15 evaporation transpiration sublimation

UNZA Pharmacy Training Lecture notes

2. 6/21/2020 2
EVAPORATION, TRANSPIRATION & SUBLIMATION

3. Evaporation,
Transpiration,Sublimation
Processes by which water
changes phase-
Liquid or solid to gas vapor

4. Learning Objectives: Evapotranspiration (ET)
•Learn what conditions are necessary for evaporation to occur
•Learn what factors control evaporation rates
•Learn how to measure ET
•Learn where to find or how to compute variables needed to
estimate ET
•Understand the difference between
potential evapotranspiration (PET) and
actual evapotranspiration (AET)
•Understand the difference between evaporation and
transpiration
•Learn what factors control transpiration
•Become aware of common equations used to estimate ET
•Understand how ET varies in time and space

5. Evaporation
•Phase change liquid to gas
•Hydrogen bonds broken – vapor diffuses
from higher to lower vapor pressure
•At an open water surface, net evaporation
= 0-bonds constantly forming and
breaking
•Most takes place over open water
surfaces such as lakes and oceans
weather.cod.edu/karl/Unit2_Lecture1.ppt

6. What controls evaporation?
1.Energy inputs
2.Temperature
3.Humidity
4.Wind
5.Water availability

7. What controls evaporation?
•Evaporation is energy intensive- latent
heat of vaporization is 540 cal/gram
•Provided mainly by
–Solar energy -radiation
–Sensible heat –temperature –transferred via
conduction and convection
–kinetic energy of water –internal energy, heat

8. Energy Budget
•Net radiation: R
netis
determined by measuring
incoming & outgoing
short- & long- wave rad.
over a surface.
•R
net can –or +
•If R
net> 0 then can be
allocated at a surface as
follows:
R
net= (L)(E) + H + G + P
s
L-is latent heat of vaporization,
E-evaporation,
H-energy flux that heats the air or
sensible heat,
G-is heat of conduction to ground and
Ps-is energy of photosynthesis.
LE-represents energy available for
evaporating water
R
net-is the primary source for ET & snow
melt
.

9. •In a watershed R
net,
(LE) latent heat and
sensible heat (H) are of
interest.
•Sensible heat can be
substantial in a
watershed, Oasis effect
where a well- watered
plant community can
receive large amounts
of sensible heat from
the surrounding dry,
hot desert.
•Advection is movement of
warm air to cooler plant-
soil-water surfaces.
•Convection is the vertical
component of sensible-
heat transfer.

10. What controls evaporation?
1.Energy inputs
2.Temperature
3.Vapor content
4.Wind
5.Water availability

11. Temperature
•Measure of heat energy
•Affects vapor pressure- Saturation vapor
pressure increases with air temperature
–Can compute with an equation if know
temperature
•Saturation vapor pressure minus actual
vapor pressure = saturation deficit
–The amount of additional water vapor that air
can hold at a given temperature

12. What controls evaporation?
1.Energy inputs
2.Temperature
3.Vapor content
4.Wind
5.Water availability

13. Measuring the Vapor Content
•There are a number of ways that we can
measure and express the amount of water vapor
content in the atmosphere:
–Vapor Pressure
–Mixing Ratio
–Relative Humidity
–Dew Point
–Precipitable Water Vapor
–Others (absolute humidity, specific humidity)

14. Humidity can be describe in many ways, for example,
Measure symbol units
Volumetric concentration c
wv mol m
-3
Vapor pressure e
a, also p
H2O kPa
(the partial pressure of H
2O vapor)
Relative humidity RH %
=(e
a/e
s)* 100, where e
s
is saturation vapor pressure
Vapor pressure deficitVPD kPa
=e
s–e
a

15. Vapor Pressure (e)
•Vapor pressure (e) is simply the amount of
pressure exerted only by the water vapor
in the air
•The pressures exerted by all the other
gases are not considered
•The unit for vapor pressure will be in units
of pressure (millibars and hectopascals
are the same value with a different name)

