Investigation of the volatilized effects for residual liquid in cylinder gas

Transcription

1 Journal of Engineering Technology and Education, Vol. 9, No.3 September 2012, pp Investigation of the volatilized effects for residual liquid in cylinder gas Ta Hwa University of Science and Technology Abstract There s over 3.4 million families use cylinder gas in Taiwan, and it s much higher than families which use liquefied natural gas. It s the problem that the gas will store at the bottom of the cylinder become the residual liquid, and this problem affects interests of mass consumer. In this study, a newly designed device is presented for solving the residual liquid. It will increase the pressure inside the cylinder, promoting the residual liquid to evaporate in case the residual liquid store at the bottom of cylinder. A Five-kilogram cylinder is used in the study. The results show that when the input electric power is 32.4 W, 57.6 W, or 90 W, the gauge pressure is 1.35, 1.72, and 2.15 times higher, respectively, than the initial pressure inside the cylinder, indicating increases of 35.1%, 71.6%, and 115.3%. It allows the liquefied petroleum gas to be fully used up without leaving behind residual liquid in the four seasons. The volatilized device promotes liquefied petroleum gas utilization and reduces unnecessary financial loss. The consumers can utilize the gas to the maximum and save some costs. Keywords: Cylinder gas, Residual liquid, Volatilized device 1. Introduction Liquefied petroleum gas (LPG) is a convenient, clean and highly efficient fuel and is used in many different applications in countries all over the world. LPG has received increasing attention since it was recognized as a reasonable energy resource and commonly used for many applications [1,2]. In Taiwan, up to 1.6 million families use natural gas, while over 3.4 million families use LPG. Including mobile street vendors, there are as many as 4.7 million cylinder gas users in the country [3,4]. LPG can filling into cylinders (commonly known as bottled gas), whether in mobile or use all the powerful convenience, so currently fuel market can be said is completely dominant LPG in Taiwan. In the hot weather of summer, gas usage is relatively low. Whenever the gas volume in a gas cylinder is low, the fire on the stove will only blaze for a few seconds before burning out. When this occurs, it indicates that there is almost no gas left in the cylinder. In the winter, however, a higher flame from the heater is necessary to heat cold water to a higher temperature. When the gas cylinder releases gas, heat is absorbed and the temperature inside the cylinder will drop accordingly. The lower the temperature in the cylinder, the lower the pressure will be. As the pressure inside the cylinder drops below 0.7 kg/cm2, the fire on the stove will burn lower. The fire will not, however, burn out in a short time. When releasing gas in a low temperature environment, the cylinder causes condensation of the surrounding moisture, creating a sweating phenomenon around the cylinder. This phenomenon indicates that there is only about 0.5 kilograms of gas left in the cylinder. Generally, it is impossible to use up all the gas in the cylinder. The remaining unused gas in the cylinder is called residual liquid or residual gas. In order to fully extract the residual liquid in the cylinder, some users turn the cylinder over on the floor 2007 National Kaohsiung University of Applied Sciences, ISSN

