Heat-Exchanger Bypass Control - American Chemical Society

Ind. Eng. Chem. Res. 2011, 50, 965–973

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Heat-Exchanger Bypass Control William L. Luyben* Department of Chemical Engineering, Lehigh UniVersity, Bethlehem, PennsylVania 18015, United States

Many methods are used for controlling temperatures in heat exchanger systems. Direct manipulation of the flow rate of either the hot or the cold stream is most often used when that stream is a utility (cooling water, steam, hot oil, or refrigerant). When the flow rates of both streams are set by process requirements, heatexchanger bypassing is widely used. A portion of one of the streams (either hot or cold) is sent through the heat exchanger, and the remainder is bypassed around the exchanger. The temperature of the mixed steam is controlled by valves in each path. This system provides very tight temperature control, since the dynamics of blending a hot stream and a cold stream are very fast. This paper explores the design and control issues when heat-exchanger bypassing is used. The design optimization variables include the fraction of bypassing, the area of the heat exchanger, and the design pressure drops over the control valves. Dynamic rangeability requires heat-transfer rates to be adjustable over a wide range. As expected, results demonstrate that a larger area and more bypassing improve the ratio of maximum-to-design heat transfer rates, which is important for dynamic controllability. An unexpected, counterintuitive result is that control valve design pressure drops have little effect on rangeability in heat-exchanger bypass systems in which variable-speed pumps are used to maintain total flow rates. 1. Introduction Heat exchangers are undoubtedly the most numerous industrial unit operation. The temperatures or phases of process streams must be changed to achieve process objectives. Hot sources of heat (steam, hot oil, or molten salt) and cold sinks of heat (cooling water, air, or refrigeration) are sent through a variety of different types of heat exchangers to heat, cool, vaporize, or condense a process stream. Process-to-process heat exchangers are widely used to reduce utility consumption, which lowers energy costs. Most heat exchangers require some type of control system to achieve process objectives. When a utility stream is involved, its flow rate is usually manipulated to control a process temperature. If phase changes are occurring in the heat exchanger (vaporization or condensation), the control objective may be to maintain vessel pressure or liquid level instead of temperature. Bypassing is sometimes used in heat exchangers in which low velocities of cooling water can result in fouling. The cooling water flow can be set at a fixed high flow rate, and the hot stream can be bypassed. When the heat exchanger involves a hot process stream and a cold process stream, manipulation of either for control purposes is usually not possible, since their flow rates are set by upstream or downstream process objectives. In this situation, heat-exchanger bypassing is frequently employed. Sometimes a portion of the hot stream is bypassed around the heat exchanger. In other situations, a portion of the cold stream is bypassed. The common heat-transfer heuristics is to bypass the stream whose outlet temperature is to be controlled. The blending of the portion of the process stream going through the heat exchanger with the portion being bypassed affects the mixed temperature almost instantaneously, so tight temperature control of the blended stream can be achieved. There are secondary, slower changes that occur as the flow rate through the heat exchanger affects the outlet temperature, but the fast * To whom correspondence should be addressed. Tel: 610-758-4256. Fax: 610-758-5057. E-mail: [email protected].

blending of hot and cold streams quickly compensates for these slow disturbances. Like most design situations, the design of heat-exchanger bypassing systems involves trade-offs. Increasing the fraction of the stream that is bypassed permits larger changes in the heat-transfer rates, which means wider dynamic rangeability to handle changing process requirements, but increasing bypassing requires a larger heat exchanger (more area) because the differential temperature driving forces are smaller than they would be if all the stream were sent through the heat exchanger. This situation is good example of the interaction (conflict) between design and control. Another example that also comes into play in bypass heatexchanger design is the issue of pressure drop. The design pressure drop through the heat exchanger is usually established by heuristics to give reasonable heat-transfer coefficients. The higher the pressure drop, the higher the velocity, the larger the film coefficients, the smaller the required heat-transfer area and the lower the capital investment. But higher pressure drops mean higher-head pumps or compressors and higher energy requirements in motors or turbines. This issue is important in heat-exchanger bypass systems because sometimes these systems are designed with a control valve in the bypass line but with no valve in the line going through the heat exchanger (see the top flowsheet in Figure 1A). The pressure drop over the bypass valve must be equal to the pressure drop through the heat exchanger. Rangeability is usually very limited in this situation, particularly when the bypass valve is opened wide because there is still flow through the heat exchanger. Much wider rangeability is achieved by having two control valves, one in the bypass line and one in the line through the heat exchangers (see the bottom flowsheet in Figure 1A). The flow rates through the two paths can be much more widely adjusted, which provides a larger ratio of maximum-to-minimum heat transfer rates. Remember, however, that in most situations, the total process flow rate is set by other process requirements. An upstream variable-speed pump is used in some liquid systems to maintain the total required flow rate. Alternatively, one of the two valves can control the total flow rate while the other

