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COST-EFFECTIVE LNG REGASIFICATION WITH MULTI-TEMPERATURE MULTI-TEMPERATURE LEVEL (MTL) A IR HEATERS—AN HEATERS—AN ECONOMIC ECONOMIC AND ENVIRONMENTALL ENVIRONMENTALL Y FRIENDLY APPRO A PPROACH ACH C ong Dinh Dinh P rocess rocess Spec S peciali ialist st J osep oseph h Cho Head of HTC Technology Manager J ay Yang Yang General Manager SK Engineering & Construction Co. Ltd. Houston, Texas USA www.skec.com
AB STRACT The The main ain ind indust ustry con concer cerns for for liq liquefie efied d natu atural gas gas (LNG) (LNG) imp import ort reg regasif asific icat atio ion n term erminal inals s ar are lowering costs while minimizing environmental impacts. There are several options that have been developed to use renewable energy in LNG regasification terminals. One viable LNG vaporization option that can mitigate industry concerns for terminals at some locations is to use ambient air, in combination with heat transfer fluids, as the heat source for LNG vaporization. This This pape paperr pro prop poses oses novel ovel reg regasif asific icat atio ion n meth ethods ods th that use multilti-ttemperat eratur ure e lev level el (MTL) (MTL) air air heaters to achieve cost savings by reducing the total number of air heater bays. This concept is similar to chilling with multi-temperature refrigerants that use the warmest refrigerants before using the coldest refrigerants. This paper describes how LNG can be heated and then vaporized using cold heat transfer fluid (HTF) before applying hot HTF to save capital cost. Normally, HTFs exit air heaters at constant outlet temperatures in conventional schemes that require narrow temperature approaches to ambient air temperatures. This results in larger air heater heat transfer areas than required. Shifting some of the heating from hot to cold HTF at higher circulation rates increases the temperature approaches and thus reduces the air heater heat transfer areas. This This paper aper disc discu usses sses econ econom omic/t ic/tec ech hnical ical adv advant antages ages of MTL air air heat eaters ers usin sing environmentally friendly HTF systems in conventional shell and tube vaporizers (STV). This paper also discusses the advantages of potassium-based heat transfer fluids which have superior low temperature thermal characteristics and reduced environmental impacts in comparison with heat transfer fluids using conventional, ethylene glycol water- based solutions.
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INTRODUCTION LNG Vaporizers
Vaporization of liquefied natural gas (LNG) in most import regasification terminals requires large quantities of heat. A diagram of a typical LNG regasification terminal is shown in Figure 1. This paper focuses on LNG vaporizers that use air heater vaporization (AHV) technologies.
Figure 1. Typic al LNG Regasific ation Terminal
Listed below are some major issues influencing the selection of LNG vaporizers: •
Proven technology
•
Capital and construction costs
•
Fuel and electricity operating costs
•
Type of heat source - air / sea water / natural gas
•
Environment impacts - CO2 and toxic emissions
•
Safety
A summary of the characteristics of air heaters for LNG vaporizers in comparison with conventional open rack LNG vaporizers is given in Table 1.
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Table 1. Summary of LNG Vapori zer Techno logy
Air Heated Amb. Air Open Rack Vaporizer (AHV) Vaporizer (AAV) Vaporizer (ORV) Heat Sourc e
Amb ien t air
Amb ien t Air
Sea w ater
Heating Medium
Heat Transfer Fluid (Indirect heat)
No ne (Direc t h eat)
No ne (Direc t h eat)
Major Equipment
STV, Air heater
Ambient vaporizer
Sea water intake facility
Key Design Parameters
Air tem per atu re, Relative humid ity
Air tem per atu re
Sea water temperature, Allo wab le tem per atu re drop
Key Issues
Air te mp eratu re variations
Defrosting, temperature variations
Sea water intake facility maintenance, Environmental impact
En vironmental Issues
HTF leakage
Fogging
Marine life, low temperature,biocide injection
Ad van tag es
Pro ven tec hn ol og y
Not used in large scale plants
Proven technolog y
Free Heat
Inexpensive source o f heat (Sea Water)
Dis ad van tag es
Inexpensive source o f heat Low emissions, low maintenance
No emissions, low maintenance
Med iu m p lo t s iz e area
L arg e p lo t area
L arg e s ea w ater In let
Perio dic d efro stin g
Perio dic c lean in g Large power load Sea water application - not permitted in USA
One use of air as the heating medium for LNG vaporizers is shown in Figure 2. This scheme uses a heat transfer fluid (HTF) in a shell-and-tube vaporizer (STV) and conventional air heaters to reheat the HTF using ambient air. An LNG import terminal using AHV technology has been operated at Dahej Terminal in India since 2004.
