Table of Contents 1.0
Introduction ............................................................................................................................ 3
2.0
Selection of Heat Exchanger .................................................................................................. 4
3.0
Specification ........................................................................................................................... 5
4.0
Physical Properties of Fluid .................................................................................................... 6
5.0
Exchanger Type and Dimensions ........................................................................................... 6
6.0
Heat Transfer Area................................................................................................................. 7
6.1 7.0
Overall Coefficient .............................................................................................................. 7 Material Selection, Sizing and Layout for Tube....................................................................... 8
7.1
Material Selection ............................................................................................................... 8
7.2
Tube and Shell Fluid ........................................................................................................... 8
7.3
Tube Dimension ................................................................................................................. 8
7.3.1
Tube size ..................................................................................................................... 9
7.3.2
Tube Thickness ........................................................................................................... 9
7.4
Tube Arrangement ............................................................................................................ 10
7.5
Number of Tubes .............................................................................................................. 11
8.0
Shell Selection and Sizing .................................................................................................... 12
9.0
Heat Transfer Coefficient ..................................................................................................... 13
9.1
Tube-side Heat Transfer Coefficient ................................................................................. 13
9.2
Shell-side Heat Transfer Coefficient ................................................................................. 14
9.3
Overall Heat Transfer Coefficient ...................................................................................... 16
10.0
Pressure Drop ...................................................................................................................... 17
10.1
Tube-side Pressure Drop .................................................................................................. 17
10.2
Shell-side Pressure Drop .................................................................................................. 17
11.0
Mechanical Design ............................................................................................................... 19
11.1
Heat Exchanger Tubes ..................................................................................................... 19
11.2
Heat Exchanger Shell ....................................................................................................... 19
11.3
Tube Sheets ..................................................................................................................... 19
11.4
Vent Point ......................................................................................................................... 19
1
11.5
Baffle number and distance .............................................................................................. 20
11.6
Weight Loads.................................................................................................................... 20
12.0
Schematic Diagram of Heat Exchanger ................................................................................ 21
13.0
Mechanical Drawing of Key Item .......................................................................................... 22
14.0
Piping and Instrumental........................................................................................................ 23
15.0
Costing ................................................................................................................................. 24
16.0
Auxiliary Equipment, pump and piping ................................................................................. 25
17.0
Mass Balance and Safety Review ........................................................................................ 26
17.1
Mass Balance Review ...................................................................................................... 26
17.2
Safety Review................................................................................................................... 26
18.0
Conclusion ........................................................................................................................... 27
19.0
References........................................................................................................................... 28
Appendix A
Nomenclature......................................................................................................... 29
Appendix B
Process Flow Diagram For Plant Design ................................................................ 30
Appendix C
Mass Balance For Plant Design ............................................................................. 31
Appendix D
Pipe Sizes – ANSI/ASME B36.19M - 1985 ............................................................. 32
Appendix E
TEMA Shell and Tube Nomenclature ..................................................................... 33
2
1.0
Introduction
The aim of this project is to design Heat Exchanger as one of the selected major equipment in the Plant Design project. The values used in the design of this equipment will be based on the revised mass balance which can be found in Appendix C with reference to Appendix B. Equations used in the calculation are presented in the Appendix A with the nomenclature right under the equation.
It should be noted that there is no best heat exchanger unit in this world as any unit of heat exchanger need to be custom made and design to suit the process. There are myriads of combination and types of heat exchanger proposed in the literature. The selection of heat exchanger type, the number of tubing, the number of plate, the size of tube, the amount of pass in tube and shell, the use of baffle, the type of material used, all are part of design consideration when making decision.
The design criteria of this equipment are as follow: I.
Safe to operate
II.
Low operating cost
III.
Low maintenance cost
IV.
Low fabrication cost
The safety aspect of the equipment will be placed before any other criteria when designing the equipment.
3
2.0
Selection of Heat Exchanger
There are many types of heat exchanger to choose from before starting the design of the equipment. Among the heat exchanger type that is widely discussed in the books and literature are Shell and Tube Heat Exchanger, Double Pipe Heat Exchanger, and Gasketed Plate Heat Exchanger.
It is known that each type of heat exchanger have its pros and cons, after weighing both pros and cons was the decision to use Shell and Tube Heat Exchanger in the design project One of the major concern in the selection of the Heat Exchanger type is whether it can sustain high pressure while maintaining good heat transfer efficiency and low cost.
