UO1-1

EXPERIMENT # UO1

DOUBLE EFFECT EVAPORATOR OBJECTIVE: The objective of this experiment is to study the performance of a double effect evaporator. Mass and energy balances will be carried out to determine steam economy and heat transfer coefficients. INTRODUCTION Evaporation is a process whereby a solution consisting of a volatile solvent and a non-volatile solute is concentrated by vaporizing the solvent. Common types of evaporators used in the industry include i.) forced circulation evaporators, ii.) natural circulation evaporators (thermosiphon), iii.) falling film evaporators, and iv.) rising film evaporators. Drying differs from evaporation in that the product in this case is solid. In many industrial applications, water is used as the solvent while steam is utilized as heating medium for the vaporization process. In such cases, given efficient heat exchange, one kilogram of water may be evaporated per kilogram of steam condensed. To improve steam economy, multiple evaporators are used. The basic principle involved is to utilize steam generated in one effect as the source of heat for the subsequent evaporator. In this way, multiple effect evaporators have the capacity to significantly reduce steam consumption. THEORETICAL BACKGROUND Steam economy, which is defined as the mass of water vapor generated per mass of steam is the process parameter most significant in evaluating the performance of multiple effect evaporators (Equation 1). The other important parameter upon which both the evaporation and condensation process depend is the heat transfer coefficient (Equation 2).

Steam Economy : s

v

mmE = (1)

Heat Transfer Rate: q = UAΔT (2)

UO1-2

ΔT for both effects should be considered. For U1 , ΔT=Ts – T1, and for U2 , ΔT=T1 – T2. MASS AND ENERGY BALANCES Assuming that the stream entering the effect is saturated and that the condensate resulting from the steam is not sub cooled, then the total heat transfer in the effect is obtained as : q ms s= .λ -(3) The heat given up by the condensing steam is absorbed by the process fluid and used for partial vaporization. From mass and enthalpy balance, q m m H m H m Hf p v p p f f= − + −( ) -(4) Values of enthalpies at the experimental conditions of temperature and pressure should be obtained from literature. The flow rates of various streams are determined from experimental data. PROCESS DESCRIPTION AND EXPERIMENTAL PROGRAM The double effect evaporator unit is shown schematically in Figure 1.The two effects are initially filled to desired operating levels. Steam is supplied to the first effect where heat transfer takes place causing partial evaporation of water in the effect. The vapor generated is then supplied to the second effect where it is used to vaporize some of the liquid water in the evaporator. Vapor produced in the second effect is subsequently condensed and collected in the receiving tank. The entire experimental unit is to be operated at steady state for analysis. At steady state, process parameters such as pressures, temperatures, mass flow rates and tank level are measured and recorded as shown in the log sheet. The measurements are repeated three times at regular intervals for reproducibility.

UO1-3

T5

P1

P2

P3

P4

T2

P5

CONDENSER CYCLONE CYCLONE

I II

mv2mv1

mp2mp1

FIRST EFFECT

SECOND EFFECT

T4

CONDENSATE TANK

C1 C2

Feed Water F1

Steam

T1

Data: Condensate Tank Diameter D = 50 cm Evaporator Heat A = 0.51m2 Transfer Area C : Condensate P : Pressure Indicator T : Temperature Indicator

FIGURE 1 : DOUBLE EFFECT EVAPORATOR

T3

C3

UO1-4

EXPERIMENTAL PROCEDURE 1. Start the unit and let it reach steady state (to be done with the help of

technician). 2. After the unit reaches steady state, record the temperature and pressure at

various points as shown in the log sheet. 3. Measure the flow rate of condensate from the first effect by collecting a

quantity of the condensate in a given time. 4. Repeat steps (3) for the second effect. 5. Measure the flow rate of the condensate flowing into the receiving tank by

monitoring the rise in the level in the tank. 6. Repeat steps (3) to (5) two more times. 7. Shut down the unit. (To be done with the help of technician). DATA ANALYSIS The following items must be covered in the data analysis and the analysis sheet filled in accordingly: 1) Tabulate all enthalpies used in analysis from literature. 2) Carry out a mass and energy balance for the two effects. 3) Determine the steam economy of the process. 4) Determine the overall heat transfer coefficients for thde two effects. Notations: A Surface area of heat transfer , m2 E Steam Economy Hv, Hf , Hp Enthalpies of the vapor, the feed and the product, W/m2 .K mf, mp, ms mass flow rate of the feed, product and steam supplied kg/hr mv1 mass flow rate of vapor from first effect, (= C2) kg/hr mv2 mass flow rate of vapor from second effect, (= C2) kg/hr q heat transfer rate, W. Tp temperature of the process fluid, K Ts temperature of the steam supplied, K U overall heat transfer coefficient, W/m2 .K λ Latent heat of vaporization, J/kg

UO1-5

References: 1.) Kern, D. Q., Process Heat Transfer, McGraw Hill, (1965) 2.) Perry, R.H. and Chilton, C.H., "Chemical Engineers' Handbook", 5th

Edition, McGraw-Hill Book Company NY, chap 20 (1973). 3.) Thompson, E.V., and Ceckler, W.H., "Introduction to Chemical

Engineering.", McGraw-Hill Book Company, NY, chap.17 (1977).

UO1-6

TABLE 1. LOG SHEET FOR EXPERIMENT UO1 Date : Signature of Instructor: Name of students: 1) 2) 3)

Tag Description Units Set 1 Set 2 Set 3

PI-1 Fresh steam pressure Psig

PI-2 Vap. pr. in the 1st effect "

PI-3 Steam pr. in the 2nd effect "

PI-4 Vapor pr. in the 2nd effect In. Hg

PI-5 Shell side pr. in the cond. "

TI-1 Vap temp in the 1st effect oF

TI-2 Vap temp in the 2nd effect "

TI-3 Vap temp at cond. Inlet "

TI-4 Temp at condenser outlet "

TI-5 Feed temperature "

FI-1 Feed flow rate LPM

Weight of cond (1st effect) kg

Flow time (1st effect) min.

Weight of cond (2nd effect) kg

Flow time (2nd effect) min.

LI-3 Rise in tank level cm

Diameter of Tank cm 50

Total Time min.

Weight of empty bucket kg

Weight of empty bucket with condensate

kg

UO1-7

TABLE 2. ENTHALPY TABULATION

Temperature Pressure Enthalpy Ref.

T1 P2 Hv1 =

" " Hp1 =

T5 1 atm. Hf1 =

T2 P4 Hv2 =

" " Hp2 =

Tsat P1 λ1 =

Tsat P3 λ2 =

TABLE 3. CALCULATION SUMMARY (Details to be shown elsewhere)

Parameter Value Units

Feed flow rate to 1st effect kg/h

Vapor flow rate from 1st effect "

Condensate flow rate from 1st effect "

Steam Economy of 1st effect

Vapor flow rate from 2nd effect kg/h

Condensate flow rate from 2nd effect "

Steam Economy of 2nd effect

Heat transfer area of H01,H02 0.51 m2

Overall heat transfer coefficient of 1st effect W/m2.K

Overall heat transfer coefficient of 2nd effect "

Overall Steam Economy of process

UO1-8