3. Chemical Design Heat Exchanger Latest

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DESIGN OF HEAT EXCHANGER

4.1 CHEMICAL DESIGN

According to Ramesh K. Shah et al, 2003, In chemical plant industry, heat exchanger is

one of the most and main important equipment. Heat exchanger is a device that transfers thermal

energy (enthalpy) between two or more fluids for solid and fluid or between particulates and fluid

at different range of temperature with thermal contact.

Most of the usage and application of the heat exchanger involves cooling, heating,

condensing, evaporating, concentrating, crystallizing, sterilizing, distilling and etc. In heat

exchanger, there are two ways of heat can be transferred which is indirect and direct transfer. The

indirect transfer, heat exchange is done using energy storage and rejection passing through the

exchanger surface.

On the other hand, for the direct transfer of heat exchanger, the fluid inside does not mix

since the fluid is separated by the walls designed in the heat exchanger. Some notes need to be

taken into consideration in designing heat exchanger. Most important and major factor in

designing heat exchanger is to know the details of fluid, phase and choosing the type of the heat

exchanger. More explanation to design the heat exchanger will be discussed further.

4.1.1 Design Specification

The liquid mixture from distallation column is low temperature. The main objective of

the heat exchanger is to heat up the temperature until 75 C from 43 C. At this stage, the first

heat exchanger heat up the stream of 2601.11 kg/hr of liquid mixture before entering the

polymerization reactor. Hence, the design of the heat exchanger is based on this flow rate rather

than of the fresh feed.

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4.1.2 Selection of Equipment

More than one type of heat exchanger is used in chemical plant and there is various types

of them. Every heat exchanger used has its own supremacy and weakness. The heat exchanger

type and specification will be described shortly in table 4.1 below.

Heat exchanger

type

Specification

Shell and tube

heat exchanger

The most common heat exchanger use in the world.

Can be easily clean.

The configuration can give a large surface area in a small

volume.

Not compact compare to other type of heat exchanger.

Can be constructed from a wide range of materials.

Well established design procedures.

Spiral and tube

Heat exchanger

Extremely compact and can handle most type of the fluid

Can be easily clean and the turbulence in the channel is high

Mostly use in the dirty process fluid and slurries

Has a high heat transfer rate compare straight tube

But ,applicable only for a small capacity duty

Mechanical cleaning of the tube is very limited and sometimes is

impossible

Very limited pressure drop 20 bar and temperature of 400C

Plate fin heat

exchanger

Low temperature approach can be used as low as 1C,compared

to shell and tube heat exchanger,5C to 10C

Plate will prefer more when cost of material is high

Suitable for high viscous fluid

But, it has a very limit operation temperature,260C

Also provide a very high pressure drop compare to the shell and

tube and heat exchanger.

Table 4.1 Summary Specification of Heat Exchanger Types

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After considering all the descriptions and specifications, shell and tube type is chosen

since it is the most widely used and can be designed for virtually any application. Furthermore, it

is relatively cheap as compared to the air-cooled and plate fin heat exchanger and it is sufficient

for this application.

Essentially, a shell and tube heat exchanger is compilation of bundle of tubes which is

compact together in a cylindrical shell. The ends of the tubes are fitted into tube sheets, which

separate the shell side and tube side fluids. Baffles are introduced inside the shell to direct the

fluid flow and support the tubes. The assembly of baffles and tubes is hold on each other with the

support of rods and spacers which can be represent in Figure 4.2 (Coulson &Richardsons

Volume 6)

Figure 4.1 Baffle Spacers and Tie Rods

4.1.3 Selection of Shell-Tube Type of Heat Exchanger

Referring to the Tubular Exchanger Manufacture Association (TEMA) classification, the heat

exchangers are as follows:

Type Reason of selection

Front and stationary head

Types

Type A: Channel and

removal cover

Removal cover without

break the flanges

Shell types Type E: One pass shell The most commonly used

Rear ends, head types Type M: Floating head with Used extensively in

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backing devices

petroleum service

Table 4.2 Selection on the types of heat exchanger

Internal Floating Head Exchanger is selected due to the following advantages:

Permitting differential movement between shell and tube and complete tube bundle

withdrawal.

Separate the shell and tube side fluid at the floating head end.

Access to the tubes and at the stationary end is obtained by removing the stationary head

cover or complete head.

