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COIL, A MODEL FOR SIMULATION OF SPIRAL WOUNDLNG HEAT EXCHANGERS

COIL : MODELE DE SIMULATION DEREVAPORISEURS DE GNL A SPIRES

“R A. Fredheim and O. J@rstad,

$75

Statoil R&D, Norway,t:$ ,. .!~ ,. .$\ >?.

“\. .. .. ., G. Owren, S. Vist and B. Neeraas,

.’,-=-. ..1 SINTEF Energy Research, Norway

ABSTRACT

The LNG heat exchangers contributes only to a small part of the total investments cost in an LNGchain, but the process design and thermal and hydraulic design of the heat exchanger are linkedtogether. Detailed know-how about the LNG heat exchanger performance is therefore necessary inorder to obtain an optimum process design and thereof cost reduction by technological developments.The spiral wound LNG heat exchanger have normally been regarded as a “black box”, and detailedinformation regarding performance and operation have been proprietor information for a limitednumber of manufactures. A in-house simulation model for multi-stream LNG heat exchangers, namedCOIL, has been developed. A correlation package for calculation of heat transfer coefficients andpressure drop is included in the model. The correlation package has been developed from in-houseexperimental data. The COIL model has been developed as a user added subroutine for ProVision ‘M,

in order to link the process design and the thermal and hydraulic design of the main heat exchanger.Use of integrated LNG heat exchanger simulation during the process design phase and during de-bottleneck studies participates to the selection of an optimum process concept.

RESUME

Les revaporiseurs de GNL ne representent qu’une faible part du coil total d’investissement clans unechalne GNL, mais la conception du precede et Ie dimensionnement thermique et hydraulique durevaporiseur sent lies. Une connaissance approfondie des performances des revaporiseurs de GNLest done necessaire pour optimiser la conception du precede et, de la, reduire Ies co(its grace a desdeveloppements technologiques, Jusqu’a present, Ie revaporiseur de GNL a spires etait considerecomme une sorte de (( boite noire )), et Ies information concernant ses performances et sa conduiterestaient la propriete d’un nombre Iimite de fabricants. Un modele interne de simulation pour desrevaporiseurs multicourants, COIL, a ete mis au point. Ce modele inclut un ensemble de correlationsdestinees au calcul des coefficients de transfert thermique et de perte de charge. Ces correlationssent Ie resultat de donnees experimentales internes. COIL a ete developpe comme un sous-programme de ProVision ‘M, afin de relier la conception de precede et Ie dimensionnement thermiqueet hydraulique du revaporiseur principal. L’utilisation de la simulation integree des revaporiseurs deGNL pendant la phase de conception de precede et pendant Ies etudes de deblocage participe a laselection d’un concept de processus optimum.

DISCLAIMER

Portions of this document may be illegiblein electronic image products. Images areproduced from the best available originaldocument.

,,

1. Introduction

The driving force for development of the LNG trade has been remote locations of gas sourcesand long distances to markets, where transportation by pipeline has been impossible or uneconomic.Development of new grass root LNG projects is very capital intensive. Reduction in capital cost bytechnological developments have been one of the main goals both for R&D activities and fielddevelopments during the last years.

The LNG heat exchangers contributes only to a small part of the total investments cost in anLNG chain, but the process design and thermal and hydraulic design of the heat exchanger are linkedtogether. Detailed know-how about the LNG heat exchanger performance is therefore necessary inorder to obtain an optimum process design and thereof cost reduction by technological developments.

The spiral wound LNG heat exchanger have normally been regarded as a “black box”, anddetailed information regarding performance and operation have been proprietor information for alimited number of manufactures.

2. Experimental data

A development program was started in 1985 in order to generate experimental data andsimulation models for spiral wound heat exchangers. Two different test facilities have beenestablished, one for measurements of heat transfer and pressure drop in condensing mixtures on tubeside, and one for measurements of heat transfer and pressure drop in evaporating refrigerant on shellside. The shell-side test plant operates with down-wards evaporation flow and the tube-side test plantoperates with curved inclined flow. The construction and building of the test plants, and the performingof the measurements, have been done at SINTEF Energy Research. Statoil has financed the plantsand the tests. The two plants are described in reference /1/ and /2/. A flow diagram for the shell sidetest facility, including the major part of equipment and instrumentation, is given in Figure 1.

During the last 10 years, an extensive test program has been carried out. Measurements insingle-phase and two-phase flow, using different pure and mixed hydrocarbons as test fluids, havebeen accomplished. An overview of the different measurements performed in the shell side testfacility is given in Table 1. About 800 measurements have been performed. An overview of thedifferent measurements performed in the tube side test facility is given in Table 2. About 250measurements have been performed.

