HEAT EXCHANGER DESIGN IN AN
ETHYLENE OXIDE PLANT

ABSTRACT

1.0 INTRODUCTION

2.0 THEORY

3.0 PROCESS MODIFICATIONS

4.0 OPTIMUM PROCESS DESIGN

5.0 OPERATING CONDITIONS AND SPECIFICATIONS

6.0 SHORT-CUT EXCHANGER DESIGN

7.0 RIGOROUS DESIGN OF PROCESS REBOILER

7.1 DETERMINATION OF HEAT EXCHANGER TYPE 7.2 ALLOCATION OF FLUID 7.3 PRESSURE DROP 7.4 SIZING

8.0 PROCESS ECONOMICS

9.0 ALTERNATIVES

10.0 CONCLUSIONS

REFERENCES

FIGURES AND TABLES

FIGURE 1. COMPOSITE CURVES

FIGURE 2. ORIGINAL ETHYLENE OXIDE HEAT EXCHANGER LAYOUT

FIGURE 3. INTEGRATED ETHYLENE OXIDE LAYOUT

TABLE 1. REACTOR FEED MODIFICATIONS

TABLE 2. ORIGINAL EXCHANGER DESIGN CONDITIONS

TABLE 3. PROCESS FLOWS

TABLE 4. DOUBLE REBOILER FLOW CONDITIONS

TABLE 5. EXCHANGER SPECIFICATIONS

TABLE 6. PROCESS REBOILER SPECIFICATION SHEET

TABLE 7. SHELL SIZING

TABLE 8: Yearly Utilities Breakdown and Capital Cost

ABSTRACT

The focus of this project is to implement a heat exchanger network for the ethylene oxide reactor and separator designs and to design shell-and-tube exchangers for the network. Feed modifications to the reactor project were made to create a continuous flow process producing 30,000 lb/hr of ethylene oxide from the separation column. Composite curve analysis revealed that five exchangers are needed for this ethylene oxide process. Three exchangers require cooling water at a total rate of 6,926 gal/min, one exchanger utilizes 55,000 lb/hr of 150 psi superheated steam, and the final exchanger is a process-process exchanger. Approximately $1.6 million are saved by this integration. Total yearly estimated utility cost is $3.5 million.

The final exchanger and the steam exchanger connected in parallel comprise the double reboiler for the distillation column. The reactor effluent is used to supply heat to approximately 1/3 of the column bottoms in the process-process exchanger. The design of this process reboiler supported by rigorous ASPEN simulations consists of 250 1.5" tubes 16 feet long encased by a 32" diameter shell for a total surface area for heat transfer of 1570 ft². The reboiler is a BEM exchanger with an expansion joint. ASPEN generated cost analysis priced the double reboiler at a total purchase cost of $93,000: $31,900 for the process reboiler and $61,100 for the steam reboiler.

1.0 INTRODUCTION

Heat optimization and integration is one of the most important aspects in the design of any chemical process. Utility costs are indeed the largest of all operational costs and can save a company millions of dollars if it is designed carefully and correctly. The main objective is to utilize as many of the process streams as possible to provide the required heat exchanges for different areas of the plant and then implement steam and cooling water when extra utilities are needed.

In the ethylene oxide plant, there are five main streams of concern, four of which need cooling and one heating. The final system is determined through heat network synthesis. This system consists of five heat exchangers, three of which are associated with final separation column. The design of the column's reboiler is modified to optimize heat exchange by creating a parallel system of two heat exchangers, one using a process stream to provide heat and one using steam utility.

Both short-cut and rigorous methods are performed in ASPEN in order to design safe, cost effective and reliable heat exchangers. Important factors such fluid allocation, pressure drop, tube size, and total surface area for heat transfer are taken into account.