The following data need to be considered when choosing a heat exchanger. I.
Can withstand 30bar of pressure
II.
Can withstand up to 150
III.
Can be cleaned or maintained in case of fouling
IV.
Can perform well while support load up to 200,000 kg/h
Ethylene oxide has been found to undergo polymerization with iron oxide and strong alkali. It has also been reported that chlorine and alkali contamination can also cause polymerization. Since the plant would be running using commercially available water, it is almost impossible to avoid any contamination of water that could potentially cause the polymerization of ethylene oxide. A periodic cleaning or maintenance will have to be carried out to prevent any safety hazard from happening.
The Gasketed-Plate Heat Exchanger has been reported to exhibit high heat transfer coefficient, but it cannot hold high pressured fluid. Also, the use of gasket is not appropriate with the high reactive fluid such as Ethylene Oxide, there have been accounts of report whereby Ethylene Oxide would attack the gasket and cause leakage. While Welded-Plate Heat Exchanger can hold high pressure fluid while exhibit high heat transfer efficiency and prevent leakage, the heat exchanger cannot be cleaned, or by any means maintained.
Double pipe heat exchanger is one of the high cost equipment that is usually used for sensible heating and cooling of relatively small area. It can withstand high pressure and temperature but the cost of the equipment out weight the advantage it can offer and will not be considered in the design.
Shell and Tube heat exchanger will be chosen in this design project as it can withstand the pressure of the cold fluid, it is relatively cheaper than double pipe heat exchanger and it can be cleaned with a proper design. 4
3.0
Specification
The equipment will have the following specification as illustrated in the diagram below:
54625 kg/hr (100% Saturated Steam & Water) 1.0135 bar 100
148798 kg/hr (4.85% mol EO & 95.15% Water) 20 bar 50
148798 kg/hr (4.85% mol EO & 95.15% Water) 20 bar 100
54625 kg/hr (100% Saturated Steam) 1.0135 bar 100
Figure 2.1 Specification of Equipment
The values in the diagram above are extracted from Appendix C. The general process flow diagram can be found in Appendix B for reference.
The heat transfer rate (duty) is part of the limiting specification which can be calculated from the inlet using the formula: ̇
(3.1)
̇
5
4.0
Physical Properties of Fluid
The physical properties of the hot fluid involved in the design of the heat exchanger are tabulated in the table below.
Steam
Inlet
Outlet
Temperature
100
100
Specific heat
1.988
1.988
Latent heat of vaporization
2259.44
2259.44
Thermal conductivity
0.016
0.016
Density (1bar)
0.59
0.59
Viscosity
0.00001297
0.00001297
Table 3.1
Unit
Physical Properties of Hot Fluid
It should be noted that the cold fluid is a mixture of Ethylene Oxide and Water, the physical properties of the mixture at 75
is calculated and tabulated in the following table.
Physical Properties
Water
Ethylene Oxide
Mixture
Specific heat
4181.3
2782
4113.35
Thermal conductivity
0.58
0.14
0.161365
Density
0.97755
0.785175
0.960636
Viscosity
0.00018
0.0003797
0.00037
Table 3.2
5.0
Unit
Pa.s
Physical Properties of Cold Fluid Mixture
Exchanger Type and Dimensions
The heat exchanger type and dimension discussed here will focus on the number of tube passes, number of shell passes. As the heat exchanger would have to be cleaned with periodic maintenance, the number of passes in both shell and tube will be limited to one only. Counter flow arrangement is preferable over parallel flow, as the mean temperature difference between the two fluids over the length of heat exchanger is maximized as reported in many literatures.
The heat exchanger will be used as a condenser with the saturated steam from the evaporator used to heat up the Ethylene Oxide and Water mixture. A simple energy balance calculation is performed to check the outlet temperature of the hot fluid.
