Multiple Effect Evaporator Experiment and Lab Report - Chemical Engineering
AIM: To study the Multiple Effect Evaporator Unit and to obtain a mass and energy balances over the operation
Introduction to multiple Effect evaporator experiment
The object of evaporation is to concentrate a solution consisting of a non-volatile solute and a volatile solvent. In the overwhelming majority of evaporation operations, the solvent is water. A portion of the solvent is vaporized to produce a concentrated solution or thick liquor during the operation.
Steam is used as the heating medium in most evaporators and after transferring heat, steam is condensed in metal tubes. Often the boiling liquid is under a moderate vacuum to reduce the boiling temperature of the liquid. It increases the heat transfer rate in the evaporator.
When only one single evaporator is used to concentrate the solution, the vapor from the boiling liquid is condensed in the condenser and discarded from the system. This method is called a single-effect evaporation system, although it is a simple design, it utilizes steam ineffectively because the evaporated vapor is not utilized again usefully though it has thermal energy.
In a double effect evaporator system, vapor from one evaporator is fed into the steam chest of the next evaporator, and vapor from the second evaporator is condensed. Then, this type of evaporator is called a double effect evaporator system.
APPARATUS
- Multiple Effect Evaporator
- Electronic Balance
- Refractometer
Safety Precautions
- There are several processes and steam valves that discharge directly into the room. Do not touch a valve until you exactly understand the equipment system properly. The tagged valves are particularly dangerous and steam burns can be very serious injuries.
- If you overfeed the evaporator above the top viewport, turn off the main steam immediately. Be sure that the condenser coolant (cooling water) is turned on before the steam enters the calendar section and that coolant is not turned off until boiling has stopped.
- When opening valves, be careful that discharging liquid will not harm the visiting people.
Procedure of the experiment
- Identify all the vessels and flow paths of the experimental setup using the equipment manual and with the support of a technical officer.
- Draw a detailed diagram to show all the important parts of the double effect evaporator system.
- Prepare 60 liters of sugar solution of 20g/L concentration. Fill the feed tank with 32L of the solution.
- Close all the valves of the setup. Reduce the pressure of the condenser and the second effect tank using the vacuum pump and fill the 1st and 2nd effects up to the specified levels.
- Then, Supply steam at a pressure of about 0.3 kg/cm2. Do not allow the steam pressure to rise above 0.5 kg/cm2. Record the levels in the four liquid measuring tanks.
- Allow the system to reach steady conditions while maintaining the specified levels in the evaporators. Observe the levels of the four liquid measuring tanks and calculate the accumulation of liquid.
- Record the supply steam pressure and temperature of the 1st and 2nd effects. Take liquid samples from the feed tank, 1st effect, 2nd effect, and the concentration tank.
- Determine the concentrations of the sugar samples collected from the receivers using the refractometer.
Observations
Pressure of steam, vapor and vacum
| Location | Pressure (kPa) | Temperature (0C) |
|---|---|---|
| Steam to effect 01 | 130 | 107 |
| E1 Vapor | 90 | 94 |
| E1 Vapor | 65.27 | 76 |
| Vacum | 58.33 | 24 |
Time taken to collect sample during steady state = 20 min
Volume collected after condensation at different locations
| Location | Volume collected (L) |
|---|---|
| Steam condensate | 3 |
| Vapour 01,Condensate | 1 |
| Vapour 02, Condensate | 1 |
| Concentrated solution | 1 |
Flow rated at different locations
| location | Flow rate (L/h) |
|---|---|
| Feed flow rate to E1 | 20 |
| Feed flow rate to E1 | 10 |
Concentrations of samples using brix value
| Sample | Brix Value (%) |
|---|---|
| Feed to Evaporator 01 | 2.5 |
| Concentrate at E1 | 3.0 |
| Concentrate at E2 | 3.5 |
| Concentrated solution | 3.5 |
Density of samples
| Sample | Density (kg/3) |
|---|---|
| Feed to Evaporator 01 | 1002.8 |
| Concentrate at E1 | 1013.1 |
| Concentrate at E2 | 1014.0 |
| Concentrated solution | 1014.0 |
| Steam condensate | 1001.0 |
- Atmospheric pressure = 720 Hgmm
- Atmospheric temperature = 300C
Notations
- PE1 – Vapor pressure of effect 1
- TE1 – Vapor Temperature of effect 1
- PE2 – Vapor pressure of effect 2
- TE2 – Vapor Temperature of effect 2
- PS – Steam pressure
- MS – Steam flow rate
- MF – Feed flow rate
- XF– Weight fraction of sugar solution in feed
- MSC – Steam condensate flow rate from effect 1
- ML1 – Concentrated sugar solution flow rate from effect 1
- XL1 – Weight fraction of sugar in concentrated sugar solution from effect 1
- MV1 – Vapor flow rate from effect 1
- ML2 – Concentrated sugar solution flow rate from effect 2
- XL2 – Weight fraction of sugar in concentrated sugar solution from effect 2
- MV2 – Vapor flow rate from effect 2
