an advanced course in food dehydration and drying · 3.6 case study #2: application of psychrometry...

AN ADVANCED COURSE INFOOD DEHYDRATION AND DRYING

Donald G. Mercer, Ph.D., P.Eng.

DRAFT COPY

Disclaimer

The author assumes no responsibility or liability for any problems of any mannerencountered through the application of the principles discussed herein.

All “Examples” and “Case Studies” are based on generic or hypothetical cases and donot represent any specific or proprietary processes in current or past use. Such

“Examples” and “Case Studies” are intended for instructional purposes only.

© Donald G. Mercer 2008Reproduction in whole or in part by any means,electronic or otherwise, is forbidden except with

the expressed permission of the author.

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Intermediate Course in Food Dehydration and Drying Outline: Page i.

AN ADVANCED COURSE IN FOOD DEHYDRATION AND DRYING

OUTLINE

CHAPTER 1: GETTING STARTED

1.1 Learning Objectives

1.2 Previous Course Material

1.3 Review: Mango Drying Case Study

CHAPTER 2: PROCESS CONTROL IN A DRYING OPERATION

2.1 Introduction

2.2 Basics of Process Control2.2.1 Moisture Targets and Limits2.2.2 Control Limits

2.3 Moisture Samling2.3.1 Grab Samples2.3.2 Continuous Monitoring

2.4 Case Study #1: Moisture Testing2.4.1 Background2.4.2 Assorted Problems and Shortcomings

2.4.2.1 Sample Size2.4.2.2 Sample Uniformity2.4.2.3 Sampling Interval and Timing2.4.2.4 Process Response Time

2.5 Process Control Mechanisms2.5.1 Feed-Back Control2.5.2 Feed-Forward Control2.5.3 Combined Feed-Forward and Feed-Back Control2.5.4 General Comment

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Intermediate Course in Food Dehydration and Drying Outline: Page ii.

CHAPTER 3: PSYCHROMETRICS

3.1 Introduction

3.2 Definitions and Associated Terminology

3.3 Psychrometric Charts

3.4 Sample Problems and Calculations3.4.1 Sample Problem #13.4.2 Sample Problem #23.4.3 Sample Problem #3

3.5 Practice Problems (with answers)

3.6 Case Study #2: Application of Psychrometry to Drying3.6.1 Problem Statement3.6.2 Solution

3.7 Case Study #3: Drying Feasibility Study3.7.1 Background3.7.2 Feasibility Study

3.8 General Comments

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Intermediate Course in Food Dehydration and Drying Outline: Page iii.

CHAPTER 4: Troubleshooting

4.1 Introduction

4.2 Drying Problems4.2.1 Uneven Drying4.2.2 Case Hardening4.2.3 Product Colour Changes4.2.4 “Sweating” in Storage Bins4.2.5 Finished Product Shrinkage4.2.6 Moisture Uptake After Drying4.2.7 Poor Dryer Performance4.2.8 Heat Losses from Dryer4.2.9 Uneven Bed Depth4.2.10 Holes in Bed of Material4.2.11 Variability Throughout the Day4.2.12 Seasonal Variations4.2.13 Lifting of the Bed4.2.14 Product-Specific Problems

4.3 General Comments

CHAPTER 5: SUMMARY COMMENTS

5.1 Introduction

5.2 Things to Keep in Mind

CHAPTER 6: SOURCES OF INFORMATION

6.1 Introduction

6.2 General References

ACKNOWLEDGEMENTS

During the preparation of the “AdvancedCourse in Food Dehydration and Drying”course manual, I have once again hadthe privilege of working with a multi-national team of dedicated professionalswho are members of the InternationalUnion of Food Science and Technology(IUFoST), the International Academy ofFood Science and Technology (IAFoST),and the South African Association ofFood Science and Technology(SAAFoST).

The support, encouragement, andenthusiasm of these individuals has beenphenomenal and I would like to publiclyacknowledge their efforts.

Dr. Daryl Lund (USA) is the Chair of theDistance Education Task Force (amongstmany other Food Science related duties).It is under his watchful eye and insightfulguidance that the concept of this DistanceEducation Initiative is becoming a reality.

Mrs. Judith Meech (Canada) is theSecretary General and Treasurer ofIUFoST. She has been a tremendousresource in establishing contacts andworking on the administrative aspects.

Dr. Walter Spiess (Germany) is a PastPresident of IUFoST and was the drivingforce in establishing the DistanceEducation Initiative for Sub-SaharanAfrica during his term as President. Hehas continued to be a strong supporterand advocate of our activities.

Dr. Ralph Blanchfield, MBE (UnitedKingdom) is the President of IAFoST. Hehas provided continuing support for thisInitiative, and reviewed the draft copy of

this course.

The South African Association of FoodScience and Technology (SAAFoST)collectively has been a dedicatedsupporter of this effort. Mr. OwenFrisby, Executive Director of SAAFoST,has worked tirelessly to promote theDistance Education Initiative. Owen’sencouragement has been extremelyimportant to me personally whilepreparing the Dehydration and DryingModule material, and I know many of theDistance Education Task Force membersshare this appreciation of Owen’s efforts.I feel very fortunate to have spent timewith Owen during my trip to Africa in 2006and I value the correspondence I havehad with him since that time.

