Document vVVLb0GGo2JqrQgr7jgyLKRx8

ABD00262714 l tj 1 i j } 1 < 1 I i Awareness of the many variables in Air Supply--Controlled by damper in the dis > charge of a fan. dryer operation is essential to the Material Throughput--Movement of the ma j design of dryer control systems. terial in a rotary kiln or dryer depends on speed, di j ameter and slope of the dryer. Belt dryer throughput ALFRED H. McKINNEY rate is easily controlled by belt speed. E. I. du Pont dc S'emours <* Co., Inc. Air Recirculation Rate--In some screen dryers, recirculated air is introduced at one or more points ! Drying operations have a number of significant vari with a separate blower. It is controllable by a damper. ables that are not easy to measure directly. There Generally, air recirculation is not deliberately intro fore, general practice is to hold measurable conditions duced or controlled in other types of dryers, but it is constant so that a uniform product will be made. This present to some extent due to convection currents. works well if there are no load changes. The basic Feed Rate--Feed rate of material to be dried is control problem is essentially the same for rotary, commonly held constant or it is determined by produc i traveling screen, moving sheet, tray and screw-con tion needs. Theoretically, it could be varied in re veyor dryers. This discussion, in terms of the typical sponse to a measurement. rotary, dryer, also applies to the other types. Spray Exhaust Damper--Air can be pushed (forced- dryers present additional problems and are discussed draft fan) or pulled (induced-draft fan) through a separately. dryer. If both types of fans are used, either but not Both temperatures and pressures are usually easy both can control air flow. The other fan can control i to measure in drying systems. These or any other pressure at a single point between the two dampers. readily available measurement can be used for control Product Recirculation--This is an infrequent since system dynamics are favorable in drying systems practice, used mostly when the material handling where there is a reasonably rapid response to change. problem becomes troublesome. It is useful in rotary A generalized drying system in the form of a rotary kilns to prevent ring formation of reacting products.' 'j> dryer is shown in the illustration. Product recirculation takes place in designs with Process control for dryers can be supplied at sever, countercurrent air at suflicient velocity to move some points: of the product to the feed end. Heat Supply--Can be steam, fuel or electrical Probably no single dryer has been built with all { energy. seven conditions automatically controlled. However, it 7 This article Is bnsv`1 on th.j manuscript for the forthcoming McGraw-Hill ,Hi'.r.>ll.o<.r: of Applkr'I Instrumentation." all of these conditions affect dryer performance to some extent. Any of the seven conditions can be held Chemical MavEngineering-- J. 1961 i 79 DRYERS . . ABD00262715 Possible control arrangements for dryers--Table I Heat Supply Air Supply Belt or Shell Speed Air Recitculalion Rate Feed Rate Exhaust Damper Product Recirculation Inlet air temperature, Ti.......... Exhaust air temperature, T;. . . Pressure, P .................................... Exhaust air/................................. wet bulb temperature, Tj. . A A A A A A A A B AA B A A AA B Temperature................................ A A A A8 Moisture........................................ A A A AB Degradation................................ A A A AB B B B Solids depth................................. A A' Retention time............................. AA Air velocity................................... A AA Evaporation rote........................ A A AA Solids/gas ratio......................... A B 'A A A one) B rpre*enl good end foir control, rejpectively, of loop determined by intersection of row ond column. Colored ^repretents vo'ioblnj commonly controlled by each measurement. A* for rolory unit and blank for screen dryer. A \ ' A/' \ \ constant either because it is not convenient to control thorn, or because there is no suitable measurement within the system to regulate these conditions. Control of all system valuables does not require each controllable condition in be manipulated auto matically from a measurement, for example, an op erator can adjust a valve in response to an appro priate measurement. Each control clement can be operated manually or automatically. Table Lists Control Arrangements Possible control arrangements for dryers are shown in Table I. The vertical columns are the seven ele ments that can be used to control the system. Hori zontal rows depict: Measurements that are commonly used. Product qualities that are desirable to control where suitable direct measurements are possible. Conditions within the dryer that can serve as guides for proper operating conditions. An A at the intersection of a row and column means that the measurement in that row can be used to con trol the condition shown in the given column. B means that closing this control loop is not as satisfactory, but is possible in most installations. Blank spaces in the table indicate that control would be unsatisfac tory. Good control of a single loop, indicated by A, assumes that the equipment is