Free The Experiment Provides Basic Knowledge On Application Of Distillation Industry And Measures To Control And Maintain Process. Report Example

Abstract

In the laboratory assignment, the experimental investigation of the distillation process is performed. The mixture of dichloromethane and trichloroethylene was separated using the glass distillation column. The influence of power input on temperatures and column flow rate was investigated. Apparently, the power input increases the efficiency of the process. The experiment had three main tasks: study of the variation of the pressure drop, application of refractometer as an express method of binary mixture composition analysis, and estimation of the column efficiency.
It has been proven that the boil-up rate is directly proportional to pressure drop. Although there is some deviation in composition obtained from refractometer data, it can be applied for efficiency assessment. Due to significant difference in boiling temperatures, the efficiency of the process is rather high and reaches 50%.

Introduction

Distillation column is an equipment used in chemical, food, and oil industry for separation of mixtures of two or more liquids. The separation process is called distillation. Distillation is a physical process of selective evaporation and condensation that results in recovery of the separate phases of the component mixtures, or phases with enriched components (Coulson, Richardson & Sinnott 2009).
Distillation has a number of industrial applications, for example primary processing of crude oil to obtain fuel and other oil fractions, separation process for purification of chemical or food products; oxygen is produced by air distillation (Benvenuto 2013).
Distillation is realized as boiling of the mixture to a certain temperature, which causes the more volatile component to evaporate. When the temperature is increased, the second component evaporates as well. As the vapour flow goes up the distillation column, its temperature decreases and less volatile component turns into liquid. Thus, two flows appear: the upward flow of vapour, and the downward flow of liquid. This allows to obtain two phases: the liquid phase with an increased concentration of the less volatile component and the gas phase with an increased concentration of volatile component (Sinnott & Towler 2009). At the top of the column, the vapour is being condensed, and liquid is obtained. The process is divided into several steps that are called plates. The plates are made as physical stages that are characterized with a certain temperature. Once it is defined, the multi-component mixture can be separated by gathering condensate from certain plates (Lei, Chen & Ding 2005).
The important part of each industrial process is efficiency. For distillation, it means the efficiency of separation, e.g. the final content of the mixture. The efficiency is determined by contact between liquid and vapour phase. Thus, the excessive foaming, liquid dumping, short-circuiting causes the decrease of plate efficiency. The column has a certain number of plates, which are meant to provide separation. However, typically the separation efficiency is lower than theoretically expected (Coulson, Richardson & Sinnott 2009). Another important factor is pressure at the column, which influences temperature.
The experiment is meant to study the basics of distillation process and factors affecting its efficiency, namely pressure difference at the top and bottom of the column, and power applied to the boiler.

Methodology

Equipment
The batch distillation column UOP3BM was used for the experiment. It consists of a process unit and a control unit.
The floor standing process unit (Figure 1) has the following compartments: 1 – floor–standing frame, 2 – adjustable feet, 3, 7, 11, 14, 16 – valves, 4 – sight glass, 5 – reboiler, 6 – level sensor, 8 – U-tube manometer, 9, 15 – two-section glass distillation column, 10 – thermocouple sensors, 12 – product collecting vessel, 13 – diaphragm valve, 17 – ratio control valve, 18 – rotameter, 19 – reflux pipe valve, 20 – glass decanter, 21 – pressure relief valve, 22 – water-cooled coil-in-shell condenser.
The glass distillation column is attached to the steel framework with adjustable feet to provide the unit portability. The column consists of eight sieve glass plates, divided into two sections (four plates each).
The reboiler 5 is made of stainless steel and insulated. It is designed to be used both in continuous and periodical feed operation. For the experiment, the periodical operation is applied; thus, valve 3 remains closed during the experiment. At the beginning, the reboiler is charged with the initial dichloromethane (DCM) and trichloroethylene (TCE) mixture. The reboiler is equipped with a level sensor 6 and sight glass 4 for overheating protection and level control, respectively.
Figure 1: Batch distillation column UOP3BM
After the distillation column, vapour is delivered to a condenser 22, cooled with water. In the condenser, vapour turns to liquid phase.
The water flow is measured by rotameter 18 and controlled by the diaphragm valve 13. The pressure relief valve 21 is designed for safety measures in case of blocked vent or cooling water failure.
The condensate comes to a phase separator 20, and then is delivered to reflux ratio control valve 19. Depending on the reflux timers, the condensate is delivered either to the top of the column or to the top product collecting vessel 12.
Temperature in a distillation column is measured by thermocouple sensors 10, located on each sieve plates to control the process temperature. The U-tube manometer 8 is used to measure the pressure drop.

