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Microchannel Heat Exchangers: Applications and Limitations

Anna Lee Tonkovich, PhDManager, Technology Development CenterVelocys, IncPlain City, OHwww.velocys.com

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Overview

Microchannel Exchanger Definition

Advantages of Microchannel Exchangers

Implementation Challenges

Microchannel Exchanger Applications at Velocys

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D ~ 0.1-0.3 mm

Characteristic dimension

Microchannel Technology Definition

Microchannel

Tube and Shell

Plate and Frame

D ~ 3-10 mm

D ~ 10-50 mm

Size:Small channels (typically < 2mm) in close proximity

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Microchannel Definition

Manufacturing:Shims or sheets with microchannel features

Diffusion bonded or welded to form hermetically-sealed microchannels

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Higher Performance• High volumetric heat flux• Modest pressure drop• Compact hardware for space critical applications (e.g., off-shore

applications, transportable systems, etc.)

Robust Design• Proven manufacturing processes• Demonstrated mechanical integrity

Scalable Technology• Repeatable Design• Effective Flow Distribution

Microchannel Technology Advantages

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dkNuh ×

=Nu: Nusselt numberh: Heat transfer coefficientd: Hydraulic diameterk: Thermal conductivity

Performance:Higher Heat Transfer Coefficients

Small channels provide high heat transfer coefficient

Small diameter results in large heat transfer coefficient in microchannels

Microchannel Heat Exchanger Conventional Heat Exchanger

High surface area/volume ratioHigh heat transfer per volume

Low surface area/volume ratio

Low heat transfer per volume

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Micro-channel Heat Exchanger Performance Comparison

400 – 200050 – 30020 – 100Heat Transfer coefficient (W/m2/K) (Gas)

LaminarTurbulentTurbulentFlow Regime

< 10°C~ 10°C~ 20°CApproach Temperature (°C)

> 70003000 – 7000~ 5000 (tube side)

Heat Transfer coefficient (W/m2/K) (liquid)

> 1500850 – 150050 – 100Surface Area Per Unit Volume (m2/m3)

Micro-channel Heat Exchanger

Compact Heat Exchanger

Shell and Tube Heat ExchangerParameter

75.1VLPp

∆ 75.1VLPp

∆ VLPp

∆

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Performance:Manageable Pressure Drop

Laminar Flow• Orderly flow – less fluctuations

•Laminar Flow

Turbulent Flow

flowh VDfLP µ)(=

∆

Turbulent Flow• Random flow – more fluctuations

•75.175.025.0)( VDf

LP

h ρµ=∆

flow

(Blasius friction factor)

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Performance:Manageable Pressure Drop

Distributed flow provide shorter flow length, overall low pressure drop

ρ24

2GdLfP ××=∆

f: Friction factorG: Mass Fluxd: Hydraulic diameterρ: DensityL : Length

Microchannel Exchanger Conventional Heat Exchanger

Distributed flow

Short length

Bulk flow

Long length

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Performance:Increased Volumetric Heat Flux

0.01

0.1

1

10

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5

Channel Gap (in)

Volu

met

ric H

eat F

lux

(W/c

m3 )

Basis: • N2 / N2 heat exchanger (gas/gas)• Stream 1: 150°C inlet, 1.5 psig• Stream 2: 50°C inlet, 1.5 psig • Approach temperature: 5°C per stream

Higher Volumetric Flux Smaller Hardware

Low system pressure drop 1.5 psig

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Feature creation Stacking

Bonding Machining

Shim

Robust:Proven Manufacturing Methods

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Robust:Mechanical Integrity

Diffusion bonded metals (stainless or high nickel alloys)

Mechanical design and validation to ASME standards

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Robust:Mechanical Integrity

Validated Mechanical Strength

Stamped Diffusion Bonded Device

Pressure hammer tests• 0 psig to 1000 psig • 30 second cycle time

Example Results of Test Specimen• First: 8,816 cycles at 850oC – no failure• Then: 14,871 cycles at 900oC – no failure • Finally, failed after 87 cycles at 950oC.