6
Latent Heat of Vaporization
= 2259.44 kJ/kg
Steam Flow rate
= 54625 kg/h
Calculation, Total energy released upon condensation
= (2259.44)(54625) = 12342.21 MJ/h = 34283.96 kW
Comparing, Energy required to heat up the cold fluid to 100
= 8500.823 kW
From the calculation above, we can see that the hot fluid have more than enough heat energy to warm up the cold fluid. Thus, it can be assumed that the hot fluid, despite undergoing condensation will remain at 100
with 24.8% of the hot fluid condensate into liquid. The hot fluid is
assumed to absorb all the heat released by the condensation and reach 100
The log mean temperature cannot be used in this situation as there are phase change occurs during the process with excess energy provided in the hot fluid. The temperature difference used in the calculation will only be based on the temperature difference of the cold fluid, that is 50
6.0
.
Heat Transfer Area
The heat transfer area calculated here is for the preliminary analysis which later will be changed again after the coefficient of heat transfer for each part of the heat exchanger is obtained in the later part. 6.1
Overall Coefficient
For Tube and Shell Heat Exchanger, the overall coefficient will be in the range of 490 – 1000 for fluids of steam to light organics. Taking the design conservatively 490 W/m2
will be used in the
preliminary calculation.
The heat transfer area can be calculated using the equation (6.1) Rearranging we have, (6.2) Substituting the values,
7
A preliminary estimation of the area needed for heat transfer is roughly 347m2.
7.0
Material Selection, Sizing and Layout for Tube
7.1
Material Selection
There are many cases of run off reaction concerning Ethylene Oxide, one of them is the polymerization of Ethylene Oxide. It is reported that rust, or iron oxide will catalyzed and initiate polymerization. The presence of loose quantity of rust could pose a significant safety hazard. The condition of metal surfaces is extremely important in determining the rate of ethylene oxide polymer formation. It has been reported that even clean carbon steel catalyzes polymerization, although at a much slower rate than rusty steel. The polymerization of ethylene oxide usually cause line and equipment plugging and off-specification product.
Stainless steel would be the best choice for materials of construction, especially when the surface to volume ratio is high. The polymerization reaction has not been found to be auto-catalytic. That is, the presence of polymer does not accelerate the polymerization process. The type of stainless steel that will be used in the fabrication of this equipment would be Stainless Steel 316.
7.2
Tube and Shell Fluid
The cold fluid which is the mixture of ethylene oxide and water has higher pressure than the hot fluid. Higher pressure fluid should be contained in the tube while the lower pressure steam will be put in the shell side.
7.3
Tube Dimension
In order to maximize the heat transfer area and increase the heat transfer efficiency, a good selection of tube dimension is important. While the calculation will be perform to check the best tube size for the equipment, it is usually not easy to get one. There are many commercially available tube size on the market which may be able to serve the need of the equipment. Hence, the next best available commercial tube that is close to the one we need will be used.
8
7.3.1
Tube size
The tube selected will be based on the ASME B36.19 standard which can be found in the Appendix D. The reason for choosing the tube size from the ASME B36.19 standard is due to the convenient of obtaining the material for fabrication while getting a fair price from the market.
The nominal pipe size of ¼ Schedule 10S will be used in the calculation. The size of the pipe is subjected to change after checking the pressure drop and heat transfer rate. A 358, Electric Fusion Welded stainless steel will be used.
According to the ANSI/ASME B36.19 standard the ¼ Schedule 10S have the following dimensions
Dimension
Value
Outer diameter
13.7 mm
Thickness
1.65mm
Table 6.1
7.3.2
Dimension of ¼ Schedule 10S
Tube Thickness
Before the selection of tube, a pressure test on the tube will need to be done. The following equation will be used to examine the minimum thickness of the tube required for the working pressure.
The minimum wall thickness of a tube is given by the equation (7.1) (7.2)
The data from the table below is used for the calculation. Properties
Value
Internal design gauge pressure,
5000000*
Pipe outside diameter,
0.0137
Basic allowable stress for pipe material,
115142446.576
Casting Quality,
1
Temperature coefficient,
0.4
Table 6.1
Properties of Stainless steel 316 for ¼ Schedule 10S
9
* Taking the calculation conservatively, an extra 250% of internal pressure is allocated for safety purposes.
The calculation is worked out as follow [
]
As calculated above, the pressure design thickness has to be at least 0.15mm to withhold the internal pressure without failing. The schedule 10S has a thickness of 1.65mm which is more than enough to hold the internal pressure and give allowance for the corrosion and erosion.
