DETERMINATION OF OPTIMUM CONDITIONS IN A CONTINUOUS … · Key Words :Drying, Relativehumidity,...

DETERMINATION OF OPTIMUMCONDITIONS IN A CONTINUOUSWALL HEATED FLUIDIZED BED

DRYER

G Srinivasa , M Mallaiahb, S Kishore Kumarc,a,b,c Chemical Engineering Department,

BV Raju Institute of technology (BVRIT),Nursapur, Medak,

Telangana state 502 313, IndiaY Pydi Settyd, dChemical Engineering Department,

NIT Warangal, Warangal,Telangana state 506004, India

[email protected], [email protected],

[email protected], [email protected]

April 30, 2018

Abstract

The drying operation is mostly used in different types ofindustries such as food, pharmaceutical and process indus-tries. Several parameters influence drying operation like gasvelocity, gas temperature, gas relative humidity, solids flowrate, initial moisture content of solids and heat load of thesystem. These parameters influence each other and indi-vidual and by operating at optimum conditions of all theseparameters, the required drying throughput can be obtainedwith optimum operational cost or else it leads to immenseproduct loss or high operational cost. In the present study

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International Journal of Pure and Applied MathematicsVolume 118 No. 24 2018ISSN: 1314-3395 (on-line version)url: http://www.acadpubl.eu/hub/Special Issue http://www.acadpubl.eu/hub/

the optimum conditions have been predicted through sim-ulation studies using Simprosys software. Simprosys soft-ware is used to study the heat, pressure and mass balanceof the drying unit operation. In the present study, effect ofall the parameters has been investigated and the optimumconditions in a wall heated fluidized bed dryer have beenobtained.

Key Words:Drying, Relative humidity, Heat load, Mois-ture content, Simprosys.

1 Introduction

Fluidized bed drying has wide range of advantages in comparisonwith the other type of dryers. Fluidized bed dryers provide highheat and mass transfer rates, higher specific surface area, high rel-ative velocities of individual phase, uniform equilibrium moisturecontent of dried product, uniform moisture and temperature distri-bution in the bed, high particle-particle and particle-wall collisionswhich keeps uniformity in bed, easy operation (no moving parts)and low cost. But fluidized bed dryers have certain disadvantageslike non uniform drying in continuous drying process, heat lossduring transportation of heating medium. Also drying of wet solidstakes place at heating medium temperature and hence the driedproduct has to be cooled to desired temperature. To operate thefluidized dryers at optimum cost the fluidized bed dryers have tobe operated at optimum conditions. In conventional fluidized beddryer, in general heating medium used is hot air and to heat the airfrom room temperature to desired temperature the required heat isvery high due to low thermal conductivity of air. To overcome thisdisadvantage Srinivas and Pydi Setty (2013) have developed thewall heated fluidized bed dryer. Several authors have investigatedthe additional heat source influence on fluidized bed performanceand some of them are listed below.

Zhang and Wei (2017) have studied the bed-to-wall heat trans-fer in a gas-solid bubbling fluidized bed with an immersed verticaltube experimentally and through computational particle fluid dy-namics simulation. Stefan et al. (2014) have studied the particleand gas convective heat transfer in a gas solid fluidized bed in-serting horizontal tube and tube bundles using different Geldart A

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International Journal of Pure and Applied Mathematics Special Issue

and Geldart D group particles. Mandal et al. (2013) have studiedthe heat transfer rates in a packed fluidization technique in whichunary packed bed of same size pebbles as a heating medium usingdifferent particles of lithium titanate and silica. Hou et al. (2012)have studied the heat transfer variation in a horizontally immersedtube in a fluidized bed. Yusuf et al. (2012) have studied experi-mentally particle and gas convective hat transfer in a wall heatedfluidized bed. Yusuf et al. (2011) have studied the hydrodynam-ics & heat transfer distribution in a fluidized bed with immersedtube-bank with horizontal tubes at different fluidization gas veloc-ities. Armstrong et al. (2010) have studied the hydrodynamics ofheat transfer in a wall-to-bed heat transfer in one side wall heatedfluidized bed at different velocities. Hou et al. (2009) have studiedthe convective and conduction heat transfer in a wall heated flu-idized bed using discrete phase model coupled with computationalfluid dynamics. Radmila et al. (2008) have studied the wall-to-bedheat transfer in a wall heated fluidized bed. Gao et al. (2007) havestudied the heat transfer near an immersed object in a gas solid flu-idized bed using double particle-layer and porous medium model.Patil et al. (2006) have studied the wall-to-bed heat transfer in agas-solid fluidized bed using glass. Kim et al. (2003) have studiedthe heat transfer and bubble frequency behaviors in a uidized bedwith immersed horizontal tube bundle. Schmidt and Renz (2000)have studied the heat transfer hydrodynamics in transient condi-tions in a tube immersed fluidized bed at different velocities usingglass beads. Several other authors also studied the heat transfer influidized beds with and without internals and developed models.

