control strategy of a pv charging station for plug-in electric vehicles

新能源存储技术对提升续航能力的作用英语作文Title: The Pivotal Role of New Energy Storage Technologies in Enhancing Endurance CapabilitiesIn the epoch of sustainable development, the quest for renewable energy sources has garnered unprecedented momentum. Amidst this transformation, new energy storage technologies have emerged as game-changers, playing a pivotal role in bolstering the endurance capabilities of various energy-dependent systems, particularly in the realm of transportation and power grids.Firstly, let us delve into the essence of energy storage. It refers to the process of capturing and preserving energy generated from renewable sources such as solar, wind, and hydro, for later use when these sources are unavailable or insufficient. This capability is crucial for overcoming the intermittency of renewable energy and ensuring a reliable, continuous supply.New energy storage technologies, including advanced batteries, flywheels, compressed air energy storage, and pumped hydro storage, have significantly advanced in recent years. Among them, batteries havegarnered particular attention due to their versatility and scalability. Lithium-ion batteries, in particular, have revolutionized the electric vehicle (EV) industry by providing extended driving ranges and faster charging times. This has not only alleviated range anxiety among consumers but also accelerated the adoption of EVs as a viable alternative to fossil fuel-powered vehicles.Moreover, these technologies are enabling the integration of renewable energy into existing power grids. By storing excess energy generated during peak production hours, they can supply electricity during off-peak or low-production periods, enhancing grid stability and resilience. This, in turn, reduces reliance on fossil fuel-based backup systems, contributing to a cleaner, greener energy mix.Furthermore, new energy storage solutions are enabling the development of microgrids and off-grid systems, particularly in remote or underserved areas. These systems can operate independently or in conjunction with the main grid, providing reliable, sustainable energy access to communities that would otherwise be left behind.In conclusion, new energy storage technologies are indispensable in enhancing endurance capabilities across various sectors. By bridging thegap between energy production and consumption, they are facilitating the widespread adoption of renewable energy sources, promoting energy security, and accelerating the transition towards a more sustainable future. As these technologies continue to evolve and mature, their impact on our energy landscape will only grow more profound.Translation:标题：新能源存储技术在提升续航能力中的关键作用在可持续发展的时代，对可再生能源的追求获得了前所未有的动力。

Operating manualSolar charge controller10.10 A / 8.8 A / 6.6 APlease read these instructions completely before installation!1. About this manualThese operating instructions are part of the product. Read these operating instructions carefully before use, keep them over the entire lifetime of the product, and pass them on to any future owner or user of this product.This manual describes the installation, function, operation and maintenance of the solar charge controller. These operating instructions are intended for end customers.A technical expert must be consulted in cases of uncertainty.2. SafetyThe solar charge controller may only be used in PV systems for charging and controlling lead-acid batteries in accordance with this operating manual and the charging specifications of the battery manufacturer.The solar charge controller may only be connected to the local loads and the battery by trained personnel and in accordance with the applicable regulations. Follow the installation and operating instructions for all components of the PV system.No energy source other than a solar generator may be connected to the solar charge controller. Follow the general and national safety and accident prevention regulations.Keep children away from PV systems. Do not use the solar charge controller in dusty environments, in the vicinity of solvents or where inflammable gases and vapours can occur. No open fires, flames or sparks in the vicinity of the batteries. Ensure that the room is adequately ventilated. Check the charging process regularly.Follow the charging instructions of the battery manufacturer. Battery