Spray Dryer Exhaust Heat Recovery
A Techno-economic Assessment Model
Tim Walmsley, M Walmsley, M Atkins, J Neale
PRES 2014
Outline
Research Motive/Context
Overarching Goal
Review of Progress
Exhaust Heat Recovery Modelling
Conclusions
New Zealand University of Waikato
Dairy
Cows
NZ Dairy Performance
0%
10%
20%
30%
0
5
10
15
20
1990 2000 2010 2020
Pe
rce
nta
ge o
f to
tal e
xpo
rts
Val
ue
of
exp
ort
s ($
bill
ion
s)
Year
FutureGrowth?
DairyProducts $$
Milk Powders $$
ExportShare %
Source: Statistics New Zealand, 2013
42.2
19.9
17.115.0
6.2
2.9 2.6
32.1
0
5
10
15
20
25
30
35
40
45
Food &beverage
Petroleum& chemical
Pulp &paper
Woodproduct
Non-metallicmineralsproduct
Metalsproduct
Othermanufacturing
Ener
gy u
se b
y N
Z m
anu
fact
ori
ng
in 2
01
2 [
PJ]
Estimated contributionfrom Dairy processing
NZ Manufacturing Process Heat Use 2012
Source: Survey of New Zealand Energy Use: Industrial and trade sector 2012
Coal & N.G.Fuel
Supply
MP Process Demand
ConversionLosses
Milk Powder Production Utility Demands
H
Condenser
Eva
po
rato
r
Eva
po
rato
r
Eva
po
rato
r
TVR
Spray Dryer
Fluidised Bed (1)
Fluidised Bed (2)
Bag
House
Milk Concentrate
Std Milk
H
H
H
Exhaust Air
CO
W (1
)
HCIP
Steam
Water
Vap. (3)
Inlet Air
CO
W (2
)
CO
W (3
)
Evaporators Spray Dryer
Clean-In-Place (Hot Water)Fluidised Bed (3)
H
Dry
Powder
MVR MVR
CIP
Dry
Powder
Milk Powder Plant
Source
Sink
To cooling
tower
C C C C
H
H
C
Vapour
Liquid
Gas
Important!
Aim: To Investigate How to Maximise Economic Heat Recovery in Milk Powder Production?
Progress to Achieving the Research Goal
4) Heat Exchanger
Design
2) Heat Recovery
Loop
1) Direct Heat Integration
3) Milk Powder Fouling
(1a) PDM Heat Integration Schemes (PRES’12)
Walmsley et al. (2013), App Therm Eng
Milk
(1a) PDM Heat Integration Schemes (PRES’12)
MER A: Split milk & direct condenser integration
13
8
53.954
Pinch
Std milk
COW
Cond. vap.
CIP
64.3
H
H
H
H
H
H
H
59.3
48.5
63.4 58.1
Exhaust air
Milk conc.
FB 1
FB 3
FB 2
Dryer inlet air
15
200
32
45
49
65 54
5575
55 1548.5
Eva
po
rato
r
Zo
ne
Dry
er
Zo
ne
CIP
Zo
ne
42.8
62.5
27.562.5
27.5
15
MER B: Cyclic milk matching & direct condenser integration
13
8
53.954
Pinch
Std milk
COW
Cond. vap.
CIP
64.3
H
H
H
H
H
H
H
44.163.4 58.1
Exhaust air
Milk conc.
FB 1
FB 3
FB 2
Dryer inlet air
15
200
32
45
49
65 54
5575
55 1545.8
Eva
po
rato
r
Zo
ne
Dry
er
Zo
ne
CIP
Zo
ne
52.8
62.5
27.562.5
27.5
48.5
53.5
15
Std milk
CO
W (1
)
Cond.
Vap.
CO
W (2
)
COW (3)
H
CIP
To drain
H
CO
W
Exhaust
Air
Dryer
Inlet AirH
Std milk
CO
W (1
)
Cond.
Vap.
CO
W (2
)
COW (3)
H
CIP
To drain
H
CO
W
Exhaust
Air
Dryer
Inlet AirH
13
8
53.954
Pinch
Std milk
COW
Cond. vap.
CIP
64.3
H
H
H
H
H
H
H
59.343.563.4 57.6
Exhaust air
Milk conc.
FB 1
FB 3
FB 2
Dryer inlet air
15
200
32
45
49
65 54
5575
55 15
Eva
po
rato
r Z
on
eD
rye
r Z
on
eC
IP
Zo
ne
48.5
41.248.5
41.2
21.6
42.8
62.5
27.562.5
27.5
15
43.5
MER D: Cyclic milk matching & indirect condenser integration
13
8
53.954
Pinch
Std milk
COW
Cond. vap.