The bench-mounted control console allows adjusting the distillation column power, as well as controlling temperature and reflux.

Experiment 1. Variation of the column pressure drop
The mixture of 5.43 l of TCE and 4.57 l of DCM was introduced into the distillation column. Stop watch and measuring cylinder were used to measure the blow up rate.
The equipment was set up as Figure 1 shows. All the valves were closed. Reflux pipe valve 19 was opened and the temperature selector switch was set to T9 on a control console. After this, valve 13 was opened to reach the cooling water rate 3 l/min. Then, reboiler and power controller were turned on until 0.75 kW was reached. As temperature increased, the vapour upward flow was observed. When the temperature readings T1-T9 became constant, they were recorded. The boil-up rate was calculated by measuring time necessary to fill the measuring cylinder.
The pressure drop readings were measured, and samples at the top and bottom of the column were collected. The readings were measured until two close replicates were obtained.
The procedures were repeated for heater power 0.85 and 0.95 kW.

Experiment 2. Mixture composition

The experiment requires allocation of refractometer, measuring cylinder, containers for mixing samples, solutions of TCE and DCM.
The refractive index (RI) of pure TCE and DCM were measured by a refractometer. Then, RI were measured for mixtures of 0, 25, 50, 75% mixtures of TCE and DCM.

Using the obtained data, the calibration graph is plotted (RI as a function of DCM mole fraction).

Experiment 3. Column efficiency
The experiment requires equipment the same as in experiments 1 and 2.
The system was heated to equilibrium when the reflux gave the steady bubbling and boil-up rate was measured. The samples from top and bottom of the column were collected, and their RI was measured. The procedure was repeated every ten minutes to collect five top and bottom samples.

Results

The experiment results are presented in Tables 1-3.
The distillation parameters are presented in Table 2.

Figure 2 presents the relationship of the logarithmic function between the pressure difference and the boil-up rate values at different power input.
Figure 2: The relationship between Log Q and Log ∆P.

The calibration line is a function of RI of DCM molar fraction (Figure 3).

Figure 3: The relationship between DCM mole fraction and refractive index.

The number of theoretical plates and data on column efficiency are presented in Table 6.

Discussion
The pressure readings are experimental data. The pressure difference is calculated as: Pressure reading difference = Pressure reading High - Pressure reading Low.

Thus, the preasure readings are the following:

For 0.75 kW: Pressure reading difference = 176 – 74 = 102 cm H2O.
For 0.85 kW: Pressure reading difference = 178 – 72 = 106 cm H2O.
For 0.95 kW: Pressure reading difference = 179.5 – 68 = 110.5 cm H2O.
Boil-up rate calculates as Boil-up rate = Boil-up volume / Boil-up time ∙ 3600 / 1000 (l/hr).
For 0.75 kW: Boil-up rate = 102 / 60 × 3600 / 1000 = 6.12 l/hr
For 0.85 kW: Boil-up rate =116 / 60 × 3600 / 1000 = 6.96 l/hr
For 0.95 kW: Boil-up rate =120 / 60 × 3600 / 1000 = 7.2 l/hr
The values of Log ∆P and Log Q are obtained by taking a logarithm of pressure reading difference and boil-up rate (values are presented in Table 2). The RI of overhead and bottom composition are experimental values obtained by measuring the samples from top and bottom of the column by refractometer.

As for the refractometer application for analysis of mixture (Table 3), it should be noted that RI dependence does not perfectly fit the line, as R2 (determination coefficient) value shows. Therefore, the application of this line will be subject to errors (Figure 3), as it is apparent that RI – DCM molar dependence is a curve, whereas the calibration is a straight line.
The DCM and TCE mole fractions calculated with application of the equation DCM molar fraction = (RI -1.4824) / (- 0.0528). The calculated DCM values at the top of the column are close to due to errors); however, this has no physical sense, so the values are written as 1.0 in Table 4.
The TCE concentration is calculated as X (TCE top) = 1 – X(DCM top) or X (TCE bottom) = 1 – X(DCM bottom).