10.1021/ie1020574  2011 American Chemical Society Published on Web 12/16/2010

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Ind. Eng. Chem. Res., Vol. 50, No. 2, 2011

Figure 1. (A) Heat-exchanger bypass with and without valve in heatexchanger line. (B) Heat-exchanger bypass with fixed-speed pump.

controls the temperature, as shown in Figure 1B. We will return to this system later in the paper to see the effect of valve design pressure drop on rangeability. The action of the two valves is different. As shown in Figure 1A, if sending all the flow through the heat exchanger is the fail-safe situation (cooling of this stream being done in the heat exchanger), the valve in the heat-exchanger line should fail wide open (air-to-close, reverse action) and the bypass valve should fail closed (air-to-open, direct action). Of course, putting a valve in the heat-exchanger circuit produces more pressure drop and increases motor work requirements. These issues are all fairly obvious, but little quantitative analysis has appeared in the literature of the several design factors involved. Little guidance is available in setting up a heatexchanger bypass system. That is the purpose of this paper. The specific application is one in which a circulating coolant stream is used to cool a chemical reactor. The coolant stream is itself cooled in a heat exchanger that has a bypass. Most control textbooks show various heat exchanger configurations and control structure alternatives, but little quantitative analysis is provided. McMillam and Toarmina1 and Riggs and Karim2 discuss a number of piping and control configurations, including bypassing, and point out some of the control advantages of the bypass configuration. Bequette3 shows the simple bypass structure with only a valve in the heat exchanger line (see the top graph in Figure 1A). Balchen and Mumme4 discuss the bypass system shown in Figure 2A. The example uses a cold utility stream to control the exit temperature of a process stream. They show only a valve in the bypass line, which is used to control the outlet temperature of the blended stream and provides tight temperature control. On top of this basic control structure, they add a “parallel” controller whose input signal is the signal sent to the bypass valve and whose output signal manipulates the position of a valve in the cold utility stream. The idea is to keep the bypass valve at ∼50% open under steady-state conditions so that tight temperature control can always be achieved by the blending of hot and cold streams. This type of structure is more descriptively termed “valve-

Figure 2. (A) Valve position control. (B) Three-heat-exchanger system with bypassing.

position control” by Shinskey,5 who demonstrated its application to minimum pressure operation of distillation columns. A quantitative study of a bypass system is given in Seider et al.,6 who examined the three-heat-exchanger system shown in Figure 2B. A bypass was added to permit the control of all three temperatures. Resiliency analysis was applied to guide in the pairing of controlled and manipulated variables in the three temperature loops. The effects of two design parameters (heatexchanger area and percent bypassing) were explored. Increasing either parameter was shown to improve rangeability (handle larger disturbances while maintaining temperature set points). Dynamic simulations were presented to demonstrate control performance. Seider et al. did not study the effect of valve design pressure drop on rangeability. In this paper, a reactor/exchanger process is considered in which heat-exchanger bypassing is used in a circulating coolant system. 2. Process Studied Figure 3 shows the flowsheet of the process considered in this paper as a numerical example. The conditions and parameters shown correspond to one particular case and will vary for different design choices. The exothermic liquid-phase reaction of aniline with hydrogen to form cyclohexyl amine (CHA) is carried out in a jacketed CSTR reactor. C6H7N + 3H2 f C6H13N R ) VRkCACH

(1)

where R is the overall reaction rate (kmol/s), VR is the reactor liquid volume (m3), k is the specific reaction rate (m3 kmol-1 s-1), CA is the concentration of aniline in liquid phase (kmol/ m3), and CH is the concentration of hydrogen in liquid phase (kmol/m3). k ) 2 × 104e-11,111 (cal/mol)/RT

(2)


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Figure 3. Circulating water system with heat-exchanger bypassing.

