Saturation Pressure of a Binary System Using Pvt Simulator

Saturation Pressure of a Binary System using PVT Simulator

Cover Letter

An experiment was conducted on September 22 in which a PVT simulator was used to study the Saturation Pressure of a Binary System. The aim of this experiment was to observe and understand the effects of the changing composition of the binary system containing a mixture of CO2 (Carbon Dioxide) and C4H10 (n-Butane) at the same temperature. While conducting this experiment, the researchers have used PVT lab simulator to gather information essential for the outline—graphs.

Table 4-a (group 1) from the manual was used. Throughout the experiment, the researchers tried to remain the temperature constant at 900 F and the pressure was set to 2000psi with a margin of (±10). The amount of liquid butane was 10cc without any gas present. The main objective initially was to achieve the bubble point by decreasing the pressure.

To make sure butane was pure and there was no gas, a step was involved in increasing the volume by eliminating Hg step. As per the lab manual, a certain amount of carbon dioxide (CO2) was also added to the visual cells, which varied from 0.1 to 0.9—mole fractions in relation to the amount of butane.  The experiment observed and recorded the bubble portions and dew portions as per the varied mole fractions when the required mixture of the two components was attained. Different bubble and dew points were recorded—that was because of the shifting arrangement of carbon dioxide as mercury was removed.

The first trace of the vapor achieved the bubble point was at the gas volume of 0.002cc due to the withdrawal of the mercury from the visual cell. Each increase in the mole fraction of carbon dioxide resulted in the increase of bubble point as well. But the withdrawal of mercury from the cell has also resulted in the expansion of volume. The gas volume increases with the decrease of the liquid volume, which was not favorable for the discovery of the dew point.

The experiment started by finding the bubble and dew point of the pure normal butane that corresponded to 0.0 a mole fraction of 0.0 of added carbon dioxide. It systematically increased by 0.1 until a mole fraction of 0.8 reached. The position of the bubble point was recorded at each increase.

Theory, Concepts and Objective of the Experiment

The objective of this experiment was to observe and understand the effects of the changing composition of the binary system containing a mixture of CO2 (Carbon Dioxide) and C4H10 (n-Butane) at the same temperature. In order to achieve this objective, several factors have to be considered throughout the experiment. Most important of them are the accurate observation and recording of the bubble and dew points.

Many dew points were collected and recorded during the steps in which they are plotted in a graph known as the pressure-mole diagram. These lines usually intersect at a certain point, also known as the critical point. And the area between the lines is called phase envelope. In this case, the critical point is the temperature above where no more condensing can occur (McCain). No more condensing equals no existence of gas in the mixture and no vaporization can occur. And the value of both bubble and dew points remains the same.

When a liquid at a certain (or constant) temperature is heated, it produces the first bubble of vapor. This temperature is the bubble point. The dew point is at which the first drop of liquid condenses when the pressure is increased at a constant temperature.  

The experiment conducted also used the pressure mole diagram (see Fig. 1). Figure 1 shows the plotting of pressure vs. mole fraction of the component. This was plotted by using the recorded pressure of butane and the mole fraction of CO2. After this, a phase envelope (that is, the area between the bubble point line and the dew point line) helps understand the combination of both gas and liquid. A gas combination is the one located right below the phase envelope, while the liquid combination is located above.

Using concept of the binary system is helpful here in understanding the effects of these changes (in combination, phase behavior of butane, and CO2) clearly. It also helps locate the bubble point and dew point of the mixture at a constant temperature and varying mole fractions of CO2. Increasing the mole fractions of added CO2 from 0.1 to 0.9 dictated the pressure value of both the bubble and dew points.

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Figure1: Pressure vs. Mole Fraction Plot

This figure shows a typical pressure-composition diagram of two component mixtures with an equilibrium tie-line (1, 2 or 3), a bubble point line and a dew point line. The space between the bubble and dew point lines is the phase envelope (with a liquid line above and a gaseous mixture line below). The main point is the critical point where these two lines intersect. That point is where no gas exists and no liquid produces.  

Experimental Procedure

Figure 2 is a simulator representation of a PVT apparatus.

[pic 2]

Figure 2: PVT Simulator

The temperature, pressure, volume, and composition were all recorded at the beginning of this experimental procedure. The temperature was set at 900 F (Group 4) while opening the PVT simulator. The pressure was set at 2000psi with a margin of ±10 and the volume of liquid butane used in the experiment was 10cc (±0.001).  

Valves 8 and 9 were opened to link the hand pump, pressure gage, and the visual cell (that is, the pressure gage is linked to the cell and the cell is then linked to the hand pump). The hand pump was useful to discover the bubble point at 900 F. It has also helped to withdraw mercury and to add the same-in-quantity CO2 from the cell. The reason of adding the same amount of CO2 was to attain a mole fraction 0.1 and to have a blend in the system.

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