16. Relative Humidity (RH)
•The relative humidity (RH) is calculated using the actual water
vapor content in the air (mixing ratio) and the amount of water
vapor that could be present in the air if it were saturated
(saturation mixing ratio)
•RH = w/w
sx 100%
•The relative humidity is simply what percentage the
atmosphere is towards being saturated
•Relative humidity is nota good measure of exactly how much
water vapor is present (50% relative humidity at a
temperature of 80 degrees Fahrenheit will involve more water
vapor than 50% relative humidity at -40 degrees)
•Relative humidity can change even when the amount of water
vapor has not changed (when the temperature changes and
the saturation mixing ratio changes as a result)

17. Dew Point (T
d)
•The dew point temperature is the temperature at
which the air will become saturated if the
pressure and water vapor content remain the
same
•The higher the dew point, the more water vapor
that is present in the atmosphere
•The temperature is always greater than the dew
point unless the air is saturated (when the
temperature and dew point are equal)

18. Precipitable Water Vapor (PWV)
•Precipitable water vapor (PWV) is the amount of
water vapor present in a column above the
surface of the Earth
•Measured in units of inches or millimeters
•It represents the maximum amount of water that
could fall down to the surface as precipitation if
all the water vapor converted into a liquid or a
solid
•Can be measured easily by weather balloons or
satellites

19. What controls evaporation?
1.Energy inputs
2.Temperature
3.Vapor content
4.Wind
5.Water Availability

20. Wind
•Creates turbulent diffusion and maintains
vapor pressure gradient
•Turbulence a function of wind velocity and
surface roughness
•Evaporation can increase substantially
with turbulence up to some limit that is a
function of energy, temperature and
humidity

21. Additional factors affecting
evaporation from free water surface
•Water quality
–More salinity means less evaporation
•Depth of water body
–Deep lakes have more evap in winter
•High heat capacity means lake water warmer that
air temperature
–Shallow lakes cool fast in fall and freeze
•No evap in winter

22. What controls evaporation?
1.Energy inputs
2.Temperature
3.Vapor content
4.Wind
5.Water Availability

23. Additional factors affecting
evaporation from free water surface
•Area of water body
–More evap from larger surface area but rate
decreases upwind as air picks up vapor
•Maximum rates from small, shallow lakes
in dry climates

24. Evaporation from soil
•Same factors drive the process as in open
water
1. Soil moisture also important
–Evap rates decrease as surface dries
2. Soil texture: affects soil moisture content
and capillary forces
–E.g., Fine soil- retains moisture, rates high at
first but then depends on capillary forces

25. Evaporation from soil
3.Soil color –affects albedo and thus energy
inputs
4.Depth to water table
-If shallow such as wetlands, almost unlimited
evaportation
5.Vegetation
-provides shade- limits insolation (energy and heat)
-reduces windspeed at ground level
-increase vapor pressure through transpiration

26. How do we measure/estimate
evaporation?
1.Direct measurement
–Pans
–Lake water balance
–Lysimeters

27. Pan evaporation
•Class A pan – 4 feet diameter, 10 inches
deep- galvanized steel – measure daily
water loss by adding water to same level
•Evap = change in water level -
precipitation
•Pan evap > lake evap why?
•Use a pan coefficient (usually 0.6- 0.8)
•Map of pan evap

31. Soil lysimeter
•Water tight box on a scale or pressure
transducer
•If only soil and water, loss of weight is due
to evaporation of water
•Evap = change in weight –precipitation
•Either prevent seepage or collect and
measure

33. Transpiration
•Evaporation from plants
•Water vapor escapes when stomata open for
photosynthesis, need carbon dioxide
•Related to density and size of vegetation, soil
moisture, depth to water, soil structure
•Of the water taken up by plants, ~95% is
returned to the atmosphere through their
stomata (only 5% is turned into biomass!)

34. Water Availability
•An open water surface provides a
continuous water source
•Transpiration can provide water up until a
certain limit based upon the plant’s ability
to pull water up through its roots and out
its stomatae (rate of transpiration)

35. Water movement in plants
•Illustration of the energy
differentials which drive
the water movement from
the soil, into the roots, up
the stalk, into the leaves
and out into the
atmosphere. The water
moves from a less
negative soil moisture
tension to a more
negative tension in the
atmosphere.

