Humidity and Drying Flashcards
1
Q
Name three advantages of drying a product
A
1) Reduce transport costs - water adds weight 2) Maintain product properties (eg. salt will flow when dry) 3) Stop corrosion
2
Q
Define absolute humidity
A
The mass ratio of water to dry air.
3
Q
Define percentage humidity
A
H0 is the humidity of the gas when it is saturated with water vapour (for a given temperature).
Percentage humidity is the ratio of the humidity to the maximum humidity at that temperature.
4
Q
Define dew point
A
The temperature at which condensation will first occur. This is the temperature at which the gas is saturated with vapour, so if it goes any cooler the liquid will come out of the air.
5
Q
Define vapour pressure
A
This is denoted by pw (w for water) and is the pressure of the water vapour in the bulk gas.
6
Q
Define saturated vapour pressure.
A
This is denoted by p0 or p* and is the partial pressure of the vapour immediately above the surface of the liquid. It is also equal to the partial pressure of the vapour when the relative humidity is 100 %.
7
Q
Show how humidity relates to partial pressure.
A
1) Show how to find kmol of vapour and dry air (pV=nRT). Pressure for vapour = pw and pressure for dry air = (p-pw)
2) Show how to find kg of vapour and dry air (n=m/Mr)
3) Substitute these equations into the definition for absolute humidity.
8
Q
Define relative humidity
A
This is not the same as absolute humidity, but relates to the partial pressure of the vapour when the gas is saturated (for a given temperature).
9
Q
Demonstrate the relationship between percentage and relative humidity.
A
Use the equations for H and H0 in terms of partial pressures and substitute into the equation for percentage humidity.
10
Q
Under what conditions does percentage humidity equal the relative humidity?
A
1) If both pw and p0 are much less than p
2) If pw and p0 are approximately equal. This usually applies to air water systems.
11
Q
What is the wet bulb temperature?
A
The temperature the air would have if it had been cooled to it’s dew point by the evaporation of water into it.
12
Q
Define humid volume
A
The volume occupied by a unit mass of dry gas and its assiciated vapour.
13
Q
How do you find the dew point using a psychrometric chart?
A
The humidity remains constant. Therefore we can follow the humidity line until 100 % relative humidity is reached and read off the wet bulb temperature.
14
Q
How do you find the vapour pressure in the mixture using a psychrometric chart?
A
For a constant temperature, follow the dry bulb temperature line to 100 % relative humidity and read off the absolute humidity. Then use the equation for relating vapour pressure to humidity, where p is equal to 101325 because the psychrometric chart is only valid at this pressure.
15
Q
What is the driving force for evaporation?
A
The difference between saturated vapour pressure and vapour pressure.
16
Q
What is the equation for the rate of evaporation?
A
in kg/s
17
Q
What is the rate of heat transfer for evaporation?
A
Q=hA(T-Tw)
Tw is the liquid temperature, and T the gas temperature.
18
Q
Write the energy balance for evaporating water.
A
Where lambda is the latent heat of vapourisation.
19
Q
What is the first step to answering a drying question?
A
Draw the BFD
20
Q
How do you find how much water is removed per pass using the psychrometric chart?
A
Find the humidity at points before and after the dryer, and find the difference.
21
Q
Which line on the psychrometric chart can be followed if there is no water added or removed?
A
The humidity line.
22
Q
Draw a tray dryer
A
23
Q
What are tray dryers used for?
A
Granular materials or individual articles such as pharmaceuticals.
24
Q
Describe briefly how a tray dryer works
A
The material is placed on trays and heated from below by a series of coils containing steam. Drying is carried out by the circulation of air over the material, and the moisture in the material trnasferring from the solid to the gas.
Recirculation often takes place, with the air being reheated between each pass. The temperature and humidity of the air changes with each pass, the humidity increasing when in contact with the solid, and temperature increasing when it is reheated.
25
Draw a diagram to explain how the humidity and temperature of air changes with each pass in a tray dryer.
The line drawn is the adiabatic cooling line for various relative humidities. The point H shows the initial temperature of the air should the same drying being achieved with a single pass.
26
State the three advantages of reheating air when drying.
1) Less air is srequired because each kg picks up more water than in a single stage system
2) The maximum temperature required is lower
3) The tendency for the air to carry away particles is reduced due to the lesser air flow
27
Give some typical dimensions of a dryer tray.
0.3-1 m2 with a depth of 10-100 mm depending on the particle size
28
Give the typical air velocities for an air dryer and the typical percentage that is recirculated.
