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3D Heat Exchanger Design - studentsouedu

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3D Heat Exchanger Design - studentsouedu

    Memo

    To: Independent Refineries Inc. Project Coordinator

    From: Mark McClendon, Senior Research Coordinator of 3D

    Date: 9/19/2011

    Re: Optimizing heat transfer equipment.

    Overview of the Enclosed Report

    In response to Independent Refineries Inc. request for consulting services on 11/15/07, Dynamic Design and Development (3D) Inc. has prepared the following report. Enclosed in the report is 3D Inc.’s simulation analysis of a new cyclohexane production equipment. After reviewing the simulations of the production unit, 3D has recommended equipment requirements specific to the unit. The heat exchanger equipment has been analyzed using a rigorous model. An economic analysis of the heat exchanger section of the unit was performed. Also included in the report are calculations that support our conclusions.

3D Website- http://students.ou.edu/H/Oluwaseun.O.Harris-1/

    1

     Dynamic Design and Development

    Equipment Analysis

    For

    Independent Refineries Incorporated

    11/28/07

    Consulting Engineers:

    Seun Harris

    Kristy Booth

    Jenny Cochran

    Mark McClendon

    James Akingbola

    2

    Table of Contents

    Overview of the Enclosed Report ............................................................................................................ 1 SUMMARY ................................................................................................................................................ 4

    PROBLEM STATEMENT ....................................................................................................................... 4 SIMULATION PROCESS ....................................................................................................................... 5 HEAT EXCHANGER MATERIAL ......................................................................................................... 9 HEAT EXCHANGER PRICE ............................................................................................................... 10 CYCLOHEXANE DISTILLATION COLUMN ..................................................................................... 10 ERROR ANALYSIS .............................................................................................................................. 11

    RECOMMENDATIONS ........................................................................................................................ 11 REFERENCES ...................................................................................................................................... 12

    APPENDIX........................................................................................................................................... 13

    3

SUMMARY

    This report analyzes the specific equipment used in the cyclohexane production plant proposed by Independent Refineries. These pieces of equipment were the heat exchangers and the distillation column. The Rigorous model was found to produce the most efficient heat exchanger design. The heat exchanger should be designed as double pass, multi-pipe heat exchangers made of stainless steel. The total fixed capital investment for the heat exchanger was found to be $530,000. The dimensions of the cyclohexane distillation column were calculated to be 70 feet tall and a diameter of 6.5 feet.

PROBLEM STATEMENT

    The objective of this study was to design heat exchangers that would satisfy the requirements of the cyclohexane plant. To accurately design the exchangers an in depth analysis was necessary. The in depth analysis includes T-Q diagrams, rigorous models, Pro-II simulations. After designing the heat exchangers there cost and FCI must be found. Another task for this project is to design a cyclohexane distillation column. Shown below is a detailed outline of the task performed in this study.

    1. Design the heat exchanger indicated in the cyclohexane plant by hand (or Excel). First, use

    single pass tubes and then double pass tubes. Create the T-Q diagram for both cases. Estimate

    the heat exchanger cost for both cases.

    2. Design the heat exchanger indicated in the cyclohexane plant with the Rigorous Model used in

    PRO/II. Maintain pressure drop on both sides smaller than 5 psi.

    3. Select material type for all the exchangers (E1, E2, E3) and suggest which fluid flows through

    which part of the exchanger (shell or tubes).

    4. Create the T-Q diagram for the other two heat exchangers (E2, E3).

    5. What is the cost for installing the heat exchangers (Fixed Capital Investment)? Use the PRO/II

    results for E1.

    6. Use a new link in your company’s web-page to upload your key findings

    7. Submit an electronic copy of the PRO/II Keyword files, and of your oral presentation (in pdf or

    power-point format).

    8. Design the column of the cyclohexane plant with PRO/II. Make sure the diameter of the

    column does not increase with height! Provide the specification sheet.

    4

SIMULATION PROCESS

    The simulation process for the cyclohexane plant was designed with the PRO/II program. The inlet and outlet heat and material balances were specified as provided by IRI engineers so the program simulation matches the actual production of the plant. A detailed report of the simulation can be found in the Appendix.

HEAT EXCHANGER DESIGN BY HAND

     Single Pass Double Pass

     Tubes Tubes

    Temperature 150 F 126 F

    Surface Area 1400 ft2 1776 ft2

    Tube Length 12 ft 12 ft

    Tube Diameter .0625 ft .0625 ft

    Number of Tubes 594 753

    Bundle/Shell Diameter 2.27 ft 2.52 ft

    Overall Heat Transfer 42.1

    22Coefficient 44.9 btu/hr/ft/F btu/hr/ft/F

    Tube Side Pressure Drop 5 psia 10 psia

    Shell Side Pressure Drop 5 psia 5 psia

     Chart 1. Heat exchanger specifications when calculated by hand.

    Multiple equations from Peters, Timmerhaus, and West were used to design the heat exchanger by hand. By looking at the previous chart, you can see that the calculations point to the single pass tube heat exchanger as a better source of heat exchanging. It is better because it requires a smaller heat exchanger surface area as well as a fewer number of tubes. These calculations were found using the following equations.

    First, an overall heat transfer coefficient was assumed using a known change in temperature and heat duty to find the heat transfer area. This assumption was carried out through the remaining calculations of independent shell side and tube heat transfer coefficients.