Based on the measurements, calculation methods have been developed and verified. Thesingle phase, heat transfer coefficient on the shell side is calculated by a method of Gnielinski 13/.Deviations between measured and calculated values are given in Figure 2. The deviation is wellwithin the accuracy of the measurements. The single phase, pressure drop on the shell side iscalculated by a method by Barbe /4/. Deviations between measured and calculated values are givenin Figure 3.Some of the two-phase measurements with propane are reviewed in reference /1/. All of the tube sidemeasurements are reviewed in reference /2/.

&r

I I (P..) I I 11;

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1/;1,11,

IIIi{II11/It,11,II11[IIIllII11{II,

L-!-M-J 1 ! i -1-i-.---,. —, ,- ,1—

W3~6 -Z. -.---L----+-L-. L&L

H:. %~-ly”----------- *------ y

A Test heat-exchanger B Separatorc Vapor blower D Orifice-meterE Vapor cooler F PumpG Turbine-meter H Liquid coolerI Propane pump J Temperature regulatorK Propane cooler L Propane expansion drumM Methane expansion drum N Philips cryogenerator

Figure 1: Flow diagram for the shell side test facility

Table 1: Overview of measurements performed in the shell-side test facility.Fluid Composition Phase Vapor Temperature Pressure Mass flow Number

[Mole %] fraction [c] [Bar] &#m2s] of

km PointsN2 100 Vaper 1.0 -18--11 4.6- 9.1 27–95 26C2 100 Vapor 1.0 -30--14 9.3- 15.0 5–125 66

cl/c2 45/55 Vapor 1.0 -16 14.9 7–109 15C2 100 Film 0.0 -66--46 3.0- 6.2 20-C2 100 Two-phaseC3 100 Film (C3 100 Twe-phase o

G/C2 45/55 Two-phase 0.1-0.89 -125--96 3.0- 6.0 49-76 61C3,Cz 90/10 Film 0-0.05 -30--15 2.2 – 3.7 36-97 14C3,,C9 85/15 Film 0.05-0.1 -18 3.8 36–85 7c3g2 90/10 Tim-phase 0.03-0.83 -19--12 3.2- 3.9 52 – 94 20

c3~2 85/15 Two-phase 0.05-0.94 -41--10 1.7- 4.7 43 – 87 66

-110 130.1-0.9 -75--47 2.0- 6.0 46-77 760.0-0.09 -30--5 1.6- 4.0 33-120 35).05-0.95 -30--5 1.6- 4.0 37– 122 144

NC&/CI 80.7/14 .3/5.0Nc.&z/cl 65.2/ 15.4/19.4

NC4G 85/15 Two-phase 0:07-0.93 -25--2 1.5- 2.5 45-71 34NC&Z 70/30 Film 0.05 -34 2.0 43-80 10Nc4z2 70/30 Two-phase 0.05-0.91 -34--3 2.0- 4.0 43 – 84 47

Two-phase 0.09-0.87 -44--1o 1.7- 3.0 45 – 69 19Two-phase 0.07-0.93 -117--46 2.5- 5.0 44 – 70 65

I Nc4C2/cl/N2 I 62;6/14,8/18.6/4.- Tttwphase 0.08-0.92 -122--63 2.5- 5.2 46 – 84 65n

., .

Table 2: Overview of measurements performed in the tube-side test facility.Fluid Composition Phase Vapor Pressure Mass flOW Number of

[Mole %] ,fiaction [Bar] ~g/m2s] Points

0 10000 20000 30000 40000 50000 60000 70000 80000

Re number

+4.5 bar N2

s 9.0 bar N2

Figure 2: Deviation between calculated and measured heat-transfer coefficient forsuperheated vapor flow.

. .

10

8

6

4

72*Us00.=.-

i -2

-4

-6

-8

-lo

o 10000 20000 30000 40000 50000 60000 70000 80000

Re number

I + N2 vapour

Figure 3: Deviation between calculated and measured heat-transfer coefficients forsuperheated vapor flow.

3. Computer model

The simulation model of the spiral-wound heat exchanger is implemented as a user-added-subroutine to ProVisionTM ‘ProVision is a flow-sheeting program from Simulation Science inc, USA).An element of the spiral-wound heat exchanger is shown in-Figure 4. The tubes are wound around-thecore tube in layers. Each layer consists of several tubes in parallel, which also may belong todifferent streams. Each layer is wound with a constant inclination angel relative to the horizontalplane. Adjacent tube layers are wound in opposite direction. The center distance betieen two tubesin axial direction is P/ while the corresponding distance in radial direction is Pr. The geometryparameters defined by Figure 4 is sufficient for the heat transfer and pressure drop correlation.

