EVALUATION OF AN ANIMAL EXCRETA DRYER

Thesis for the Degree 0f M. S.
WCHiGAN STATE UNIVERSITY
TRUMAN CARL SURBROOK
1969

 

 

 

 

ABSTRACT

EVALUATION OF AN ANIMAL EXCRETA DRYER
By

Truman Carl Surbrook

Considerable research has been conducted on develOp-
ing methods of animal excreta management, but little of
this research has been concerned with finding means of
drying the excreta on the farm. A poultry excreta dryer
was developed for farm use, but a lack of data made it
impossible to evaluate this dryer's performance while
processing poultry and other animal excreta. This re-
search was concerned with the production rate, fuel con-
sumption, and thermal efficiency as well as a study of
the drying process within the machine. Bulk densities
and particle size distributions were determined for each
dried excreta.

Based on a forty hour week, one dryer would process
the excreta from 22 bovine weighing 1000 pounds, l8h hogs
weighing 100 pounds, or 7800 laying hens. The dryer
would also handle limited amounts of litter. Fuel oil
consumption averaged 2.5 gallons per hour, and the
electrical demand was “.2 kilowatts. Bulk densities
ranged from 10.9 pounds per cubic foot for bovine excreta,
3.9 per cent straw, to 23.“ pounds per cubic foot for

poultry excreta. A very small percentage of dried

Truman Carl Surbrook

excreta particles were outside the size range of 0.01 to
0.1 inches. The high drying temperature caused a reduc-
tion in the nutrient content of the excreta in most cases.
Drying was accomplished with air temperatures as
high as 1000° F in one area. Seventy-five per cent of
the drying was found to result from a pneumatic process
where the mass transfer coefficient for an idealized
situation varied directly with absolute temperature and

inversely with particle diameter.

    

Approved $.10]: W

Department Chairman

EVALUATION OF AN ANIMAL EXCRETA DRYER

By

Truman Carl Surbrook

A THESIS

Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

MASTER OF SCIENCE

Department of Agricultural Engineering

1969

ACKNOWLEDGMENTS

I wish to express my appreciation to Dr. James S.
Boyd, Dr. A. M. Dhanak and Dr. Howard C. Zindel for their
guidance and valuable assistance while conducting this
research.

I also wish to express appreciation to the techni—
cians in the Agricultural Engineering and Poultry Science
Departments for their many hours of faithful service, and
to the Dryer Corporation of America for supplying the
excreta dryer and technical assistance when needed.

My special thanks go to my parents, Mr. and Mrs.
Floyd G. Surbrook and Mr. and Mrs. w. Earl Holman. Their
advice, council, encouragement and help proved invaluable
to the completion of this research.

And to the one for whom appreciation cannot be
expressed in words, my wife Mary. Her smile would have
been enough, but with it she gave unboundless help and

encouragement.

ii

TABLE OF CONTENTS

Page

ACKNOWLEDGMENTS . . . . . . . . . . . . 11

LIST OF TABLES. . . . . . . . . . . . . 1V

LIST OF FIGURES . . . . . . . . . . . . vi

LIST OF SYMBOLS . . . . . . . . . . . . viii
Chapter

I. INTRODUCTION . . . . . . . . . . 1

II. LITERATURE REVIEW . . . . . . . . 3

III. THE ANIMAL EXCRETA DRYER . . . . . . 8

IV. OBJECTIVES OF THE RESEARCH . . . . . 18

V. EVALUATION PROCEDURES . . . . . . . 21

VI. RESULTS AND DISCUSSION. . . . . . . 23

6.1 Performance of the Dryer. . . . 23

6.2 Properties of the Dried Excreta . 33

6.3 Study of the Drying Process. . . 42

VII. SUMMARY. . . . . . . . . . . . 57

Conclusions. . . . . . . . . . 58

Suggestions for Further Research. . . 59

REFERENCES . . . . . . . . . . . . . . 61

iii

LIST OF TABLES

Table Page
1. Comparison of three poultry excreta dryers . . 7

2. Production figures for the dryer while
processing different kinds of animal 29
excreta . . . . . . . . . . . . .

3. Production figures forthe animal excreta
dryer expressed in terms of one ton of
wet excreta. . . . . . . . . . . . 30

A. Excreta voided by poultry and livestock
Maddex (1965) . . . . . . . . . . . 32

5. Projected number of animals that can be served
with the excreta dryer operating forty hours

per week. . . . . . . . . . . . . 32
6. Solid material released from the dryer stack . 3A
7. Particle size distribution of solid

material released from the the stack . . . 3A
8. Bulk density of dried animal excreta . . . . 35

9. Animal excreta particle size distribution
expressed in per cent of total weight . . . 39

10. Poultry excreta nutrient levels before and
after drying . . . . . . . . . . . A0

11. Feed value of poultry excreta before and
after drying . . . . . . . . . . . Al

12. Bovine excreta nutrient levels before and
after drying . .‘ . . . . . . . . . Al

13. Swine excreta nutrient levels before and
after drying . . . . . . . . . . . A2

1A. Nutrient levels of animal excreta converted

to per cent oven dry basis from Benne
et al. (1961) . . . . . . . . . . . “3

iv

Table Page

15. Water removed during each phase of drying

poultry excreta . . . . . . . . . 50
16. Per cent of total drying occurring during

each phase of drying poultry excreta . . 50

Figure

NGU‘I

(I)

10.

ll.

l2.

13.

1A.

15.

16.

17.

LIST OF FIGURES

Flow-through rotating drum dryer

Raymond Flash Dryer with pneumatic drying
and conveying . . . . . . . .

Air and material flow through the animal
excreta dryer . . . . . . . . .

Interior of the excreta dryer
Exterior view of the dryer . . . .
Dryer screening plate . . . . . . .

The hammer mill and feedback operating
during drying . . . . . . .

The intermediate drying area. . . .
Excreta and feedback control system . .

Poultry excreta mixture, 18.3 per cent wood
Chip litter. O C C O O C O C 0

Bovine excreta mixture, 3.9 per cent straw

Granules of dried poultry, bovine and swine
excreta . . . . . .

Particle size distribution of dried poultry
excreta . . . . . . . . . . .

Particle size distribution of dried bovine
excreta . . . . . . . . . . .

Particle size distribution of dried swine
excreta . . . . . . . . . .

Time required for excreta to travel through
the dryer . . . . . . . . . . .

Average peak temperatures measured while
processing poultry excreta. . .

vi

Page

10
15
15

16
16
17

27

27

36

36

37

37

AA

Figure Page

18. Temperature variation with time within

the dryer . . . . . . . . . . . A5
19. Temperature variation over the surface of

the dryer . . . . . . . . . . . A7
20. Diagram of excreta flow through the dryer . A9

vii

:p

3 '3‘ Q H) Q: U U O f9

3’

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LIST OF SYMBOLS
area, ft2
as subscript, air
moisture concentration, 1bm H20/ft3

mass diffusivity of H 0 into air, ft2/hr

2
as subscript, diffusion
diameter of particle, ft

as subscript, final

mass flow of fluid, 1bm/hr—ft2
mass transfer coefficient, ft/hr

moisture content, fraction

mass flow rate of H20, 1bm/hr

number of particles per unit volume

as subscript, initial, time zero or before
as subscript, particle

as subscript, solid

temperature, °R

velocity, ft/hr

volume, ft3

volume flow rate, ft3/hr—ft2
oven dry weight, grams

wet weight, grams

fraction void

viii

viscosity of fluid, lbm/hr-ft2

kinematic viscosity u/p , ft2/hr
density, lbm/ft3

fraction solid (1 - 5)

ix

CHAPTER I

INTRODUCTION

In many areas of the United States, the problem of
the utilization or disposal of animal excreta without
creating a health hazard or public nuisance has become
serious. As greater numbers of poultry and livestock
are confined in smaller areas, the situation becomes
increasingly crucial. "Farm Animals in the United
States produce ten times as much waste as the human pOpu-
1ation."*

At present, the greatest portion of animal excreta
is put on the land in an untreated form. With the rising
rural non-farm population and increasing emphasis on
reducing environmental pollution, this means of waste
disposal is uncertain. Society is no longer willing to
accept the odor, and questions have been raised as to the
possible pollution of streams and underground water sup-
plies by this material. In some localities, ordinances
limiting the disposal of animal excreta on land have
caused some poultrymen and livestock raisers much expense

and inconvenience.