The volume of the reactor is 34 m3, the operating temperature TRX is 400 K and the pressure is 20 atm. The fresh feed of aniline is 50 kmol/h. An excess of hydrogen is fed (200 kmol/ h) to maintain a high conversion of aniline. The exothermic reaction requires that 516.2 kcal/s of heat be removed by the circulating treated-water system flowing through the jacket at a temperature TJ of 350 K. The circulation rate, FJ, is high enough so that a uniform jacket temperature can be assumed. The vessel aspect ratio is 2, so the diameter is 2.8 m and the jacket heat-transfer area is 50.9 m2. An overall heat-transfer coefficient of 0.885 kW K-1 m-2 is assumed. Jacket volume is 10 m3. The circulating water is pumped through two parallel paths. One flow, FHX, is fed to a water-cooled heat exchanger. The other flow, FBY, flows around the heat exchanger. The two streams are combined and circulate back to the jacket. There are two control valves, one in each line. The valve in the heatexchanger line is air-to-close (fails open for maximum cooling). The bypass valve is air-to-open. The total circulation flow rate is 6000 kmol/h with 3714 kmol/h bypassing the heat exchanger, so the fraction bypassed is 62%. As we demonstrate in the following sections, the fraction bypassed is uniquely set in this system by the total circulation flow rate and the heat-transfer area in the heat exchanger. The flow rate of the cooling-tower water on the cold side of the heat exchanger is set so that the cooling water exit temperature is 315 K with a 305 K inlet cooling water temperature. The required water flow rate to remove 522.0 kcal/s in the heat exchanger is 9670 kmol/h (770 gpm). The heat duty in the heat exchanger is slightly larger than in the reactor because of the pump work. Pump motor work is 22 kW in this case with a 5 atm pump head. With a 1 atm pressure drop over the heat exchanger, the design pressure drop over the bypass valve is 5 atm and over the heat-exchanger valve is 4 atm. The case shown in Figure 3 has a heat exchanger area of 200 m2, a total coolant circulation of 6000 kmol/h, and a bypass control valve design pressure drop of 5 atm. Other cases are explored in the following sections to see their effects on rangeability. 3. Advantages of Bypass Configuration The circulating water system has several advantages in chemical reactors over the alternative of adding cooling-tower water directly

into the jacket for once-through operation. First, the fairly high reactor temperature (400 K) would result in high water exit temperatures even at normal loads, which could result in corrosion problems. At low loads with small cooling water flow rates, the water could even boil. Second, the dynamics of the once-through system are much slower than the circulating system. The water in the jacket must be displaced by the incoming water before jacket temperatures change. In the circulating system, the hot bypass flow and the cold heat-exchanger flow are mixed and immediately change the blended temperature. The high circulation rate changes the jacket temperature very quickly. So reactor temperature control is much tighter in the circulating system. However, the downsides of the circulating system are that a pump is required, a larger heat exchanger is needed, and more cooling-tower water is consumed. These make the circulating system more expensive. In addition, the maximum heat-removal rate is smaller, since there are two heat-transfer resistances (one through the reactor jacket and one through the tubes in the heat exchanger) between the high-temperature source and lowtemperature sink. Is the improvement in dynamic control worth the added expense? In many reactor systems, the answer is a definite yes. Reactor temperature control is usually critically important to maintain both stability and selectivity. Temperature deviations can lead to runaways. Not maintaining optimum reactor temperature can lower yields, produce undesirable byproducts or degrade catalyst. Potential corrosion or environmental problems can also become issues. The reactor example considered in this paper is just one of many types of applications of heat-exchanger bypassing. Feedeffluent heat exchanges with bypassing are widely used. Condensers and pumparounds on distillation columns sometimes use bypassing to adjust heat removal. Heat-integrated systems frequently use heat-exchanger bypassing. 4. Bypass Design Issues Now that the process has been established, there are several decisions that must be made to complete the design: (1) What fraction of the total circulation should be bypassed at design conditions? (2) How large should the heat exchanger be? (3) How much pressure drop should be taken over the control valves at design conditions? Thus, there are three design optimization variables.

