Assignment 6 Final (Autosaved)

7/28/2019 Assignment 6 Final (Autosaved)

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CRIMSON ENGINEERING ASSOCIATES, LLC 12008

CrimsonEngineeringAssociates,

LLC

[HEAT TRANSPORT

EQUIPMENT]


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Table of ContentsExecutive Summary ....................................................................................................................................... 3

Background ................................................................................................................................................... 4

Heat Exchanger Design ................................................................................................................................. 4

Analytical Design of Heat Exchanger E1.................................................................................................... 4

Simulation Design of Heat Exchanger E1 .................................................................................................. 7

Material Selection ..................................................................................................................................... 8

Temperature-Duty Diagrams for Heat Exchangers E2 and E3 .................................................................. 9

Heat Exchanger Cost Evaluation ................................................................................................................. 10

Purchase Costs ........................................................................................................................................ 10

Installation Costs ..................................................................................................................................... 11

Column Design ............................................................................................................................................ 11

References .................................................................................................................................................. 12

Appendix ..................................................................................................................................................... 13


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Executive Summary

The following study was conducted as a follow up to a previous consultation performed forIndependent Refineries, Inc. (IRI) by Crimson Engineering Associates (CEA). The project

concerns the sizing and cost analysis of heat exchange equipment in the proposed cyclohexaneplant. The purpose of this study was to utilize analytical calculations combined with simulationsoftware to provide rigorous design specifications for the heat exchanger cascade.

Analytical calculations revealed the optimal heat exchanger was a double pass shell and tubeexchanger with effective transfer area of 1001 ft2. This result agreed closely with the simulationoutput which suggested an effective heat transfer area of 1004 ft2. The estimated purchase costof the unit was approximately $10,200. Combined with direct and indirect costs associated withthe units installation, the total fixed capital investment was $195,000.

The final component of CEAs consulting work consisted of sizing the distillation column used

for product purification. The column consisted of 30 trays with the feed located at tray 16. Thetray diameter calculated at the feed location was 84 inches, and tray spacing was calculated attwo feet.


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Background

The current project continues CEAs previous consulting work for Independent Refineries, Inc.Based on CEAs previous recommendations, IRI has started construction on its proposedcyclohexane production plant. Before completing construction, a more rigorous evaluation of

the heat transfer equipment necessary for plant operation was required. CEA was charged withdesigning the heat exchange units for use in the plant and determining their cost of constructionand instillation. The design calculations will be performed by hand based on previous simulationdata and confirmed with further simulations using PRO/II. The following report contains theresults of this study. Additionally, further description of the distillation column used for productpurification is included. For the labeling associated with the various heat exchangers, pleasereference process flow diagram in the Appendix.

Heat Exchanger Design

Analytical Design of Heat Exchanger E1Design of the heat exchanger began with an analytical evaluation based on fundamentalequations. The first step to designing the heat exchanger was to define the flow rates,temperatures, pressures, and enthalpies (which give the duty of the exchanger) of the inlet andoutlet streams. These values were taken from the PRO/II report when the project was initiallysimulated. The next step was to determine the thermodynamic data, such as density, thermalconductivity, viscosity, and heat capacity for each stream. These values may be found in theAppendix. The third step was deciding on the type of heat exchanger that would be best for theprocess. Since shell and tube heat exchangers are cheap and well understood, this type waschosen. There were no limiting factors, such as extreme temperatures and pressure or sizeconstraints that would cause another type of exchanger to be chosen over the shell and tube type.

