Control Policy for the Startup, Semi-Continuous and Continuous Operation of a Reactive Distillation Column

Control Policy for the Startup, Semi-Continuous and Continuous Operation of a Reactive Distillation Column

Copyright (\') IFAC Advanced Control of C hemical Processes. Banff. Canada. 1997 Control policy for the startup, semi-continuous and continuous opera...

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Copyright (\') IFAC Advanced Control of C hemical Processes. Banff. Canada. 1997

Control policy for the startup, semi-continuous and continuous operation of a reactive distillation column

1. L. Baldon(2), 1. 1. Strifezza(2), M. S. Basualdo(i,2)(O) and C. A.Ruiz(i) (1) GIAIQ. urN -FRR (2) Departamento de Electr6nica. FCEIyA. UNR Rosario. Argentina. E-mail: [email protected]. FAX 54 41-821772

Abstract- Dynamic behavior of a controlled reactive distillation column for startup and continuous operation is

analyzed here through simulation accounting with experimental data reported in the literature. Previous studies about simulation of column startup can be found in the literature, however no papers have been found describing the startup period of a reactive distillation column. In this work an esterification case for obtaining ethyl acetate is analyzed by assuming different startup strategies in order to achieve a minimwn startup time for an specific product quality. In addition a control system based on Dynamic Matrix Control (DMC) design is applied for controlling the semi- continuous and continuous stages. A special indicator is used to switch from startup control policy to the semi- continuous and continuous control strategy in order to have under automatic control the hole operation. Several preliminary simulation results are presented here by using the software package called READYS. This rigorous mathematical model is found on the conservation laws, the equilibriwn relationship as well as pressure, holdup and reaction terms. KEYWORDS Reactive distillation. Dynamic simulation. Startup strategies . DMC applications

the optimal switching time from total reflux to operation at the desired reflux ratio which minimizes the time from the initial startup to the steady state operation. Dynamic simulation has been used increasingly in recent years, especially in connection with reactive distillation columns. In the present work a simulation tool for column startup is developed using a model equilibriwn stage. This program called READYS is described in detail in Ruiz et al. (1995) and Basualdo and Ruiz (1995). This software package proves to be a valuable tool in planning the startup strategy. Based on this the amount of extensive experimental studies can be minimized. The program facilitates an extended view in these complicated process and helps to understand it. READYS allows the calculation of temporal changes of temperature, concentration., holdup. liquid and vapor flow profiles along the column height. The dynamic simulation of startup operation of reactive distillation columns is interesting from the view point of modeling, design. control and operability. Here attempts are made to answer the question of which valves have to be switched at what time in order to achieve the demanded product purities within the shortest possible time in a safe manner. The development and application to startup and ShUtd0\\11 of such simulation program have been reported earlier by Ruiz, (1986): Gani et al. (1987), Ruiz et al. (1988) and Ruiz and Gani (1990) but only for conventional distillation columns.

1- INTRODUCTION The startup behavior is interpreted as the change of the column from its initial state to a steady-state when a defined product composition is obtained. The startup procedure represents the most complex process in industrial practice because of the simultaneous changes in many relevant process variables. Because these dynamic transitions are always non-productive procedures the aim of investigations is to minimize the time needed. The problems which arise in attempting to minimize startup times of industrial plants, such as distillation columns, are in the domain of minimwn time control, and research of this aspect has been reported in Coward, (1967) and Yasuoka et 01. (1969); for distillation columns. However, algoritluns for control systems using this approach are usually very complicated, and only with great difficulty they can be adapted for use with low cost systems employing microcomputers. Nonnal startup of a distillation column involves the following simple reflux-switching operations: i) initial operation at total reflux, ii) switchover at the proper time to operation at specified reflux ratio, and iii) changeover from the startup operation to steady state operation once the liquid composition on each tray has attained a steady state. The optimal startup problem then becomes of determining

(") Author to whom all correspondence should be submitted at Riobamba 326. 2000 Rosario. Santa Fe. Argentina

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Practical ex-periences and notes concerning colwnn startup operation have been featured in the open literature by production engineers. here the concern has ranged from the startup strategy, a checklist for control equipment to practical hints and experience in troubleshooting [Kister, 1979; Fulks, 1982; Meier, 1982]. 10 this paper a semiemphirical indicator for optirnizing the time of total reflux operation is used. The validity of this indicator is examined by simulation, using a reactive distillation for ethyl acetate production. The algoritlun, taken from Yasuoka et al. (1987); is then used as the basis for an automatic on line control system for the startup operation. These authors have applied that methodology on conventional distillation columns using a 10w-<:OSt control system based on microcomputers. This indicator for determination of the optimal switching time is characterized by a function which can be readily determined from internal measurements within the distillation colwnn at each moment. The time corresponding to the minimum in this function approximates the optimal switching time, by assuming the colwnn is started at total reflux. However, the authors did not consider the role played by plate hydraulics during the startup period. In addition this indicator is used to switch to the control system for semi
reaction is computed by a user added routine. allowing to define the plate(s) where reaction occurs, reaction order. stoichiometric coefficients and rate constants (i.e., evaluated via an Arrhenius type equation). Instantaneous, non reversible reactions, can be modeled by fixing appropriately the reaction rate constants. 10 this work, condenser, reboiler and tray holdups are considered. The following basic assumptions are made: (i) molar vapor holdup is negligible compared to the molar liquid holdup; (ii) liquid and vapor leaving each plate are in thermal equilibrium; (iii) the definition of Murphree plate efficiency applies for each plate irrespective of the chemical reactions in the liquid phase; (iv) all chemical reactions occur in liquid phase; (v) liquid and vapors are perfectly mixed. The assumption (i) implies that the vapor stream from one stage to the following goes directly to the liquid phase without any inter-phase mass transfer. So (i) is reasonably valid at low column pressures. About condition (iii) no methods are reported for efficiency evaluation with simultaneous chemical reactions. Then, the classical Murphree definition is employed. From the "chemical point of view" assumption (v) is equivalent to consider each plate as a completely mixed stirred tank reactor. Hence, mixing models for liquid phase are not used. Many different types of trays in the colwnn can be simulated by READYS. Chemical reactions can occur elsewhere in the column or in specific locations along it. The set of procedures involves the thennodynamic packages for the equilibrium prediction. Newton-Raphson method is used to update temperatures and pressures on "equilibrium" calculation. The hydraulic routine calculates the liquid and vapor flow rates, checks for flooding, weeping or entrainment conditions and finds out the weeping and entrainment rates. The correlations used for determining the hydraulic variables are mainly empirical. The particular set employed in this work is the typically used in industry. Sieve plates are used for this example, the plate pressures are constant (for the simplified procedure) so the vapor flow rates are determined algebraically from energybalance by considering neglected the enthalpy variations. The ODEs are integrated by modified Gear method.

2- SIMULATION MODEL The underlying numerical techniques used in READYS have been presented in detail in earlier works cited above and are therefore discussed only briefly here. This rigorous model, capable to simulate simultaneous chemical reactions, takes into account variations in internal liquid and vapor rates, variable tray liquid holdups including plate hydraulics. The model consists in a set of ordinary differential equations (ODEs) representing the mass and energy balances related to the system of column/s under study. These balances are made around each plate of the column/s, reboiler, condenser, accwnulators and controllers. It may include the integral of the error, if any PI or PID controller is used. The "procedure variables " include the hydraulic variables (flow rates, pressure drops. etc.), thennodynamic variables (plate compositions, temperatures, pressures, etc.), chemical reaction evaluations and the input / output specifications. In addition, the "design variables 11 include details of colwnn geometry, colwnn configuration and optional calculations. The time is the independent variable. The program READYS can work with different user selectable thennodynamic packages, such as those based on Soave-Redlich-Kwong state equation, Grayson and Streed correlation or UNIFAC group contribution method. Production or consumption of components due to chemical

2. 1 System studied To illustrate the implementation of READYS package for simulating the startup period for the ethyl acetate production the simplified version was run. This problem was taken from Holland (1981) and also proposed to evaluate the ASPEN simulator perfonnance in the work of \enkataraman et af. (1990). Here the same example is analyzed consisting on the ethyl acetate production by separating the components in a continuous reactive distillation fonn. The details of the system are given in Table l. The following reaction is assumed to occur in the liquid phase on all the stages including the condenser and reboiler.

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to steady state reflux value. From Fig. I it can be seen that for the three inicialization cases the same time for the infinite refiux ratio is done. Then the refilL" ratio changes at the same value for overall strategies. When each one achieves the steady state the temperature profiles are recorded. This useful infonnation is accounted in the calculations of the optimal switching time. Therefore, a quantitative indicator, presented by Yasuoka et al. (1987): for determining the optimal switching time from total refilL" to steady state operation is used here.

where acetic acid (A), ethanol (B), and water (C) compositions are the same given in the cited works and D corresponds to the main product, ethyl acetate. Table I: Description of the test problem Reference: General DaIa: Number of Trays Number ofComponerts Model Option Equilioown Model: Total Number of ODEs

Venkataraman al. (1990) I3

4 simplified UNlFAC 52

(a)

".-------------------,

Operational DaIa: Type ofCondenses' partial Type of Reboiler par1ia1 Colwnn Top Press. (KPa) 101.2 Reboiler Heal Duty (KW) 0.156 Reflux Ratio: !01art up period 10 production period Feed Composition (mole fraction) : Comp. A 0.4963 Comp. B 0.4808 Comp. C 0.0229 Comp. D 0.000 Notes

";.. ---

kO! EOD KmollmJ sec Joulelgmol 123.00 59445.1

Reaction Direct ExponerIIS «l) Reaction Inverse ExponerIIS (8)

A 1.0 0.0

: .:..-~:.

" 0t.~--"n-~...... lQ --30'--~.....
(a)

D.E(i-fl.) 10 _------------~

Reaction Sdleme:

kOD KmollmJ sec 483.33

----.-.-._., .-

" I "

EO! Joulelgmol 59445.1

B 1.0 0.0

C 0.0 1.0

\

\ \

\

,

0 ..

D

~

0.0 1.0

.-.. ~ . : .---

"

Here the holdup varies slightly tray by tray due to the hydraulic calculation (Francis weir fonnula is used). The startup behavior is then simulated with READYS by considering three different ways, working at infinite reflux ratio. The same composition along the colwnn is assumed. The strategies proposed for the startup period consist in considering the column initially filled with the feed composition (acetic acid, ethanol and water) for the rest of this work it will be called "feed". The other is the colwnn filled with acetic acid only (called "acetic acid" strategy) and then the same feed is introduced. The third case is done by considering each tray, condenser and reboiler with ethanol only (called "ethanol" strategy) and then the same feed composition enters in the column. Preliminary tests were done by working at infinite reflux ratio for about 30 hours. Then the production reflux is adopted until the system composition reaches its new stable point (30 hours more). In Figure I the top compositions for the three startup strategies are shown. The time required to attain the steady state is taken as the time where the top composition product fall in the ±2% range of the product specification.

00

<0

D.E(i-fl)

(b) _ .. - . ACEOCACD - . - . ElH'Kl.

---wo._ "

-

:i \

1.. 06

~

ElHI'l ACETA lE

,

.

.. .

-~

~' -

_.-._.-

- - : .=--':"---

00 60

(c)

Figure I : a) startup with feed initial charge; b) with pure ethanol; c) with pure acetic acid, for infinite (30 hs.) and production refilL"" (30 hs.)

It is defined by the following function : N

M

I

=

L

IT j - TjST I

(I)

j = I

where TJ is the temperature of the jth tray during total refilL" and ~ is the corresponding steady-state value for the steadystate refilL'>,;, N is the number of the plates including the condenser (N = I) and the reboiler (N = 13). In general, Mt variation is done as that given in Figure 2, it decreases with time to a minimum and then increases to constant values.

3- DEFINmONS OF THE CHARACTERISTIC FUNCTIONMT The objective here is to evaluate which strategy presents the minimum time for switching from total refllL"

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DMC structure will be designed over the initial data obtained from these startup conditions.

3-1 Behavior of the function Mt for different startup strategies The relationship between the switching time from total reflux to steady state operation and the Mt function defined previously is examined here. Here the Mt ~on is used as a selection criteria since it can be calculated easily by measuring the temperature of the liquid on each tray at various times. In Figure 2 the Mt for the three strategies are compared. As can be seen the option of startup with f~ initial charge represents the optimal switching time for this operation. By using this structure the switching time can be reduced in about 50% comparing with the other two possibilities. This difference is so relevant to conclude the superiority from the "feed" strategy to each other.

D

)(f

B

4- DESIGN OF AN AUTOMATIC ON-LINE STARTUP CONTROL SYSTEM. Yasuoka et al. (1987); have demonstrated that the function is a good tool for indicating the time when the refllLx must be changed at the startup period. They proposed a control structure which simplified version for the distillation column is shown in Figure 3. The temperature measure is converted properly in a digital signal which serves for Mt calculations in a microcomputer. Note that in the most of practical cases the plate temperatures are not a noisy signal so it is easy. to detect the minimum value for the Mt parameter. OthelWlse, if the temperature measure contains noise a proper filter can be used in order to detect clearly the moment when Mt reaches its minimum value. Thinking that when Mt is closed to zero is associated to finish the startup period means that is the best operation point to

Figure 3: schematic diagram of an automatic optimal startup operation

Mt

~ r-------------------------~

Tt.£(t-6)

Figure 2: numerical results of Mt vs. switching time for the three strategies.

5- THE DMC IMPLEMENTATION OVER THE REACTIVE DISTILLATION COLUMN The DMC technique first appeared in the open literature in 1979 when Cutler and Ramaker, and Prett and Gillette reported its application to a fluid catalytic crac~g unit. Here a multiple-input multiple-output DMC IS designed for the reactive distillation column for the ethyl acetate production. The DMC algorithm used in this work consists in finding the sequence of control moves that minimize the squared deviation of the predicted output from their respective set-points. This is done by first solving the unconstrained optimization with the quadratic objective function explicitly, obtaining the least square solution. The constraint handling capability is then added by iteratively checking for constraint violation and resolving the modified problem if violation occurs. More details about this subject can be seen at Ogunnaike and Ray (1995) and other references cited there. Therefore, the following steps are considered for implementing DMC over the reactive system : . a) the reference trajectory is specified as a step of the desired set point value, . b) process output prediction is carried out using the ~ep­ response model with the dynamic matrix B as the pnnclpal operator , then

fP (k + 1) = fpo(k) + ~U(k) + W(k + 1) (2) here, Y"" is the initial value of the output Y for the actual instant (k), W represents the unmeasured disturbance term

achieve quickly the new desired steady state. Therefore, any other over or under specified condition, which probably involves a waste of energy, is avoided. From the cwves shown in Figure 2 it is clear that for "feed" case the optimal switching time is 7.47 hs., for the "ethanol" strategy 12.65 hs. and for "acetic acid" case the minimum is done at 13 .76 hs. Hence the optimal startup time for this example is the corresponding to the "feed" strategy. Therefore, the

and

tJ. U ( k)

are the sequence of control moves from

actual instant k to m. been m = 12 in this example. Matrix B for the multivariable case is obtained from the coefficients B~ which model the unit step response function for each input of the system as is shown in Figure 4. Then, B,) relates the output variable i with input variable j . In this example the manipulated variables are distillate (0) and vapor (V) flow rates and controlled

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identification technique. The open loop system needs 30 hours to reach the steady state, then the ~ coefficients are calculated each 0.2 hOUTS. The software package for the system identification was presented by Striffezza et al. (1996) and it was developed in C language. Matrix B is then obtained and the algebraic operations indicated in Eq. (3) about product and inversion of the matrix is calculated by using mathematic routines which can be found elsewhere. The manipulated variables for the finite reflux period are the distillate flow rate and the reboiler heat duty. The selection of the ethyl acetate bottom composition as one of the controlled variable is proposed in order to obtain all the product from the top by generating enough vapor for doing it. In addition the feed flowrate and feed composition are considered as perturbations. The move suppression factors (k) are chosen so as the system presents a good trade-off between stability and performance response. The constrains accounted here are the minimum and maximum values for the two manipulated variables. As an example it can be seen from Figure 6 the incidence for different k values which affect the distillate flow rates movements. Finally an interesting result can be seen from Figure 7 where the use of the Mt factor for switching the time where DMC begins its control action. It must be note that the linear DMC strategy demonstrates its capability for handle efficiently the semi non linear behavior of the transition stage between the startup and continuous periods.

variables are top (XD) and bottom (XB) composition of ethyl acetate.

Figure 4: the unit-step response function. c) Control action computation: it is calculated by solving the unconstrained optimization problem to obtain the least squares solution expressed by:

tlU(k) =(BT rBfl BT r £(k + 1)

(3).

Here, the weighting matrix r is accounted in order to recognize the importance of scaling the changes in the various output variables to treat them in equal form. Matrix r is defined as:

(4) where n is the scaling matrix. For the example studied here, although the two controlled variables are the top and bottom ethyl acetate composition, the relative changes for each one differ in about one order of magnitude, therefore is necessary to chose an appropriate scaling factor which is also considered as a tuning parameter. In addition, the main tuning parameters known as the move suppression factor are also stated in order to run the example presented here. d) Model prediction update is achieved by providing estimates for W(k+l) (see Eq. (2» using the single correction term calculated at time instant k. 00 01"

0'80 , -_ _ _ _ _ _ _ _ _ _ _ _- - - ,

0)18

k= 00

r---------------, DO - SO

O,O t:61

0368

00 "' 5

§

D,OOS3

00 "'0

~ 0 .0 0 "

ili {

~

1

0.00)\

oo . ~

Q,o a-,g

0 0 """

Figure 6: the effect of the move suppression factor over the dynamic response of the top ethyl acetate composition.

00 _35

OO . 3()

ODQ)()

.

0

50

00

f4

..

7- CONCLUSIONS 00 _10

""

The optimal switching time from total to steady state refllLx in the startup operation of a reactive distillation column was shown, by dynamic simulation, using a complex software called READYS. In this work a simple methodology for determining the optimal switching time is used as a tool to evaluate different startup strategies for the ethyl acetate production. The function called M can be calculated in a very simple manner and by plotting their values at various times its minimum magnitude indicates the optimal switching time. This algorithm was used to design an automatic on line control system using a small, low -cost control structure based on a microcomputer. This system has been successfully tested also experimentally but only applying over conventional distillation columns. Although this methodology does not consider the role played by plate

Figure 5: top ethyl acetate composition response for feed flow rate step changes. 6- APPLICATION RESULTS

In this section preliminary results about DMC design over the column are presented. Firstly for the construction of the dynamic matrix B several step changes over the manipulated and disturbance variables are done. For the example analyzed here feed flow and feed composition are assumed as disturbance variables. In Figure 5 the time responses of top ethyl acetate composition for step changes in feed flow rates are shown. In this case the system presents the less typical response to be modeled by DMC

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hydraulics during the startup period it was very useful for indicating the starting point for the DMC action for semicontinuous and continuous stages. Hence the results presented here must be seen as preliminary analysis since studies considering variable pressures and plate hydraulics will be part of future researches. top ethyl acetate XO

",----.--------------,

lO

time (hs .)

