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COST-EFFECTIVE LNG REGASIFICATION WITH MULTI-TEMPERATURE LEVEL (MTL) AIR HEATERS—AN

ECONOMIC AND ENVIRONMENTALLY FRIENDLY APPROACH

Cong Dinh Process Specialist

Joseph Cho Head of HTC Technology Manager

Jay Yang General Manager

SK Engineering & Construction Co. Ltd. Houston, Texas USA

www.skec.com

ABSTRACT

The main industry concerns for liquefied natural gas (LNG) import regasification terminals 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 paper proposes novel regasification methods that use multi-temperature level (MTL) 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 paper discusses economic/technical advantages of MTL air heaters using 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. Typical LNG Regasification 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 Vaporizer Technology

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 of LNG using HTF for an Air Heated Vaporizer (AHV)

Air Heated Vaporizer (AHV)

Amb. Air Vaporizer (AAV)

Open Rack Vaporizer (ORV)

Heat Source Ambient air Ambient Air Sea water

Heating Medium Heat Transfer Fluid (Indirect heat) None (Direct heat) None (Direct heat)

Major Equipment STV, Air heater Ambient vaporizer Sea water intake facility

Key Design Parameters Air temperature, Relative humidity Air temperature

Sea water temperature, Allowable temperature

drop

Key Issues Air temperature variations

Defrosting, temperature variations

Sea water intake facility maintenance,

Environmental impact

Environmental Issues HTF leakage FoggingMarine life, low

temperature,biocide injection

Advantages Proven technology Not used in large scale plants Proven technology

Inexpensive source of heat Free Heat Inexpensive source of

heat (Sea Water)Low emissions, low

maintenanceNo emissions, low

maintenance

Disadvantages Medium plot size area Large plot area Large sea water Inlet

Periodic defrosting Periodic cleaning

Large power load

Sea water application - not permitted in USA

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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 Japan/Korea and some European countries but it is prohibited in the USA because of environmental issues.

Figure 4. Typical 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 Combustion 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 Availabilities

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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Figure 6. Ambient Temperature Exceedance Curve at a Hypothetical Site

Figure 7. Relative Humidity Exceedance Curve at a Hypothetical 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

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

22 23 24 25 26 27 28 29 30 31 32 33 34

App

roxi

mat

e %

of y

ear t

empe

ratu

re is

exc

eede

d

Site Ambient Temperature (°C)

0

10

20

30

40

50

60

70

80

90

100

0

10

20

30

40

50

60

70

80

90

100

22 23 24 25 26 27 28 29 30 31 32 33 34 35

% E

xcee

denc

e

Rela

tive

Hum

idity

(%

)

Temperature (oC)

Maximum RHAverage RHMinimum RH% Exceedence

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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 with Various Ambient Air Design Temperatures

Heat Transfer Fluids for 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 Requirements for Low Temperature HTFs

Ambient Air Temperature

(°C)

Air Heater Inlet

Design Temp. (°C)

Actual Accumulated

hours per year (hours)

Actual Accumulated % per year

STV outlet temp. at 420

t/h with 3 Units (°C)

Available Regas Capability with 3 Units (t/h) at

STV Outlet Temp.

STV outlet temp. at 420 t/hwith 4

Units (°C)

Available Regas

Capability with 4 Units (t/h) at STV Outlet

Temp.22 20 4 0.05 10.8 317 @14°C 14.0 420 @14°C23 21 55 0.63 12.3 399 @14°C 15.0 420 @15°C24 22 322 3.68 14.0 420 @14°C 15.0 420 @15°C25 23 8379 95.65 15.0 420 @15°C 15.0 420 @15°C

Desc ript ion Des irable Propert ies EG 36% K- HTFOperational Temp. Range Low Freez ing

TemperatureFreez ing po int

- 20°CFreez ing po int

- 20°CThermal Conduc tivity High Low High

Viscos ity Low High LowSpec if ic Heat High High Low

Tox ic ity Low High NoneVo lat ility Low Low Low

So lubility in Water So luble in all proport ions So luble So lubleCorros ion rate Low High Low

Odour None None NoneEnvironmental Friendly Non Friendly Friendly

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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).

Figure 8. Viscosities of K-HTF vs. EG 36 wt% Solutions

Figure 9. Thermal Conductivities of K-HTF vs. EG (36 wt %) Solutions

0.0

1.0

2.0

3.0

4.0

5.0

6.0

‐10 ‐5 0 5 10 15 20 25

Viscosity (cP)

Temperature (C)

EG 36%

K‐HTF

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

‐10 ‐5 0 5 10 15 20 25

Thermal Con

ductivity [W

/m‐k]

Temperature (C)

K‐HTF

EG 36%

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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

• Vaporize 500,000 Nm3/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

• 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)

• 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

• Case B - K-HTF at a Single Outlet Temperature

• Case C – K-HTF at Two Different Outlet Temperatures

• Case D – K-HTF at Two Different Outlet Temperatures with Modified STV (Add 3rd nozzle)

• Case E – K-HTF at Two Different Outlet Temperatures using a NG Super-heater (SH) and STV

• Case F – K-HTF at Two Different Outlet Temperatures with a Modified STV (Add 3rd nozzle) and Air Heaters in Series

• Case G – K-HTF at Three Different Outlet Temperatures with Modified STV (Add 3rd 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.

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Table 4. Case Study Definitions

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 AHVs

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).