Next, a trial value for U, the overall heat transfer coefficient, was chosen. Based on Table 14-5in Peters et al, the average value of the range given was chosen. For a heat exchanger with lightorganics on each side, the recommended values are 300-425 W/m2K, which gives an averagevalue of 362.5 W/m2K. The design was initially based off of this value, and then this value waschecked at the end of the design process and updated to 296 W/m2K for a single pass exchangerand 308 W/m2K for a double pass exchanger. The next step is to obtain a value for the meantemperature difference. For a single pass exchanger, this is simply equal to the log-meantemperature difference. For the two pass exchanger, the log-mean temperature difference ismultiplied by a factor found in Peters et al Figure 14-4. Once this value has been obtained, theheat exchanger area may be found from the formula:

(1)

After the area is found, the specifics of the exchanger are defined. The number of tubes requiredwas found by dividing the area by the surface area of the outside of one tube. An outsidediameter of 1 in was chosen for the single and the double pass exchanger. A triangular pitch wasused to give the largest heat exchange for the bundle area; also because the pressure drop was not


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significant enough to make a square pitch necessary. The shell diameter was then calculatedusing the formula:

(2)

Where K1 and n1 are constants specific to the type of exchanger. The pitch was set at 1.25 timesthe outside diameter of the tubes. The number of baffles was chosen by how many would fit inthe length of the shell, rounding down from the guess of 100% of the shell diameter as the lengthbetween baffles. The baffle spacing was then calculated from the number of baffles and thelength of the exchanger. A summary of the major results for both the single and double passexchanger can be found in Tables 1 and 2. Purchase costs were calculated from Figure 14-16.The detailed calculations can be found in the Appendix.

Specification E1 (single pass)

Tube Passes 1

Tube Length, ft 20

U, BTU/hr-ft2-F 52.2

LMTD, F 171

Effective Heat Transfer Area, ft2 885

Shell Inner Diameter, in 24

Pitch, in 1.25

Layout Triangular

Baffle Spacing, in 20.6

Purchase Cost $9,500Table 1. Specifications for Single Pass Exchanger

Specification E1 (double pass)

Tube Passes 2

Tube Length, ft 20

U, BTU/hr-ft2-F 54.3

LMTD, F 171


Shell Inner Diameter, in 22Pitch, in 1.25

Layout Triangular


Purchase Cost $10,200

Table 2. Specifications for Double Pass Exchanger


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Next, the tube side heat transfer coefficient, tube side pressure drop, shell side heat transfercoefficient, and shell side pressure drop are found. The correlations for these calculations,respectively, are given below.

Tube Side: (3, 4)

Shell Side:

(5, 6)

In the first equation, C is assumed to equal 0.021, since the flow is mostly vapor. The Reynoldsnumber is based on G, the mass velocity, which is the mass flow rate divided by the areaavailable for flow. (/w)

0.14 is neglected because it is assumed to be close to unity. In thesecond equation, Bi has to do with the reversal of flow, and it was assumed to be equal to one forthese calculations. np is the number of passes, which is equal to one for the single pass and twofor the double pass exchanger. The value i is equal to 1.02 for turbulent flow. In the third

equation, ao is equal to 0.33 for the staggered tube configuration used for the design. Fs is asafety factor which is usually equal to 1.6. In the final equation, Bo is the number of times theshell side fluid changes direction based on the number of baffles. Nris the number of tube rows,determined from the pitch of the tubes and the shell diameter.

Once all of these values have been calculated, the corrected value for U may be found. Theequation used for this calculation is below:

( )

(7)

Where 1/hod equals 1/hid and both are equal to 6x10-4

m2

-K/W according to Peters et al Table 14-5. kw is the thermal conductivity of the steel tubes. A value of 43.3 W/m-K was found in tableD-5 for carbon steel at 300C. After several iterations, a value of Uo which matched the initialguess of U was found. These values are mentioned above and all values used may be found inthe Appendix. The temperature-heat duty (T-Q) diagrams for both the single pass and doublewere based on simulation data from PRO/II. Due to the similarity of the T-Q diagrams, theinformation has been condensed into one graph depicted in Figure 1.


