cstr 40l

UNIVERSITI TEKNOLOGI MARA FAKULTI KEJURUTERAAN KIMIA PROCESS ENGINEERING LABORATORY II (CPE554) NAME : SHEH MUHAMMAD AFNAN BIN SEH HANAFI STUDENT ID : 2013210382 GROUP : EH2214A EXPERIMENT : CONTINUOUS STIRRED TANK REACTOR (CSTR) 40L DATE PERFORMED : 17TH MARCH 2015 SEMESTER :4 PROGRAMME/CODE : BACHELOR OF CHEMICAL ENGINEERING (HONS.) IN PROCESS ENGINEERING / EH221 SUBMIT TO : CIK HABSAH BINTI ALWI NO Title Abstract/Summary Introduction Aims Theory Apparatus Methodology/Procedures Results Calculations Discussion Conclusion Recommendations Reference Appendix TOTAL Remarks: 1 2 3 4 5 6 7 8 9 10 11 12 13

Allocated Marks (%) 5 5 5 5 5 10 10 10 20 10 5 5 5 100

Marks

Checked by:

Rechecked by:

______________

______________

Date:

Date:

ABSTRACT

This experiment is carried out in the model of BP 143 (SOLTEQ). In this model, the Continuous Stirrer Tank Reactor (CSTR) 40 L is used for second experiment which is effect of temperature on the reaction in a CSTR. The objectives in this experiment are to determine the effect of temperature onto the reaction extent of conversion and lastly to determine the reaction’s activation energy. In this experiment, 0.1 M of acetyl acetate, 0.1 M sodium hydroxide and hydrochloric acid, HCL (0.25M) for quenching were prepared. Then adjust the valves V5 and V10 to give a flow rate of 0.20 L/min. Make sure that both flow rates are the same for the whole experiment. Firstly, the temperature of the water was set at 40°C by switch on the thermostat T1. This is to ensure that the reactor has reached steady state. that will be the same as the reactor and reactant temperature. Then, the stirrer was switched on and the stirrer speed was set about 200 rpm. After 5 minutes later, conductivity was observed and valve V12 is open to collect a 150 mL sample. Carry out a back titration procedure to manually determine the concentration of NaOH in the reactor and extent of conversion for every 50 mL. Make sure that the flow rates of both solutions are maintained at 0.20 L/min. The reading was recorded and the steps were repeated for temperature of 50°C after we collected the mixture in a beaker. Last but not least, after finishing all the steps in the experiments, mixture inside the reactor was drained off and the reactor was clean properly. All liquid waste (mixture) was dispose immediately after each experiment. As for result, the temperature in term of conversion which was have the lower conductivity will made the high conversion. For example, at temperature 40 oc have lower conversion and high conductivity which is 2.73 mS/cm. In terms of rate of reaction, as the temperature increasing the rate of reation will increase due to increasing of its activation energy. There are some recommendation to increase the efficiency of the experiment which are the titrations is repeat for two or three times because a lot of error comes from titration or use another method other than titration. Beside that, take conductivity reading when the conductivity not changes in time because it can change rapidly in short of time.

INTRODUCTION

The unit used in this experiment, which is SOLTEQ-QVF Continuous Stirred Tank Reactor (Model: BP 143). The experiment was conducted to study the effect of temperature on saponification reaction of ethyl acetate and sodium hydroxide in batch reactor. A batch reactor was a reactor which characterized by its operation. Batch reactor is a reactor reached steady state which was a generic term for a type of vessel widely used in the process industries. Reactor is one of the most important parts in industrial sector. Reactor is equipment that changes the raw material to the product that we want. A good reactor will give a high production and economical. One of criteria to choose or to design a good reactor is to know the effectiveness of the reactor itself. There a many types of reactor depending on the nature of the feed materials and products. One of the most important we need to know in the various chemical reaction was the rate of the reaction. Continuous stirred tank reactor or known as CSTR is a most common ideal reactor type in chemical engineering .In a continuous stirred tank reactor (CSTR), reactants and products are continuously added and withdrawn from the reactor. The CSTR is the idealized opposite of the weel-stirred batch and tubular plug flow reactors. Analysis of selected combination of these reactors types can be useful in quantitatively evaluating more complex solid, gas-, and liquid- flow behaviours. A stirred tank reactor (STR) may be operated either as a batch reactor or as a steady state flow reactor (CSTR). The key or main feature of this reactor is that mixing is complete so that properties such as temperature and concentration of the reaction mixture are uniform in all parts of the vessel. Material balance of a general chemical reaction described below.The conservation principle requires that the mass of species A in an element of reactor volume dV obeys the following statement: (Rate of A into volume element) - (rate of A out of volume element) + (rate of A produced within volume element) = (rate of A accumulated within vol. element) By studying the saponification reaction of ethyl acetate and sodium hydroxide to form sodium acetate in a batch and in a continuous stirred tank reactor, we can evaluate the rate data needed to design a production scale reactor.