7.3.3
Tube Length
A longer tube length is usually favourable over shorter one as longer tube length would result in less number of tubes and smaller diameter of the shell. The decision of having longer tube could be justified as the cost of larger diameter of shell is usually high. A standard 6 meter pipe length is usually readily available on the market and will be used in the design. 7.4
Tube Arrangement
Triangular tube arrangement will be used in this design project. It is reported that triangular pitch gives greater turbulence than square pitch, under comparable conditions of flow and tube size, the heat transfer coefficient for triangular pitch are roughly 25% greater than that of square pitch.
As the heat transfer process includes condensation, condensate on the tube will flows by gravity onto lower tubes in bundle. The condensate thickness around the lower tube will increase and hence decrease the heat transfer coefficient. It was found out by theory and calculation that triangular, staggered arrangement of tubes generally yields an average heat transfer coefficient that is generally larger than that for a square in-line arrangement.
The tube bundle will also be inclined by 5 degree in the design as it is found out by Shklover and Buevich that inclination of the tube bundle increases the average heat transfer coefficient as much as 25%.
10
7.5
Number of Tubes
The number of tube will be calculated based on the data specified above which is summarized in the table below.
Dimension
Value
Outer diameter of tube 13.7mm Length of tube Table 8.1
6.0 m Dimension of Tube
Area of one tube Number of tubes
The number of tubes will be rounded up to 1350. The tube-side velocity can be calculated using the formula ̇
(7.3)
Substituting the values we have,
This velocity of the fluid should be fast enough to achieve turbulent flow, a check on the Reynolds’ number will be carried out to determine the fluid flow profile. The Reynolds number of the flow can be calculated using the formula (7.4)
Substituting the values, we have
As the Reynolds number is more than 4000, the fluid flow in tube is considered as turbulent and hence is acceptable by most standards. The purpose of achieving turbulent flow is to have higher heat transfer coefficient, low fouling rate, corrosion rate and low deposition rate.
11
8.0
Shell Selection and Sizing
As the heat transfer process involves condensation of steam, it is therefore good to use J type shells, which have two inlet for vapour phase and one central outlet for condensate. It has been reported that J-Shell has approximately 1/8 pressure drop of a comparable E-Shell. The Shell type and its alphabetical character is standardized by TEMA and can be seen in Appendix F. In addition to that a type B front-end stationary head will be used along with type M rear-end stationary head in the design for the ease of cleaning and maintenance.
8.1
Bundle and Shell Diameter
The bundle diameter can be calculated using the equation (
)
(8.1)
For triangular pitch with one pass,
Substituting values into the equation we have,
(
)
For an Outside packed head exchanger, the typical shell clearance is 38mm, so the shell inside diameter is
As the shell diameter is 714mm, which fall under category of the nominal shell range 610-740, for alloy steel the minimum thickness is 4.8mm. An extra 0.5mm will be allocated for fouling, corrosion and erosion of the shell wall. Hence the shell outside diameter would be,
, after
rounding up.
A check up on the dimension of Shell diameter and Tube length is done by referring to a figure illustrating a graph of tube outside surface area as a function of shell inside diameter and effective tube length (Bell, 1998). For tube heat transfer area of 350m2 and tube length of 6m, the appropriate shell inside diameter is 737mm, very close to our calculation.
12
9.0
Heat Transfer Coefficient
9.1
Tube-side Heat Transfer Coefficient
Numerous Nusselt numbers correlation where proposed in the literature. In order to choose a suitable correlation that fits the flow profile of the design, it is important to calculate the Prandtl numbers and Reynolds number.
The Reynolds number is as calculated before which is 8743.36
Prandtl number can be calculated using the following equation (9.1)
Substituting the values into the equation, we have
Since the Re is more than 2300 and below 5000000, with Pr more than 0.5, Gnielinski’s correlation will be used in this case. Gnielinski’s correlation is as follow (9.2)
Where,
(9.3)
Substituting the values, we have
Substituting the f values into Gnielinski, we have
From the Nusselt numbers we can calculate the heat transfer coefficient with the equation (9.4)
13
Substituting the values, we have
9.2
Shell-side Heat Transfer Coefficient
As the shell-side heat transfer involves condensation, a different heat transfer correlation must be used in this case. For partial condenser, the effect of film condensation must be taken into account when calculating the heat transfer coefficient. Two types of correlation can be used in determining the heat transfer coefficient, laminar film condensation and forced convection.