 

*"Restoring the Quality of Our Environment," The
White House (November, 1965), 170-171.

1

 

Considerable research has been conducted on develop-
ing methods of animal excreta management, but little has
been concerned with drying excreta on the farm. A flow-
through type rotating drum dryer was used on a poultry
farm in England, but a report on the dryer was not com-
plete enough to evaluate the process. A flash dryer using
pneumatic conveying was also tested in England, but on a
scale too large for the average farm.

Recently, however, a commercial poultry excreta
dryer was developed for farm use in this country. A lack
of data made it impossible to evaluate this new dryer's
performance while processing poultry and other animal
excreta. Late in 1967 a dryer was consigned to Michigan
State University for testing and evaluation. Preliminary
trials were started in January, 1968 and testing continued
until March, 1969.

The aim of the research conducted on the new dryer
was as follows: (1) evaluate the performance of the dryer
while processing poultry and livestock excreta; (2) deter-
mine some of the physical characteristics of the dried
product; and (3) study the drying process itself to
determine the primary drying mechanism and parameters
involved. The intent of this research is not to dwell
upon the economics of drying with this machine, or to

speculate as to the utilization of the dried product.

CHAPTER II

LITERATURE REVIEW

Few researchers have investigated the possibilities
of drying animal excreta on the farm. 0f the published
reports, few specific details were given. From the
standpoint of general appraisal of the processes them-
selves, these published reports are significant and worthy
of consideration.

Two types of excreta dryers have been tested on
poultry farms in England. The first is a flow-through
type rotating drum dryer. A variable speed auger feeds
the fresh excreta into the upper end of an inclined
rotating drum. Hot air is circulated around the outside
of the drum and then directed through it, entering at the
upper end and leaving at the lower end. The flue gases
are passed through an afterburner to reduce odor. The
heated air is circulated around the lower end of the drum
before it is released. The flow-through rotating drum
dryer is diagrammed in Figure 1. Cross wires fitted in
the drum prevent balls of material from forming.

A second type of system used to dry poultry excreta
in England was a flash mixer dryer using pneumatic con-
veying of the solids. The flash type excreta dryer was

3

not described in the literature, but a similar flash
process has been used to dry potato granules.

Various types of flash dryers for producing potato
granules are described in literature on food processing
and technology. A slurry in the order of 70 per cent
moisture is mixed with a large amount of dried granules
of about 10 per cent moisture. The product from this
mixture is a material of about 35 to A0 per cent moisture.
In the Raymond Flash Drying System, a disintegrator is
used to granulate the agglomerated mixture before it
enters the vertical pneumatic drying and conveying tube.
The granules are retained in the vertical tube in the
order of 6 to 10 seconds after which time the moisture
content has been reduced from near A0 per cent down to
10 per cent. Air and granules are separated in a cyclone,
and a large portion of the dried granules are used as
"add-back" for the mixer. A fan at the exhaust is used
to induce air flow through the system. The Raymond Con—
tinuous Flash Dryer is diagrammed in Figure 2. Neel
gt_al (195A) reported that up to 85 per cent of the dried
product was used as "add-back" to the mixer in a slightly
different flash dryer. Flash dryers using horizontal
pneumatic conveying have been used, but higher air
velocities were required to prevent the granules from

settling in the drying tube.

 

 

 

 

 

 

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Figure l. Flow-through rotating drum dryer. (1) fresh
excreta hopper; (2) burner; (3) fan; (A) rotating drying
drum; (5) dust precipitator; (6) fan; (7) deodorizing
afterburner.

 

 

 

 

 

 

 

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Figure 2. Raymond Flash Dryer with pneumatic drying and
conveying. (1) fresh excreta hOpper; (2) mixer; (3) burner;
(A) disintegrator; (5) vertical drying tube; (6) cyclone
separator; (7) fan.

 

Combo

 

Ryder (1967) discussed the various types of dryers
and concluded that the flow—through rotating drum and
flash dryers showed the greatest promise for processing
poultry excreta. The thermal efficiency of the drum
dryer was reported to be low as compared to the flash
dryer. The uniformity of drying was questionable with
the drum dryer especially if measures were not taken to
prevent balling of the material. For the flash process
uniformity of drying was reported to be good, especially
if some means of particle attrition was included in the
system. As far as capital investment is concerned, the
drum dryer was most favorable for farm drying. The flash
process became competitive only on a large scale such as
a farm with 200,000 or more laying hens. High power
requirements were necessary to operate the fan and
particle disintegrator on the flash drying unit. Power
requirements were comparatively low for the drum type
dryer. Both dryers were reported to render the product
sterile after drying.

Performance figures were given for two flow-through
rotating drum dryers and a flash dryer, although precise
measurements were not made during the trials. The esti-
mated dryer production rates given in the literature were
converted to a A0 hour work week and listed in Table 1.
Physical dimensions were given only for the smallest flow-

through drum dryer, however, investment cost estimates

were given for all three. Therefore, capital investment
was included in Table l as a crude means of comparing the

size of the dryers.

TABLE 1. Comparison of three poultry excreta dryers.

Two flow-through rotating drum dryers and a flash dryer.

Dried product output is based on A0 hours of operation
per week.

 

 

Druml Drum2 Flash2
Estimated
Capital $7,000 $17,000 $38,000
Investment
Estimated

Fresh Excreta in 8 tons/wk. 20 tons/wk. 57 tons/wk.
70% m.c. (w.b.)

Estimated
Dried Product Out 3 tons/wk. 7 tons/wk. 20 tons/wk.
15% m.c. (w.b.)

 

1"High Temperature Dryer for Poultry Manure," Farm
Mechanization and Buildings (May, 1968), 27.

2C. Ryder, "Water Costs Money," English paper
unpublished (December, 1967), 9-10.

 

CHAPTER III

THE ANIMAL EXCRETA DRYER

The object of this research was to evaluate a
commercial animal excreta dryer. The dryer operated on a
principle similar to the flash dryer. The main differ-
ence was that with this unit, granulated material was
thrown into the air stream by a hammer mill, rather than
being suspended by the air itself. The dryer also em—
ployed a different method of facilitating final drying.

The dryer consists of five internal inclined sur-
faces rigidly connected together (Figure A). These dry-
ing surfaces and their side walls are constructed of 16
gauge stainless steel. One end of the internal drying
chamber is connected to an eccentric which applies a
horizontal shaking action. The shaking causes material
to slide down the surfaces during drying. The hammer
mill for mixing and pulverizing is located at the lower
end of the top inclined drying surface. The hammer mill
is eight inches in diameter and turns at 2250 revolutions
per minute. A plate with either l/A inch or 3/8 inch
holes acts as a screen to limit maximum particle size

(E in Figure A). The material must pass through holes in

 

 

 

 

 

 

 

 

 

 

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STORAGE

 

 

Air and material flow-through the animal excreta

dryer. Legend:
back mechanism;

(A) excreta feed mechanism;

(C) initial drying area;

(B) dried feed-

(D) hammer mill;

(E) screening plate with l/A or 3/8 inch holes; (F) inter—

mediate drying area;

(I) fan.