Acid splashes on skin or clothing should be immediately rinse with plenty of water. Seek medical advice.Do not operate the solar charge controller when it does not appear to function at all. The solar charge controller or connected cables are visibly damaged or loose. In these cases immediately remove the solar charge controller from the solar modules and battery.3. FunctionsThe solar charge controller monitors the state of charge of the battery bank, controls the charging process, controls the connection/disconnection of loads. This optimises battery use and significantly extends its service life.The following protection functions are part of the basic function of the controller: Overcharge protection ; Deep discharge protection ; Battery undervoltage protection ; Solar module reverse current protection.4. Installation4.1 Mounting location requirementsDo not mount the solar charge controller outdoors or in wet rooms. Do not subject the solar charge controller to direct sunshine or other sources of heat. Protect the solar charge controller from dirt and moisture.Mount upright on the wall (concrete) on a non-flammable substrate. Maintain a minimum clearance of 10 cm below and around the device to ensure unhindered air circulation. Mount the solar charge controller as close as possible to the batteries (with a safety clearance of at least 30 cm).4.2 Fastening the solar charge controllerMark the position of the solar charge controller fastening holes on the wall.Drill 4 Ø 6 mm holes and insert dowels. Fasten the solar charge controller to the wall with the cable openings facing downwards, using 4 oval head screws M4x40 (DIN 7996).4.3 ConnectionUse an wire size suited to the current ratings of the charge controller, e.g. 6mm² for 10A, 5 mm² for 8A, 4 mm² for 6A, 3 mm² for 5A for cable length of 10 m.An additional external 20A fuse (not provided) must be connected to the battery connection cable, close to the battery pole. The external fuse prevents cable short circuits.Solar modules generate electricity under incident light. The full voltage is present, even when the incident light levels are low. Protect the solar modules from incident light during installation, e.g. cover them.Never touch not isolated cable ends. Use only insulated tools. Ensure that all loads to be connected are switched off. If necessary, remove the fuse.Connections must always be made in the sequence described below.1st step: Connect the batteryConnect the battery connection cable with thecorrect polarity to the middle pair of terminalson the solar charge controller (with the batterysymbol).If present, remove any external fuse. Connectbattery connection cable A+ to the positivepole of the battery. Connect batteryconnection cable A– to the negative pole ofthe battery. Insert the external fuse in thebattery connection cable.If the connection polarity is correct, the infoLED illuminates green.2nd step: Connect the solar moduleEnsure that the solar module is protected from incident light (cover it or wait for night).Ensure that the solar module does not exceed the maximum permissible input current.First connect the M+ solar module connection cable to the correct pole of the left pair ofterminals on the solar charge controller (with the solar module symbol), then connect theM– cable. Remove the covering from the solar module.3rd step: Connect loadsFirst connect the L+ load cable to the correct pole of the right pair of terminals on the solarcharge controller (with the lamp symbol), then connect the L– cable. Insert the load fuse orswitch on the load.Notes : Connect loads that must not be deactivated by the solar charge controller deepdischarge protection, e.g. emergency lights or radio connection, directly to the battery.Loads with a higher current consumption than the device output can be directly connectedto the battery. However, the solar charge controller deep discharge protection will no longerintervene. Loads connected in this manner must also be separately fused.4th step: Final workFasten all cables with strain relief in the direct vicinity of the solar charge controller(clearance of approx. 10 cm).5. LED displaysLED Status Meaningilluminates green normal operationInfo LEDflashes slowly red* system fault- too high charging current- overload / short circuit- overheatedtogether with red LED :- too low battery voltagetogether with green LED :- too high battery voltageflashing quickly* battery empty, low voltage disconnectionprewarning, loads still onBatteryredLED flashing slowly* deep discharge protection active (LVD), loadsdisconnectedilluminates battery weak, loads are onBatteryyellowLEDflashes slowly yellow* LVD reconnection setpoint has not yet beenreached, loads still disconnectedilluminates battery goodBatterygreenLEDflashes quickly green* battery full, charge regulation active*flashing slowly: 0,4Hz: 4 times in 10 second, flashing quickly: 3Hz: 3 times in 1 second6. GroundingThe components in stand-alone systems do not have to be grounded – this is not standardpractice or may be prohibited by national regulations (e.g.: DIN 57100 Part 410: Prohibitionof grounding protective low voltage circuits). Ask your dealer for technical assistance.7. Lightning protectionIn systems subjected to an increased risk of overvoltage damage, we recommendinstalling additional lightning protection / overvoltage protection to reduce dropouts.Ask your dealer for technical assistance.8. MaintenanceThe solar charge controller is maintenance-free.All components of the PV system must be checked at least annually, according to thespecifications of the respective manufacturers. Ensure adequate ventilation of the coolingelement. Check the cable strain relief. Check that all cable connections are secure. Tightenscrews if necessary. Check corrosion on terminals.9. Faults and remediesNo display : Check battery polarity and external fuse. Or battery voltage is too low orbattery defective.Battery is not charged : Check if solar modul is connected with correct polarity or if shortcircuit at the solar input. If solar module voltage is lower than battery voltage or if solarmodule is defective the battery cannot be charged.Battery displays jumps quickly : Battery voltage changes quickly. Large pulse currentscause voltage fluctuation. Battery is too small or defective. Ask your dealer for technicalassistance.The following faults do not destroy the controller. After correcting the fault, the device willcontinue to operate correctly:* solar module short circuits * reverse solar module polarity *2* short circuits at load output * excessive load current* reversed battery polarity *1* solar module overcurrent* device overtemperature * overvoltage at the load output10. Legal guaranteeAccording to the German legal requirements, for this product the customer has a 2 yearlegal guarantee.The seller will remove all manufacturing and material faults that occur in the product duringthe legal guarantee period and affect the correct functioning of the product. Natural wearand tear does not constitute a malfunction.Legal guarantee does not apply if the fault can be attributed to third parties, unprofessionalinstallation or commissioning, incorrect or negligent handling, improper transport, excessiveloading, use of improper equipment, faulty construction work, unsuitable constructionlocation or improper operation or use.Legal guarantee claims shall only be accepted if notification of the fault is providedimmediately after it is discovered. Legal guarantee claims are to be directed to the seller.The seller must be informed before legal guarantee claims are processed.For processing a legal guarantee claim an exact fault description and the invoice / deliverynote must be provided. The seller can choose to fulfil the legal guarantee either by repair orreplacement.If the product can neither be repaired nor replaced, or if this does not occur within asuitable period in spite of the specification of an extension period in writing by thecustomer, the reduction in value caused by the fault shall be replaced, or, if this is notsufficient taking the interests of the end customer into consideration, the contract iscancelled. Any further claims against the seller based on this legal guarantee obligation, inparticular claims for damages due to lost profit, loss-of-use or indirect damages areexcluded, unless liability is obligatory by German law.11. Technical DataSteca Solsum F 6.6F 8.8F 10.10FSystem voltage 12 V (24 V)Own consumption < 4 mADC input sideOpen circuit voltage solar module(at minimum operating temperature)< 47 VModule current 6 A 8 A 10 ADC output sideLoad current 6 A 8 A 10 AEnd of charge voltage 13.9 V (27.8 V)Boost charge voltage 14.4 V (28.8 V)Reconnection voltage (SOC / LVR) *³> 50 % / 12.4 V … 12.7 V(24.8 V … 25.4 V)Deep discharge protection (SOC / LVD) *³< 30 % / 11.2 V … 11.6 V(22.4 V … 23.2 V)Operating conditionsAmbient temperature -25 °C … +50 °CFitting and constructionTerminal (fine / single wire) 4 mm2 / 6 mm2 - AWG 12 / 9Degree of protection IP 32Dimensions (X x Y x Z) 145 x 100 x 24 mmWeight approx. 150 g*1Solsum is protected against reverse battery polarity together with polarity protectedloads. Reverse battery polarity combined with short circuited or polarised load couldcause damages in load or regulator*2Avoid reverse module polarity in a 24V system*3Lower value for nominal current, higher value for lowest currentInfo LED Battery LEDsManufactured in aDIN EN ISO 9001:2000 facilitySolsum / Z02 / Version 1104/ 730.930。

第41卷第1期2013年1月Vol．41No．1 Jan．2013风光储联合发电系统调频控制策略研究李鹏，黄越辉，许晓艳，刘德伟，马烁（中国电力科学研究院，北京100192）摘要：针对风光储联合发电系统的运行特点，基于分段调频控制的理念，提出了一种风光储联合发电系统参与电力系统二次调频的控制策略。

该控制策略根据区域控制偏差ACE就调频控制的紧急程度进行划分，在不同控制区域使用不同的有功控制方式，实现对联合发电系统出力的精细化控制，最大程度利用风电及光伏发电，保障储能电池SOC运行在合理范围。

仿真分析验证了所提调频控制策略的可行性、有效性及经济性。

关键词：风光储联合发电系统；调频控制策略；充放电控制；有功功率作者简介：李鹏（1985-），男，硕士，工程师，研究方向为新能源发电调度运行与控制技术。

中图分类号：TM761文献标志码：A文章编号：1001-9529（2013）01-0144-04基金项目：国家科技支撑计划项目（2011BAA07B03）；国家电网公司科技项目Research of Frequency Control Strategy for Wind-PV-Storage Power Generation SystemLI Peng，HUANG Yue-Hui，XU Xiao-Yan，LIU De-Wei，MA Shuo（China Electric Power Research Institute，Beijing100192，China）Abstract：This paper proposes a control strategy of the wind-PV-storage power generation system taking part in second control of power system based on partition frequency control considering operating characteristics of the wind-PV-stor-age power generation system．This control strategy distinguishes different emergency degree of frequency control ac-cording to area control error（ACE），utilizes different active power control mode in different control area，exerts de-tailed control on the joint generating system，reduces the limitation on wind power and solar power and guarantees the SOC operating within reasonable limits．Simulation analysis verifies the feasibility，effectiveness and economy of the proposed strategy．Key words：wind-PV-storage power generation system；frequency control；strategy；charge and discharge control；ac-tive powerFoundation items：The National Key Technology R＆D Program of the Ministry of Science and Technology （2011BAA07B03）目前，对于风电、光伏发电以及储能技术已有较多研究［1-7］，但就以上3个单元的联合运行控制技术的研究才刚刚起步。

Control of reactive distillation productionof high-purity isopropanolSan-Jang Wanga,*,David S.H.WongbaDepartment of Chemical and Material Engineering,Ta Hwa Institute of Technology,Chiunglin,Hsinchu,307Taiwan,ROCbDepartment of Chemical Engineering,National Tsing Hua University,Hsinchu,300Taiwan,ROCReceived 2December 2004;received in revised form 14April 2005;accepted 22June 2005AbstractThe process characteristics and control strategy of a high-purity IPA reactive distillation column were investigated.A robust nominal operation was found by maintaining an excess of propylene feed to the column and recycling the unreacted propylene to the feed instead of the top stage.Stage temperature and propylene composition with one-to-one relationship with reboiler duty and propylene feed are selected as controlled variables for maintaining bottom purity and feed ratio in the presence of possible mea-surement bias respectively.High nonlinearity between selected input–output pair was reduced by using variable transformation.Dynamic simulations demonstrated that such a control scheme with nonlinear transformed variable was capable of providing much superior control performance than the one using natural variable.Ó2005Elsevier Ltd.All rights reserved.Keywords:Reactive distillation;Nonlinearity;Multiplicity1.IntroductionReactive distillation column has become an attractive unit operation in recent years due to its ability to reduce energy consumption and the number of equipment unit,as well as its ability to break azeotropes.Research and development of reactive distillation has been compre-hensively reviewed by Doherty and Buzad [1]and Taylor and Krishna [2].One of the main disadvantages of a reactive distillation column is that the process exhibits nonlinear characteristics such as multiple steady states and high sensitivity to operating variables due to the coupling between separation and chemical reaction [3–13].Hence special consideration must be given to the design of its control system.For example,two-point control scheme [14],nonlinear control strategy [15,16],adaptive control [17]and robust PI [18]control configu-ration were used in controlling ETBE,ethylene glycol and ethyl acetate reactive distillation processes,respec-tively.The keys to controlling a reactive distillation col-umn in which the reaction is equilibrium controlled are to maintain the product composition as well as stoichi-ometric balance between the reactant feeds.When there is no measurement bias of the absolute feed flow rate,a ratio control of the feed flow rates coupled with the usual temperature control for distillation product purity is sufficient for the reaction in an equilibrium control re-gime.However,Al-Arfaj and Luyben [19]pointed out that such a feed-ratio control scheme is one with no assurance that correct stoichiometric balance is main-tained in case of feed flow measurement bias.The use of a composition analyzer in the reactive zone to main-tain stoichiometric balance was advocated.They also have studied a number of reactive distillation columns [20–24]and found that effective control can be provided by the simple PI control ing a MTBE0959-1524/$-see front matter Ó2005Elsevier Ltd.All rights reserved.doi:10.1016/j.jprocont.2005.06.015*Corresponding author.Tel.:+88635927700x2853;fax:+88635927310.E-mail address:cewsj@.tw (S.-J.Wang)./locate/jprocontJournal of Process Control 16(2006)385–394synthesis column as an example,Wang et al.[25]found that interaction multiplicity between stoichiometric bal-ance and product quality control loops must be elimi-nated for such a linear control strategy to work.In an equilibrium controlled reactive distillation column,the reaction rate is infinite.Therefore the net extent of reac-tion,which is nonzero[26],will not be affected by the residence in the reactive section.The nominal tempera-ture and composition profiles will not be affected if all flow rates were increased in the same proportion.When the reaction is kinetically controlled,the reaction rate is finite and the temperature and composition profiles will change even if stoichiometric balance of the feeds can be maintained[27].Set-point adjustment of the internal composition control loop by eitherfinal product purityfeedback[27]or direct throughput rate feedforward con-trol[28]becomes necessary.Isopropanol(IPA)is a widely used solvent in chemi-cal and cosmetics industries.It is used as a feedstock for the manufacture of acetone and other compounds. Direct hydration of propylene is widely used in the manufacture of IPA[29].However,a complex distilla-tion scheme is required to recover IPA.The synthesis reaction of IPA is an equilibrium-limited reaction. Therefore,reactive distillation can be used to improve conversion and simplify separation.In this study,the process characteristics and control strategy of a reactive distillation for the production of high-purity IPA (99.9mol%)by direct hydration of propylene are inves-tigated.The main purpose is to show that a simple control strategy coupled with a nonlinear variable trans-formation,will keep the IPA column operating at high product purity even though the overall process exhibits steady state multiplicities and highly nonlinear dynamics.2.Base case column designThe reactive distillation column shown in Fig.1is used as the basis of process characteristic and control study.A similar column has been studied by Xu et al.[30].It consists of28stages,including a total condenser,a partial reboiler and26column stages.The column is operated at20atm.and its pressure drop is assumed to be0.135atm.There are three zones in the column: rectification zone(stages1and2)and stripping zone (stages6–28)purifying top and bottom products.Pure water and propylene,high and low boiling reactants, at298K and20atmospheres are fed to the stages3 and5,respectively,numbering from column top.IPA is formed in the reaction zone between stages3and5 according to the following reversible reaction and with-drawn from column bottom:waterþpropylene IPAð1ÞDi-isopropyl ether(DIPE),a by-product,is produced by the following reaction:IPAþIPA waterþDIPEð2ÞThe simulation of the IPA column is carried out using the rigorous distillation model provided by ChemCad software.The liquid phase activities were calculated by using UNIFAC.Redlich–Kwong equation of state was used to predict the nonideal vapor phase behavior. The IPA yield is strongly limited by the equilibrium con-version.The reversible reaction needs to be catalyzed by strong acids.K a1and K a2,the equilibrium constants of the main and side reactions,given by Xu[31],are used ln K a1¼À18:46þ7592Tð3Þln K a2¼133:7À46103Tð4ÞEvery stage is assumed to be in chemical reaction equi-librium.A designed water feed rate of100mol/s is used.In IPA synthesis,the lightest component is propylene and the heaviest component is water.IPA is withdrawn from the bottom.Hence it is desirable to keep an excessNomenclatureK a reaction equilibrium constant K c proportional gainK cu ultimate gainn odd number P u ultimate periodT temperature(K)T I integral timeh logarithmic temperature386S.-J.Wang,D.S.H.Wong/Journal of Process Control16(2006)385–394of propylene,otherwise water will be found at the bot-tom of the column,limiting the IPA purity.The unre-acted propylene can be returned to the column by near total reflux at the top of the column or recycled back to the feed location.If all the unreacted propylene is refluxed at the top of the column,the propylene feed and water feed should be at stoichiometric ratio.High purity of IPA can be obtained at a reboiler duty of9.4MJ/s(Fig.2a).How-ever output multiplicity can be found and the design is extremely sensitive to whether exact stoichiometric bal-ance is maintained.If propylene feed is in slight excess, there will be a large increase in reboiler duty(Fig.2b).If there is a slight excess in water,the purity of IPA can be decreased significantly(Fig.2c).Alternatively one can use a small reflux ratio and re-cycle some excess propylene using a recycled feed. Fig.3a shows that for a recycled propylene feed ofS.-J.Wang,D.S.H.Wong/Journal of Process Control16(2006)385–39438750mol/s (based on a water feed and a fresh propylene feed of 100mol/s)and a reflux ratio of 30,high-purity IPA can be obtained.The reboiler duty required to achieve this purity is higher (19.4MJ/s)compared with the case of total reflux.The output multiplicity can be eliminated,although input multiplicity remains to be found.This design is much less sensitive to errors in water feed rate.If there is less water feed,high purity can be achieved with higher reboiler duty (Fig.3b).If there is more water in the feed,the same purity can be achieved at a lower reboiler duty (Fig.3c).Hence there is a trade offbetween energy efficiency and robustness of operating condition to error in feed rate.One can run this operation at different excess combinations of recy-cled feed rate and reflux ratio,and obtain a high purity of IPA.In general,the better the energy efficiency,the range of operability will be smaller.We have chosen the design with 50mol/s of recycle feed and a reflux ratio of 30to ensure that high-purity IPA is obtainable even when there are +10%fluctuations in water feed rate.Table 1lists the conditions of this base case design.3.Temperature +feed-ratio controlAlthough there is no output multiplicity in the given base case design,there is still input multiplicity which makes direct control of IPA purity impossible.Alterna-tive candidates for the controlled variable are the tem-perature of internal stages.The relationship between stage temperature and reboiler duty with reflux ratio fixed at 30is shown in Fig.4.Very low sensitivity and near-constant value of temperature can be found at stages 1and 2.Input multiplicity occurs at the tempera-tures of stages 3and 4and between the temperatures of stages 7–28around nominal operating condition.Temperatures of stages 5and 6are the only two vari-ables that have one-to-one relationship with reboiler duty.Both temperatures show very strong nonlinearity.They have low sensitivity when reboiler duty is below19.409MJ/s and above 19.450MJ/s,but very high sen-sitivity when reboiler duty is between 19.409MJ/s and 19.450MJ/s.The response of temperature of stage 6is found to be faster than that of stage 5.Hence tempera-ture of stage 6is chosen as the controlled variable of temperature control loop.The propylene and water feed ratio is designed to be 1.5:1in order to eliminate output multiplicity.TheTable 1Nominal operating condition of IPA reactive distillation column Feed condition Column specificationFeedWater Propylene Rectification stages 2Feed tray35Reaction stages 3Temperature (K)298298Stripping stages23Pressure (atm.)2020Overhead pressure (atm.)20Flow rate (mol/s)100150Column pressure drop (atm.)0.135Reflux ratio30Reflux rate (mol/s)1497Bottom rate (mol/s)99.96Reboiler duty (MJ/s)19.4Water conversion (molar basis)0.9997Distillate propylene purity (mol%)99.93Bottom IPA purity (mol%)99.9388S.-J.Wang,D.S.H.Wong /Journal of Process Control 16(2006)385–394simplest way to maintain this ratio is to use feed-ratio control.Fig.5illustrates the proposed temperature+ feed-ratio control scheme.Both theflow rates of water feed and fresh propylene feed are measured and con-trolled.The set point of total propylene feedflow is adjusted to1.5times that of the water feedflow by ratio control.A buffer tank containing large inventory of pro-pylene is installed at recycle line of propylene.It ensures that sufficient recycle propylene feed is available in case of large increase in water feed.Column pressure is con-trolled by manipulating coolantflow rate.Reflux-drum level and base level are maintained by changing reflux flow rate and bottomflow rate respectively.Reflux ratio is controlled by manipulating distillateflow rate.Tem-perature control loop is implemented by manipulating the steam supply to the reboiler.Temperature of stage 6is used as the controlled variable of the loop.All con-trol valves are designed to be half open at the initial stea-dy state.Dead time of5s is assumed for the temperature measurement with a transmitter span of100°C.Dead time offive minutes is assumed for the concentration measurement.The transmitter span of composition measurement is taken as0.6.Theflow measurement has no dead time and a span equals to the difference of1.5and0.5time the nominal operatingflow.To avoid control stability problems induced by high nonlinearity, the tuning parameters of temperature control loop are obtained by trial-and-error.Fig.6shows the dynamic responses of the control scheme when the set point of the waterflow loop is changed by±20%with no bias in feedflow measure-ment.In the case of feedflow increase,the controlled stage temperature and bottom IPA product purity can quickly be settled at their set point and designed operat-ing value,respectively.This is because the temperatureS.-J.Wang,D.S.H.Wong/Journal of Process Control16(2006)385–394389of stage6will initially decrease from its nominal set point of404.2K into the region with low sensitivity. However,in the case when feedflow decreases,the tem-perature of stage6initially increases from404.2K into the region(404.5–431.4K)which is highly sensitive to the reboiler duty.Hence the temperature of stage6oscil-lates sharply around431.4K for about20h before returning sharply back to slightly below404.2K.Once the temperature is below404.2K,the system becomes insensitive to reboiler duty and takes a long time to return gradually to its set point.The purity of IPA in the bottom also shows similar trends.4.Variable transformationThe temperature of stage6exhibits highly nonlinear characteristic with reboiler duty.The use of nonlin-ear control element becomes necessary to improve performance.Bequette[32]surveyed nonlinear control techniques ranging from ad hoc or process-specific strat-egies to predictive control approaches.One of nonlinear control techniques is variable transformation approach.A common transformation variable used in high purity column is the logarithmic temperature calculated by the following equation[33]:h¼lnTÀT topT botÀTð5Þh and T are the logarithmic and original temperatures, respectively.T top and T bot represent column top and bottom temperatures,respectively.However,in this column case,nonlinear characteristic of the relation between temperature of stage6and reboiler duty remains unchanged if the logarithmic transform is used (Fig.7).The relation between temperature of stage6 and reboiler duty shown in Fig.4resembles the function tanh(x)1/n,where n is an odd number.After scaling and translation,the following transformation formula for temperature of stage6is used:50ÁtanhÀ1123:639ðT6À417:89Þ11þ420ð6ÞFig.8shows the relationship between reboiler duty and original temperature and transformed temperature of stage6.It can be found the transformed temperature390S.-J.Wang,D.S.H.Wong/Journal of Process Control16(2006)385–394has more linear behavior with reboiler duty than the origi-nal one.For the transformed temperature loop,the controller was tuned using a sequential design approach[34]. Relay-feedback test[35]is performed to obtain ultimate gain and ultimate frequency.The following equations are then used to calculate the tuning parameters of PI controllers:K c¼K cu=3ð7ÞT I¼P u=0:5ð8Þwhere K c and T I represent proportional gain and integral time respectively,and K cu and P u are ultimate gain and ultimate period respectively.The dynamic responses of the control scheme using transformed temperature as controlled variable are shown in Fig.9 when there is aÀ20%change in waterflow -pared with Fig.6,it is obvious that the control scheme with transformed temperature provides much superior performance.5.Temperature+composition controlAl-Arfaj and Luyben[19]pointed out that a feed-ratio control scheme is a feedforward scheme,that has no guarantee of thefinal product quality.When there is a measurement bias in feedflow rate,the feed-ratio control scheme will not be able to keep the desired feed ratio.Fig.10illustrates the dynamic responses of the transformed temperature+feed-ratio control scheme to±10%flow measurement bias in water feed rate. The bottom IPA purity cannot return to its designed value,even though the controlled stage temperature can settle at its set point.To overcome the shortcomings of a feed-ratio control,Al-Arfaj and Luyben[19]sug-gested that the composition of some column stage be controlled by the feedflow.Although online composi-tion measurement is more expensive and has a longer dead time,it does not depend on the absolute value of the measurement.However,the problem is tofind a proper candidate for controlled variable.Fig.11shows the effect of propylene feed on propylene compositions at different stages.Very low sensitivity and near-constant value of propylene composition are found in stages1and2.Low propylene compositions are found in stages8–28.The candidates left are stages3–7. Fig.12shows the effect of reboiler duty on propyleneS.-J.Wang,D.S.H.Wong/Journal of Process Control16(2006)385–394391composition.It can be found that interaction multiplic-ity occurs between reboiler duty and propylene compo-sitions of stages3and4.Thus propylene compositions of stages5–7are the only candidates for the controlled variables of the composition loop.The propylene com-position of stages6and7has slower response to propyl-ene feedflow.Hence the propylene composition of stage 5is chosen as the controlled variable of composition loop.Very strong nonlinearity occurs around nominal operating condition between propylene composition of stage5and propylene feedflow.To have better control performance,the propylene composition of stage5,c5,is transformed by the similar procedures as those for the variable transformation of stage temperature by using the following equation:tanhÀ110:235ðc5À0:435Þ11þ0:45ð9ÞFig.13shows the difference between original composi-tion and transformed composition.Much linear behav-ior is obtained using the transformed variable.The dynamic responses under the control scheme are shown in Fig.14when there is aflow measurement bias ±10%in water feed rate.Both controlled variables,pro-pylene composition of stage5and temperature of stage 6,can return to their set point respectively.Bottom purity of IPA can also return to its nominal operating value.392S.-J.Wang,D.S.H.Wong/Journal of Process Control16(2006)385–3946.ConclusionsIn this study,we have used the steady-state and dynamic simulations to investigate the process charac-teristics and control strategy of a reactive distillation column for IPA synthesis by direct hydration of propyl-ene.The keys to a robust operation are maintaining an excess of propylene feed to the column and recycling the unreacted propylene to the feed instead of the top stage. Quality can be maintained by controlling the tempera-ture of some column stage.Input multiplicity between bottom IPA purity and reboiler duty can be avoided by choosing the stage temperature having monotonic relationship with reboiler duty.Maintenance of feed ratio in the presence offlow measurement bias can be implemented by controlling propylene composition of an intermediate stage which has monotonic relationship with propylene feed.Variable transformation is used to overcome the strong nonlinearity existing in tempera-ture and composition loops.Simulation results show that the transformed temperature and composition con-trol scheme can provide superior control performance. 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