum air velocity for ash¼ 15.5 cm min¹1 3 103.

Figure 3 Effect of slurry velocity on minimum air velocity for ash¼ 40.7 cm min¹1 3 103.

then minimum fluidization velocity decreases (Figure 1)Similarly with a particular ash and pressure drop if coal tooil ratio increases then minimum fluidization velocitydecreases.

DEVELOPMENT OF MODEL

If modified slurry velocity (V3) is plotted against air velocitya polynomial curve is obtained. The equation of the curve is

Coal suspension in a multiphase reactor: A. Mukherjee et al.

1686 Fuel 1998 Volume 77 Number 14

Figure 4 Effect of pressure drop on minimum air velocity for ash¼ 10.6 cm min¹1 3 103.

Figure 5 Effect of pressure drop on minimum air velocity for ash¼ 15.5 cm min¹1 3 103.

given asSuperficial air velocity

¼ ¹0:08733 V2

0:7713 ln(0:3255A)þ 0:59 (5)

¼ ¹0:08733 V1

0:7713 ln(0:32553 A) 3 0:99463 Ln1167

R

þ 0:59

¼ ¹0:11623 V1

ln11:167

R3 ln(0:32553 A)

þ 0:59

The multiple correlation coefficient of the equation is foundto be 0.92.

By plotting re-modified pressure drop (P3) and superficialair velocity a general relation may be obtained as follows

Superficial air velocity¼ 0.67443 exp(¹0.03863 P3)

Coal suspension in a multiphase reactor: A. Mukherjee et al.

Fuel 1998 Volume 77 Number 14 1687

Figure 6 Effect of pressure drop on minimum air velocity for ash¼ 40.7 cm min¹1 3 103.

Figure 7 Agreement between the actual minimum fluidization velocity and predicted minimum fluidization velocity.

or

P3 ¼ ¹ 25:8963 lnAir velocity

0:6744ReplacingP3 by P2 andP1 we get

P3 ¼P2

0:30443 A0:4923

¼P1

1:9243 ln6:73

R3 0:30443 A

0:4823

(6)

¼ ¹ 25:8963 lnSuperficial air velocity

0:6744

so

Pressure drop(P1) ¼ ¹ 15:16733 A0:4823 3 ln6:73

R

3 lnAir velocity

0:6744

The multiple correlation coefficient is found to be 0.89.The agreement between the actual values and those pre-dicted from eqns (5) and (6) are shown inFigures 7 and 8.

OPTIMIZATION OF PROCESS VARIABLES FOR AIRVELOCITY

In this study, air velocity was minimized with respect to theprocess variables, i.e. slurry velocity, coal to oil ratio andash of coal. The minimum air velocity optimized processvariables are given inTable 3.

Coal suspension in a multiphase reactor: A. Mukherjee et al.

1688 Fuel 1998 Volume 77 Number 14

Figure 8 Agreement between the actual pressure drop and predicted pressure drop.

Table 3 Optimized process variables with respect to minimum superficial air velocity

Range Optimium values of variables Minimum superficialair velocity(cm min¹1 3 103)V1 A R V1 A R

0.1–2 10–25 2–3 2 14.3 2.99 0.47

0.1–1 15–25 1–2 0.99 18.7 1.89 0.55

0.01–0.1 15–25 2–3 0.09 18.43 2.98 0.585

Table 4 Optimized process variables with respect to the minimum pressure drop

Range Optimum values of variables Minimum pressuredrop (cm of Hg)

V1 A R V1 A R

0.1–2 10–25 2–3 0.1 13.11 2.99 23.77

0.1–1 15–25 2–3 0.1 17.75 2.99 27.86

0.1–1 15–25 1–2 0.1 17.87 1.99 41.27

Optimization of process variables for pressure dropFurther study was carried out to optimize the process

variables to obtain minimum pressure drop. The variablesconsidered were ash in coal slurry velocity and coal to oilratio. The minimized pressure drop with respect to theprocess variables are given inTable 4.

CONCLUSIONS

The following conclusions may be drawn from theinvestigation.

(1) For any coal to oil ratio and any ash, if the superficial airvelocity is plotted against the slurry velocity, separatestraight lines with negative slopes are obtained. Theselines if superimposed on a single line using a standardmethod, a model for calculating air velocity with differ-ent variables can be developed.

(2) For any ash and coal to oil ratio, if the superficial airvelocity is plotted against the pressure drop, separatepolynomial curves are obtained. These seperate curvescan be superimposed into a single curve to develop

models for calculating pressure drop incorporating theeffects of different variables

(3) For a particular range of ash, coal to oil ratio and slurryvelocity, the superficial air velocity and pressure dropcan be minimised using the above method.

(4) The model is similar for a ash level of coal from 14 to47 wt% and the coal to oil ratio of 4.

REFERENCES

1 Nigam, K.D.P. and Schumpe, A.,AICHE J., 1987,33, 328–330.

2 Heek, J. and Onken, U.,Chem. Engng Sci., 1987,42, 1211–1212.

3 Zweitering, Th.N.,Chem. Engng. Sci., 1958,8, 244.4 Koide, K., Yasuda, T., Imato, S. and Fukada, E.,J. Chem.

Engng Jpn, 1983,16, 7–12.5 Roy, M., Guha, D.K. and Rao, M.N.,Trans. Ind. Inst. Chem.

Engrs, 1963,April , 23–28.6 Grootscholten, P.A.M. and De Jong, E.J.,Chem. Engng.

Sci., 1985,40, 151–154.

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