Course Number: ChE-302

Course Title: Chemical Engineering Laboratory-II

Experiment Number: 04

Name of the Experiment: Drying of solids

Submitted by:

Mahe Rukh

Student Number: 1202036

Section: A2 Group Number: 02

Department of Chemical Engineering

BUET

Partners’ Student Numbers: 1202037

1202038

1202039

1202040

Date of

Performance:10

/10/15

Date of Submission:

31/10/15

Submitted to:

Dr. Md. Tanvir Sowgath

Assistant Professor

Department of Chemical Engineering

BUET

Summary Drying of solid is of great importance in industrial production and in chemical engineering application.

Various kind of process for drying are of day to day use in various fields. That is why conducting this

experiment is very effective as it gave us vivid idea of the phenomena- drying of solids e.g. sand. For this

purpose, two trays were used. Sand was first poured on them and then water was mixed. In a vacuum

dryer the trays were heated and moisture content from sand was removed by steam supplied from

steam generator where the flow rate of steam was maintained at 30psi. After every five minute trays

were drawn out and moisture content in them was calculated by weighing their weight and comparing it

to initial weight of tray and sand. From the collected data drying rate, moisture content was measured

and graphs were drawn focusing drying characteristics of sand.

Introduction The removal of relatively small amounts of water or other liquid to reduce the content of

remaining liquid to acceptably low amount in a solid matter is known as drying of solids. this is a

mass transfer process were liquid is removed by means of evaporation. So, we can see drying of

solids need both mass transfer and heat transfer simultaneously. A source of heat and an agent is

involved in this process. Most commonly, a gas stream such as air applies the heat by convection

and carries away the liquid in solid as vapor.

There are various methods of drying e.g. direct drying, indirect drying, dielectric drying, freeze

drying, supercritical drying and natural drying. During thermal drying two things occur at a time.

Transfer of moisture content from internal space to solid surface with subsequent evaporation

and Transfer of heat to raise the wet solid temperature and to evaporate the moisture content. The

drying rate depends on how these two processes proceed. Heat transfer process occurs due to

conduction, convection or radiation mode of heat flow or in some cases as a result of a

combination of any of these effects. In our experiment we have used indirect method of drying

where it is important to have the maximum contact between heated dryer metal surfaces and the

solid undergoing drying and maximum movement at areas of contact. Here, the drying operation

does not depend upon the effect of convection from heated air or gases. Air flow through such

dryer is only required to remove moisture as vapor from the system as it is liberated in the course

of drying.

2

External conditions such as temperature, humidity, water velocity relates with drying of solids.

Theoretical models often consider not only external conditions but also the mechanisms of

internal moisture movement and its consequent effect. The solids fed to dryer can be in the form

of flakes, granular, crystals and may have widely different properties. There are over 100 distinct

types of dryers. Of them tray dryer is most common. Tray dryer is one kind of batch dryer which

can handle both direct drying and indirect drying. The dryer operates under a vacuum, and heat

transfer is largely by conduction or by radiation. The trays are enclosed in a big cabinet, which is

evacuated. Water vapor produced is usually condensed, so that the vacuum pumps need only to

deal with non-condensable gases.

Experimental work The following things were used in this experiment-

1. Two trays

2. Sand

3. Water

4. Vacuum pump

5. Vacuum dryer

3

Figure 01: A schematic diagram of vacuum dryer

Figure -02: Schematic diagram of vacuum dryer.

Procedure 1. At first the weight of the two empty trays were taken.

2. Tray weight with sand was taken. Then water was added to moisten the sand and trays were reweighted.

4

3. Trays were vacuum dried by putting inside the drier.

4. At an interval of five minutes’ vacuum drying was stopped by releasing vacuum drying to

take the reading of weight of tray containing sand and water

5. Reading was taken until constant weight readings were achieved.

6. Data for calculation were noted.

Observed Data Weight of each empty tray = 6.5 lbm

Tray 1

Weight of Tray + sand = 14 lbm

Weight of Tray + sand + water = 15 lbm

Tray 2

Weight of Tray + sand = 15.0 lbm

Weight of Tray + sand + water = 16.5 lbm

Table 1: Representing weight of Tray +Sand + Water at different time

No of Obs

Time

(min)

Weight ( lbm) of

Tray 1+Sand+Water

Weight(lbm) of

Tray 2 +Sand+ Water

01. 5 14.88 16.31

02. 10 14.75 16.07

03. 15 14.50 15.81

04. 20 14.25 15.68

05. 25 14.19 15.50

06. 30 14.13 15.25

07. 35 14.00 15.13

08. 40 14.00 15.00

5

09. 45 14.00 15.00

Calculated data

Table 2: Drying Rate and Moisture Content of Tray 1 at Different Time

Observation No. Time

(Min)

Moisture decrease rate

C =(W0-Wt)/t

lb/min

Drying rate, R

lb/ft2.min

Moisture content

lb water/ lb solid

1 5 0.024 .0090 0.117

2 10 0.026 0.010 0.10

3 15 0.05 0.0183 0.067

4 20 0.05 0.0183 0.033

5 25 0.012 0.0044 0.025

6 30 0.012 0.0044 0.0173

7 35 0.026 0.0096 0.0

8 40 0.0 0.0 0.0

9 45 0.0 0.0 0.0

Table 3: Drying Rate and Moisture Content of Tray 2 at Different Time

Observation No. Time

(Min)