    More specifically, the following equation was used to find the tube side heat transfer coefficient:

    5

    Using data from previous simulations, we were able to solve for the Nusselt number which is directly related to the heat transfer coefficient. After finding the tube side heat transfer coefficient, we found the shell side coefficient after calculating the bundle diameter from the following equation:

    We assumed a negligible difference in the bundle and shell diameter and used this value to find the shell side heat transfer coefficient from the following equation, where once again the Nusselt number is directly related to the heat transfer coefficient:

    Finally, we back calculated using the individual heat transfer coefficients to find a new overall heat transfer coefficient with the following equation:

    By inserting the new number into the original guess for the heat transfer coefficient, we were able to find a correct overall heat tranfer coefficient upon the convergence of the data. This graph represents the T-Q Diagram (temperature and heat duty) for the heat exchanger examined above.

    TQ Diagram E1

    500

    450

    400F)o350

    300Hot side 250Cold side200

    150

    Temperature (100

    50

    0

    0246810

    Duty (MMBTU)

    6

HEAT EXCHANGER DESIGN BY RIGOROUS MODEL

    Heat exchangers were designed for the cyclohexane plant using a rigorous model with the aid of the PRO II simulation program. The rigorous model is useful in determining some important specifications that will needed for the building the heat exchangers. The table below gives a list of the values obtained from the simulation; it is important to know that these values are estimations of what the actual specifications will be on site.

    Rigorous Model Heat Exchanger Specifications

    Heat Area Shell Tube outer Passes Material Number of tubes 2Exchanger (ft) Diameter (in) Diameter (in)

    1 766 2 316 Stainless Steel 195 19 0.75

    2 1502 2 317 Stainless Steel 390 26 0.75

    3 3128 2 318 Stainless Steel 818 36 0.75

    The heat exchanger area depends greatly on the amount of heat transferred required to produce the necessary outlet temperatures of the fluids. This amount of heat transferred also determined the total number of tubes needed to achieve the required area of the exchanger. For each exchanger, the materials flowing through both the shell and tube sides were determined based on their corrosiveness and temperature. A more detailed report of the material selection is contained in the heat exchanger material section. Also attached to the report is the PRO II report that was generated for the rigorous design, it can be found in appendix 4.

    7

T-Q DIAGRAMS

    The following tables and charts show the data extracted from PRO II for heat exchangers E2 and

    E3. This data was used to optimize the design of the heat exchangers. The temperature of the hot

    side in heat exchanger 2 remains constant because the heat from condensing steam is a result of

    phase change.

    E2

    Duty (MMBTU) 2.167

    Hot side Cold side (Steam)

    oInlet (F) 388.16 300

    oOutlet (F) 388.16 250

    TQ Diagram E2

    400

    380

    360

    F)o340

    320Hot side (Steam)300Cold side280

    260Temperature (240

    220

    200

    00.511.522.5

    Duty (MMBTU)

    8

    E3

    Duty (MMBTU) 5.337

    Cold side Hot Side (Cooling water) oInlet (F) 227.289 90 oOutlet (F) 120 60.001

    TQ Diagram E3

    250

    200F)o

    150Hot Side

    100Cold side

    (Cooling water)Temperature (50

    0

    0246

    Duty (MMBTU)

HEAT EXCHANGER MATERIAL

    The heat exchangers should be made of 316 stainless steel due to the corrosive properties of the methane, benzene and cyclohexane. Stream 5 is heated by Stream 7 in Exchanger 1. Since both streams have extremely high pressures from 489 psig 509 psig, the pressure difference is of little

    consequence. Temperature is the dominant property for determining flow patterns. Since Stream 7 is being cooled from 434ºF, it should flow tube-side. Stream 5 should flow shell-side since it is heated to only 250 ºF.

    Exchanger 2 is used to heat Stream 4 from 107ºF to 300ºF. The stream’s corrosive and fouling tendencies indicate that it should be flown tube-side. Steam, flown shell-side, will heat the fluid at a maximum rate due to the expanded contact between the tubes and steam.

    Exchanger 3 is used to cool Stream 9 from 227ºF to 120ºF. Cold water at approximately 65ºF is flown shell-side and while it cools the liquid. The tubes are more easily cleaned than the shell, so the corrosive fluid should be flown tube-side.

    9

HEAT EXCHANGER PRICE

    3The cost of heat exchangers was found using the correlation to surface area shown in figure 14-16. The cost was then found in 2007 using the Marshal and Swift index shown in the appendix 3. The

    total fixed capital investment was found using the unit cost factors shown in appendix 2.

    Heat Exchanger Cost

    Heat Area Cost in Cost in FCI 2Exchanger (ft) 2002 2007

    1 766 $15,000 $16,600 $87,200

    2 1502 $26,000 $28,800 $151,200

    3 3128 $50,000 $55,400 $290,900

CYCLOHEXANE DISTILLATION COLUMN

    The cyclohexane distillation column was found to be 70 feet tall with a diameter of 6.56 feet. The

    major assumptions and calculations are shown below. Assumptions:

     Tray spacing of 2 feet

     80% flooding

     Surface tension of 6 dynes/cm

    Known values from PROII

    3L=0.904 lbmol/hour V=0.9033 lbmol/hour ρ=0.18 lb/ft ρ=47.6 lb/ft VL

    1Using the equation 15-5, the following value is calculated:

    0.50.5.904.18149LV==0.0618 .903347.566V(((L(

    1By the correlation found in figure 15-5 C=0.1 m/sec (The tray spacing is estimated as 2 feet) sb

    α is assumed to be 6 dynes/cm

    0.50.20.20.5;647.6;0.18LV VC0.11.276m/snfsb20200.18(((V(

    Assuming 80% flooding

    V= 0.8V= 1.02 m/s n nf

    10

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