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Scaleable:“Numbering-up” vs Scaling-up

Reduce time and cost to commercialization

Number up Scale up

Identical channel hydrodynamics at

all scales

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Scaleable:Development Methodology

Full-scaleReactor

CELL

Cell• Internal channel dimensions same

as commercial chemical processor• Number of channels increase;

size of channels does not

Multi-Cell• Many channels• 10-100 lb/hr

Full-Scale• >1000 channels• 1000-5000 lb/hr

Full-Scale Reactor is the basic building block of a commercial plant

Gas flow

MULTI

CELL

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Scaleable:Flow Distribution Strategy

Inlet

1 2 3

max min1

max

100%m mQm−

= ×

2 3

Pressure Drop in flow circuits can be tailored to achieve sufficient flow distribution

Pressure Drop in flow circuits can be tailored to achieve sufficient flow distribution

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Scaleable:Validated Flow Distribution

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Scaleable:Channel Flow Distribution

Sufficient flow distribution measured in test deviceIrregular gasket on half of test device

Sufficient flow distribution measured in test deviceIrregular gasket on half of test device

Run 16: 214.0 SLPM of air

0.0E+00

1.0E-05

2.0E-05

3.0E-05

4.0E-05

5.0E-05

6.0E-05

7.0E-05

8.0E-05

9.0E-05

1.0E-04

0 12 24 36 48 60 72

Channel number

Mas

s flo

w ra

te (k

g/s)

ModelExperiment

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Implementation Challenges

• Cost

• Reliability

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Implementation of Microchannel Exchangers

Overall costs determined by• Equipment costs• Installation costs• Process productivity

Attractive costs for applications that • Require expensive materials of construction, e.g.,

high nickel alloys• Involve multiple streams• Demand close approach temperatures

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Installation Costs are Lower for Compact Equipment

0

1

2

3

4In

stal

latio

n Fa

ctor

Column VerticalVessels

HorizontalVessels

Shell &Tube HXs

Plate HXs Pumps,Motors

Source: Chemical Engineering, “Sharpen your Capital Cost Estimating Skills,” Oct. 2001

Average Installation Factors for Land-based Facilities

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Implementation of Microchannel Exchangers

Three performance aspects impact reliability• Fouling/plugging• Corrosion• On-stream factor

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Reliability: Fouling

With appropriate design, microchannels can be used in some

‘fouling’ services but not others

Fouling in microchannels depends upon service and …

• Particulate size

• Surface chemistry

• Solids content

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Fouling expected in some operating services: water boiling

Vaporizer after 2000 hours operation

Significant performance degradation over 2000 hours

Solids at 15 ppm, 80% initial vapor quality

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Fouling expected in some operating service but not detected

Solids at 1ppm, 30% vapor quality

00.20.40.60.8

11.21.41.61.8

2

0:00:00 2400:00:00 4800:00:00 7200:00:00 9600:00:00Time (hh:mm:ss)

Del

ta P

(psi

g)

020406080100120140160180200

Tem

pera

ture

(C) ,

%

Stea

mDelta P

Steam Quality

Outlet Air Temp (C)

Vaporizer after 9600 hours operation

No performance degradation over 9600 hours

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Pitted areasPitting observed after 100 hr of testingPitting observed after 100 hr of testing

Non-Aged Surface After 100 hrs

Reliability:Corrosion from Unprotected Metal Surface

Coupon tested for hot corrosion at 960oC,1 atm, 20% (O2 + steam), balance inert

Coupon tested for hot corrosion at 960oC,1 atm, 20% (O2 + steam), balance inert

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Reliability: Corrosion Resistance with Protected Surface

Non-Aged Surface After 1000 hrs

No visible difference in surface between fresh and 1000 hrs. No visible difference in surface between fresh and 1000 hrs.