0000 0 .... 0002

time (hs .)

Figure 7: dynamic response of the top and bottom ethyl acetate composition from the startup to the continuous operation Wlder automatic control. REFERENCES Basualdo M. S. and Ruiz. C. A (1995)" "Dynamic Simulation of a Batch Reactive Distillation for the Ethyl Acetate Production" Latin American Applied Research , 25/s, pp 29-34. Coward 1., (1%7) Chem Eng. Sc. 22, P 503 Cutler C. and Ramaker B., "Dynamic Matrix Control - A computer Control Algorithm". AlChE National Meeting, Houston, 1979. Fieg G., G . Wozny, Ch. Kruse (1993) "Experimental and Theoretical Studies of the Dynamics of Startup and Product Switchcover Operations of Distillation Columns" . Chem. Eng. Proc. 32, 283-290. Fulks B. D . (1982) "Planing and Organizing for Less Troublesome Plant Startup" . Chem. Eng., 6, 96106. GanL R, Ruiz, C. A , Carneron, 1. T., (1987) "Studies in the Dynamics of Startup and Shutdown Operations of Distillation Columns", Proc. Chemical Engineering FW1damentals Conference '87: The Use of Computers in Chemical Engineering', Taonnina, Italy, . Holland C.D. ; (1981) "FWldamentals of Multicomponent Distillation", McGraw-Hill, New York. . Kister H. Z. (1979) "When Tower Startup Has Problems" Hydrocarbons Process, 58, 89-94. Komatsu, H. 1., (1977) "Application of the Relaxation Method for Solving Reacting Distillation Problems", 1. Chem. Eng. Japan 10, 200 ..

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Kruse Ch. , G. Fieg, G. Wozny and L. Jeromin (1995) " Development and Experimental Verification of a Simulation Tool for Column Startup" Proceedings of DYCORD+'95, Dennmark,. Meier F., (1982) "Is your Control System Ready to Startup?" Chem. Eng. 12, 1-14,. Ogunnaike B. and Ray H. , (1995) "Process Dynamics, Modelling and Control", Chapter 27. Prett D. and Gillette , "Optimization and Constrained Multivariable Control of a Catalytic Cracking Unit" . AlChE National Meeting, Houston, 1979. Ruiz, C. A, (1986) "DesarrolIo de Wta Politica de Control para Operaciones de Puesta en Marcha de Colurnnas de Destilacion", Ph.D. Chem. Eng. Thesis, Universidad Nacional del Sur, Bahia Blanca, Argentina.. Ruiz, C. A, Carneron, 1. T., Gani, R, (1988) "A Generalized Dynamic Model for Distillation Columns, Part III: Study of Startup Operations", Computers and Chemical Engineering. Ruiz, C. A, Gani, R , (1990) "Dynamic Simulation and Design of Distillation Columns, Part 11: Single Column Startup and Shutdown Strategies", Lat. Am. Res., 20, 113-128. Ruiz, C. A, Basualdo M. S. and Scenna N. (1995) "Reactive Distillation. Dynamic Simulation". Transactions of the Institution of Chemical Engineers (ICHEM E). 73, A4, Mayo. Strifezza 1., Basualdo M. and Ruiz c., (1996) " La Simulacion Diruimica de Procesos para la Enseftanza de Control; I. Aplicacion aI Control Predictivo . Proceedings of7° C. L. C. A IFAC. ~nkataraman S., Chan K. and Boston J .F., (1990) "Reactive Distillation using ASPEN PLUS". Chem. Eng Progr., pp.45-54. Yasuoka, H., Iguichi T., Nakanishi E. and Kunigita E., 1. (1969) Japan Assoc. Automatic Control 13, p. 683 . Yasuoka H., Nakanishi E. and Kunigita E., (1987) "Design of an on-line starup system for a distillation column based on a simple algorithm" . International Chem. Eng., 27, 3.