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Figure 1. T-Q Diagram for Single and Double Pass Exchanger

Simulation Design of Heat Exchanger E1

In addition to the analytical method of heat exchanger design PRO/II simulation was utilized toperform rigorous heat exchanger design. The simulation utilized available stream data alongwith user inputs to perform a predictive design of the heat exchanger. For both the tube and shellside the desired pressure drop was specified to be 5 psi or less. The cold stream was selected for

the tube side because it was higher pressure than the hot stream. A summary of the major resultsmay be found below in Table 3. The complete simulation report is available in the Appendix.

Specification Simulation E1

Tube Passes 2

Tube Length, ft 20

U, BTU/hr-ft2-F 226

LMTD, F 120


Shell Inner Diameter, in 23

Pitch, in 1.0

Pitch Pattern 90


Table 3. Specifications from Rigorous Heat Exchanger Modeling


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Material Selection

The selection of heat exchanger material is determined by the temperature and corrosiveness ofeach exchanger. In addition, the material should have good thermal conductivity for the tube sidefor the heat exchanger to function efficiently. Table 4 shows the material type and fluidcomponent in each exchanger.

Heat Exchanger Side Fluids Material Type

E1 Tube Benzene Carbon Steel

Cyclohexane

Hydrogen

Methane

Shell Benzene Carbon Steel

Cyclohexane

Hydrogen

Methane

E2 Tube Benzene Stainless Steel 309

Cyclohexane

Hydrogen

Methane

Shell Steam Stainless Steel 309

E3 Tube Benzene Stainless Steel 309

Cyclohexane

Hydrogen

Methane

Shell Cooling Water Stainless Steel 309

Table 4. Component and Material Breakdown for Heat Exchangers

In exchanger E1 the fluids flowing are all organic. Thus, an exchanger with carbon steel is ideal.Additionally, carbon steel is readily available and has a low cost. In E1, stream 5 is heated by


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stream 7. Therefore, stream 5 should flow on the shell side given that it has an operatingtemperature of 250 F, while stream 7 should flow on the tube side with an operatingtemperature of 435 F.

In exchanger E2 and exchanger E3 stainless steel 309 is used. Stainless steel 309 has goodcorrosion resistance, good thermal conductivity and mechanical properties. Both E1 and E2 havewater is present on the shell side. Therefore, stainless steel 309 is ideal because it will preventrust.E2 is used to heat stream 4 which has an operating temperature of 300 F, while E3 is usedto cool stream 9 from 250 F to 120 F.

Determination of shell and tube side fluids was based on the pressure of the two streams. Ingeneral, it is preferable that the higher pressure fluid flow on the tube side, while the lowerpressure fluid flow on the shell side.

Temperature-Duty Diagrams for Heat Exchangers E2 and E3In order to size the draw the T-Q diagrams for heat exchangers 2 and 3 for the plant, the datafrom the plant needed to be taken. The data is presented as follows:

Table 5. Exchanger 2 Data Table 6. Exchanger 3 Data

Hot Feed Inlet (F) S4

Hot Feed Outlet (F) S6

Duty (Btu/hr) 4057506

U*A (Btu/hr-F) 225418268

Inlet Temp F 232Outlet Temp F 300

Using the equation for heat duty:

mTAUq (8)

where Tm is:

1221

1221

ln

tT

tT

tTtTTT lmm

(9)

After taking this data from tables above, a solver program can be used to solve for what thetemperatures of other stream satisfy Tm numbers of .018 for exchanger 2 and 642 for exchanger3. These solutions will give a possible temperature range. Knowing that these points are steamfor exchanger 2 and water for exchanger 3 allowed us to say whether or not the possiblesolutions for T-Q diagram are valid.

Hot Feed Inlet (F) S9

Hot Feed Outlet (F) S1

Duty (Btu/hr) 7113381

U*A (Btu/hr-F) 11072

Inlet Temp F 250Outlet Temp F 120


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Figure 2. T-Q Diagram Exchanger 2

Figure 3.T-Q Diagram for Exchanger 3

The hot and cold streams fall in line with the possible conditions of steam on hot side of one andcold on hot side of other. This makes the above graphs valid for the heat exchangers in the plant.