.AIMS

1) To determine the effect of temperature onto the reaction extent of conversion. 2) To determine the reaction’s activation energy. THEORY A stirred-tank reactor (STR) may be operated either as a batch reactor or as a steadystate flow reactor (better known as Continuous Stirred-Tank Reactor (CSTR)). The key or main feature of this reactor is that mixing is complete so that properties such as temperature and concentration of the reaction mixture are uniform in all parts of the vessel. Material balance of a general chemical reaction is described below. The conservation principle required that the mass of species A in an element of reactor volume ∆V obeys the following statement:

Rate of A into

Rate of A -

out of

Rate of A +

produced

Rate of A =

Accumulated

volume

Volume

within volume

within volume

element

Element

Element

Element

The usual agitator arrangement is a centrally mounted shaft with an overhead drive unit. Impeller blades are mounted on the shaft. A wide variety of blade designs are used and typically the blades cover about two thirds of the diameter of the reactor. Where viscous products are handled, anchor shaped paddles are often used which have a close clearance between the blade and the vessel walls.(Christe John Geankoplis. Transport Process and Separation Process Principle) Most batch reactors also use baffles. These are stationary blades which break up flow caused by the rotating agitator. These may be fixed to the vessel cover or mounted on the side walls. Despite significant improvements in agitator blade and baffle design, mixing in large batch reactors is ultimately constrained by the amount of energy that can be applied. On large vessels, mixing energies of more than 5 Watts per liter can put an unacceptable burden on the cooling system. High agitator loads can also create shaft stability problems. Where mixing is a critical parameter, the batch reactor is not the ideal solution. Much higher mixing rates can

be achieved by using smaller flowing systems with high speed agitators, ultrasonic mixing or static mixers. (H. Scott Fogler, Elements of Chemical Reaction Engineering) A batch reactor is used for small-scale operation, for testing new processes that have not been fully develop, for the manufacture of expensive products, and for processes that are difficult to convert to CSTR. The reactor can be charged through the holes at the top. A batch reactor has neither inflow nor outflow of reactants or products while the reaction is being carried out: Fjo = Fj = 0. In – Out + Generation = Accumulation V

FA 0  FA 

r

A

dV 

dN A dt

V dN A   rA dV dt

(H. Scott Fogler, Elements of Chemical Reaction Engineering) The rate of reaction of component A is defined as: -rA = 1/V (dNA/dt) by reaction = [moles of A which appear by reaction] [unit volume] [unit time] By this definition, if A is a reaction product, the rate is positive; whereas if it is a reactant which is consumed, the rate is negative. Rearranging equation (3), (-rA) V = NAO dXA Dt

Integrating equation (4) gives, t = NAO ∫ dXA__

(-rA)V where t is the time required to achieve a conversion XA for either isothermal or nonisothermal operation.

1/-rA Area = t

There are some advantage and disadvantage for using batch reactor. For advantages it production of high cell densities due to extension of working time (particularly important in the production of growth-associated products). Next, it controlled conditions in the provision of substrates during the fermentation, particularly regarding the concentration of specific substrates as for example the carbon source. As for disadvantages, it requires previous analysis of the microorganism, its requirements and the understanding of its physiology with the productivity. Beside that, it requires a substantial amount of operator skill for the set-up, definition and development of the process. Lastly in a cyclic fed-batch culture, care should be taken in the design of the process to ensure that toxins do not accumulate to inhibitory levels and that nutrients other than those incorporated into the feed medium become limiting, Also, if many cycles are run, the accumulation of non-producing or low-producing variants may result. (H. Scott Fogler, Elements of Chemical Reaction Engineering)

APPARATUS

The unit used in this experiment was Solteq-QVF Continuous Stirred Tank Reactor (Model : BP 143).