(G) final

drying'area;

(H) fire box;

10

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11

this screening plate in order to get to the lower drying
surfaces.

Hot dry air is produced by burning fuel oil in a
fire box. Additional air is forced into the dryer by a
blower (I in Figure A) and enters from the lower side walls
near the fire box. Air movement through the dryer results
from the combination of back pressure of air entering,
convection, and draft stimulated by outside air blowing
across the top of the stack.

The walls consist of an inner drying chamber wall
cfi‘stainlesssteel, a 1/2 inch air space, fiberglass insu—
lation and the outer steel skin. An air duct pressurized
by the blower (I in Figure A) extends along both sides and
one end of the dryer. Air passes downward from this duct
through the 1/2 inch air space between the inner drying"
chamber wall and the insulation. An opening along the
lower walls allows this air to enter the machine, and be-
come part of the drying air. This air cools the surface
of the dryer and, at the same time, is preheated before
becoming part of the drying air. Some additional air
enters by convection.

The hammer mill, shaking drying chamber, blower for
conveying dry product, and elevator are powered by a three
horsepower single phase electric motor. Additional 120
volt, single phase fractional horsepower electric motors
are used to power the fresh excreta feed drag chain, air

inlet fan, and burner pump and fan.

12

The burner is controlled by an adjustable thermostat.
The sensing bulb for the thermostat is located on the side
wall of the dryer near the hammer mill.

The exhaust stack of the dryer extends through the
roof to the outside of the building. The stack Opening
is fourteen feet above the ground and about level with
the peak of the roof. The stack is covered to prevent
snow and rain from entering.

Fresh excreta is fed intermittently into the dryer
when needed. An adjustible current sensing relay in the
line to the main drive motor determines when more fresh
excreta is needed. The fresh material drOps onto the
upper end of the tOp inclined drying surface (C'in Figure
A) and slides downward toward the hammer mill. At the
same time fresh excreta is entering the machine, dried
granules are fed back and dropped into the hammer mill
(B in Figure A). The dried feedback, fresh excreta, and
partially dried residual are mixed by the action of the
hammer mill and subjected to the drying air. The ex-
creta circulates in this area until it finally drOps
through the holes in the screening plate.

The intermediate drying area consists of a series
of two inch wide channels with a 1/2 inch slot between
them through which air passes (Figure 8). The greatest
amount of air and the hottest air is directed onto the

excreta granules on these channels. The air then passes

13

through the slots and on to the initial drying area. The
temperature in this area is high enough to cause some
sparking during normal drying.

Final drying is accomplished as the material moves
down the last three inclined drying surfaces (G). Smaller
quantities of lower temperature air are directed over
these surfaces. Actually, the final surface acts more as
a cooling area with very little additional drying taking
place. After leaving this final surface, the material
is moved horizontally by an auger and then elevated to a
small storage hOpper from which dried material is augered
into the initial area as feedback with the remainder
transferred to permanent storage by a blower.

Dryer controls consist of two sub—systems. One
sub-system senses the current drawn by the three horse-
power motor powering the dryer. When this current falls
below a preset value, the feed mechanism is activated.
When the current again surpasses this preset value, the
feed mechanism is shut off. Both dried feedback and
fresh excreta enter the dryer when the feed mechanism
Operates.

The second control sub-system operates an elevator
that keeps the dryer hopper supplied with excreta. A
pressure switch senses a nearly empty hopper and acti-
vates the control relays to start the elevator. This

second control sub-system was not used during these tests.

1A

When an elevator is used to keep the hOpper sup-
plied with excreta, the dryer operates automatically.
Manual labor is not required during the drying operation.
The unit will shut down automatically when the elevator

no longer keeps the hopper supplied with material.

15

 

Figure 5. External view of the excreta dryer. Fresh
excreta is loaded into the hopper at the top by a
separate elevator.

 

Figure 6. Dryer screening plate. The granulated material
leaves the initial drying area by falling through the
holes in the screen plate. This plate is marked (E) in
Figure A.

l6

 

Figure 7. The hammer mill and feedback operating during
drying. The hammer mill mixes and pulvarizes the excreta,
and throws it into the drying air. The dried feedback is
also visable coming from the auger above the hammer mill.

 

Figure 8. The intermediate drying area. The granular
material moves down these two inch wide channels. Air
passes through the 1/2 inch slots between the channels.

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Figure 9. Excreta and feedback control system. Legend:

(1) main relay; (2) feed mechanism relay; (3) three second
time delay relay; (A) drive motor current sensing relay
(closes when current drops below preset value); (5) pressure
switch; (6) fresh excreta feed motor; (7) dried feedback
solenoid operated clutch; (8) drive motor; (9) dried feed-
back auger; (10) hammer mill.

CHAPTER IV

OBJECTIVES OF THE RESEARCH

The work reported on previous dryers was incomplete

with only rough estimates of the capabilities of these

dryers.

If an animal excreta dryer is to be fitted to a

farm situation, the performance of the unit must be known

exactly.

The first major objective of this evaluation was

to determine the overall performance of the dryer while

processing animal excreta. The questions to be answered

were as follows:

1.

5 .

Can the dryer be used to process bovine and
swine excreta?

Will the dryer handle poultry excreta containing
wood chip litter and bovine excreta containing
straw?

What are the production rates, fuel oil and
electricity consumptions, and efficiencies while
drying different animal excreta?

Is solid material released from the stack into
the air?

Does the machine give off an odor during drying?

Animal excreta is obviously difficult to handle

especially with a minimum of labor. If the ease of

18

19

handling is to be evaluated, certain physical characteris-
tics of the dried excreta must be known. The second major
objective of this research, then, is to determine some
physical characteristics of the dried excreta.

6. What are the bulk densities of the dried pro-

ducts collected during the production trials
of various animal excreta?

7. What are the particle size distributions of the

dried animal excreta?

8. Is there an odor to the dried excreta?

9. Is a change in the levels of nitrogen,

Ephosphorus, potassium, and protein detected
before and after drying?

Little was known about the actual drying process
taking place within the machine itself when the evalua—
tion began. A logical approach to dryer evaluation from
a design standpoint would be an investigation to deter—
mine the key drying mechanism and possible parameters
involved. Specific questions to be answered that would
help accomplish this third major objective were as
follows:

10. What is the temperature gradient through the
dryer?
11. How does temperature vary with time at any

one location within the dryer?

l2.

13.

1A.

20

What is the temperature of the surface of
the machine during drying?

What per cent of total drying takes place at
the various areas within the dryer?

What parameters most significantly affect

the drying rate?

CHAPTER V

EVALUATION PROCEDURES

The animal excreta was put into buckets and weighed
on a platform scale before manually filling the dryer
hopper. The weight of fresh excreta was determined to
the nearest one half pound. The dried excreta out of the
machine was also weighed on the platform scale.

A one gallon (U. S.) graduated cylinder was used to
measure the fuel oil consumed by the dryer. A stop-
watch recorded the time to consume the one gallon of
fuel. When the cylinder was emptied, the time was re-
corded, the stop—watch reset, and the cylinder quickly
refilled with fuel from a previously filled one gallon
container. The amount of electricity was measured with
a kilowatt—hour meter.