36. The driving force
of transpiration is
the “vapor
pressure
gradient.” This is
the difference in
vapor pressure
between the
internal spaces in
the leaf and the
atmosphere
around the leaf

37. Stomatal conductance balances the
atmospheric demandfor evaporation with the
hydraulic capacity to supplywater
SUPPLY
Flow of liquid water =
(Ψleaf –Ψsoil) * K
DEMAND: VPD
Transpiration =
≈VPD * LAI * leaf conductance

38. Leaf Conductance
•Ease of water loss affected by leaf
conductance
•Conductance a function of
–light,
–carbon dioxide concentration,
–vapor pressure deficit,
–leaf temperature and
–leaf water content

41. Effects of Vegetative Cover

42. Evapotranspiration -ET
•Hard to separate evaporative loss from
transpiration loss in wildland situations
•Look at ET (evapotranspiration)
•AET –Actual ET
•PET –Potential ET

43. PET –Potential Evapotranspiration
•Rate at which ET would occur in a
situation of unlimited water supply, uniform
vegetation cover, no wind or heat storage
effects
•First used for climate classification criteria
•Usually assume short grass as the uniform
vegetation
•Compute as function of climate factors

44. Actual Evapotranspiration
•Amount actually lost from the surface
given the prevailing atmospheric and
ground conditions
•Provides information of soil moisture
conditions and the local water balance
•Measured by a lysimeter (difficult to
maintain, not many in existence) that
weighs the grass, soil, and water above

45. PET equations
•Penman- Monteith (based on radiation balance)
•Jensen- Haise (developed for dry, intermountain
west)
•Priestly- Taylor (based on radiation balance)
•Thornthwaite (based on temperature)
•Hamon, Malstrom (based on T and saturated
vapor pressure)
•See table 4.3 p 95 in text

46. Physically-based theoretical
methods-e.g. Penman Monteith
•Energy budget
–Mass balance on energy inputs and outputs
–Incoming solar radiation –reflected solar
radiation (albedo) –net longwave radiation +
net energy advected to vegetation = ET
energy (latent heat) + sensible heat transfer
from veg to air + changes in energy storage in
heating soil and veg
–Can measure all but latent heat which equals
ET

47. Physically-based methods
•Turbulent mass transfer
–Function of wind speed and vapor pressure deficit
–Evap = k u
z( e
w –e
z)
–K is a constant, U is wind velocity, e is vapor
pressure, z is some reference height, w is level at
water surface
•Can only measure precisely over short distances
–Useful only for experimental situations

48. AET equations
•Blainey- Criddle
–Good for crops and ag situations
–f = tp/100
•f is consumptive factor, t is mean monthly air temperature in
Fahrenheit (tmax + tmin/2)
•p is mean monthly percentage of annual daytime hours
•Compute f for each month of interest
–U = KΣf
i
•Where U is total consumptive use in inches per season
–K is crop coefficient, sum over the number of months of growth

49. Variables used in common
ET models
Model T RH or e Lat Elev Rad. Wind
Penman x x x x x
Priestly- Taylor x x
Jensen- Haise x x x
Blainey-Criddle x x
Thornthwaite x

50. JAWRA 2005
(mm/yr)

51. fine soils with
ample soil-moisture
storage, warm
summers, cool
winters, and little
change in
precipitation
throughout the year
coarse soils
with limited soil-
moisture storage,
warm, dry summers,
and cool, moist
winters.

52. Available Soil Water

53. Evapotranspiration
•> 70% annual
precipitation in the US
•In General: ET/P is
–~ 1 for dry conditions
–ET/P < 1 for humid
climates & ET is
governed by available
energy rather than
availability of water
•ET affects water yield
by affecting antecedent
water status of a
watershed high ET
result in large storage
to store part of
precipitation
http://www.ctahr.hawaii.edu/faresa/courses/nrem600/10-02%20Lecture.ppt

54. Human effects
•Change in vegetation affects ET
–Agriculture, horticulture, urbanization,
deforestation, etc.
•Change in climate will affect ET
–Think about the factors that affect ET
•Reservoir storage affects ET
–By 2000, Evap losses were greater than total
domestic use in 1950 and is increasing