SOLTEQ-QVF Continuous Stirred Tank Reactor (Model: BP 143) 1. 2. 3. 4. 5.

Continuous stirred tank reactor. Model: BP 143 50 mL burette 200 mL beaker Conical flask Solution : ISodium hydroxide, NaOH (0.1M) IIEthyl acetate, Et (Ac) (0.1M) IIIDeionized water IVPhenolphthalein 6. Conductivity probe 7. 100 mL measuring cylinder.

PROCEDURES General start-up Procedures: 1. The following solution were prepared:

i40L of sodium hydroxide, NaOH (0.1 M) ii40 L of ethyl acetate, Et (Ac) (0.1M) iii1 L of hydrochloric acid, HCl (0.25M) , for quenching. 2. All valves were initially closed. 3. The feed vessels were charged as follows: iThe charge port caps for vessels B1 and B2 were opened. iiThe NaOH solution was carefully poured into vessel B1 and Et (Ac) solution

4. 5. 6. 7. 8. 9.

was poured into vessel B2. iiiThe charge port caps for both vessels were closed. The power for control panel was turned on. Sufficient water in thermostat T1 was checked. Refill as necessary. Cooling water V13 was opened and are let to flow through condenser W1. The overflow tube was adjusted to give a working volume of 10L in the reactor R1. Valves V2, V3, V3, V7, V8 and V11 were opened. The unit was ready for experiment.

General shut-down Procedures: 1. The cooling water valve V13 was kept open to allow the cooling water to continue flowing. 2. Pumps P1 and pumps P2 were switched off. Stirrer M1 was switched off. 3. The thermostat T1 was switched off. The liquid in the reaction vessel R1 was let to cool down to room temperature. 4. Cooling water V13 was closed. 5. Valves V2, V3, V7, and V8 were closed. Valves V4, V9 and V12 were opened to drain any liquid from the unit. 6. The power for control panel was turned off.

Preparation of Calibration Curve for Conversion vs. Conductivity 1. The following solution were prepared: i1 L of sodium hydroxide, NaOH (0.1M) ii1 L of sodium acetate , Et (Ac) (0.1M) iii1 L of deionized water, H2O. 2. The conductivity and NaoH concentration for each value were determined by mixing the following solution into 100 mL of deionized water. i0% conversion : 100 mL NaOH ii25% conversion : 75 mL NaOH + 25 mL Et (Ac) iii50% conversion : 50 mL NaOH + 50 mL Et (Ac) iv75% conversion : 23 mL NaOH + 75 mL Et (Ac) v100% conversion : 100 mL Et (Ac)

Back Titration Procedures for Manual Conversion Determination: 1. A burette was filled up with 0.1 M NaOH solution. 2. 10 mL of 0.25 M HCl was measured in a flask. 3. A 50 mL sample was obtained from the experiment and immediate the sample was added to the HCl in the flask to quench the saponification reaction. 4. A few drops of pH indicator were added into the mixture. 5. The mixture was titrated with NaOH solution from the burette until the mixture was neutralized. The amount of NaOH titrated was recorded.

EXPERIMENT 2: Effect of temperature on the Reaction in a CSTR 1. The general start-up was performed. 2. Pumps P1 and P2 was switched on simultaneously and valves V5 and V10 was opened to obtain the highest possible flow rate into the reactor. 3. The reactor are let to fill up with both the solution until it is just overflowed. 4. The valves V5 and V10 are readjusted to give a flow rate of 2.0 L/min. Both flow rates are ensured recorded at the same time. 5. The stirrer M1 are switched on and the speed are set at about 200 rpm. 6. The thermostat T1 are switched on and the water temperature was set to 40 ºC. 7. The conductivity value at Q1-401 was started to monitor and the temperature value at T1-101 until no changed over time. To ensured the reactor had reached to steady state. 8. The steady state conductivity and the temperature values was recorded and the concentration of NaOH in the reactor and the extent of reaction of conversion from calibration curve was found.

9. Sampling valves V12 was opened and a 50 mL sample was collected. The titration procedures was carried out back to manually determine the concentration of NaOH in the reactor and extent of conversion. 10. The experiment was repeated ( steps 7 to 10 ) for different reactor temperatures by setting the thermostat temperature to 50, 60,70 and 80 ºC. The flow rates of both solutions was ensured maintained at 0.20 L/min.