The inside of the tubes may be cleaned in situ and complete bundle may remove for

cleaning the outside of the tubes or repairs.

The split-backing ring floating head type accommodates a smaller number of tubes than

fixed tube sheet and U tube types having the same shell diameter.

4.1.4 Selection of Shell or Tube Side for the Fluids

Fluid in shell side: Steam water

Fluid in tube side: Organic mixture

It is advisable that the Organic mixture flows in the pipe and the water flows in the opposite

direction in the annular space between the two pipes. With this arrangement, the outer surface of

the equipment will be at the lowest possible temperature and thus, heat loss to surroundings will

be minimal. The organic mixture should be place in the tube side rather than shell side as this

confines the organic mixture to the tube side and shell side will not affected.

4.1.5 Fouling

Thermal resistance of the fouling layers on the inside and outside heat transfer surfaces should be

taken into consideration in the calculation of the overall heat coefficient. The fouling layers

increase in thickness with time during operation and have lower thermal conductivity than the

fluids or the tube material, thereby increasing the overall thermal resistance.

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4.1.6 Process Background of Heat Exchanger Heater, H-102

These sections discuss the design of suitable heat exchanger after the distillation process stage.

The heat exchanger to be design is first heat exchanger (H-102). The purposed of heat exchanger

is to increase high temperature from distillation column (T-102) at 43 C until 75 C to the

polymerization reactor (R-103). A comprehensive design study is to determine the heat exchanger

mechanical details and physical characteristics and anticipated performance. This section contains

the operating criteria, the equipment selection, and the result of the thermal design and

mechanical design of heat exchanger.

Figure 4.2 Process conditions for heat exchanger

Name of Equipment : Heater H-102

Equipment Purpose : To heat up the process in the shell to achieve the desire temperature by

using steam water in the tube.

Type : Shell and Tube, Internal Floating Head

4.1.6.1 Specification and Chemical Properties

The process liquid will be in the tube side which will be heat up by the steam water from outside

that will be flowed through the shell side of the heat exchanger. The value of chemical properties

for both stream are as shown below.

Component Organic mixture (Tube) Water steam (Shell)

Type Cold Hot

Temperature inlet, (C) t1 = 43 T1 = 90

Temperature outlet, (C) t2 = 75 T2 = 54

Specific heat, Cp (kJ/kg.C) 17.82 4.200

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Thermal conductivity (W/m.C) 0.6376 0.1150

Density (kg/m3) 19.2667 8.9194

Viscosity (Ns/m2) 3.36 10-5 1.3 10-3

Table 4.3 Data information of process condition

4.1.6.2 Mean Temperature Difference (LMTD)

Counter flow arrangement is selected, as the temperature difference is greater as compared to

cross flow. For the LMTD involved, the following assumptions are made:

The overall coefficient of heat transfer is constant

The rate of flow of each fluid is constant

There is an equal amount of cooling surface in each pass

Figure 4.3 Temperature profiles for counter current flow

Organic mixture Water

In Out In Out

t1 (C) t2 (C) T1 (C) T2 (C)

43 75 90 64

T1

T2

t2

t1

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Where,

T1 = Inlet shell side fluid temperature

T2 = Outlet shell side fluid temperature

t1 = Inlet tube side fluid temperature

t2 = Outlet tube side fluid temperature

True Temperature Difference,

Temperature correlation factor, Ft is determined in Figure 12.19 Ray Sinnote & Gavin Towler

Chemical Engineering Design, 5th Edition, 2009 (Figure D.1 Appendix D). At R = 0.81 and S =

0.68, Ft = 0.8. Therefore, it allowed to be install in 1 shell pass since the correction factor is high

enough and satisfy (Towler and Sinnot, 2009)

Therefore,

Where,

Tm = True temperature difference

Ft = Temperature correction factor

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4.1.6.3 Heat Transfer Area

From the shell side chemical properties, the value of power (Q) could be calculated

After Q value is obtained, the flow rate of steam water used can be calculated by using the same

equation.

Assume that Qtube = Qshell,

Thus, 3.7727 kg/s of steam water are needed to heat up the cold stream of 43 C to 75 C.

The overall coefficient that suitable with this heat exchanger condition will be in range 500

1000 W/m2.C for organic solvent, determined from Table 19.1 Ray Sinnote & Gavin Towler

Chemical Engineering Design, 5th Edition, 2009 (Table D.2 Appendix D), the starting value was

selected to be 500 W/m2.oC.