The Simprosys software can be used for various purposes andfew are described here. Gong and Mujumdar (2010) have studiedthe drying of non aqueous system using Simprosys software. Theyfound that the Simprosys can be used to examine, modifying andevaluate new and old systems for better performance. Gong etal. (2011) have performed the energy audit of a fiberboard dryingproduction line using Simprosys software. They have applied theindustrial operating conditions in simulation and effect of variousparameters such as heat input and fresh air inflow at various con-ditions studied and suggested various methods for optimization ofenergy savings. Gong and Mujumdar (2014) have simulated thecombustion drying system with Simprosys software. They found

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that Simprosys can be successfully used to implement the combus-tion drying system and also optimization can be carried out withthis software.

Like a conventional continuous fluidized bed dryer, the wallheated fluidized bed dryer has several influencing parameters like airvelocity, temperature, relative humidity, solids flow rate and heatinput to the dryer. The optimum conditions have been investigatedin the present study using the Simprosys software.

2 Modeling and simulation

There are several authors have conducted experiments and devel-oped mathematical models for continuous fluidized bed dryers andsome of them are listed in Table 1.

TABLE 1. Continuous fluidized bed drying investigation byseveral authors

Ragi* - Elesine coracana Lin

The model governing equations for a gas-liquid system in sim-ulation of drying system are presented in equations 1 to 4. Thedesired quantities for equations 1 to 4 have been calculated from

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Keey (1978), Pakowski and Mujumdar (2007), Perry (1997), Fulleret al (1966&1969) and Poling et al.(2001).

t− tWB

Y − Ys,WBT

=−∆hv,WBT

CH

Le−2

3φ (1)

Le =λg

CpρgDAB

(2)

φ =MA/MB

Y ∗ −Y ln(1 +Y ∗ −Y

MA/MB + Y) (3)

DAB =0.01013T 1.75( 1

MA+ 1

MB)1/2

P [(∑

A vi)13 + (

∑B vi)

13 ]2

(4)

3 OPTIMIZATION USING SIMPROSYS

Experimental details for the present study have been taken fromSrinivas and Pydi Setty [1] and simulation studies have been per-formed using the Simprosys software. The Simprosys software toolcan be used for heat, mass, pressure and humidity balance for dif-ferent types of drying equipments like solid dryers, liquid dryers,air filter, scrubber etc. In the software there are different modulesthat are available for drying. In the present study the heat loadto the system is given by heat load supplied to the fluidized bed ofSrinivas and Pydi Setty [1]. The software consists of different datasheets for input and output streams along with specification datasheet for the unit operation. Simulation input stream conditionssuch as gas flow rate wet basis, pressure, dry bulb temperatureand relative humidity of air have been taken from the experimen-tal conditions. The outlet conditions for simulation stream suchas dry bulb temperature is also taken from the experimental resultand the input for the material stream such as mass flow rate ofsolids dry basis, temperature and the moisture content of solids ondry basis have been taken from the experimental conditions andthe simulation outlet condition of moisture content on wet basis istaken from the experimental results. The information of gas pres-sure drop, heat loss, heat input, work input, heat loss by transportdevise, moisture evaporation rate, initial gas temperature, specificheat consumption and thermal efficiency are provided to simulate

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the drying operation. List of contents has been presented in figures1, 2, 3 and 4 as shown below.

Fig. 1. Input stream conditions for gas stream

Fig. 2. Input stream conditions for material stream

Fig. 3. Drying unit operation conditions

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Fig. 4. Model flow sheet of drying equipment

In the above figures the sample data has been presented and inthe simulation, the data has been varied according to the experi-mental conditions. Rheostat has been used to calculate the heatload to the fluidized bed dryer and the gas inlet and out let streamconditions have been measured with humidity meter HTC HD 304and solids moisture content is evaluated on dry basis. Instead ofgas at high temperature, the gas at room temperature is used inexperimentation and also in simulation. Instead of several mod-els specified in the table 1 the Simprosys software can be used forinitial evaluation process.