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Figure 4. Control structure with heat-exchanger bypassing.

If we considered only steady-state economics, these decisions would be easy. Nothing should be bypassed. This would give the smallest circulation rate, smallest pump, and least heatexchanger area. However, good dynamic control and rangeability are vital in this reactor system, so we must consider how the selection of the three design optimization variables impact control. The approach is to select a range of values of these three variables and see what rangeability in heat removal they provide. Rangeability is judged by the maximum possible feed flow rate to the reactor while still maintaining the desired 400 K reactor temperature. Of course this maximum heat-removal occurs when the bypass valve is completely shut (no bypass flow, everything going through the heat exchanger for maximum cooling). 5. Simulation Setting up this process for dynamic simulation is not a trivial job. Aspen simulation software is used. The steady-state process is configured in Aspen Plus and then exported into Aspen Dynamics to evaluate dynamic performance. 5.1. Steady-State Simulation. The RCSTR model is used in Aspen Plus with the Direct heat-transfer option selected. The liquid level in the reactor is set at 80% by selecting the setup option of reactor. Reactor temperature is 400 K at design. The reactor jacket is simulated using a Flash2 model with a heat stream input coming from the reactor. Jacket temperature is 350 K at design conditions with an aniline feed flow rate of 50 kmol/ h. At higher throughputs, the jacket temperature must decrease to increase heat removal from the reactor. The valve on the vapor line in the Flash2 model used for the jacket is completely closed. The heat exchanger uses a HeatX model with counter-current flow of cooling-tower water and circulating water. An overall heat-transfer coefficient of 0.85 kW K-1 m-2 is used in the heat exchanger. 5.2. Dynamic Simulation. The Aspen Plus file is exported into Aspen Dynamics as a pressure-driven simulation. The heattransfer rate between the reactor and the jacket is calculated using the relationships given in eq 3. QJ ) UAJ(TRX - TJ) QRX ) -QJ

(3)

The Aspen Dynamics Flowsheet Equations capability is used to set up these relationships during the dynamic simulations.

Both reactor temperature, TRX, and jacket temperature, TJ, are changing with time. 5.3. Control Structure. The control scheme used in this system is shown in Figure 4. The following loops are set up using conventional PI controllers: 1. Aniline feed is flow-controlled. 2. Hydrogen feed is flow-controlled with the flow controller set point ratioed to the aniline feed flow rate. 3. Reactor level is controlled by manipulating liquid product using a proportional controller with KC ) 4. 4. Reactor pressure is controlled by manipulating vapor product. 5. Jacket temperature is controlled in a cascade control structure by manipulating both the bypass and the heatexchanger valves. The jacket temperature controller is reverse acting. A 1-min deadtime is inserted in the temperature measurement. With the jacket temperature controller on automatic (not cascade), relay-feedback tests and Tyreus-Luyben tuning are used to find the gain and integral time. These controller settings change from case to case as process parameters (heat-exchanger area and circulation rate) are varied. 6. Reactor temperature is controlled by manipulating the set point of the jacket temperature controller in this cascade structure. The reactor controller is reverse-acting. A 1-min deadtime is inserted in the temperature measurement. With the jacket temperature controller on cascade and the reactor temperature controller on automatic, relay-feedback tests and Tyreus-Luyben tuning are used to find the gain and integral time of the reactor temperature controller. These controller settings change from case to case as process parameters (heatexchanger area and circulation rate) are varied. 7. The total circulation flow rate is flow controlled by manipulating pump speed. Note that a variable-speed pump is assumed in the simulation. 8. The flow rate of cooling-tower water fed to the heatexchanger is fixed. Note that the two valves in the circulation loop have opposite action. All valves in Aspen Plus are direct (AO). In Aspen Dynamics the action can be selected, so the valve in the heatexchanger line is specified to be reverse (AC). Their size depends on their design flow rates, pressure drops, and fraction open. All valves are assumed to be 50% open at design. Valve pressure drop depends on the pump head specified at design. Aspen default pump characteristics are used. Flow rates depend