Figure 2. Indirect Heating o f LNG usin g HTF for an Air Heated Vaporizer (AHV)
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The Ambient Air Vaporizer (AAV) shown in Figure 3 illustrates a vaporizer that uses air as a heating medium. This type of vaporizer uses air in direct contact with LNG. However, the major issues with this type of vaporizer are the requirements for periodic defrosting and large plot space. It can also produce a fog of condensed water vapor that can be a nuisance. Currently, there are no LNG import gasification terminals using only this type of vaporizer.
Figure 3. Direct Heating of LNG with Air using Ambient Air Vaporizers (AAV)
Sea water is used as the heating medium in Open Rack Vaporizers (ORV) as shown in Figure 4. LNG is vaporized inside tubes when seawater flows on their outside surfaces. The use of ORV’s is common in J apan/Korea and some European countries but it is prohibited in the USA because of environmental issues.
Figure 4. Typic al Open Rack Vaporizer (ORV)
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Combustion of natural gas is the heat source used in Submerged Combustion Vaporizers (SCV) as shown in Figure 5. LNG is vaporized inside stainless steel tubes in a submerged-water bath. The water bath is heated and maintained at a certain temperature by the hot flue gases. SCVs have long been used in many LNG regasification terminals but usually only as back-up facilities due to their higher operation costs.
Figure 5. Typical Submerged Comb usti on Vaporizer (SCV)
One viable LNG vaporization option as given in Table 1, which can mitigate industry concerns for terminals at some locations, is to use ambient air as a heat source for LNG vaporizers. This option addresses the main objectives to lower costs while minimizing environmental impacts. The use of an AHV is preferred over an AAV because it is a proven technology and because AAVs have not yet been used for LNG regasification terminals. An example of AHV technology is at the Dahej Terminal in India (see Figure 2), which uses ethylene glycol (EG) and water as a HTF. The environmental impact of using ethylene glycol and water as a HTF when leakage occurs can be addressed by using potassium-based HTFs (K-HTF). This type of HTF is biodegradable. The plot space areas of air heaters can also be reduced by up to 50% using multiple air heaters with different outlet temperatures in combination with K-HTF as the replacement for EG.
PERFORMANCES OF AHVS AHV A vailabil it ies
In LNG regasification terminals that use AHVs, LNG is vaporized and superheated in shell and tube vaporizers (STV). The heat transfer fluid (HTF) provides the heating medium for vaporizing and superheating of the LNG. The air heater heats the HTF with ambient air, which is a renewable energy source. One concern with AHV technology is the availability of air that is warm enough as a heat source throughout the year to satisfy air heaters requirements. If the ambient temperature exceedance curve at a hypothetical LNG regasification terminal site is as shown in Figure 6, then AHVs are feasible. Based on this figure, the availability of air with dry bulb temperatures above 25 C is 90%. The case studies in this paper are therefore based on using ambient air at 25 C. Assuming a 2 C cold air recirculation temperature, the air temperature entering the air heaters is 23 C. The case studies in this paper further assume that the relative humidity exceedance curve is as given in Figure 7. The 90% exceedance relative humidity (RH) is at the minimum RH of 83% for an ambient air temperature of 25 C. ˚
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100%
d e d e e c x e s i e r u t a r e p m e t r a e y f o % e t a m i x o r p p A
90% 80% 70% 60% 50% 40% 30% 20% 10% 0% 22
23
24
25
26
27
28
29
30
31
32
33
34
Site Ambient Temp erature (°C) Figure 6. Ambient Temperature Exceedance Curve at a Hypothetic al Site
100
100
90
90
80
80
70
70
) % ( 60 y t i d i m 50 u H e v i t 40 a l e R
e c n e d 50 e e c x E % 40
60
Maximum RH
30
30
Average RH Minimum RH % Exceedence
20