4 RESULTS AND DISCUSSION

In any type of dryer the drying characteristics are influenced byvarious parameters such as air velocity, air temperature, air rela-tive humidity, solids flow rate, initial moisture content, equilibriummoisture content and solids residence time in the dryer. To en-hance the performance of the fluidized bed dryers, the dryers haveto be operated at optimum conditions such as optimum air veloc-ity, air temperature, relative humidity of air, solids flow rate andinitial moisture content of solids to reduce the operational cost. Inthe present study the optimum conditions have determined throughsimulation using the experimental data. In the present study theoptimum values of different parameters have been estimated by con-sidering the solids outlet temperature and relative humidity of theoutlet air matching with the ambient conditions.

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4.1 Optimum air flow rate

In general, drying kinetics represents, with increase in air flow ratethe drying rate increases due to enhanced gas to solids contactratio and high solids circulation rate at higher velocities [11]. Influidized bed dryers, the gas stream velocity above the minimumfluidization velocity has to be used for good gas to solid contactratio. In the present study the optimum air flow rate has beendetermined by evaluating the solids outlet temperature compilingwith ambient conditions. If the air flow rate used is above optimumair velocity, the drying rate increases slightly and the quantity of airmass increases which will result in increased operational cost. In thepresent study air flow rate is varied from 10 kg/h to 100 kg/h withan interval of 5 kg/h and outlet solids temperature is monitoredin all the cases. In the present case the given input conditions forsimulation are initial moisture content of solids on dry basis 10%,Mass flow rate of solids (dry basis) 10 kg/h, dry bulb temperatureof air is 40 ◦C, relative humidity of air is 0.7 and the air velocity isvaried from 10 kg/h to 100 kg/h with an interval of 5 kg/h. Theunit operation input information provided is gas pressure drop 1atm, heat input 0.4 kw and heat loss by transport devise is 0.1kw. Figure 5 presents the results of solids outlet temperature atdifferent air flow rates and from the figure it can be observed thatwith increase in air velocity the drying rate increases and also theoutlet solids temperature increases. From the results it has beenobserved that the optimum air flow rate is 60 kg/h. At optimumair flow rate of 60 kg/h the outlet solids temperature has beenobserved as room temperature. From this it can observed thatsolids are drawn from the dryer at desired moisture content at roomtemperature. If the outlet solids temperature is significantly higherthan room temperature then the solids need to cooled down and inthe process the equilibrium moisture may vary. If the outlet solidstemperature is significantly less than the room temperature thenin attaining the equilibrium the solids moisture may vary. Hence ithas been suggested that the air flow rate at which the outlet solidstemperature is matching with the room temperature can be usedas optimum air flow rate to avoid variation of moisture in solids.

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Fig. 5. Solids outlet temperature at different air

4.2 Optimum Air temperature

The air temperature plays an important role in the drying of solids.With increase in air stream temperature, the solids outlet tempera-ture increases. With increase in air temperature above the optimumtemperature, the outlet solids temperature increases which requiredcooling of the dried product. For cooling of the dried product thetime and energy both result in the increased cost of drying and alsoto increase the temperature above optimum the required energy in-creases which will be the additional cost. The product moisturemay also vary after cooling. In the present study, the inlet air tem-perature is varied from the 25 to 40 ◦C with increment of 1 ◦C withkeeping remaining parameters constant. Air velocity at 60 kg/h onwet basis and 15% solids initial moisture content on wet basis areconsidered and outlet solids temperature is monitored and resultswere presented in Figure 6. From the figure it can be observed thatwith increasing the inlet air stream temperature the outlet solidstemperature increase and is increasing and in the present study theoptimum is observed in between the 28 to 29 ◦C where the solidsoutlet temperature is also matching with the room temperature.

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Fig. 6. Outlet solids temperature at different input airtemperatures

4.3 Optimum air relative humidity

The Relative Humidity (RH) is defined as the ratio of the partialpressure of water vapor to the equilibrium vapor pressure of waterat a given temperature. The relative humidity plays an importantrole in drying of solids [11]. Generally the relative humidity ofair stream is maintained constant with the help of moisture trapsystem. With increase in the relative humidity of air the drying ratedecreases and it is not possible to use bone dry air in large quantitiesfor the drying purpose and hence optimum relative humidity has tobe determined to attain the required equilibrium moisture contentof solids with optimum cost. The specific enthalpy is defined as thetotal energy in a system due to pressure and temperature per unit ofmass in that system [12]. With increase in the relative humidity thespecific enthalpy increases and also to remove the moisture contentfrom air, the required energy also increases which increases the cost.To avoid this, the optimum relative humidity has to be used. Inthe present study, the relative humidity of air is varied from 10 to100% with interval of 10% and remaining parameters have beenkept constant.

