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chemical engineering research and design 8 9 ( 2 0 1 1 ) 187–196

Contents lists available at ScienceDirect

Chemical Engineering Research and Design

journa l homepage: www.e lsev ier .com/ locate /cherd

ynamics and control of a biodiesel processy reactive absorption

ostin Sorin Bildeaa, Anton A. Kissb,∗

University “Politehnica” of Bucharest, Centre for Technology Transfer in Process Industries, Polizu 1-7, 011061 Bucharest, RomaniaAkzoNobel Research, Development & Innovation, Process Technology ECG, Velperweg 76, 6824 BM, Arnhem, The Netherlands

a b s t r a c t

Integrated biodiesel processes based on reactive separations powered by solid acid/base catalysts are available

nowadays, offering significant advantages such as minimal capital investment and operating costs, as well as no

catalyst-related waste streams and no soap formation. However, the controllability of the process is just as impor-

tant as the capital and operating savings. In such processes the small number of degrees of freedom is a drawback

which makes it difficult to correctly set the ratio of reactant feeds and consequently to avoid impurities in the prod-

ucts. This work considers the process control of biodiesel production by reactive absorption, the main result being

an efficient control structure that ensures the excess of methanol that is necessary for the total conversion of the

fatty acids and for prevention of the difficult separations, while maintaining high purity of the water by-product.

Rigorous simulations were performed – using Aspen Plus and Aspen Plus Dynamics as efficient computer-aided pro-

cess engineering tools – for a plant producing 10 ktpy biodiesel from waste vegetable oil with high free fatty acids

content, using solid acids as green catalysts. This reactive absorption process eliminates all conventional catalyst-

related operations, and efficiently uses the raw materials and the reactor volume in an integrated setup that is well

controllable in spite of the reduced number of degrees of freedom.

© 2010 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

Keywords: Reactive absorption; Plantwide control; Dynamic modeling; Green catalysts; Biofuels

duction by reactive absorption. The results are given for a plant

. Introduction

iodiesel is an alternative sustainable fuel produced fromreen sources such as waste vegetable oils, animal fat or evenrying-oils from the food industry (Encinar et al., 2005; Kulkarnind Dalai, 2006; Bowman et al., 2006; Balat and Balat, 2008).uch waste raw materials can contain a substantial amount ofree fatty acids (FFAs), up to 100%. At present, employing wastend non-edible raw materials is mandatory to comply withhe ecological and ethical requirements for biofuels (Demirbasnd Balat, 2006). Moreover, the ‘food versus fuel’ competitionan be avoided when the raw materials used are waste oils oron-food crops such as Jatropha (Kumar and Sharma, 2005) orahua (Puhan et al., 2005).

As a non-petroleum-based diesel fuel, biodiesel consistsf fatty acid methyl esters (FAME), currently produced by

cid/base-catalyzed (trans-)esterification, followed by severaleutralization and purification steps (Ma and Hanna, 1999;

∗ Corresponding author. Tel.: +31 26 366 1714, fax: +31 26 366 5871.E-mail addresses: s bildea@upb.ro (C.S. Bildea), tony.kiss@akzonobeReceived 16 February 2010; Received in revised form 12 May 2010; Acc

263-8762/$ – see front matter © 2010 The Institution of Chemical Engioi:10.1016/j.cherd.2010.05.007

Meher et al., 2006). Nevertheless, all the conventional methodssuffer from problems associated with the use of homogeneousacid or base catalysts, leading to serious economical and envi-ronmental consequences, especially considering the recentgrowth of the overall biodiesel production scale (Hanna et al.,2005). Nowadays, modern plants replaced the homogeneouscatalysts with solid bases or acids, thus eliminating the saltwaste streams and simplifying the downstream processingsteps. Moreover, integrated processes based on reactive distil-lation (Kiss et al., 2006a,b, 2008; Dimian et al., 2009) or reactiveabsorption (Kiss, 2009) are now available, offering significantadvantages such as minimal capital investment and operat-ing costs, as well as no catalyst-related waste streams and nosoap formation.

This work considers the process control of biodiesel pro-

l.com, tonykiss@gmail.com (A.A. Kiss).epted 28 May 2010

producing 10 ktpy biodiesel from waste vegetable oil with highfree fatty acids content (up to 100%), using solid acids as green

neers. Published by Elsevier B.V. All rights reserved.

188 chemical engineering research and design 8 9 ( 2 0 1 1 ) 187–196

duct

Fig. 1 – Flowsheet of a biodiesel pro

catalysts. Compared to reactive distillation, the absence of areboiler and a condenser makes reactive absorption a sim-pler process. However, the drawback is the small number ofdegrees of freedom that makes it difficult to set the reactantsfeed ratio correctly and consequently to avoid impurities in theproducts. Aspen Plus and Aspen Plus Dynamics were used asefficient computer-aided process engineering tool to performrigorous simulations for testing various control structures. Anefficient control structure is presented, that ensures the ratioof reactants that fulfills an excess of methanol. This excess issufficient for the total conversion of the fatty acids thereforepreventing difficult separations, while keeping high the purityof the water by-product.

2. Process description

The fatty components were lumped into one fattyacid and its fatty ester, according to the reaction:R–COOH + CH3OH ↔ R–COO–CH3 + H2O. Lauric acid/ester

Table 1 – Mass balance of a 10 ktpy biodiesel process based on

F-ACID F-ALCO BTM T

Temperature [◦C] 160 65.5 135.5 162.Pressure [atm] 0.987 1.036 1.017 0.98Vapor fraction 0 1 0 1Mole flow [kmol/h] 5.885 6.111 6.111 5.88Mass flow [kg/h] 1176.5 195.8 1257.6 114.Volume flow [l/min] 25.1 2730.6 23.5 3555Enthalpy [Gcal/h]] −0.948 −0.291 −0.902 −0.3

Mass flow [kg/h]Methanol 1.51 × 10−4 195.8 9.20 8.83Acid 1176.2 0 0.168 9.52Water 0.246 0 9.36 × 10−4 105.FAME 8.62 × 10−5 0 1248.2 8.6 ×

Mass fractionMethanol 0 1 7.32 × 10−3 7.7 ×Acid 0.9998 0 1.34 × 10−4 0.08Water 2.0 × 10−4 0 0 0.91FAME 0 0 0.9926 0

Mole fractionMethanol 0 1 0.047 4.7 ×Acid 0.9976 0 1.37 × 10−4 0.00Water 2.32 × 10−3 0 0 0.99FAME 0 0 0.953 0

ion process by reactive absorption.

was selected as lumped component due to the availabil-ity of experimental results, kinetics and VLLE parametersfor this system (Kiss, 2009). The assumption of lumpingcomponents is very reasonable since fatty acids and theircorresponding fatty esters have similar properties. Thisapproach has been successfully used during the last decadeto simulate other fatty esters production processes, based onreactive distillation for example (Kiss et al., 2008). The nextsimulations consider the UNIQUAC property model (AspenTechnology, 2009a,b,c), sulfated zirconia as solid acid catalyst(Kiss et al., 2006a,b, 2008; Dimian et al., 2009) and a simplebut reliable kinetic expression (Kiss, 2009): r = kCAcidCAlcohol

where CAcid and CAlcohol are the mass concentration ofreactants, k = A exp(−Ea/RT), A = 250 kmol m3 kg−2 s−1 andEa = 55 kJ/mol. The production rate of the process consideredin this work is 10 ktpy fatty acid methyl esters manufacturedfrom 100% fatty acids and methanol. Fig. 1 presents the flow-

sheet of a biodiesel process based on a reactive absorption(RA) column, while Table 1 presents the mass and energybalance.

reactive absorption.

OP REC-TOP VAP Water FAME

9 51.9 135.5 51.9 307 0.987 0.2 0.987 0.2

0 1 0 05 0.061 0.255 5.824 5.8577 9.747 11.3 104.9 1246.3.7 0.19 711.7 1.81 20.937 −0.009 −0.013 −0.395 −0.954

× 10−3 1.51 × 10−4 7.61 8.68 × 10−3 1.599.50 1.39 × 10−4 16.8 × 10−3 0.168

2 0.246 8.28 × 10−4 104.91 1.07 × 10−4

10−5 8.62 × 10−5 3.708 0 1244.5

10−5 1.55 × 10−5 0.672 8.27 × 10−5 1.28 × 10−3

3 0.975 1.23 × 10−5 1.6 × 10−4 1.35 × 10−4

7 0.025 7.32 × 10−5 0.9998 00 0.328 0 0.9986

10−5 7.73 × 10−5 0.932 4.65 × 10−5 8.50 × 10−3

8 0.776 0 1.44 × 10−5 1.43 × 10−4

2 0.224 1.81 × 10−4 0.9999 00 0.068 0 0.991

chemical engineering research and design 8 9 ( 2 0 1 1 ) 187–196 189

vent

ocamwrem0t

tevcidcfTa

3

RaaHatat

fhKteat

Fig. 2 – Control structure CS-1a: in addition to basic in

The column has 15 theoretical stages with a liquid holdupf 25 L. The catalyst loading is 6.5 kg per reactive stage. The RAolumn is operated in the temperature range of 135–160 ◦C,t ambient pressure, with a maximum reaction rate in theiddle of the reactive zone. As phase splitting may occurhen the molar fraction of water is higher than 0.25, the

eactive-absorption column is modeled using VLLE data. Nev-rtheless, as revealed latter by the composition profiles, theolar fraction of water in the liquid phase does not exceed

.05 and therefore phase splitting does not occur here, underhe designed process conditions.

The fatty acid is pre-heated then fed as a hot liquid in theop of the reactive column while an amount of alcohol whichnsures the complete conversion of the fatty acid is injected asapor into the bottom of the column, thus creating a counter-urrent flow regime over the reactive zone. Water by-products removed as top vapor, then condensed and separated in aecanter from which the fatty acids are recycled back to theolumn while water by-product is recovered at high purity. Theatty esters are delivered as bottom product of the RA column.he hot product is flashed to remove the remaining methanol,nd then it is cooled and stored.

. Problem statement

eactive absorption offers indeed significant advantages suchs minimal capital investment and operating costs, as wells no catalyst-related waste streams and no soap formation.owever, the controllability of the process is just as importants the capital and operating savings. Therefore, it is importanto note that reactive absorption has less degrees of freedomnd therefore more difficult to control than reactive distilla-ion (Nagy et al., 2007).

In processes based on reactive distillation or absorption,eeding the reactants in a correct ratio is essential to achieveigh products purity (Nagy et al., 2007; Dimian et al., 2009;iss et al., 2010). Thus, the fatty acid is completely converted

o fatty esters when there is an excess of methanol, but the

xcess of methanol becomes an impurity in the top streamnd thereafter in the water by-product. On the contrary, whenhere is an excess of fatty acids, the methanol is completely

ory control, a ratio controller fixes the inlet flow rates.

converted and the purity of water by-product is high, but theconversion of fatty acids is incomplete. In the later case thebottom product contains unreacted fatty acids that cannot beremoved from the final product by simple flashing. Since theseparation of fatty acids from fatty esters is more difficult thanthe separation of fatty acids from water, this situation shouldbe avoided. It is important to remark that this constraint mustbe fulfilled not only during the normal operation, but also dur-ing the transitory regimes arising due to planned productionrate changes or unplanned disturbances.

4. Results and discussion

Plantwide control concerns the strategy of solving thedynamic problems of an entire process, with the goals ofsafety and a required production (rate and quality), whileminimizing the operating cost. In this section, two workablecontrol structures are developed by addressing these tasks ofplantwide control. To achieve this goal, dynamic simulation(performed in Aspen Plus Dynamics) will be used to under-stand the dynamic behavior of the process and to prove theeffectiveness of the proposed control structures.

4.1. Basic control

In general, the safety of a process is achieved by a combinationof inherent safe design, safety relief valves and instrumentprotective functions. By controlling liquid levels and gas pres-sures, therefore ensuring that all the material is containedwithin the process boundaries, the control system supportssafety. For the reactive-absorption column presented in Fig. 1,the levels of organic and aqueous phases in the decanter, thelevel in the sump of the column, and the level in the flashvessel should be controlled. These tasks can be achieved byemploying, as manipulated variables, the flow rates of acidrecycle, water by-product, column bottom and FAME product,respectively.

The pressure at the top of the column can be controlled

by the vapor product rate. Besides the safety of the process,controlling this variable is important because the pressuredetermines the temperature profile inside the column (boiling

190 chemical engineering research and design 8 9 ( 2 0 1 1 ) 187–196

Fig. 3 – Dynamic simulation results for control structure CS-1a. Production rate changes are easily achieved. At t = 2 h, theol/h

fresh acid flow rate is increased by 10%, from 5.824 to 6.4 km

5.2 kmol/h.

point of the reacting mixture) and the vapor flow rate. Thesecontrol loops are presented in Fig. 2.

The effluents (products) of the reactive distillation pro-cess are FAME, water and unreacted methanol. The amountsof methanol and acid in the FAME product must be keptlow because this is the main product and its quality deter-mines the performance of the process. The concentration ofmethanol is the result of the vapor-liquid equilibrium in theauxiliary flash vessel. The temperature of the separation iseasily controlled by the flash duty.

One way to achieve the required vacuum is by means of apositive displacement compressor. In this case, the pressurecan be controlled by manipulating the flow rate of a small inletstream of inert gas, as shown in Fig. 2. These control loopsalso ensure that a limited amount of FAME is lost with themethanol by-product stream. However, low acid concentra-tion in the FAME product cannot be achieved and controlledthrough a simple vapor-liquid equilibrium, because of the verytight requirement. To accomplish this goal, the control sys-tem must ensure that the reaction proceeds fast enough andthere is sufficient methanol such that the entire amount ofacid fed in the process is consumed in the reaction. As a firsttry, the ratio between the column-inlet flows of alcohol and

acid is kept constant, in a feed-forward manner: the setpointof the flow controller on alcohol stream is calculated as the

Fig. 4 – Control structure CS-1a: composition, reaction rate, and tfor different production rate changes (±10%).

. At t = 20 h, the fresh acid flow rate is decreased to

measured column-inlet acid flow multiplied by the desiredalcohol/acid ratio (Fig. 2).

At the top of the column, the amount of acid found in thewater by-product is the result of its solubility. The solubilityis determined by temperature, which can be controlled withthe cooling duty. However, methanol and water are completelymiscible. As a result, the entire amount of methanol foundin the vapor outlet of the column will end in the water by-product.

The simplest way to set the production rate is by changingthe amount of fatty acid that is fed into the process. Becauseonly tiny amounts of fatty acid should leave the process withFAME, water and excess methanol (stream VAP in Fig. 1), theflow rate of fresh acid (stream ACID in Fig. 1) directly deter-mines the process throughput. It should be remarked that theproduction rate is not feedback-controlled, but it is being setas a consequence of changing the amount of fresh acid fed tothe process. This strategy works well when the per-pass con-version of the limiting reactant is large (Kiss et al., 2002, 2003,2007; Dimian and Bildea, 2008; Kiss, 2010), condition that isfulfilled here. Note that the buffer vessel where the streams offresh and recycled acid are mixed (Fig. 2) is not necessary and itcan be replaced by a simple mixer. However, it allows switching

the flow and level control loops. Thus, the alternative controlstructure CS-2 shown in Fig. 13 and discussed later sets the

emperature profiles along the reactive-absorption column,

chemical engineering research and design 8 9 ( 2 0 1 1 ) 187–196 191

Fig. 5 – CS-1a: purity versus the alcohol/acid ratio at various temperatures of the column-inlet acid feed (left); Optimum ratioversus temperature of the column-inlet acid feed (right).

F bottor

pic

stfflabocoh

Fo

ig. 6 – Control structure CS-1b: the concentration of acid inatio.

roduction rate in an indirect manner, as a result of chang-ng the amount of acid that is fed to the reactive-absorptionolumn.

Fig. 3 shows results of dynamic simulation when controltructure CS-1a is applied. Starting from the steady state, atime t = 2 h, the fresh acid flow rate is increased by about 10%,rom 5.824 to 6.4 kmol/h. The ratio control leads to a higherow rate of the alcohol feed. The new FAME production rate ischieved in several hours and the new operating point is sta-le. At time t = 20 h, the fresh acid flow rate is decreased to 90%f the original value (5.2 kmol/h). Again, the production rate

hanges as expected. The second plot of Fig. 3 shows the purityf the FAME and water streams. Although the purity remainsigh, during the large-production operating regime the purity

ig. 7 – Dynamic simulation results for control structure CS-1b. Pf FAME stream is maintained.

m stream is controlled by manipulating the alcohol/acid

of the FAME product stream is below the 99.8% requirement.Moreover, there is too much methanol in the water streamduring the low-production operation.

Fig. 4 shows reaction rate, temperature and liquid phasecomposition profiles along the reactive-absorption column,for different production rates. When the production rate ishigh, the reaction front moves to the bottom of the column.Because no reaction takes place in the upper part of the col-umn, the acid is pushed downwards, with the result of largeconcentration of acid in the FAME product stream. Simulta-neously, the temperature in the upper part of the column

increases because of the higher fraction of acid. Note thatthe numbering of stages starts from the top, increasing to thebottom of the column, as illustrated by Fig. 1.

roduction rate changes are easily achieved and the purity

192 chemical engineering research and design 8 9 ( 2 0 1 1 ) 187–196

and t

Fig. 8 – Control structure CS-1b: composition, reaction rate,for different production rate changes (±10%).

4.2. Control of FAME purity

The profiles presented in Fig. 4 suggest that keeping the alco-hol/acid ratio at the steady-state design value is not sufficientto achieve high purity of the FAME and water product streamswhen the production rate is modified. The influence of thealcohol/acid ratio on the purity of the FAME and water streamsis presented in Fig. 5, at steady state and for different valuesof the Acid inlet temperature. It can be seen that at low valuesof the alcohol/acid ratio, the FAME purity is low but the waterpurity is high. The purities are reversed when the alcohol/acidratio is high, namely high purity of the FAME stream and lowpurity of the water stream. Fig. 5 also shows that there is anarrow range of alcohol/acid ratio values, depending on theacid feed temperature, for which both FAME and water prod-ucts have a high purity. In addition to the drawback pointedout by dynamic simulation, the sensitivity analysis showsthat control structure CS-1a lacks robustness with respect

to measurement inaccuracies because any error of the acidflow rate measurement leads to deviation of the alcohol/acid

Fig. 9 – Control structure CS-1c: concentration of acid in bottom sThe temperature of the column-inlet acid stream is manipulated

emperature profiles along the reactive-absorption column,

ratio from the required value and therefore to impure prod-ucts.

These results suggest that the manipulation of the alco-hol/acid ratio could be used to limit the amount of acidthat arrives in the bottom of the column during the high-production operating regime. This change implies adding aconcentration controller as shown in CS-1b presented in Fig. 6.

Fig. 7 presents the dynamic simulation results for the con-trol structure CS-1b when the same scenario as in Fig. 3 isapplied. As with CS-1a, the operation is stable and productionrate changes can be easily achieved. In addition, the qual-ity of the FAME product is radically improved. However, theacid-concentration controller overreacts and large amountsof methanol can be found in the water stream during thehigh-throughput operating regime.

Fig. 8 shows the reaction rate, temperature and liquidcomposition profiles along the reactive-absorption column,for different production rates and control structure CS-1b,

when production rate changes are applied. The variabilitywith respect to the base case (design) is reduced and only fine

tream is controlled by manipulating the alcohol/acid ratio.by a methanol concentration controller.

chemical engineering research and design 8 9 ( 2 0 1 1 ) 187–196 193

Fig. 10 – Dynamic simulation results for control structure CS-1c. Production rate changes are easily achieved and theproducts purity is maintained at high values.

F andf

athFttb

Ff

ig. 11 – Control structure CS-1c: composition, reaction rate,or different production rate changes (±10%).

djustments seem to be necessary. It should be remarked thathe large amount of methanol in the water stream during theigh-throughput regime is a steady-state effect, as shown byig. 7 (right). Therefore, it is not caused by a wrong tuning ofhe ratio controller. In order to keep the methanol concentra-

ion at the required value, an additional process variable muste used.

ig. 12 – Control structure CS-1c: composition, reaction rate, andor different catalytic activities.

temperature profiles along the reactive-absorption column,

4.3. Control of water purity

The right diagram of Fig. 5 shows that an increase of the acidtemperature reduces the amount of alcohol that must be fedto the column in order to keep the purity of both products

at high values. This effect can be explained by the additionalcontribution of higher temperature to higher reaction rate and

temperature profiles along the reactive-absorption column,

194 chemical engineering research and design 8 9 ( 2 0 1 1 ) 187–196

Fig. 13 – Control structure CS-2: the column-inlet flow rate of acid is fixed. The fresh acid controls the level in the buffer tank.

Fig. 14 – Dynamic simulation results for control structure CS-2. Production rate changes are easily achieved and theproducts purity is maintained at high values. At t = 2 h, the column-inlet acid flow rate is increased by 10%, from 5.887 to6.5 kmol/h. At t = 20 h, the column-inlet acid flow rate is decreased to 5.24 kmol/h.

Table 2 – Controller tuning parameters.

Controller Controlaction

Range ofmanipulated

variable

Range ofcontrolledvariable

Setpoint Bias Gain [%/%] Integral time[min]

Level (buffervessel)

Direct 0–2350 kg/h 0–1.5 m 0.75 m 1176.8 kg/h 10 60 min

Level (decanter,organic liquid)

Direct 0–20 kg/h 0–0.75 m 0.37 m 10.12 kg/h 1 60 min

Level (decanter,aqua. phase)

Direct 0–215 kg/h 0–0.75 m 0.35 m 105.7 kg/h 1 60 min

Level (columnsump)

Direct 0–2517 kg/h 0–1.5 m 0.75 m 1259 kg/h 1 60 min

Level (flash) Direct 0–2490 kg/h 0–1.5 m 0.75 m 1245 1 60 minPressure(column)

Direct 0–12 kmol/h 0.95–1.05 bar 1 bar 5.91 kmol/h 2 12 min

Pressure (flash) Reverse 0–1.22 kmol/h 0.18–0.22 bar 0.2 bar 0.31 kmol/h 2 12 minTemperature(acid inlet)

Reverse 0–18.7 × 104 kcal/h 140–180 ◦C 160.6 ◦C 9.36 × 104 kcal/h 2 20 min

Mole fraction(methanol,column top)

Direct 140–180 ◦C 3–5 × 10−3 4 × 10−3 160.6 ◦C 0.05 20 min

Mole fraction(acid, columnbottom)

Direct 0.1–0.2 2–8 ppm 6 ppm 0.1688 1 20 min

Temperature(methanol inlet)

Reverse 0–12 × 104 kcal/h 60–80 ◦C 65.4 5.9 × 104 kcal/h 1 20 min

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chemical engineering research a

herefore to complete acid conversion. Consequently, in theontrol structure CS-1c (Fig. 9), a second concentration con-roller adjusts – in a cascade fashion – the setpoint of thecid stream temperature controller in order to maintain theoncentration of methanol in the vapor outlet below a cer-ain value. During the high-throughput regime, increasing theemperature in the upper part of the column has the effect ofncreased reaction rate, and therefore reduces the need of anxcess of methanol.

Fig. 10 shows the dynamic simulation results for the controltructure CS-1c. The same scenario was tested as for CS-1a andS-1b. The production rates are changed as desired. The purityf the FAME product is practically unchanged throughout theynamic regime, while the purity of water is significantly bet-er compared to the previous cases. The temperature, reactionate and liquid composition profiles are shown in Fig. 11. It cane seen that by making fine adjustments to the temperaturerofile – by means of the acid feed temperature – the con-rol system keeps constant the position of the reaction front,hile the reaction rate is modified according to the requiredroduction rate.

.4. Influence of the catalyst activity

inally, the performance of the control system was tested fordeactivating catalyst. The investigated scenario assumeduniform decrease of the pre-exponential factor from

50 kmol m3 kg−2 s−1 (at t < 2 h) to 200 kmol m3 kg−2 s−1 (at= 40 h). The control system succeeded to keep the through-ut and product quality unchanged. Fig. 12 shows profilesbtained at different moments during the dynamic simula-ion. The control system compensates lower catalyst activityy higher temperature inside the column, with the result of aonstant reaction rate.

We conclude that control structure CS-1c achieves stableperation at different production rates and is robust withespect to uncertainty of the reaction rate.

.5. Indirect setting of the production rate

iss et al. (2007) discuss another option for controllinghe inventory of reactants in the process, namely fixinghe flow rate of the reactant at the inlet of the reac-ive section (stream F-ACID in Fig. 1) and bringing theeactant into the process by means of a level controller.his control structure, denoted by CS-2, is illustrated inig. 13.

When the level and control loops around the buffer tankere switched, dynamic simulation showed that the gain ofoth controllers in the concentration–temperature cascade athe top of the column had to be reduced in order to attain sta-le operation. Fig. 14 shows results of dynamic simulation forhe following scenario: at t = 2 h, the fresh acid rate is increasedy 10%, from 5.824 to 6.5 kmol/h. At t = 20 h, the fresh acid flowate is decreased to 5.24 kmol/h. The production rate can beasily manipulated (although in an indirect manner) by chang-ng the setpoint of the acid-flow controller. The purity of theroduct streams remains high, although, compared to Fig. 10,slight degradation of the performance is observed. However,

his could be as well the result of a wrong selection of the tun-ng parameters, and not a consequence of switching the levelnd flow control loops.

sign 8 9 ( 2 0 1 1 ) 187–196 195

4.6. Controller tuning

The control loops were tuned by a simple version of the directsynthesis method (Luyben and Luyben, 1997). According tothis method, the desired closed-loop response for a giveninput is specified. Then, with the model of the process known,the required form and the tuning of the feedback controller areback calculated.

For all controllers, the acceptable control error, �εmax, andthe maximum available control action, �umax, were specified.Then the controller gain, expressed in engineering units, wascalculated as Kc = �umax/�εmax and translated into percentageunits.

First order open-loop models were assumed, in order tocalculate the integral time of the pressure, temperature andconcentration control loops. As rough evaluations of theprocess time constants �, 12, 20 and 20 min were used, respec-tively. It can be shown (Luyben and Luyben, 1997) that thedirect synthesis method requires that the reset time of a PIcontroller is equal to the time constant of the process, �i = �. Forthe level controllers, a large reset time �i = 60 min was chosenas no tight control is required. An overview of the controllerstuning parameters is given in Table 2.

5. Conclusions

Integrated biodiesel processes based on reactive absorptionhave fewer degrees of freedom as compared to reactive distil-lation. This makes it difficult to correctly set the reactants feedratio and consequently to avoid impurities in the products. Acontrol structure that achieves this objective is the main resultof this study.

• Basic quality control is achieved by keeping constant theratio alcohol/acid at column inlet, in addition to inventorycontrol loops (CS-1a, Fig. 2).

• When the process is disturbed or there are uncertainties inthe measurements, measuring the acid concentration in thebottom stream of the column and adjusting the alcohol/acidratio (CS-1b, Fig. 6) is necessary for maintaining a very smallconcentration of acid in the FAME product, an importantquality feature of the biodiesel product.

• High purity of the water by-product requires control of thealcohol concentration in the column top stream by manip-ulating the column-inlet acid temperature (CS-1c, Fig. 9). Inthis way, the correct excess of alcohol is achieved resultingin high products purity. As an alternative, the column-inletacid flow rate can be fixed (CS-2, Fig. 13).

The performance of the proposed control structure wasdemonstrated (Figs. 7, 10 and 14) by rigorous dynamicsimulations using Aspen Plus Dynamics as an efficientcomputer-aided process engineering tool.

Acknowledgements

C.S. Bildea acknowledges the support of CNCSIS – UEFISCSU,project number PNII–IDEI 1543/2008 – “A nonlinear approachto conceptual design and safe operation of chemical pro-

cesses”. We also thank A. C. Dimian (University of Amsterdam)and A.B. de Haan (Eindhoven University of Technology, TheNetherlands) for the helpful discussions.

and

196 chemical engineering research

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