1-10 m/s with 85-95 % recirculated
29
State the typical amounts of steam required by a tray dryer for each kg of moisture removed.
2.5-3 kg/kg moisture removed
30
State the 3 things that tray dryer capacity depends on.
material, loading method and external conditions
31
Is a tray dryer a batch or continuous process?
Batch
32
Does a tray dryer have a high or low labour requirement?
High
33
What kinds of quantities and material is a tray dryer suitable for?
High quantities of valuable products
34
Draw a diagram of a tunnel dryer.
35
Draw a diagram of a conveyor dryer
36
How do you change the configuration of a tunnel dryer from counter-current to co-current?
Change the position of the heater and blower.
37
How does material move through a tunnel dryer?
On a series of trays
38
How does drying take place in a tunnel dryer?
In a current of warm air
39
What are tunnel and conveyor dryers used for?
Large throughputs where individual cabinets in a tray dryer are too labour intensive.
40
What are the costs like for a tunnel or conveyor dryer?
High initial and maintenance costs.
41
How does the material travel through a conveyor dryer?
On a conveyor belt
42
Give one advantage of a tunnel or conveyor dryer
Flow and temperature are easy to control
43
What operating condition is suitable for a converyor dryer.
Vacuum operation
44
What is the recommended maximum length of a converyor dryer?
30 m
45
What is a requirement for the solids put through a tunnel or conveyor dryer?
Must have an open, porous structure to achieve good rates, eg. granules or seeds
46
Draw a diagram of a rotary dryer including louvres and flights
47
Describe how a rotary dryer works
The solids are coveyed along a rotating, inclined cylinder which is sometimes heated. The feed is dried by contact with hot air.
48
State the costs of a rotary dryer
Low capital and labour costs
49
What kind of material is a rotary dryer suitable for?
Free-flowing granular materials
50
Name a problem with rotary dryers
Non-uniform residence times can cause segregation and varying moisture content of the products.
51
What is a suitable capacity for a rotary dryer?
Above 1 tonne/hr
52
What is a suitable rotating speed for a rotary drier?
up to 0.4 Hz
53
Can a rotary drier be switched to counter current flow?
Yes, by putting the air in flow at the opposite end to the feed inlet.
54
How cann indirect heating be used in a rotary drier?
By containing the material in an inner shell, and hot gas is injected into an external cylinder, or the inner shell is surrounded by steam tubes.
55
Describe the construction of a rotary dryer.
The shell is made of welded roll plate of mild and alloy steels which is thick enough to support the transmission of the torque required for the rotation and strong enough to hold the total weight.
It is often also coated with plastics to avoid contamination of the product.
The shell is supported on tyres that run on wide rollers.
56
Describe the principles of countercurrent operation in a rotary dryer
The gases are exhausted by a fan which means it operates at a slight vacuum, which makes sure dusty gases are prevented from escaping, and the solids are discharged at a high temperature.
Hot gases come into immediate contact with dried material, which means a lot of energy and space is used heating up the material rather than drying it, which means the solid can become excessively hot. This means the rate of heat transfer at the feed end is at a minimum and the solid is dried to a minimum overall.
57
What equipment does the exit gas of a rotary drier often go through?
A cyclone or scrubber to remove fine particles.
58
What kind of heating is used for a rotary drier?
Gas or oil fired heating.
59
What are the advantages of cocurrent operation in a rotary drier?
1) Rate of passage of material is greater
2) Rapid surface drying occurs - good if the material tends to stuck to the walls or if crystallisation is needed
3) Product leaves the dryer at a lower temperature than in countercurrent operation
4) Better for heat sensitive materials as the temperature of the gas falls rapidly
60
Describe the general function of a lifter in a rotary dryer
Distributes the matrial through the air so it has a more even contact time
61
Describe the two different types of lifter in a rotary drier
1) Flights - lift the material and shower it in the gas stream
2) Louvres - the gas stream enters via these at a certain angle, they are slim channels in the shell. They carry away the moisture at the end. This means the material constantly meets fresh air - which means this is neither co or countercureent flow, but through flow. The rotation of the shell maintains the agitation.
62
Give two examples of indirectly heated rotary dryer arrangements.
1) A single shell in a brick housing
2) Two shells in a housing, where it can also be arranged that there is direct heating on the return pass
63
Give one advantage and one disadvantage to using a rotary drier that is indirectly heated.
A) Better heat recovery
B) More expensive
64
Explain how steam tubes can be used in a rotary drier
They are fitted along the shell in concentric circles and rotate with the shell. The wet material comes into contact with very humid air, so surface drying is minimised, which could cause the solids to stick. It has a high thermal efficiency and can be made from corrosion resistant materials.