- MSC2 – Steam condensate flow rate from effect 2
- XV1 – Weight fraction of sugar in vapor from effect 1
- XV2 – Weight fraction of sugar in vapor from effect 2
- hg – Enthalpy of saturated water vapor
- hf - Enthalpy of saturated water liquid
- C1 – Specific heat capacity of concentrate from effect 1
- Cp – Specific heat capacity of concentrate of feed
Calculations
Prepare a material balance for sugar and water for the double effect evaporator
Calculate steam flow rate to the 1st effect using heat balance
 balance.jpg "Calculate steam flow rate to the 1st effect using heat (energy) balance")
Determine steam flow rates using level gauges in collecting tanks
Determine heat transfer coefficients of 1st and 2nd effects
Discussion on multiple effect evaporator
Explain different types of evaporators
Evaporation in chemical engineering is transforming the solvent of the solution into a vapor state while concentrating the solute. Evaporators are the equipment that is used for evaporation. In evaporators, the process of boiling a liquid in order to get the concentrated solution is done. There are many types of evaporators. Single-effect evaporators and multiple-effect evaporators are two of them.
single-effect evaporator
A single-effect evaporator consists of only one evaporator. These are used when the throughput is low and a cheap steam supply is available. The vapor produced in the evaporator is contaminated and thus it cannot be reused.
Single effect evaporators may be used for batch processing, semi-continuous processes, or continuous processes. But in the batch process, it is rarely used, since there are some difficulties.
multiple-effect evaporator
A multiple-effect evaporator consists of two or more vertical tube evaporators connected together. The heat source and the feed water are supplied only at the first effect. The feed in the first effect gets boiled and the produced vapor acts as the heat source for the next effect.
The concentrated solution of the first effect is transferred to the next effects respectively to achieve the desired concentration. The pressure in every effect is lower than the last effect it is connected to in order to lower the boiling point of the solution.
Comparison of multiple-effect evaporator and single-effect evaporator
A multiple-effect evaporator gives a higher rate of heat transfer and a higher concentration than a single-effect evaporator since the single-effect evaporator does the concentrating process once and the other does it several times. Also, multiple-effect evaporators don't require high operating temperatures and large heating surfaces, unlike a single-effect evaporator. Therefore it can be used for large-scale and continuous evaporating processes.
The vapor produced in a multiple-effect evaporator is used efficiently for the next effect whereas the vapor is contaminated in the single-effect evaporator. The overall energy requirement of a multiple-effect evaporator is lower than that of a single-effect evaporator since the multiple-effect evaporator uses the vapor from each effect to the next effect as the heating source.
Multiple effect evaporator obtains the maximum recovery of solid used. The economy of an evaporator is the ratio between the quantity of vapor produced and steam applied. Normally in a Single effect evaporator, 1kg of steam produces 1kg of vapor. But unlike it, a multiple-effects evaporator gives vapor proportional to the number of effects it has from 1kg of steam. Thus, the economy of a multiple-effect evaporator is higher than a single-effect evaporator.
Compare calculated and measured steam flow rate to the 1st effect at comment on your results.
- Measured value of steam flow rate = 9.009 kg/h
- Calculated value of steam flow rate = 12.474 kg/h
From the above data, a deviation between the measured steam flow rate and the calculated steam flow rate can be seen. There are many reasons that may cause the deviation.
It was assumed that the specific heat capacity of the concentrated liquid and the feed is equal to the specific heat capacity of water. Since those values are not equal, it may cause the deviation of the above values.
Also, it was assumed that the weight fraction of the sugar in the vapor is zero. But practically it cannot be zero. Not only that but also, the Brix values were taken from the sample from the tanks. Maybe those samples are not pure, there may be some impurities in those samples. The deviation can result from that also.
Moreover, it is assumed that there are no heat losses. Since there is no equipment that is perfectly insulated in practical use, there may be a heat loss to the surrounding via pipes and evaporators and it may also cause this deviation.
When taking the measurements, there may be some errors. Human errors may have occurred when taking measurements such as parallax error. Also since the instruments are old, they may not have shown the exact values.
When considering all those reasons, it is clear that the deviation of the measured steam flow rate and calculated steam flow rate may have happened due to those reasons.
Questions