A check on the velocity of the fluid flow in the shell,
Effective cross sectional area = cross sectional area of shell – cross sectional area of tube bundle *
(
) +
*
(
)
+
This fluid velocity is clearly too high for a pipe flow, it is also known that the steam feed is in excess, one way to reduce the velocity of the steam is to reduce the feed. Since from the calculation above that only 24.8% of the total gas is condensed into fluid, a reduction of feed by 75% would tremendously reduce the gas velocity in the shell. Hence, by limiting the steam feed to 13656.25kg/h. we get
Hence,
A gas velocity of 32m/s is more reasonable compared to the previous calculation. A high velocity could cause serious erosion in the shell side.
14
To calculate the forced convection on the tube and the effect of condensation, a modified version of Shekriladze and Gomelauri equation on the surface shear convection by Butterworth using interpolation formula will be used in the calculation. This formula satisfies gravity-controlled and shear-controlled condensation and is considered a more conservative approach compared to other approach.
The Butterworth equation is as follow (
̃
)
(9.5)
Where F is given by (9.6)
The two-phase Reynolds number is ̃
(9.7)
Calculating the F values gives
Calculating the two-phase Reynolds Number gives ̃
Rearranging and substituting both ̃ and ( (
) (
into equation (9.5) gives
̃ )
)
From the equation (9.4) the mean heat transfer coefficient can be calculated
The heat transfer coefficient is appropriate for a condensing steam. 15
9.3
Overall Heat Transfer Coefficient
The overall heat transfer coefficient can be calculated using the equation
(
)
(9.8)
The condensing steam from the evaporator is assumed to have traces of light hydrocarbon in it, a fouling coefficient of 5000 W/m2
will be used in the calculation.
There are insufficient amount of data for the fouling coefficient for the cold fluid mixture of ethylene oxide and water. Taking that the fouling factor for water and ethylene oxide is addictive, that is, independent of each other, a combine fouling factor of 10000 W/m2
would be taken for
conservative calculation.
The following data will be used to calculate the overall heat transfer coefficient. Coefficient type Values (W/m2K 17240 5000 10000 1048.5 Table 9.1
Heat transfer coefficient for heat exchanger
Substituting the values into the equation (9.8) we have (
)
16
10.0
Pressure Drop
10.1
Tube-side Pressure Drop
Tube side pressure drop can be referred from the Tube-side friction factors in the Sinnot & Towler book. The Reynolds number of cold fluid in tube is 8743.4. From the figure, this correspond to friction factor of
.
The pressure drop can be calculated using the equation [
( )
]
(10.1)
Substituting the known values into the equation (10.1) we have [
(
)
]
The pressure drop is very low and is way below the recommended pressure drop guideline outlined by Sinnot & Towler, hence it is acceptable. 10.2
Shell-side Pressure Drop
As the shell-side involves condensation which reduces volume dramatically, the pressure drop should be inspected. The pressure drop for shell-side associated with condensation will be calculated using Kern’s method to make an approximate estimate.
Area for cross-flow is given by (10.2)
Using a baffle spacing of that equal to the internal diameter of Shell with 45% cut lined vertically. Since
, the equation can be further simplified to
(10.3)
Substituting the values into equation (10.3) we have
17
Mass flow rate over area
= (13656.25/3600)/0.1023 = 37.08kg/s m2
Calculating the shell-side equivalent diameter, for a triangular arrangement, the following equation can be used. (10.4)
Substituting the values, we have
The Reynolds Number is,
Using the friction factor diagram found in Sinnot and Tawlor
Fluid speed in the shell-side is (10.5) Where
Take pressure drop as 50% of that calculated using the inlet flow, neglect viscosity correction. We have the equation (
)(
)( )
(10.6)
Substituting the values, we have (
)(
)(
)
The amount of pressure drop in the shell-side is small compared to the minimum pressure drop value recommended of 0.5 atm. Hence the design is acceptable
18
11.0
Mechanical Design
11.1
Heat Exchanger Tubes
As mentioned above during the calculation, the heat exchanger tube will be made with the following material, specification and dimension
Material
Stainless Steel, 316
Nominal Size
¼
Schedule
10S
Internal Diameter 13.7mm Thickness Table 11.1
1.65mm
Material, specification and dimension of tubes
These tubes are to be welded to the fixed-tube sheet and rolled in the tube sheets. As the tube-side liquid is high pressured, special caution has been taken to measure the minimum allowable thickness of the tube. The calculation can be referred above. To minimize the vibrational damage, wide grid bars will be resting on the tube wall. The baffles in the shell will also act as a vibrational cushion to the tubes. The tubes should also be inclined 5 degree as mentioned above to facilitate the film condensate drop flow and increase the overall heat transfer coefficient. 11.2
Heat Exchanger Shell
The heat exchanger shell will have the following material, specification and dimension
Material
Stainless Steel, 316
Internal Diameter 715.2mm Thickness Table 11.2
11.3
4.8mm
Material, specification and dimension of shells
Tube Sheets
The tube sheet is extremely important in this design as it separate two pressure sections. The thickness of the tube sheet has to be constant. The tubes will be fastened in a tube sheet by welding and roller expansion. The tube must be fixed before welding to prevent eccentric weld joints. 11.4
Vent Point
As condensation process is in the shell-side, a special attention is needed to vent the system properly, the steam will be drawn off at vent point but screened off to prevent the non-cooled heating steam from taking the direct path to the vent system.