CIP
64.3
H
H
H
H
H
H
H
59.3
43.5
63.4 57.6
Exhaust air
Milk conc.
FB 1
FB 3
FB 2
Dryer inlet air
15
200
32
45
49
65 54
5575
55 15
Eva
po
rato
r Z
on
eD
rye
r Z
on
eC
IP
Zo
ne
48.5
18.548.5
18.5
21.6
42.8
62.5
27.562.5
27.5
15
MER C: Split milk & indirect condenser integration
43.5
Exhaust
Air
Std milk
CO
W (1
)
Cond.
Vap.
CO
W (2
)
COW (3)
H
CIP
To drain
Dryer
Inlet AirH
H
CO
W
FB 1,2,3
Exhaust
Air
Cond.
Vap.
CIP
Dryer
Inlet AirH
HFB 1,2,3
Std milk
CO
W (1
)
CO
W (2
)
COW (3)
H
To drain
CO
W
9 % Cost reduction, 30% Heat Recovery (HR) improvement
Walmsley et al. (2013), App Therm Eng
(1b) The Cost Derivative Method (PRES’13)
Stream
data
Cost
data
ΔTcont
Pinch
targets
HEN
structure
HE
selection
Adjust
soft
data?
Yes
No
Yes
No
Pinch Design Method
Add UE’s to all
streams
HEN design
(CDM)
De
sig
na
te a
RE
to
act a
s a
TE
Cost Derivative Method
Find θ for RE’s
to UE’s
Iteratively solve
Initialisation
No
No
Yes
Yes
HEN design(s)
using ΔTcont
HE
N s
tru
ctu
re
Relax
network?
Limiting
Tt?
Eliminate
UE?
1
C
H
x
+dTy2
-dQCx
-dQHy
+dQ1
-dTx2
y
+dA1
Hot streams
Co
ld s
tre
am
s
+dTyn
-dTxn
Other
network HE’s
-dCC,ut
-dCH,ut
Tx,s
Ty,s
Tx,t
Ty,t
TC1
TC2
TH1
TH2
-dACx
-dAHy
1
)(
1
)(
1
1
1 dA
dS
dA
dCC
dA
dCC
dA
dTC iutiut
(a) PDM
13 °C
8 °C
53.9 °C54 °C
Milk
COW
Vap.
CIP
64.3 °C1
32
H1
63.4 °C
55 °C 15 °C
(b) CDM
12748 kW
1214 kW 1198 kW
0.11
1452 kW
357 kW
12.1 °C
8 °C
53.9 °C54 °C
Milk
COW
Vap.
CIP
64.3 °C1
32
63.4 °C
55 °C 15 °C
12977 kW
916 kW 1496 kW
0.07
1521 kW
59 kW
H2
H1
H2
5 % Costreduction
Walmsley et al. (2014), App Therm Eng
(1b) The Cost Derivative Method (PRES’13)
+
+ -
-+
+
-
-
-
-
+dA, +dQ
(1b) The Cost Derivative Method (PRES’13)
Walmsley et al. (2014), App Therm Eng
(a) Non-self-interacting open loop
C1
H1
H2
C2
(b) Self-interacting closed loop
C1
H1
H2
No flow-on
C
C
H
H
C
C
H
H
(2) New Heat Recovery Loop Design Method for Improved Inter-plant Heat Integration (PRES’13)
Processing Site
Cold
storage
Process A
H1
Process B
H2
Process C
C1
Process D
H3
Hot
storage
Cold supply
Cold return
Hot supply
Hot return
Process E Process F
C3C2
Thermal
storage
C C H
C H H
Spray Dryer
Exh
0
2
4
6
8
10
12
0 25 50 75 100
C[k
W/°
C]
Time [h]
0
2
4
6
8
10
12
0% 25% 50% 75% 100%
C[k
W/°
C]
Ordered C
Time-ave C
Median C
Peak C
Walmsley et al. (2014), Energy
(2) New Heat Recovery Loop Design Method for Improved Inter-plant Heat Integration (PRES’13)
Stream
data
Select
Qr
Composite
curves
Calculate
Cl(h)
and Cl(c)
Calculate
ΔTmin
Select
Tlc and T
lh
DetermineT
ho and T
co
Scale Cl
CalculateHE areas
Transient
HRL model
T
ΔH
Tco
Tho
Qr
Tlc
Tlh
Cl(h)
Cl(c)
Cold Storage Pinch
T
ΔH
Tco
Tho
Qr
Tlc
Tlh
Cl(h)
Cl(c)
Hot Storage Pinch
ΔTmin
ΔTmin
ΔTmin
ΔTmin
ΔTadd
Walmsley et al. (2014), Energy
(2) New Heat Recovery Loop Design Method for Improved Inter-plant Heat Integration (PRES’13)
7
8
9
10
11
12
0 200 400 600 800 1000
Ave
rage
he
at r
eco
very
an
d s
ola
r h
eat
ing
[MW
]Storage tank volume [m3]
CTS
VTS
VTS with solar
CTS with solar
New HRL Design Method
12.3 MW
Conventional HRL Design Method
9.2 MW
SOLAR
SOLAR
Walmsley et al. (2014), Energy
0
20
40
60
0%
25%
50%
75%
100%
25 27 29 31 33 35 37 39
Sto
rage
te
mp
erat
ure
[°C
]
Ho
t st
ora
ge le
vel [
%]
Day