2 282 and shake it or pour hot water on it to force the residual liquid out. The LPG gas in the gas cylinder is a combination of liquid and vapor in a saturated vapor pressure condition. When the cylinder is placed upright, the surface of contact of the liquid and vapor in the cylinder is small and safe. When the cylinder is placed horizontally, however, the surface of contact of the liquid and vapor becomes relatively large and unsafe. Also, a horizontally placed cylinder can roll easily, increasing the amount of friction at contact surface. This causes vaporization of liquid and the elevation of pressure in the cylinder, thus compromising the safety of the gas cylinder. LPG is a mixture of propane and butane, which is in vapor form under room temperature and atmospheric pressure. It may be turned to liquid form under high pressure or after being cooled to be stored in cylinders. When they vaporize, liquid propane and butane absorb heat and lower the surrounding temperature, cooling the gas cylinder. When the temperature of the LPG drops, the vapor pressure also drops accordingly and the vaporization rate decreases. When the vaporization rate declines below the rate of gas release (gas use), there will be insufficient butane gas for burning and the fire on the stove will naturally turn low. LPG needs to absorb heat from the environment to vaporize. When the volume of gas in the cylinder is low, there will be insufficient pressure to provide gas to high consumption burners such as high speed cooker stove and heater. These burners will not have sufficient burning power. The condition will be worst if the cylinder is placed at a temperature below 15 C. Traditional high-pressure steel cylinder is highly vulnerable to factors such as external temperature, altitude, ongoing or intermittent burning, and the duration of burning. The stability of the gas flow is easily compromised by these conditions. The presence of residual liquid is therefore relatively common. Solving the problem of having residual liquid in the cylinder is important not only to the safety of the user and his/her properties, but also to the welfare of the society. There are growing concerns over energy resource issues currently and it is foreseen that such concerns will continue to mount in the future as the price of energy resources rise continuingly in the world. It is worth our time and effort to find ways to use up the entire content of a gas cylinder and not have residual liquid [5,6]. The volatilized device proposed in this study is safe and can effectively vaporize the residual liquid. This is good news to the consumers as they can maximize the cylinder gas and minimize expenses. To the vendors, this can reduce risks and danger when transporting and storing returned gas cylinders. The gas itself is not harmful but can cause disastrous events in the event of negligence. Residual liquid in gas cylinder is relatively hazardous and the public should be educated about the characteristics of residual liquid to prevent accidents from happening. However, the key is to eliminate the existence of residual liquid. 2. Materials and Methods The experimental device proposed in this study is shown in Figure 1. The core of the experiment is the volatilized device illustrated in Figure 2. The volatilized device consists of 4 parts; namely the strip packaging unit (#1), the electric heating unit (#2), the pressure measuring unit (#3), and the control unit (#4). The main part of the strip packaging unit is a waterproof canvas belt (#11), affixed with N pole (#12) and S pole (#13) magnets at both ends respectively. The canvas belt is tightly wrapped around the cylinder. The electric heating unit is a heating belt (#21). The inside of the belt is connected to a temperature control switch. The heating unit is made of silicon rubber heater of 24 V, 120 W, and size of 120 x 600 mm. The flexible heating plate can be tightly stitched to the belt. The heating plate is connected to the control unit with a flexible wire (#22). The control unit is then linked to

3 Investigation of the volatilized effects for residual liquid in cylinder gas 283 the power plug (#24) with another flexible wire (#23). Figure 1. Experimental apparatus Figure 2. Schematic diagram of volatilized device The pressure measuring unit (#3) is a digital pressure gauge (#31) installed on the cylinder head. It measures the pressure inside the cylinder and sends the reading to the control unit via the pressure signal transmission line (#32). When the residual liquid in the cylinder becomes relatively low and the pressure detected by the pressure gauge falls below a certain value, the pressure signal processing unit (#43) in the control unit (#41) will automatically activate the heating unit via the temperature control switch (#42) to heat up the gas cylinder. The temperature control switch in the control unit is installed with short circuit and overload protection. When the heating plate exceeds a certain temperature, the temperature control switch in the control unit will automatically cut off the power supply. In this study, experimental factors include input current Ii (A) and input voltage Vj (V) of the power supply and pressure Po (kg/cm2) and temperature To ( C) inside the cylinder. The input current and input voltages are independent variables and are fixed by the researcher. Pressure and temperature inside the cylinder are dependent variables that change with Ii and Vj. To comply with the electrical specifications of the heating plate, 5 sets of series voltage of 6 V, 12 V, 18 V, 24 V, and 30 V are adopted as the independent variables in the experiments. In