Assume that U = 500 W/m2C

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Where,

q = Heat transferred per unit time, W

U = The overall heat transfer coefficient, W/m2.C

A = Heat transfer area, m2

Tm = the mean temperature difference, C

4.1.6.4 Number of Tube Calculation

TEMA design standard allows tube diameters in the range of 6.4 mm to 50 mm, but the common

used diameter in industry within 16 mm to 50 mm. While the favorable tubes length for heat

exchanger are 1.83 m, 2.44 m, 3.66 m, 4.88m, 6.10 m and 7.32 m (Towler and Sinnot, 2009). The

optimum tube length to shell diameter ratio usually fall within the range of 5 to 10. The following

tube dimension has been selected show in table below.

Selected tube characteristic Reasons

Tube length selected: 4.88 m It provides an adequate heat transfer surface

area and pressure drop is below the allowable

pressure drop.

The outer diameter selected, Do: 20 mm

The inner diameter selected, Di: 16 mm

It is a common tube used

Pitch selected; 23.81 mm (Triangular pitch) It cause lower pressure drop compare the

square pitch.

Birmingham wire gage (BWG): 16 It provides flow area and wall thickness to

withstand significant pressure drop.

Table 4.4 Selection of Tube

Area of tube,

Thus, the number of tube is

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The fluid in the tube usually directed to flow back and forth in a number of passes through

groups of tubes arranged in parallel in order to increase the length of the flow path. The number

of passes is selected to give the required tube side design velocity. In this design, single passes

are chosen to decrease cost of construction.

Tube per pass = 151 tubes

4.1.6.5 Area per Pass

4.1.6.6 Heat Transfer Factor, jh

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From Reynolds Number, it shown that the flow is turbulent.

So,

From figure 12.23 Ray Sinnote & Gavin Towler Chemical Engineering Design, 5 th Edition 2009

(Figure D.3 Appendix D), the value for heat transfer factor is jh = 3.9 10-3. Therefore,

For turbulent flow,

Where,

hi = inside coefficient, W/m2.C

de = equivalent diameter (or hydraulic mean), m

kf = fluid thermal conductivity, W/m.oC

= fluid viscosity at the bulk fluid temperature, Ns/m2

w = fluid viscosity at wall

Neglecting the,

the result becomes:

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4.1.7 Bundle and Shell Diameter Calculation

4.1.7.1 Tube Arrangements

Usually, tubes in heat exchanger are arranged I equilateral triangular or square or rotated square

pattern. A square or rotated square pattern arrangement is used for heavily fouling fluid, where it

is necessary to mechanically clean the outside of the tubes. The triangular pattern gives higher

heat transfer rates than square pattern. Therefore, this heat exchanger goes to triangular pattern.

Figure 4.4 Tube pattern

The recommended tube pitch (distance between tube centers) is 1.25 times the tube outside

diameter and this will normally be used unless process requirement dictate otherwise.

Thus, triangular pitch,

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From Ray Sinnote & Gavin Towler Chemical Engineering Design, 5 th Edition 2009, at Table 12.4

(Table D.4 Appendix D), the triangular pitch properties are listed below by referred to number of

tube passes.

The number of tube passes = 1

K1 = 0.319

n1 = 2.142

Thus, the bundle diameter is

4.1.7.2 Bundle Clearance and Shell Diameter

Type of heat exchanger that been chosen is split-ring floating head. The bundle clearance for this

type of exchanger determined by referring to Figure 12.12 in Ray Sinnote & Gavin Towler

Chemical Engineering Design, 5th Edition 2009 (Figure D.5 Appendix D). It is 64mm (0.064m)

Therefore, the shell diameter is

4.1.8 Shell-Side Heat Transfer Coefficient

The complex flow pattern on the shell side, and the greater number of variables involved,

make it difficult to predict the shell-side heat transfer corfficient and pressure drop with complete

assurance.

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Kerns method was choose to determine these heat transfer in shell side. This method is

base on experimental work on commercial exchanger with standard tolerances and will give a

reasonably satisfactory prediction of the heat transfer coefficient for standard designs.

In order to calculate the heat transfer on the shell side, the number of baffle spacing must

be estimate first. Baffle spacing are used in the shell to direct the fluid stream across the tubes, to

increase the fluid velocity and so to improve the rate of transfer. The most commonly used type

of baffle is the single segmental baffle spacing. Take the baffle spacing equal to 5 because this

spacing should give good heat transfer.