RESULT Reactor volume = 10 L Concentration of NaOH in feed vessel = 0.1 M Concentration of Et(Ac) in feed vessel = 0.1 M

Temperature (oC)

40

50

60

70

80

Flow rate of NaOH (mL/min)

200

200

200

200

200

Flow rate of Et (Ac) (mL/min)

200

200

200

200

200

400

400

400

400

400

Conductivity

2.73

2.60

2.51

2.45

2.34

Volume of NaOH titrated, V1(mL)

23.1

23.7

24.2

24.4

24.7

25

25

25

25

25

9.88

9.52

9.20

9.00

8.76

Total flowrate , Fo (mL/min)

Residence time, τ (min) Volume of unreacted quenching HCl, V2 (mL) Volume of HCl reacted with NaOH , V3 (mL)

‘ 9.88

0.48

0.8

1.0

1.24

92.4

94.8

96.8

97.6

98.8

127.98

280.47

756.25

1355.56

5488.89

ln k

4.85

5.64

6.63

7.21

8.61

1/T

0.025

0.02

0.017

0.014

0.013

2365.47

2441.05

2516.62

2592.2

2667.77

Conservation , X (%) Rate Constant ,k (M-1s-1)

Ea ( J/mol )

CALCULATION When the flowrate of both solution is 0.1 L/min (Column 1 of Table 1), the known quantities are :

F0 = 0.2+0.2 = 0.4 mL/min Volume of sample,Vs

50 mL

Concentration of NaOH in the feed vessel, CNaOH,f

0.1 M

Volume of HCl for quenching, VHCl,s

10 mL

Concentration of HCl in standard solution, CHCl,s

0.25 mol/L

Volume of NaOH titrated, V1

23.1 mol/L

Concentration of NaOH used for titration, CNaOH,s

0.1 mol/L

For T = 40°C i-

ii-

iii-

iv-

vvi-

vii-

Concentration of NaOH that entering the reactor, CNaOH 0. CNaOHo = ½ CNaOHs = ½ (0.1) = 0.05 mol/L Volume of unreacted quenching HCl,V2 V2 = (CNaOHs / CHCls) x V1 = (0.1/0.25) x 23.1 = 9.24 mL Volume of HCl reacted with NaOH in sample, V3 V3 = VHCls – V2 = 10 – 9.24 = 0.76 mL Moles of HCl reacted with NaOH in sample, n1 n1 = (CHCls x V3) / 1000 = 0.25 x 0.76/1000 = 0.00019 mol Moles of unreacted NaOH in sample, n2 n2 = n1 = 0.00019 mol Concentration of unreacted NaOH in the reactor, CNaOH CNaOH = n2/Vs x 1000 = 0.00019/50 x 1000 = 0.0038 mol/L Conversion of NaOH in the reactor, X X = (1- CNaOH / CNaOHo) x 100% = (1 – 0.0038/0.05) x 100%

= 92.4 % viii-

Residence time, τ

ix-

τ = VCSTR / Fo = 10 / 0.4 = 25 min Reaction rate constant, k k = ( CAo – CA) / τCA2 = ( 0.05 – 0.0038) / (25 x 0.00382) −1 = 127.98 M min -1

For T = 50°C i-

ii-

iii-

iv-

Volume of unreacted quenching HCl,V2 V2 = (CNaOHs / CHCls) x V1 = (0.1/0.25) x 23.7 = 9.48 mL Volume of HCl reacted with NaOH in sample, V3 V3 = VHCls – V2 = 10 – 9.48 = 0.52 mL Moles of HCl reacted with NaOH in sample, n1 n1 = (CHCls x V3) / 1000 = 0.25 x 0.52/1000 = 0.00013 mol Moles of unreacted NaOH in sample, n2

v-

vi-

vii-

viii-

n2 = n1 = 0.00013 mol Concentration of unreacted NaOH in the reactor, CNaOH CNaOH = n2/Vs x 1000 = 0.00013/50 x 1000 = 0.0026 mol/L Conversion of NaOH in the reactor, X X = (1- CNaOH / CNaOHo) x 100% = (1 – 0.0026/0.05) x 100% = 94.8 % Residence time, τ τ = VCSTR / Fo = 10 / 0.4 = 25 min Reaction rate constant, k k = ( CAo – CA) / τCA2 = ( 0.05 – 0.0026) / (25 x 0.00262) −1 = 280.47 M min -1