Coupon tested for hot corrosion at 960oC,1 atm, 20% (O2 + steam), balance inert

Coupon tested for hot corrosion at 960oC,1 atm, 20% (O2 + steam), balance inert

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Reliability: On-Stream Factor

Frequency of Servicing

Maintenance access

Sequential service of modules with partial plant capacity reduction

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Case Studies

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• Formed in 2001 by Battelle Memorial Institute to commercialize microchannel technology

• Located in a 27,000 ft2 facility near Columbus, Ohio

• Established alliances with engineering and manufacturing firms

• Total S.A., Dow, ABB and other strategic partners have invested over $75 million

• More than 50 granted patents & 80 patent applications in process

Velocys Introduction

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Cryogenic Applications

Microchannel Exchanger Developmentat Velocys

High temperaturereactions

Integrated phase change

Distillation

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LNG Application

• Microchannel Heat Exchanger increases LNG process productivity through decreased pressure drop:• Shorter flow length• Laminar Flow• High surface area-to-volume

• Higher ROI for monetizing stranded natural gas• Lower capital cost and operating cost per throughput, or• Increase plant capacity for the same compression capacity

• Small foot-print an additional advantage of microchannel heat exchangers

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Case Study: Simplified LNG Cycle

Natural gas50,000 metric tons/year

LNG

J-TValve

Compressor Condenser

Three stream main heat exchanger

153°C331.3 psig

29.4°C323.3 psig

-153.9°C318 psig -158.3°C

29.95 psig

32.2°C635.3 psig

-153.9°C5 psig

20.9°C19.95 psig

322.8 psig

27.75 psig

-155.3°C

Compression Ratio• Before 10• After ~8

CompressionSavings = 18-22%

Compression Ratio• Before 10• After ~8

CompressionSavings = 18-22%

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Heat Transfer Comparison for LNG

201Relative Length

500-1500>1500Core Area, m2/m3

<1>10Core Heat Flux, W/cm3

Conventional Plate-fin HXVelocys HX

Significant reduction in hardware volume

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Why the advantage?

Stainless steel (bonded) versus aluminum reduces axial conduction and allows shorter lengths

Optimized multi-stream microchannel exchanger

Short lengths reduce pressure drop

Flow Flow

Benchmark

Velocys

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Length AdvantageAlternate Material Selection Reduces Heat Exchanger Length By Reducing Axial Conduction

LkAc∝λAxial Conduction Parameter

Channels

Ac

L

For same heat duty,• Aluminum plate fin heat exchanger, L = 6.7 m• Velocys Stainless Steel heat exchanger, L = 0.3 m

Shorter Heat Exchanger Length• Lower thermal conductivity• Smaller metal area

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Conventional Technology

Conventional Steam Reformer20 million standard cubic feet hydrogen per day

Plot ~ 30m x ~30m x ~30m

High temperature exchangers required

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Velocys® Technology ReactorsMicrochannel Steam Methane Reformer

Identical capacity

Plot < 10 m x 10m x 10 m (< 10% original)

~25% reduction in overall plant costs

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Integrated Reactor and Exchanger: Steam Reforming

Reactor Section

Multi-streamHeat Exchanger Section

Internal Manifold Section

ReactantProductFuel

Air

Exhaust

100900

Temperature (C)

Length (m)

0.9

40

30

40

50

60

70

80

0 2 4 6 8 10 12Contact time, msec

%

CO selectivity, %

CH4 Conv, %

P = 20 atm, T = 860 C, 2:1 steam:C

Equilibrium

Steam Reformer Performance

Near equilibrium conversion and selectivity at millisecond contact times

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Target Segment

Microchannel Cost Advantage

Hydrogen Production Capacity (MMSCFD)5 5010 15 20 25 30 35 40 45

Cap

ital C

ost (

$/SC

FD)

Velocys

Conventional

00

0.5

1.0

1.5

2.0

2.5

Substantial capital cost

savings

Substantial capital cost

savings

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Methane Steam ReformerDevelopment Status

Commercial partnership• Announced joint development project with Total S.A.

for gas-to-liquids technology• Focused on large, land-base applications

Demonstration Facility• Construction of industrial steam methane reforming

demonstration will begin in late 2006• Start-up of demonstration plant expected in late 2007• Site selection and preliminary engineering completed

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GTL Process:Phase change controls FT reactor

