MAKERERE UNIVERSITYCOLLEGE OF NATURAL SCIENCESDEPARTMENT OF CHEMISTRYUNIT OPERATIONS LABORATORYPRACTICAL REPORT ON A PLATE DRYERGROUP 2EXPERIMENT ONENAME REG NO. SIGNATUREKWEZI MWAKA JULIUS 12/U/494BUKULU ERIC 12/U/4808/PSWEKKA JAMES AGGREY 12/U/15404/PSNABADDA REBECCA 12/U/9674/PSNANKYA MASTULA NAKIGGUDE 12/U/11917/PS LECTURER : Assoc. Prof. G.W.NYAKAIRUDATE OF SUBMISSION : 17TH- MARCH- 2014

ABSTRACTThe objectives of this experiment are; to remove water from wet sand by evaporation using dry air in a tray drier, determining the rate of drying of the wet sand under constant exit temperature and humidity conditions of the drying air. Drying in general means the removal of water or another solvent by evaporation from the material, water is usually removed as a vapour by air. Amount of water was put in the pan upon the dry sand and the weight of water was recorded every fifteen minutes until there was no more water in the sample (Reaching the equilibrium). By plotting the drying rate and moisture content, the trend shows the following behaviors, the graph increases gradually and then rapidly, then it increased slowly to maximum where is almost remains constant, hence the slope and rate are constant during this period. Then the drying rate starts decreasing in the falling rate period. . The falling rate period can take a far longer time than the constant rate period even though the moisture removal may be less. The moisture is decreasing with time and with time it become totally dry

TABLE OF CONTENTSContentsMAKERERE UNIVERSITYiABSTRACTiNOMENCLATURE6INTRODUCTION7REVIEW OF THEORY8EXPERIMENTAL PROCEDURE10RESULTS11TREATMENT OF RESULTS12DISCUSSION16CONCLUSION17RECOMMENDATIONS18REFERENCES19APPENDIX19

NOMENCLATUREA : Drying surface area, m2AU : Non drying surface area of drying solid , m2Am : Average solid surface area , m2CS : Saturated specific heat of air, J/kg0CG : Mass velocity of gas, kg/m2secR : Drying rateNC : Constant drying rate , kg/m2secLs : Mass of dry solidTg : Absolute temperature of gas (dry bulb),KZm : Metal thicknessTr : Absolute temperature of radiating surface ,KTs : Absolute temperature of solid surface ,KMin: minutesTID: Temperature of the inlet dry air, CTo: exit temperature, CTIW: temperature of the inlet wet air, C

INTRODUCTIONDrying in general mean usually means removal of relatively small amounts of water from material. In drying the water is usually removed as a vapor by air.In some cases water maybe removed by mechanically from solid materials by presses, centrifuging and other methods. This is cheaper than drying by thermal means for removal of water. The moisture content of the final dried product varies depending upon the type of product.Drying is usually the final processing step before packing and makes many materials, such as soap powders and dyestuffs, more suitable for handling.Drying processes can be classified as batch, where the material is inserted to the drying equipment and drying proceeds for a given period of time, or as continuous, where the material is continuously added to the dryer and dried material continuously removed.There are different types of drying equipments like: Tray dryer. Vacuum-shelf indirect dryers Continuous tunnel dryers. Rotary dryers. Drum dryers. Spray dryers.Heat is added by different ways, such as, by direct contact with heated air at atmospheric pressure and the water vapor formed I removed by air. The other way is to add the heat indirectly by contact with a metal wall or by radiation.The batch tray dryer requires large amounts of labor to load and unload the product for weighing hence its time consuming; also the drying times are long. A large dryer capacity is required for uniform air flow over the trays, which is expensive. Warpage of the trays can cause poor distribution of drying air hence poor dryer performance.

REVIEW OF THEORYDrying is a mass transfer process consisting of removal of water or another solvent by evaporation. And it consists two fundamental and simultaneous processes; heat transfer in evaporating the solid and then mass transfer as a liquid or a vapour from the solid. These can be represented as heat balance and mass balance respectively.For mass balance; Total mass entering = total mass leaving the plateMass in = mass out + mass storedMass of dry sand + water = mass of dry sand + mass of water vapourMws = Mmv + Ml (1)For energy balance; From law of conservation of energyTotal heat supplied = Convective heat + Radiation heat + Conduction heatFor a plate dryer. Q = hc(Tg Ts) + hr(Tr Ts) + hk(Tg Ts) (2)Where hc , hr , hk are the heat transfer co-efficient for convection , radiation , and conduction respectively also Q is the rate of heat transfer in J/s. Energy consumed for drying is a large component of the overall costs. This is shown from the dryer efficiency which is the ratio of the minimum energy needed to the energy actually consumed and for a plate dryer is given as; = (3)where T1, T2, Ta are inlet air temperature, outlet air temperature and ambient air temperature respectively.In drying there is loss of water from the sample in form of moisture and the amount of water in the sample can be termed as moisture content. Moisture content (Xt) is one of drying terms and is the amount of water contained in a material. Moisture content affects the rate of drying such that as time proceeds the amount of water in sand reduces.As time , Xt X*Where X* is Equilibrium Moisture Content. (4) (5)Where: R, LS, W, A, Xt Rate of drying [kg water/m2 hr], Weight of dry solid [kg dry solid], Weight of water [kg water], Surface area [m2], and Moisture content at time t [kg water/kg dry solid] respectively.DESCRIPTION OF THE APPARATUS