19
11.5
Baffle number and distance
Each baffle will have a spacing of 0.715m, for a rough estimate on the number of baffle needed. Total spacing = 6m/0.715m = 8.39 Number of baffle needed
=8–1 =7
Rounding down the numbers, a total of 7 baffles will be used. 11.6
Weight Loads
The dead weight of the heat exchanger consist of several parts as described below I.
Shell
II.
Shell Cover
III.
Tubes
IV.
Baffle
V.
Tube Sheets
Approximate weight of the heat exchanger will be calculated by assuming it as a cylindrical vessel with domed ends and uniform thickness. The following equation can be used
(11.1) Substituting the values,
The weight of the tube can be calculated as follow,
Adding up the tube and shell, we have an approximate weight of 10801.12kg.
11.7
Nozzle
The nozzle of the heat exchanger will have the internal diameter of 5 inch with 8 bolts and nuts for tightening. Special gasket will be used as a seal when the nozzle is connected to a pipe.
20
12.0
Schematic Diagram of Heat Exchanger
21
13.0
Mechanical Drawing of Key Item
22
14.0
Piping and Instrumental
23
15.0
Costing
The equipment costing can be approximate using correlation data by Sinnot and Tawlor. For the estimation of heat exchanger, the U-tube shell and tube heat exchanger correlation data will be used.
The data of correlation is as follow Equipment
Unit for size Slower Supper a
U-tube shell and tube Area, m Table 15.1
2
10
1000
b
n
24000 46 1.2
Equipment cost correlation for U-Tube Shell and Tube
Using the equation (15.1) The area of the heat exchanger is 346.97m2 , substituting the values into the equation, we have
Since stainless steel 316 is used instead of carbon steel, the estimated cost need to be multiplied by 1.3 for stainless steel factor. This would total up the estimation cost to
This again should be coupled with escalation of cost since the estimated cost is for the year 2007. A cost index is referred in the Sinnot and Towler design book with the current cost index extrapolated to be 2500
A correction to the cost estimation with cost index gives
24
16.0
Auxiliary Equipment, pump and piping
There is no auxiliary equipment needed for the operation of the heat exchanger. There is, however, a need for pump and compressor in order for the heat exchanger to work.
It is calculated that the tube side pressure drop is
which is negligible. The pipe
that carry the feed into heat exchanger would have to be stainless steel due to the nature of fouling. The heat exchanger will be operating from the gound. The internal diameter of the pipe used will have 5 inch to match the nozzle of the heat exchanger. Assume that the compressor is situated 20 meter away.
Cross-sectional area of pipe = Fluid velocity Reynolds Number
=1119942
Absolute roughness of pipe =0.000015m Relative roughness
= 0.000118
From the diagram of pipe friction versus Reynolds number and relative roughness, the friction factor is 0.001625.
Friction loss in pipeline,
(
)
(
)
Since the pressure before the compressor is atmospheric pressure, the pressure difference is,
The energy balance is (1898675/960.63)+(1256.5453/960.63)
= 1977.8J/kg
The required power Power = 109965.53W Taking the compressor efficiency as 70%, Power = 157093.624kW
This would require a heavy duty compressor as the duty is as high as 157kW.
