Pressure dew point

Compressed air is used for various purposes. Air which might come into contact with the product must be free of odor, particles, and oil, have a pressure dew point at less than 2°C, and be sterile. The air is normally compressed to 1000 kPa (8—10 atm) and cooled direcdy after the compressor. If needed, the air is then further dried by an absorbtion unit and the small particles and germs are removed through a set of filters of declining pore si2e. 

For a process system that involves a single condensable component, a vapor-liquid phase change, and specified or requested values of feed or product stream properties (temperature, pressure, dew point, relative saturation or humidity, degrees of superheat, etc.), draw and label the flowchart, carry out the degree-of-freedom analysis, and perform the required calculations. 

It is Eqs. 10.3-5 and 10.3-6 that are used in the algorithm of Fig. 10.3-5. Computer programs and MATHCAD worksheets for bubble point temperature, bubble point pressure, dew point temperature, dew point pressure, and isothermal flash calculations using the Peng-Robinson equation of state with generalized parameters (Eqs. 6.7-1 to... 

Fig. 13. Correlation of saturation pressure (dew point) of gas-condensates with SF(P o+) and absence of correlation with bubble point in oils.

Dew point temperature is the temperature at which condensation occurs when the air is cooled at a constant humidity ratio and at constant pressure. Dew point is read on the dry bulb axis corresponding to the saturation relative humidity curve. 

For bubble-point and dew-point pressure calculations, the appropriate forms are, respectively ... 

The bubble and dew-point temperature calculations have been implemented by the FORTRAN IV subroutine BUDET and the pressure calculations by subroutine BUDEP, which are described and listed in Appendix F. These subroutines calculate the unknown temperature or pressure, given feed composition and the fixed pressure or temperature. They provide for input of initial estimates of the temperature or pressure sought, but converge quickly from any estimates within the range of validity of the thermodynamic framework. Standard initial estimates are provided by the subroutines. 

At low or moderate pressures,a Newton-Raphson iteration is not required, and the bubble and dew-point pressure iteration can be, respectively. 

The computer subroutines for calculation of vapor-liquid equilibrium separations, including determination of bubble-point and dew-point temperatures and pressures, are described and listed in this Appendix. These are source routines written in American National Standard FORTRAN (FORTRAN IV), ANSI X3.9-1978, and, as such, should be compatible with most computer systems with FORTRAN IV compilers. Approximate storage requirements for these subroutines are given in Appendix J their execution times are strongly dependent on the separations being calculated but can be estimated (CDC 6400) from the times given for the thermodynamic subroutines they call (essentially all computation effort is in these thermodynamic subroutines). 

BUDET calculates the bubble-point temperature or dew-point temperature for a mixture of N components (N < 20) at specified pressure and liquid or vapor composition. The subroutine also furnishes the composition of the incipient vapor or liquid and the vaporization equilibrium ratios. 

Bubble-point and dew-point pressures are calculated using a first-order iteration procedure described by the footnote to Equation (7-25). 

Once the bubble point is reached (at point B), the first bubble of ethane vapour is released. From point B to C liquid and gas co-exist in the cell, and the pressure is maintained constant as more of the liquid changes to the gaseous state. The system exhibits infinite compressibility until the last drop of liquid is left In the cell (point C), which is the dew point. Below the dew point pressure only gas remains in the cell, and as pressure is reduced below the dew point, the volume increase is determined by the compressibility of the gas. The gas compressibility is much greater than the liquid compressibility, and hence the change of volume for a given reduction in pressure (the... 

If the experiment was now reversed, starling from A and increasing the pressure, the first drop of ethane liquid would appear at point C, the dew point of the gas. Remember that throughoufthis process, isothermal conditions are maintained. 

The experiment could be repeated at a number of different temperatures and initial pressures to determine the shape of the two-phase envelope defined by the bubble point line and the dew point line. These two lines meet at the critical point, where it is no longer possible to distinguish between a compressed gas and a liquid. 