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Figure 5. Effect of area on maximum heat removal.

on the total circulation rate and heat exchanger area specified in a given case. 6. Effect of Design Parameters A range of circulation rates, heat-exchanger areas, and valve design pressure drops are explored. We are interested in finding the maximum heat removal in each case or, equivalently, the maximum feed flow rate to the reactor while still maintaining the 400 K reactor temperature. The maximum heat removal is obtained when the bypass valve is completely shut and the heat-exchanger valve is wide open. The total circulation goes through the heat exchanger. The dynamic simulation is used to find the maximum. The jacket temperature controller is placed on manual, and its output signal is set to zero percent, which shuts the bypass valve completely and opens the heat-exchanger valve completely. Then the control structure is changed so that the reactor temperature controller’s output signal changes the set point of the aniline flow controller. The aniline flow controller is put on cascade. The final steady state when reactor temperature is driven to 400 K gives the maximum feed flow rate and heat removal. 6.1. Heat-Exchanger Area. Figure 5 shows the effect of using heat exchangers of different sizes for two different circulation flow rates (4000 and 6000 kmol/h). As the area increases, the maximum heat removal and maximum feed flow rate increase, as we would expect. Higher circulation rates give higher heat removal and maximum feed flow rates. However, the curves flatten out for areas above about 200-300 m2. The point of diminishing returns is somewhere in this range. Heat-transfer area represents capital investment. Investing capital can usually be justified to save energy and, particularly, to improve product yield. The 200 m2 heat exchanger shown in Figure 3 costs about $ 230 000 (Douglass7). Increasing to 300 m2 raises the price to about $300 000. The dynamic rangeability required of the process has to be considered to decide the appropriate size. 6.2. Coolant Circulation Flow Rate. Figure 6 shows the effect of designing with different circulation flow rates. Heatexchanger area is fixed at 200 m2 for these results. The top graph shows that increasing circulation increases the maximum heatremoval rate.

The design heat-removal rate is 516.2 kcal/s. Designing for a circulation flow rate of 6000 kmol/h gives a maximum heatremoval rate of 643.6 kcal/s. Thus, the ratio of maximum-todesign heat removal rates is 1.25 for this design case. The ability to remove at least 25% more heat in reactor systems is often required, so this design is not overly conservative. Setting the total circulation flow rate establishes the fraction of bypassing for fixed heat exchanger area and exit temperature. The flow rate through the heat exchanger is the same for all circulation rates, since the inlet and exit temperatures to the heat exchanger and the heat duty are the same in all cases. Therefore, the bypass flow rate changes directly with the total circulation flow rate. The lower graph in Figure 6 shows that the fraction of the total circulation that is bypassed (FBY/FJ) over the range of total circulation flow rates. Notice that for a 6000 kmol/h circulation flow rate, the fraction bypassed at design is 62%. More circulating coolant is bypassed than goes through the heat exchanger under design conditions. Trying to convince a project engineer that this much bypassing is required can take a lot of quantitative arguments. The role of dynamic simulation in this type of analysis is invaluable. 6.3. Control Valve Design Pressure Drop. A. Circulating Water System. The third design optimization variable is valve pressure drop at design conditions, which is established by the pump head specified. In all the results shown so far, the pressure drop over the control valve in the heat exchanger line has been 4 atm. The pressure drop over the heat exchanger is assumed to be 1 atm, so the pressure drop over the bypass valve is 5 atm. A pump head of 5 atm is therefore required, which gives a motor work of 22 kW with a circulation rate of 6000 kmol/h. At design conditions, only a portion of the circulation flows through the heat exchanger. This portion decreases as total circulation flow rate is increased, as discussed in the previous section. For all circulation flow rates, 2289 kmol/h go through the heat exchanger with an assumed pressure drop of 1 atm. When the process is at maximum heat-removal conditions, the entire circulation goes through the heat exchanger with nothing flowing through the bypass. The pressure drop through the heat exchanger increases as the square of the flow rate, since turbulent flow exists in this unit. The result is a very large increase in