The solid material leaving the stack was collected
in screened containers. The openings in the screen were
larger than the average particle size collected, but
the screen very quickly became coated with solid material
thus improving the ability of the screen to catch the
small solid material. Actually, little of the material
was lost. The material collected was removed from the
screen containers and weighed on a spring scale.

21

22

The air velocity profile was determined in the
stack by a vane anemometer. The air in the stack
directly above the initial drying chamber contained much
floating material which interfered with other types of
instruments. This floating solid material did not affect
the vane anemometer. Average air velocity measurements
were taken over a period of two minutes.

Temperatures in excess of 1000° F were anticipated,
therefore, Chromel-Alumel (type K) thermocouple wire was
selected. Chromel-Alumel thermocouples have a range of
0° to 2500° F. Glass insulation was chosen that would
withstand a temperature of 1800° F. A two channel con-
tinuous potentiometric recorder with an internal tempera-
ture compensating reference junction was used. One
channel recorded over the range 0° to 800° F while the
second recorded over the range 0° to 2500° F. A special
thermocouple switch was used when temperature readings
were taken for many different locations.

Moisture content was determined by weighing several
samples of material before and after drying in a labora-
tory oven at 220° F, for twenty four hours. All moisture

contents in this work are expressed on a wet basis.

Moisture Content (per cent, w.b.) = -————— x 100

CHAPTER VI

RESULTS AND DISCUSSION

6.1 Performance of the Dryer

 

The dryer had been used to process poultry excreta
on several farms in the Mid-west before this research
began. It was known that the machine would do a satis-
factory job of drying poultry excreta, but would the
dryer satisfactorily process other animal excreta?

Bovine excreta, both dairy and beef, containing no
straw was dried successfully. Tests were conducted with
l/A inch and 3/8 inch screening plates. Dried excreta
particle size was somewhat larger when the 3/8 inch
screen was used. The production rate was higher with the
3/8 inch screen, but the difference was not determined.
Complete drying can be accomplished when the 3/8 inch
screening plate is used with a resulting higher produc—
tion rate, therefore, this size is recommended over the
l/A inch screen.

Swine excreta was tested successfully, but with
some difficulty. The partially dried excreta granules
were quite coarse and rough, causing a great deal of
friction. As a result, the excreta did not slide down
the floor of the initial drying area easily. A slight

23

2A

build up of excreta at the leading edge of the hammer
mill was often enough to restrict the flow of material
into the hammer mill. Even then, the hammer mill did not
throw the material very far up into the initial drying
area limiting granule exposure to the drying air.

This sluggishness of flow resulted in a concentra-
tion of material at the lower end of the initial drying
area near the hammer mill, and the upper end of the
screening plate was not covered with excreta granules.
Under these conditions, some of the hot air short cir-
cuited through these holes and did not come in contact
with excreta. Short circuiting of hot air was greatest
when the 3/8 inch screening plate was used, but was not
satisfactorily reduced when the 1/A inch screen was put
in place. This short circuiting effect also occurred,
to some extent, when the 3/8 inch screen was used during
the drying of bovine excreta. Simply eliminating the tOp
few rows of holes in the screening plate would probably
correct this problem.

The control system was over ridden during trials
with swine excreta to increase the amount of dry excreta
feedback. This extra amount of feedback increased the
fluidity of the material in the initial drying areas.
The feedback, however, dropped into the center of the
hammer mill in a narrow stream. Therefore, distribution

across the dryer was poor. This did not cause any

25

complication with poultry or bovine excreta, but did
cause trouble when processing swine excreta. The material
flowed well in the center of the dryer where feedback was
applied, but did not flow well along the edges. A more
even distribution of feedback across the dryer should
eliminate this lack of uniform flow.

Some sticking occurred as the highly fluidized
swine excreta slurry moved down the first inclined sur-
face towards the hammer mill, although it did not pose
much of a problem. Drying rate probably could be main-
tained and sticking eliminated if the plate temperature
could be reduced.

Dryer performance while processing swine excreta
seemed questionable, but the machine performed quite well
under these difficult circumstances. By using a 3/8 inch
screening plate with the tOp few rows of holes elimi-
nated, and a greater amount of more evenly distributed
dried excreta feedback, the dryer would probably do a
good job of processing swine excreta.

During one trial, the swine excreta entered the
dryer in frozen chunks as much as three inches across.

No difficulty occurred during the drying, but extra heat
was needed to thaw out the frozen excreta.

Mink excreta was dried successfully, and no diffi-
culty resulted when the l/A inch screening plate was used.

Only one trial was conducted with mink excreta, and the

26

dryer temperature setting was a little too low. A
drier final product and a higher production rate could
have been achieved if the machine had been Operated at
the same settings as used for poultry excreta. The mink
excreta did, however, contain some litter which tended to
lower the production rate of the dryer.

The first dryer test with litter in the excreta
was conducted with straw in bovine excreta. The mixture
was mainly wet soggy straw. The dryer could not handle
this excessive amount of straw. Later the dryer was
tested with bovine excreta containing lesser amounts of
straw. Under these new conditions, the dryer operated
smoothly. One test was run with bovine excreta of which
2.0 per cent of the dry matter was straw, and another with
3.9 per cent straw. As the amount of straw increased, the
production rate and efficiency decreased. Tests were
made with a 3/8 inch screening plate in the dryer. These
percentages of straw in the excreta may seem insignifi-
cant, but straw is largely void, and a large quantity has
very little weight.

Poultry excreta containing litter of wood chips
was tested. The dry matter in the mixture consisted of
18.3 per cent litter. There was some difficulty with
this high concentration of litter. Some of the larger
pieces of wood (as large as 5 1/2 inches long, 1/2 inch
wide, l/A inch thick) did not break down sufficiently to

pass through the 3/8 inch holes in the screening plate.

27

 

Figure 10. Poultry excreta Figure 11. Bovine excreta
mixture containing 18.3 per mixture containing 3.9 per
cent wood chip litter. cent straw.

 

Figure 12. Granules of dried poultry (left),
bovine (center), and swine (right) excreta.
Each small division on the scale is 1/32 inch.

28

As a result, an accumulation occurred in time (roughly

30 minutes) after which the bulk of the unpulverized wood
chips had to be removed by hand. Other than the problem
of repeated accumulation of larger chips, the wood litter
passed through the dryer quite well.

Turkey excreta containing feathers was tested. The
feathers were quite brittle and the hammer mill effec-
tively pulverized them. There was no noticeable accumu-
lation of feather material above the l/A inch screening
plate.

A higher individual production rate on poultry
excreta was obtained with the l/A inch screening plate,
but higher overall production could be obtained by using
a 3/8 inch screen. Complete production figures are listed
in Tables 2 and 3.

The production and efficiency of the dryer were
greatly reduced as the amount of litter was increased.

If bovine excreta without any straw was processed in the
dryer using a 3/8 inch screening plate with the upper few
rows of holes eliminated, the production rate and effi—
ciency should be higher.

Although higher production was indicated for swine
excreta when the l/A inch screening plate was used, this
most likely would not have been the case if a 3/8 inch
screening plate was used with the tOp few rows of holes

eliminated.

