Chemical Process Control
Chemical Process Control CHBE 4400
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This 0 page Class Notes was uploaded by Hazel Simonis on Monday November 2, 2015. The Class Notes belongs to CHBE 4400 at Georgia Institute of Technology - Main Campus taught by Jay Lee in Fall. Since its upload, it has received 9 views. For similar materials see /class/233973/chbe-4400-georgia-institute-of-technology-main-campus in Chemical Engineering at Georgia Institute of Technology - Main Campus.
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Date Created: 11/02/15
Lecture 2A Implementation of Control Sensors and Transmitters CHE44OO Professor Jay H Lee Georgia Inst Technology What Do We Need Sensor eg thermocouple Transmitter e g signal converter ampli er conditioner Transmission Line e g electrical line air tube data line Controller e g computer Actuator e g control valve Typical Setup Disturbance P t C Process input MV rocess OUtpu V CT Desired Controller V output 420mA or 315 psig Exemplary Control Loop Actuator F2 39 lt T2 Sensor 4mowell Thermocouple millivolt signal V T DCS 4 20 mA Operator 5quot Control Transmitter Console Computer Figure 22 Schematic of the control system on the CST thermowell mixer showing each component along with the various signals Implementation Modes 0 Pneumatic Transmission Pressure signals 315 psig Control Calculation Mechanical device e g tubes bellows Only very basic calculation can be performed Safe in general Electronic Analog Transmission Analog electric signal 420 mA Control Calculation Analog circuit More advanced calculation but still not very exible Susceptible to contamination Cannot be used in safetysensitive processes Digital Implementation t w Ea D A SAMPLER AD l A I Input Sequence Output sequence CONTROL ALGORITHM A v v v 13 y0y1y2 0 1 2 quotu vovtsV2tS Y0Ytsy2ts Implementation Modes 0 Digital Transmission either analog signal which gets converted to digital signal just before entering the controller or digital signal sequence of 01 binary pulses AD converts analog signal to digital signal DA converts digital signal to analog signal Controller digital computer can be complex High exibility and easy recon guration Easy access of remote data and past data Advantages of Digital Implementation Increased exibility much easier to manipulate data on computers than analog signals through analog devices Signal conditioning eg noise filtering Control calculation algorithm Easy storage and display More precision Little signal degradation Opportunity for improved control Smart actuators and sensors More sophisticated control algorithms Easily recon gurable Sensors Physical Properties gt Signals appropriate for electric and mechanical processing Common sensors Temperature thermocouple Pressure bellows bourdon tube diaphragm Differential pressure same as above Flowrate ori ce venturi vane magnetic ow meter Liquid level free oat xed oat differential pressure pH pH meter electrode Viscosity differential pressure under constant owrate Chemical composition Gas chromatograph Temperature Sensors l Thermocouple Two metal junctions at different temperatures generating a voltage that is proportional to the temperature A lt C 95 gtZN Afftectetdtby the t 39 am 1611 em era ure T J T P V C Tad Electric T quotquot rce39PuiHrquotquot39 h A Electric ice point generates emf that compensates for the ambient temperature effect Chromelalumel Ktype most popularly used Ironconstantan J type higher emf 0Copperconstantan TType cryogenic temperature l3 RhPt Pt Rtype high temperature gt9000C Temperature Sensors 2 Resistance Thermometer Detector RTD Resistance of certain metals depends strongly upon their temperature Platinum and nickel are typical choices More accurate and repeatable but more expensive and less rugged than TCs Used for important temperature control points reactors distillation columns 1mA Temperature Sensors 3 Optical Pyrometer Estimates from the wavelength for the radiation of a hot gtlOOO C body eg furnace tubes Does not require thermal contact 0 Thermistors Temperature dependence of its resistance Limited in range eg ilOOF Less expensive than RTD Gas liquid bimetal devices Temperature Sensors 4 Summary of temperature sensors and chracteristics Principle Type Usable Range 0 Remarks Thermal expansion gas expansion 23 600 N2 liquid 200 350 oil bimetal 50 500 Resistance PtlOO Q 200 500 accurate linear selfheating Thermistor lt 300 cheap inaccurate nonlinear emf thermocouple 200 1600 low sensitivity IC temp sensor 100 150 high voltage accurate linear Radiation Pyrometer very wide noncontacting need accurate calibratio Pressure Measurement DP Cells Balance bars BaSiC Primin 0f DP C6115 DP Cell from FisherRosemont Strain Gauges High pressure measurements Stretched wire elastic a Increased Resistance Flow Measurement 1 Signal represenling differential pressure Jl Crosssectional area squot of orifice DIHEIEI IUBI plessuve transmitter NO 6 lt Inert gas purge Iine quot Supply 55555 m Q 2 2341 J1 ltLzAAgtV p Diaphragm 000000 I 000000 J We Crosssectlonal plate Orifice and DP Measurement area 0f Pipe What s the basic relationship between owrate and pressure drop How do we size the ori ce ow range vs AP range Choose the maximum pressure drop according to the range of the DP measurement device and calculate the opening for the maximum ow rate 1 Maximum pressure drop should be lt 4 of the total line pressure 2 Diameter ratio of the openings should be between 02 and 07 3 Reynolds number between 104107 If 13 are not satis ed choose a different DP cell Example Sizing an Ori ce Meter Size an ori ce meter for service on a 25 inch schedule 40 pipe 2469 in