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Course Number: ChE-302 Course Title: Chemical Engineering Laboratory-II

Experiment Number: 05 Name of the Experiment: Study of Spray Drying

Submitted by:

Submitted to:

Mahe Rukh

Dr. Md. Tanvir Sowgath

Student Number: 1202036

Assistant Professor

Section: A2

Department of Chemical Engineering

Group Number: 02

Department of Chemical Engineering

BUET

Partners’ Student Numbers: 1202037

Date of

1202038 Performance: 1202039 31/10/15 1202040 Date of Submission: 7 /11/15

Summary The purpose of this experiment is to study spray dryers and spray drying procedure. Spray drying is one of the best drying approaches to convert the fluid materials into solid or semi-solid particles in a single step. This experiment helped us to understand spray drying technique, the mechanism of spray drying, working principle of spray dryer, operating variables of spray drying, types of material handle in spray dryer and advantages and disadvantages of spray dryer over other types of dryer. We also studied different parts of a spray dryer and their functions. This process is rapid and labor cost is low. Spray dryers employ atomizer or spray nozzle to break the bulk of liquid or slurry into controlled drop size. This method of drying is applied in food, pharmaceuticals and other industries for its unique features. In this experiment, a solution of 1000.1 g liquor milk with 50.1g milk powder was spray dried and after the drying process 13g milk powder was recovered. Hence, 26 % milk powder was recovered and the drying rate was 38g/min. the recovery is not satisfactory. Probable reasons for low recovery has been explained and modification techniques are also discussed in this report.

Introduction Spray drying, as the name suggests is a process of drying utilizing spray mechanism. This process is a largely used industrial operation found in various industries. It is engaged in producing powders from liquid feedstocks in a single step. The liquid feedstock which can be a solution, emulsion or suspension is sprayed through a nozzle in a drying chamber where hot air is flown simultaneously. It can be used for equally heat-resistant and heat sensitive products. In the spray dryer heated gas mixes with an atomized liquid stream to accomplish evaporation easily. And dry powder is produced with controlled average particle size at last. The unit operation of spray dryer includes following steps: 

Atomization of liquid stream- by atomization we mean separating something into fine particles. It is the process of breaking bulk liquids into droplets. Primary functions of atomization are gaining high surface to mass ratio resulting in high evaporation rates,



production of particles of desired shape, size and density. Heating of gas/air- Blower is set on the top of the dryer which draws air. Drawn air is heated using heating coils. 1



Mixing of gas/air and atomized stream- This should be done for adequate residence time



for achieving high heat and mass transfer. Separating and collecting the dried powder from the gas stream.

The unique feature of a spray dryer is the surface area per unit weight generated by atomization of the liquid feed which enables easy and higher evaporation rate. To see a real life exam, let’s look at milk. The ability to keep cold large quantities of a product like milk is limited. Moreover, milk has 80-90% water, by spray drying it, one can reduce its bulk weight. Spray drying 100kg of milk can possibly reduce that weight to 12kg, which offers savings in shipping expenses. In this experiment, 5% solution of milk and water is fed to the spray dryer through atomizer. The atomized stream then comes in contact with compressed air and creates mist. The mist comes into contact with hot air in drying chamber. The temperature of fine droplets increases instantaneously and solvent vaporizes. Increase in temperature of solvent and subsequent vaporization counts for the heat transfer in the process. Once solvent vaporizes, it passes into dry air which can be accounted as mass transfer. As a result, it can be said that spray drying includes both heat and mass transfer processes. The humid air passes to the atmosphere and dry air and powder mixture enters cyclone separator from which powder is recovered. In cyclone separator centrifugal action is used to separate powder from the mixture. The flow of air to the dryer can be co-current or counter-current to the atomizer direction. In the latter case, the residence time of the particles in the chamber is greater than the former case. In our experiment air flew in the counter current direction to the atomizer. Spray drying process operates in one step which makes it very popular these days as in food industries rapid drying is required. This process is versatile and particle size can be controlled too. By changing inlet temperature, hot air flow rate, fluid volume, fluid velocity the quality of final product can be controlled. This feature made spray drying very widespread in pharmaceutical industries too. But in this process high maintenance is required while dealing with nozzles used. Nozzles are prone to clogging and abrasion can also occur at the opening of nozzles. Rotary disc atomizers if used, there is chance of internal corrosion. Finally, powder may stick to internal chamber contributing to cleaning cost and low recovery.

Experimental work 2

Apparatus 

        

Spray Dryer o Drying Chamber o Air blower o Atomizer o Air heater Feed tank Temperature gauge Pressure gauge Beaker Milk-water solution Stop watch Thermometer Cyclone separator Dry air

Experimental Set up

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Figure-01: Schematic diagram of experimental setup of a spray drying system

Figure 02: Two fluid nozzle

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Figure-03: Cyclone Separator

Experimental Procedure   

About 50g milk powder was taken in a beaker by weighing 950gm of water was added to the beaker and mixed to prepare 5% solution The spray dryer was started by by turning the hot air blower on. The heater was adjusted

  

according to requirements. The operating atomizer pressure was set to 20 psi. The milk powder was separated in a cyclone separator and was collected in a bottle. Weight of recovered powder was measured and by comparing to initial weight percentage of recovery was calculated.