Finally, I would like to acknowledge thesupport of my wife, Jane, without whosepatience and understanding I would nothave been able to do this work.

Donald G. Mercer

March 2008.

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Advanced Course in Food Dehydration and Drying Chapter 1: Page 1.

ADVANCED COURSE IN FOOD DEHYDRATION AND DRYING

CHAPTER 1: GETTING STARTED

1.1 Learning Objectives

This “Advanced Course in FoodDehydration and Drying” is intended to bea follow-up to the “Introductory” and“Intermediate” Food Dehydration andDrying Courses. It is offered as the finalcourse in this three-part module.

A number of topics will be covered tocomplement those already studied in theprevious two drying courses. Now thatthe basics of drying have hopefully beenmastered, a more “hands-on” approachwill be taken. Case studies based onpersonal practical experiences have beenincluded to illustrate the application ofpsychrometrics to drying.

Throughout this course, we will look at“troubleshooting” problems in dryingoperations and examine ways toovercome these problems. Please keepin mind that food drying is not a subjectthat can be learned solely from coursemanuals. Much of the learning comesfrom experience in an actual foodprocessing environment. Nothing cantake the place of this experience, and noinstruction manual, no matter howthoroughly it attempts to treat the topic,can ever hope to cover every singlesituation that may present itself to adrying process operator.

The chapter entitled “SummaryComments” also provides an overview ofdrying from a general perspective.

After completing the “Advanced Course inFood Dehydration and Drying”, youshould:

C understand the basic principles ofprocess control as applied to dryingoperations.

C be able to identify problemsassociated with improper drying offood products.

C be able to suggest potential causes ofimproperly dried products.

C be able to recommend possiblemeasures to correct drying problems.

C be able to assess the validity ofproduct testing for taking correctiveaction in a food drying process.

C be able to avoid (or minimize) many ofthe problems encountered by dryeroperators who are unfamiliar with fooddrying.

C understand the principles associatedwith psychrometrics and be able toperform basic associated calculations.

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1.2 Previous Course Material

A quick look at the topics covered in the“Introductory” and “Intermediate” dryingcourses is included here to refresh yourmemories and establish what subjectmatter should have been studied prior tobeginning this course.

Introductory Course

C Reasons for drying foodsC Factors influencing dryingC Effects of drying on productsC The “unit operation” approachC Process flow diagramsC Organization in problem solvingC Dimensional analysisC Wet and dry basis moisturesC Basic calculations

Intermediate Course

C Thermal properties of waterC Thermal properties of foodsC Sensible and latent heatsC Heat transfer mechanismsC Calculating the heat to dry a productC Stages of dryingC Critical moisture contentC Drying curvesC Scale-up of dryersC Types of dryersC Dryer design featuresC Water removal capacity of a dryer

1.3 Review: Mango Drying Case Study

Perhaps one of the best ways to reviewthe concepts from the previous twocourses is through the use of a dryingcase study.

Scenario

A mango grower has taken the“Intermediate Course in FoodDehydration and Drying” and knows theimportance of understanding thebehaviour of food products as theyundergo drying.

In order to prepare a commercial driedmango product for sale, the grower did aseries of drying trials using a smalllaboratory-scale tray dryer. During thedrying test run, the weight of the mangoeswas followed over time. The initialmoisture of the mangoes was determinedprior to the test run using a laboratorymoisture meter.

Test Results

Table 1-1 shows the weight of the mangosample for the full 18 hours of the dryingtrial. During the first four hours of thetest, the weight loss is fairly rapid. Forthis reason, weights are shown for every15 minutes for times up to four hours, andfor every thirty minutes for the remainderof the test.

The initial moisture content of themangoes was 78.72%, expressed on awet basis. The starting weight of themangoes was 243 grams.

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TABLE 1-1: Weight of Mangoes During the

Drying Test Run

Time(hours)

Weight(grams)

Time(hours)

Weight(grams)

0 243 8.5 77

0.25 237 9.0 74

0.5 230 9.5 71

0.75 223 10.0 68

1.0 216 10.5 66

1.25 209 11.0 64

1.5 202 11.5 63

1.75 196 12.0 61

2.0 188 12.5 60

2.25 181 13.0 59

2.5 174 13.5 58

2.75 167 14.0 57

3.0 160 14.5 56

3.25 153 15.0 55

3.5 147 15.5 55

3.75 142 16.0 55

4.0 136 16.5 54

4.5 126 17.0 54

5.0 117 17.5 54

5.5 110 18.0 53

6.0 102

6.5 96

7.0 90

7.5 85

8.0 81

Your Tasks

Answer the following questions:

1. How long does the constant ratedrying period last ?

2. What is the rate of moisture lossduring the constant rate dryingperiod?

3. What is the critical moisture contentof the mangoes ?

4. How long did it take the mangoes toreach a final target moisture of 12%(wet basis) under the given dryingconditions ?

5. Why was the drying test run for 18hours when a shorter time periodwould seem to have been adequate ?

6. Comment on any irregularities youmay see in the data.

Calculations

While the information in Table 1-1 doesshow how quickly moisture is lost fromthe 243 grams of fresh mangoes, itdoesn’t really tell us very much about theoverall drying process.