4 284 the input tests, input current and input voltages are the controllable factors. To prevent noise factors such as the surrounding temperature (Ta) and surrounding humidity (φ) from over influencing the test results, air conditioning is used in the laboratory to ensure the stability and consistency of the surrounding temperature and surrounding humidity. As a safety precaution, all the 5 kg gas cylinders are emptied prior to conducting the test. Temperature is measured with digital thermometers and 4 type K thermocouples which are respectively secured outside the cylinder, as shown in Figure 1. T1 is secured between the cylinder and the heating belt to measure the temperature of the heating belt. T2 and T3 are secured with metal rods at 3 mm and 17 mm respectively deep inside the cylinder to measure the temperature inside the cylinder. T4 is secured at least 30 cm away from the body of the cylinder to measure the surrounding temperature (Ta). Data is collected once every second. The sampling rate is 1 Hz. The sampling time is 20 minutes. The uncertainty of the temperature measurement calculated with the uncertainty analysis is ± C, while the coefficient of variation is 0.36%. 3. Results and Discussion First, the specifications of the heating plate are measured and examined. Figure 3 shows that the input current (Ii) and the input voltage (Vj) has a linear transformation relationship; i.e. the voltage of the heating plate is proportional to its current, V/I = constant. According to the Ohm s law, V R I (1) where, the proportionality constant (R) is the resistance measured in Ohm (Ω). Actual measurement shows that the resistance of the heating plate is 10 Ω. The other line in Figure 3 represents the input electric power (Pe). Figure 3. Relationships between I, Pe and V for electric heating unit According to the Joule`s law, Pe = V.I (2)

5 Investigation of the volatilized effects for residual liquid in cylinder gas 285 where, Pe is measured in watt (W). When there is resistive or linear load, the Ohm s law may be integrated into Equation (2): 2 2 V Pe = I R = R (3) The actual measurement of electric power is compared with the theoretical value and the variation between the two is within ±1.6%. It is shown in the Figure 3 that the higher the voltage, the higher the input electric power is; and the higher the voltage, the steeper the line is. At V = V, the electric power rises to 25 W. This is an important parameter for the selection of alternative energy source in the future. This parameter is especially notable when considering solar energy as an energy source, for the electric power of the solar cell has to be taken into consideration. Experiment findings show that the higher the input voltage, the higher the input electric power is. The temperature of the heating plate also rises faster and higher accordingly. Figure 4 shows that at input voltage of 6 V, it takes 1018 seconds for the temperature to rise to 30.9 C, a mere 3.5 C of increase. The heat generated by the heating plate reaches the inside of the cylinder through the steel cylinder itself. During this process, much of the heat is absorbed by the steel barrel, while some heat is dissipated into the surrounding. The actual amount of heat that reaches the inside of the cylinder is substantially reduced. Therefore, at 3 cm (T1) and 17 cm (T2) deep inside the cylinder, the temperature measured is merely 28.6 C and 28.1 C respectively. The increase in temperature is 1.3 C and 1.0 C respectively. However, the amount of time it takes to rise to such temperature is relatively long; namely 1171 seconds and 1083 seconds respectively. The input voltages are then raised to the pre-determined values, and the heating effect is found to change accordingly, as shown in Figure 4. From the curve of the line chart, it is observed that the higher the output power, the faster and higher the temperature rises. The curve of the line chart is similar to that at voltage 6 V. Figure 4. Variation of temperatures in cylinder for different electric voltage

6 286 As the surrounding temperature (Ta) of each experiment is inconsistent, the inconsistency is minimized by adopting the relative temperature ( T ). The relative temperature is defined as: T T T T a a Figure 5 shows the impact of heating on the relative temperature. The vertical coordinate represents the relative temperature after dimensionless conversion. The 3 lines in the chart represent the temperatures of the heating plate (T1) and that at 3 cm (T2) and 17 cm (T3) deep inside the cylinder. From the slope of the heating plate temperature (T1), it is found that the temperature increases with input voltage. However, the graph shows a turning point when the input voltage is 12 V. When V is < 12 V, the relative temperature slope is V-1. When V is 12 V, the slope is V-1, which is 51.8% steeper than the first half of the slope. From the slope of the temperature inside the cylinder, it is found that the temperature inside the cylinder increases with the input voltage. However, the relative temperature slope is much lesser than the heating plate temperature slope. The T2 slopes are V-1 (V<12V) and V-1 (V 12 V) respectively, while the T3 slopes are V-1 (V < 12V) and V-1 (V 12 V) respectively. In conclusion, to raise the temperature inside the cylinder (of for example a 5 kg cylinder), the input voltage should be above 12 V for better heating results. Figure 5. Variation of dimensionless temperatures for different electric power According to the ideal gas law, changes in temperature will directly alter the gas density and pressure, as shown in the below equation: P = ρr o T (4) where, P is the absolute pressure, ρ is the density, Ro is the gas constant, and T is the absolute temperature (K). In standard air, Ro = kj/kg.k. With equation (4), the pressure inside the cylinder under different temperatures can be calculated. The vapor pressure of propane and butane mixture inside a gas cylinder under