55. Evaporation for
pharmaceutical processes
It is the vaporization from the surface of liquid.
The rate of vaporization depends on:
The diffusion of vapor through the boundary layers above the
liquid ' takes a long time ' the liquid will be rotten.
Therefore, the process needs heat to:
1. Provide the latent heat of vaporization.
2. Enhance rate of evaporation (no heat slow vaporization).
Evaporation:
is the removal of liquid from a solution by boiling the liquor in a
suitable vessel and withdrawing vapor leaving a conc liquid.
The rate of evaporation is controlled by:
the rate of heat transfer.
Evaporators are designed to give maximum heat transfer to
liquid by:
(i) increasing the surface area
(ii) reducing the boundary layers.

56. •The rate of heat transfer (q) through the
heating surface of an evaporator:
q = U A ΔT
q= rate of heat transfer (quantity/ time),
A= area of heat transfer,
U= overall heat transfer coefficient,
ΔT= mean temp difference between heating
element and liquid to be evaporated.

57. A-NATURAL CIRCULATION EVAPORATORS
1-Evaporating Pan
•Itconsistsof:Aninnerpan(liner)
envelopedbyouterpan(Jacket).
•Theyarejoinedsoastoenclosea
spacethroughwhichsteamispassed.
•Theinnerpanismadefromcopper
(highthermalconductivity).
•Ifacidicmaterialisused,coppermust
betinnedorusestainlessstealpan.
•Thejacketismadefromcastiron(↓↓
thermalconductivity)to↓heatloss.
•Thepanishemisphericaltogive
bestsurfacearea/volumeforheating.
Advantages:
1-Simple, 2-Cheap for construct,
3-Easy to use, clean and to maintain.

58. •Disadvantages:
1-Natural circulation only  ↓in rate of heat
transfer.. WHY?
(i) Overall heat transfer coefficient will be poor,
(ii) Solids deposit on liner surface decomposition.
2-The heating surface of liner (pan) is limitedand
the surface area decreased as the product becomes
concentrated.
3-Thepanisopenedthevaporpassesto
atmospheresandsaturateitslowevaporation
processanddiscomfortforworkers.
4-Notsuitableforthermolabilesubstancesor
thosedissolvedinorganicsolvents(alcohols)
Thesolutionisheatedalltime.
5-Limiteduseforaqueoussolutionand
thermostableliquors(Ext.ofliquorice).

59. 2-Evaporating Still
•Itisverysimilarto
evaporatingpan in
compositionwithcover
thatconnectstoa
condenser.. WHY?
Thevaporofsolutioncan
berecovered(economic)
 widelyusedin
pharmaceuticalindustry.

60. •Advantages:
1-Simpleinconstruct,easytocleanand
maintenance.
2-Condensedvaporspeedstheevaporationand
allowstheequipmenttobeusedforsolventsother
thanwater.
3-Receiverandvacuumpumpcanbefittedtothe
condenserusefulforthermolabilesubstances.
•Disadvantages:
1-Naturalcirculationonly.
2-Allliquidisheatedallthetime.
3-Theheatingsurfaceislimited.

61. A-Horizontal tube evaporator
•Thebodyofevaporatorisavertical
cylinderclosedattopandbottom.
•Steamisintroducedintosteam
compartmentanditflowsthroughthe
tubesitwashesbothnon-condensed
gasesaswellascondensateandnolive
steamgetoutside.
•Thefeedentersatmiddlepointoverthe
tubes.
•Thevaporescapesatthecenterofthe
top.
•Thickliquoriscollectedfromtheliquor
outlet.

62. •Advantages:
1.Relativelowinitialcost.
2.Requirelowheadroomandeasytoconstruct.
3-Usedaseitherbatchorcontinuosevaporators.
Disadvantages:
1.Poorcirculation(natural).
2. Not suitable for:
•Viscous liquids.
•Solutions that crystallize on concentration.
•
Thermolabile substances because evaporation takes a
long time.