Case A B C D E F G

Option EG K- HTF K- HTF 2 Levels

2 Levels w/ Mod

STV

2 Level w/ Add SH

2 Level w/ AH Series

3 Level w/ Mod STV

HTF EG 36 wt% K- HTF K- HTF K- HTF K- HTF K- HTF K- HTFHTF Temp. Level 1 1 2 2 2 2 3Air Heater Parallel Parallel Parallel Parallel Parallel Series ParallelAir Heater Bays/unit 10 8 7 6 6 6 5STV No Mod No Mod No Mod Add Nozzle No Mod Add Nozzle Add NozzleSuperheater (SH) No No No No Yes No No

Case Case A Case B Case C Case D Case E Case F Case G1 Temp. 1 Temp. 2 Temp. 2 Temp. 2 Temp. 2 Temp. 3 Temp.

LNG Flow of Each Unit ton/hr 140 140 140 140 140 140 140STV Duty MW 29.9 29.9 29.9 29.9 29.1 29.9 29.9SH Duty MW - - - - 0.8 - -STV+SH Duty MW 29.9 29.9 29.9 29.9 29.9 29.9 29.9STV UA (note 2) kJ/C- h * 1E6 note 1 2.07 2.11 2.28 1.82 2.27 2.29SH UA (note 2) kJ/C- h * 1E6 - - - - 0.48 - -STV+SH UA (note 2) kJ/C- h * 1E6 2.07 2.11 2.28 2.30 2.27 2.29% STV+SH UA of Case B % 100 102 110 111 110 111STV HTF Hot Inlet T °C 19 19 19 19 19 19 19STV HTF Warm Inlet T °C - - - - - - 14STV HTF Cold Inlet T °C - - 16.5 15 15 13.7 12STV HTF Return Outlet T °C 2 2 2 2 2 2 2STV LNG Inlet T °C - 156 - 156 - 156 - 156 - 156 - 156 - 156STV NG Outlet T °C 15 15 15 15 7 15 15NG SH Outlet T °C - - - - 15 - -HTF Flow gpm 1,634 1,610 1,725 2,029 2,031 2,190 2,364% HTF Flow of Case B % 101 100 107 126 126 136 147HTF Type EG 36% K- HTF K- HTF K- HTF K- HTF K- HTF K- HTFHTF Pump Hydraulic Power MW 0.27 0.27 0.32 0.38 0.38 0.41 0.44Air Heater Hot Duty MW 29.7 29.7 16.6 4.2 4.2 1.8 5.2Air Heater Warm Duty MW - - - - - - 11.3Air Heater Cold Duty MW - - 13.0 25.3 25.4 27.6 13.0Air Heater Total Duty MW 29.7 29.7 29.5 29.5 29.5 29.5 29.5Air Heater Hot UA (note 2) kJ/C- h * 1E6 note 1 14.30 7.23 1.82 1.82 1.81 2.26Air Heater Warm UA (note 2) kJ/C- h * 1E6 3.69Air Heater Cold UA (note 2) kJ/C- h * 1E6 - - 5.32 8.80 8.81 8.90 3.66Air Heater Total UA (note 2) kJ/C- h * 1E6 14.30 12.55 10.62 10.63 10.71 9.61% AH UA of Case B % 100 88 74 74 75 67Air Heater Bays/ Unit 10 8 7 6 6 6 5 MW 0.64 0.64 0.64 0.64 0.64 0.64 0.64

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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.

Figure 10. Single Outlet Temperature (Case A and Case B)

Case B - K-HTF at a Single Outlet 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

HTF Pump

NGM Air Heater

Return HTFLNG Vaporizer

(STV)

LNG

10 Bays (Case A)8 Bays (Case B)

Hot HTFTrim Heater

(optional)

Case A and Case B

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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 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.

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.

Case Case A Case B Case GUnits EG 36 wt % K-HTF K-HTF w/3 Temp.

EquipmentsEstimated Air Heater Cost/Unit $million USD 2.84 2.56 1.60

Estimated STV Cost/Unit $million USD 2.42 2.18 2.32Estimated subtotal Cost /Unit $million USD 5.26 4.74 3.92

No. of Units 4 (3+1) 4 (3+1) 4 (3+1)Estimated grand total cost $million USD 21.04 18.94 15.68

HTFHTF Cost basis $/gallon 3.6 9.5 9.5

Initial HTF Fill-up for 4 units gallons 105,000 105,000 105,000Estimated HTF Total cost $million USD 0.38 1.00 1.00

Grand Total $million USD 21.42 19.94 16.68Potential Saving vs. Case A $million USD Base 1.48 4.74Potential Saving vs. Case B $million USD Base 3.26

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Figure 11. Two Different 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 Outlet Temperatures with Modified 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%.

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.

HTF Pump

NG M

Air Heater

Return HTFLNG Vaporizer

(STV)

LNG

M

Air Heater

4 Bay

3 Bays

Hot HTF

Cold HTF

Case C

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Figure 12. Two Different Outlet Temperatures with Modified STV (Case D)

Case E – K-HTF at Two Different Outlet Temperatures using 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%.

Figure 13. Two Different Temperatures HTF using a NG SH and with STV (Case E)

HTF Pump

M

Air Heater

LNG Vaporizer

(STV)

LNG

M

Air Heater

1 Bay

5 Bays

Hot HTF

Cold HTF

Trim Heater(optional)

NG

Case E

Return HTF

NG Superheater

(SH)

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Case F – K-HTF at Two Different Outlet Temperatures with a Modified STV (Add 3rd nozzle) and Air Heaters in 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 Different Outlet Temperatures with a Modified STV (Add 3rd nozzle) and Air Heaters in Series (Case F)

Case G – K-HTF at Three Different Outlet Temperatures with 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.

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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Figure 15. Three Different Outlet Temperatures with Modified 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.