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Fig. 7. Specific enthalpy variation with varying relative humidityof air

Figure 7 presents the results of variation of specific enthalpy atdifferent values of relative humidity of air with 11.1% of solids ini-tial moisture content in wet basis with solids flow rate of 10 kg/h ondrying basis at air velocity of 60 kg/h on wet basis. The optimumrelative humidity can be found based on the ambient relative hu-midity conditions and mostly it may be in between 40 to 60%. It isexpensive process to maintain the relative humidity lower than the40% by considering the ambient conditions. If the relative humid-ity is above 60% in the case the solids outlet equilibrium moisturecontent may be affected due to the reasons the optimum has beenchosen in between the 40 to 60% of relative humidity. In the presentstudy using the relative humidity in between 40 to 60% the out-let solids temperature can be obtained at room temperature withdesired equilibrium moisture contents.

4.4 Optimum solids flow rate

In general in the drying kinetics, with increase in the solids flowrate the drying rate decreases as same amount of heat is suppliedat different high flow rates. At high flow rates the mass of solidsin the bed increases which lead to decrease in drying rate. At lowflow rates of solids the solids in the fluidized bed may dry fast andcarryover of small particles may also happen due to excess heatavailable. Hence the fluidized bed dryers have to be operated atoptimum solids flow rate. In the present study solids flow rate isvaried from 3 to 11 kg/h with an interval of 1 kg/h and solids

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outlet temperature is monitored for initial moisture content of 10%on dry basis at air velocity of 50 kg/h with inlet air temperatureof 35 ◦C with relative humidity of 50% and the heat supplied is1.3 kw and results were presented in Figure 8. From these resultsit can be observed that with increase in solids flow rate the solidsoutlet temperature is changing drastically. For a fixed heat inputto the system the optimum is determined in the present study toattain the solids outlet temperature to the room temperature withdesired equilibrium moisture content and in the present study atsolids flow rate of 3 kg/h the maximum temperature is observed andat solids flow rate of 11kg/h the minimum temperature is observedand optimum is obtained at 10 kg/h of solids flow rate.

Fig. 8. Outlet solids temperature variation with varying solidsflow rate

4.5 Optimum heat load and initial moisture con-tent

The optimum heat load to the system can be found for a specificinitial moisture content of solids and with increase in the initialmoisture content of solids the required heat input to the systemalso increases to attain the same equilibrium moisture content ofsolids [11]. The figure 9 presents the results at solids flow rate of 10kg/h with air temperature and relative humidity of 40 ◦C and 50%RH respectively at air flow rate of 60 kg/h and from the results, itcan be observed that with increase in the initial moisture content

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the heat input to the system increases. Considering the solids outlettemperature and drying air relative humidity with respect to theambient conditions the optimum heat load to the system can befound for a specific initial moisture content of solids.

Fig. 9. Heat load variation with varying initial moisture contentof solids on dry basis

5 Conclusion

The influence of various parameters such as air velocity, inlet airtemperature, inlet air relative humidity, solids flow rate, initialmoisture content of solids and heat load on drying behavior havebeen studied and the optimum conditions have been predicted forall parameters. Instead of new model developing for drying in sev-eral different unit operations this software (Simprosys) can be usedeffectively for evaluation of the system for basic calculations.

AcknowledgmentAuthors would like to express special gratitude and thanks to

Simprotek Corporation (Simprosys) for providing the software.Nomenclaturet Dry bulb temperature (K)

tWB Wet bulb temperature (K)Y Absolute humidity (kg/m3)Ys,WBT Saturation humidity at wet bulb temperature (kg/m3)Latent heat of evaporation at wet bulb temperature (kJ/kg)CH Humid heatLe Lewis numberHumidity-potential coefficient

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Thermal conductivity of humid gas (W/mK)CP Specific heat humid gas (J/kgK)Density of the humid gas (kg/m3)MA molar mass of moisture gasMB molar mass of dry gasY* saturation humidity (kg/m3)DAB binary diffusivity between the moisture and the gas (m2/sec)T temperature of the liquid-gas system (K)P pressure of the liquid-gas system (Pa)

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