2.0 THEORY The composite curve analysis is an effective way of determining exactly how many utility heat exchangers are required [1]. The composite curves for the four hot streams and the cold stream are generated in MATLAB. This is done by first calculating all the FCp's (flow rate*specific heat capacity) for each temperature interval in order to find the required or available heats. Phase changes are treated separately because there is no temperature change. The subroutine entH2 generates the information necessary to create the s from the calculated heats and given temperatures [2]. Since the curves produced may not intersect, a shift analysis is performed on the cold composite curve to find out how much utilities are needed. The point at which the two curves cross is called the pinch point. In this design, the streams above the pinch point do not need utilities and streams below the pinch point do.

3.0 PROCESS MODIFICATIONS

Modifications were made to the initial reaction process in order to create a continuous process flow diagram (Figure 2) between the reactor design and the separations design [3,4]. The reaction process was designed to produce a larger quantity of ethylene oxide than the separations column was built to handle. The oxygen and ethylene feed streams to the reactor were adjusted until equal flows of ethylene oxide from the reactor exit and to the feed for the separation were obtained. In addition, the methane injection feed was reduced. These two modifications coupled with the decision to keep the original operating conditions (reactor pressure and temperature) ensured that oxygen level in the process streams did not rise to explosive levels, a concern of the reactor design project. Table 1 contains the original and adjusted rates.         Table 1: Reactor Feed Modifications Feed stream Original feed to reactor (lbmol/hr) Adjusted feed to reactor (lbmol/hr) Ethylene 3,460 2,160 Oxygen 3,160 1,990 Blanket methane 88 51

4.0 OPTIMUM PROCESS DESIGN

The existing layout in Figure 2 calls for five heat exchangers, four which are to be cooled with on-site cooling water and the one using steam as a heating medium. The four cooling exchangers are the reactor feed cooler, the reactor effluent cooler, the column feed cooler, and the condenser. The remaining exchanger is the reboiler.

Table 2 dictates the original requirements for heating and cooling solely using utilities. Cooling water is available on-site at ambient pressure and feeds at 85 F. The cooling tower can process the water from 115 F to 85 F [5]. Steam is supplied from cogeneration units at 150 psi superheated to 400 F. The results are obtained from ASPEN using the short-cut method for heat exchangers. Since both the reactor design project and the separations project select NRTL as the thermodynamic package of choice, NRTL is also used for this project [3,4].

Exchanger	Inlet/Outlet Temp (F)	Duty (MMBtu/hr)	Cooling Water (gal/min)	Steam (lb/hr)	Process pressure (psi)
Reactor feed cooler	320.3/270	9.4	640	--	230
Reactor effluent cooler	490/380	23.4	1677	--	230
Column feed cooler	218.3/130	32.3	2286	--	65
Condenser	114.2/114.2	52.6	4000	--	50
Reboiler	292.8/292.8	72.2	--	81,600	60

From the information generated in constructing composite curves, only one of the cooling streams has high enough temperatures to be incorporated for efficient heat exchange in the reboiler. The design in Figure 3 takes advantage of the only usable heat source, the reactor effluent, in order to reduce the total amount of steam needed to maintain the 72.2 MMBtu/hr heat duty for the column. This integrated plan utilizes a double reboiler consisting of two heat exchangers, one using the reactant effluent to vaporize a fraction of the bottoms of the column and a second using steam to provide the remaining heat for the rest of the bottoms flow.

The exchangers are connected in parallel for flexibility if the need for maintenance or expansion arises.

5.0 OPERATING CONDITIONS AND SPECIFICATIONS

The integrated layout still contains the same operating conditions specified in Table 1 for the reactor feed cooler, the column feed cooler, and the condenser. The need for cooling water to cool the reactor effluent stream is eliminated as is 33% of the steam supplied to the reboiler. Table 3 gives the process flows for each exchanger. Table 4 supplies the new process flow conditions for the double reboiler.