The unit consists of a floor standing tunnel in one end of which is mounted an axial flow fan. Down-stream of the fan is electrically heated elements which heat the air flowing to the drying chamber. The chamber contains a rack of trays (suspended from a balance mounted on the top of the drier, though this balance was not used). Ducting up stream and down-stream of the drier is designed to provide a uniform air flow over the trays and an aspirated psychrometer positioned at the upstream of the tray measures the dry bulb and wet bulb temperatures of the air. The fan is controlled through the switch at the control panel so that the air flow rate can be set to the desired value. The control panel is fitted with a heater control switch which can be used to control the power input to the heater, which heats up the air entering the duct. This allows the operator to manipulate the temperature of the entering air to the drier. Also the drier trays are tight fitted in the cabinet to prevent air from passing the materials to be dried. EXPERIMENTAL PROCEDUREThe dryer was heated to the exit temperature of 600C and kept constant.Sand (0.96kg) and water (100g) were prepared on a pre-weighed tray.The two were mixed thoroughly and spread uniformly over the tray using hands.The minimum and maximum temperatures were observed due to automatic controller.The wet bulb thermometer was wetted thoroughly.The unit was first run for 15minutes without any sample inserted and the readings of all temperatures at an interval (5minutes) were taken, that is, TID, TIW and TO.The fan was stopped and measurements were recorded in the table.The tray with the sand mixture was inserted into the unit, the door closed and the experiment run. Readings were taken at intervals (5minutes), that is, of temperatures while the tray weight was taken at an interval of 15minutes and the mass of the sand was computed.The above procedure was repeated until there was no change in the mass of the sand on the plate. The results obtained were tabulated.The heater was switched off while the fan was left to run for another 10 minutes.The fan was stopped and the mains were switched off.

RESULTS Weight of plate (g)3840

Weight of plate + dry sand (g)4800

Weight of dry sand (g)960

Weight of plate + wet sand (g)4900

Weight of wet sand (g)1060

Width of the plate (cm)40.6

Length of the plate (cm)81.0

weight of water (g)100.0

Area of a plate, A = width x length = 40.6cm x 81.0cm = 3288.6 cm2 = 0.32886 m2Results without sampleTime(min)TID(0C)To(0C)TIW(0C)

5586056

10566258

15565857

Results with sampleTime(min)TID(0C)TO(0C)TIW(0C)Mass of tray + wet sand(kg)

06060564.90

56064604.84

10626664

15636462

206262604.82

25596057

30626260

356362604.81

40595756

45606058

506566624.81

55605858

60605856

65565656

705454524.81

TREATMENT OF RESULTSAt t = 0 min:From equation (5):Xt = = = 0.1042 kg water/kg dry sand.X* = Xt (t = ) = Xt (t = 70 min) = = = 0 kg water/kg dry sand.X = Xt X* = 0.1042 0 = 0.1042 kg water/kg dry sand.X(t=15min) = = kg water/kg of dry sand = = = -0.00278From equation (4) = 8.12 kg water/m2hrTime (min)Weight of water, W (kg water)Moisture content, X (kgwater/ kg sand)dX/dtRate of drying (kg water/m2hr)

00.10.1041700

150.060.0625-0.002780.008109469

300.020.02083-0.002780.008109469

450.010.01042-0.000690.002025908

6000-0.000690.002027854

From equation (20), Plate efficiency, = T1= 60.4667 T2= 60.333 Ta= 58.867 = = 83.75%

DISCUSSIONFrom the graph of rate of drying against moisture content, generally, the drying rate first increases, becomes constant, and then falls. The increase shows the heat transfer to the material, and the drying rate increases drastically as free moisture is being removed.The constant area shows the reduced rate of evaporation as a result of moisture persistence on the surfaces. The drying rate is maximum at this stage.The falling stage shows when the movement of moisture from in-between the particles to the outer surface becomes the limiting factor and reduced moisture content.The moisture content graph against time generally shows a decrease of moisture content with time.

CONCLUSIONThe moisture is decreasing with the time and by time it becomes totally dry.The falling rate period can take a far longer time than the constant rate period even though the moisture removal may be less

RECOMMENDATIONS The following recommendations may be considered to obtain better results; Computer controlled dryers may be used as they reduce the amounts of energy used. A smaller plate may be used other than the larger one in order to reduce the rate of evaporation so as to obtain better results. The drying time can be reduced by increasing air flow or air temperature, but the air temperature must not be set too high as it reduces the quality of the dried product.

REFERENCES Warren L. McCabe, Julian C. Smith, Peter Harriott. (2005). Unit Operations of Chemical Engineering. (7th Edition). The Mc-Graw Hill Companies, Inc. New York. (796-797, 816-817, 828-829). Sinnott. R. K. (1983). Chemical Engineering.An Introduction to Chemical Engineering Design. (volume 6). A wheaton and company Ltd, Exeter. Great Britain. (333-334). http://www.anotes.com/how-products-encyclopedia.RB steward.WE, and Lightfoot,EN, transport phenomena, John Wiley and sons, Inc. 1960. At 15:30pm 14th March, 2014.

APPENDIX QJ/s

hcJ/sK

hrJ/sK

hkJ/sK

%

dry-bulb temperaturewet-bulb temperaturedew point temperaturerelative humidity enthalpy humidity ratio: