Dynamics and control of a heat pump assisted … · Dynamics and control of a heat pump assisted...

Dynamics and control of a heat pump assisted E-DWC for bioethanol dehydration Patraşcu, Bîldea, Kiss

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Dynamics and control of a heat pump assisted extractive dividing-1

wall column for bioethanol dehydration 2

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Iulian Patraşcu,1 Costin Sorin Bildea,1 Anton A. Kiss,2.3* 4 1 University “Politehnica” of Bucharest, Polizu 1-7, 011061 Bucharest, Romania 5 2 AkzoNobel Research, Development & Innovation, Process Technology SRG, Zutphenseweg 6

10, 7418 AJ Deventer, The Netherlands. E-mail: [email protected] 7 3 Sustainable Process Technology Group, Faculty of Science and Technology, University of 8

Twente, PO Box 217, 7500 AE Enschede, The Netherlands 9 * Corresponding author: [email protected], Tel: +31 26 366 9420 10

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Keywords 12

Bioethanol dehydration; process dynamics; process control; integrated design; heat pumps 13

14

Highlights 15

• Highly integrated design leading to challenges in process dynamics and control 16 • Efficient control structure ensuring stable operation of VRC assisted E-DWC 17 • Disturbances in feed flowrate and composition can be effectively rejected 18

19

Abstract 20

Recently, a novel heat-pump-assisted extractive distillation process taking place in a dividing-21

wall column was proposed for bioethanol dehydration. This integrated design combines three 22

distillation columns into a single unit that allows over 40% energy savings and low specific 23

energy requirements of 1.24 kWh/kg ethanol. However, these economic benefits are possible 24

only if this highly integrated system is also controllable to ensure operational availability. 25

This paper is the first to address the challenges related to process dynamics and control of this 26

highly integrated system. After showing the control difficulties associated with the original 27

design owing to thermal unbalance, an efficient control structure is proposed which introduces 28

a by-pass and an additional external duty stream to the side reboiler. The range of the external 29

duty is rather small, about 5% of the combined duty of the reboilers, but sufficient to stabilize 30

the system by controlling the temperature on the pre-concentration side of the column. Two 31

quality control loops ensures product purity when the system is affected by feed flowrate and 32

composition disturbances. 33

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2

1. Introduction 1

Bioethanol is a renewable fuel produced in largest amounts, by various routes: corn-to-2

ethanol, sugarcane-to-ethanol, basic and integrated lignocellulosic biomass-to-ethanol. In all 3

cases, the raw materials are pre-treated and then fermented to yield diluted bioethanol of 5-4

12% concentration (Vane, 2008; Huang et al., 2008; Frolkova and Raeva, 2010). This needs 5

to be further concentrated to 99-99.8 %wt (depending on the standard), in an energy intensive 6

process that involves distillation combined usually with extractive distillation (Kiss and Ignat, 7

2013). The concentration of diluted streams from fermentation is not an issue related only to 8

ethanol, but a more generic problem encountered in biorefineries (Kiss et al., 2016). 9

Various energy improvements of the distillation process have been proposed for the ethanol 10

separation and purification, such as the use of internally heat-integrated distillation columns 11

(HIDiC) that are based on the vapor recompression principles (Kiss and Olujic, 2014; Ponce 12

et al., 2015). However, the HIDiC technology is hindered by large equipment costs (leading to 13

long payback times) and it is also limited to producing only hydrated ethanol. 14

Dividing-wall column (DWC) is a proven process intensification distillation technology that 15

can deliver 25-35% savings in energy and investment costs (Dejanovic et al., 2010; Yildirim 16

et al., 2011; Kiss, 2013). In spite of the integration of two distillation columns into a single 17

shell, DWC proved to be controllable providing that a suitable control scheme is selected – 18

based on simple PID or more advanced controllers (Serra et al., 1999; Diggelen et al., 2010; 19

Kiss and Bildea, 2011; Rewagad and Kiss, 2012). The addition of heat pumps to dividing-20

wall column technology was also proposed for various processes but the process dynamics 21

was not investigated (Chew et al., 2014; Luo et al., 2015; Liu et al., 2015; Shi et al., 2015). 22

Using DWC for various processes, including bioethanol dehydration by azeotropic and 23

extractive distillation was proposed by several authors (Bravo-Bravo et al., 2010; Sun et al., 24

2011; Kiss and Suszwalak, 2012). Later, the control analysis of extractive dividing-wall 25

columns revealed satisfactory controllability properties of such integrated systems (Tututi-26

Avila et al., 2014; Zhang et al., 2014). 27

In a follow-up study, Kiss and Ignat (2012) proposed a novel extractive DWC configuration 28

that integrates the three distillation columns of a classic process – pre-concentration 29

distillation column (PDC), extractive distillation column (EDC), and solvent recovery column 30

(SRC) – into a single unit. On top of this integrated extractive DWC system, Luo et al. (2015) 31

added also a vapor recompression (VRC) heat pump to further increase the energy savings up 32

to 40% (see Figure 1). This elegant solution was also featured at the joint PSE-2015 & 33

ESCAPE-25 conference (www.pse2015escape25.dk) among the best contributions in the 34


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process design section. But the authors were also challenged to prove the controllability of 1

such a highly-integrated system. This paper addresses for the first time the challenges related 2

to the process dynamics and control of the proposed integrated heat pump assisted E-DWC. 3

4

2. Problem statement 5

The smart combination of heat pumps (e.g. vapor recompression) with extractive dividing-6

wall column (E-DWC) technology offers significant advantages in terms of low total annual 7

costs and major energy savings of about 40%. But the controllability of the process is just as 8

important as the cost savings, considering that the plant must be available and fully operable 9

in order to deliver the expected design savings. The problem is that a more integrated system 10

has less flexibility and fewer degrees of freedom hence being more difficult to control. To 11

solve this problem, we investigate the dynamics of the process and propose a rather simple 12

but effective process control structure that is able to reject common disturbances, within short 13

settling times and low overshooting. 14

15

3. Results and discussion 16

Plantwide control deals with the strategy of solving the dynamic problems of a full process, 17

under the production (rate and quality) and safety constrains, while minimizing the costs. In 18

this work, an efficient control structure is developed by addressing these tasks of plantwide 19

control. To achieve this goal, dynamic simulations (carried out in Aspen Dynamics) were 20

used to understand the dynamics of the process and to prove the effectiveness of the proposed 21

control structures. The starting point is the VRC assisted E-DWC design proposed by Luo et 22

al. (2015), which achieves 40% energy savings compared to the conventional separation 23

sequence. Note that further energy integration could be attempted by using the product 24

streams for feed pre-heating. However, as there is no liquid flow from the main column to the 25

pre-fractionator, the feed stream acts as reflux. Therefore, feeding below the boiling point 26

improves separation efficiency. Figure 1 presents for details of the VRC assisted E-DWC, 27

including the mass and energy balance. Steady state temperature and composition profiles are 28

shown in Figure 2. The reader is referred to the original paper (Luo et al., 2015) for more 29

details concerning the principle of operation and the efficiency of the system. 30

31

3.1 Process dynamics 32

The original Aspen Plus steady state simulation was exported to Aspen Dynamics. The 33

simulation consists of two separate RADFRAC models which correspond to the PDC and 34


4

EDC-SRC sides of the dividing-wall column and are inter-connected by vapor and liquid 1

streams. The dynamic simulation model is presented in Figure 3, which includes the setpoint 2

of the main controllers. 3

The safety of a process is typically achieved by a combination of inherent safe design, safety 4

relief valves and instrument protective functions. The process control system supports safety 5

by controlling the liquid levels and gas pressures, thus ensuring that all the material is 6

contained within the process boundaries. The default inventory controllers provided by Aspen 7

Plus / Aspen Dynamics are: 8

• Level of the PDC sump, by manipulating the bottoms flow rate. According to the 9

steady state boilup ratio V/B, a fraction of this stream is withdrawn as product, while 10

the rest is sent to the side reboiler. 11

• Level of the EDC-SRC sump, by manipulating the bottoms flow rate. 12

• Pressure of the PDC column (top), by manipulating the vapor distillate rate. Note that 13

this controller, required by the flow-driven dynamic simulation, is not part of the 14

practical implementation where the pressures above the top tray of the PDC and on the 15

EDC-SRC side are equal. 16

• Pressure of the EDC-SRC column (top), by manipulating the vapor distillate rate. 17

Some basic changes were made to the dynamic simulation exported by Aspen Plus: 18

• The flow rate of solvent entering the EDC-SRC column was set on flow control. The 19

solvent make-up becomes the manipulated variable in a level control loop. 20

• A heat exchanger was added to allow controlling the temperature of the solvent 21

entering the EDC-SRC column. The duty of this heat exchanger is nearly zero at 22

design condition, but changes when the system is affected by disturbances. 23

• A vapor-split controller was added. This controller is not part of the practical 24

implementation, but is necessary to ensure that the vapor split remains at the design 25

value in the flow-driven dynamic simulation. 26

• A compressor-outlet pressure controller was added. By manipulating the compressor 27

brake-power, the pressure of the compressed vapor is kept at the design value. The 28

practical implementation of controlling the outlet pressure and flow is briefly 29

described in next section. 30

After implementing the basic inventory control, the simulation can be run. Starting from the 31

steady state, as long as the system is not disturbed, the process variables (flow rates, 32

temperatures, pressures) stay at their nominal values. However, the simulation breaks for any 33


5

small disturbances. Quality control loops were added, as follows: 1

• A temperature controller was added on the condensate stream. 2

• A temperature controller was added in the EDC-SRC. Temperature on stage 33 (the 3

most sensitive tray) is controlled by the EDC-SRC reboiler duty. 4

After these changes, the simulation is successful as long as it starts from the steady state and 5

no disturbance is introduced. However, the simulation still breaks for any small increase of 6

the feed rate, which shows that the operating point is unstable. What happens is the following. 7

Increasing the flow rate of PDC feed (cold liquid) leads to a lower vapor flow rate along PDC. 8

As a result, the pressure drops and less vapor is fed to the EDC-SRC section (the PDC 9

pressure controller closes the vapor distillate valve). The pressure on the EDC-SRC section 10

drops and the EDC-SRC pressure controller decreases the EDC-SRC vapor distillate rate. 11

Then, less heat is transferred in the (heat-integrated) side reboiler, followed by further 12

decrease of the vapor flow rate along the PDC. Simultaneously, more EG is found in the 13

bottom section of the EDC-SRC which leads to further temperature increase (despite the 14

lower pressure). The SRC-EDC temperature controller reacts and decreases the reboiler duty, 15

leading to an additional reduction of the vapor flow along the EDC-SRC section (and 16

therefore to further decrease of the SRC-EDC vapors distillate flow). Clearly, an appropriate 17

control structure is required. 18

The key to stabilizing the system is adding a small duty to the last stage of the PDC column. 19

This duty can then be used as new manipulated variable in a control loop which ensures that 20

the vapor flow rate from the PDC to the SRC-EDC side is constant when the system is 21

affected by disturbances. When the valve-position controller VPC (Figure 3) is added, feed 22

flow rate and composition disturbances can be introduced without affecting the stability of the 23

process. However, the control structure shown in Figure 3 is not able to maintain the quality 24

of the products, namely the purity of the water and ethanol streams. Moreover, measuring the 25

flow rate of the vapor going from the PDC to the SRC-EDC side is difficult in practice. 26

Dynamic simulations also reveal that the solvent feed and PDC bottoms heat exchangers 27

require heating or cooling, depending on the disturbance. Therefore, the control structure 28

presented in Figure 3 is further refined, as explained in the next section. 29

30

3.2 Process control 31

As the original design of the VRC assisted E-DWC has control difficulties associated with the 32

thermal unbalance, we propose here a novel efficient control structure that introduces an 33

additional duty stream to the side reboiler. The range of the additional duty is rather small, 34


6

about 5% of the combined duty of the reboilers, but this is sufficient to counteract the possible 1

thermal unbalance. Moreover, the design changes and additional controllers ensures the 2

quality of the water and ethanol product streams. 3

Figure 4 shows the control structure for the VRC assisted E-DWC. For convenience, the 4

setpoint of several controllers are included, while detailed information about controller 5

settings can be found in Table 1. The production rate is set by changing the setpoint of the 6

flow controller manipulating the feed stream flow rate. A ratio controller ensures that the 7

amount of solvent used (make-up and recycle stream) is proportional to the feed flowrate. 8

This ratio is set by a quality controller which achieves constant mass fraction of water 9

impurity in the ethanol product. The temperature of the solvent added to the E-DWC unit is 10

controlled by a split range setup, in which one of the heat exchangers cools-down or heats-up 11

the solvent stream, as required. The compressor is operated at constant outlet pressure – the 12

pressure at which the valve on the outlet line opens – and variable flow rate, which is 13

achieved by means of an internal recycle or other means. The duty of the PDC (external) 14

reboiler is provided by a fraction of the compressed vapors (about 94% at design conditions) 15

and by hot utility. The external duty provided by the hot utility is used to control the 16

temperature on the PDC side of the dividing-wall column. When the tuning of this loop is 17

sufficiently aggressive (large gain, small integral time), the system is stabilized. The ‘valve-18

position control’ (VPC) loop ensures both rapid response to disturbances and energy 19

efficiency. Thus, when higher duty is required by the PDC temperature controller, the flow 20

rate of hot utility is increased. This has a fast effect on the controlled variable. Afterwards, the 21

VPC controller gradually increases the flow rate of compressed vapors passing through the 22

reboiler (by-pass fraction is reduced), until the utility flow rate returns to its design / set point 23

value. The set point of the PDC temperature controller is given, in cascade fashion, by a 24

quality controller keeping constant the mass fraction of ethanol impurity in the water product 25

stream. Note that the dynamics of the concentration transducers (measurements) on the 26

ethanol and water streams was properly modelled including 1 minute sampling interval and 1 27

minute dead-time. 28

In practice, the feed contains small amounts (about 0.1%wt.) of dissolved CO2, which have to 29

be removed from the system. To this end, a small purge stream from the reflux drum could be 30

used as manipulated variable, in a pressure control loop (Loy et al., 2015; Batista et al., 2012). 31

Table 1 provides the controller tuning parameters. The control loops were tuned by a simple 32

version of the direct synthesis method (Luyben and Luyben, 1997), according to which, the 33

desired closed-loop response for a given input is specified. With the model of the process 34


7

known, the required form and the tuning of the feedback controller are then back-calculated. 1

For all controllers, the acceptable control error (∆εmax) and the maximum available control 2

action (∆umax) were specified. Afterwards, the controller gain (expressed in engineering units) 3

was calculated as Kc = ∆umax/ ∆εmax and translated into percentage units. First order open-loop 4

models were assumed, in order to calculate the integral time of the pressure, temperature and 5

concentration control loops (Bildea and Kiss, 2011). As rough evaluations of the process time 6

constants τ, 12 min, 20 min and 40 min were used, respectively. It can be shown (Luyben and 7

Luyben, 1997) that the direct synthesis method requires that the reset time of a PI controller is 8

equal to the time constant of the process (τi = τ). For the level controllers, a rather large reset 9

time τi = 60 min was chosen, as no tight control is required. When the PDC temperature 10

control loop is sufficiently fast, the system is stabilized and disturbances can be successfully 11

rejected. To tune the concentration controllers, the ultimate gain and the period of oscillations 12

at stability limit were found using the ATV (Auto Tuning Variation) method. These values 13

were further used to calculate the controller parameters according to Tyreus-Luyben settings 14

(Luyben and Luyben, 1997). 15

Figure 5 and Figure 6 show the dynamic response of the system for feed flow rate 16

disturbances. Starting from the steady state, the feed rate is ramped up by 10% (from t = 2 h 17

to t = 4 h), brought back to the initial value (at t = 12 h), decreased by 10% (at t = 22 h), and 18

returned again to the initial value (at t = 32 h). Both the ethanol and water flow rates change 19

accordingly to their feed rate and reach their new steady state values in a rather short time 20

with almost no overshooting (Figure 5, left). It is remarkable that the composition of the 21

ethanol and water stream products remains practically unchanged (Figure 5 right, where the 22

mass fraction of solvent in water is shown, as this variable is not measured / controlled). 23

Figure 6 shows the key internal variables, for the same scenario. The various internal flow 24

rates follow the change of the feed rate (plot a). The ethanol purity is maintained by changing 25

the solvent/feed ratio (plot b). When the feed rate increases (from t = 2 h to t = 12 h), higher 26

side-reboiler duty is required. This is achieved by increasing the external duty (plot c) and by 27

reducing the by-pass around the side-reboiler (plot d). Note that when lower duty is required 28

(from t = 22 h to t = 32 h), this is achieved by increasing the by-pass, while the external duty 29

remains at the minimum value of 2 GJ/h. 30

The system can also withstand larger feed flow rate disturbances, for example +/- 20%, if they 31

occur as ramp with the maximum slope of about 5% / hour. Moreover, a larger range of the 32

external duty is required (0-16 GJ/h). Although this seems to be a limitation of the 33


8

controllability of this highly integrated system, fast changes of the feed rate (for example in 1

the form of step signals) are not likely to be encountered in practice. 2

Figure 7 and Figure 8 show the dynamic response to various changes of feed ethanol 3

concentration, from the initial value of 10 %wt. to 12 %wt., and 8 %wt., similarly to the 4

previously described scenario. The flowrates of ethanol and water products follow the 5

amounts existing in the feed (Figure 7 - left). The changes in the concentration of impurities 6

(solvent in water, and water in ethanol) are quite minor and the system stabilizes shortly 7

(Figure 7 - right). The key internal variables shown in Figure 8 have limited variations, and 8

return to their initial values when the disturbance is removed. 9

Figure 9 and Figure 10 show dynamic simulation results for changing the setpoint of the 10

ethanol concentration controller from the initial value of 99.8 %wt to 99.5 %wt (at t = 2 h), 11

and to 99 %wt (at t = 12 h), after which the setpoint is brought back to the initial value in two 12

steps (at t = 22 h and t = 32 h). The setpoint changes are implemented as ramp signals, over 8-13

hour period. The product flow rates change according to the mass balance (Figure 9 – left), 14

while the ethanol concentration follows it setpoint (Figure 9 – right). The internal variables 15

have limited changes (Figure 10) and return to initial values once the disturbance is removed. 16

Tututi-Avila et al. (2014) analyzed the control properties of a conventional extractive 17

dividing-wall column used for ethanol dehydration, where the feed stream is a concentrated 18

ethanol solution (93 %wt). The control structure involved conventional temperature, pressure 19

and level control loops. Use of the vapor split as manipulated variable in a temperature 20

control loop was necessary to properly reject step feed flow rate and composition 21

disturbances, product purities returning close to their specifications in about 1.5 h. For most 22

disturbances, the VRC assisted E-DWC is equally fast. However, the dynamics is slower 23

when the ethanol feed concentration decreases, the new steady state being achieved in about 5 24

hours. By using concentration controllers, the control structure of the VRC assisted E-DWC 25

achieves better composition control, without the need of vapor split manipulation. In contrast 26

to the conventional E-DWC system, the VRC assisted E-DWC cannot withstand step 27

disturbances, due to higher degree of integration. 28

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4. Conclusions 30

Integrated processes combining vapor recompression with dividing-wall column technology 31

have more interactions of variables and fewer degrees of freedom as compared to classic 32

DWC systems. This makes it challenging to control and questions the expected benefits. 33


9

In particular, the system is unstable even when appropriate basic inventory (level and 1

pressure) controllers are provided. The reason is the positive feedback due to vapor-2

recompression: decrease of the pre-fractionator temperature leads to lower flow rate of the 3

recompressed vapor, which results in less heat being transferred in the side reboiler, and 4

further temperature decrease. Some design changes are necessary to provide the manipulated 5

variables necessary for stabilizing the system. Thus, the side reboiler has an additional 6

external duty, used for pre-fractionator temperature control. Moreover, only a fraction of the 7

recompressed vapor is used for heating the side reboiler. Manipulating the by-pass provides 8

additional heat when needed, but also ensures that the external duty is not shut down and is 9

kept to a minimum value. Concentration measurements are necessary to achieve good quality 10

control of the ethanol and water streams. 11

An effective control structure that achieves the control objective is the main result of this 12

study, which is the first to prove the controllability of vapor recompression assisted extractive 13

DWC. The system is robust to ramp feed rate and feed composition disturbances and to ramp 14

changes of ethanol purity setpoint. As step changes are not tolerated, this study provides a 15

clear warning to process and control engineers that such changes should be implemented in a 16

slow fashion. 17

18

Acknowledgement 19

Financial support of the European Commission through the European Regional Development 20

Fund and of the Romanian state budget, under the grant agreement 155/25.11.2016 (Project 21

POC P-37-449, acronym ASPiRE) is gratefully acknowledged. 22

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24

References 25

1. Batista F. R. M., Follegatti-Romero L. A., Bessa L. C. B. A., Meirelles A. J. A., 26

Computational simulation applied to the investigation of industrial plants for bioethanol 27

distillation, Computers and Chemical Engineering, 46 (2012), 1-16. 28

2. Bildea C. S., Kiss A. A., Dynamics and control of a biodiesel process by reactive 29

absorption, Chemical Engineering Research and Design, 89 (2011), 187-196. 30

3. Bravo-Bravo C., Segovia-Hernández J.G., Gutiérrez-Antonio C., Duran A.L., Bonilla-31

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optimization, Industrial & Engineering Chemistry Research, 49 (2010), 3672-3688. 33

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sustainable distilling, Chemical Engineering and Processing, 49 (2010), 559-580. 4

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dividing-wall columns, Industrial & Engineering Chemistry Research, 49 (2010, 288-307. 6

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Foundations of Chemical Engineering, 44 (2010), 545-556. 8

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technologies in current and future biorefineries, Separation and Purification Technology 10

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columns, Chemical Engineering and Processing, 50 (2011), 281-292. 13

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azeotropic distillation in dividing-wall columns, Separation & Purification Technology, 15

86 (2012), 70-78. 16

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dividing-wall column, Separation & Purification Technology, 98 (2012), 290-297. 18

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13. Kiss A.A., Advanced distillation technologies - Design, control and applications. Wiley, 21

Chichester, UK, 2013. 22

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extractive dividing-wall column and pressure swing adsorption: An economic comparison 33

after heat integration and optimization, Separation and Purification Technology, 149 34


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(2015), 413-427. 1

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2213. 4

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12

Tables 1

2

Table 1. Controller tuning parameters 3

Controller PV, value & range OP, value & range Kc, %/%

Ti, min

Solvent feed to PDC

FC Flow rate = 16.736 t/h Not modelled in flow-driven simulation

QC Water in ethanol product = 800 ppm solvent : feed = 0.1339 0.581 17.1

400 … 2000 ppm 0…0.3

LC Level Modeled as instantaneous unit

TC Temperature = 52.5 °C Cooling duty = 0 GJ/h 10 60

45 …62 °C -4…0 GJ/h

TC Temperature = 52.5 °C Heating duty = 0 GJ/h 10 50

40…70 °C 0…4 GJ/h

Fresh feed Not modelled in flow-driven simulation

FC Flow rate = 125 t/h

Column

PC Column pressure = 1 bar Vapor flow rate = 1142 kmol/h

20 12

0 … 2 bar 0…2284 kmol/h

PC Compressed vapor = 3.7 bar Compressor power = 1.8 MW

1 10

3.5…4 bar 0…3.6 MW

RC Reflux ratio L/D = 3.20

LC Sump level = 2.05 m EG recycle = 16.44 t/h 10 60

0…4.1 m 0…32.88 t/h

TC Tray 33 temperature = 184.18 °C Reboiler duty = 37.7 GJ/h

1 20

170…200 °C 0…75 GJ/h

TC Reflux temperature = 105.3 °C Cooling duty = -4.233 GJ/h

10 30

95…115 °C -24…0 GJ/h

Side reboiler

LC Level Modelled as instantaneous unit

PC Pressure = 1.25 bar Modelled as instantaneous unit

TC PF Tray 6 temperature = 100.1 °C Trim duty = 2 GJ/h 49.3 9.2

90…110 °C 0…8 GJ/h

QC Ethanol in water product = 89 ppm SP of tray 6 TC = 101 °C 0.14 6.6

0…200 ppm 90...110 °C

VPC Trim duty = 2 GJ/h By-pass = 0.061 1 10

0…8 GJ/h 0…0.5


13

Figure captions (auto-updated) 1

2

Figure 1. Flowsheet of the VRC assisted extractive-DWC for bioethanol dehydration 3

4

Figure 2. Steady state temperature and liquid phase composition profiles of pre-fractionator 5

(dashed lines) and SRC-EDC (continuous lines) 6

7

Figure 3. Dynamic simulation model of the VRC assisted extractive-DWC for bioethanol 8

dehydration including the control loops necessary for stabilizing the process 9

10

Figure 4. Control structure of the VRC assisted E-DWC for bioethanol dehydration 11

12

Figure 5. Dynamic response of products flow rate and purity, for ±10% disturbance of feed 13

flow rate 14

15

Figure 6. Key internal variables for ±10% flow disturbance. a) Flow rates; b) Ethanol product 16

composition controller; c) Prefractionator temperature controller; d) Side-reboiler external 17

duty controller (VPC) 18

19

Figure 7. Dynamic response of products flow rate and purity, for +/- 2% disturbance of feed 20

concentration 21

22

Figure 8. Key internals variables for +/- 2% disturbance of feed concentration a) Flow rates; 23

b) Ethanol product composition controller; c) Prefractionator temperature controller; d) Side-24

reboiler external duty controller (VPC) 25

26

Figure 9. Dynamic response of products flow rate and purity, for 99.8 to 99.0%wt range 27

disturbances of ethanol product purity 28

29

Figure 10. Key internals variables for 99.8 to 99.0%wt range disturbance of ethanol product 30

purity. a) Flow rates; b) Ethanol product composition controller; c) Prefractionator 31

temperature controller; d) Side-reboiler external duty controller (VPC) 32

33

Figure 1

Ethanol

Water

Feed

rL

rV

E-DWC

Solvent (EG)

1

1

13

18

30

37

4

31

17

EDC

PD

C

SR

C

FEHE

S-REB

COMP

Solvent (recycle)

Q = 1947 kW

25°C, 1.2 bar125000 kg/h10%w EtOH90%w H2O

39.7°C

204.3°C, 1.25 bar16487.3 kg/h 100%w EG

25°C, 250 kg/h 100%w EG

52.5°C

Q = 12024 kW

104.7C, 1.19 bar112750 kg/h99.8%w H2O00.2%w EG

Reflux ratio = 0.646Boilup ratio = 2.587rL=0.0, rV=0.548Qreb = 10043 kW

115.3°C, 3.7 bar12500 kg/h99.9%w EtOH00.1%w H2O

78°C, 1 bar

115.3°C3.7 bar52542 kg/h99.9%w EtOH00.1%w H2O

149.7°C, 3.7 bar

W = 1808 kWQcnd = –267 kW

Figure 2

50

90

130

170

210

250

0 5 10 15 20 25 30 35 40

Tem

pe

ratu

re /

[°C

]

Stage

0.00

0.20

0.40

0.60

0.80

1.00

0 5 10 15 20 25 30 35 40

Ma

ss f

ract

ion

Stage

WATER

ETHANOL

ETHYLENE GLYCOL

Figure 3

1

4

17

31

37

VPC

LCFC

PC

PC

PC

TC33

TC

TC

FC

1

13

Solvent make-up

Feed

Ethanol

Water

Solvent recycle

LC

L/D = 3.203

V/B = 0.173

3.7 bar

1 bar

1.128 bar

125 t/h

153.3 °C

701.15

kmol/h

115.3 °C

52.5 °CLC

X

32

FC

LC

PDC EDC-SRC

Figure 4

TC

LC

FC

PC

PC

TC

TC

TC

FC

Solvent

make-up

Feed

Ethanol

Water

Solvent recycle

LC

L/D = 3.203

3.7 bar

125 t/h

184.18 °C

115.3 °C

52.5 °CLC

LC

VPC

QC

89 ppm

Ethanol

2 GJ/h

QT

QTQC

800 ppm

Water

TC

Splitrange

1

4

17

31

33

37

6

1 18

13 30

1 bar

By-pass

PC

19

1.25 bar

x

Figure 5

10000

11000

12000

13000

14000

15000

100000

105000

110000

115000

120000

125000

0 5 10 15 20 25 30 35 40 45

Flo

w r

ate

/ [

kg

/h]

Time / [h]

Water

Ethanol

0.00

0.10

0.20

0.30

0.40

0.50

0 5 10 15 20 25 30 35 40 45

Ma

ss f

ract

ion

/ [

%w

t]

Time / [h]

Solvent in Water

Water in Ethanol

10000

11000

12000

13000

14000

15000

100000

105000

110000

115000

120000

125000

0 5 10 15 20 25 30 35 40 45

Flo

w r

ate

/ [

kg

/h]

Time / [h]

Water

Ethanol

0.00

0.10

0.20

0.30

0.40

0.50

0 5 10 15 20 25 30 35 40 45

Ma

ss f

ract

ion

/ [

%w

t]

Time / [h]

Solvent in Water

Water in Ethanol

Figure 6

0

15000

30000

45000

60000

75000

0 5 10 15 20 25 30 35 40 45

Flo

w r

ate

/ [

kg

/h]

Time / [h]

Reflux

Ethanol

Distilate (HP)

to Reboiler

Bypass

0

1

2

3

4

5

98.0

99.0

100.0

101.0

102.0

103.0

0 5 10 15 20 25 30 35 40 45

Du

ty /

[G

J/h

r]

Tem

pe

ratu

re /

[°C

]

Time / [h]

PV

SP OP

(a) (b)(a) (b)

(c) (d)

(a) (b)

(c)

(a) (b)

(d)(c)

(a) (b)

0.130

0.131

0.132

0.133

0.134

0.135

790

800

810

820

830

840

0 5 10 15 20 25 30 35 40 45

So

lve

nt

/ Fe

ed

ra

tio

Wa

ter

/ [p

pm

]

Time / [h]

PV

SP

OP

0.90

0.92

0.94

0.96

0.98

1.00

1

2

3

4

5

6

0 5 10 15 20 25 30 35 40 45

Sp

lit

/ [-

]

Du

ty /

[G

J/h

r]

Time / [h]

PV

SP OP

Figure 7

10000

11000

12000

13000

14000

15000

108000

110000

112000

114000

116000

118000

0 5 10 15 20 25 30 35 40 45

Flo

w r

ate

/ [

kg

/h]

Time / [h]

Water

Ethanol

0.00

0.10

0.20

0.30

0.40

0.50

0 5 10 15 20 25 30 35 40 45

Ma

ss f

ract

ion

/ [

%w

t]

Time / [h]

Solvent in Water

Water in Ethanol

Figure 8

0

15000

30000

45000

60000

75000

0 5 10 15 20 25 30 35 40 45

Flo

w r

ate

/ [

kg

/h]

Time / [h]

Reflux

Ethanol

Distilate (HP) to Reboiler

Bypass

0

2

4

6

8

10

94.0

96.0

98.0

100.0

102.0

104.0

0 5 10 15 20 25 30 35 40 45

Du

ty /

[G

J/h

r]

Tem

pe

ratu

re /

[°C

]

Time / [h]

PV SP

OP

(d)(c)

(a) (b)

0.00

0.05

0.10

0.15

0.20

0.25

600

700

800

900

1000

1100

0 5 10 15 20 25 30 35 40 45

So

lve

nt

/ Fe

ed

ra

tio

Wa

ter

/ [p

pm

]

Time / [h]

PV

SP

OP

0.80

0.84

0.88

0.92

0.96

1.00

1

2

3

4

5

6

0 5 10 15 20 25 30 35 40 45

Sp

lit

/ [-

]

Du

ty /

[G

J/h

r]

Time / [h]

PV

SP

OP

Figure 9

0.00

0.40

0.80

1.20

1.60

2.00

0 5 10 15 20 25 30 35 40 45

Ma

ss f

ract

ion

/ [

%w

t]

Time / [h]

Solvent in Water

Water in Ethanol

12300

12400

12500

12600

12700

12800

112500

112600

112700

112800

112900

113000

0 5 10 15 20 25 30 35 40 45

Flo

w r

ate

/ [

kg

/h]

Time / [h]

Water Ethanol

Figure 10

0.00

0.05

0.10

0.15

0.20

0.25

0

4000

8000

12000

16000

20000

0 5 10 15 20 25 30 35 40 45

So

lve

nt

/ Fe

ed

ra

tio

Wa

ter

/ [p

pm

]

Time / [h]

PV

SP

OP

0

15000

30000

45000

60000

75000

0 5 10 15 20 25 30 35 40 45

Flo

w r

ate

/ [

kg

/h]

Time / [h]

Reflux

Ethanol

Distilate (HP) to Reboiler

Bypass

0

1

2

3

4

5

98.0

99.0

100.0

101.0

102.0

103.0

0 5 10 15 20 25 30 35 40 45

Du

ty /

[G

J/h

r]

Tem

pe

ratu

re /

[°C

]

Time / [h]

PV SP

OP

(d)(c)

(a) (b)

0.90

0.92

0.94

0.96

0.98

1.00

1

2

3

4

5

6

0 5 10 15 20 25 30 35 40 45

Sp

lit

/ [-

]

Du

ty /

[G

J/h

r]

Time / [h]

PV

SP

OP

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