20
10
10
0
0 22
23
24
25
26
27 28 29 30 Temperature (oC)
31
32
33
34
35
Figure 7. Relative Humidit y Exceedance Curve at a Hypotheti cal Site
Based on the exceedance curve in Figure 6, the actual accumulated hours per year at different temperatures, for a hypothetical site, are given below in Table 2. The ratings of air heaters and STVs for various ambient temperatures indicate that air heaters could provide the
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heating duty to the STV at a full capacity of 420 ton per hour with 3 units in operation for +99% of the time or 100% of the time for 4 units in operation in order to meet an STV outlet temperature of 14 C. ˚
For operation with ambient temperatures of 23 C or colder, the STV can reach full capacity with either all four units in operation, with supplemental heating using a SCV or use of a trim heater. ˚
Table 2. Capacities of STVs wi th Various Ambi ent Air Desig n Temperatur es
Air Heater Actual STV outlet Ambient Air Actual Inlet Accumulated temp. at 420 Temperature Accumulated Design hours per year t/h with 3 (°C) %per year Temp. (°C) (hours) Units (°C) 22 23 24 25
20 21 22 23
4 55 322 8379
0.05 0.63 3.68 95.65
10.8 12.3 14.0 15.0
Available Available Regas STV outlet Regas Capability with temp. at Capability with 3 Units (t/h) at 420 t/h 4 Units (t/ h) STV Outlet with 4 at STV Outlet Temp. Units (°C) Temp. 317 @14°C 399 @14°C 420 @14°C 420 15°C
14.0 15.0 15.0 15.0
420 @14°C 420 @15°C 420 @15°C 420 15°C
Heat Transfer Fluid s fo r AHVs
The selection of a HTF for an air heater vaporizer directly affects its sizing, reliability, operating cost and environmental impact. HTFs are chosen on the bases of operating temperature ranges (-1 to 99 C), low freezing points, and flammability. Other physical properties that should also be considered are freezing points, densities, thermal conductivities, viscosities and specific heats. ˚
The Dahej Terminal in India that is operating with AHV technology uses a 36% (by weight) ethylene glycol (EG) in water solution as a HTF. One alternative HTF that can be substituted for ethylene glycol is a potassium-based aqueous coolant designated in this study as K-HTF. K-HTF is an environmentally friendly low to medium temperature heat transfer fluid that can operate efficiently within the range of -50°C up to 218°C. It is a non-combustible fluid that does not support bioactivity and has better thermo-physical qualities than aqueous solution of ethylene glycol. It is also virtually odor free, biodegradable, CFC free and is considered non-toxic. The general requirements for low temperature HTFs are given below in Table 3. Table 3. General Requir ements f or Low Temperature HTFs
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K-HTF was selected in this study as the HTF of choice rather than EG (36 wt %) because it has better thermal transport properties and can provide the same freezing point of -20 C. In addition, K-HTF has lower viscosities (see Figure 8) especially at low temperatures and has better thermal conductivities (see Figure 9). ˚
6.0
EG 36%
5.0 K‐HTF
4.0
) P c ( y t i s3.0 o c s i V 2.0
1.0
0.0
‐10
‐5
0
5
10
15
20
25
Temperature (C)
Figure 8. Visc osit ies of K -HTF vs. EG 36 wt % Solut ions
0.7
0.6
0.5
] k m /
‐
K‐ HTF
0.4
W [ y t i v i t c 0.3 u d n o C l 0.2 a m r e h T 0.1
EG 36%
0.0
‐10
‐5
0
5
10
15
20
25
Temperature (C)
Figure 9. Thermal Conduc tivi ties of K-HTF vs. EG (36 wt %) Solu tio ns
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Use of K-HTF in air heaters and shell and tube vaporizers reduces their heat transfer surface areas. Tube wall temperature on the HTF sides of these shell and tube vaporizers are higher because of the larger film heat transfer coefficients of K-HTF. This means K-HTF is more likely to prevent freezing in shell and tube vaporizers than ethylene glycol solutions. Changing HTFs from EG (36 wt %) to K-HTF can improve heat transfer properties and reduce STV and air heaters sizes. In addition, K-HTF prevents corrosion caused by possible degradation products of ethylene glycol.
DESIGN BASES OF LNG REGASIFICATION TERMINALS USING AHVs Process •
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Vaporize 500,000 Nm /hr LNG (420 metric ton/h) in shell and tube vaporizers (STVs) using either HTFs of heated ethylene glycol (EG)-water solutions or of heated K-HTF at a hypothetical South East Asia location
•
4 units (3 units in operation +1 standby) of 140 metric t/h capacity each
•
Send-out LNG temperature of 14 C at 40 barg
•
•
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Air heaters designed for an ambient air temperature of 25 C with a site-specific exceedance probability of 90% (see Figure 6). °
2 C air heater temperature correction decrease due to cold air recirculation. Inlet air temperatures to LNG air heaters of 23 C (25 C ambient air temperature with 2 C air recirculation temperature correction) ˚
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•
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Site-specific relative humidity of 83% for ambient air temperature of 25 C °
Case Definitions •
Case A – EG 36 (wt %) HTF with a Single Outlet Temperature
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Case B - K-HTF at a Single Outlet Temperature
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Case C – K-HTF at Two Different Outlet Temperatures
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Case D – K-HTF at Two Different Outlet Temperatures with Modified STV (Add 3 nozzle)
•
•
•
rd
Case E – K-HTF at Two Different Outlet Temperatures using a NG Super-heater (SH) and STV rd
Case F – K-HTF at Two Different Outlet Temperatures with a Modified STV (Add 3 nozzle) and Air Heaters in Series rd
Case G – K-HTF at Three Different Outlet Temperatures with Modified STV (Add 3 nozzle)
CASE STUDIES OF LNG REGASIFICATION TERMINALS USING AHVs The case studies described in this paper improve AHV technologies. Case A, using 36 wt % EG as the HTF, is compared with Case B which uses K-HTF. K-HTF is used in all the other cases. Case B is used as the base case for all the other cases C to G. All cases are compared in Table 4. 9
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Table 4. Case Study Defin iti ons Case EG
K- HTF
EG 36 wt%
K- HTF
K- HTF
K- HTF
K- HTF
K- HTF
K- HTF
1 Parallel 10
1 Parallel 8
2 Parallel 7
2 Parallel 6
2 Parallel 6
2 Series 6
3 Parallel 5
No Mod
No Mod
No Mod
Add Nozzle
No Mod
No
No
No
No
Yes
Option HTF
2 Levels w/ Mod STV
K- HTF 2 Levels
HTF Tem . Level Air Heater Air Heater Ba s/unit STV Su erheater SH
2 Level w/ 2 Level w/ 3 Level w/ Add SH AH Series Mod STV
Add Nozzle Add Nozzle No
No
Operating Data for AHVs
The operating data used in this study for each type of AHV unit are given below in Table 5. UAs of Cases C to G are compared against Case B. Table 5. Operating Data for A HVs Case LNG Flow of Each Unit STV Duty SH Duty STV+SH Duty STV UA (note 2) SH UA (note 2) STV+SH UA (note 2) % STV+SH UA of Case B STV HTF Hot Inlet T STV HTF Warm Inlet T STV HTF Cold Inlet T STV HTF Return Outlet T STV LNG Inlet T STV NG Outlet T NG SH Outlet T HTF Flow % HTF Flow of Case B HTF Type HTF Pump Hydraulic Power Air Heater Hot Duty Air Heater Warm Duty Air Heater Cold Duty Air Heater Total Duty Air Heater Hot UA (note 2 ) Air Heater Warm UA (note 2 ) Air Heater Cold UA (note 2) Air Heater Total UA (note 2 ) % AH UA of Case B
ton/ hr MW MW MW kJ/ C- h * 1E6 kJ/ C- h * 1E6 kJ/ C- h * 1E6 % °C °C °C °C °C °C °C gpm % MW MW MW MW MW kJ/ C- h * 1E6 kJ/ C- h * 1E6 kJ/ C- h * 1E6 kJ/ C- h * 1E6 % MW
Case A 1 Temp. 140 29.9 29.9 note 1 -
Case B 1 Temp. 140 29.9 29.9 2.07 2.07 100 19 2 - 156 15 1,610 100 K- HTF 0.27 29. 7 29. 7 14. 30
Case C 2 Temp. 140 29.9 29.9 2.11 -
Case D 2 Temp. 140 29.9 29.9 2.28 -
Case E 2 Temp. 140 29.1 0.8 29.9 1.82 0.48
Case F 2 Temp. 140 29.9 29.9 2.27 -
Case G 3 Temp. 140 29.9 29.9 2.29 -
19 16.5 2 - 156 15 1,725 107 K- HTF 0.32 16. 6 13. 0 29. 5 7. 23
19 15 2 - 156 15 2,029 126 K- HTF 0.38 4.2 25 .3 29. 5 1. 82
19 15 2 - 156 7 15 2,031 126 K- HTF 0.38 4. 2 25. 4 29. 5 1. 82
19 13.7 2 - 156 15 -
19
-
14. 30 100
5. 32
8. 80
8. 81
8. 90
13. 0 29. 5 2.2 6 3.6 9 3. 66
0.64
0.64
0.64
0.64
0.64
0.64
0.64
19 2 - 156 15 1,634 101 EG 36% 0.27 29 .7 29 .7 note 1
1. 8 27. 6 29. 5 1. 81
12 2 - 156 15 -
5.2
Note 1: The heat transfer area requirement for Case A in comparison with Case B is based on actual heat exchanger ratings since the heat transfer properties of EG 36 wt% are different from K-HTF. Note 2: UA is the product of U (overall heat transfer coefficient) and A (heat transfer surface area).
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Case A – EG 36 (wt %) HTF with a Single Outlet Temperature
Case A, with a single outlet temperature from the air heater, uses EG 36 wt% as a HTF and is shown in Figure 10. Case B with a single outlet temperature from the air heater and which uses K-HTF is also shown in Figure 10. Case A serves as the basis for HTF comparisons since this is the HTF being used at the Dahej Terminal. Case A will result in a requirement for 10 bays of air heaters for each unit or a total of 40 bays for 4 units. The number of bays per unit in this study is different from the number used for Dahej Terminal because the capacity and air heater design criteria are different. In case A, the air heater design at a single outlet temperature will require a large heat transfer area due to the relatively close approach temperature between the ambient air temperature and the HTF temperature. This fact will translate into a relatively high cost and large plot space requirement for Case A compared to other cases. The use of a trim heater is optional in Case A depending on the natural gas (NG) battery limit specifications and the actual ambient air temperatures. The convention method is to provide a trim heater on the natural gas stream which would require a separate heating source such as hot water since direct fire heating is undesirable. The use of a trim heater on the HTF stream is not recommended since it would require a large heat duty. To simplify the scheme for discussion, the trim heater is discussed but omitted from further analysis in this study.
NG M
Air Heater
Hot HTF
Trim Heater (optional)
10 Bays (Case A) 8 Bays (Case B) Return HTF
LNG Vaporizer (STV)
HTF Pump
LNG
Case A and Case B
Figure 10. Single Outl et Temperature (Case A and Case B) Case B - K-HTF at a Sin gle Outl et Temperature
Case B with a single outlet temperature using K-HTF is shown in Figure 10. Case B will reduce the number of bays to 8 (compared to 10 for Case A) for each unit since K-HTF has improved HTF heat transfer properties compared to EG solutions. The use of K-HTF can potentially save $1.5 million in initial capital investment for an import regasification terminal as given in Table 6. In Table 5, Case B is the base for comparison of its UA with the UAs of other cases. It is expected that the total fan power remains the same for all cases since the total air heater duties
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and air delta temperatures remain the same. Larger fan motors would be required if the number of bays decreased to provide the same total air throughput for each unit. Table 6. Economic Comparison Case
Units
Case A EG 36 wt %
Case B K-HTF
Case G K-HTF w/3 Temp.
Estimated Air Heater Cost/Unit
$million USD
2.84
2.56
1.60
Estimated STV Cost/Unit
$million USD
2.42
2.18
2.32
Estimated subtotal Cost /Unit
$million USD
5.26
4.74
3.92
4 (3+1)
4 (3+1)
4 (3+1)
$million USD
21.04
18.94
15.68
HTF Cost basis
$/gallon
3.6
9.5
9.5
Initial HTF Fill-up for 4 units
gallons
105,000
105,000
105,000
Estimated HTF Total cost
$million USD
0.38
1.00
1.00
Grand Total
$million USD
21.42
19.94
16.68
Potential Saving v s. Case A
$million USD $million USD
Base
1.48 Base
3.26
Equipments
No. of Units Estimated grand total cost HTF
Potential Saving v s. Case B
4.74
Case C - K-HTF at Two Different Outlet Temperatures
Case C with two different outlet temperatures using K-HTF is given in Figure 11. Dividing the total heat duty for the HTF into a hot stream and a cold stream at a higher total circulation rate for the same overall total duty requirement will reduce the heat transfer area of the air heaters since it increases the temperature approaches. The air heaters are grouped into two sections operating in parallel with one section requiring four bays and the other section requiring three bays. Each air heater section will have different HTF outlet temperatures. The cold HTF stream is used at the LNG inlet end of the STV and the hot HTF stream is used at the outlet end for superheating the natural gas. The single LNG Shell and Tube Vaporizer (STV) shown in Figure 11 utilizes inlet nozzles for the HTF at both ends of the shell. The HTF nozzle at the LNG inlet end is required to prevent excessive ice layer build-ups of HTF on the surfaces of the tubes since the LNG entering temperature is at -156 C. The HTF nozzle at the NG outlet is required to provide for superheating. The hot HTF temperature remains at 19 C going into the NG outlet end while the cold HTF temperature going into the LNG inlet end is at 16.5 C. The combined HTF stream that returns to the air heaters remains the same as Case B at 2 C. ˚
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The vaporization of the LNG at -156 C at the cold end of the STV will have little impact whether the HTF is at 19 C or 16.5 C since this is only about a 2% change in LMTD. However, it has large impact on the air heater design since the ambient air is at 23 C and the HTF outlet is at 19 C for hot HTF or 16.5 C for cold HTF. The HTF outlet temperature approach to the inlet ambient air is therefore improved from 4 C for hot HTF to 6.5 C for cold HTF. This fact has a significant impact on the air heater design. ˚
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Air Heater M
Hot HTF 4 Bay Air Heater M
NG Cold HTF
3 Bays
LNG Vaporizer (STV)
Return HTF HTF Pump
LNG
Case C
Figure 11. Two Diff erent Outlet Temperatures (Case C)
Changing the air heater design in Case B to the design in Case C and as shown in Figure 11 reduces the total number of bays from 8 to 7 bays per unit or a 12% reduction in total heat transfer area when compared against Case B. The STV heat transfer area increases by 2% and the HTF flow and pump power increases by 7%. This scheme will require a rearrangement of the air heaters to provide two HTF streams at two outlet temperatures at the outlet of the air heaters. Minimal changes in the designs of the STV and HTF pumps will occur. Case D – K-HTF at Two Different Outl et Temperatures w ith Modifi ed STV (Add 3rd nozzle)
Case D shown in Figure 12 describes an improvement to the air heater design given in Case C that uses HTFs at two different outlet temperatures and involves a modification to the design of the STV. The air heaters are grouped into two sections operating in parallel with one section requiring five bays and the other section requiring one bay. Changing the cold HTF to 15 C coupled with a higher HTF circulation rate and maintaining the hot HTF at 19 C will reduce the air heater design from 8 to 6 bays per unit or a 25% reduction in total heat transfer area when compared against Case B. This will require however, a STV with a larger heat transfer area and an additional HTF inlet nozzle as shown on Figure 12. The heat transfer area of the STV increases by 10%. The total HTF flow rates and pump power increase by 26%. ˚
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This option will allow the trim heater to be located on the hot HTF stream which uses only a small flow compared to the total flow of required HTF. The trim heater in the hot HTF loop could eliminate the hot water loop if a fired heater were to be used for heating of the hot HTF. The use of a fired heater as a trim heater is not recommended if the trim heater is located on the main NG circuit as shown in Figure 10.