Therefore, take the baffle spacing equal to 5, hence:

Shell side mass velocity, Gs

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Linear velocity of shell, Us

The baffle cut is the height of the segment removed to form the baffle, express as a percentage of

the baffle disc diameter. The term baffle cut is used to specify the dimension of a segmental

baffle. The optimum baffle cut is about 20% to 25%, which giving good heat transfer rates

without excessive pressure drop. In this case, 25% is taken as the optimum for the baffle cut.

Based on the Figure 12.29 from Ray Sinnote & Gavin Towler Chemical Engineering Design, 5 th

Edition 2009 (Figure D.6 Appendix D), the value of heat transfer factor, jn, can be obtained by

Reynolds number, Re.

Therefore,

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Jn = 4.0 10-3

Neglecting the viscosity correction,

The overall coefficient and the individual coefficients relationship is given by:

Where,

ho = outside fluid film coefficient, W/m2.C

hi = inside fluid film coefficient, W/m2.C

hod = outside dirt coefficient (fouling factor), W/m2.C

hid = inside dirt coefficient, W/m2.C

kw = thermal conductivity of the tube wall material, W/m2.C

di = tube inside diameter, m

do = tube outside diameter, m

Parameter value

outside fluid film coefficient, ho W/m2.C 987.22

inside fluid film coefficient, hi W/m2.C 1736.6

outside dirt coefficient (fouling factor), hod W/m2.C 4000

inside dirt coefficient, hid W/m2.C 5000

thermal conductivity of the tube wall material, kw W/m2.C 55

tube inside diameter, di m 0.016

tube outside diameter, do m 0.020

Table 4.5 Value for calculate overall coefficient

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From the overall coefficient been assume at earlier stage to calculated coefficient, the

relative error is about 8.86 % errors, therefore our design is determined to be feasible in range of

overall coefficient from 250 - 750 W/m2.C.

4.1.9 Pressure drop

In order to determine the pressure drop on tube and shell side, the viscosity correction is

neglect as in fluid mechanism; a boundary layer is the layer of fluid in immediate vicinity of

bounding surface where the viscosity is significant. Allowable pressure drop varies with total

system pressure and the phase fluid (Thakore & Bhatt, 2007).

Allowable pressure drop:-

Tube side = 0.01 bar

Shell side = 0.64 bar

4.1.9.1 Tube side pressure drop

Re = 1.14 104

From Figure 12.24 Ray Sinnote & Gavin Towler Chemical Engineering Design, 5th Edition 2009

(Figure D.7 Appendix D), tube side friction factor, jf = 1.02 10-3

By neglecting viscosity correction:

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4.1.9.2 Shell side pressure drop

Re = 1.83 104

From Figure 12.30 Ray Sinnote & Gavin Towler Chemical Engineering Design, 5 th Edition 2009

(Figure D.8 Appendix D), the 25% baffle cuts were taken, the friction factor, jf = 2.87 10-4

By neglecting viscosity correction:

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4.1.10 Summary 0f Chemical Design

Item: Heater

Item no.: H-102

Design by:

Mohd Zhariff Bin Mohd Kamar

Function: heating cold stream to achieved desired temperature in order to enter the

polymerization reactor, R-103

Operation: Continuous

Type: Shell and Tube; Internal Floating Head

Heat Duty, Q: 411.98 kW

Heat transfer area: 46.21 m2

Tube Side

Fluid handle: Organic mixture

Flowrate: 19.2667 kg/s

Pressure: 19.50 bar

Temperature: 43 75 C

Heat transfer coefficient: 1736.63 W/m2C

Pressure drop: 0.01 bar

Tube Details

Inside diameter, di: 20 mm

Outside diameter, do: 16 mm

Length, L: 4.88 m

Bundle diameter:0.687 m

No of tube, Nt: 151 tubes

Shell Side

Fluid handle: Steam water

Flowrate: 3.7729 kg/s

Pressure: 19.80 bar

Temperature: 90 - 64 C

Heat transfer coefficient: 987.22 W/m2C

Pressure drop: 0.62 bar

Shell Details

Diameter, Ds: 0.715 m

Overall Coefficient, Uo: 544.30 W/m2C

Table 4.6 Chemical design summarization