63. B-Vertical tube evaporator
•Thebodyofevaporatorresembles
thehorizontaltubeevaporator.
•Theheatingelement[calandria]
consists ofannularsteam
compartmentintowhichissealeda
nestofnumerousverticaltubes
havingbothendsopen.
•Asthefeedisintroducedintothe
evaporator,itrisesintothetubesand
willbeheatedbythesurrounding
steam.
•Whensufficientlyhot,acirculationis
setwherethefeedboilsandspouts
upwardoutofthetopoftubes(like
liquidboilsintesttube)andreturn
downthecentraldowntakeandthe
processcontinues.

64. •Advantages:
1. Large heating area (tubular calandria).
2. Suitable forliquid forming deposits (sugar industry) because
vigorous circulation scrapes any solid from the surface.
3-Can be used as either batch or continuos process.
4-Condenser, receiver and vacuum pump could be used.
Disadvantages:
1-The evaporator is complicated, expensive to construct,
difficult in cleaning.
2-Not suitable for thermolabile substancesas a result of
increased pressure at the bottom of the vessel caused by the
head of liquid and the liquor depth (~ 2 meter).
3-Certain amount of liquid is heated for long time..
This could be prevented by slowly removing concentrated
liquor from bottom.

65. B-FORCED CIRCULATION EVAPORATORS
•Theyarenaturalcirculation
evaporatorswithmechanical
agitation.
•Itisanevaporatingpaninwhich
thecontentsareagitatedby
stirring.
•Agitationcandonebyeither:
apropeller(paddle)mountedin
pan
oracirculatingpump.
•Theliquoriscirculatedbymeans
ofthepumpandasitisunder
pressureinthetube,sotheboiling
pointiselevated.

66. •Astheliquorleavesthetubesandentersthebodyof
evaporator,thereisadropinpressureandvaporflashes
givingsuperheatedliquor.
Advantages:
1.Highdegreeofconcentrationisachieved.
2. Used for:
a. Scale forming agentsbecause movement removes scales.
b. Liquids tending to froth.
c. Viscous materialsas it can work under reduced pressure.
d. Thermolabile substancesdue to the rapid evaporation rate.

67. C-FILM EVAPORATORS
1-Climbing Film Evaporator
•Anumberofverticaltubesabout6m
lengthand5cmindiameterare
enclosedinanouterjackettowhich
steamissupplied.
•Thefeedispreheatedandadmittedat
thebase,afilmofliquidformsonthe
wallsandrisesupthetubes(climbing).
•Attheupperend,themixtureofvapor
andconcentratedliquorentersa
separator,
Thevaporpassingoncondenser
Theconcentratedliquortoreceiver.

68. Advantages:
1-The film of high velocity giving better heat transfer.
2-Large surface area of heat transfer (6 m).
3-Used for thermolabile substances and foaming forming
substances.
4-Time of contact between liquid and heating surface is very
short.
5-Evaporation rate is high due to its large surface area.
Disadvantages:
1 -Expensive in manufacture.
2-Difficult to clean (since replacement of tubes required high
headroom).
3-If feed rate is too high, the concentration will be
insufficient
If feed rate is too slow, the film can not maintained (dry on
the wall)

69. 2. Falling Film Evaporator
•Itresemblestheclimbingfilm
evaporatorbutinverted.
1.Thefeedentersoverawireat
thetopofthetubes,
2.Thevaporandconcentrate
leavefromthebottom.
Advantages:
•Usedforviscousmaterialsdue
totheirmovementhelpedby
gravity.

70. 3. Horizontal Film Evaporator
•This overcomes the
difficultyofreplacementof
7mlongtubes.
•Severalparallel2-5mlong
tubesarejoinedtogetherin
series.
•Thefeedpassesintheinner
tube.
•Eachtubeisenclosedinan
outertubethroughwhich
steamispassedinthe
oppositedirection[2opposite
directioncountercurrently].

71. Any Questions or Additions

72. THANK YOU

73. Definethefollowingterms:
[Evaporation,Sublimation, etc]
Respondtothefollowingquestions:
Giveadetailedaccountof………………
Explainindetailstheprocessof…………..
Describeindetailswithexamplesthe…………
Withexamples,illustratethepharmaceuticalapplicationsof……………

74. Groupworkdiscussionalquestions:
Explainindetailstheprocessof………
Describewithexamplesindetailsthe…………..
Withexamples,illustratethepharmaceuticalapplicationsof…….