The calculation of overhead molar fraction composition:

For 0.75 kW: X(DCM top) = (1.4240 -1.4824) / (- 0.0528) = 0.999.
X (TCE top) = 1 - 0.999 = 0.001.
For 0.85 kW: X(DCM top) = (1.4251-1.4824) / (- 0.0528) = 0.999.
X (TCE top) = 1 - 0.999 = 0.001.
For 0.95 kW: X(DCM top) = (1.4235-1.4824) / (- 0.0528) = 0.999.
X (TCE top) = 1 - 0.999 = 0.001.

The calculation of bottom molar fraction composition:

For 0.75 kW: X(DCM bottom) = (1.4710-1.4824) / (- 0.0528) = 0.22.
X (TCE bottom) = 1 - 0.22 = 0.78.
For 0.85 kW: X(DCM bottom) = (1.4690-1.4824) / (- 0.0528) = 0.25.
X (TCE bottom) = 1 - 0.25 = 0.75.
For 0.95 kW: X(DCM bottom) = (1.4700-1.4824) / (- 0.0528) = 0.23.
X (TCE bottom) = 1 - 0.23 = 0.77.
The relative volatility is obtained from the average boiling temperature of the mixture. The average value of temperature calculates as: Tb = (313 + 360) / 2 = 336.5 K. The same way, the average enthalpy of vaporization is calculated: ∆HVAP = (29230 + 35010) / 2 = 32120 J/kg.
Next, β = ∆HVAP / RTb = 32120 / (8.3142∙336.5) = 11.48. The relative volatility basing on boiling temperatures: αaverage = =4.98. The results are presented in Table 5.
n = (log ((X (DCM) top / X (TCE) top) × ( X (TCE) bottom / X (DCM) bottom)) / log (αaverage)) – 1.
Using data from Tables 4 and 5, the following is obtained. For power 0.75 kW:
n = log ((0.999 / 0.001) × (0.78 / 0.22) / log (4.98)) – 1 = 4.

For power 0.85 kW:

n = log ((0.999 / 0.001) × (0.75 / 0.25) / log (4.98)) – 1 = 3.9.

For power 0.85 kW:

n = log ((0.999 / 0.001) × (0.77 / 0.23) / log (4.98)) – 1 = 4.03.
Theoretically, according to the calculations, four plates are required to perform the efficient distillation process. However, the experimental column is designed to have eight plates and provides the same efficiency. Hence, the efficiency is calculated:
E = (n / Z) × 100, where n is the theoretical number of plates and Z is the actual number of plates (8 for the performed experiment).
Therefore, E = (4 / 8) × 100 = 50 %. Since all numbers of theoretical plates for different powers are the same, the average efficiency is 50%.
This is an experimental proof that the number of plates should be at least two times more that the number of theoretical plates.

Conclusions

The experiment showed that there is direct proportional dependence between heating power and temperature in the distillation column.
The foaming observed at the trays is not violent. Only at 0.95 kW the mixture gently foams over the whole tray.

The relationship between the boil-up rate and the pressure drop is directly proportional.

There is relation between the mole fraction of DCM and the refractive index of the mixture. However, dependence is not directly proportional, and there are some uncertainties at determining the mixture composition.
The DCM was observed in the vapour phase, and there was almost none in the liquid phase. This happened due to low boiling point of DCM. Hence, the concentration of DCM approached 100% at the top of the distillation column, while TCE concentration increased at the bottom of the distillation column.
The experimentally found efficiency is about 50% for all the studied heating powers. This is because there is difference in practical and theoretical process flow, and the calculations applied have some limitations.

Reference List

.Benvenuto, MA 2013, Industrial Chemistry. De Gruyter, Berlin.
Coulson, JM, Richardson, JF, & Sinnott, RK 2009, Chemical engineering, Pergamon: Oxford.
Poling, BE 2008, Perry's chemical engineers' handbook, McGraw-Hill, New York.
Sinnott, RK & Towler, G 2009, Chemical engineering design, Butterworth-Heinemamn, Oxford.
Lei, Z, Chen, B & Ding, Z 2005, Special distillation processes. Elsevier, Amsterdam.