A mathematical model description is shown in Figure 5. The hatched block represents thedown-flowing cold shell-side stream while the other blocks are the tube streams. There is heatexchange between the cold shell-side stream and each of the hot tube-side streams. As the tube-sidestreams do not heat exchange with each other, the distribution of the streams on different tube layersdo not influence the heat transfer calculations. Two tube-side streams may only influence each otheronly through heat exchange with the cold shell side stream.

As shown in Figure 5, the main variables in the equation systems are the specific enthalpiesof the individual streams for i=l. .rfg+l. i=l at the cold end of the heat exchanger while i=ng+f is thehot end. Each of the blocks in Figure 5 represents control volumes where a heat balance equation isapplied. The equations for the tube-side control volumes are as follows:

Similar equations are easily deduced for the shell-side stream. The index I is the grid-pointnumber and indexj is the stream number where j=l always is the cold shell-side stream. As thenumber of unknown variables is (ng+7)o ns, while the number of control volumes and equations isngons ,itisnecessarytoadd m specifications.The standard way of implementing unit operation in

ProVisionm is to specify the variables on the feed streams and let the unit operation calculate theproduct streams. This scheme is used and the number of specifications at the cold end cs is equal tothe number of cold feed streams, which most often is one for spiral-wound heat exchangers.

If the UA-values are constant, and the enthalpy-temperature relation is linear andindependent of pressure, the equation system ((ng+f)ms equations) will become linear. Since thereis only heat exchange between neighboring control volumes, the equation system will consequentlybe a banded system. The bandwidths are shown in Table 3.

Table 3: Properties related to the band equation system

Number of control volumes nga ns

Cold end specifications CsHot end specifications hs=ns-csBandwidth 2ms – fLeft bandwitdh CsRight bandwidth 2.ns - I – cs

The linear band equation system is solved efficiently by a factorization algorithm that takesadvantage of the banded structure. The real non-linear equations system is solved by repeatedlysolving the linear band system. The heat transfer coefficients are updated in each iteration by usinglocal heat transfer correlation. Pressures are also updated from local pressure drop calculations.

B.s

-—<——.- .———.___

A “x

.

Figure 4: Element of a spiral wound heat exchanger.

ns :Nuber of streamscs: Number ofspeciications cold end

i

Equation

i+ 1

1 A

I@i-l)ns1 Ihi-l)ns 2 T&i-l). +3

-L(i-l)n +3-l-es

Q3-1Qi.,

A

h(i.l)ns+z

(i-l)ns- -4+C

hi ~@

Figure 5: Mathematical model description for the Spiral routine.

4. Bundle simulation

The user-added-subroutine COIL is a rating model, able to calculate one bundle at the time.The user must specify all the input streams while COIL calculates the output streams. The heatexchanger geometry has to be specified by the user. The subroutine calculates the heat transfer andpressure drop, where the thermodynamic and physical properties are calculated with flashing andproperty routines available in ProVisionTM.

The UAS model COIL has several calculation options. The geometrical input section is quitelarge. Basically two different geometrical input schemes are available:

1. A simplified method where all tubes have the same length, and consequently the same inclinationangle. The user must specify the diameter on the core and shell tube, and COIL will fill in themaximum number of tubes.

2. A flexible method where the different streams may have different tube lengths and the respectivetube lengths must be specified.

The heat transfer coefficient and pressure drop may be specified for each stream or it can beestimated by a correlation. A screen dump of the UAS COIL input picture is given in Figure 6.

Figure 6: A screen dump of the UAS COIL input picture

The importance of using correlation for calculation of heat transfer and pressure drop isdemonstrated in Figure 7. The difference in stream temperatures calculated with fixed heat transfercoefficients and with the correlation package is shown, along a bundle where the natural gas iscondensed and sub-cooled. The relative length is used as the coordinate. The temperature differenceis greatest in the region with phase change, between two-phase flow and pure liquid flow.

5

z

o0 -5g

g

% -10a

–-mm-- Vap HPL3~g -15

E

c

-20

-25

0 0.2 0.4 0.6 0.8 1

Relative length (-); O - cold end and 1 - warm end

Figure 7: Difference in calculated temperature profile along a bundle. The temperatureestimated with heat transfer coefficient model is subtracted from the temperature calculated

with fixed heat transfer coefficient

. .