Tlh
Tlc
Storagelevel
0
20
40
60
0%
25%
50%
75%
100%
25 27 29 31 33 35 37 39
Sto
rage
te
mp
erat
ure
[°C
]
Ho
t st
ora
ge le
vel [
%]
Day
Tlh
TlcStorage
level
(3) Milk Powder Fouling of Flat Plates
Walmsley et al. (2014), J Food Eng
(3) Milk Powder Fouling Model
SMP Deposition model
0.1
1
10
100
1000
10000
20 30 40 50 60 70
γ s[J
/m2 ]
(T - Tg)crit* [°C]
This work - normal impact
This work - oblique impact
Zhao (2009), corrected
Murti et al. (2009)
Paterson et al. (2007)
Hogan et al. (2010)
Murti et al. (2010)
Hennigs et al. (2001)
Particle gunPaterson and co-workers
Fluidised bed tests
Mechanical stir test
Particle gun tests
Model
Upper bound
Lower bound
n
g
n
n
g
n
critg
G
BY
a
YvrD
G
BY
a
YvrD
TT
22525653
1
22525653
2
tan*4
*11
*2442.0log
tan*4
*11
*2442.0log
*)(
Walmsley et al. (2014), J Food Eng
(3) Milk Powder Fouling of Tubes
Front Back
Walmsley et al. (2014), HXF&C Conference
(3) Milk Powder Fouling of Tubes
Round Tube Elliptical Tube
Walmsley et al. (2014), HXF&C Conference
(3) Milk Powder Fouling of Fins
Walmsley et al. (2013), Adv Powder Tech
(3) Model Validation for Tube Fouling
0
10
20
30
40
50
60
70
80
90
30 40 50 60 70 80
Cri
tica
l im
pac
t an
gle
[°]
T – Tg* [°C]
Round tube (4.5 m/s)
Elliptical tube (4.5 m/s)
Model (4.5 m/s)
90°
30°
60°
0°
45°
45°
Air Flow
c)
Air Flow
90°
30°
60°
0°
60°
30°
b)
Air Flow
44°
71°
90°
30°
60°
0°
a)
90°
30°
60°
0°
45°
45°
Air Flow
c)
Air Flow
90°
30°
60°
0°
60°
30°
b)
Air Flow
44°
71°
90°
30°
60°
0°
a)
(3) Application of Fouling Results
0
20
40
60
80
100
40 50 60 70 80
Air
mo
istu
re c
on
ten
t (g
/kg)
Air temperature (°C)
Outlet temperature
Significant deposition
initiates
Dryer exhaust
Rear section of heat exchanger
Front section of heat exchanger
High Fouling Region
Low Fouling Region
Curve dependent on velocity
57 °C
(4) Exhaust Heat Exchanger Design Problem
Heat Recovery Savings
High Air Flow Resistance
High Fouling
High air speed
Low air speed
Few tuberows
Max. Qr
No Qr
Many tube rows
Design
Plant Specific Analysis System Integration
Design Parameters
Air Velocity (HX Face Size)
Tube Diameter
Tube Spacing◦ Transverse, Longitudinal
Fin Dimensions◦ Height, Pitch, Thickness
Number of passes
Loop flow rate
Walmsley et al. (2014), CET (PRES’14)
Spreadsheet Optimisation Model
User-Defined Inputs
Coupled HX System Design◦ Face size (i.e. air velocity)
◦ Tube & fin dimensions
◦ Pass arrangement
◦ Pump, piping & Buffer tank
◦ Fan
Economic Parameters◦ Steam & electricity price
◦ Capital cost formula
◦ Discount rate
◦ Price inflation
◦ Production hours per annum
◦ Cost to clean
Fluid Flow Specifications◦ Exhaust air temperature, flow rate,
humidity
◦ Inlet air temperature, flow rate, humidity
◦ Intermediate fluid (loop) flow rate
Fouling & Cleaning Parameters◦ Powder concentration
◦ Particle size distribution
◦ Run length
◦ Wash length
◦ Model time step
Spreadsheet Optimisation Model
Fouling Prediction Model
Milk powder
particle size
distribution
Fouling and
cleaning
parameters
Calculate critical impact
angle
Calculate tube area
coverage
Calculate probability of
sticking
Estimate probability of
impact
Determine mass
deposited
Next particle size
Next time step
Next tube row
Estimate temperature