Process Block Diagram 5

Air

Blower

Heater

Water Hot air Air Spray Mixer Milk Powder

Dryer Milk Solution (5%)

Compressor

Compressed air

Air Milk Powder

Exhaust air Cyclone Separator

Product Milk Powder

Figure-04: Process block diagram of a spray drying system

Process Flow Diagram 6

Blower

Feed container

Air

Water

Milk

Heater Feed

Mixing unit

solution Hot air

Flow controller

Spray dryer Exhaust air

Compressed air Dried milk & air

Pressure controller Air

Cyclone Separator Recovered milk powder Compressor Figure-05: Process flow diagram of a spray drying system

Observed Data Weight measured 7

Weight of milk powder taken, w1 = 50 g Weight of water, w2 = 950 g So, wt % of prepared milk solution = 5% Weight of the container, wc = 561.6 g Weight of container + recovered milk powder, wc+m = 574.6 g Temperature measured Initial inlet temperature of air,TIi = 150°C Initial outlet temperature of air, TIo = 80°C Final inlet temperature of air, TFi = 150°C Final outlet temperature of air, TFo = 66.5°C Pressure measured (P) Working pressure of atomizing air = 20 psia Time measured Required time for drying the solution, t = 25 minutes

Calculation Data calculation: Weight of liquor milk = W1 + W2 = (50.1+950.) g = 1000.1 g Weight of milk powder recovered, = (Weight of beaker with recovered milk-Weight of empty beaker) Wp= Wc+m-Wc = (574.6-561.6) g =13 g

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Percentage of milk powder recovered = (Weight of milk powder recovered)/(Weight of milk powder initially taken)×100 % = (13/50.1)×100 % = 26 % Weight of water W2= 950 g Total time required for drying, t = 25.0 min = (25×60) sec = 1500 sec Drying rate = (Weight of water, Mw)/( Total time required for drying ,t) = 950/25 = 38 g min-1

Result Percentage recovery of milk powder = 26 % The drying rate = 38 g/min

Discussions The percentage of recovery was 26% and drying rate is 38g/min. from the values we can easily tell that the recovery of powder is not satisfactory. The possible reasons of this low recovery are enlisted below:   

Some dried milk may accumulate in the apparatus The flow of feed was controlled manually which resulted in low recovery Dry milk powder is assumed to pass to the atmosphere with exhaust air which resulted in



loss. In spray dryer heat transfer mode is convection and so it shows low thermal efficiency.



Heat is lost in the discharged air. A good amount of particles had been attached within the walls of spray dryer and also at cyclone separator. 9



Feed may have entered the dryer at high flow rate which reduced contact period between



droplets and dry air and thus affected evaporation process. The pressure of the atomizing air and the working pressure of spray dryer should have been maintained at constant value. If the atomizing air pressure were higher, the feed would have been broken into finer particles, resulting in efficient drying.

Modifications To make drying more efficient in spray drying process the following measures can be taken. Such as      

Allowing lengthier contact time. Using bigger separator. Lower flow rate feed. Increasing atomizing pressure. Ensuring better insulation. Using a dryer with smoother wall, and using smooth pipes with fewer bends. Replacing spray nozzle with rotary atomizer can increase contact area between



drying air and feed ensuring more mass transfer. If flowing area of drying agent can be increased, more vaporization will occur.

Some instrument can be used for better result. Such asBag filter

10

Figure-06: Bag Filter

Bag filters work very simply; air is pulled through the filter drawn by the exhaust fan powder collects on the bags while air is allowed to pass through the filter medium. A compressed air 11

jet frees the powder from the bag and drops into a hopper and out of the system. It reduces particle degradation and it is useful for low density powders. Wet scrubber:

Figure-07: Wet Scrubber

Dust collectors that use liquid are commonly known as wet scrubbers. In these systems, the scrubbing liquid (usually water) comes into contact with a gas stream containing dust particles. The greater the contact of the gas and liquid streams, the higher the dust removal efficiency. It ensures high efficiency and low maintenance cost. Higher level of instrumentation is usually required for density and level control.

Electrostatic precipitators (ESP)

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An ESP is an emission control unit largely used in various industries e.g. power/electric, cement, chemical, metal, paper. in many industrial plants, particular matter created in the industrial process is carried as dust in the hot exhaust gases. These gases pass through an electrostatic precipitator which removes most of the dust. Cleaned gas then gets out of the precipitator and passes to the atmosphere. Precipitators usually accumulate 99.9% or more of the dust from the gas stream. Precipitators function electrostatically by charging the dust particles of the gas stream. The charged particles are then attracted to and deposited on plates or other collection devices. When enough dust has been collected, the collectors are shaken to remove the dust. The dust is then removed by a conveyor system for discarding or reusing.

References Books 1.Foust et al: Principles of Unit Operations, 2nd ed, John Wiley & sons, New York (1994) 2.Masters, K.: Spray Drying Handbook, 4th ed., George Godwin, London, (1985) 3. McCabe, W.L., Smith, J.C. Harriot, P.: Unit Operation of Chemical Engineering, 6th ed., Mcgraw-Hill, Singapure, (2001) Websites https://www.academia.edu/3192734/Spray_Drying_as_an_Appropriate_Technology_for_ the_Food_and_Pharmaceutical_Industries  http://www.eurotherm.com/industries/life-sciences/applications/spray-drying/  https://www.neundorfer.com/knowledge_base/electrostatic_precipitators.aspx  https://www.corrosionpedia.com/definition/120/atomization  http://www.fortitechpremixes.com/formulation/what-is-spray-drying/  http://www.spraydrysys.com/spray-dryers/spray-dryers.htm

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Nomenclature Symbol

Significance

Unit

T

Temperature

⁰C

W

Weight

g

t

Time

second

Marking Scheme: Normal Report Name: Mahe Rukh Student number: 1202036 Section and marks allocated

Marks 14

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)

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