 

29

 

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m.a: :.m mza m.ca m.mw mam =w\m mposoxm osfism

H.:: :.m mma m.ma m.m> mmm =:\H mposoxo osfism

:.w: o.m oza m.sa 2.mm mma =m\m smspm Rm.m
mummoxm msfi>om

o.Hm o.m :ma o.mH :.mm mzm =m\m sanpm so.m
mumsoxo ocfi>om

5.2: m.m mma s.w z.mm mmm :m\m smpufia gm.wa
machoxm mmpasom

w.as :.m mam H.HH m.os osm =m\m sumsoxs aspasos

Illoém Rum. Rim. PI; FBI. A film. .mss

H.mz m.m mma s.oa n.2n mam

m.co o.m pea H.m m.ms emm

o.mm o.m omm m.w H.ms emm

m.wm o.m mom w.m m.cm smm

m.sm s.m mom e.s o.ms mm: =:\H sadness sspasos

t e... is. ...: .2; “Mean
soap Samsoo oo>oEmm asapmfloz asapmfioz . Hdfihtpmz
mo:Ofi0flmgm Hmsm smpmz Hmcflm HmeHCH opmsoxm compom

ps3

 

.mamflsp Ham CH soon had masonnppmsoafix m.: omwmso>m soapdESmcoo mpHOHspooam .mposoxm
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3O

 

 

s.mH Hm. m: :.mH m.ca =:\H spssoxs xcflz
m.ca In- mm m.mm m.m =m\m sumsoxs msfism
m.ma In: mm m.Hm m.m =:\H sumsoxm ssflsm
m.sH mm. m: o.mm o.HH =m\m gasps sm.m
mumsoxm mcfl>om
o.mH AH. mm m.Hm m.w =m\m smspm so.m
whopoxm ocfl>om
s.m mm. mm m.cm o.w smxm ssssfia sm.ma
muosoxo msofisom
H.HH mm. mm m.sH m.m smxm dsmsoxs Assfisos
w.s mm. em o.mH m.e =:\H sumsoxs assasos
A.o.3 sv mpowwmmewmz Am3xv Aammv Asnv Oman
OLMWMMME Lo cop pom COB\.pooam :oB\Hmsm :09\oEHB coowom Hmfisopmz

mposoxm omfiso

 

.mpmsoxm no: mo cop oco mo
mEsou CH commosdxm somso moosoxo Hmeflcm who sow mosswflm QOHposoosm .m mqm<e

31

In Table 2, pounds of water removed from the excreta

per hour was determined by the following expression:

Pounds H20 Removed per hour

(1 - MO) Mf

= (Production Rate) MO _

 

And, overall thermal efficiency of the dryer was deter-

mined from the following:

Overall Thermal Efficiency, per cent

(1b. water evaporated)(latent heat, Btu/lb)(100%)

 

(gal. fuel used)(net heating value of fuel, Btu/gal)

The net heat value of the fuel oil was taken as lA0,000
Btu per gallon, and the latent heat of vaporization of
water taken as 970 Btu per pound.

A variation in final moisture content of from five
per cent to about twenty five per cent could be obtained
by changing the dryer thermostat setting. The prOper set-
ting for a particular output moisture content was deter-
mined easily in a few minutes of experimentation.

The wet excreta production rates of the dryer take
on real meaning when compared with the amounts of ex-

creta voided by poultry and livestock.

32

TABLE A. Excreta voided by poultry and livestock,
Maddex (1965).

 

 

Weight of Weight of Per cent Moisture

Animal Animal excreta/day Feces-Urine Content

(lb) (lb) (per cent) (per cent)
Hens A to 5 l/A 100 - 0 76
Bovine 1A00 90 70 - 30 8A

1000 6A

750 A8
Swine 175 12.5 73 - 27 8A

100 7.0

 

From the figures in Table 5, the dryer has potential
for processing excreta produced by poultry and swine, but
the amount of excreta voided by bovine seems too great

for the capacity of this dryer.

TABLE 5.--Projected number of animals that can be served with
the excreta dryer Operating forty hours per week.

 

 

Weight of Weight of A0 hour Animals
Animal Animal excreta/wk. capacity of served by

(lb) (lb) dryer (lb) dryer
Hens A to 5 1.75 13,600 7,800
Bovine 1,A00 630 9,720 15
1,000 AA8 9,720 22
750 336 9,720 29
Swine 175 88 9,000 102

100 A9 9,000 18A

 

33

Exhaust analysis revealed that solid material was
released from the stack during drying. Up to six per cent
of the dry matter entering the dryer escaped from the
stack. This solid material was very light and fine. It
consisted of fines and hair from feathers in the case of
poultry excreta, and animal hair and fines in the case of
bovine excreta. This material could be removed by drawing
the exhaust air through a cyclone separator, or eliminating
it with an afterburner.

A slight odor was released form the stack, but it
did not resemble the fresh excreta odor. A more thorough
evaluation of the level of odor from the stack should be
conducted using impartial observers. Ostrander (1969)
described the odor from an animal dehydrator as less
offensive than fresh excreta. He stated that these odors
are not serious because there are means of eliminating
them at the dehydrator.

Even though the exhaust air was released to the
outside of the building, there was considerable dust in
the air around the dryer. The operator used a face mask
to reduce intake of dust. Dust accumulation in nasal

passages became offensive if the face mask was not used.

6.2 Properties of the Dried Excreta
Bulk density is an important factor to consider
where handling and storing of materials is concerned.

Sobel (1966) reported a bulk density of 66 pounds per

3A

TABLE 6. Solid material released from the dryer stack.

 

Fresh Stack
Screen Excreta Material
Material size Dry Dry

Moisture Stack Loss
Content of Per cent

 

. Stack Mat. of Total

(in) M?E£§r Matger (per cent) Dry Matter
Poultry l/A 137 8.2 37.9 6.0
120 “.7 20.5 3.9
avg. 29-2 H.9
Poultry 3/8 169 2.5 22.7 1.5
Bovine 3/8 120 I 3.6 3A.? 3.0

 

TABLE 7.-—Particle size distribution of solid material
released from the dryer stack. The material larger than
0.0930 inches was primarily clumps of fines matted

. together during storage.

 

 

Particle Poultry Poultry Bovine
size l/A" Screen 3/8" Screen 3/8" Screen
(in) % % %

.0000
Al.0 36.6 29.3

.0165
23.A 25.3 27.7

.0232
' 15.2 21.3 22.0

.0328
7 9 11.2 12 1

.0550
2.7 1.8 2.6

.0930
9.8 3.8 6.3

 

35

cubic foot for bovine excreta. Comparing these values
with the ones in Table 8, the bulk density of the dried
granular excreta is about one-third the value for the
fresh excreta. Also, as would be expected, the bulk

density increases as granule size decreases.

TABLE 8. Bulk density of dried animal excreta at 10 per
cent moisture content, wet basis.

 

 

Dried Material Sgiizn Bulkbggngity
Poultry excreta l/A" 23.A
Poultry excreta 3/8" 16.8
Poultry excreta

18.3 % litter 3/8" 17.A
Bovine excreta l/A" 19.0
Bovine excreta

2.0% straw 3/8" 12.3
Bovine excreta

3.9% straw 3/8" 10.9
Swine excreta l/A" 20.9
Swine excreta 3/8" l9.A
Mink excreta l/A" 22.7

 

The dried excreta in all cases was a fine granular
material. The degree of fineness is illustrated in
Figures 13 to 15. It is interesting to note that par—

ticle size distribution remained relatively constant for

36

 

Poultryl

 

- -- -Poultry2

-o—o-a-a

L.---__ ----- Poultry3

 

 

50"
P I
(19 4
m 4°".
3 .
a] 4
._ 30-.
2
8 20‘" I-o—-d
h I
Z
8 , ---d
K
LIJ
Q.