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Figure 6. Effect of circulation rate. Table 1. Control Valve Design Pressure Drop Circulating Coolant design pressure drop in HX line (atm) pump head (atm) motor work (kW) valve size coefficient (m3)1.5 g0.5 s-1 atm-0.5 design QHX (kcal/s) maximum FA (kmol/s)

1 2 9.08 32 920 518.6 61.5

2 3 13.6 23 250 519.1 61.5

4 5 22.7 16 410 522.0 61.5

heat exchanger pressure drop from 1 atm up to 6.9 atm. The pressure drop over the control valve in the heat exchanger line also increases, even though the valve is wide open. The control valve that has been designed for a 4 atm pressure drop at design conditions (2289 kmol/h and 50% open) gives a pressure drop of 6.9 atm at maximum conditions (6000 kmol/h and 100% open).The pump head must increase to provide these increased pressure drops, increasing from 5 to 13.8 atm by increasing motor speed. Pump work increases linearly with head since flow rate is constant. Designs with control valve pressure drops of 2 and 1 atm were evaluated to see if rangeability was affected. In conventional piping systems, increasing valve design pressure drop permits more flow to pass through the valve when wide open, so rangeability is increased; however, results showed that there was almost no effect on rangeability. Control valve size increases and pump work requirements decrease as valve pressure drop is decreased. Table 1 gives results for three designs with different valve design pressure drops. B. Simple Heat-Exchanger Bypass System. The result found in the circulating-coolant bypass system is unexpected and counterintuitive. To see if this result applies only to the circulating-coolant bypass system or is a general result for other bypass system, a simple heat-exchanger bypass system is simulated. Figure 7 gives the flowsheet conditions. A hot process stream at a flow rate of 100 kmol/h and a temperature of 400 K is cooled to a mixed temperature of 340 K by sending 76.68% through a 11.52 m2 heat exchanger and bypassing 23.32%. The exit temperature of the hot stream out of the heat exchanger is 320 K. Cooling water at a flow rate of 643.9 kmol/h is needed to remove the 60 960 kcal/s of heat with cooling water inlet and outlet temperatures of

Figure 7. Simple heat-exchanger bypass system. Table 2. Simple Bypass System pump discharge pressure (atm) bypass valve pressure drop (atm) bypass valve COmax (m3)1.5 g0.5 s-1 atm-0.5 heat-exchanger valve pressure drop (atm) heat-exchanger valve COmax (m3)1.5 g0.5 s-1 atm-0.5 maximum throughput (kmol/h)

7 6 953.5 5.5 3408 134.8

10 3 670.3 2.5 2302 134.8

305 and 320 K, respectively. An overall heat-transfer coefficient of 0.57 kW K-1 m-2 is assumed. Pressure drop through the heat exchanger is assumed to be 0.5 atm. The pressure of the mixed process stream is 4 atm. The pressure drops of the two valves depend on the pump head. The valves are 50% open at design. A flow controller maintains constant total process flow through the system by manipulating motor speed. Table 2 gives results for two designs with different pump heads. As design pump head is increased, valve pressure drops increase and valve sizes (COmax) decrease. The maximum throughput while still achieving the 340 K process exit temperature is found when the bypass valve is closed completely and the heat-exchanger valve is opened completely. For both the large and small control valve pressure drop designs, the set point of the process flow controller can be increased from 100 to 134.8 kmol/h and still hold the outlet process temperature at the 340 K set point. Thus, the effect of valve design pressure drop in bypass systems in general appears to be negligible.


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Figure 8. 20% increase with and without flow control of circulation.