Compound	Reactor feed cooler (vapor)	Reactor effluent (vapor)	Column feed cooler (mixed)	Condenser (vapor)	Steam reboiler (liquid)
Ethylene	8352	6240	--	--	--
Oxygen	1994	--	--	--	--
Ethylene oxide	--	1738	1738	5993	--
Process water	--	750	4040	1	5700
Carbon dioxide	727	1747	--	--	--
Methane	6263	6263	--	--	--
Acetaldehyde	--	--	0.599	0.01	--
Formaldehyde	--	--	1.044	1	--

Reboiler	Column Bottoms	Heating Medium
Steam exchanger	5700 lbmol/hr	150# steam, 400 F	55,000 lb/hr
Process exchanger	2730 lbmot/hr	Reactor effluent	19,200 lbmol/hr

The two-exchanger reboiler for the ethylene oxide/water column consists of a steam exchanger and a two-stream process exchanger. In order to maintain the molar reboiler ratio of 1.07, the total duty 72.2 MMBtu/hr is required. The bottoms fluid enters the exchangers as 8430 lbmol/hr of liquid at 292.8 F where approximately 51.8 mol% of the liquid feed is vaporized before returning to the last stage of the column. The reactor effluent is able to provide enough heat to 32.4 wt% of the total feed. The remaining 67% of the bottoms feed is partially vaporized in the steam exchanger.

6.0 SHORT-CUT EXCHANGER DESIGN

ASPEN is implemented for short-cut design of all the exchangers excluding the process reboiler. Table 5 includes the total surface area for heat transfer and the overall heat transfer coefficient for each exchanger as produced by ASPEN. By inspection, the condenser has an enormous surface area required. This is attributed to the small temperature difference between the cooling water (85-105 F) and the condenser (114 F), thus producing a low driving force for heat transfer. Possible improvements are to implement an air-cooled heat exchanger, a chiller with refrigerant , or the use another process stream available in the preliminary separation section at a lower temperature for heat removal.

Exchanger	Total surface area (ft2)	U (Btu/hr/ft2/F)
Reactor feed cooler	302	150
Column feed cooler	2792	155
Condenser	38,531	150
Steam reboiler	???	???

7.0 RIGOROUS DESIGN OF PROCESS REBOILER

ASPEN is used for the rigorous design of the process reboiler. A full specification sheet is detailed in Table 6. The exchanger has a total surface area of 1570 ft² for heat transfer with an overall heat coefficient of 109.3 Btu/hr/ft²/F (dirty) and 131.6 Btu/hr/ft²/F (clean).

Service of unit Process reboiler Item No. 3
Size 30-192 Type BEM Position Horizontal
Surface per unit 1570 sq. ft. Shells per unit 1 Surface per shell 1570 sq. ft.
No. of units 1 Shell arrangement Engrs. GAA, LTA, CYH
PERFORMANCE OF ONE UNIT
	Shell Side	Tube Side
Fluid circulated	water	ethylene oxide
Total fluid entering lb/hr	49,200	4,306,000
Vapor lb/hr		4,306,000
Liquid lb/hr	49,200
Steam lb/hr
Non-condensables lb/hr
Fluid vaporized of condensed lb/hr	28,485
Steam condensed lb/hr
Gravity-liquid S.G. oAPI
Viscosity-liquid CP.@avg. T.
Molecular Weight-Vapors	18	26
Enthalpy-Btu/lb	-6,590	-810
Temperature in F	292.8	490
Temperature out F	292.8	380
Operating pressure lb/in2 abs.	60	230
Number of passes Per shell	One	One
Velocity ft/sec
Pressure drop (max. allow.) lb/in2	0.4 (1.0)	1.8 (5)
Fouling resistance	0.0005	0.001
Heat exchanged-Btu/hr 23,400,000 MTD (corrected) 136 F
Transfer rate-service 109.3 Btu/hr/sq.ft./F
CONSTRUCTION-EACH SHELL
Design pressure lb/in2	90	260
Test pressure lb/in2	135	390
Design temperature F	350	520
Tubes TP-304-SS No. 250 O.D. 1.5" BWG 13 Length 16 ft. Pitch 30o triang
Shell C. Steel Floating head cover
Channel: Channel cover
Tube sheets-stationary S.S. Floating
Baffles-segmental 8 Tube supports
Type joints-shell Gasket and Packed Tubes
Gaskets-shell Fltg. hd. Channel
Connections-shell-in 3" Out 10" Series
channels-in 20" Out 20" Series
Corrosion allowance-shell side Tube side
Code requirements ASME Code Stamped TEMA Class C
Weights-each shell and bundle Bundle only Full of water
	Unit Price	Amount
Remarks:	$31,900	$31,900