For T = 60°C i-

ii-

iii-

ivv-

vi-

vii-

Volume of unreacted quenching HCl,V2 V2 = (CNaOHs / CHCls) x V1 = (0.1/0.25) x 24.2 = 9.68 mL Volume of HCl reacted with NaOH in sample, V3 V3 = VHCls – V2 = 10 – 9.68 = 0.32 mL Moles of HCl reacted with NaOH in sample, n1 n1 = (CHCls x V3) / 1000 = 0.25 x 0.32/1000 = 0.00008 mol Moles of unreacted NaOH in sample, n2 n2 = n1 = 0.00008 mol Concentration of unreacted NaOH in the reactor, CNaOH CNaOH = n2/Vs x 1000 = 0.00008/50 x 1000 = 0.0016 mol/L Conversion of NaOH in the reactor, X X = (1- CNaOH / CNaOHo) x 100% = (1 – 0.0016/0.05) x 100% = 96.8 % Residence time, τ τ = VCSTR / Fo = 10 / 0.4

viii-

= 25 min Reaction rate constant, k k = ( CAo – CA) / τCA2 = ( 0.05 – 0.0016) / (25 x 0.00162) −1 = 756.25 M min -1

For T = 70°C i-

ii-

iii-

ivv-

vi-

vii-

viii-

Volume of unreacted quenching HCl,V2 V2 = (CNaOHs / CHCls) x V1 = (0.1/0.25) x 24.4 = 9.76 mL Volume of HCl reacted with NaOH in sample, V3 V3 = VHCls – V2 = 10 – 9.76 = 0.24 mL Moles of HCl reacted with NaOH in sample, n1 n1 = (CHCls x V3) / 1000 = 0.25 x 0.24/1000 = 0.00006 mol Moles of unreacted NaOH in sample, n2 n2 = n1 = 0.00006 mol Concentration of unreacted NaOH in the reactor, CNaOH CNaOH = n2/Vs x 1000 = 0.00006/50 x 1000 = 0.0012 mol/L Conversion of NaOH in the reactor, X X = (1- CNaOH / CNaOHo) x 100% = (1 – 0.0012/0.05) x 100% = 97.6 % Residence time, τ τ = VCSTR / Fo = 10 / 0.4 = 25 min Reaction rate constant, k k = ( CAo – CA) / τCA2 = ( 0.05 – 0.0012) / (25 x 0.00122) −1 = 1355.56 M min -1

For T = 80°C i-

Volume of unreacted quenching HCl,V2 V2 = (CNaOHs / CHCls) x V1

ii-

iii-

ivv-

vi-

vii-

viii-

= (0.1/0.25) x 24.7 = 9.88 mL Volume of HCl reacted with NaOH in sample, V3 V3 = VHCls – V2 = 10 – 9.88 = 0.12 mL Moles of HCl reacted with NaOH in sample, n1 n1 = (CHCls x V3) / 1000 = 0.25 x 0.12/1000 = 0.00003 mol Moles of unreacted NaOH in sample, n2 n2 = n1 = 0.00003 mol Concentration of unreacted NaOH in the reactor, CNaOH CNaOH = n2/Vs x 1000 = 0.00003/50 x 1000 = 0.0006 mol/L Conversion of NaOH in the reactor, X X = (1- CNaOH / CNaOHo) x 100% = (1 – 0.0006/0.05) x 100% = 98.8 % Residence time, τ τ = VCSTR / Fo = 10 / 0.4 = 25 min Reaction rate constant, k k = ( CAo – CA) / τCA2 = ( 0.05 – 0.0006) / (25 x 0.00062) −1 = 5488.89 M min -1

