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Heat recovery from a boiler exhaust to pre-heat air to a spray dryer
Heat RecoverySystems Vol. 6. No. 1. pp. 11-23, 1986 Printed in Great Britain.
0198-7593/86 $3.00 + 0.00 Pergamon Press Ltd
HEAT RECOVERY FROM A BOILER EXHAUST TO PRE-HEAT AIR TO A SPRAY DRYER* A DEMONSTRATION
PROJECT
AT BIP CHEMICALS
LTD
Abstract--This demonstration project at BIP Chemicals Ltd, is one of a package of projects being funded by the Energy Efficiency Office of the Department of Energy, under the Energy Efficiency Demonstration Scheme. It involves the recovery of waste heat to pre-heat spray dryer inlet air. It is estimated that the adoption of this technology within the UK will lead to energy savings worth £1.5M/year by 1990. The project at BIP demonstrates the use of a heat pipe exchanger to recover the heat from a boiler exhaust in order to pre-heat the inlet air. Other projects in the package are at Clayton Aniline Ltd and ABM Chemicals Ltd. In these the heat is recovered from the spray dryer exhaust using a run-around coil and a glass tube heat exchanger respectively. The performance of the system at BIP was independently monitored by the Hatwell Laboratory, and this report details the performance of the exchanger and the resulting energy and cost savings at the spray dryer. The total cost of the installation was £22,000, of which £5500 was funded by the Energy Efficiency Office. The heat recovery system has resulted in a 27% reduction in the specific energy consumption of the spray dryer. For BIP in 1983, this was worth £11,300/year giving a simple payback period of 1.9 years. The savings associated with this type of heat recovery system are dependent upon a number of factors including: (i) The quantity of available flue gas---at the maximum flue gas flow-rate, the savings to BIP would be worth £14,500/year, giving a project payback of 1.5 years; (ii) The frequency of operation of the spray dryer--BIP operated their dryer for 3677 h in 1983. Assuming a notional 6000 h, the savings, at maximum flue gas availability would be worth £22,000 with a payback of one year; (iii) The mode of operation--BIP installed the exchanger in a co-current flow mode. In the more efficient counter-current orientation, savings worth £31,000 could have been achieved giving a payback period of just eight months. BIP installed the exchanger in a cocurrent mode because of the simplicity of installation. A number of operational factors associated with the heat exchanger have been identified in the course of this project: (i) It is essential to ensure that the heat pipe heat exchanger cannot be subjected to temperatures in excess of the design limit. During the course of this project this led to tube fracture which has necessitated the replacement of the exchanger unit: (ii) To enable good efficiency to be maintained, it is essential that provision be made to allow for routine cleaning of the heat exchanger. In addition, to maximise the cost savings, dryer operation should be as efficient as possible. A number of areas found worthy of investigation in order further to improve performance on this particular dryer are as follows: (i) Leakage through the explosion relief doors; (ii) Excessive start-up and shut-down periods; (iii) Inefficient scheduling of dryer operation. These areas have now been improved with significant further operational savings. This project has shown that the use of a separate boiler exhaust, even if located some 60 m from the dryer, is a practical and cost-effective means of improving the efficiency of a spray dryer.
1. I N T R O D U C T I O N
Background to the project S p r a y d r y e r s a r e u s e d in m a n y i n d u s t r i e s , l a r g e l y b e c a u s e t h e y p r o d u c e a f r e e - f l o w i n g s p h e r i c a l p r o d u c t . T h e r e a r e b e t w e e n 150 a n d 200 s u c h d r y e r s in t h e U . K . T h e f e e d to a s p r a y d r y e r has to be in t h e f o r m o f a l i q u i d o r p u m p a b l e slurry, w h i c h i n v a r i a b l y m e a n s t h a t it h a s a r e l a t i v e l y *Prepared by the Energy Technology Support Unit, AERE Harwell, Oxfordshire, acting on behalf of the Energy Efficiency Office of the Department of Energy, from reports by the Harweil Laboratory. 11
12
ENERGY TECHNOLOGY SUPPORT UNIT
high moisture content compared with many dryer feedstocks. To evaporate this moisture requires very large quantities of energy. Operating a spray dryer is generally much more expensive than the alternative of evaporating or filtering the feed and using some other type of dryer. This penalty is accepted for reasons of improved product quality. Spray drying in the U.K. consumes of the order of 500,000 tonnes of coal equivalent per year (tce/year). Pre-heating the inlet air to a spray dryer can be extremely beneficial as it leads to a direct reduction in the fuel consumed by the dryer's heating system. Nationally, potential energy savings are estimated to be of the order of 20,000 tce/year. The pre-heating can be achieved by two methods. First, heat can be recovered from the dryer's own exhaust air. There are currently two demonstration projects on this topic. One, at Clayton Aniline, is demonstrating the use of a run-around coil heat exchanger and the other, at ABM Chemicals, the use of a glass tube exchanger. Alternatively, heat can be recovered from some external source, such as boiler flue gases. This is the subject of the BIP Chemicals Ltd demonstration project described in this report. BIP Chemicals Ltd, part of the Turner and Newall Group of Companies, manufacture a range of moulding materials and resins at their Oldbury Works. A proportion of the aqueous malamine and urea resins is dried in a Niro Atomizer A/S spray dryer. Also, on the same site, BIP manufacture formaldehyde in two plants adjacent to the spray dryer. The residual gases from these plants contain approximately 20% hydrogen and traces of methanol and formaldehyde. For environmental reasons these trace components must be eliminated and, as the gas has a significant calorific value, it is burnt in a "tail-gas incinerator" to produce steam. The temperature of the flue gas leaving the incinerator is about 250°C. In April 1982, BIP installed a heat pipe exchanger, supplied by Isoterix Ltd, in the boiler exhaust to recover heat from the flue gas to pre-heat the inlet air to the spray dryer. The annual energy consumption of the dryer was initially 13,875 GJ (524tce). The new heat recovery system was expected to reduce this to 9904 GJ (374 tce), an annual saving of 3971 GJ (150 tce), worth £16,150.
Assistance under the Energy Efficiency Demonstration Scheme BIP applied for support under the Energy Efficiency Demonstration Scheme (ED) which is funded by the Energy Efficiency Office of the Department of Energy. The scheme is designed to provide up to 25% of the installation costs and 100% of the monitoring costs for energy saving projects which satisfy certain criteria. Grants are awarded under the Scheme for the demonstration project only; replication projects need to be cost-effective without Government support. The grant is intended to encourage the host company to adopt the new energy-saving technique and helps to offset any perceived technical risks. BIP's application for support under the Scheme was approved in 1981. The Energy Efficiency Office made a grant of £5500 towards the total installation cost of £22,000. In addition the EEO met the cost of the independent monitoring of the performance of the system by the Harwell Laboratory. Mr A. C. Mercer of the Energy Technology Support Unit (ETSU) was appointed to represent the interests of the EEO in this project. Companies involved in the project are listed in Appendix 1. 2. DESCRIPTION OF THE PLANT
Spray drying system The equipment used is a Niro Atomizer A/S spray dryer. The chamber is 5.6 m in diameter and has an overall height of 8.5 m. The unit has been operating since 1962. The air to the dryer is heated by an Urquart direct gas-oil burner mounted in a torroidal-flow combustion chamber. The gas-oil is atomized into the combustion chamber by compressed air. The combustion air is drawn from the outside through a bank of filters. Originally, the dilution air was taken through an adjacent, separate bank of filters and entered the combustion chamber downstream of the combustion zone. This dilution air is now drawn by a new fan through a new set of filters and passes into the waste heat exchanger, before flowing along a 0.8 m diameter duct for some 60 m to the air heater. A detailed description of the waste heat exchanger is given below. The feedstock is prepared in batches and stored in tanks adjacent to the spray dryer. Its
Heat recoveryfrom a boiler exhaust to pre-heat air to a spray dryer
13
temperature is raised from the storage Value oi~ 3 ~ ° C tO the d r ~ inlet temperature of approximately 70°C by a small steam heater. The feed is sprayed into the drying chamber by a rotary disc atomizer. The product and exhaust gas leave the chamber through a common discharge. An air broom sweeps the conical bottom section of the chamber to aid the free flow of powder out of the unit. The solids/air mixture then divides between four primary collection cyclones. The exhaust gases from the cyclones are remixed and passed through a bughouse for secondary cleaning before being released to atmosphere, The powder discharged from the bottom of the four cylones is pneumatically conveyed by warm air to a single secondary collection cyclone. It is then sieved by vibrating screens to remove any oversize particles, and mixed in a double-cone blender before being bagged off. The drying chamber incorporates additional air streams to modify the gas-flow pattern and to minimise the amount of powder hitting the roof and walls. Filtered ambient air is used to sheath the atomizer and to produce a cool annulus around the hot air inlet. The pneumatic conveying air which is separated from the product in the secondary collection cyclone is used as swirl air in the lower part of the chamber.
Tail-gas incinerator The residual gases from the formaldehyde plants contain approximately 20% by weight of hydrogen together with traces of methanol and formaldehyde. The gas has a gross calorific value of 2700 kJ/m 3, and it is burnt in a Scotch-type firetube boiler manufactured by Alfred Allen Maxeta Ltd. The boiler, which has been operating since 1972, is fitted with a Wellman Base Quip 9 burner. Typically, production of steam is between 700 and 2500 kg/h at 13.8 bar. The temperature of the flue gas leaving the boiler is usually in the range 235-255°C: its dewpoint temperature is approximately 50°C. The supply of flue gas depends on the operation of the two formaldehyde plants: it can range from zero, up to an actual volumetric flowrate of 12,500 m3/h at a temperature of 255°C when both plants are working at maximum capacity. Originally, the gas was discharged straight to atmosphere. It now passes through the lower half of the new waste heat exchanger. The original boiler induced draught fan was replaced, and the boiler stack was repositioned.
Heat exchanger The waste heat recovery unit uses heat pipes to transfer heat from the hot boiler flue gas to the dilution air for the heat on the spray dryer. This unit was designed and manufactured by Isoterix Ltd, Tonbridge, Kent. A heat pipe is an evacuated, sealed tube containing a small amount of fluid, in this case water. One end of the tube is heated, which vapourises the liquid. The steam produced then passes to the condensation section and reverts to a liquid, hence transferring its latent heat to the dilution air. This operating cycle is shown diagrammatically in Fig. I. The unit supplied to BIP consists of copper tubes, with aluminium fins, supported in a galvanized steel casing. There were essentially two units mounted side-by-side, each containing six rows of 17 heat pipes. The unit was installed with the cold inlet air stream above the hot exhaust, and hence the condensed water in the tubes returns to the evaporator section by gravity. To simplify the associated ductwork, BIP installed the exchanger with the flow of exhaust gas in the same direction as the flow of cold air. A schematic of the installation is shown in Fig. 2. The maximum exposure temperature of the exchanger was specified at 250°C and hence BIP installed a small dilution air fan in the exhaust line. This is controlled by the temperature of the exhaust and, if it becomes too high, cold air is blown in to reduce the temperature to the exchanger. The heat exchanger represents only part of the total heat recovery installation, the full cost of which amounted to £22,000 (Table 1). 3. MONITORING PROGRAMME A number of different types of urea and melamine resins are dried in the spray dryer, and the operating conditions are set to suit the particular product being processed. The feedstock used throughout the monitoring programme was melamine resin.
14
ENERGYTECHNOLOGYSUPPORTUNIT ,j Heol~
OU'IC
w
'•
t,.
Condensolion
l
• Heot out
~.
T
1
1
I l
1
1
l
Co~en$ote return in wick Table I. Breakdown of project costs Item
VO~U r
flow
/I
\ i
7 Evo~:orot~
Heat in
/I
•
\
D
Heat in
9
Total
Fig. I.
Operating cycle of heat pipe.
(£) 10,510 3469 1929 1913 1425 I 154 1044 340 216 £22,000 Cost
Ductwork* Heat exchanger Instrumentation BIP staff time Civil work* Electrical Fans and inlet filter Pipework Miscellaneous stores items
*Sub-contracted-costs include labour and materials
The plant was monitored prior to any modifications in December 1981. The waste heat exchanger was installed in April 1982 and the subsequent monitoring was performed in September 1982. In the first two runs of the post-monitoring the heat recovery system was operating with 'tail' gas coming from both formaldehyde plants. Two further runs were then performed with the boiler inoperative and no heat being transferred to the dilution air via the heat recovery system. Comparison of these runs should give an accurate measure of the heat recovery system. A schematic diagram of the drying system indicating the position of the measurement points is shown in Fig. 3. The measurements made at each location, together with the techniques used, are listed in Table 2, while the measurement techniques employed are described in detail in Appendix 2. Two complete sets of measurements were taken to check their reproducibility. All the data collected are given in Appendix 3. They were analysed to yield material balances over the dryer and heat balances over exchanger and the dryer. There are several ways in which the performance of the dryer can be quantified. The measure used in this report is the specific energy consumption. It can be defined in two ways: Es (the quantity of fuel consumed per kg of product) and Es' (the quantity of fuel consumed in evaporating 1 kg of water). The former is essentially a measure of the cost of drying the product and is often used by dryer operators. The latter is a measure of the efficiency with which water is evaporated from Combution air
Gos
Feed :ii::r S~
FormaLdehyde plant
I
,I
~ - ~ Air
I'"
1,1 I'1 -IT.!L-Qos boeter
I ~'~ pi,. h..~
~L)
excnor~er
Product +
exhaust DiLution air fan
Fig. 2.
Schematic of heat recovery
Heat recovery from a boiler exhaust to pre-heat air to a spray dryer
15
Air
Air
( ~ ~'~ ~inlet ~Wostehentoi
•
Gos oil
v
~, air -~,
]
Combustion chomber
Stee~
Combustion oir
Atomiser cooling oir
Air inlet
cooling olr
Spray dryer
o"
Fxhoust oir tO ~ hOUSe
Air inlet
Coml~esear
Steom gi~gi, T]
. "~ . con.yi.0 oir
Locotion of u4dltionOL measurementpoint
Btand~,,~
Dust hood exl~roctoir
Product Fig. 3. Schematic diagram o f spray dryer.
Measurements Gas flowrate Gas temperature Gas humidity
Gas-oil flowrate Gas-oil temperature Feedrate Solids moisture content Solids temperature Stack condensate flowrate
Table 2. Measurements and Locations I, 2, 8, 9, 10, 12, 14, 15, 16, 21 I, 2, 5, 8, 9, 10, I1, 12, 13, 14, 15, 16, 18, 19, 20, 21 I, 2, 3, 8, 9, 18 14, 16 15 4 4
6 6,17 7,17 22
techniques Technique Pitot tubes and magnehelic gauges Type K thermocouples and digital thermometers Wymark hair hygrometer Endress and Hauser dewpoint hygrometer Wet and dry bulb hygrometer Gravimetric hygrometer Kent integrating meter Type K thermocouple and digital thermometer Feedtank level measurement Vacuum oven analysis Type K thermocoupk's and digital thermometer Measuring cylinder
16
ENERGY TECHNOLOGY SUPPORT UNIT
the feed and is more useful in comparing the performance of different dryers or in assessing the success of modifications to a given dryer. The performance of the heat exchanger is usually described in terms of a percentage temperature efficiency, Fro. Isoterix, and most other manufacturers of heat recovery equipment, define it as: Fm=
T,-T2 x 100 T,-~
where T~ and T2 are the hot stream inlet and outlet temperatures respectively, and T3 is the cold stream inlet temperature. In a counter-current exchanger this definition is a simplified expression for the ratio of the actual energy recovered from the hot stream to the energy that would be recovered from it if its outlet temperature equalled the inlet temperature of the cold stream (i.e. as in an infinitely large exchanger). It also assumes no condensation in the unit and no heat loss from it. However, in an infinitely large co-current exchanger, the outlet temperatures of the hot and cold streams become equal, thereby reducing the maximum possible amount of heat that can be recovered from the hot stream. Therefore, in a co-current exchanger, Fm represents the ratio of the actual heat recovered to the maximum heat that could be recovered in a counter-current unit. 4. M O N I T O R I N G
R E S U L T S AND D I S C U S S I O N
Dry air mass balance around dryer Before the energy balance around the dryer can be calculated, it is necessary to determine the extent of any air leaks into or out of the system from a mass balance analysis. The flowrates of the various air streams entering and leaving the drying system are listed in Table 3. They are given on a dry basis, i.e. they exclude any water vapour associated with the stream. The balances are closed by assuming an air leak into the dryer. The most likely location of this is the explosion doors in the roof of the drying chamber. An error analysis of the flow measurement technique employed shows that the air in-leakage can be calculated to within _+2% of the sum of all the flowrates into and out of the dryer; namely some _+0.27 kg/s. The flowrates of the cooling air streams in runs 3 to 6 were either negligible or extremely low as the inlet filter on the common supply fan was severely fouled. This had no obvious effect on the performance of the dryer or on product quality. However, the absence of these cold air streams may lead to the build up of deposits on the roof and uppermost parts of the chamber. As no internal inspection of the chamber was possible, this could not be substantiated. The increase in the dust hood air flowrates in runs 3 to 6 was the result of replacing the original ducting with new ducting of a large diameter. There is quite a variation in the measured exhaust air flowrates. This could have been caused by several factors. First, the exhaust fan must force air through the baghouse. The flowrate will therefore depend on the pressure drop over the baghouse, which is a function of the condition of
Table 3. Dry gas mass balance around the dryer
Post-monitoring Pre-
monitoring Run number
No tail gas
Tail gas
1
2
3
4
5
6
4.43 0.06 0.20 0.52 0.44 0.68
&37 0,06 0,21 0,51 0.44 0.68
4.48 0.04 . 0.50 0.73 0.72
3.80 0.04
4.48 --
4.67 0.03
0.47 0.79 0,72
0.48 0.77 0.67
0.51 0.79 0.71
5.82 + 0.12
6.40_+0.13
6.71 +0.13
Flowrates (kg/s)
Streams in Hot combustion gas Atomiser cooling air Chamber wall cooling air Air broom Dust hood air Pneumatic conveying air Total Streams o u t Exhaust gas Difference
6.33+0.13
6.27_+0.13
6.47+0.13
.
.
.
6.63+0.13
6.63+0.13
7.20+_0.14
7.86_+0.16
7.45_+0.15
7.30+-0.15
0.30 +- 0.26
0.36 + 0.26
0.73 _+0.27
2.04 _+0.28
1.05 + 0.28
0.59 _ 0.28
17
H e a t r e c o v e r y f r o m a b o i l e r e x h a u s t t o p r e - h e a t a i r to a s p r a y d r y e r Table 4. Energy balance around the dryer Run number
1
~
2
3
4
5
6
792.5 100.2 I 1.8 3.7 8.8 2.8 43.0 32.9 14.7 8.1 29.4 0.3
482.8 348.3 12.9 3.1 . 2.9 48.3 39.1 29.9 27.5 29.9 0.2
491.9 284.8 14.0 3.3 . 2.3 46.3 39.6 26.8 85.4 28.9 0.2
696.9 144.8 18,3 4.5
651.4 190.8 21.9 5.1
-47.0 39.4 31.0 39.8 37.2 0.3
2.1 53. I 42.4 37.5 27.2 29.2 0.3
1048.2+-31.5
1024.9+-30.7
1023.5+-30.7
1059.2+31.8
1061.0 + 31.8
966.9 3.3
947.8 2.6
962.3 4. I
1048.2 4.5
1076.3 5.9
1012.2 4.9
Total
970.2 +- 29.1
950.4 +- 28.5
966.4 +- 29.0
1052.4 + 31.6
1082.2 +- 32.5
1017,1 + 30.5
Difference
76.0 +- 60.5
97.8 + 60.0
58.5 + 59.7
- 2 9 . 2 +- 62.3
- 2 3 . 0 + 64.3
43,9 + 62.3
Energy flowrates (kW) Streams in Combustion of fuel Dilution air Combustion air Fuel atomisins air Chamber wall cooling air Atomiser cooling air Air broom Pneumatic conveying air Dust hood air Air leak Feed Fuel
813.2 83.8 10.4 3.1 7.6 2.5 42.4 31.4 12.9 6.7 31.9 0.3 Total 1046.2+31.4
Streams out Exhaust gas Product
.
.
the bags. Secondly, the size of any air leaks into the system may vary. In particular, BIP have experienced difficulties sealing the explosion relief doors in the roof of the drying chamber.
Energy balance around the dryer The energy balances around the dryer are presented in Table 4. In runs 3 and 4 the effect of the heat recovery system can be seen clearly vis-a-vis runs l, 2, 5 and 6 by the increased energy of the dilution air stream, and the associated reduction in the energy needed from the combustion of the fuel. The difference between the sums of all the energy flows entering and leaving the dryer should represent the heat loss from the system. An error analysis of the measurement techniques shows that the heat losses can be calculated with an overall accuracy of + 3% of the sum of all the energy flows entering and leaving the dryer, namely some + 60 kW. The heat loss difference between runs is therefore within the experimental error.
Energy balance around the heat exchanger The energy balances around the heat exchanger are presented in Table 5 together with the amounts of energy recovered and the thermal efficiencies of the unit.
Specific energy consumption The measured specific energy consumptions Es and Es" for all the runs are presented in Table 6. Comparison of runs 1 and 2 shows that the energy consumption was higher in run number 2. Subsequent investigations showed that this was probably due to rain water ingress through the Table 5. Energy balance around the heat exchanger Run number
3
4
152.6 774.3
123.3 774.3
Energy flowrates (kW) Streams in Cold dilution air Hot flue gas
Total 926.9 + 27.8 Streams out Hot dilution air Stack flue gas Heat loss by difference Energy gained by dilution air Percentage temperature efficiency (%) H.R.S. 6/I--B
362.3 500.5
897.6 +- 26.9 292.6 500.5
Total 8 6 2 . 8 + 2 5 . 9
793.1 +_23.8
64.1 +- 53.7
104.5 +- 50.7
209.7 +- 6.3
169.3 +- 5.1
68.0
68.0
18
ENERGY TECHNOLOGY SUPPORT UNIT Table 6. Specific energy consumption of the spray dryer
Run No.
Es (kg oil/ kg product)
Es" (kg oil/ kg evap. water)
Original dryer (December 1981)
1 2
0.215 -4-_0.011 0.233_+0.012
0.198 ___0.010 0.215 -+ 0.011
Co-current exchanger (August 1982) Tail-gas incinerator on Tail-gas incinerator off
3 d
0.123 _+0.006 0.125 _+0.006
0.140 _+0.007 0.149 _ 0.007
5 6
0.142 + 0.007 0.165_+0.008
0.168 _+0.008 0.195+_0.010
Table 7. Comparison of actual to expected heat exchanger performance Flue gas flowrate (% of max) Case no I
2 3
Description lsoterix design calculation Measured run 3 Design based on conditions monitored during run 3 Design based on case 3 but for countercurrent flow of air and exhaust
58%
100%
Energy recovered by heat exchanger kW [240] 210 [220]
300 [260] [275]
247
[375]
NB: The values given in square brackets are estimated values, the others were either measured in the monitoring or calculated by the equipment supplier.
explosion protection doors in the roof of the drying chamber. Therefore Es" in run number 1 is a better measure of the performance of the original dryer. In fact, all the subsequent runs were performed on dry days thereby eliminating this problem. Comparison of run number 1 with runs 3 and 4 shows that the heat recovery system was saving a considerable amount of energy. Es' had fallen by some 27%. In run number 5 Es' is significantly lower than in run number 1. However, there is good agreement between runs 6 and 1. The measured solids feedrate in run number 5 appears to be anomolously high and so this run will be discounted in the subsequent discussion. Comparison of Es in runs 1 and 6 shows a much lower value for run number 6, probably due to the lower solids inlet moisture content which in turn leads to a lower evaporative load. This highlights the need to maintain as low as inlet moisture content as is practicable in any thermal drying process.
Performance of the heat recovery system The amount of energy transferred to the dilution air stream is highly dependent upon the quantity of flue gas coming from the tail-gas incinerator. During the monitoring tests, the measured flue gas flowrate was approximately 1.8 kg/s, compared with the original design specification of 3.1 kg/s. The effect of this on the potential for heat recovery is shown in Table 7. In this table, case number 1 is the Isoterix design specification. Case number 2 is the amount of energy savings measured in run number 3 of the monitoring, the recovery for 100% flue gas flowrate being estimated from the results at 58% of the flowrate. Case number 3 is the estimate that the design calculations would have predicted for the energy savings under the actual conditions measured in run 3. Also included, as case number 4, are the energy savings which could have been achieved if the heat recovery system had been arranged with countercurrent flow of the exhaust and air streams. Comparison of the measured results in case number 2 with the design calculations in case 3, shows that the amount of heat being recovered by this heat pipe heat exchanger is almost up to design. 5. ECONOMIC ANALYSIS The total cost of this project was £22,000 and the payback period for BIP was just under 2 years. For this type of heat recovery from a separate exhaust, the economics are dependent not only upon the availability of that exhaust, but also upon the frequency of operation of the spray dryer. At BIP, operation of the formaldehyde plant generating the tail gas to the incinerator is continuous.
Heat recovery from a boiler exhaust to pre-heat air to a spray dryer Percentoge of moximum flue Qoe fLowrote
19
Co-current flow
58 % 77%. \ 100%
o
J I
, 2
,
3
,
4
+,
+
+
+
Dryer operol:lng hours/yeor ('000) Fig. 4. Effect of flue gas flowrate on payback period--co-current flow.
The only variation is in the quantity of exhaust gas supplied. The monitored situation of a flue gas rate of 58% of the maximum represents a low case, and hence paybacks on that basis are conservative. Again, for BIP, the spray dryer was operated for only 3677 h in 1983 and hence the annual energy savings at 210 kW are 2.8 TJ (Table 7). This gives a reduction in gas oil consumption worth £11,300/year, and a simple period of 1.9 years. The effect of variations in both the spray dryer utilisation and the availability of flue gas on the payback period are shown in Fig. 4. This shows that the economics of this system are attractive even with low spray dryer utilsation. Paybacks of 2 years are possible even where the spray dryer is only used for 2800 h per annum. In this installation the flows of inlet air and exhaust were in the same direction through the exchanger, i.¢. in a co-current mode. It is more usual, and efficient, to arrange for the flows to be counter-current. The effect of this on the heat recovery has been included in Table 7 which shows that, at the monitored condition, 247 kW would have been recovered (design calculations). The payback period against spray dryer operating hours for various flue gas flowrates, for countercurrent operation, is shown in Fig. 5. For this installation, the simple payback would have been reduced to 1.7 years for 3677 h/year at 58% flue gas rate. These results were obtained with the tail-gas incinerator operating at less than its maximum rate. The flue gas flowrate corresponded to 58% of the maximum specified rate of 3.1 kg/s. The effect of the flue gas flowrate is discussed in detail later. 6. OPERATING EXPERIENCES In operation, BIP experienced no problems with starting up, running or shutting down the dryer with the integrated heat recovery system. In this particular installation, the flow of hot exhaust was essentially continuous through the heat exchanger. Hence for the periods when the spray dryer was not operational, the temperature of the heat pipes will have approached the temperature of the exhaust. Heat pipes are essentially small pressure vessels and therefore there is a maximum operating temperature, in this case 250°C. If subjected to temperatures above this there will be overpressurisation and, in severe cases, this will lead to failure of the tubes. In an attempt to safeguard against this BIP incorporated a small dilution air fan into their system. An alternative approach would have been to incorporate a bypass around the exchanger and then if the temperature of the tubes were to rise too high a portion or, if necessary, all of the exhaust could
20
ENERGY TECHNOLOGY SUPPORT UNIT
Counter-current flow PercentoQe of maximum flue Qos f Lowrate
o
100%
0
I )
I 2 Dryer
I I I 3 4 5 operctin¢ hours/year
I 6 1'000|
I 7
I 8
Fig. 5. Effect of flue gas flowrate on payback period--counter-current flow,
be routed away from the exchanger. Despite the protection apparent overheating led to tube fracture which has necessitated replacement of the exchanger unit. Correct operation and protection of heat pipe systems cannot therefore be overemphasised. In the design of the ducting for this exchanger, no provision was made for access to the unit for cleaning, or for it to be easily removed from its working location. Hence the exchanger was not clearned during the 6 months prior to the monitoring. Visual examination of the exchanger was carded out some 2 years laters, and this revealed some fouling of the heat transfer surfaces particularly on the hot side, but this was not at all severe. There was some evidence of rust deposition due to corrosion of the galvanised steel ductwork; however this formed loose deposit which proved easy to remove. For this particular installation there was no real problem due to fouling, but this may not always be the case and provision should therefore always be made for access to the exchanger for cleaning. One reason why there was limited fouling on the cold air side of the exchanger was the inclusion of filters in the inlet air. To obtain the full benefit of this, a manometer should be fitted and regular readings taken so that the filters can be renewed, or cleaned, when the measured pressure drop exceeds the manufacturer's recommendations. In addition to these experiences related to the heat recovery unit, in the course of the monitoring a number of points relating to the spray dryer were apparent. The drying chamber is protected from the potential damage of an explosion by means of hinged panels in its roof. These are easily distorted by people walking on them and are prone to air and rain-water leaks. Protective covers should be fitted to the p~ els mainly to prevent water running into the dryer. However, the covers must not affect the expl, sion relief performance of the panels. Many spray dryers are started up and shut down on a water feed to prevent the production of off-specification material. To maintain high efficiency of operation it is essential that the length of these procedures is kept fairly short, typically 15-30 min. The majority of spray dryers are only operated for a limited number of hours per year. Careful attention should be given to the scheduling of the dryer operation so that it runs for extended periods. This reduces the wastage of energy and, often, off-specification product associated with start up and shut down. 7. CONCLUSIONS AND RECOMMENDATIONS The heat pipe heat recovery system has saved a significant amount of energy and money. The
Heat recovery from a boiler exhaust to pre-heat air to a spray dryer
21
specific energy c o n s u m p t i o n o f the s p r a y d r y e r has been r e d u c e d by 2 7 % , l e a d i n g to a n n u a l savings w o r t h £11,300. T h e p a y b a c k p e r i o d o f the project was 1.9 years. A d d i t i o n a l energy a n d cost savings w o u l d be o b t a i n e d if: 1. T h e s p r a y d r y e r was o p e r a t e d for m o r e t h a n 3677 h/year; 2. T h e h o t flue gas flowrate was higher. F o r 6000 h s p r a y d r y e r o p e r a t i o n , at m a x i m u m flue gas rate, the p a y b a c k p e r i o d is o n l y one year. W h e n e v e r practical, h e a t p i p e h e a t e x c h a n g e r s s h o u l d be installed in a c o u n t e r - c u r r e n t flow m o d e . This gives m o r e efficient o p e r a t i o n a n d hence increased energy savings. A t the m a x i m u m flue gas rate a n d 6000 h d r y e r o p e r a t i o n , the p a y b a c k for c o u n t e r - c u r r e n t o p e r a t i o n is 18 m o n t h s a c c o r d i n g to design calculations. T h e h e a t p i p e h e a t e x c h a n g e r has o p e r a t e d satisfactorily in recovering h e a t f r o m the 230-250°C e x h a u s t for a p e r i o d o f 18 m o n t h s . T h e e x h a u s t gas s u p p l y d u c t w o r k m u s t be designed such t h a t u n d e r n o c i r c u m s t a n c e s c a n the h e a t p i p e h e a t e x c h a n g e r be subjected to t e m p e r a t u r e s in excess o f the design m a x i m u m . T h e o p t i m u m system w o u l d have a n e x c h a n g e r b y p a s s with flow d i r e c t i o n c o n t r o l . A n a l t e r n a t i v e is a d i l u t i o n air fan b l o w i n g c o l d air into the exhaust. W i t h this, however, the fan has to be large e n o u g h to c o p e with all p o s s i b l e situations. P r o v i s i o n s h o u l d be m a d e for access to the surfaces o f the h e a t e x c h a n g e r s to a l l o w for p e r i o d i c cleaning. This c o u l d be either access d o o r s cut into the d u c t i n g or, alternatively, (the o p t i o n r e c o m m e n d e d b y BIP) s o m e m e a n s o f easily r e m o v i n g the e x c h a n g e r f r o m its w o r k i n g location. T o o b t a i n the full benefit o f a n y h e a t r e c o v e r y system the p e r f o r m a n c e o f the s p r a y d r y e r s h o u l d be as efficient as possible. A n u m b e r o f a r e a s for i m p r o v e m e n t which were identified d u r i n g the c o u r s e o f the p r o j e c t were: 1. A i r / w a t e r l e a k a g e t h r o u g h the e x p l o s i o n relief d o o r s ; 2. Excessive s t a r t - u p a n d s h u t - d o w n p e r i o d s ; 3. Inefficient scheduling o f the o p e r a t i o n o f the dryer.
APPENDIX Host organisation BIP Chemicals Ltd Popes Lane Oldbury West Midlands B69 4DP
1
Equipment manufacturer Isoterix Ltd Mill Works Mill Crescent Tonbridge Kent TN9 IPE Tel. No.: 0732 358483 Telex No.: 957312 Mr P. I. Patrickson
APPENDIX
2. M E A S U R E M E N T
Monitoring contractor Harwell Laboratory Didcot Oxfordshire OXI 1 0RA Tel. No.: 0235 24141 Extn 4889 Telex No.: 83135 Mr R. E. Bahu
TECHNIQUES
Gas flowrate Airflow developments modified ellipsoidal-nosed pitot tubes were employed as recommended in BS. 1042 (Part 2A). Either 4 or 8 mm diamter versions were used depending on the diameter of the duct. In all cases, the length of straight duct upstream of the measurement point equalled or exceeded six duct diameters. The difference between the total and static pressures was measured on a Dwyer magnehelic differential pressure gauge. This gauge had been previously calibrated against an inclined manometer; the agreement between the two was better than 1%. In the circular ducts, two traverses were made on diameters mutually at right angles to each other at each location. Ten readings were taken on each traverse according to the log linear rule. For each rectangular duct, a single traverse was made according to the log Tchebyscheff rule. Six readings were taken on the traverse. The static pressure at each location was measured by either a Dwyer magnahelic gauge or by an Airflow Developments "Slim Jim"-type vertical manometer filled with coloured kerosene having a specific gravity of 0.784 at 20°C. The atmospheric pressure was measured by a Negretti and Zambra aneroid barometer model number M2606. The instrument has an accuracy of _+0.3 mm of Hg at 20°C. There is automatic temperature compensation from 5 to 35°C. The unit was calibrated by the Harwell Instrument Loan Pool. An overall accuracy of _+2% on the gas flowrate measurements can be obtained if the differential pressures do not vary by more than +25% from the mean value and if the pressure difference is at least l0 N/m 2. These conditions were met in all the measurements made.
22
ENERGY TECHNOLOGY SUPPORT UNIT
Temperature Type K Ni/Cr vs Ni/AI laboratory grade thermocouples were employed. Manufactured by Harwell's Engineering Division to conform with BS 4937; Part 4: 1973, they were mineral-insulated and sheathed in 18/8/1 stainless steel. The accuracy of the thermocouples is +0.75°/,. The thermocouples were connected individually to a Comark digital thermometer type 3002. This device is calibarated by the manufacturer for use with type K thermocouples. The quoted accuracy is +_0.2% of reading. The overall accuracy of the temperature is therefore + 1%. The gas temperature at each location in the circular ducts was determined by taking the arithmetic average of six readings made on each of two traverses at right angles to each other. In the rectangular duct, the average of three readings on a single traverse was taken. At all locations, the maximum variation observed in the measured gas temperature was + 1%. The hot gas temperature entering the dryer was also noted from the temperature indicator on the dyer control panel. The accuracy and calibration of the system was not checked as the information is not used in the subsequent energy balance analysis. The temperature of the feed was measured by attaching a thermocouple to the surface of the syrup feed pipe with a jubilee clip and covering it with insulating glass fibre tape. The product temperature was determined by immersing a thermocouple in the solids on the screen. The gas-oil temperature was taken to be the average of the ambient temperatures recorded at locations I, 2 and 3.
Gas humidity The ambient air humidity at locations 1, 2 and 3 was measured by a Wymark hair thermo-hygrometer and by an Endress and Hauser dewpoint hygrometer model No. WMT 9170. The Wymark had been calibrated against an EG & G 660 automatic optical dewpoint hygrometer. The Endress and Hauser is a primary instrument with an accuracy of +0.5%C on the dewpoint between - 10 and 35°C dry bulb temperature. The agreement between the two instruments was +0.2°C on the dewpoint. The ambient air humidity at locations 8 and 9 was determined only by the Wymark hygrometer. The humidities of the swirl air (point 16) and the dust hood extract air (point 14) were measured by a wet and dry bulb hygrometer. The device was desgined and constructed at Harwell. It consists of two type K thermocouples, one of which is covered by a tightly fitting clean wick. Distilled water is supplied continuously to the tip of the wick through a hypodermic needle. For satisfactory operation of this and similar instruments, the air velocity in the duct should exceed 3 m/s. This was checked by a vane anemometer. The method requires careful attention to measurement details. However, given this, an accuracy of + 2 % RH can be achieved. The humidities of the exhaust gas from the dryer (point 15) and the exhaust flue gases from the heat exchanger (point 21) were measured by gravimetric analysis. The apparatus was designed and constructed at Harwell according to the British Standard 1756: Part 4: 1971. A sample of the gas is drawn through a trace-heated line to the device. It passes through a filter tube containing silica wool which removes any dust in the gas. The water vapour is absorbed onto a mixture of phosphorus pentoxide and glass powder contained in three glass vessels in series. The volume of gas passing through the apparatus is measured by an Alexander Wright & Co Ltd wet test meter. An electrically-driven pump downstream of the wet test meter is used to draw the gas through the device. The British Standard states that, using this apparatus, it is possible to achieve an accuracy of +0.5%.
Solids flowrate The tlowrate of solids through the dryer was measured by recording the level of syrup in the feed tank at the start of the test and at various times during it. The level was measured simply with a tape measure. The density of the syrup was determined using a hand hydrometer supplied by BIP. The accuracy of the level measurement was +0.2% and that of the hydrometer was + 1%. The observed variation in the feed rate during the tests was +2*/,.
Solids moisture content Three samples of syrup were withdrawn from the feed tank and three samples of product were collected from the screens. They were placed in air-tight containers and returned to Harwell for analysis in the Engineering Sciences Division laboratories. The samples were dried at 70°C under vacuum to constant weight. A temperature of 70°C was used to prevent chemical decomposition of the material. The accuracy of this moisture measurement technique was +0.01%. The observed variation in the feed moisture content was 1.10 + 0.011 kg/kg and that of the product was 0.0066_ 0.0013 kg/kg.
Gas-oil flowrate A new Kent Meters ~ in Merton type A standard integrating flowmeter was installed by BIP. The meter has a specified accuracy of + 1%. Readings were taken at the start of each of the tests and at various times during them. The observed variation in the gas-oil flowrate was +2%.
Electrical energy consumption The power consumption of the electrical motors was measured by tong ammeters; a Yew Model 2433 with a range 0 to 20A and a Ferranti ammeter with a range 0 to 1000A. The electrical supply has a voltage of 450V.
23
H e a t r e c o v e r y f r o m a b o i l e r e x h a u s t t o p r e - h e a t a i r to a s p r a y d r y e r
APPENDIX
~ 3.
Run Number Date Measurement point 2 (see Figure 6) Dilution air temperature (°C) Dilution air humidity (kg/kg) Mass flowrate of dilution air (kg/s) Measurement point 2 Combustion air temperature CC) Combustion air humidity (kg/kg) Mass flowrate of combustion air (kg/s) Measurement point 3 Mass flowrate of atomising air (kg/s) Atomising air humidity (kg/kg) Atomising air temperature (°C) Measurement point 4 M ass flowrate of diesel oil (kg/s) Measurement point 5 Dryer air inlet temperature (°C) Measurement point 6 Mass flowrate of melamine syrup (kg/s) Measurement point 7 Temperature of melamine syrup feed to spray dryer (°C) Melamine syrup feed moisture content (kg/kg) Measurement point 8 Mass flowrate of atomiser cooling air (kg/s) Temperature of atomiser cooling air (°C) Humidity of atomiser cooling air (kg/kg) Measurement point 9 Mass flowrate of chamber wall cooling air
RESULTS
OF
MEASUREME~TS
1 l I/2/82
2 12/2/82
3 14/9/82
4 15/9/82
5
8 0.0054 3.81
11 0.0062 3.78
64 0.0085 4.04
64 0.0082 3.33
19 0.0072 3.88
25 0.0085 4.09
8 0.0054 0.48
11 0.0062 0.45
16 0.0085 0.35
16 0.0082 0.38
20 0.0072 0.48
24 0.0085 0.47
0.14 0.0054 8
0.14 0.0062 It
0.09 0.0085 16
0.09 0.0082 16
0.12 0.0072 20
0.11 0.0085 24
0.0179
0.0174
0.0106
0.0105
0.0153
0.0143
210 0.0827 66
210
165
0.0741 68
165
0.085
165
0.085
69
168
0.106
70
0.91
6 16/9/82
72
0.87
0.085 70
1.10
1.10
0.88
0.88
0.06 28 0.0054
0.06 31 0.0061
0.04 54 0.0088
0.04 43 0.0085
No flow ---
0.03 48 0.0096
0.20
0.21
No flow
No flow
No flow
No flow
(ks/s) Temperature of chamber wall cooling air (°C) Humidity of chamber wall cooling air (kg/kg) Measurement point 10 Air broom mass flowrate (kg/s) Measurement point I 1 Air broom temperature (downstream of compressor) (°C) Measurement point 12 Mass flowrate of the pneumatic conveying air (kg/s) Temperature of the pneumatic conveying air (°C) Measurement point 13 Temperature o f dryer exhaust air CC) Measurement point 14 Mass flowrate of dust hood air extract
24
27
0.0054
0.0061
0.52
0.51
68
0.68
93 0.44
68
0.68
.
.
.
.
.
.
.
.
0.50 77
0.47 75
0.64
0.48 78
0.67
0.51 82
0.69
0,66
32
33
39
38
42
93
91
91
91
91
0.44
0.73
0.79
0.77
0.79
(kg/s) Temperature of dust hood air extract (°C) Humidity of dust hood air extract (kg/kg) Measurement point 15 Humidity of exhaust air to the baghouse
18 0.0044
17 0.0064
--
w
87
87
21 0.0080
20 0.0053
0.0252
0.0265
20 0.0078
24 0.0093
--
--
84
83
(m~/s) Temperature of exhaust air to the baghouse (°C) Volumetric flowrate of exhaust air to the baghouse (m3/s) Measurement point 16 Mass flowrate of dryer swirl air (kg/s) Temperature of dryer swirl air (°C) Humidity of dryer swirl air (kg/kg) Measurement point 17 Outlet solids moisture content (kg/kg) Outlet solids temperature CC) Measurement point 18 Dilution air temperature (°C) Dilution air humidity (kg/kg) Measurement point 19 Dilution air temperature (°C) Measurement point 20 Flue gas temperature (°C) Measurement point 21 Flue gas temperature CC) Flue gas humidity (kg/kg) Mass flowrate of flue gas Measurement point 22 Stack condensate flowrate (kg/s) ( k g / s )
82
83
7.30
7.42
7.82
8.56
8.00
8.13
0.65 35 0.0059
0.67 36 0.0053
0.72 47 0.0089
0.72 45 0.0093
0.67 47 0.0141
0.71 49 0.0101
0.0066 32
0.0066 28
0.0181 37
0.0179 40
0.0181 44
0.0177 46
---
---
15.5 0.0085
16.2 0.0082
---
---
--
--
67
66
--
--
--
--
210
210
- -
- -
---
----
--
--
- -
--
--
78 0.076 1.79 0.0033
78 - -
- -
- -
--
--
--
--
--
--
0198-7593/86 $3.00 + 0.00 Pergamon Press Ltd
HEAT RECOVERY FROM A BOILER EXHAUST TO PRE-HEAT AIR TO A SPRAY DRYER* A DEMONSTRATION
PROJECT
AT BIP CHEMICALS
LTD
Abstract--This demonstration project at BIP Chemicals Ltd, is one of a package of projects being funded by the Energy Efficiency Office of the Department of Energy, under the Energy Efficiency Demonstration Scheme. It involves the recovery of waste heat to pre-heat spray dryer inlet air. It is estimated that the adoption of this technology within the UK will lead to energy savings worth £1.5M/year by 1990. The project at BIP demonstrates the use of a heat pipe exchanger to recover the heat from a boiler exhaust in order to pre-heat the inlet air. Other projects in the package are at Clayton Aniline Ltd and ABM Chemicals Ltd. In these the heat is recovered from the spray dryer exhaust using a run-around coil and a glass tube heat exchanger respectively. The performance of the system at BIP was independently monitored by the Hatwell Laboratory, and this report details the performance of the exchanger and the resulting energy and cost savings at the spray dryer. The total cost of the installation was £22,000, of which £5500 was funded by the Energy Efficiency Office. The heat recovery system has resulted in a 27% reduction in the specific energy consumption of the spray dryer. For BIP in 1983, this was worth £11,300/year giving a simple payback period of 1.9 years. The savings associated with this type of heat recovery system are dependent upon a number of factors including: (i) The quantity of available flue gas---at the maximum flue gas flow-rate, the savings to BIP would be worth £14,500/year, giving a project payback of 1.5 years; (ii) The frequency of operation of the spray dryer--BIP operated their dryer for 3677 h in 1983. Assuming a notional 6000 h, the savings, at maximum flue gas availability would be worth £22,000 with a payback of one year; (iii) The mode of operation--BIP installed the exchanger in a co-current flow mode. In the more efficient counter-current orientation, savings worth £31,000 could have been achieved giving a payback period of just eight months. BIP installed the exchanger in a cocurrent mode because of the simplicity of installation. A number of operational factors associated with the heat exchanger have been identified in the course of this project: (i) It is essential to ensure that the heat pipe heat exchanger cannot be subjected to temperatures in excess of the design limit. During the course of this project this led to tube fracture which has necessitated the replacement of the exchanger unit: (ii) To enable good efficiency to be maintained, it is essential that provision be made to allow for routine cleaning of the heat exchanger. In addition, to maximise the cost savings, dryer operation should be as efficient as possible. A number of areas found worthy of investigation in order further to improve performance on this particular dryer are as follows: (i) Leakage through the explosion relief doors; (ii) Excessive start-up and shut-down periods; (iii) Inefficient scheduling of dryer operation. These areas have now been improved with significant further operational savings. This project has shown that the use of a separate boiler exhaust, even if located some 60 m from the dryer, is a practical and cost-effective means of improving the efficiency of a spray dryer.
1. I N T R O D U C T I O N
Background to the project S p r a y d r y e r s a r e u s e d in m a n y i n d u s t r i e s , l a r g e l y b e c a u s e t h e y p r o d u c e a f r e e - f l o w i n g s p h e r i c a l p r o d u c t . T h e r e a r e b e t w e e n 150 a n d 200 s u c h d r y e r s in t h e U . K . T h e f e e d to a s p r a y d r y e r has to be in t h e f o r m o f a l i q u i d o r p u m p a b l e slurry, w h i c h i n v a r i a b l y m e a n s t h a t it h a s a r e l a t i v e l y *Prepared by the Energy Technology Support Unit, AERE Harwell, Oxfordshire, acting on behalf of the Energy Efficiency Office of the Department of Energy, from reports by the Harweil Laboratory. 11
12
ENERGY TECHNOLOGY SUPPORT UNIT
high moisture content compared with many dryer feedstocks. To evaporate this moisture requires very large quantities of energy. Operating a spray dryer is generally much more expensive than the alternative of evaporating or filtering the feed and using some other type of dryer. This penalty is accepted for reasons of improved product quality. Spray drying in the U.K. consumes of the order of 500,000 tonnes of coal equivalent per year (tce/year). Pre-heating the inlet air to a spray dryer can be extremely beneficial as it leads to a direct reduction in the fuel consumed by the dryer's heating system. Nationally, potential energy savings are estimated to be of the order of 20,000 tce/year. The pre-heating can be achieved by two methods. First, heat can be recovered from the dryer's own exhaust air. There are currently two demonstration projects on this topic. One, at Clayton Aniline, is demonstrating the use of a run-around coil heat exchanger and the other, at ABM Chemicals, the use of a glass tube exchanger. Alternatively, heat can be recovered from some external source, such as boiler flue gases. This is the subject of the BIP Chemicals Ltd demonstration project described in this report. BIP Chemicals Ltd, part of the Turner and Newall Group of Companies, manufacture a range of moulding materials and resins at their Oldbury Works. A proportion of the aqueous malamine and urea resins is dried in a Niro Atomizer A/S spray dryer. Also, on the same site, BIP manufacture formaldehyde in two plants adjacent to the spray dryer. The residual gases from these plants contain approximately 20% hydrogen and traces of methanol and formaldehyde. For environmental reasons these trace components must be eliminated and, as the gas has a significant calorific value, it is burnt in a "tail-gas incinerator" to produce steam. The temperature of the flue gas leaving the incinerator is about 250°C. In April 1982, BIP installed a heat pipe exchanger, supplied by Isoterix Ltd, in the boiler exhaust to recover heat from the flue gas to pre-heat the inlet air to the spray dryer. The annual energy consumption of the dryer was initially 13,875 GJ (524tce). The new heat recovery system was expected to reduce this to 9904 GJ (374 tce), an annual saving of 3971 GJ (150 tce), worth £16,150.
Assistance under the Energy Efficiency Demonstration Scheme BIP applied for support under the Energy Efficiency Demonstration Scheme (ED) which is funded by the Energy Efficiency Office of the Department of Energy. The scheme is designed to provide up to 25% of the installation costs and 100% of the monitoring costs for energy saving projects which satisfy certain criteria. Grants are awarded under the Scheme for the demonstration project only; replication projects need to be cost-effective without Government support. The grant is intended to encourage the host company to adopt the new energy-saving technique and helps to offset any perceived technical risks. BIP's application for support under the Scheme was approved in 1981. The Energy Efficiency Office made a grant of £5500 towards the total installation cost of £22,000. In addition the EEO met the cost of the independent monitoring of the performance of the system by the Harwell Laboratory. Mr A. C. Mercer of the Energy Technology Support Unit (ETSU) was appointed to represent the interests of the EEO in this project. Companies involved in the project are listed in Appendix 1. 2. DESCRIPTION OF THE PLANT
Spray drying system The equipment used is a Niro Atomizer A/S spray dryer. The chamber is 5.6 m in diameter and has an overall height of 8.5 m. The unit has been operating since 1962. The air to the dryer is heated by an Urquart direct gas-oil burner mounted in a torroidal-flow combustion chamber. The gas-oil is atomized into the combustion chamber by compressed air. The combustion air is drawn from the outside through a bank of filters. Originally, the dilution air was taken through an adjacent, separate bank of filters and entered the combustion chamber downstream of the combustion zone. This dilution air is now drawn by a new fan through a new set of filters and passes into the waste heat exchanger, before flowing along a 0.8 m diameter duct for some 60 m to the air heater. A detailed description of the waste heat exchanger is given below. The feedstock is prepared in batches and stored in tanks adjacent to the spray dryer. Its
Heat recoveryfrom a boiler exhaust to pre-heat air to a spray dryer
13
temperature is raised from the storage Value oi~ 3 ~ ° C tO the d r ~ inlet temperature of approximately 70°C by a small steam heater. The feed is sprayed into the drying chamber by a rotary disc atomizer. The product and exhaust gas leave the chamber through a common discharge. An air broom sweeps the conical bottom section of the chamber to aid the free flow of powder out of the unit. The solids/air mixture then divides between four primary collection cyclones. The exhaust gases from the cyclones are remixed and passed through a bughouse for secondary cleaning before being released to atmosphere, The powder discharged from the bottom of the four cylones is pneumatically conveyed by warm air to a single secondary collection cyclone. It is then sieved by vibrating screens to remove any oversize particles, and mixed in a double-cone blender before being bagged off. The drying chamber incorporates additional air streams to modify the gas-flow pattern and to minimise the amount of powder hitting the roof and walls. Filtered ambient air is used to sheath the atomizer and to produce a cool annulus around the hot air inlet. The pneumatic conveying air which is separated from the product in the secondary collection cyclone is used as swirl air in the lower part of the chamber.
Tail-gas incinerator The residual gases from the formaldehyde plants contain approximately 20% by weight of hydrogen together with traces of methanol and formaldehyde. The gas has a gross calorific value of 2700 kJ/m 3, and it is burnt in a Scotch-type firetube boiler manufactured by Alfred Allen Maxeta Ltd. The boiler, which has been operating since 1972, is fitted with a Wellman Base Quip 9 burner. Typically, production of steam is between 700 and 2500 kg/h at 13.8 bar. The temperature of the flue gas leaving the boiler is usually in the range 235-255°C: its dewpoint temperature is approximately 50°C. The supply of flue gas depends on the operation of the two formaldehyde plants: it can range from zero, up to an actual volumetric flowrate of 12,500 m3/h at a temperature of 255°C when both plants are working at maximum capacity. Originally, the gas was discharged straight to atmosphere. It now passes through the lower half of the new waste heat exchanger. The original boiler induced draught fan was replaced, and the boiler stack was repositioned.
Heat exchanger The waste heat recovery unit uses heat pipes to transfer heat from the hot boiler flue gas to the dilution air for the heat on the spray dryer. This unit was designed and manufactured by Isoterix Ltd, Tonbridge, Kent. A heat pipe is an evacuated, sealed tube containing a small amount of fluid, in this case water. One end of the tube is heated, which vapourises the liquid. The steam produced then passes to the condensation section and reverts to a liquid, hence transferring its latent heat to the dilution air. This operating cycle is shown diagrammatically in Fig. I. The unit supplied to BIP consists of copper tubes, with aluminium fins, supported in a galvanized steel casing. There were essentially two units mounted side-by-side, each containing six rows of 17 heat pipes. The unit was installed with the cold inlet air stream above the hot exhaust, and hence the condensed water in the tubes returns to the evaporator section by gravity. To simplify the associated ductwork, BIP installed the exchanger with the flow of exhaust gas in the same direction as the flow of cold air. A schematic of the installation is shown in Fig. 2. The maximum exposure temperature of the exchanger was specified at 250°C and hence BIP installed a small dilution air fan in the exhaust line. This is controlled by the temperature of the exhaust and, if it becomes too high, cold air is blown in to reduce the temperature to the exchanger. The heat exchanger represents only part of the total heat recovery installation, the full cost of which amounted to £22,000 (Table 1). 3. MONITORING PROGRAMME A number of different types of urea and melamine resins are dried in the spray dryer, and the operating conditions are set to suit the particular product being processed. The feedstock used throughout the monitoring programme was melamine resin.
14
ENERGYTECHNOLOGYSUPPORTUNIT ,j Heol~
OU'IC
w
'•
t,.
Condensolion
l
• Heot out
~.
T
1
1
I l
1
1
l
Co~en$ote return in wick Table I. Breakdown of project costs Item
VO~U r
flow
/I
\ i
7 Evo~:orot~
Heat in
/I
•
\
D
Heat in
9
Total
Fig. I.
Operating cycle of heat pipe.
(£) 10,510 3469 1929 1913 1425 I 154 1044 340 216 £22,000 Cost
Ductwork* Heat exchanger Instrumentation BIP staff time Civil work* Electrical Fans and inlet filter Pipework Miscellaneous stores items
*Sub-contracted-costs include labour and materials
The plant was monitored prior to any modifications in December 1981. The waste heat exchanger was installed in April 1982 and the subsequent monitoring was performed in September 1982. In the first two runs of the post-monitoring the heat recovery system was operating with 'tail' gas coming from both formaldehyde plants. Two further runs were then performed with the boiler inoperative and no heat being transferred to the dilution air via the heat recovery system. Comparison of these runs should give an accurate measure of the heat recovery system. A schematic diagram of the drying system indicating the position of the measurement points is shown in Fig. 3. The measurements made at each location, together with the techniques used, are listed in Table 2, while the measurement techniques employed are described in detail in Appendix 2. Two complete sets of measurements were taken to check their reproducibility. All the data collected are given in Appendix 3. They were analysed to yield material balances over the dryer and heat balances over exchanger and the dryer. There are several ways in which the performance of the dryer can be quantified. The measure used in this report is the specific energy consumption. It can be defined in two ways: Es (the quantity of fuel consumed per kg of product) and Es' (the quantity of fuel consumed in evaporating 1 kg of water). The former is essentially a measure of the cost of drying the product and is often used by dryer operators. The latter is a measure of the efficiency with which water is evaporated from Combution air
Gos
Feed :ii::r S~
FormaLdehyde plant
I
,I
~ - ~ Air
I'"
1,1 I'1 -IT.!L-Qos boeter
I ~'~ pi,. h..~
~L)
excnor~er
Product +
exhaust DiLution air fan
Fig. 2.
Schematic of heat recovery
Heat recovery from a boiler exhaust to pre-heat air to a spray dryer
15
Air
Air
( ~ ~'~ ~inlet ~Wostehentoi
•
Gos oil
v
~, air -~,
]
Combustion chomber
Stee~
Combustion oir
Atomiser cooling oir
Air inlet
cooling olr
Spray dryer
o"
Fxhoust oir tO ~ hOUSe
Air inlet
Coml~esear
Steom gi~gi, T]
. "~ . con.yi.0 oir
Locotion of u4dltionOL measurementpoint
Btand~,,~
Dust hood exl~roctoir
Product Fig. 3. Schematic diagram o f spray dryer.
Measurements Gas flowrate Gas temperature Gas humidity
Gas-oil flowrate Gas-oil temperature Feedrate Solids moisture content Solids temperature Stack condensate flowrate
Table 2. Measurements and Locations I, 2, 8, 9, 10, 12, 14, 15, 16, 21 I, 2, 5, 8, 9, 10, I1, 12, 13, 14, 15, 16, 18, 19, 20, 21 I, 2, 3, 8, 9, 18 14, 16 15 4 4
6 6,17 7,17 22
techniques Technique Pitot tubes and magnehelic gauges Type K thermocouples and digital thermometers Wymark hair hygrometer Endress and Hauser dewpoint hygrometer Wet and dry bulb hygrometer Gravimetric hygrometer Kent integrating meter Type K thermocouple and digital thermometer Feedtank level measurement Vacuum oven analysis Type K thermocoupk's and digital thermometer Measuring cylinder
16
ENERGY TECHNOLOGY SUPPORT UNIT
the feed and is more useful in comparing the performance of different dryers or in assessing the success of modifications to a given dryer. The performance of the heat exchanger is usually described in terms of a percentage temperature efficiency, Fro. Isoterix, and most other manufacturers of heat recovery equipment, define it as: Fm=
T,-T2 x 100 T,-~
where T~ and T2 are the hot stream inlet and outlet temperatures respectively, and T3 is the cold stream inlet temperature. In a counter-current exchanger this definition is a simplified expression for the ratio of the actual energy recovered from the hot stream to the energy that would be recovered from it if its outlet temperature equalled the inlet temperature of the cold stream (i.e. as in an infinitely large exchanger). It also assumes no condensation in the unit and no heat loss from it. However, in an infinitely large co-current exchanger, the outlet temperatures of the hot and cold streams become equal, thereby reducing the maximum possible amount of heat that can be recovered from the hot stream. Therefore, in a co-current exchanger, Fm represents the ratio of the actual heat recovered to the maximum heat that could be recovered in a counter-current unit. 4. M O N I T O R I N G
R E S U L T S AND D I S C U S S I O N
Dry air mass balance around dryer Before the energy balance around the dryer can be calculated, it is necessary to determine the extent of any air leaks into or out of the system from a mass balance analysis. The flowrates of the various air streams entering and leaving the drying system are listed in Table 3. They are given on a dry basis, i.e. they exclude any water vapour associated with the stream. The balances are closed by assuming an air leak into the dryer. The most likely location of this is the explosion doors in the roof of the drying chamber. An error analysis of the flow measurement technique employed shows that the air in-leakage can be calculated to within _+2% of the sum of all the flowrates into and out of the dryer; namely some _+0.27 kg/s. The flowrates of the cooling air streams in runs 3 to 6 were either negligible or extremely low as the inlet filter on the common supply fan was severely fouled. This had no obvious effect on the performance of the dryer or on product quality. However, the absence of these cold air streams may lead to the build up of deposits on the roof and uppermost parts of the chamber. As no internal inspection of the chamber was possible, this could not be substantiated. The increase in the dust hood air flowrates in runs 3 to 6 was the result of replacing the original ducting with new ducting of a large diameter. There is quite a variation in the measured exhaust air flowrates. This could have been caused by several factors. First, the exhaust fan must force air through the baghouse. The flowrate will therefore depend on the pressure drop over the baghouse, which is a function of the condition of
Table 3. Dry gas mass balance around the dryer
Post-monitoring Pre-
monitoring Run number
No tail gas
Tail gas
1
2
3
4
5
6
4.43 0.06 0.20 0.52 0.44 0.68
&37 0,06 0,21 0,51 0.44 0.68
4.48 0.04 . 0.50 0.73 0.72
3.80 0.04
4.48 --
4.67 0.03
0.47 0.79 0,72
0.48 0.77 0.67
0.51 0.79 0.71
5.82 + 0.12
6.40_+0.13
6.71 +0.13
Flowrates (kg/s)
Streams in Hot combustion gas Atomiser cooling air Chamber wall cooling air Air broom Dust hood air Pneumatic conveying air Total Streams o u t Exhaust gas Difference
6.33+0.13
6.27_+0.13
6.47+0.13
.
.
.
6.63+0.13
6.63+0.13
7.20+_0.14
7.86_+0.16
7.45_+0.15
7.30+-0.15
0.30 +- 0.26
0.36 + 0.26
0.73 _+0.27
2.04 _+0.28
1.05 + 0.28
0.59 _ 0.28
17
H e a t r e c o v e r y f r o m a b o i l e r e x h a u s t t o p r e - h e a t a i r to a s p r a y d r y e r Table 4. Energy balance around the dryer Run number
1
~
2
3
4
5
6
792.5 100.2 I 1.8 3.7 8.8 2.8 43.0 32.9 14.7 8.1 29.4 0.3
482.8 348.3 12.9 3.1 . 2.9 48.3 39.1 29.9 27.5 29.9 0.2
491.9 284.8 14.0 3.3 . 2.3 46.3 39.6 26.8 85.4 28.9 0.2
696.9 144.8 18,3 4.5
651.4 190.8 21.9 5.1
-47.0 39.4 31.0 39.8 37.2 0.3
2.1 53. I 42.4 37.5 27.2 29.2 0.3
1048.2+-31.5
1024.9+-30.7
1023.5+-30.7
1059.2+31.8
1061.0 + 31.8
966.9 3.3
947.8 2.6
962.3 4. I
1048.2 4.5
1076.3 5.9
1012.2 4.9
Total
970.2 +- 29.1
950.4 +- 28.5
966.4 +- 29.0
1052.4 + 31.6
1082.2 +- 32.5
1017,1 + 30.5
Difference
76.0 +- 60.5
97.8 + 60.0
58.5 + 59.7
- 2 9 . 2 +- 62.3
- 2 3 . 0 + 64.3
43,9 + 62.3
Energy flowrates (kW) Streams in Combustion of fuel Dilution air Combustion air Fuel atomisins air Chamber wall cooling air Atomiser cooling air Air broom Pneumatic conveying air Dust hood air Air leak Feed Fuel
813.2 83.8 10.4 3.1 7.6 2.5 42.4 31.4 12.9 6.7 31.9 0.3 Total 1046.2+31.4
Streams out Exhaust gas Product
.
.
the bags. Secondly, the size of any air leaks into the system may vary. In particular, BIP have experienced difficulties sealing the explosion relief doors in the roof of the drying chamber.
Energy balance around the dryer The energy balances around the dryer are presented in Table 4. In runs 3 and 4 the effect of the heat recovery system can be seen clearly vis-a-vis runs l, 2, 5 and 6 by the increased energy of the dilution air stream, and the associated reduction in the energy needed from the combustion of the fuel. The difference between the sums of all the energy flows entering and leaving the dryer should represent the heat loss from the system. An error analysis of the measurement techniques shows that the heat losses can be calculated with an overall accuracy of + 3% of the sum of all the energy flows entering and leaving the dryer, namely some + 60 kW. The heat loss difference between runs is therefore within the experimental error.
Energy balance around the heat exchanger The energy balances around the heat exchanger are presented in Table 5 together with the amounts of energy recovered and the thermal efficiencies of the unit.
Specific energy consumption The measured specific energy consumptions Es and Es" for all the runs are presented in Table 6. Comparison of runs 1 and 2 shows that the energy consumption was higher in run number 2. Subsequent investigations showed that this was probably due to rain water ingress through the Table 5. Energy balance around the heat exchanger Run number
3
4
152.6 774.3
123.3 774.3
Energy flowrates (kW) Streams in Cold dilution air Hot flue gas
Total 926.9 + 27.8 Streams out Hot dilution air Stack flue gas Heat loss by difference Energy gained by dilution air Percentage temperature efficiency (%) H.R.S. 6/I--B
362.3 500.5
897.6 +- 26.9 292.6 500.5
Total 8 6 2 . 8 + 2 5 . 9
793.1 +_23.8
64.1 +- 53.7
104.5 +- 50.7
209.7 +- 6.3
169.3 +- 5.1
68.0
68.0
18
ENERGY TECHNOLOGY SUPPORT UNIT Table 6. Specific energy consumption of the spray dryer
Run No.
Es (kg oil/ kg product)
Es" (kg oil/ kg evap. water)
Original dryer (December 1981)
1 2
0.215 -4-_0.011 0.233_+0.012
0.198 ___0.010 0.215 -+ 0.011
Co-current exchanger (August 1982) Tail-gas incinerator on Tail-gas incinerator off
3 d
0.123 _+0.006 0.125 _+0.006
0.140 _+0.007 0.149 _ 0.007
5 6
0.142 + 0.007 0.165_+0.008
0.168 _+0.008 0.195+_0.010
Table 7. Comparison of actual to expected heat exchanger performance Flue gas flowrate (% of max) Case no I
2 3
Description lsoterix design calculation Measured run 3 Design based on conditions monitored during run 3 Design based on case 3 but for countercurrent flow of air and exhaust
58%
100%
Energy recovered by heat exchanger kW [240] 210 [220]
300 [260] [275]
247
[375]
NB: The values given in square brackets are estimated values, the others were either measured in the monitoring or calculated by the equipment supplier.
explosion protection doors in the roof of the drying chamber. Therefore Es" in run number 1 is a better measure of the performance of the original dryer. In fact, all the subsequent runs were performed on dry days thereby eliminating this problem. Comparison of run number 1 with runs 3 and 4 shows that the heat recovery system was saving a considerable amount of energy. Es' had fallen by some 27%. In run number 5 Es' is significantly lower than in run number 1. However, there is good agreement between runs 6 and 1. The measured solids feedrate in run number 5 appears to be anomolously high and so this run will be discounted in the subsequent discussion. Comparison of Es in runs 1 and 6 shows a much lower value for run number 6, probably due to the lower solids inlet moisture content which in turn leads to a lower evaporative load. This highlights the need to maintain as low as inlet moisture content as is practicable in any thermal drying process.
Performance of the heat recovery system The amount of energy transferred to the dilution air stream is highly dependent upon the quantity of flue gas coming from the tail-gas incinerator. During the monitoring tests, the measured flue gas flowrate was approximately 1.8 kg/s, compared with the original design specification of 3.1 kg/s. The effect of this on the potential for heat recovery is shown in Table 7. In this table, case number 1 is the Isoterix design specification. Case number 2 is the amount of energy savings measured in run number 3 of the monitoring, the recovery for 100% flue gas flowrate being estimated from the results at 58% of the flowrate. Case number 3 is the estimate that the design calculations would have predicted for the energy savings under the actual conditions measured in run 3. Also included, as case number 4, are the energy savings which could have been achieved if the heat recovery system had been arranged with countercurrent flow of the exhaust and air streams. Comparison of the measured results in case number 2 with the design calculations in case 3, shows that the amount of heat being recovered by this heat pipe heat exchanger is almost up to design. 5. ECONOMIC ANALYSIS The total cost of this project was £22,000 and the payback period for BIP was just under 2 years. For this type of heat recovery from a separate exhaust, the economics are dependent not only upon the availability of that exhaust, but also upon the frequency of operation of the spray dryer. At BIP, operation of the formaldehyde plant generating the tail gas to the incinerator is continuous.
Heat recovery from a boiler exhaust to pre-heat air to a spray dryer Percentoge of moximum flue Qoe fLowrote
19
Co-current flow
58 % 77%. \ 100%
o
J I
, 2
,
3
,
4
+,
+
+
+
Dryer operol:lng hours/yeor ('000) Fig. 4. Effect of flue gas flowrate on payback period--co-current flow.
The only variation is in the quantity of exhaust gas supplied. The monitored situation of a flue gas rate of 58% of the maximum represents a low case, and hence paybacks on that basis are conservative. Again, for BIP, the spray dryer was operated for only 3677 h in 1983 and hence the annual energy savings at 210 kW are 2.8 TJ (Table 7). This gives a reduction in gas oil consumption worth £11,300/year, and a simple period of 1.9 years. The effect of variations in both the spray dryer utilisation and the availability of flue gas on the payback period are shown in Fig. 4. This shows that the economics of this system are attractive even with low spray dryer utilsation. Paybacks of 2 years are possible even where the spray dryer is only used for 2800 h per annum. In this installation the flows of inlet air and exhaust were in the same direction through the exchanger, i.¢. in a co-current mode. It is more usual, and efficient, to arrange for the flows to be counter-current. The effect of this on the heat recovery has been included in Table 7 which shows that, at the monitored condition, 247 kW would have been recovered (design calculations). The payback period against spray dryer operating hours for various flue gas flowrates, for countercurrent operation, is shown in Fig. 5. For this installation, the simple payback would have been reduced to 1.7 years for 3677 h/year at 58% flue gas rate. These results were obtained with the tail-gas incinerator operating at less than its maximum rate. The flue gas flowrate corresponded to 58% of the maximum specified rate of 3.1 kg/s. The effect of the flue gas flowrate is discussed in detail later. 6. OPERATING EXPERIENCES In operation, BIP experienced no problems with starting up, running or shutting down the dryer with the integrated heat recovery system. In this particular installation, the flow of hot exhaust was essentially continuous through the heat exchanger. Hence for the periods when the spray dryer was not operational, the temperature of the heat pipes will have approached the temperature of the exhaust. Heat pipes are essentially small pressure vessels and therefore there is a maximum operating temperature, in this case 250°C. If subjected to temperatures above this there will be overpressurisation and, in severe cases, this will lead to failure of the tubes. In an attempt to safeguard against this BIP incorporated a small dilution air fan into their system. An alternative approach would have been to incorporate a bypass around the exchanger and then if the temperature of the tubes were to rise too high a portion or, if necessary, all of the exhaust could
20
ENERGY TECHNOLOGY SUPPORT UNIT
Counter-current flow PercentoQe of maximum flue Qos f Lowrate
o
100%
0
I )
I 2 Dryer
I I I 3 4 5 operctin¢ hours/year
I 6 1'000|
I 7
I 8
Fig. 5. Effect of flue gas flowrate on payback period--counter-current flow,
be routed away from the exchanger. Despite the protection apparent overheating led to tube fracture which has necessitated replacement of the exchanger unit. Correct operation and protection of heat pipe systems cannot therefore be overemphasised. In the design of the ducting for this exchanger, no provision was made for access to the unit for cleaning, or for it to be easily removed from its working location. Hence the exchanger was not clearned during the 6 months prior to the monitoring. Visual examination of the exchanger was carded out some 2 years laters, and this revealed some fouling of the heat transfer surfaces particularly on the hot side, but this was not at all severe. There was some evidence of rust deposition due to corrosion of the galvanised steel ductwork; however this formed loose deposit which proved easy to remove. For this particular installation there was no real problem due to fouling, but this may not always be the case and provision should therefore always be made for access to the exchanger for cleaning. One reason why there was limited fouling on the cold air side of the exchanger was the inclusion of filters in the inlet air. To obtain the full benefit of this, a manometer should be fitted and regular readings taken so that the filters can be renewed, or cleaned, when the measured pressure drop exceeds the manufacturer's recommendations. In addition to these experiences related to the heat recovery unit, in the course of the monitoring a number of points relating to the spray dryer were apparent. The drying chamber is protected from the potential damage of an explosion by means of hinged panels in its roof. These are easily distorted by people walking on them and are prone to air and rain-water leaks. Protective covers should be fitted to the p~ els mainly to prevent water running into the dryer. However, the covers must not affect the expl, sion relief performance of the panels. Many spray dryers are started up and shut down on a water feed to prevent the production of off-specification material. To maintain high efficiency of operation it is essential that the length of these procedures is kept fairly short, typically 15-30 min. The majority of spray dryers are only operated for a limited number of hours per year. Careful attention should be given to the scheduling of the dryer operation so that it runs for extended periods. This reduces the wastage of energy and, often, off-specification product associated with start up and shut down. 7. CONCLUSIONS AND RECOMMENDATIONS The heat pipe heat recovery system has saved a significant amount of energy and money. The
Heat recovery from a boiler exhaust to pre-heat air to a spray dryer
21
specific energy c o n s u m p t i o n o f the s p r a y d r y e r has been r e d u c e d by 2 7 % , l e a d i n g to a n n u a l savings w o r t h £11,300. T h e p a y b a c k p e r i o d o f the project was 1.9 years. A d d i t i o n a l energy a n d cost savings w o u l d be o b t a i n e d if: 1. T h e s p r a y d r y e r was o p e r a t e d for m o r e t h a n 3677 h/year; 2. T h e h o t flue gas flowrate was higher. F o r 6000 h s p r a y d r y e r o p e r a t i o n , at m a x i m u m flue gas rate, the p a y b a c k p e r i o d is o n l y one year. W h e n e v e r practical, h e a t p i p e h e a t e x c h a n g e r s s h o u l d be installed in a c o u n t e r - c u r r e n t flow m o d e . This gives m o r e efficient o p e r a t i o n a n d hence increased energy savings. A t the m a x i m u m flue gas rate a n d 6000 h d r y e r o p e r a t i o n , the p a y b a c k for c o u n t e r - c u r r e n t o p e r a t i o n is 18 m o n t h s a c c o r d i n g to design calculations. T h e h e a t p i p e h e a t e x c h a n g e r has o p e r a t e d satisfactorily in recovering h e a t f r o m the 230-250°C e x h a u s t for a p e r i o d o f 18 m o n t h s . T h e e x h a u s t gas s u p p l y d u c t w o r k m u s t be designed such t h a t u n d e r n o c i r c u m s t a n c e s c a n the h e a t p i p e h e a t e x c h a n g e r be subjected to t e m p e r a t u r e s in excess o f the design m a x i m u m . T h e o p t i m u m system w o u l d have a n e x c h a n g e r b y p a s s with flow d i r e c t i o n c o n t r o l . A n a l t e r n a t i v e is a d i l u t i o n air fan b l o w i n g c o l d air into the exhaust. W i t h this, however, the fan has to be large e n o u g h to c o p e with all p o s s i b l e situations. P r o v i s i o n s h o u l d be m a d e for access to the surfaces o f the h e a t e x c h a n g e r s to a l l o w for p e r i o d i c cleaning. This c o u l d be either access d o o r s cut into the d u c t i n g or, alternatively, (the o p t i o n r e c o m m e n d e d b y BIP) s o m e m e a n s o f easily r e m o v i n g the e x c h a n g e r f r o m its w o r k i n g location. T o o b t a i n the full benefit o f a n y h e a t r e c o v e r y system the p e r f o r m a n c e o f the s p r a y d r y e r s h o u l d be as efficient as possible. A n u m b e r o f a r e a s for i m p r o v e m e n t which were identified d u r i n g the c o u r s e o f the p r o j e c t were: 1. A i r / w a t e r l e a k a g e t h r o u g h the e x p l o s i o n relief d o o r s ; 2. Excessive s t a r t - u p a n d s h u t - d o w n p e r i o d s ; 3. Inefficient scheduling o f the o p e r a t i o n o f the dryer.