7.1 DETERMINATION OF HEAT EXCHANGER TYPE

A shell-and-tube heat exchanger was chosen as the design basis for the process reboiler because these types of heat exchanger are most commonly used for two fluid heat transfer. Generally, reboilers are designed as kettles or thermosiphons [6]. However, because ASPEN does not have these designs available, a BEM one-pass heat exchanger with an expansion joint was opted. The exchanger is a fixed tubesheet exchanger with removable heads. Because of the temperature difference desired in the reactor effluent of 110 F, an expansion joint is necessary to avoid cracking of the exchanger due to stress and strain.

A one-pass arrangement was selected because of the high flow rate encountered in the reactor effluent. For reasons discussed in the next section, the reactor effluent is allocated to the tube-side of the exchanger. If a two-pass exchanger were selected, the velocities in the bend of the tubes could accelerate corrosion and cracking of the tubes that would result in tube leakage. Replacing tubes, especially those tubes inside the bundle, in a U-tube exchanger can be difficult if not impossible. Also, bundle replacement can be costly. One-pass exchangers are more easily cleaned and repaired.

7.2 ALLOCATION OF FLUID

As mentioned earlier, the reactor effluent is allocated to the tube-side for a variety of reasons. The effluent is 100-200 degrees higher than the column bottoms. By placing the effluent in the tubes as opposed to the shell, less insulation will be needed around the exchanger, thus resulting in a safer environment for operators. In addition, ethylene oxide is corrosive and should be restricted to the tube-side. Finally, the high flow rates encountered would cause larger pressure drops if the fluid is placed in the shell of the exchanger.

Placing the column bottoms in the shell also has advantages. Composed of mostly pure water, shell-side allocation of the bottoms fluid eliminates the need to have a stainless steel heat exchanger. Instead, only stainless steel tubes are needed, thus reducing the base cost of the exchanger. The column bottoms runs at a considerably lower pressure (60 psi) than the reactor effluent (230 psi). This will allow the pressure rating for the shell to be lower than the tube-side pressure, thus saving additional cost in shell thickness. Also, since the bottoms is undergoing a partial phase change, the shell provides more room for phase change.

7.3 PRESSURE DROP

The main problem to eliminate in any heat exchanger design is pressure drop. Distillation columns are extremely pressure sensitive so it is vital to minimize the pressure drop in the bottoms stream. A 0.4 psi pressure drop was achieved by two adjustments. The baffle cut was enlarged to 45% from 40%, a design guideline. In addition, correctly sized inlet and outlet nozzle diameters of 3" and 10" respectively allowed for the partial phase change inside the exchanger. Eight single segmental baffles support the tubes for a baffle-to-baffle length of 21". This length will provide enough support for the tubes as well as minimize turbulent buffeting.

On the tube-side, the reactor effluent is not as pressure drop limited. This effluent is upstream of preliminary separation processes that occur at lower pressures. A 1.8 psi pressure drop was achieved by increasing the nominal tube size from commonly used 1.0" tubes to 1.5" tubes to allow more room for flow. Although a larger tube size will require a higher cost per tubes, the number of tubes needed were reduced by using 1.5" tubes from 433 1.0" tubes to 250 1.5" tubes.

7.4 SIZING

A majority of the base cost of an exchanger comes from the type of shell used, namely the shell diameter. Hence, a major point in heat exchanger design comes with using the smallest shell diameter possible, without having an infinitely long exchanger, and with preserving a desirable pressure drop and acceptable tube-side and shell-side velocities. This exchanger has a 32" diameter, less than three feet, and has a tube length of 16 feet. The smaller 30" exchanger was eliminated based on high tube-side velocities from having a fewer number of tubes to accommodate the required reactor effluent flow.