5. System simulation

The main heat exchanger system of an APCI propane pre-cooled MCR process with twospiral-wound heat exchanger bundles was simulated to test the calculation program. The system isshown in the ProVisionm spreadsheet in Figure 8. The natural gas (stream 30) and the MCR (stream225) are pre-cooled to 238.1 5K MCR flow (stream 225) is flashed and led into the “ho~ heatexchanger bundle together with the natural gas stream (stream 30). The tube streams are cooled bythe evaporating mixed refrigerant (stream 255) on the shell side of the heat exchanger. The naturalgas from the hot bundle (stream 32) is cooled from 146K to 116K in the cold bundle (stream 33).

Figure 8: LNG process with two heat exchanger bundles.

The composition, flow rate, temperature and pressure for the natural gas and MCR inputstreams are shown in Table 4.The natural gas flow rate corresponds to an LNG production ofapproximately 3.7 million tones per annum.

Table 4: Composition, flow rate, temperature and pressure for the natural gas (30) and mixedrefrigerant (225) input streams.

Natural Gas Mixed Refrigerant

Nitrogen [70] 2.71 5.93Methane (Cl ) [%] 88.76 40.48Ethane (C2) [%] 5.16 35.21Propane (C3) [%] 2.27 18.38Butane (C4) [%] 0.32Isobutane (IC4) [%] 0.57Pentane (C5) %0 0.21

Flow Rate [kmollh] 23323 30398Temperature [K] 238.15 238.15Pressure [bar] 60 42.5

The simulation of the LNG-process is conducted in ProVisionTM and the solution of theiteration of the two tearing streams (streams 255 and 242) are usually converging in five to ten trials.The geometry data for the heat exchanger bundles are given in Table 5.

. .

Table 5:Heat exchanger geometry data.

Hot Bundle Cold Bundle

Total surface area [~] 27664 3660.1Bundle Length [m] 14.3 4.08Shell diameter [m] 3.85 2.842Core diameter [m] 1.498 1.498Inside tube diameter lmml 7.9 7.9Outside tube diameter [mm] 9.5 9.5Radial tube pitch [mm] 14.0 14.0Longitudinal tube pitch [mm] 11.0 11.0Number of tube layers [-] 84 48Tube inclination angle ~ 7.128 7.128Number of tubes f-l 8022 3720

I Tube Length [m] I 115.24 I 32.88 I

The subroutine calculating the spiral-wound heat exchangers gives the temperaturedistribution through the heat exchangers. The temperature distribution in the hot and the cold bundle isshown in Figure 9 and Figure 10. -

240

230

220-

g210g200

flK)- ../--! q~ /------

y~~ 170

160

140, I 1 I 1 1 1 I

o 2 4 6 8 10 12 14 16Bmda Length[m]

-----Tube lsbeam —A-Tube2slEarn — Tube 3 skeam —-–– Shellsbeam

Figure 9: Calculated temperatures in the hot bundle for the three tube streams and theshell side stream.

150 , 1

145

g 140

115

110 P I I I 1

0 1 2 4 5Length [m] 3

l+ TubelsWam —Tube2stEam +Shellskarnl

Figure 10: Calculated temperatures in the cold bundle for the two tube streams and the shellside stream.

The heat exchanger subroutine also calculates the pressure drop inside the tubes and on theshell side of the tubes, using pressure drop correlation. The static and frictional pressure drop arelisted in Table 6.

Table 6: Calculated static and frictional pressure drop in the two heat exchangers [kPa].

Hot Bundle Cold Bundle

Static [kPa] Frictional [kPa] Static [kPa] Frictional [kPa]

Shell side -1.34 69.6 -0.89 2.98Tube 1 446 36.6 104 17.5Tube 2 397 28.6 63.5 19.6Tube 3 349 69.7

The total local heat-transfer coefficients between the hot and the cold streams are shown forthe two bundles in Figure 11.

Figure 11: Total local heat transfer coeftlcient between hot and cold streams in the hot and

I

%+

o 1 2 3 4 5

Length [m]

I + Tube 1 -B- Tube 2

.,”

REFERENCES

1- Fredheim, A.O. (1994). Thermal design of coil-wound LNG heat exchangers, shell-side heattransfer and pressure drop. Dr.hg. avhand/ing, NW

2- Neeraas, B.O. (1993). Condensation of hydrocarbon mixtures in coil-wound LNG heatexchangers, tube-side heat transfer and pressure drop. Dr.ing. avhand/ing, NTH

3- Heat Exchanger Design Handbook. (1983) Hemisphere Publishing Corporation

4- Barbe, C., Mordillat, D., Roger, D.(1 972) Pertes de charge en ecoulement monophasique etdiphasique clans la calendre des exhangeurs bobines}. X// Journees de /’Hydrau/ique, Park