Steam Reforming CO / H2 Products

Diesel Product

Air

H2 H2O

Gas Recycle

Steam

Fischer Tropsch

Local Natural

Gas

Hydro-Cracking

Natural Gas

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Integrated Heat Transfer in Fischer-Tropsch Reactor

Fischer-Tropsch Synthesis (highly exothermic)• Remove heat via integrated microchannel steam generation• Stable partial boiling in 0.6-m microchannel demonstrated• Excellent temperature control enables short contact time

- Conventional Fixed Bed: ~10 seconds- Velocys reactor: < 0.4 second contact time

851210Reactor productivity, bl/te300-5001,400-1,7001,800-2,000Reactor wt, tonnes35,00019,00019,000Capacity, bpdVelocys

Tubular Fixed Bed *Slurry *

* Source: Hoek, Shell, CatCon2003

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Phase Change Demonstrated: Commercial Length Microchannels

230

233

236

239

242

245

248

251

254

257

260

0 5 10 15 20 25Distance to Inle t, inch

Wal

l tem

pera

ture

, o C

q"=5.8 W/cm2q"=3.8 W/cm2q"=1.4 W/cm2Inlet temperature: Saturation temperature (P=522 psia)

2.5 oC

2.9 oC

End effectEnd effect

Stable phase change in long microchannels demonstrated

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FT Demonstration:Commercial Length Reactors

>0.9α number>500 hoursTime on Stream

7.6%Methane Selectivity

70%Conversion per pass

330 psigPressure< 225 °CTemperature~ 330 msContact Time0.9 mReaction Channel

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FT Reactor Assembly:Commercial Microchannel Reactor Within Assembly

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Impact of Compact Hardware:Transportable Synthetic Fuel Production

Design Basis: •350 barrels/day•Containerized modules

A mobile fuel production plant is an application where size matters

Velocys SMR

Velocys FT

De-sulfurization

Fired Feed Preheater

Heat Exchangers

Steam Turbine Generator

Boiler

Engine Generator

Water Treatment

Product Storage

Sponsor: U.S. Army’s National Automotive Center

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Impact of Compact Hardware:Off-shore applications

10,000 – 35,000 BPD Land-based or ship

mounted with conventional marine hulls

Initial design for floating, production, storage and off-loading facility

Design project completed by external engineering firm

Off-shore platforms cannot use autothermal reforming and must be compact

600 ft

150 ft

Conventional Distillation

Ethane-Ethylene Fractionation

“C2 Splitter”

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Microchannel Distillation

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Microchannel Distillation creates short HETP

diffusion to/from vapor-liquid interface

Vapor channel

Liquid film Heat exchange

t: Characteristic diffusion timed: Hydraulic diameterD: DiffusivityD

ddiff

2)2(=τ

• Liquid film flows along liquid removal structure• Integrated heat management: Add or remove heat

near desired temperature and quantity.• Short HETP from enhanced mass transfer

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Microchannel Distillation Demonstrated:Hexane/Cyclohexane Test Separation

Liquid Removal Structure

Gas-Liquid disengagement

Vapor channel

Assembled device

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Distillation Results:HETP < 1 inch

Liquid In

Liquid OutVapor In

Vapor Out

Vap

or

Liquid

Liquid In

Liquid OutVapor In

Vapor Out

Vap

or

Liquid

1015# Equilibrium Stages

0.500.33HETP, inches

73%80%Vapor Out

9%7%Liquid Out

Mole % hexane

68°C69°CVapor Out

75°C76°CLiquid Out

Temperature, °C

10347-2(2 X Base Case

Flow)

10347-1(Base Case)

Experimental Run

5 in

ches

84% n-hexane16% Cyclohexane

9% n-hexane91% Cyclohexane

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Summary

Microchannel technology• Enables process performance improvements• Is robust and scaleable

Breadth of applications under development• High performance heat exchangers• Compact reactors• Distillation units

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Contact Information

A. Lee Tonkovich, Ph.D.Mgr. Technology Development CenterVelocys Inc.7950 Corporate Blvd.Plain City, OH 43064Phone: (614) 733-3300Email: tonkovich@velocys.com

www.velocys.com