What we need to do is find out how fastwater is being lost on a constant basis ofmaterial. For this purpose, it is best tobase our calculations on a unit mass ofdry material. In the case of the datapresented in Table 1-1, a basis of onegram of dry solids would be appropriate.

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Since the initial moisture content of themangoes is 78.82% (wet basis), thepercentage of dry material will be 21.18%(100% - 78.82%).

Weight of = Initial Sample x % SolidsDry Solids Weight 100%

= 243 g x 0.2118

= 51.47 g

. 51.5 g

Since no solids are being lost during thedrying process, there will always be 51.5grams of solids present in the dryer.

Knowing the weight of the mango sampleat any time and subtracting the weight ofsolids present will allow us to calculatethe amount of water present at that time.

Weight of = Sample - Weight of Water Weight Solids

If we know the weight of water and solidspresent in the sample at a given time, wecan then calculate the dry basis moistureat that time.

Dry Basis = Weight of Water Moisture Weight of Dry Solids

Table 1-2 shows the calculated values forthe dry basis moistures at each time. Inaddition, the raw data from Table 1-1 hasbeen included. These values werecalculated using a spreadsheet programand were transcribed into Table 1-2.Therefore, they are subject to someeffects of round-off.

The information provided in Table 1-2,provides the necessary starting point to

answer most of the questions listedabove. By plotting the dry basis moisturecontent of the mangoes versus time, asshown in Figure 1-1, we can visualizehow the dry basis moisture contentchanges during the drying process.

The constant rate drying period is definedas that period during the drying processwhen water is removed from the materialat a constant, or uniform, rate. This isrepresented in Figure 1-1 by the linearportion of the curve at the start of thedrying process. By drawing a straight linethrough the linear portion of the curve inFigure 1-1, we can see where the curvebegins to deviate from linearity, whichmarks the end of the constant rate dryingperiod and the beginning of the fallingrate drying period. In Figure 1-1, thisappears to be somewhere between 3hours and 4 hours after the mangoeswere placed in the dryer. For ourpurposes, we’ll estimate that the constantrate drying period lasts for approximately3.5 hours.

We have been asked to calculate the rateof water removal during the constant ratedrying period. This can be done in atleast two ways. The first way is based onthe straight line drawn in Figure 1-1 (thesecond method will be discussed later).

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TABLE 1-2: Calculated Values for the Mango Drying Test Run

Time(hours)

Weight(grams)

Solids(grams)

Water(grams)

Dry Basis Moisture(g water / g dry solids)

0 243 51.5 191.5 3.72

0.25 237 51.5 185.5 3.60

0.5 230 51.5 178.5 3.47

0.75 223 51.5 171.5 3.33

1.0 216 51.5 164.5 3.20

1.25 209 51.5 157.5 3.06

1.5 202 51.5 150.5 2.92

1.75 196 51.5 144.5 2.81

2.0 188 51.5 136.5 2.65

2.25 181 51.5 129.5 2.52

2.5 174 51.5 122.5 2.38

2.75 167 51.5 115.5 2.24

3.0 160 51.5 108.5 2.11

3.25 153 51.5 101.5 1.97

3.5 147 51.5 95.5 1.86

3.75 142 51.5 90.5 1.76

4.0 136 51.5 84.5 1.64

4.5 126 51.5 74.5 1.45

5.0 117 51.5 65.5 1.27

5.5 110 51.5 58.5 1.14

6.0 102 51.5 50.5 0.98

6.5 96 51.5 44.5 0.87

7.0 90 51.5 38.5 0.75

7.5 85 51.5 33.5 0.65

8.0 81 51.5 29.5 0.57

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TABLE 1-2 (continued): Calculated Values for the Mango Drying Test Run

Time(hours)

Weight(grams)

Solids(grams)

Water(grams)

Dry Basis Moisture(g water / g dry solids)

8.5 77 51.5 25.5 0.50

9.0 74 51.5 22.5 0.44

9.5 71 51.5 19.5 0.38

10.0 68 51.5 16.5 0.32

10.5 66 51.5 14.5 0.28

11.0 64 51.5 12.5 0.24

11.5 63 51.5 11.5 0.22

12.0 61 51.5 9.5 0.19

12.5 60 51.5 8.5 0.17

13.0 59 51.5 7.5 0.15

13.5 58 51.5 6.5 0.13

14.0 57 51.5 5.5 0.11

14.5 56 51.5 4.5 0.09

15.0 55 51.5 3.5 0.07

15.5 55 51.5 3.5 0.07

16.0 55 51.5 3.5 0.07

16.5 54 51.5 2.5 0.05

17.0 54 51.5 2.5 0.05

17.5 54 51.5 2.5 0.05

18.0 53 51.5 1.5 0.03

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The slope of the straight line in Figure 1-1has units of “grams of water per gram ofdry solids per hour”.