operating with all con trol elements in a fixed position and that the one cle ment under question is varied. For example, in column 1. A appears opposite the measurements for inlet air temperature, exhaust air temperature, exhaust air wet-bull) temperature, prod uct temperature, product moisture, product degrada tion and evaporation rate. This means that all of these measurements, actual or potential, would be in fluenced by a change in the position of the valve in the heat supply. Thus, almost any one of these meas urements, if available, can be used to control the heat supply. However, the moisture content of the final product would be ideal in principle. In Table I, the A values printed in color indicate variables commonly controlled by each measurement. The three in color are: (1) inlet air temperature, 7\ that controls heat supply; (2) pressure, P that con trols the exhaust damper and (3) exhaust air wetbulb temperature, T3 that controls air recirculation rate. Only the measurements T,, T3 and P will be con trolled. The others vary within certain limits. Examine Variables That Affect Control In most dryer systems, the following variables in fluence the performance of the control system and, hence, are of interest to us: Dry Bulb Temperatures--Thermocouples, resist ance thermometers or filled-system thermometers are used according to the range of temperatures encoun tered. It is important to locate the measuring element at a representative point and to protect it from cor rosive and abrasive conditions. Signal response time is not usually critical so that heavy-duty protective wells are common. In hightemperature gas streams, the temperature-sensing clement must be properly shielded from cold surfaces, which would cause radiation errors. Suction pyrom eters or very small, shielded, bare thermocouples are necessary to obtain measurements approaching the true gas temperature. SO Afnv !, Cukmkxm. Fncinkkiung \ ?? i i i ABD00262716 tent is above 2-3% and the handling characteristics are favorable. Thermal Degradation--This degradation of the product is a serious problem in many drying opera tions. It has rarely been successfully measured continuously and with sufficient rapidity for good con trol. Maximum temperature and minimum flow con ditions are usually the basis for safe operating limits. Pressure--Dryer pressure measurements vary from fractions of inches of water to much higher values. For dusty conditions, it is good practice to purge the line between the point of measurement and the instru ment. This prevents the .accumulation of dust in the stagnant pipe run. Pressure can be controlled equally well from either of the dampers in the suction fan duct or the forced-draft fan duct. The damper not used to control pressure must be operated by some other measurement within the system. In combina tion, the two dampers will control the air flow and also the system pressure at the point of measurement. The pressure measurement can be located at any place between the two dampers. Controlled pressure may be either positive or nega tive within the range of the two fans. If pressure is to be controlled close to atmospheric at a single point in the system, the pressure will differ from atmos pheric at every other point in the system. A chimney effect will occur between different heights if the gas density inside the dryer differs from atmospheric density. There will also be a pressure drop due to air motion inside the dryer. Humidity--Exhaust air humidity is usually meas ured with a wet-bulb psvehrometer. Several electrical humidity-sensing elements are also available. Product Temperature--Commonly used to guide op erators in controlling dryers, this is a very useful measurement when it can be obtained. Product Moisture Content--This measurement is used extensively for drying continuous sheets. How ever, moisture content of granular solids and powders is difficult to measure continuously. Dielectric con stant has been used as a moisture content measure ment of powders where the final desired moisture con Operating Conditions in Dryers The screen dryer in Fig. 1 shows a single drying section only. The controls on each zone of a multi section dryer would he the same except that pressure drop through the material, measured in only one sec tion. is used to control the belt speed. A belt-speed change may be required to hold constant pressure drop through the material being dried if its con sistency or throughput rate changes. The uncon trolled variable is feed rate. With the_drnm dryer shown in Fig. 2. the product moisture content is measured and used to control steam input to the drum. The evaporation from the drum depends upon the external air condition and velocity as well as the drum temperature. The feed rate is controlled to hold the level in the feed hopper. The feed stream will vary with the capacity of the drum at various speeds, web thickness, and tempera ture conditions. 