 

 

 

 

 

 

 

I l v 1 I I r r *1

.04 .06 .08 .I0 .I 2

PARTICLE DIAMETER (inches)

Figure 13. Particle size distribution of poultry excreta.
(l) l/A inch screen; (2) 3/8 inch screen; (3) 3/8 inch
screen, 18.3 per cent wood chip litter.

 

PERCENT OF TOTAL WEIGHT

0 V T

 

 

Bovine'

- - ---Bavihe2

 

 

 

V v I t f T

.03 .0'6 .03 .l0 .l2

PARTICLE DIAMETER (inches)

Figure 1A. Particle size distribution of bovine excreta.

(l) l/A inch screen; (2

) 3/8 inch screen, 2.0 per cent

straw; (3) 3/8 inch screen, 3.9 per cent straw.

37

 

 

 

 

 

 

 

 

 

5 i
01 SwMe'
+- I
I . 2
$240- ---------- ----- Swine
w . ...-
3 j ~----
_. . ————— Mink'
E 301
E : I- ..
“[20"
o 4
E : .-‘::1 ........... at
E§IO‘
a: ‘ -- -----
m :P ooooo -
a. , ____ %-———._._.-.-.-
O f v v r f f ' V V V ' '
.02 .04 .06 .08 IO J2
PARTICLE DIAMETER (inches)
Figure 15. Particle size distribution of swine and mink

excreta. (1) 1/A inch screen; (2) 3/8 inch screen.

 

 

 

 

 

Figure 16. Time required for excreta to travel through the
dryer. The total estimated average exposure time for
poultry excreta was 3.5 minutes. Legend: (C) initial dry-
ing area; (F) intermediate drying area; (G) final drying
area; (*) for poultry excreta, l/A inch screen.

38

a particular animal excreta regardless of the amount of
litter or size screening plate used except in the case of
poultry excreta where the 3/8 inch screen was used. Table
9 compares the particle size distribution for all of the
animal excreta tested.

A Ro-Tap machine and Tayler sieves were used to
determine the particle size distribution of the dried
excreta. A standard test was conducted where 100 i 1
gram of excreta was sifted for three minutes.

The odor of the dried poultry and mink excreta did
not resemble the fresh excreta. There was resemblance
between the odor of the dried as compared to the fresh
bovine and swine excreta, however, much of the odor was
removed. When dried poultry excreta was applied on the
surface of flower beds in a residential area, the odor
was offensive for a few days although it did not resemble
the fresh poultry excreta. The odor was, however, con-
fined to the area of the flower beds and could not be
detected a short distance away. After a few days the
dried excreta became odorless. Also, when the dried
excreta was rewetted, the odor increased.

The fresh and dried excreta was analyzed to deter-
mine whether the nutrient levels were altered by the
high temperatures near 1000° F for up to 15 seconds.

The fresh and dried analysis were expressed on an oven

dry basis so they could be compared directly.

39

 

 

 

s.m 2.:H H.sm s.mm m.mH m.o =s\H wetness xcfiz
m.m p.03 s.mm m.ca H.e H.m =m\m «esteem edflsm
m.m o.mm s.os m.sH :.e H.m =:\H wetness esfism
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A0

Only one set of the bovine and swine excreta were
analyzed. Therefore, the figures serve only as indi-
cators of possible trends and cannot be cited as the
rule. Notice for poultry excreta in Table 10 the level
of phosphorus decreased slightly (3.6%), and nitrogen
decreased a much greater amount (26%). This apparent
reduction in nitrogen may render the dried poultry
excreta more suitable as a lawn and garden fertilizer.
Potassium was also found to decrease (17%) and protein
(26%). Three additional sets of data in Table 11 further
support these trends. These figures show that for poultry
excreta high temperature appears to decrease nutrient
level. Tables 12 and 13 give nutrient levels for bovine
and swine excreta.

TABLE 10. Poultry excreta nutrient levels before and
after drying.

 

 

Crude
N P K fiber H20 Protein

% as
received

FRESH 1.22 .623 .570 A.31 7A.89 7.63

DRIED 3.A8 2.30 1.81 15.81 3.60 21.75
% oven
dry basis

FRESH A.86 2.A8 17.16 -— 30.39

2.27
DRIED 3.61 2.39 1.88 16.A0 —- 22.56

 

A1

TABLE 11. Feed value of poultry excreta before and after
drying. Expressed in per cent on an oven dry basis.

 

Non—

 

, Crude
Ca1c1um Phosphorus Protein . Protein
Nitrogen fiber
FRESH (1) 5.85 2.86 2.7A 19.57 12.03
(2) 7.A9 2.85 2.57 2A.72 15.0A
(3) 8.35 3.19 A.75 1A.96 12.71
avg. 7.23 2.97 3.35 19.75 13.26
DRIED (l) 6.16 2.77 1.5A 18.15 11.21
(2) 8.17 2.89 1.55 21.87 10.97
(3) 7.31 2.97 2.A9 16.75 12.A6
avg. 7.21 2.88 1.86 18.92 11.55

 

TABLE 12. Bovine excreta nutrient levels before and after

 

 

drying.
Crude
N P K fiber H20 Protein
% as received
FRESH .A89 .136 .012 5.21 81.56 3'06
DRIED 2.56 1.12 .79 27.61 3.02 16.00
% oven dry
basis
FRESH 2.65 .738 .65 28.25 -- 16.59

DRIED 2.6A 1.15 .81 28.A7 -- 16.50

 

A2

TABLE l3.--Swine excreta nutrient levels before and after

 

 

drying.
Crude .
N P K Fiber H20 Prote1n

% as received
FRESH 1.05 .515 .A26 3.1A 75.21 6.56
DRIED 3.3A 2.12 1.31 13.12 6.A9 20.88
% over dry
basis
FRESH A.2A 2.08 1.72 12.67 -- 26.A6
DRIED 3.57 2.27 1.A0 1A.03 -— 22.33

 

The excreta used for these drying trials was ob-
tained from the university farm and undoubtedly some of
the animals were on special feed rations. This may have
resulted in abnormal levels of some of the nutrients.
Table 1A was included as a comparison with levels of

nutrients obtained by Benne et a1 (1961).

6.3 Study of the Drying Process
Temperature measurements at different locations
within the dryer revealed the extremes of the stress
conditions applied to the machine and showed where heat
was being applied and to what intensity. Air, sometimes
at a temperature exceeding 1000° F, made contact with the

partially dried material falling through the holes in

A3

the screening plate. This same high temperature air was
directed onto the granules of excreta on the channeled
tray. Sparks were always present in the air leaving

this intermediate drying area during normal drying. When
a 3/8 inch screening plate was used in the dryer, sparking
was reduced but not eliminated. A lower temperature was
applied to the bottom two drying surfaces. Therefore,

the excreta granules were allowed to cool before leaving

the dryer.

TABLE 1A. Nutrient levels of animal excreta converted to
per cent oven dry basis from Benne et a1 (1961).

 

 

Excreta Nitrogen Phosphorus Potassium
Poultry 3.A8 0.87 0.76
Dairy cattle 2.67 0.A8 2.38
Fattening cattle 3.50 1.00 2.25
Swine 2.00 0.57 1.52
Horse 1.72 0.25 1.50
Sheep A.00 0.60 2.85

 

The temperature gradient can be used as a rough
approximation of the drying taking place. For example,
a temperature drop of about 250° F occurred across the
lower drying area and 600° F across the initial drying

area.

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I50°
“30°
5C”
AJR '
C?
Figure 18.

Temperature variation with time within the dryer.
Each curve is representative of temperature at the point,
but the curves were not obtained simultaneously.