7 Investigation of the volatilized effects for residual liquid in cylinder gas 287 different temperatures can also be estimated. When the volatilized device is powered and starts to heat up, the heat passes through the steel cylinder via heat conduction and is transferred to the fluid inside the cylinder via heat convection. When part of the fluid inside the cylinder is heated, its volume expands and its density reduces. It will gradually rise and the surrounding fluid which has lower temperature and higher density will take its place. Such fluid will also be heated and rise and the surrounding fluid will take its place. The cycle continues and the heat is transferred to the entire cylinder by the flow of the fluid. This natural convection effect causes temperature variation in the cylinder. As shown in Figure 4, the thermocouple placed at the upper part of the cylinder (T2) obviously measures a higher temperature than the thermocouple at the lower part of the cylinder (T3). To simplify the calculation of pressure inside the cylinder, it is assumed that the liquid density is homogeneous. The pressure inside the cylinder is calculated with Tm, which is the average of T2 and T3. Therefore, T2 + T3 T m = 2 According to the ideal gas law, once the volatilized device is activated, the pressure inside the cylinder will begin to rise. Values measured in Figure 4 can be placed directly into equation (4) to calculate the air pressure variation in the cylinder. It is assumed that the air density is a fixed value (ρ@25 = kg/m3) [7]. In Figure 6, the vertical coordinate represents the gauge pressure (Pg), measured in kg/cm2. The horizontal coordinate represents the heating duration, measured in seconds. The distribution of the line chart shows that when the input electric power (Pe) is relatively low, the gauge pressure increases linearly over time. At heating duration of 1200 s and electric power of 3.6 W and 14.4 W respectively, the gauge pressure is only 1.57 and 3.46 times higher than the initial pressure in the cylinder respectively. When Pe is > 14.4 W and the heating duration is 1200 s, pressure inside the cylinder rises sharply. Under such conditions, when the electric power is 32.4 W, 57.6 W, and 90 W respectively, the gauge pressure is 6.24, 10.99, and times higher than the initial pressure inside the cylinder respectively. Figure 6. Dependences of the gauge pressure on the input electric power for empty cylinder