diameter carrying water with a maximum ow rate of 180GPM and a minimum ow rate of 60 GPM Assume that the line pressure is lSOpsig Use C 010 6 Choose a DP cell with a 02psi range Calibrate the DP cell so that 180GPM9133psi 67 of the full range gt09 of the line pressure but A2Al0742 use the previous equation to calculate this so requirement 2 is not satis ed Choose a DP cell with a 05psi range Calibrate the DP cell so that 180GPMH333psi 67 of the full range 322 of the line pressure and A2Al0573 Flow Measurement 2 Vortex shedding ow meter Inserting an obstruction in the pipe and measuring the frequency of downstream pulses created by the ow past the obstruction Clean low viscosity liquids and gases More expensive than ori ce meter but easier to maintain Should watch out for cavitation partial vaporization of liquid causing noise Accurate for Regtl0000 Magnetic ow meter Electrically conducting uid passing through a magnetic field created by the device Voltage generated is proportional to the ow rate Viscous uids slurries highly corrosive materials etc e g in waster water treatment facilities Flowmeters are typically installed before in the upstream of the control valve less noise etc Flow Measurement 3 Vortex ow meter Magnetic ow meter Yokogawa Sparling Level Measurement 1 Closed Tank Gala Value A Max Level INSTAL LAHON WITH DRY LEG Level Measurement 2 Open Tank JIM Level Measurement 3 Open Tank Consta nt Pressure ir or Gas Supply L 000 Chemical Composition Analyzers Gas chromatograph Volatile sample with a carrier gas Different residence times for various components due to different af nities for the sorbent column packing Separated stream passes through a detector thermal conductivity detector or hydrogen ame ionization detector IR NIR Raman and UV spectrophotometers Concentrations Each compound absorbs radiation at certain frequencies and the degree of absorption depends on concentrations Used also for paper and coating thickness basis weight Expensive to purchase and maintain gtlOOKyr Sample preparation long analysis delay 10 min NIR Spectra for Different Compounds and Concentrations NaOH NaOH soln 80120 glL and 80 quotC 3005 01 7005 01 600E 01 5005 01 5 4005 01 l 5 2 3005 01 e K o m 2005 01 g A 1005 01 mm 12 13 15 16 17 10 19 2 21 22 1005 01 wavelength um 2005 01 15B Measured V Slupe 011521 onquot 1 ElElEEElEl l E7EerEE El 999EEE El 999981 rEl ElElE4El3 El 999789 39 lEIEle 5Ele D PVleCIQd V r El ZEI AEI E PEquot 7 EEI lElU lZ EI 1 MEI Fred lcted measured Lignin Lignin soln 2060 gIL and 80 quotC Na2CO3 Nazcogsoln 2060 gIL and 30 quotc Actual Data from A Pulp Mill NaOH 9 Nazo L 6 5 as s 3 s s s e s N159 1515 99999 989 99 9 99 gt9 9 0 9 3 0 r 9 9quot 5 9 99 59 N59 a 5 a 3 0 0 6 9 9 9 quot quot Q R Date amp Time IazCOa 35 s 3 9 of 659 G 9 55 e9 5 6 3 Da e 8 Time eqycbqycgyeqyeqyoqyc eqy 93793 3717 a tyqy qy ey 9 gt 36gt 96 9 Qquot a a Qquot a Q 9 0 Na2CO3 Trend Lines NIR Analyzer Prediction vs Actual Test Results More Sensors Various other optical sensors 7 Particle size distribution 7 Crystal size and morphology Imaging 7 Video Camera Microscopes SEM AFM etc 7 Example Furnace burning ef ciency analyze the shape and color of the ame MEMs Micro Electro Mechanical devices 7 Very small in size 7 Ultrahigh sensitivity Biological warfare detection of toxic materials 7 Biomedical applications diagnosis drug delivery etc Soft sensors Inference from other process variables 7 Least Squares regression 7 Arti cial Neural Networks Transmitters Sensor signal typically in mV gt signal that can be transmitted and accepted by the controller e g 420mA Example 500C 100 OCgt4mA 20mA Gain of the transmitter 16 mA 500C 016mA OC Zero of the transmitter value when the output is at the minimum 4mA 50 OC Question Why make the minimum output 4mA instead of OmA Ideal Transmitter Ideally a transmitter would behave linearly throughout H Span 100 4 gt39I Zero 50quot I 50 100 150 T 39C Figure 94 A linear instrument calibration showing its zero and span 0 In this case the gain and zero speci es the entire behavior Real Transmitter Real transmitter is likely to show some nonlinear behavior l I I I I Operating boini 2 I I I I I I I I I l I I I I 4 Operating pomtl I 150 Tt39Cl Figure 95 Gain oi a nonlinear transducer as a function of operating point The nonlinear behavior could also be due to the sensor as well as the transmitter Possible Transmission Arrangements for Digital Control Transmit and then digitize Into Ifowl I d Analog Signal I Computer Slgna Ampli er A gt a Transmission Line 0 Digitize and then transmit 170W Digital Signal 51311 31 gt Directly Into Transmission Line Compmer Multiplexing Using multiplexing multiple signals can be ampli ed by a same ampli er transmitted through a same line and digitized by a single AD converter Sensor signals AmPIi ed 10 mV 500 X amp signals 5 V Sequentially t rrrr mitted Multiplexer b Multiplexing followed by ampli cation Dynamics of sensors and transmitters are relatively fast compared to typical chemical process dynamics but they Dynamics can sometimes be signi cant d I Tm ym 2ka dt ym y i Zia 70 aw kw J Sensor Transmitter Errors Error Bias Variance Bias systematic error true value mean Variance repeatability quotNumber of oowmnccs of a particular reading h ute 914 Analysis of types ot error for a flow instrument whose range is o to 4 ow units 0 Most likely Tme value measured valug Total error i E l maximum I l Systematic l l I error bias li I Random Llt I i error gtIlt gt1 repeatability I I l i I I I l 39 x I I 39 x l x I I l I I I I I l X I I I I I l I l l I I I I I I I I r I olzol I I 1025 30 035 040 q ow units Ciykaulcj
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