Moisture decrease rate

C =(W0-Wt)/t

lb/min

Drying rate, R

lb/ft2.min

Moisture content

lb water/ lb solid

1 5 0.038 0.0140 0.154

2 10 0.048 0.0177 0.125

6

3 15 0.052 0.0191 0.095

4 20 0.026 0.0096 0.080

5 25 0.036 0.0132 0.059

6 30 0.05 0.0183 0.029

7 35 0.024 0.0088 0.015

8 40 0.026 0.0096 0.0

9 45 0.0 0.0 0.0

Sample calculation Calculation for observation no 1 of tray 1:

Initial weight of sand+ tray 1+ water after 5 min, W0 = 14.88 lb

Weight of sand+ tray+ water after 10 min, Wt = 14.75 lb

Moisture decrease rate , C= (Wo-Wt)/time= (14.88-14.75)/5

= 0.026 lb/min

Area of tray 1, A = 29x 13.5 inch2

= 391.5 inch2

= 2.71875 ft2

Drying rate R= C/A

=

0.01252.71875 lb/min ft2

= 0.0096 lb/min ft2

Moisture content =

lb of waterlb of dry solid

= (weight of sand+ water+ tray at 5 minutes)-(weight of sand+ tray)

(initial weight of sand + tray)- (weight of tray)

= (14.88—14)/ (14-6.5)

7

=0.1173

Graphical representation

Graph 01: Drying rate versus time

8

Graph 02: Moisture content vs time

Graph-03 : Drying rate versus moisture content

9

Results In case of both Tray 1 & Tray 2 100% removal of the moisture was observed. For tray-1 moisture

content reduced to zero after 35 minutes and for tray-2 it took 40 minutes.

Discussion

Discussion on result

Moisture decrease rate should have been same for two tray-1 and tray-2 for same number of

observations as they were dried in same dryer. But from data it is seen that they are not same.

Discussion on graphs

By using the calculated data, following three graphs are drawn for both trays:

1) Plot of Drying rate vs. Time

2) Plot of Moisture Content vs. Time

3) Plot of Drying Rate vs. Moisture Content

Plot of Drying Rate vs. Time

the drying rate increased for some times at first, then started decreasing and finally became

constant in both cases. But drying rate does not decrease with decreasing of moisture content,

nevertheless drying rate fluctuate too much

Plot of Moisture Content vs. Time

From graphs 1 and 2, we see that moisture content decreases with time as usual and

consequently it becomes zero which was supposed to be obtained.

Plot of Drying Rate vs. Moisture Content

There should have been constant drying period but from graphs we see there was not any.

Errors that may have occurred in the experiment-

10

1. the pressure gauge that measured steam pressure showed fluctuations during the

experiment but the flow rate should have been constant.

2. The measuring device was operated manually and weight measured using this device may

have caused inaccuracy.

3. The trays were taken out in every 5 minutes. So there is chances that some moisture was

lost during weight measurement.

4. In the vacuum dryer maintaining constant pressure was a struggle. So it affected heat

transfer between the wet solid and steam. Also vacuum could not be maintained perfectly

as the door of the dryer had to be opened and thus air entered the dryer every time and it

was difficult to evacuate it by the vacuum pump so fast, so the drying conditions might

have changed.

5. During the mixing some weight loss may have occurred and even a small loss of sand

particle may cause error in the experiment.

Ways to improve the experiment-

1. Mixing should be done with care

2. Using digital weight measuring device instead of manual one.

3. Constant pressure at both steam inlet to dryer and in the vacuum dryer should be

maintained.

Nomenclature

Symbol Name Unit(If Applicable)

A Area of the tray ft2

C Moisture Decreasing rate lbm/minFt Feet - lbm Pound mass -

11

T Time minuteW Moisture Content lbm/ lbm

References 1. Foust, Alan S.; Wenzel, Leonard A.; Clump, Curtis W.; Maus, Louis; and

Andersen, L. Brice: Principles of Unit Operations, 2nd ed., John Wiley & Sons,

New York, 1994.

2. McCabe, Warren L.; Smith, Julian C.; and Harriott, Peter: Unit Operations of

Chemical Engineering, 6th ed., McGraw-Hill, New York, 2001.

3. Genskow, L.R.; Beimesch,W.E.; Hecht, J.P.; Kemp,I.C.; Langrish,T.;

Schwartzbach, C.; Smith: Chemical Engineers' Handbook. Mc Graw Hill

Professional,2007.

4. Drying, Wikipedia, Retrieved from https://en.wikipedia.org/wiki/Drying

5. Industrial and engineering chemistry, ACS publications ,Retrieved from

http://pubs.acs.org/doi/abs/10.1021/ie50238a021

12

Marking Scheme: Normal Report

Name: Mahe Rukh

Student number: 1202036

Section and marks allocated Marks

Summary(1)

Introduction (1)

Experimental Work (1.5)

Observed Data (1)

Calculated Data (1)

Sample Calculation (1)

Graphs (1)

Results and Discussion (1)

References and Nomenclature (0.5)

Writing Quality and Style (1)

Total (10)

13

14