When the two components are mixed together (say in a mixture of 10% ethane, 90% n-heptane) the bubble point curve and the dew point curve no longer coincide, and a two-phase envelope appears. Within this two-phase region, a mixture of liquid and gas exist, with both components being present in each phase in proportions dictated by the exact temperature and pressure, i.e. the composition of the liquid and gas phases within the two-phase envelope are not constant. The mixture has its own critical point C g. 

Using this mixture as an example, consider starting at pressure A and isothermally reducing the pressure to point D on the diagram. At point A the mixture exists entirely in the liquid phase. When the pressure drops to point B, the first bubble of gas is evolved, and this will be a bubble of the lighter component, ethane. As the pressure continues to drop, the gas phase will acquire more of the heavier component and hence the liquid volume decreases. At point C, the last drop of liquid remaining will be composed of the heavier component, which itself will vaporise as the dew point is crossed, so that below... 

The initial temperature of a gas condensate lies between the critical temperature and the cricondotherm. The fluid therefore exists at initial conditions in the reservoir as a gas, but on pressure depletion the dew point line is reached, at which point liquids condense in the reservoir. As can be seen from Figure 5.22, the volume percentage of liquids is low, typically insufficient for the saturation of the liquid in the pore space to reach the critical saturation beyond which the liquid phase becomes mobile. These... 

Gas is produced to surface separators which are used to extract the heavier ends of the mixture (typically the components). The dry gas is then compressed and reinjected into the reservoir to maintain the pressure above the dew point. As the recycling progresses the reservoir composition becomes leaner (less heavy components), until eventually it is not economic to separate and compress the dry gas, at which point the reservoir pressure is blown down as for a wet gas reservoir. The sales profile for a recycling scheme consists of early sales of condensate liquids and delayed sale of gas. An alternative method of keeping the reservoir above the dew point but avoiding the deferred gas sales is by water injection. 

The diagram (Fig. 5.21) shows that as the pressure is reduced below the dew point, the volume of liquid in the two phase mixture initially increases. This contradicts the common observation of the fraction of liquids in a volatile mixture reducing as the pressure is dropped (vaporisation), and explains why the fluids are sometimes referred to as retrograde gas condensates. 

Black oils are a common category of reservoir fluids, and are similar to volatile oils in behaviour, except that they contain a lower fraction of volatile components and therefore require a much larger pressure drop below the bubble point before significant volumes of gas are released from solution. This is reflected by the position of the iso-vol lines in the phase diagram, where the lines of low liquid percentage are grouped around the dew point line. 

When a customer agrees to purchase gas, product quality is specified in terms of the calorific value of the gas, measured by the Wobbe index (calorific value divided by density), the hydrocarbon dew point and the water dew point, and the fraction of other gases such as Nj, COj, HjS. The Wobbe index specification ensures that the gas the customer receives has a predictable calorific value and hence predictable burning characteristics. If the gas becomes lean, less energy is released, and if the gas becomes too rich there is a risk that the gas burners flame out . Water and hydrocarbon dew points (the pressure and temperature at which liquids start to drop out of the gas) are specified to ensure that over the range of temperature and pressure at which the gas is handled by the customer, no liquids will drop out (these could cause possible corrosion and/or hydrate formation). 

Facilities for the treatment and compression of gas have already been described in earlier sections. However, there are a number of differences in the specifications for injected gas that differ from those of export gas. Generally there are no technical reasons for specifications on hydrocarbon dew point control (injected gas will get hotter not cooler) although it may be attractive to remove heavy hydrocarbons for economic reasons. Basic liquid separation will normally be performed, and due to the high pressures involved it will nearly always be necessary to dehydrate the gas to avoid water drop out. 

Table 11.4 Solutions for Maintaining Constant Humidity Table 11.5 Concentration of Solutions of H2SO4, NaOH, and CaCi2 Giving Specified Vapor Pressures and Percent Humidities at 25°C Table 11.6 Relative Humidity from Wet and Dry Bulb Thermometer Readings Table 11.7 Relative Humidity from Dew Point Readings...

Dew-point Temperature (DPT). DPT is the temperature at which the condensation of water vapor in a space begins for a given state of humidity and pressure as the temperature is reduced. It is the temperature corresponding to saturation (100% rh) for a given absolute humidity at constant pressure. 

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