Slope of a straight line = Rise Run

By selecting two points on the straight linein Figure 1-1, we can calculate its slope.At time t = 0, the dry basis moisture isapproximately 3.75 g water per g drysolids. At time t = 6 hours, the dry basismoisture is about 0.5 g water per g drysolids.

Slope = Rise Run

= (3.75 - 0.5) g water/g dry solids (6.0 - 0.0) hours

= 3.25 / 6.0 g water / g dry solids / hour

= 0.54 g water / g dry solids / hour

Therefore, the mangoes lose water at arate of approximately 0.54 grams of waterper gram of dry solids per hour during theconstant rate drying period, which lastsapproximately 3.5 hours.

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The critical moisture of a material is thewet basis moisture content at which theconstant rate drying period ends and thefalling rate drying period begins. Thissignifies that moisture loss has changedfrom occurring at the surface of thematerial to being controlled by diffusion ofmoisture from the inner portions of thematerial.

We have estimated that the constant ratedrying period ends approximately 3.5hours after the start of drying under theconditions used in this test. From Figure1-1, this looks like the dry basis moisturewould be about 1.8 grams of water pergram of dry solids. We now need toconvert this to its equivalent wet basismoisture.

Wet basis = weight of water x 100%moisture total wt of material

The total weight of material with 1.8grams of water per gram of dry solidswould be 2.8 grams (i.e., 1.8 g water +1.0 g dry solids).

Wet basis = 1.8 g water x 100%moisture 2.8 g material

= 64.29%

Therefore, the critical moisture content ofthe mangoes under these dryingconditions would be about 64% on a wetbasis.

The next question asks how long it wouldtake the mangoes to reach a finalmoisture content of 12% on a wet basis.Since we have the moisture contents inFigure 1-1 expressed on a dry basis, thisappears to be a slight problem. However,

we can convert the 12% wet basismoisture to a dry basis value and thenproceed.

Dry basis = weight of water moisture weight of dry solids

In 100 grams of a 12% moisture material,we would have 12 grams of water and 88grams of dry solids. We can then usethese values to determine the equivalentdry moisture value.

Dry basis = 12 grams of water moisture 88 grams of dry solids

= 0.136 g water / g dry solids

. 0.14 g water / g dry solids

From Figure 1-1, the dry basis moisturefalls to 0.14 g water per g dry solids atapproximately 14 hours. From Table 1-1,we can see that the dry basis moisturereached this level some time between 13hours and 14 hours.

Even though the mangoes required 13 to14 hours to reach their final targetmoisture of 12% (wet basis), we had torun the test for longer than this, since wedid not know in advance how long it mighttake to achieve this final value. In dryingtests such as this, it is always better to letthe test proceed for too long than toterminate it too early and miss seeingwhat happens during the later stages ofdrying. One little trick that I use is tocalculate the weight of material thatshould be present at the final targetmoisture and run the test until the weightfalls below this point. I can then decidewhether to end the drying test or keep itgoing for a longer time.

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Before leaving this case study example,there are a few additional things that wecan do as review which might bebeneficial.

Spreadsheet programs have provided uswith the ability to do complex calculationswith very little effort. Among the featuresthey offer is the calculation of theequations of the “best-fit” straight linethrough data in a graph. Knowing thatdata from the first 3.5 hours of the mangodrying trial appear to be linear (see Figure1-1), we can take these data points andplot them on a separate graph as shownin Figure 1-2. By taking the equation ofthe “trendline” through these data points,we get the following:

y = -0.5398x + 3.7324

where:

-0.5398 is the slope of the line withunits of “grams of water per gram ofdry solids per hour”

3.7324 is the y-intercept of the line

This means that at time t = 0, the drybasis moisture content of the mangoes isapproximately 3.73 grams of water pergram of dry solids (to three significantdigits) and that the rate of water loss overthe time covered by the graph isapproximately 0.540 grams of water pergram of dry solids per hour. The negativesign in front of the slope term in theequation indicates that this is a loss.

You may recall that in a previouscalculation based on the slope of thestraight line drawn through the linearportion of Figure 1-1, we arrived at awater removal rate of 0.54 grams of waterper gram of dry solids per hour. These

two values are in very close agreement(actually they are identical).

From Table 1-2, we can see that theactual initial dry basis moisture for themangoes was 3.72 grams of water pergram of dry solids. Again, this is in veryclose agreement to the y-intercept valuecalculated in Figure 1-2.

I have also taken the liberty of calculatingthe rate of water removal based on thedata presented in Table 1-2. By takingthe differences between the dry basismoistures at the start and end of eachtime interval and dividing thesedifferences by the length of time (inhours), we can get the rate of waterremoval in grams of water per gram of drysolids per hour.

For example:

At t = 1.0 hours, the water removal rate is3.20 g water / g dry solids / hour.

At t = 1.25 hours, the water removal rateis 3.06 g water / g dry solids / hour.

The rate of water removal for this quarterhour interval is:

(3.20 - 3.06) g water/g dry solids (1.25 - 1.0) hours

= 0.56 g water / g dry solids / hour

Values for all the time intervals are plottedin Figure 1-3. As can be seen, the initialwater removal rate remains relativelyconstant for the first 3.5 hours and thenbegins to decrease. This corresponds tothe start of the falling rate drying periodand agrees with our findings from Figure1-1.