65
Describe a drum dryer
A slowly rotating steam heated drum that is heated on the inside, with the wet solids spread on the outside, which is where the evaporation takes place.
66
Describe the three main components of a drum dryer
feed method - may be a drip tray with an agitator to stop the solids settling, a pan or a splash type
spreader - sometimes used to produce a uniform coating on the drum
knife - used for removing the dried material
67
What type of feed is suitable for a drum dryer?
Very wet solids that act as a free-flowing material
68
What type of feed method is used is a light fluffy product is required, and why?
A splash-feed because it will be thrown against the revolving cylinder with a uniform coating where the materials will not stick to the hot drum.
69
Give advantages and requirements for a top feed to a drum dryer.
It gives a larger capacity because it gives a thicker coating on the drum. This is only possible with a double drum arrangement.
70
Draw a drum dryer with a single drum and a pan feed.
71
Draw a drum dryer with a double drum and a top feed.
72
What are drum dryers typically made of?
Usually cast iron, but chromium-plated or alloy steel is used when contamination is to be avoided.
73
Which way do the drums turn on a double drum dryer?
Upwards where they join, with the knife at the bottom to avoid dry solid coming into contact with the condensate.
74
What condition must a drum dryer be kept under if temperatures are to remain low, and how is this achieved?
Under a vacuum. This is achieved by putting the dryer in an outer casing with a vapour outlet.
75
Give the typical contact time and heat transfer coefficient for a drum dryer.
6-15s, 1-2 kW/m2K
76
What is the approximate maximum drum surface area for a drum dryer?
35 m2
77
Describe how a spray dryer works.
A mixture (liquid or dilute slurry) is sprayed into a bessed through which a current of hot gases is passed.
78
Give an advantage of a spray dryer and the consequences of this advantage.
Due to the spraying, the feed is given a large interfacial area. This means a high rate of evaporation, but high heating requirements.
79
What kind of material is a spray dryer suitable for?
Substances that may deteriorate if their temperatures rise too high (the drop temperatures will remain below the wet bulb temperature until drying is almost complete).
80
In what state is the material obtained from a spray dryer?
A finely divided state
81
What is the performance of a spray dryer dependent on?
It is critically dependent on the drop size produced by the atomiser and the manner in which the gas mixes with the drops.
82
What is the definition of an atomiser?
A device which causes liquid to be disintegrated into drops lying within a specified size range, and which controls their spatial distribution.
83
Describe how the drying of drops in a spray dryer occurs.
The drying depends on the rate of evaporation and the contact time between the drop and the gases.
84
What does the contact time in a spray dryer depend on?
The velocity of the fall of the drop and the length of the path it takes through the vessel.
85
What does the terminal velocity of a drop in a spray dryer depend on, and how does this affect how we model the evaporation?
The flow conditions around the drop and the angular position. This is very complex so we treat a drop as if it evaporates uniformly from all surfaces.
86
Describe the two main periods on evaporation in a spray dryer.
1) High transfer rates initially as initial velocity is much greater than the surrounding gas
2) Slower transfer rates when terminal velocity is achieved - most of the evaporation happens in this period, so period 1 can often be ignored
87
What kind of product is produced by a spray dryer?
A high grade, granular product.
88
What is the most commonly used atomiser?
Rotating disk
89
What are the differences between countercurrent and cocurrent and spray atomisers?
Countercurrent have high thermal efficiencies but product temperatures are higher.
Cocurrent have low thermal efficiencies but low product temperatures unless back-mixing occurs.
90
What kinds of gases are used in spray dryers?
Combustion gases are frequently used directly. If contamination is an issue, undirectly heated air is used, which gives lower maximum temperatures.
91
Draw a countercurrent spray dryer.
92
Draw a mixed flow spray dryer
93
Draw a cocurrent spray dryer.
94
Draw a parallel flow spray dryer.
95
Describe the four types of atomiser and the feed they are suitable for.
1) pressure nozzle (up to 50MN/m2) - suitable for low viscosity (up to 0.01 m2/s)
2) rotary nozzle - suitable for slurries and pastes in co-current flow
3) twin-fluid nozzle - suitable for all combinations
4) spinning cup and gas blast - suitable for high viscosity solutions, slurries and pastes in cocurrent flow
96
What would happen if you put a slurry through a pressure nozzle?
It may cause severe erosion - providing cause for frequent replacement.