profile in HE
Estimate Rf and ΔP
Calculate payback,
NPV and IRR
Calculate input
particle size
distribution to row
Heat
exchanger
geometry
Process
stream data
Milk powder
particle size
distribution
Fouling and
cleaning
parameters
Calculate critical impact
angle
Calculate tube area
coverage
Calculate probability of
sticking
Estimate probability of
impact
Determine mass
deposited
Next particle size
Next time step
Next tube row
Estimate temperature
profile in HE
Estimate Rf and ΔP
Calculate payback,
NPV and IRR
Calculate input
particle size
distribution to row
Heat
exchanger
geometry
Process
stream data
Key Model Outputs
Average heat exchanger duty
Temperature profiles within a heat exchanger
Fouling over time◦ Mass deposited
◦ Thermal and hydraulic resistance over time
◦ Duty over time
Cost estimations
Economic Indicators (NPV, IRR, Payback)
Fouling Model Results
0
100
200
300
400
500
0.0
0.5
1.0
1.5
2.0
2.5
3.0
0 100 200 300 400 500 600 700
Exh
aust
pre
ssu
re d
rop
[P
a]
He
at r
eco
very
[M
W]
Time in operation [h]
CF CB EB
CF CB EB
Qr :
ΔP :
Effect of Number of Tube Rows
NPV IRR
-$1.5
-$1.0
-$0.5
$0.0
$0.5
$1.0
$1.5
$2.0
$2.5
$3.0
0 10 20 30 40
Ne
t P
rese
nt
Val
ue
[$
mill
ion
s]
Number of tube rows in the exhaust heat exchanger
CF - NPV CB - NPV EB - NPV
0%
20%
40%
60%
80%
100%
0 10 20 30 40
IRR
Number of tube rows in the exhaust heat exchanger
CF - IRR CB - IRR EB - IRR
Effect of Exhaust Air Velocity
$0.0
$0.5
$1.0
$1.5
$2.0
$2.5
$3.0
0 2 4 6 8 10
Ne
t P
rese
nt
Val
ue
[$
mill
ion
s]
Exhaust heat exchanger face velocity [m/s]
CF - NPV CB - NPV
EB - NPV
0%
20%
40%
60%
80%
100%
120%
0 2 4 6 8 10
IRR
Exhaust heat exchanger face velocity [m/s]
CF - IRR CB - IRR EB - IRR
NPV IRR
Site Specific Factors Affecting Economics
◦ Inlet air temperature (outside 15°C or inside 33°C)
◦ Inlet air absolute humidity (outside or inside)
◦ Exhaust air temperature (+5° → +25% HR)
◦ Bag filters (low powder conc.)
◦ Inlet and exhaust fan capacity (reduce cost by ~25%)
◦ Existing pre-heaters using utility
◦ Existing heat recovery to dryer inlet air
Site Specific Factors Affecting Economics
◦ Re-usable existing ducting (reduce cost by ~20%)
◦ Operating and production hours
◦ Price of energy (varies by 30 – 50%)
◦ Space
◦ Inlet air heater bottleneck
◦ Good attitude to change
10
100
1,000
10,000
100 1,000 10,000 100,000
Hea
t Tr
ansf
er t
o P
ress
ure
Dro
p R
atio
Reynolds Number
Effect of velocity
Effect of number of rows
Effect of longitudinal spacing
Effect of transverse spacing
Effect of fin pitch
Effect of fin height
Effect of fin thickness
Present Work: Design Optimisation
Overall Conclusions
• New Zealand milk powder plants can economically increase HR by ~20 %; exhaust heat recovery (HR) is the single largest opportunity
• Exhaust HR can play an integral part in supplying heat to neighbouring plants via HRLs
• For SMP, a final exhaust air temperature above 55 °C can minimise particulate fouling problems
• Exhaust HR is economic for good sites (payback time of ~1.6 years, NPV of NZ$2.9 million and IRR of 71 %)
•Site selection is important
Thesis Link & Any Questions?
Heat Integrated Milk Powder Production
By Tim Walmsley
http://hdl.handle.net/10289/8767