A6

One double nozzel oil burner was used as the heat
source for the dryer. When the thermostate tripped the
burner off, a dramatic temperature drOp in the drying area
resulted. In the lower portion of the dryer, vapor could
be seen rising from the surface of the excreta granules
when the burner was off. Vapor was not visible when heat
was applied. A two burner system, where one remains on
continuously and a second is operated by a thermostat,
would probably result in a higher and more even drying
rate. When the air temperature drops, the water holding
capacity of the air also drops. This is evidenced by
the visible vapors.

In most areas, the surface of the dryer did not get
hot enough to cause discomfort if a person touched it.

The two main exceptions were the area on the end near the
burner, and the area near the hammer mill. In neither of
these areas was there air circulated in the wall to carry
away the heat as was the case in the side walls. The
distribution of heat over the surface of the dryer is
shown in Figure 19.

The quantity flow of air in the dryer stack about
three feet above the initial drying area was determined
from average air velocity measured with a vane anemometer.
The air flow was found to vary between 1200 and 1500 cubic

feet per minute. The flow rate was to some extent

A7

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A8

dependent on the outside air speed across the top of the
stack.

There was actually three modes of drying. Fresh
excreta and dried granules were fed into the initial area
and mixed with about thirty pounds of 35 to A0 per cent
moisture residual already in this area. The dried
poultry excreta feedback rate was 3.3 pounds of dry
matter per minute for lA.8 minutes out of each hour.
Pneumatic drying was the primary mode in the initial area.

The second mode consisting of a combination of
pneumatic and tray drying occurred in the intermediate
area. Air at about 1000° F was blown directly onto the
granular material which remained in this area for approxi-
mately 15 seconds.

Tray drying was the final mode which occurred on the
last three inclined surfaces. A lesser amount of lower
temperature air passed over the surface of the granules
on these trays. The material remained in this area for
about 75 seconds.

A schematic flow diagram of the material during the
drying process is shown in Figure 20. Table 15 shows the
amount of moisture removed during each step of drying
poultry excreta where a 1/A inch screening plate was used
in the dryer. Table 16 gives the breakdown in per cent
of total drying that took place in each area of the

dryer. It is interesting to note that pneumatic drying

FRESH
EXCRETA

A9

DRIED

FEEDBACK

 

 

 

MIXING

INITIAL
DRYING

INTERMEDIAT E
DRYING

 

 

 

Figure 20. Diagram of excreta flow through the dryer.
poultry excreta and 1/A inch screening plate, feedback was
3.3 pounds of dry matter per minute for 1A.8 minutes out

of each hour.

For

50

TABLE 15. Water removed during each phase of drying
poultry excreta (l/A inch screening plate).

 

 

Total Dry
Drying Process Weight Mgiitgre ??E§r Matter

(lb) ° ' ' (1b)

Mixing
Fresh excreta 237 75.6 179 58
Dried feedback 52 5.1 3 A9
Stack loss (A.9% m.c.) - A 29.2 - 1 — 3
Residual 30 32.7 10 20
Before initial drying 315 60.6 191 12A
After initial drying 18A 32.7 60 12A
Before intermediate drying 15A 32.7 50 10A
After intermediate drying ' 117 10.9 13 10A
After final drying 112 6.9 8 10A
After elevating 110 5.1 6 10A
Storage 58 5.1 3 55

 

TABLE 16. Per cent of total drying that occurred during
each phase of drying poultry excreta (l/A inch screening

 

 

plate).
Total
Water Per cent of
Drying Process Weight
(1b) (1b) Total Drying
before 315 191
Initial Drying . i,
(120 SEC.) (131) (3%
after 18A 60
before 15A 50
Intermediate Drying
(15 sec.) (37) 21%
after 117 13
before 117 13
Final Drying
(75 sec.) (5) 3%
after 112 8
before 112 8
Elevating (2) 1%
after 110 6

Total Water Removed 175

 

51

accounted for 75 per cent of the total, and 21 per cent
occurred in an area where exposure time was no greater
than 15 seconds. Similar results were obtained in addi-
tional trials.

From these results, most of the water was removed
by pneumatic drying. The parameters controlling pneu-
matic drying would be the parameters controlling drying
in this machine.

In the initial.area of the dryer, fresh excreta was
attached to dried and partially dried granules as these
ingredients were mixed by the hammer mill. Drying took
place as the material passed through the hot air after
leaving the hammer mill.

Olson (1953) and Neel (195Afain studies on
drying of potato granules did not attempt to analyze the
process from a theoretical standpoint. Without such an
analysis, it is difficult to determine the most signifi-
cant parameters controlling drying. In an attempt to
determine the effect of temeprature, air flow, particle
diameters, and particle cloud density on pneumatic drying
in the excreta dryer, the following idealized theoretical

analysis was made.

Assumptions
1. steady state.
2. random distribution of particles in cloud.

3. spherical particles.

52

A. uniform moisture content throughout particle.
5. negligible moisture gradient Within particle.
6. heat and mass transfer between particles and

between particles and walls can be neglected.

m = hD A (CD - Ca) (1)

D
-———->
hD d
(neglect internal moisture gradient)

holds true if: 10 (2)

hD = 3'8h (Sherwood number) (3)
D = 0.779 x 10"Ll T3/2 (A)

for general equation Rohsenaw & Choi, p. 382.

Sh = 3D (Re Sc)1/3 (5)

Re = 239- . (Reynolds number) (6)

S _ v (Schmidt number) (7)
O‘D- .

The following are values of the Colburn jD factor ex-
pressed for fluid flow through a cloud of spherical par-

ticles as reported by 800 (1967).

 

d GO ~ d GO -l—.AA
fi7j‘:f27’> 30 3D = 1'77 u(l — :11 (8a)
d GO d G0 -]-.78

 

67—1—37 < 3° 36 = 5'7 11(1 - e5] (8“

53

The mass flow of the fluid is based on the unobstructed

flow area.

(9)

e = l — ¢ (10)

The number of particles per unit volume (Np) must be

determined experimentally.
_ W 3
Vs _ 6 Np d (11)

v
¢ = 3% (12)

Combining the above equations and neglecting the
dependence of the kinematic viscosity v on the tempera-
ture, the result is the following expression for the mass
transfer coefficient.

_ 3.22 x 10'3 (v¢)'”“ T

_ (13)
D U.ll d1.11

 

In the temperature range (6000 to 1200° R) during
initial drying, the following linear expression closely
approximates the dependence of kinematic viscosity on

temperature.

v = -1.058 - 0.0028A T (1A)

5A
Substituting equation (1A) into equation (13):

2.A2 x 10"!4 ¢'uu (T1.AA - 13.7 T )
h = . (15)
U.11 d1.11

 

It appears that the two primary parameters con-
trolling mass transfer are temperature of the air and
particle diameter. The fraction solid is also a signifi—
cant parameter but to a lesser extent. Air speed is

relatively unimportant unless a large change is initiated.
h ~. if? ' (16)
- D d‘ .

It is important to keep in mind that the preced-
ing analysis of pneumatic drying is set forth for a highly
idealized case. It does, however, give some indication
of the importance each parameter plays in the actual dry-
ing process.