97
What happens if you put a high viscosity feed through a spinning disc atomiser?
A stringy product would be produced and the nozzle must be designed to minimise incrustation.
98
Why are simple twin-fluid nozzles not yet used in industry?
They tend to produce a dusty product.
99
How is the product collected in a spray dryer?
Small particles that exit with the exhaust gases can be collcted using a cyclone, bag filters or electrostatic separators.
100
How would you collect a heat sensitive, sterile material from a spray dryer?
With streams of cool air and mechanical devices to reduce sticking, but this can cause breakage.
101
Describe the cost of a spray dryer.
Relativelt expensive, especially when using indirect heating or low inlet temperatures.
102
Describe how a pneumatic dryer works.
Feed is introduced by a mechanical rotating screw, or an extrusion machine with a high speed guillotine that will introduce short lengths of material. Hot gases from a furnace or oil burner enter at the bottom and pick up the solid particles, carrying them up the column. The particles will dry on this journey. The mix leaves through a cyclone (or occasionally a bag filter) where the dried solids are collected.
103
What must the condition of the feed to a pneumatic dryer be, and why?
In a state of fine division to keep the surface area per unit volume high.
104
Is a pneumatic dryer suitable for heat-sensitive materials and why?
Yes, because the particle temperature does not reach that of the gas stream.
105
Comment on the thermal efficiency of a pneumatic dryer.
The thermal efficiency is low because the particles are almost relatively stationary to the air. This means that heaps of air is being used and released. However, the evaporative capacity is high and depends on the solid-air ratio.
106
Draw a diagram of a pneumatic dryer.
107
When and why is a mill used in a pneumatic dryer?
Used before the dryer duct to break up agglomerates of a feed that is a mixture of the wet feed an dry product to obtain an initial flash evaporation - this creates a good recycle of solids.
108
How is the hot gas input rate controlled in a pneumatic dryer?
By measure the temperature of the exhaust coming from the dust collector or scrubber before they are vented to the atmosphere.
109
What kind of material is a fluidised bed dryer used for?
Granular, crystalline materials with a particle size of 1-3 mm.
110
Is a fluidised bed dryer used in continuous or batch operation?
Both
111
What is the heat transfer like in a fluidised bed dryer?
Rapid, uniform heat transfer which gives a short drying time.
112
What is the power requirement like in a fluidised bed dryer?
High
113
Decribe how a fluidised bed dryer works.
Heated air or hot gas is passed through a diffuser plate, through nozzles that prevent back-flow of solids, upwards into a fluidised bed of material, then into a dust separator and out through the exhaust. Wet material is fed continuously via a rotary valve and mixes immediately with air.
114
Describe the two configurations that solids can be transferred in a fluidised bed dryer.
Either a thick bed of solids contained by a weir, or a long thin bed.
115
Describe the mixing of the feed into the bed of a fluidised bed dryer.
Homogenous mixture
116
Describe the residence time distribution in a fluidised bed dryer.
Similar for all particles
117
Describe the heat transfer rate in a fluidised bed.
High heat transfer rates make it possible to maintain the whole bed in a dry condition.
118
Give examples of the type of feed used in a fluidised bed dryer.
fertilisers, plastics, sand, salts, tablet granulations
119
Draw a diagram of a fluidised bed dryer.
120
Draw the graph of dry-basis moisture content against time for the drying of a solid.
121
Describe the 3 drying stages for a solid. What is the state of the solid at the end of stages 2 and 3?
Stage 1: the material warms up, the evaporation rate increases as the temperature increases
Step 2: constant drying rate period. There is a pseudo-steady state between the surface water on the particle and the wet bulb temperature. At the end of this stage the critical moisture content has been reached - water has run out on the surface of the solid.
Step 3: falling rate period. Surface temperature increases so the drying rate falls. Water is removed from the pores of the material. At the end of this stage the final moisture content has been reached, in equilibrium with the air. This depends on the material, temperature and humidity of the contacting air, so will not get to 0.
122
What is the free moisture content?
This is the moisture content above the equilibrium moisture content. This is the moisture that can be removed from the solid.
123
Draw the graph of drying rate against the dry basis moisture content (derivative of the drying rate curve).
124
How do you find the duration of the constant rate drying period from the derivative of the drying curve?
If rate=R, D=weight of bone dry material, DX=weight of the water in the material, A=surface area
125
How do you find the duration of the falling rate drying period from the derivative of the drying curve?
Assume a straight line of gradient 1. Therefore (R/RC)=(X/XC), rearranged for R, with dt as a variable on it's own.