Figure 9. Circulation 4000 and 6000 kmol/h; 20% increase; 200 m2.

It is important to remember that the flow rate of the total stream is controlled in both of these systems. The set point of the total flow controller would be set to achieve some upstream or downstream process objective. For example, the liquid level in an upstream vessel could by controlled by manipulating this flow rate. C. Bypass System with Fixed-Speed Pump. As briefly discussed earlier and shown in Figure 1B, there are many liquid systems that do not use variable-speed pumps. In this situation, the upstream liquid level must be controlled by one of the valves.

If we consider the same system shown in Figure 7, more flow goes through the heat-exchanger than through the bypass, so process control wisdom (Richardson’s Rule)8 says that the valve in the heat-exchanger line should be used to control level. An air-to-open valve would be used so that the level in the upstream vessel is not pumped dry in the event of instrument failure. The bypass valve must be used to control the mixed temperature. It would also be air-to-open so that the hot material is not bypassed around the cooler in an emergency. In the simulation of this example, a fixed-speed pump is used with Aspen default pump characteristics.

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Figure 10. Circulation 4000 and 6000 kmol/h; 20% decrease; 200 m2.

Figure 11. Circulation 4000 and 6000 kmol/h; 20% increase; 300 m2.

The effect of valve design pressure drop in this type of system is exactly what we would expect. The higher the design pressure drop, the larger the rangeability. For example, if the pressure drop over the valve in the heat-exchanger line is designed to be 3 atm, the flow rate through the system can be increased from 100 to 120 kmol/h. The valve in the heat-exchanger line is wide open. The bypass valve is 35.8% open, and the 340 K mix temperature is maintained. If the pressure drop over the valve in the heat-exchanger line is designed to be 2 atm, the flow rate through the system can be increased from 100 to 119.3 kmol/h. The valve in the heatexchanger line is wide open. The bypass valve is 40.06% open, and the 340 K mix temperature is maintained. If the pressure drop over the valve in the heat-exchanger line is reduced to 1 atm, the flow rate through the system can only be increased from 100 to 114.5 kmol/h with the valve in the heat-exchanger line wide open. The bypass valve is 45.18% open to maintain the 340 K mix temperature. These results demonstrate that in this type of bypass system with a fixed-speed pump, more

pressure drop over control valves at design conditions provides a modest increase in rangeability. 7. Dynamic Results Dynamic simulations are run to test the performance of the process and the control structure for the reactor/circulating coolant process. 7.1. Circulation Flow Control. First, let us see what the rangeability is when the circulation flow rate is not controlled. The Aspen default pump characteristics are used, which assume a fixed-speed pump. Therefore, as the opening of the control valve in the bypass line is reduced by the jacket temperature controller, pump discharge pressure increases, and the flow rate decreases (following the pump curve). If a circulation flow controller is used, the pump speed can be adjusted to maintain a constant circulation, despite changes in the required pump head.