APPENDIX Host organisation BIP Chemicals Ltd Popes Lane Oldbury West Midlands B69 4DP
1
Equipment manufacturer Isoterix Ltd Mill Works Mill Crescent Tonbridge Kent TN9 IPE Tel. No.: 0732 358483 Telex No.: 957312 Mr P. I. Patrickson
APPENDIX
2. M E A S U R E M E N T
Monitoring contractor Harwell Laboratory Didcot Oxfordshire OXI 1 0RA Tel. No.: 0235 24141 Extn 4889 Telex No.: 83135 Mr R. E. Bahu
TECHNIQUES
Gas flowrate Airflow developments modified ellipsoidal-nosed pitot tubes were employed as recommended in BS. 1042 (Part 2A). Either 4 or 8 mm diamter versions were used depending on the diameter of the duct. In all cases, the length of straight duct upstream of the measurement point equalled or exceeded six duct diameters. The difference between the total and static pressures was measured on a Dwyer magnehelic differential pressure gauge. This gauge had been previously calibrated against an inclined manometer; the agreement between the two was better than 1%. In the circular ducts, two traverses were made on diameters mutually at right angles to each other at each location. Ten readings were taken on each traverse according to the log linear rule. For each rectangular duct, a single traverse was made according to the log Tchebyscheff rule. Six readings were taken on the traverse. The static pressure at each location was measured by either a Dwyer magnahelic gauge or by an Airflow Developments "Slim Jim"-type vertical manometer filled with coloured kerosene having a specific gravity of 0.784 at 20°C. The atmospheric pressure was measured by a Negretti and Zambra aneroid barometer model number M2606. The instrument has an accuracy of _+0.3 mm of Hg at 20°C. There is automatic temperature compensation from 5 to 35°C. The unit was calibrated by the Harwell Instrument Loan Pool. An overall accuracy of _+2% on the gas flowrate measurements can be obtained if the differential pressures do not vary by more than +25% from the mean value and if the pressure difference is at least l0 N/m 2. These conditions were met in all the measurements made.
22
ENERGY TECHNOLOGY SUPPORT UNIT
Temperature Type K Ni/Cr vs Ni/AI laboratory grade thermocouples were employed. Manufactured by Harwell's Engineering Division to conform with BS 4937; Part 4: 1973, they were mineral-insulated and sheathed in 18/8/1 stainless steel. The accuracy of the thermocouples is +0.75°/,. The thermocouples were connected individually to a Comark digital thermometer type 3002. This device is calibarated by the manufacturer for use with type K thermocouples. The quoted accuracy is +_0.2% of reading. The overall accuracy of the temperature is therefore + 1%. The gas temperature at each location in the circular ducts was determined by taking the arithmetic average of six readings made on each of two traverses at right angles to each other. In the rectangular duct, the average of three readings on a single traverse was taken. At all locations, the maximum variation observed in the measured gas temperature was + 1%. The hot gas temperature entering the dryer was also noted from the temperature indicator on the dyer control panel. The accuracy and calibration of the system was not checked as the information is not used in the subsequent energy balance analysis. The temperature of the feed was measured by attaching a thermocouple to the surface of the syrup feed pipe with a jubilee clip and covering it with insulating glass fibre tape. The product temperature was determined by immersing a thermocouple in the solids on the screen. The gas-oil temperature was taken to be the average of the ambient temperatures recorded at locations I, 2 and 3.
Gas humidity The ambient air humidity at locations 1, 2 and 3 was measured by a Wymark hair thermo-hygrometer and by an Endress and Hauser dewpoint hygrometer model No. WMT 9170. The Wymark had been calibrated against an EG & G 660 automatic optical dewpoint hygrometer. The Endress and Hauser is a primary instrument with an accuracy of +0.5%C on the dewpoint between - 10 and 35°C dry bulb temperature. The agreement between the two instruments was +0.2°C on the dewpoint. The ambient air humidity at locations 8 and 9 was determined only by the Wymark hygrometer. The humidities of the swirl air (point 16) and the dust hood extract air (point 14) were measured by a wet and dry bulb hygrometer. The device was desgined and constructed at Harwell. It consists of two type K thermocouples, one of which is covered by a tightly fitting clean wick. Distilled water is supplied continuously to the tip of the wick through a hypodermic needle. For satisfactory operation of this and similar instruments, the air velocity in the duct should exceed 3 m/s. This was checked by a vane anemometer. The method requires careful attention to measurement details. However, given this, an accuracy of + 2 % RH can be achieved. The humidities of the exhaust gas from the dryer (point 15) and the exhaust flue gases from the heat exchanger (point 21) were measured by gravimetric analysis. The apparatus was designed and constructed at Harwell according to the British Standard 1756: Part 4: 1971. A sample of the gas is drawn through a trace-heated line to the device. It passes through a filter tube containing silica wool which removes any dust in the gas. The water vapour is absorbed onto a mixture of phosphorus pentoxide and glass powder contained in three glass vessels in series. The volume of gas passing through the apparatus is measured by an Alexander Wright & Co Ltd wet test meter. An electrically-driven pump downstream of the wet test meter is used to draw the gas through the device. The British Standard states that, using this apparatus, it is possible to achieve an accuracy of +0.5%.
Solids flowrate The tlowrate of solids through the dryer was measured by recording the level of syrup in the feed tank at the start of the test and at various times during it. The level was measured simply with a tape measure. The density of the syrup was determined using a hand hydrometer supplied by BIP. The accuracy of the level measurement was +0.2% and that of the hydrometer was + 1%. The observed variation in the feed rate during the tests was +2*/,.
Solids moisture content Three samples of syrup were withdrawn from the feed tank and three samples of product were collected from the screens. They were placed in air-tight containers and returned to Harwell for analysis in the Engineering Sciences Division laboratories. The samples were dried at 70°C under vacuum to constant weight. A temperature of 70°C was used to prevent chemical decomposition of the material. The accuracy of this moisture measurement technique was +0.01%. The observed variation in the feed moisture content was 1.10 + 0.011 kg/kg and that of the product was 0.0066_ 0.0013 kg/kg.
Gas-oil flowrate A new Kent Meters ~ in Merton type A standard integrating flowmeter was installed by BIP. The meter has a specified accuracy of + 1%. Readings were taken at the start of each of the tests and at various times during them. The observed variation in the gas-oil flowrate was +2%.
Electrical energy consumption The power consumption of the electrical motors was measured by tong ammeters; a Yew Model 2433 with a range 0 to 20A and a Ferranti ammeter with a range 0 to 1000A. The electrical supply has a voltage of 450V.
23
H e a t r e c o v e r y f r o m a b o i l e r e x h a u s t t o p r e - h e a t a i r to a s p r a y d r y e r
APPENDIX
~ 3.
Run Number Date Measurement point 2 (see Figure 6) Dilution air temperature (°C) Dilution air humidity (kg/kg) Mass flowrate of dilution air (kg/s) Measurement point 2 Combustion air temperature CC) Combustion air humidity (kg/kg) Mass flowrate of combustion air (kg/s) Measurement point 3 Mass flowrate of atomising air (kg/s) Atomising air humidity (kg/kg) Atomising air temperature (°C) Measurement point 4 M ass flowrate of diesel oil (kg/s) Measurement point 5 Dryer air inlet temperature (°C) Measurement point 6 Mass flowrate of melamine syrup (kg/s) Measurement point 7 Temperature of melamine syrup feed to spray dryer (°C) Melamine syrup feed moisture content (kg/kg) Measurement point 8 Mass flowrate of atomiser cooling air (kg/s) Temperature of atomiser cooling air (°C) Humidity of atomiser cooling air (kg/kg) Measurement point 9 Mass flowrate of chamber wall cooling air
RESULTS
OF
MEASUREME~TS
1 l I/2/82
2 12/2/82
3 14/9/82
4 15/9/82
5
8 0.0054 3.81
11 0.0062 3.78
64 0.0085 4.04
64 0.0082 3.33
19 0.0072 3.88
25 0.0085 4.09
8 0.0054 0.48
11 0.0062 0.45
16 0.0085 0.35
16 0.0082 0.38
20 0.0072 0.48
24 0.0085 0.47
0.14 0.0054 8
0.14 0.0062 It
0.09 0.0085 16
0.09 0.0082 16
0.12 0.0072 20
0.11 0.0085 24
0.0179
0.0174
0.0106
0.0105
0.0153
0.0143
210 0.0827 66
210
165
0.0741 68
165
0.085
165
0.085
69
168
0.106
70
0.91
6 16/9/82
72
0.87
0.085 70
1.10
1.10
0.88
0.88
0.06 28 0.0054
0.06 31 0.0061
0.04 54 0.0088
0.04 43 0.0085
No flow ---
0.03 48 0.0096
0.20
0.21
No flow
No flow
No flow
No flow
(ks/s) Temperature of chamber wall cooling air (°C) Humidity of chamber wall cooling air (kg/kg) Measurement point 10 Air broom mass flowrate (kg/s) Measurement point I 1 Air broom temperature (downstream of compressor) (°C) Measurement point 12 Mass flowrate of the pneumatic conveying air (kg/s) Temperature of the pneumatic conveying air (°C) Measurement point 13 Temperature o f dryer exhaust air CC) Measurement point 14 Mass flowrate of dust hood air extract
24
27
0.0054
0.0061
0.52
0.51
68
0.68
93 0.44
68
0.68
.
.
.
.
.
.
.
.
0.50 77
0.47 75
0.64
0.48 78
0.67
0.51 82
0.69
0,66
32
33
39
38
42
93
91
91
91
91
0.44
0.73
0.79
0.77
0.79
(kg/s) Temperature of dust hood air extract (°C) Humidity of dust hood air extract (kg/kg) Measurement point 15 Humidity of exhaust air to the baghouse
18 0.0044
17 0.0064
--
w
87
87
21 0.0080
20 0.0053
0.0252
0.0265
20 0.0078
24 0.0093
--
--
84
83
(m~/s) Temperature of exhaust air to the baghouse (°C) Volumetric flowrate of exhaust air to the baghouse (m3/s) Measurement point 16 Mass flowrate of dryer swirl air (kg/s) Temperature of dryer swirl air (°C) Humidity of dryer swirl air (kg/kg) Measurement point 17 Outlet solids moisture content (kg/kg) Outlet solids temperature CC) Measurement point 18 Dilution air temperature (°C) Dilution air humidity (kg/kg) Measurement point 19 Dilution air temperature (°C) Measurement point 20 Flue gas temperature (°C) Measurement point 21 Flue gas temperature CC) Flue gas humidity (kg/kg) Mass flowrate of flue gas Measurement point 22 Stack condensate flowrate (kg/s) ( k g / s )
82
83
7.30
7.42
7.82
8.56
8.00
8.13
0.65 35 0.0059
0.67 36 0.0053
0.72 47 0.0089
0.72 45 0.0093
0.67 47 0.0141
0.71 49 0.0101
0.0066 32
0.0066 28
0.0181 37
0.0179 40
0.0181 44
0.0177 46
---
---
15.5 0.0085
16.2 0.0082
---
---
--
--
67
66
--
--
--
--
210
210
- -
- -
---
----
--
--
- -
--
--
78 0.076 1.79 0.0033
78 - -
- -
- -
--
--
--
--
--
--