Conditions vary considerably in the.initia1 drying
area,where a form of pneumatic drying takes place. Par-
ticle cloud density is not uniform.. The fraction void
5 is different for each location in the initial area, and
at any one location, a varies with time in a cyclic manner
with a period of about two minutes for poultry excreta
with a one-quarter inch screening plate. The granules
were only approximately spherical with rough surfaces,
and a wide range of diameters. As the granules dried,

average size was reduced by hammer mill action. Average

55

granule size changed in a cyclic manner with a period of
about two minutes for poultry excreta. The moisture con-
tent of the granules was not uniform. High moisture
material was attached to the surface of partially dried
granules. The granules themselves attained an initial
velocity of approximately seventy feet per second at the
hammer mill, while the air speed was in the range of 10
to 15 feet per second. Temperature gradient through the
dryer was great due to rapid mass transfer taking place.
Other parameters such as particle shape, surface roughness
and porosity can be linked to the drying rate.

Even under these varied conditions, one can assume
that the temperature and particle size are key parameters
controlling the drying rate in the initial area. Analy-
sis of conditions in the intermediate area were not made,
but it is safe to assume that temperature and particle
diameter are also key parameters controlling drying in
that area.

To give an idea of the approximate magnitude of the
mass transfer coefficient, equation (15) is evaluated for
the extreme high and low temperatures in the initial dry-
ing area. The granular excreta is exposed to the highest
temperature as it leaves the hammer mill. The approxi-

mate conditions are as follows:

56

T = 1200 0 R

U = 150,000 ft/hr
0 = 0.5

d — 0.00A ft

Mass Transfer Coefficient, h = 2A0 ft/hr

D

About three feet into the initial area from the hammer
mill, the conditions have changed greatly. The following

are approximate values:

T = 700° R

U = 36,000 ft/hr
0 = 0.5

d = 0.00A ft

Mass Transfer Coefficient, h = 7A ft/hr

D

If equation (1) is to be used to determine the rate
of mass transfer, HgLH must be greater than ten. The
values estimated above do not quite meet these condi-
tions, however, for a first approximation equation (1)

can be assumed valid.

CHAPTER VII

SUMMARY

The animal excreta dryer was tested with poultry,
bovine, swine and mink excreta to determine whether these
materials could be dried satisfactorily with this machine.
Poultry excreta containing wood chip litter and bovine
excreta containing straw were also tested. For each of
these materials, the production rate, fuel oil and
electricity consumption, and overall thermal efficiency
were determined.

Some attention was focused on the properties of the
dried excreta. The bulk density and particle size dis-
tribution were determined for each of the materials
tested. The effect of high temperature on nutrient con-
tent of poultry, bovine and swine excreta was also deter-
mined.

Temperature gradient through the dryer and tempera-
ture variation with time at several key locations were
determined. For poultry excreta, the per cent of total
drying in each of the three major areas was determined.
An idealized theoretical analysis of the primary drying
mechanism was undertaken to determine key parameters
controlling drying.

57

58

Conclusions

 

l. The dryer successfully processed poultry, bovine
and swine excreta, although some difficulty was incurred
during trials with swine excreta (p. 23).

2. Poultry excreta of which 18.3 per cent of the
dry matter consisted of wood chip litter, and bovine
excreta with 2.0 and 3.9 per cent straw were dried
successfully (p. 26).

3. Production rates for the dryer ranged form 178
pounds per hour for bovine excreta consisting of 3.9 per
cent straw up to 3A0 pounds per hour for poultry excreta.
Efficiencies ranged form A1.1 to 71.8 per cent (p. 29).

A. Up to 6 per cent of the dry matter entering the
dryer escaped from the stack (p. 33).

5. An odor was released during drying, but it was
different than the fresh excreta (p. 33). There also was
an odor in the dried product, but it was less intense than
the fresh excreta (p. 38).

6. Bulk densities ranged from 10.9 pounds per cubic
foot for bovine excreta consisting of 3.9 per cent straw
up to 23.A pOunds per cubic foot for poultry excreta
(p. 35).

7. A very small percentage of the dried excreta
particles were outside the size range of 0.01 to 0.1

inches (p. 39)-

59

8. In most cases nitrogen, phosphorus, potassium
and protein levels decreased as a reuslt of drying (p. A0).

9. The temperature gradient through the drying
area varied from 1100° F to 200° F (p. AA).

10. Temperature varied widely with time at dis—
tinct locations within the dryer (p. A5).

11. In most areas the temperature on the surface
of the dryer was not high enough to cause personal in-
Jury (p. A7).

12. About 75 per cent of drying occurred in the
initial area by a pneumatic process (p. 50).

13. The most significant parameters controlling
drying were temperature and particle diameter. The mass
transfer coefficient for idealized pneumatic drying
varied directly with absolute temperature and inversely

with particle diameter (p. 5A).

Suggestions for Further Research

1. The time required for combustion to occur for
various temperatures and moisture contents is needed to
establish the upper bounds on drying conditions.

2. A thorough study of pneumatic drying is needed
to investigate the full potential of this process.

3. Air flow through the excreta dryer was not
studied. Undoubtedly an improved air flow pattern would
aid drying. Perhaps an exahust fan should be used to

induce air flow.

60

A. The highest temperatures were applied to par-
tially dried excreta, and on occasion combustion resulted.
A study could be made to determine the effect of applying
the hottest air to the initial drying area.

5. The composition of the exhaust gases from the
dryer should be determined to see if undesirable levels
of air pollutants are being released.

6. Determine the comparative economics of this
and other methods of animal waste management.

7. A thorough study of drying with a flow-through
rotating drum dryer would be desirable for comparison.

8. The consumer market should be tested to deter-
mine whether dried excreta would be accepted as lawn and

garden fertilizer.

REFERENCES

61

REFERENCES

Benne, E. J., C. R. Hoglund, E. D. Longnecker, and R. L.
Cook. Animal Manures, circular bulletin 231.
Michigan Agricultural Experiment Station (1961).

Cooley, A. M., D. E. Steverson, D. E. Peightal, and
J. R. Wagner. Studies on Dehydrated Potato Gran-
ules. Food Technology, 8 (5) (195A), 236-268.

Hall, C. W. Drying Farm Crops. Edwards Brothers, Inc.,
Ann Arbor, Michigan. (1957), p. 121.

High-Temperature Dryer for Poultry Manure. Farm Mechani-
zation and Buildings, May (1968), 27.

Maddex, R. L. Liquid Manure Systems. Information series
no. 150, file l8.A2, Agricultural Engineering
Department, Michigan State University (1965).

Neel, G. H., G. S. Smith, M. W. Cole, R. L. Olson, W. O.
Harrington, and W. R. Mullins. Drying Problems in
the Add-Back Process for Production of Potato
Granules. Food Technology, 8 (5) (195A), 230-23A.

Olson, R. L., W. O. Harrington, G. H. Neel, M. W. Cole,
and W. R. Mullins. Recent Advances in Potato
Granule Technology. Food Technology, 7 (A) (1953),
177-181.

Ostrander, C. E. Waste Disposal Management. Cornell
Agricultural Animal Waste Conference, Syracuse,
N. Y., January (1969).

Restoring the Quality of Our Environment. The White
House. November (1965), 170-171.

Rohsenow, W. M., and Harry Choi. Heat, Mass and Momentum
Transfer. Prentice-Hall, Inc., Englewood Cliffs,

N. J. (1961), p. 382.

Ryder, C. Water Costs Money. Unpublished paper on drying
of chicken manure, England. December (1967).

Shade, F. H. Food Processing Plant. Leonard Hill
Books, London (1967), pp. 336—3A5.

62

63

Sobel, A. T. Physical Properties of Animal Manures
Associated with Handling. Proceedings National
Symposium on Animal Waste Management, Michigan
State University (1966), 27-32.

Soo, S. L. Fluid Dynamics of Multiphase Systems.
Blaisdell Publ. Co., Walthem, Mass. (1967), p. 190.

 

 

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