Figure 8 compares the performances of the two cases. A 20% increase in aniline feed flow rate is the disturbance (top left graph). The solid lines are for circulation flow control with a variable-speed pump. The dashed lines are with a fixed-speed pump so flow is not maintained. The design circulation flow rate is 6000 kmol/h, 3714 kmol/h through the bypass and 2286 kmol/h through the heat exchanger. The bottom right graph shows that the bypass flow goes to zero when there is no circulation flow control (dashed lines). Everything is going through the heat exchanger, but the flow rate has dropped to ∼4000 kmol/h because we have backed up on the pump curve. Reactor temperature (top right graph in Figure 8) is not maintained at its set point. When the circulation flow controller is in service (solid lines), the total circulation flow is maintained at 6000 kmol/h, 4500 kmol/h through the heat exchanger and 1500 kmol/h through the bypass. Reactor temperature is maintained at the desired 400 K. 7.2. Effect of Design Circulation Flow Rate on Performance. Figure 9 gives responses for a 20% increase in the set point of the aniline flow controller at time equal 0.5 h for the two designs based on circulation rates of 4000 and 6000 kmol/ h. Heat exchanger area is 200 m2, and valve design pressure drop is 4 atm in these results. The 4000 kmol/h design is not able to maintain the reactor temperature at its 400 K set point. The bypass valve goes completely shut, and the entire 4000 kmol/h is sent through the heat exchanger. Reactor temperature lines out at 402 K. The 6000 kmol/h design handles the 20% increase well, returning the reactor temperature to 400 K in about 2 h. Figure 10 shows that both designs can successfully handle a 20% decrease in throughput. Notice that both the initial and the final flow rates through the heat exchanger, FHX, are identical in both designs, since the same amount of heat must be transfered in the heat exchanger. Only the bypass flow rates differ. 7.3. Effect of Heat-Exchanger Area on Performance. Figure 11 gives results for two designs with 4000 and 6000 kmol/h but with larger heat exchangers (300 m2 instead of 200 m2 used in the previous section). The disturbance is the same 20% increase in throughput. Both systems can handle the disturbance and return the reactor temperature to 400 K. Notice that the bypass flow rate, FBY (lower right graph in Figure 11), in the 4000 kmol/h design goes to zero for a period until eventually lining out at ∼400 kmol/h. 8. Conclusion The results of this study illustrate that dynamic controllability and rangeability are improved in heat-exchanger bypass systems by using more heat-transfer area and higher circulation flow rates. Control valve design pressure drop has essentially no effect in bypass systems when variable-speed pumps are used to

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maintain total flow rates, which is an unexpected and counterintuitive result. Nomenclature AC ) air-to-close AJ ) heat-transfer area of reactor jacket (m2) AO ) air-to-open CA ) concentration of aniline (kmol/m3) CH ) concentration of hydrogen (kmol/m3) CHA ) cyclohexyl amine COmax ) control valve size coefficient ((m3)1.5 g0.5 s-1 atm-0.5) CSTR ) continuous stirred-tank reactor CW ) cooling water FA ) flow rate of aniline (kmol/h) FBY ) flow rate around bypass (kmol/h) FHX ) flow rate through heat exchanger (kmol/h) FJ ) flow rate of circulating coolant through jacket (kmol/h) FT ) flow transmitter k ) specific reaction rate (m3 kmol-1 s-1) KC ) controller gain (dimensionless) LC ) level controller PC ) pressure controller PDis ) pump discharge pressure (atm) QJ ) heat-transfer rate into jacket (kcal/s) QRX ) heat-transfer rate from reactor (kcal/s) TC ) temperature controller TJ ) jacket temperature (K) TRX ) reactor temperature (K) U ) overall heat transfer coefficient (kW K-1 m-2) VPC ) valve-position controller VR ) reactor volume (m3)

Literature Cited (1) McMillam, G. K.; Toarmina, C. M. AdVanced Temperature Measurement and Control; Instrument Society for Measurement and Control: Research Triangle Park, N.C., 1995. (2) Riggs, J. B.; Karim, M. N. Chemical and Bio-Process Control; Ferret Publishing: Austin, TX, 2007. (3) Bequette, B. W. Process Control: Modeling, Design and Simulation; Prentice-Hall: Upper Saddle River, N.J., 2003, p465. (4) Balchen, J. G.; Mumme, K. I. Process Control Structures and Applications; Van Nostrand Rheinhold: New York, 1988, p 33. (5) Shinskey, F. G. Process Control Systems: Application, Design and Applications; McGraw-Hill: New York, 1988. (6) Seider, W. D.; Seader, J. D.; Lewin, D. R. Product and Process Design Principles, 2nd Ed.; Wiley: New York, 2003, p 741. (7) Douglas, J. M. Conceptual Design of Chemical Processes; McGrawHill: New York, 1988. (8) Luyben, W. L.; Tyreus, B. D.; Luyben, M. L. Plantwide Process Control; McGraw-Hill: New York, 1999, p 58.

ReceiVed for reView October 10, 2010 ReVised manuscript receiVed November 26, 2010 Accepted December 2, 2010 IE1020574

Heat-Exchanger Bypass Control - American Chemical Society

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