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In the final question, we are asked tocomment on any irregularities in the data.

There are only a few such irregularitiesthat you may notice and these are seen inFigure 1-3. If you look at the calculatedwater removal rates for the first 3.5 hoursin Figure 1-3, you will see that there aretwo values which lie well away from thestraight line. One of these is below andthe other is above the value of 0.55grams of water per gram of dry solids perhour. The reason for this type of thinghappening is due to the reporting of theweights of the mangoes. The balanceused in these trials was only capable ofmeasuring to the nearest gram. Thismeans that adjacent pairs of numbersmay have one value as slightly high andthe other value as slightly low due to therounding off of the weights on thebalance. When the calculated valuesbased on these data are plotted, theround-off effects are quite noticeable.This effect is also evident from the saw-tooth nature of the curve in other placesin the graph.

You may also notice that the value Icalculated for the water removal rate (i.e.,0.56 grams of water per gram of drysolids per hour) is slightly different thanthe 0.55 g water / g dry solids / hourshown in Figure 1-3, or the 0.54 grams ofwater per gram of dry solids per hourcalculated from Figure 1-1. Again, this isdue to the fact that the computerspreadsheet carries extra decimal placesin its memory, even though the printedvalues are formatted to give anappropriate number of significant digits.Essentially there is little we can do aboutthis and we should be prepared to livewith this small problem.

It should also be recognized that

differences in the second decimal placefor these water removal rate values arealmost insignificant.

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ADVANCED COURSE IN FOOD DEHYDRATION AND DRYING

CHAPTER 2: PROCESS CONTROL IN A DRYING OPERATION

2.1 Introduction:

No matter what is being processed, it isalways necessary to have a method ofmonitoring various product attributes ateach major step in the process.

Finished product moisture is generally thekey item that we need to measure in adrying process. As we shall soon see, itwould also be helpful to know themoisture of the product entering the dryer.In addition, a number of other productproperties should be measured todetermine whether or not the final driedproduct is meeting all finished productstandards. It is critical to know that theproduct will perform properly for theconsumer when it is consumed.

For these reasons, it is necessary to testthe product at regular intervals. In thischapter, we will focus on moisture testingand control. It is also the responsibility ofthe processor to know how variations inmoisture can affect other properties of thefinished product. This has beenexamined briefly in the previous dryingcourses, but as was pointed out, moisturecan affect various products in manydifferent ways, and we cannot discuss allof these variations here.

We will begin our examination of “processcontrol” by looking at some basicapproaches to process control in general.From here we will go on to examine somefundamental control strategies that maybe employed in tracking product moistureleaving a dryer.

2.2 Basics of Process Control

It is not our purpose to make you allexperts in process control methods.However, an understanding of thefundamental approaches is definitelyuseful.

2.2.1 Moisture Targets and Limits

For example purposes, let’s consider aproduct being dried on a continuousthrough-circulation dryer. Through testingof product at different moisture levels, anacceptable range of moistures can beestablished which we are prepared toaccept from the drying process. For thisexample, we will assume that any productmoistures from 10.0% to 14.0% will givea product that is satisfactory to our needs.Based on these values, we are thenprepared to say that the minimumallowable moisture of the product wouldbe 10.0% and the maximum allowablemoisture would be 14.0%. Generally, atarget moisture would be established inthe middle of this acceptable range. Forthis example, the target finished productmoisture would be 12% (on a wet basis).

In some cases, the target moisture maybe determined first and then anacceptable range may be establishedabove and below the target value.

Although the target moisture is usually atthe mid-point of the moisture range, thisdoes not have to be the case. There maybe instances where the lower moisture

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limit is not as critical as the uppermoisture limit. With a product such asthis, you may see a target moisture of12.0%, with a lower moisture limit of 7.0%for example, and an upper moisture limitof 14.0%. In spite of this range, the bestor optimum moisture for the product maybe 12.0%, which is not at the middle ofthe range. For our discussions here, wewill use a moisture range with the targetmoisture at its mid-point.

We have now established that ourproduct target moisture is 12.0%. Theupper limit on the moisture is 14.0% andthe lower limit is 10.0%. We could thenwrite our moisture specification as being12.0% ± 2.0% to indicate the target andacceptable moisture range.

In order to follow the moisture content ofthe product over time, we could draw agraph with moisture on the vertical axisand time on the horizontal axis (seeFigure 2-1). To make the target moisturestand out on the graph, we could draw aheavy line across the graph at 12.0%moisture. To make the upper and lowermoisture limits more obvious, we couldalso draw horizontal lines at 10.0% and14.0% moisture, as done in Figure 2-1.

Finished product moisture values couldbe measured over time to follow thevariations in moisture. If the moisturestarted moving away from the targetmoisture (i.e., getting too high or too low),the trend could then be detected. Figure2-1 shows how the moisture is varying,but is staying within the acceptablemoisture limits as previously determined.