Heat Exchanger Cost Evaluation

Purchase Costs

Reference Figures 1 and 2 for Purchase Costs of the Exchanger.

232

247

262

277

292

307

322

337

8.57E+06 9.07E+06 9.57E+06 1.01E+07 1.06E+07 1.11E+07

Temperature(F)

Q (BTU)

Cold Side

Possible Hot Side

50

70

90

110

130

150

170

190

210

230

250

3.79E+06 4.79E+06 5.79E+06 6.79E+06 7.79E+06

Temperature(F)

Q (BTU)

Hot Side

Possible Cold Side


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Installation Costs

In order to figure out the Fixed Capital Investment (FCI) on the heat exchangers to be purchased,total surface area had to be calculated using the sizes solved from the Pro II analysis. Thisanalysis came up with a surface area of 1004 ft2. Using Figure 14-16 and 14-17 on page 681 in

PT&W, lead to a cost of $22,500 for a heat exchanger with all stainless steel. This same area canbe used so solve for cost of heat exchangers 2 and 3, which are made of carbon steel shells andstainless steel tubes, leading to a cost of $20,850. This comes out to a total purchased anddelivered equipment cost of $64,200. Using Table 6-9 on page 251 in PT&W, the FCI of theproject can be estimated. The figure below shows this:

Total Purchased Equipment Delivered $64,200

Direct Cost

Installation .47 of Purchased Equipment $30,174

Instrumentation and Controls .36 of Purchased Equipment $23,112Yard Improvements .10 of Purchased Equipment $6,420

Service Facilities .70 of Purchased Equipment $44,940

Total Direct Costs $104,646

Indirect Costs

Engineering and Supervision .33 of Purchased Equipment $21,186

Construction Expenses .41 of Purchased Equipment $26,322

Contractor's Fee .22 of Purchased Equipment $14,124

Contingency .44 of Purchased Equipment $28,248

Total Indirect Costs $89,880

Fixed Capital Investments $194,526

Table 7.Installation Costs for Heat Exchanger

Column Design

The Pro/II report from assignment 3 was used to setup the component flow rates, reflux ratio,and feed temperatures and pressures. The parameters used for sizing the column include a tray

spacing of 2 feet, a vapor velocity of .6 of the flooding velocity, and all trays being sieve trays.The results from running the simulation yields tray diameters of 84 inches for trays 2-15 and 90inches for trays 16-29. The reason why trays 16-29 are larger than the others would be that thefeed is located at tray 16. The trays need to be wider to accommodate the higher amount ofliquid. The Soave-Redlich-Qwong thermodynamic model was used for property calculations.After successful convergence of the simulation, the flow rate of the overhead was 36.1 lb-mol/hour with an exit temperature and pressure of 156.5 F and 14.7 psia. The detailed reportcan be found in the Appendix.


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References

Perry, Robert H., et. al. Perrys Chemical Engineers Handbook. Seventh Edition. McGraw-

Hill. Pages 2-95, 2-322, 6-10, 6-11.

Welty, Wicks, Wilson, Rorrer. Fundamentals of Momentum, Heat and Mass Transfer. FourthEdition. John Wiley and Sons, Inc. Pages 188-190.

Peters, Max S, Klaus D Timmerhaus, and Ronald E West.Plant Design and Economics forChemical Engineers. 5th Edition. St. Louis: McGraw-Hill, 2003.


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Appendix

Figure A1. Process Flow Diagram for Cyclohexane Process

M1 E1

E2

F1

SP1

C1

R1

E3

BENZENE

HYDROGEN

S3

S5

S4

S6

S7

S9 S10

S11

S12

S13

S1

500 psig

Eff. 80%

Purge Gas

Product

DP=15 psi

300 F

120 F

Recycle

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Assignment 6 Final (Autosaved)