25
17.0
Mass Balance and Safety Review
17.1
Mass Balance Review
Throughout the design process, the mass balance has been reviewed again and again by out group. While performing energy balance to determine the duty of the heat exchanger, it was found out that there are excessive steam energy from the evaporator and only 24.8% of them is fully utilized.
Additional steam has to be diverted to other stream due to the limitation in the designing of the equipment such as keeping the shell-side velocity within desirable range. In that regard, the mass balance will probably need to be adjusted again for better thermal efficiency.
17.2
Safety Review
The usage of gasket and O-ring for pump and heat exchanger is usually inevitable. There have been reports of ethylene oxide chemically attack the gasket and O-ring, especially those made from asbestos or PTFE. This would usually resulted in leakage and in other serious case, a hazard.
Special O-ring and Gasket need to be used when handling ethylene oxide, the item listed below have high resistance and stable against ethylene oxide. Only this items and this item only should be used, other Gasket Polycarbon Sigraflex BTCSS Flexible Compressed Graphite – Laminated on Stainless Steel Tang Sheet O-ring
Chemraz 505 Kalrez 2035 Kalrez 6375 Parker EPDM-740-75 Parker EPDM-962-90 Parker E-515-8-EPM Table 17.1
Item compatible with Ethylene Oxide
As the ethylene oxide is a hazardous substance, the use of pressure relief valve along the pipes that contain the liquid is not encouraged. Appropriate ethylene oxide leak detecting system should be employed, online pressure monitoring probe should be install where ethylene oxide is present in the pipe line. For insulated flanges, it is useful to install leak detection tubes. Sealed stainless steel bands installed around the flange can help prevent leakage of liquid along the piping.
26
18.0
Conclusion
This equipment was design with the priority of safety over the other factors and many design consideration was made by making calculation conservatively. The heat exchanger can withstand high pressure load even if there is a sudden fluctuation of pressure during the operation. The equipment has both the capacity to heat up the feed as well as low operating cost due to the low pressure drop over the entire unit.
The use of Stainless steel would ensure low fouling rate in both the tube and the shell, while maintenance should be carried out periodically. Overall, I think the design of this heat exchanger would satisfy the need of the plant to operate without much concern about time lost as well as saving the cost in maintenance.
27
19.0
References
Bott T.R. 1995. Chemical Engineering Monographs 26: Fouling of Heat Exchanger. Amsterdam: Elsevier Science Kakac S. & Liu H.T. 2002. Heat Exchangers Selection, Rating, and Thermal Design 2nd Ed. Florida: CRC Press Kuppan T. 2000. Heat Exchanger Design Handbook. Basel: Marcel Dekker Ramesh K.S & Dusan P.S. 2003. Fundamentals of Heat Exchanger Design. New Jersey: Wiley & Son Sinnot.R & Towler. C. 2009. Chemica Engineering Design 5th Ed. China: Butterworth – Heinemann Podhorsky M. & Krips H. 1998. Heat Exchanger: A Practical Approach to Mechanical Construction, Design, and Calculation. New York: Begell House
28
Appendix A Nomenclature
Tube-side heat transfer coefficient Tube-side fouling Shell-side heat transfer coefficient Shell-side fouling Area of Internal Diameter Specific heat at constant pressure, J/kg·K Bundle Diameter Specific gravity constant Number of tubes Overall Heat transfer coefficient Outside diameter Enthalpy of gas Thermal conductivity ̇
Fluid mass flow rate, kg/s constant Minimum thickness Pressure thickness Pressure loss in pipe friction, N/m2
A U
Pressure difference in Shell-side, N/m2 Pressure difference in Tube-side, N/m2 Mean temperature Pressure difference, N/m2 Local temperature difference between two fluids, Area, m2 Heat transfer coefficient Casting quality factor Heat transfer rate, W Basic allowable stress Sum of mechanical allowance Gravity Velocity Temperature coefficient Viscosity Density
,K
29
Appendix B Process Flow Diagram For Plant Design
30
Appendix C Mass Balance For Plant Design
31
Appendix D Pipe Sizes – ANSI/ASME B36.19M - 1985 Dimensions and Weights per metre – stainless steel pipe
32
Appendix E
TEMA Shell and Tube Nomenclature
33