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2.2.2 Control Limits

As seen in Figure 2-1, the moisture iscertainly within the acceptable limits.However, there may be times when itappears that it is climbing towards theupper limit. If the operator does not takeany action to address the rising moisturecontent of the product, it may increase toa level above the acceptable upper limit.

The big question now is, “When shouldthe operator react to the moisturechange?”

Another question might be, “How much ofan adjustment should the operator maketo the dryer when he or she decides totake action?”

In a well-designed process, nothingshould be left to chance, and proceduresshould be specified for the processoperators to follow. If the operator doesnot take action to reduce the risingmoisture of the product, it will soon beabove the acceptable upper limit and theproduct will not be able to be sold. If noaction is taken until the moisture actuallyreaches the upper limit, it will still continueto rise until the effects of the adjustmentsto the drying conditions have had achance to take affect on the product.Remember, when changes are made to adryer, the results are not instantaneous.It takes some time for the material in thedryer to show signs of response.Therefore, action should be taken beforethe moisture reaches the upper limit (orthe lower limit).

For this reason, in control strategy, anupper control limit and a lower controllimit are established to indicate whenaction should be taken to addressmoisture changes in the dryer. These are

often abbreviated to UCL, for uppercontrol limit, and LCL, for lower controllimit.

The UCL is set at some moisture valuebetween the target moisture value andthe maximum acceptable moisture limit ofthe product. The LCL is set at a moisturevalue between the target moisture andthe lower acceptable moisture limit of theproduct. One way of establishing thesecontrol limits is to simply set them atpoints half-way between the targetmoisture value and the acceptableproduct moisture limits.

Since we have a range acceptablemoistures of 2% above and below thetarget moisture, we could set our controllimits as being 1% above and below theproduct’s target moisture. This wouldmean that the moisture could varybetween 11.0% and 13.0% moisturewithout any action being taken. However,as soon as the moisture went outside thisrange, some remedial action would betaken in an attempt to bring the moistureback towards the target moisture. Whilethis action is being taken, the moisturecontent would probably drift beyond thecontrol limit, but because the control limitsare well inside the acceptable moisturelimits of the product, the product moisturewould still be okay. In this way, themoisture would not go beyond theacceptable limits and waste would bekept to an absolute minimum.

Figure 2-2 shows the upper and lowercontrol limits as dashed lines togetherwith the information that was included inFigure 2-1.

In drying systems equipped withautomated moisture analyzers, it ispossible to have the dryer controls linked

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to a computer system which interpretssignals from the moisture detectors andtakes action automatically. In caseswhere no such systems are in place, it isstill possible to follow the trends inmoisture and make adjustments to thedryer manually.

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2.3 Moisture Sampling

2.3.1 Grab Samples

In many processing applications, a qualitycontrol technician (QC technician) takesa small sample of product off the end ofthe dryer once every 30 minutes, orperhaps once every hour. The samplesthat are taken are often referred to as“grab samples” due to the fact that theQC technician literally “grabs” a samplefrom the end of a dryer and takes it to thelab for moisture analysis. Of this sample,perhaps only 10 grams is actually placedin the moisture balance.

In cases where the dryer operator takeshis or her own samples, the time intervalmight be as frequent as every 20 or 30minutes.

The sample is then placed on anautomated moisture balance which driesthe sample using an infrared lamp (asdescribed in a previous course), andgives a readout of the wet basis moisturein approximately ten to fifteen minutes.The results of the moisture tests are thencommunicated to the process operatorwho may make adjustments to the dryerif they are felt to be needed.

2.3.2 Continuous Monitoring:

While it is physically impossible for aQuality Control technician to check themoisture of every single piece of productleaving a dryer, it is possible to get ahigher level of testing done using anautomated method. To do this, acontinuous monitoring system may beused such as an infrared monitoring unit.

Basically, the system involves the use ofan infrared emitter/detector unit mountedon a set of rails across the discharge endof the dryer. The unit travels back andforth along these rails and directs a beamof infrared light down onto the bed ofmaterial just before it leaves the dryer.By having the infrared light properlycalibrated, the unit can determine themoisture of the product based on thedifferences in the frequencies of theemitted light sent down to the product bedand the reflected light bounced back fromthe product to the infrared unit.

The infrared unit not only providescontinuous information about the productmoisture, but it does so across the dryerbed as well as over the course of time asthe material comes out of the dryer. Thisis considerably better than just taking asingle sample at one location every hour.In cases where an infrared unit is inplace, hourly grab samples may also betaken as a check that the infrared unit isfunctioning properly.

The infrared unit can give the dryeroperator continuous feedback as to theproduct moisture. Any moisture changescan be detected as they occur, withoutany delay, and not just every hour.Armed with this information, the operatorcan see how the process is functioningand take appropriate action when the

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need arises.

In some cases, the infrared unit can belinked to a computer which will adjustdrying conditions without the having theoperator making the adjustments.

Continuous moisture monitoring systemsare expensive to purchase and mayrequire sophisticated maintenance andcalibration that is not possible for manydrying operations.

2.4 Case Study #1: Moisture Testing

2.4.1 Background

A drying process runs at a rate of 1,360kg of wet product (70% moisture) perhour, 24 hours per day, five days perweek. The retention time of product inthe dryer is about 25 minutes.

