Experiment for adsorption characteristics

This experimental setup is used for measuring adsorption uptake of ethanol onto the developed EG contained composite adsorbents. It comprises mainly an adsorber, an evaporator, two constant temperature water baths, and two water circulators as shown in Fig. 4.1. Adsorber is a small stainless steel cylinder of 40 mm diameter and 40 mm height whilst K-type thermocouple is inserted to measure adsorbent temperature. The evaporator

CHAPTER 4 APPLICATION TO LOW PRESSURE ADSORPTION SYSTEMS

Page | 101 is a glass tube of 10 mm outer diameter, 1.6 mm wall thickness and 174 mm height where the K-type thermocouple is inserted to measure the evaporator temperature. Two constant temperature water baths of type TBN302DA along with two water circulators of type CTP3000 have been used to control the adsorber and evaporator temperatures. A rotary vacuum pump of type ULVAC (GCD-136X) has been used to evacuate the system. A Pirani gauge of type PG-D5A is used to test the vacuum condition of the system whilst AP10-32 type pressure transducer is used to measure the system pressure. A digital microscope of type NRM-D-2XZ with a minimum resolution of 0.01 mm has been used to measure the height of ethanol level in the evaporator before and after conducting adsorption process. The weight of adsorbent samples has been measured using Sartorius Practum 124-1S analytical lab balance with 120 grams weight capacity and readability of

±0.1 mg. To avoid any condensation during the adsorption process, a tape heater surrounds the connecting tubes between evaporator and adsorber.

The experimental procedure can be summarized as follows:

i) Prior to conducting adsorption uptake measurements, the composite adsorbent is dried using an oven for about 12 hours at 120°C. After that, the sample weight is measured.

ii) The composite adsorbent is then placed into the adsorber and regenerated under vacuum condition for more than 3 hours at a temperature of 80°C.

iii) The evaporator temperature is kept constant at 5°C whilst the adsorber temperature is maintained at 70°C. The height of ethanol onto the evaporator is then measured using the digital microscope. The valve between the evaporator and adsorber is opened where ethanol vapor flows to the adsorbent sample. After reaching the equilibrium condition, the ethanol height inside the evaporator is measured again to determine the amount of ethanol adsorbed.

iv) The valve between the evaporator and adsorber is closed and the adsorber temperature is cold down to 60°C. The valve is then opened and adsorption process is carried out. The same procedure is repeated at adsorption temperatures of 50, 40, 30 and 20°C.

v) To measure the isobaric adsorption corresponding to the evaporator temperature of 10°C and 15°C, steps (i) and (iv) have been repeated.

CHAPTER 4 APPLICATION TO LOW PRESSURE ADSORPTION SYSTEMS

Page | 102 Fig. 4.1 Schematic diagram of the experimental apparatus for measuring adsorption uptake of ethanol onto composite adsorbents.

4.2.2 Gravimetrically adsorption uptake measurement

The gravimetric based magnetic suspension adsorption measurement unit (Rubotherm of type MSB-VG-S2), supplied by MicrotracBEL, Japan is employed for measurement of equilibrium uptake and kinetics of ethanol on rest of the studied adsorbents. Fig. 4.2 shows the schematic diagram of the magnetic suspension adsorption measurement unit. The major components of the experimental system are magnetic suspension balance unit; isothermal circulation oil baths to maintain adsorber and evaporator temperatures; isothermal air bath to avoid condensation through connecting tubes; a series of vacuum pumps including diaphragm, rotary, and turbo-molecular types.

Weight measurement repeatability of magnetic balance is ±30μg with a relative error of

±0.002% of the reading. The system employs nitrogen gas to operate the valves whilst pressurized helium gas is used for checking the leakage as well as to compute the void volume that is essential for buoyancy measurement. Three types of absolute pressure gauges (1) 3500 kPa, Keller PAA-35XHTT-35; (2) 133.3 kPa, MKS 628B13TBE1B; and (3) 1.33 kPa, MKS 628B12TBE1B are used in parallel to measure the ethanol vapor pressure with an accuracy of 0.15-0.25%F.S. The temperature of refrigerant has been

1: Adsorber; 2: Evaporator; 3: Constant temperature water bath; 4: Valve;

5: Refrigerant injection tube; 6: Water circulator; 7: Tape heater P: Pressure sensor;

T: Thermocouple.

To vacuum pump

T

6 3

2 7 P

6

4

T

3

1

P

5

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Page | 103 measured and maintained precisely by platinum resistance temperature element (100Ω) together with pressure instruments which assist in temperature tuning to the corresponding to ethanol vapor pressure [16,55,104].

The system executes the whole experiment automatically by following the instruction given in the software. It can be mentioned that buoyancy effect on the measurements is considered automatically by the system. Adsorption characteristics of ethanol onto prepared ACs have been investigated at five different temperatures namely 30, 40, 50, 60, and 70°C with various evaporator pressures. Experimental procedure can be described as follows; at first, the adsorbent is put into the adsorbent cell basket. The adsorbent is then regenerated at 120°C for 3 hours under vacuum condition for degassing the adsorbents. After regeneration, adsorbents are cooled to the specified adsorption temperature, and meanwhile, the ethanol (refrigerant) temperature is maintained at first set point of evaporation temperature. Once the temperatures on adsorbent and refrigerant side become steady, the valves are opened for adsorption automatically. The system records data for every second until the equilibrium condition is reached and then the refrigerant temperature is increased to next measuring point. Similarly, ethanol adsorption is recorded for all sets refrigerant temperatures. The same procedure has been adopted for every isotherm measurement.

CHAPTER 4 APPLICATION TO LOW PRESSURE ADSORPTION SYSTEMS

Page | 104 Fig. 4.2 Schematic diagram of the experimental apparatus for measuring ethanol adsorption characteristics.

4.3 Theory of adsorption isotherm and kinetics models