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Figure 12. Two Dif ferent Outlet Temperatures w ith Modifi ed STV (Case D) Case E – K-HTF at Two Different Outlet Temperatures u sing a NG Super-heater (SH) and STV
Case E is shown in Figure 13. The additional of a NG super heater (SH) would allow the implementation of an MTL air heater design without any modifications to the STV. The operating conditions for Case D and Case E are nearly identical. The main differences are the addition of a SH on the NG stream and the additional nozzle as given in Case D and shown in Figure 12. Case E may be required if modifications to the STV are not possible. Case E is also useful to revamp the design of an existing plant. It would require modifications to the air heater as far as alignments, larger air heater fans and piping are concerned. The additional surface area of the STV for increase in capacity could be added to the SH. Modification of the HTF pumps and piping would be required for an increase of 26%. Trim Heater (optional)
Air Heater M
Hot HTF 1 Bay
NG
Air Heater M
Cold HTF
NG Superheater (SH)
5 Bays
LNG Vaporizer (STV)
Return HTF
HTF Pump
LNG
Case E
Figure 13. Two Diff erent Temperatures HTF using a NG SH and w ith STV (Case E)
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Case F – K-HTF at Two Different Outl et Temperatures w ith a Modified STV (Add 3rd nozzle) and Air Heaters i n Series
Case F shows a rearrangement of air heaters in series to provide two different HTF temperatures with a hot HTF air heater in series with a cold HTF air heater. This arrangement serves the same general objective as Case D with air heaters in parallel. However, the parallel approach in Case D would be preferable since it would allow independent flows from each air heater, a lower total HTF circulation rate and lower pump powers (See Table 5). Operation of the air heaters in parallel as given in Case D is the recommended option rather than operation of air heaters in the series as given in Case F.
Figure 14. Two Dif ferent Outlet Temperatures wi th a Modif ied STV (Add 3rd nozzle) and Ai r Heaters in Series (Case F) Case G – K-HTF at Three Different Outlet Temperatures w ith Modified STV (Add 3rd nozzle)
The use of K-HTF at three different temperatures is given in Figure 15. The different temperatures of HTF are given as hot, warm and cold. The HTF temperatures are 19 C for hot, 14 C for warm, 12 C for cold and 2 C for return. The air heaters are grouped into three sections of one bay for hot, two bays for warm and two bays for cold. ˚
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Using three different temperatures reduces the number of air heater bays per unit from 8 to 5 bays or a 38% reduction in plot space area when compared against Case B. The single air heater bay for hot HTF would require about 15% higher heat transfer area. The total heat transfer area is reduced by 33% when compared against Case B. The total STV heat transfer area however, increases by 11% and the total HTF flow rates and pump powers increase by 47%. Case G can potentially save $3.2 million in initial capital investment for an import regasification terminal when compared to Case B as given in Table 6.
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Paper PS3-Spare
Figure 15. Three Different Outl et Temperatures wit h Modif ied STV (Case G)
CONCLUSIONS The use of K-HTF to replace EG (36 wt %) can potentially reduce the heat transfer areas of air heaters by 20%. This represents a potential cost savings of $1.5 million in initial capital investment for a LNG regasification terminal. In AHV technology, the heating medium is air and the utility service uses a HTF. Using HTFs in AHVs at several different temperatures reduces significantly the total capital costs of the air heater bays with only some increment costs for the STVs, HTF pumps and piping. The implementation of three different HTF temperatures, for example, hot, warm and cold, as described in Case G reduces the total number of air heater bays from 8 to 5 per unit. This is significant because the plant has 4 units. Therefore, the total number of bays for the plant is reduced from 32 to 20 when comparing Case B with Case G. This represents a potential cost savings of $3.2 million in initial capital investment and a reduction in air heater plot space of 38% when compared against Case B. The actual equipment costs, energy savings and economic evaluations will be dependent on the actual site location and other economic criteria. If the benefits of both K-HTF and MTL air heaters are combined as evidenced by comparing Cases A and G, the total number of bays can be reduced from 40 to 20 bays. This represents a potential cost savings of $4.7 million in initial capital investment or a reduction in plot space of 50% when compared with Case A. This study has shown the benefits of an MTL approach in AHV design that increases the number of HTF outlet streams, each with a different temperature. The MTL approach for design of AHVs can also be applied for expansions or revamps of existing plants to maximize their air heating capabilities.
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