Every 30 minutes, the Quality Control(QC) lab technician takes a “grab-sample”from the dryer discharge and does amoisture test on it. The test takes 15minutes to complete and the results arenot communicated to the processoperator for five minutes after that.

The test sample weighs about 250 gramsand the technician uses about 15 gramson the moisture balance for the actualmoisture test. The remainder of thesample is kept in a sealed plastic bag foradditional follow-up testing, if necessary.“Spot checks” are done using the vacuumoven method once a week on threesamples randomly selected from eachday’s retention samples.

When the process operator receives themoisture test results from the lab, adecision is made as to whether or not

adjustments to the dryer are required.For our purposes, we will consider thatthe target moisture of the finished driedproduct is 10.0% ± 2.0%.

2.4.2 Associated Problems andShortcomings

Consider the following points:

2.4.2.1 Sample Size:

The sample size of 250 grams (each halfhour) represents one sample from 680 kgof wet material entering the dryer duringeach half-hour period. The weight of finalproduct from this amount of wet materialis about 226.5 kg every half hour (or 453kg every hour, or 7.56 kg per minute).

250 grams is equal to about one-tenth ofone percent of the total dried productmade during each half-hour period.However, only 15 grams of product isactually tested. This means, that thefunctioning of the process which producesabout 226.5 kg every half-hour is beingbased on the results from a test on 15grams, which is equivalent to about0.007% of the finished product.

2.4.2.2 Sample Uniformity

There is no indication as to how uniformor representative the sample really is.

Consider these questions:

Does the sample represent the overallconditions of the product leaving thedryer?

Was the sample taken from an overly wetspot or an excessively dry spot?

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Did the dryer conditions fluctuate duringthe time preceding the taking of thesample?

How can anyone know what this “grab-sample” truly represents?

Even though all of these questions exist,very few processors stop to think aboutthe fact that they are basing the quality oftheir entire dried product on samples ofless than one one-hundredth of onepercent of their finished product. This isa very risky position to take, to say theleast.

2.4.2.3 Sampling Interval and Timing

As stated above, during the thirty minutesbetween samples, about 680 kg of wetmaterial will be feed into the dryer andabout 226.5 kg of dry product will beproduced. This is equivalent to about7.56 kg of dried product per minute.

If the QC technician takes 20 minutes toreport the results of a moisture sample,not only will the actual material tested be“history” by that time, but an additional151 kg of dried product would have leftthe dryer by the time the operator getsword of the moisture test results.

2.4.2.4 Process Response Time

If the process operator decides to make achange to the dryer at the exact momenthe or she receives the moisture testresults, it will be an additional 25 minutesbefore the full effects of that change canbe realized. During this 25 minute, about189 kg of product will have passedthrough the dryer and will haveexperienced varying degrees of exposureto the new drying conditions.

What makes this situation even worse isthat the sampling frequency by the QClab may not detect the results of thischange for an additional period of time -unless the process operator can have a“special” sample taken for testing.

Let’s examine what this process wouldlook like on an actual timeline as shown inTable 2.1 below.

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Table 2-1: Comments Regarding Sampling Timeline for Case Study Drying Process

Time Event Comments

3:00 pm Sample taken Sample tested represents about 0.007% of product

leaving dryer.

3:20 pm 3:00 pm results reported

(assume results are okay)

151 kg of product have left dryer since sample was

taken. Product represented by sample has been

out of dryer for 20 minutes.

3:30 pm Sample taken

3:50 pm 3:30 pm results reported

(action is required)

151 kg of product have left dryer since sample was

taken. Product represented by sample has been

out of dryer for 20 minutes.

3:50 pm Dryer adjustments made

4:00 pm Sample taken This sample will have experience 15 minutes of

original dryer conditions plus 10 minutes of new

dryer conditions (assuming changes were made to

entire length of dryer)

4:15 pm Product from 3:50 pm

adjustment leaves dryer

This is the first product to leave the dryer after

changes were made based on 3:30 pm sample.

340 kg of product have left the dryer since the 3:30

pm sample.

4:20 pm 4:00 pm results reported This sample is potentially misleading as it has been

subjected to changing dryer conditions, but not

steady-state conditions.

4:20 pm Operator may be tempted to

make adjustments.

Operator must make a judgement call based on

direction of response, but not the actual moisture

value.

4:30 pm Sample taken This is the first sample that will include all of the

effects of the changes made to the dryer at 3:50

pm, which were based on the 3:30 pm sample. 454

kg of product have left the dryer since 3:30 pm.

4:50 pm 4:30 pm results reported This is the first true indication the operator will have

as to whether the 3:50 pm changes based on the

3:30 pm sample were effective. 605 kgof product

have now left the dryer since 3:30 pm.

5:00 pm Sample taken The trend continues.

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2.5 Process Control Mechanisms

2.5.1 Feed-Back Control

In the example discussed above, themoisture of the product was monitored atthe end of the dryer as the finishedproduct left the dryer. The moisturecontent of the product gave the processoperator an indication of what to do to thedryer settings if action was required.