8 288 It is also observed in Figure 6 that when Pe is > 14.4 W, the gauge pressure ascends sharply with time. Generally, the steepest slope appears when the heating duration reaches 420 seconds. As gas cylinders are made of steel plates welded on with rolled steels (CNS 2947) or materials of equivalent or better quality, when the temperature of the heating plate increases, the heat is rapidly transferred to the fluid inside the cylinder. As transmission of heat of the steel cylinder via heat conduction is way faster than that of the fluid inside the cylinder via heat convection, the heat generated by the heating plate initially is entirely absorbed by the cylinder. Therefore, the gauge pressure increases linearly over time initially, and only after a certain period of heating duration that the steepest slope occurs. The study continues to explore the effect of the volatilized device on the actual domestic gas cylinders. Results in Figure 4 correspond to the vapor pressure of the propane and butane mixtures. The conversions are shown in Figure 7. The distribution of the chart shows that when the input electric power is relatively low, the gauge pressure increases over time to form a curve similar to that in Figure 6. In the experiments where the heating duration is 1200 seconds and the input electric power is 3.6 W or 14.4 W, the gauge pressures are merely 1.03 and 1.15 times higher, respectively, than the initial pressure inside the cylinder, which is equivalent to increases of 2.79% and 14.55%. In the experiments where the electric power is 32.4 W, 57.6 W, or 90 W, the final gauge pressures are 1.35, 1.72, and 2.15 times higher, respectively, than the initial pressure inside the cylinder, indicating increases of 35.09%, 71.59%, and %. This shows that the input electric power exerts a rather pronounced effect on the pressure increase inside the cylinder. When considering solar energy as an alternative energy source in the future, it would be wise to select solar cells with electric power of 15 W to 25 W. Figure 7. Dependences of the gauge pressure on the input electric power for empty cylinder It is worth noting that although the vertical coordinates in Figures 6 and 7 represent gauge pressure measures in kg/cm2, the two sets of values are different by approximately 200 times. Under a particular temperature, gas (50% propane + 50% butane) responds more quickly to temperature variation than air. This indicates that the heating belt designed by this study can markedly increase the pressure of the cylinder and can therefore efficiently solve the residual liquid problem of the gas cylinders.

9 Investigation of the volatilized effects for residual liquid in cylinder gas Conclusions This study designed an innovative volatilized device to solve the residual liquid problem of gas cylinders. Five-kilogram cylinders are used in the study. Findings show that when the input electric power is 32.4 W, 57.6 W, or 90 W, the gauge pressure is 1.35, 1.72, and 2.15 times higher, respectively, than the initial pressure inside the cylinder, indicating increases of 35.09%, 71.59%, and %. The volatilized device is a simple structured device that can be installed directly to the outer body of the steel cylinder. It allows the LPG to be fully used up without leaving behind residual liquid in the four seasons. This promotes LPG utilization and reduces unnecessary financial loss. The consumers can utilize the gas to the maximum and save some costs. The vendors, on the other hand, can enjoy minimized risks when transporting and storing the returned gas cylinders. Although Taiwan is among the countries which offer the lowest gas prices in the world, it is a country highly dependent on the import of energy resources. The utilization of energy sources should be maximized while energy-saving methods should be promoted and sought. All returned gas cylinders contain residual liquid of about 2% in the summer and as much as 20% in the winter. The residual liquid is a financial loss to the users, and this may result in a substantial amount in the long run. To solve this problem, this study has designed a method to solve the residual liquid problem of gas cylinders, which is beneficial to both the consumers and the vendors. If commercialized, this product is expected to have a rather huge market potential and we look forward to its realization. Acknowledgement This study was supported by the National Science Council, Taiwan through the industry- academy cooperation Project No. NSC E CC3. Reference [1] Dokupil, M., Spitta, C., Mathiak, J., Beckhaus, P. and Heinzel, A., Compact Propane Fuel Processor for Auxiliary Power Unit Application, Journal of Power Sources. Vol.157, pp , [2] Zakaria, Z. and Mustafa, A., Gas Reticulation Optimization for Domestic Sector in Malaysia, International Journal of Oil, Gas and Coal Technology. Vol. 3. No.2, pp , [3] Lin W.C., Case Study on Gas Explosion Incident, National Fire Agency Epaper, Vol. 327, Ministry of the Interior, Taiwan, [4] Chen C.L., Cylinder Gas Needs Transparency and Open Policy, NPF Commentary , National Policy Foundation, Taiwan, [5] Zakaria, Z. and Mustafa, A., The Effect of Surrounding Temperature on Liquefied Petroleum Gas Behavior During Exhaustion Process, International Journal of Oil, Gas and Coal Technology. Vol.3. No.2, pp , [6] Zakaria, Z. and Mustafa, A., The Influence of Compositions on Liquefied Petroleum Gas Residue in Storage, International Journal of Research and Reviews in Applied Sciences. Vol.7. No.4, pp , [7] Incropera, F.P. and DeWitt D.P., Fundamentals of Heat and Mass Transfer. 5th ed., John Wiley & Sons, 2002.