Arhenius equation : −E

k ( t )= A e RT

ln k =ln A−

E 1 ( ) R T

y=c+mx From graph eq : y = 0.909x + 3.861

c = 3.861=ln A A= e

3.861

=47.51

−E M=0.909= RT So for Arhenius equation :

k ( t )=( 47.51) e 0.909 = 117.91

Reaction’s activation energy 1. For 40

℃ ,

−E 0.909= RT E = 0.909(8.314)(40+273) = 2365.47 J/mol 2. For 50 ℃ ,

−E 0.909= RT E = 0.909(8.314)(50+273) = 2441.05 J/mol 3. For 60 ℃ ,

−E 0.909= RT E = 0.909(8.314)(60+273) = 2516.62 J/mol 4. For 70 ℃ ,

−E 0.909= RT E = 0.909(8.314)(70+273) = 2592.2 J/mol 5. For 80 ℃ ,

−E 0.909= RT E = 0.909(8.314)(80+273) = 2667.77 J/mol

DISCUSSION Based on the experiment that had been conducted, the two objectives are determine the effect of temperature onto the reaction extent of conversion and determine the reaction’s

activation energy . From the data collected from the result, two graph had been plotted which are first one is conversion versus temperature and the last one is ln k versus

1 T .

For the purpose of achieving that particular target, the experiment is designed so that two reactants which are Sodium Hydroxide, NaOH and Ethyl Acetate, Et(Ac) react with each other in the saponification process. The reactor used is CSTR since the property that is to be varied is the temperature. As the flow rate of NaOH and Et(Ac) same throughout the experiment, the residence time is also the same which is 25 min Residencetime , τ =

V CSTR F0

where VCSTR refers to the volume of the reactor (in this case 10 L) and F 0 is the total flowrate of the feed which is 400 mL/min to get same residence time,

τ . And that is exactly what

was done. The temperature in the experiment was varied to be 40, 50, 60, 70 and 80 ° C . A graph between temperature and the conversion of the reactant (in this case NaOH) has to be formed in order to study the relationship between the conversion of NaOH and temperature. The values of temperature are known, as explained before, and the values of conversion, X of NaOh can be determined by

X

(

=

1−

C NaOH C NaOH ,0

)

x 100%

From the first graph, the conversion is increase proportionally to the temperature. As we know the hypothesis that conversion is higher if the temperature is higher. But there are certain fluctuate peak and the graph is not smoothly increase which due to the error from titration reading or maybe lack of skill when titration that may be affects the result and graph respectively. From the second graph, it can be seen that the relationship of these two parameters which is ln k versus

1 T

is almost linear. This is because when two reactants is react in high

temperature, the rate of reaction will increase. By this graph, the value for Arhennius equation is 117.91 which is calculated from line equation y = 0.909x + 3.861. CONCLUSION Based on the objectives of this experiment, which is to determine the effect of temperature onto the reaction extent of conversion, the relationship conversion and temperature was directly proportional. From the calculated data, the conversion increasing when the temperature is higher. We can conclude that the experiment was successfully conducted since we get the right conclusion. By using a Continuous Stirred Tank Reactor, CSTR, these two substances were flowed into the reactor, mixed and let to react for a certain by different temperature. By doing that, saponification process was completed. The experiment also targets to determine the reaction activation energy. From arhennius equation, the reaction activation energy for 40 ℃ , 50 ℃ , 60 ℃ , 70 ℃

and 80 ℃ is 2365.47 J/mol, 2441.05 J/mol, 2516.62 J/mol, 2592.2 J/mol and

2667.77 J/mol respectively. This show that reaction of rate increasing in high temperatures.

RECOMMENDATION  Make sure CSTR 40 liters machine is running appropriately, it to prevent harm to the 

machine and individual that used the machine. Repeat titrations two or three times because a lot of error comes from titration or use



another method other than titration. Divide into two teams which is the first team in charge of the CSTR 40 liters machine



while the second team would carry out the back titration procedures. Take conductivity reading when the conductivity not changes in time because it can

change rapidly in short of time.  The indicator should be mixed with the acid first, then the sample.  When the sample is being collected, the first few mililiters should be thrown away, for 

it is the remaining of the previous sample trapped in the pipe. Pumps should never be run dry.

REFRENCES       

Sullivan, J.A (1997). Fluid Power : Theory and Application. http://www.solution.com.my/pdf/BP143(A4).pdf http://en.wikipedia.org/wiki/Continuous_stirred-tank_reactor McCabe. (2005). Unit Operations of Chemical Engineering. McGuire, J.T. (1990). Pumps for Chemical Processing. Fogler. H.S (2005). Elements of Chemical Reaction Engineering. Lab manual CPE554-CSTR40L

APPENDICES