If the moisture of the finished product wastoo high, the operator could turn up thetemperature of the heated air to providemore drying capacity. This wouldprobably best be done at the start of thedryer in the first zone (if indeed the dryerdid have separate zones). However, ifthe material was sensitive to temperature,the operator could reduce the amount ofwet material fed into the dryer, whichwould reduce the amount of water thedryer would have to remove in a givenperiod of time. The operator couldpossibly even slow down the speed of thedryer belt to increase the time the wetmaterial spent inside the dryer. Thiswould increase the thickness of the bed inthe dryer, which might create otherproblems. The operator could even do acombination of these things, if it was feltto be warranted.

If the material leaving the dryer was toodry, the operator would probably justreduce the temperature of the heated airgoing into the dryer to allow the moisturelevels to rise slightly.

When we monitor the moisture of theproduct leaving the dryer and makeadjustments in the drying conditionsaccording to these moistures, we refer tothis as feed-back control. What we are

really doing is gathering information fromthe end of the process and takingcorrective action by feeding thisinformation back to the drying processthat gave us these results in the firstplace.

If the process was fully automated, amoisture analyzer or detector wouldautomatically measure the moisturecontent from the end of the process andsend this value to a computer that wouldadjust the temperature of the air cominginto the dryer. Figure 2-3 shows a three-zone dryer with feed-back control from amoisture analyzer unit. The processcontroller, or computer, is set up to openor close valves on the gas lines to each ofthe three drying zones. If more heat isneeded, the gas valves are openedslightly more to allow more gas to get tothe burners and create a hotter flame towarm the air more. If less heat isneeded, the valves are closed slightly toallow less gas to get to the burners andreduce the temperature of the air.

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2.5.2 Feed-forward Control

Now that we have looked at feed-backcontrol, we might want to consider thefact that we are actually controlling theprocess using results of things that havealready happened. We use moisturereadings from product that has alreadygone through the dryer to adjust theconditions in the dryer. Although thismethod does work, it relies on possiblemistakes that you have made in thedrying process to tell you whether or notyou need to take corrective action. Thisis a reactive approach, rather than a pro-active approach.

From a personal perspective, you maywant to look ahead and anticipateproblems while you can still do things to

prevent them. Feed-back control only letsus correct problems after they haveoccurred. For this reason, we should takea serious look at what is called feed-forward control.

With feed-forward control in a dryingprocess, we look at the moisture of thematerial going into the dryer rather thanproduct leaving it. If the feed to the dryeris increasing in moisture, we know thatchanges must be made to anticipate theextra water going into the dryer. Changescan be made to the temperature of the airin the first drying zone as the wet productbegins to travel through the dryer. Oncethe wet material has reached the end ofthe first drying zone, conditions can bechanged in the second zone. Similarly,as the wet material reaches the end of

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the second zone, conditions in the thirdzone may be adjusted accordingly.

By not making changes to all three zonesin the dryer at the same time, we are notgoing to “over process”, or “over-dry”, thematerial that was already in the dryerwhen the wet product started to enter thedryer. Keep in mind that it may take thirtyor forty minutes for product to travelthrough the complete length of the dryer.

If feed material with a lower moisture thannormal is encountered, a similarprocedure can be used; except this timetemperatures would be reduced to allowthe product moisture to increase by thedesired amount.

Figure 2-4 shows how feed-forwardcontrol would be used in relation with theburners on a dryer.

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2.5.3 Combined Feed-forward andFeed-back Control

Figure 2-5 shows a drying process set upwith both feed-forward and feed-backcontrol. In this highly instrumentedsystem, control is based on monitoringthe moisture of the incoming feedmaterial and the finished productmoisture. Knowing that there is a timedelay between the time the feed materialenters the dryer and the product from thatfeed material leaves the dryer, it ispossible to set up a control program tomake adjustments to conditions in thedryer.

The moisture of the incoming feedmaterial would be the more importantcontrol factor. As a result, the feed-forward mechanism would be given themost “power” in controlling the process.and the feed-back control mechanismwould be given much less “power” inmaking decisions as to what to do toadjust conditions in the dryer. It is veryimportant that these two systems worktogether and do not work against eachother. If the two control systems did notwork in harmony with each other, totalchaos could result inside the dryer.

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2.5.4 General Comment

We have used the moistures of feedmaterial and finished product as keyvariables to show how feed-forward andfeed-back control work in a dryingoperation. We have chosen airtemperature as the input variable tocontrol. We controlled the airtemperature by adjusting the flow of gasas a fuel to the burners in the dryer.

It should be noted that these controlmechanisms are not just used withdryers, and that moisture is not the onlyprocessing variable that can be monitoredfor controlling a process.

Different steps in different processes forthe manufacture of various products willeach have their own important input andoutput variables that need to bemonitored and controlled. You, as aprocess operator, must decide which arethe critical output variables to monitor andwhich are the most important inputvariables to control. Once you haveidentified these items, you can work withequipment suppliers to set up whatever isnecessary to accomplish the desiredobjectives.

an advanced course in food dehydration and drying · 3.6 case study #2: application of psychrometry...

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