Shell Diameter	No. of Tubes	Tube Length	Tube-side Pressure Drop
30"	200	18 feet	2.9 psi
32"	250	16 feet	1.8 psi
34"	290	16 feet	1.5 psi

8.0 PROCESS ECONOMICS

Exchanger	Cooling Water * (gal/min)	Steam** (lb/hr)	Yearly cost***	Installed Cost
Reactor feed cooler	640	--	$ 38,700	$ 11,000
Column feed cooler	2286	--	$ 140,000	$ 44,500
Condenser	4000	--	$ 242,000	$ 394,000
Process reboiler	--	--	--	$ 31,900
Steam reboiler	--	55,000	$ 3,086,000	$ 60,100
Total			$ 3,506,700	$ 541,500

* cooling water cost based on $0.12/Kgal

** steam cost based on $6.68/Klb, 150#

*** calculations based on a 350 day operating year

The estimated yearly utility cost with heat integration is approximately $3.5 million. Using the original design layout without heat integration resulted in a yearly utility cost of $5.12 million. Thus, $1.6 million are saved per year. The majority of these savings is due to the 33% decrease in the use of steam.

The total cost generated by ASPEN for the double reboiler system is $93,000, $31,900 for the reactor effluent exchanger and $61,100 for the steam exchanger. The biggest capital cost is the condenser. As explained earlier, other alternatives for the condenser may reduce capital and utility costs.

9.0 ALTERNATIVES

Due to the nature of the operating temperatures of each hot or cold stream, the only heat exchanger network possible is the one provided in this study. However, the actual heat exchanger design for the double reboiler should not be limited to a BEM exchanger. As mentioned earlier, the use of ASPEN for heat exchanger design is limited since ASPEN does not provide options for the kettle, 'K', reboiler.

The use of a more rigorous heat exchanger rating and design program is recommended, such as HTRI's ST (sensible heat transfer), CST (latent heat transfer), or RK1 (reboiler design) packages. HTRI is the most commonly used design package for heat exchangers in industry. It has an excellent physical property databank and provides a more in depth analysis of heat exchanger fluid flow. HTRI programs will consider low-induced vibration, flow fractions, turbulent buffeting, and thermal resistances not available in ASPEN. HTRI can also perform rigorous film calculations to better predict phase change scenarios [7].

10.0 CONCLUSIONS

The heat integration for the four hot streams and one cold stream in this ethylene oxide facility involves using the hot reactor effluent stream to partially supply heat to the reboiler. The reboiler of the column is a double exchanger reboiler, one using reactor effluent as the heating medium and the other using 150 psi steam superheated at 400 F. A BEM one-pass exchanger with an expansion joint is used for the process reboiler. The total surface area of 1570 ft² is achieved with a 32" shell diameter with 250 tubes of 1.5" nominal diameter.

The total utility cost is $3.5 million per year, $420,000 for cooling water and $3.1 million for steam. Approximately $1.6 million per year is saved by integrating the reactor effluent to supply heat to the reboiler. Total purchase cost for the double reboiler is $93,000.

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[2]. http://www.owlnet.rice.edu/~ceng403/

[3]. Betton, S. "Ethylene Oxide Purification." CENG 403 Project 2 Report, Fall 1997.

[4]. Coombs, J. "Celanese Clear Lake Ethylene Oxide Reactor Revamp." CENG 403 Project 1 Report, Fall 1997.

[5]. GPSA Engineering Data Book. Gas Processors Suppliers Association, 1987.

[6]. Branan, C. Rules of Thumb for Chemical Engineers. Houston, Texas: Gulf Publishing Company, 1994.

[7]. Sensible Heat Transfer and Condensation, Training Course on the Design and Rating of Heat Exchangers, Amoco Corporation.

[8]. Gerhartz. W. et. al. Ullmann's Encyclopedia of Industrial Chemistry. 5th edition. Federal Republic of Germany: VCH, 1985.