'fable II shows the control loops possible between the various controllable conditions and the measure ments that might be made in spray dryer operation. A commonly used system for control of spray dryers is shown in Fig. 3. Air inlet temperature controls the heat input. Exhaust air temperature controls the feed rate. Pressure drop through the atomizing nozzle will also alter the average size and size dis tribution of the drops of liquid as well as the feed rate. Size distribution may be the most important factor in spray drying operation because the smaller drops dehydrate more rapidly. Also, the smaller the drops, the more variable is the distribution. If the air flow is interrupted, an interlock is desir able to shut off the feed quickly. This arrangement avoids accumulation of wet material in the bottom of the dryer. Alarms are sometimes provided for high exit gas temperatures or pluggage of the product col lecting system. Dead-times for some spray dryers are shorter (re sponse is rapid) than for other types and control may be easier. Faster measuring and control elements may be required. The system is usually sealed and there is no need to balance pressures. The system pressure is usually not controlled. An interlock is usually provided in the spray dryer to shut off the feed if temperature falls below operating limits. Provide Adequate Safe Operations Two types of specific safety problems may arise in the use of dryers. The first is fiame failure detec tion in direct-fired units. The equipment generally provided to guard against flame failure is substan- Cnr.Mir.A). --Mav 1, 1961 81 DRYERS . ABD00262717 Control chart tor spray dryers--Table II Heat Supply Inlet Air Damper Atomizer Speed Air Recirculotion Feed Rate Inlet oir temperature............ Exhaust air temperoture. . . A A A A BA '\ Exhaust Damper Product Recirculation A B Temperature........................... A B AA B Moisture................................... A A A A AA B Thermo! degradation........... A A B B AA A Size........................................... A Size distribution..................... A Pressure .................................... Exhaust air humidity............. Retention time......................... Air velocity.............................. Evaporation rate................... Sofids/gas rclio..................... A A A A A A A A B A A 8 A Bond represent ccod ond fclr <ontrol, repe:!iHy, of loop determined by intersection of tow ond column. Colored Arepfttonfi voricbles commonly controlled by ecch measurement. A A A A A A tially more elaborate than the remaining instrumen tation. Failure of combustion safeguards because of man ual cutout arrangements, etc., is often so serious that dual control systems .should be used. The first sys tem detects the existence of a flame in the conven tional manner. The second system continuously an alyzes the combustion products in the dryer for explosibility. Either system can .shut down the equipment. The second hazard exists where dusts or solvent vapors can form combustible mixtures with the air used to dry the materials. This condition has caused serious accidents even though no source of ignition presumably existed within the system. Inert atmospheres are used to dry hazardous mate rials in some cases. Where this is impractical, the solvent concentrations have been controlled by con tinuous analyzers that sample the atmosphere in the dryer and control the drying rate to keep the mix ture below the lower explosive limit. The sample line leading io the gas analyzer must be kept above the dew point of the solvent (or com bustible material) throughout its length to prevent erroneous readings. Both automatic blanketing with inert gases and shut down of the operation are trig gered if the analyzer detects mixtures approaching the explosive limit. Alarm signals and interlock ar rangements should be provided. They are actuated whenever unsafe conditions exist or are developing. Control system for spray dryer-- Fig-. ') fiC-0 Inlei air temp. IV C Feed"! shut-off! Fefd ! E*hausf ! oir temp L lTFS] Reference ]. Aikman, A. !L, "Frcquencv--Response and Controllability of a Chemical Plant," Trans. ASM!-'. 70, 1312-23 (1934). 2. Hickman, M. "Experimental Instrumentation for Dryers," 7. Inst. Fuel, 28, 113-16 (1955). 3. Holzmnnn, K G., "Dynamic Analysis of Chemical Proc esses." Trans. AS.MK. 7fi, 251 (1956). 4. Kramers, H. and G. Alberda, "Frcriuency Response Analysis of Continuous Flow Systems," Chcm. Eng. Science, 2. 173-81 (1953). 5. Marshall. W. R., Jr., "Atomization and Spray Drying," Chcm. Eng. I'rog. Monograph Series So. 2, .r*n, (1954). 6. Mozley. J. M., "Predicting Dynamics of Concentric Pipe Heat Exchangers," Ind. Eng. Chem., 48, p. 1035 (1956). 7. Saeman, IV. C. and T. R. Mitchell. Jr., "Analysis of Rotary Drver and Cooler Performance," Chcm. Eng. Frog., 50, 467-75 ( 1 954 ). Meet the Author ACKNOWLEDGEMENTS The following kindly supplied descriptions of rec ommended control systems for dryers and their assist ance is gratefully acknowledged: J. R. Boyd, Louis ville Dryer Div., General American Transportation Corp.; V. A. Cheney, Link-Belt Co. and E. H. Ras mussen, Nichols Engineering & Research Corp. ALFRED H. McKINNEY is a consuRniit in the instru ment section of the Engineering Service Division of E. I. du Pont dc Kcmours ct Co., Inc., Wilmington. He is presi dent of the Wilmington section of ISA and in an active member of AIChE and AC'S. Ue is also licensed as a Professional Engineer ip Pa. Mr. McKinney received a II. S. degree in chemical engineering from Dreycl Tech. <S2 i\hiv I, 1961--Chemical Enoinei: iunc.