Dew-point Adjustment

Dew-point adjustment (see Fig. 7.10) serves the reduction of the concentration of water and heavy hydrocarbons in natural gas to such an extent that no condensation occurs during the ensuing transport in the pipeline. This would lead to a multiphase flow that places higher demands on the design and laying of the pipelines. 

By liquefied petroleum gas, also called LPG, a mixture of propane and butane is understood that is often sold in small containers as cylinder gas. Above all, it is important for a sufficient fuel gas supply of infrastructurally weak areas without connection to a gas pipeline. In some countries, LPG is very popular as fuel for motor vehicles. Moreover, LPG is a valuable raw material in the petrochemical and chemical industry 

The commercial recovery of liquefied petroleum gas from natural gas is carried out in plants with a capacity range of 10 000 m, h up to over 1 000 000 h  

Simple process versions such as in the case of dew point adjustment are not suitable here, since at a tjrpical crude gas pressure of 50 to 100 bar and a single-stage, partial condensation with ensuing separation of the developed fluid, too much propane remains in the gas phase, thus limiting the propane separation from the sales gas. 

All modem plants dispose of a cryogenic separation in the core of the process. Earlier concepts, based on the scmbbing of light hydrocarbons from the cmde gas by means of oil, are no longer market-relevant 

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The natural gas after appropriate treatment for acid gas reduction, odorization, and hydrocarbon and moisture dew point adjustment (ASTM D1142) would then be sold within prescribed limits of pressure, calorific value, and possibly, the Wobbe Index [(calorific value)/(specific gravity)]. 

Dew-Point Method For many applications, the dew point is the desired moisture measurement. VHien concentration is desired, the relation between water content and dew point is well-known and available. The dew-point method requires an inert surface whose temperature can be adjusted and measured, a sample gas stream flowing past the surface, a manipulated variable for adjusting the surface temperature to the dew point, and a means of detecting the onset of con-densation. 

Where e, is in units of inches of water per day. Up is the wind speed 2 feet above the ground expressed in miles per day, and e and e, are the saturation vapor pressures at mean air and mean dew-point temperatures, respectively (expressed in inches of mercury). For development of the wind function, an adjustment in the psychrometric constant is generally made to account for the sensible heat conducted... 

Two methods of changing the humidity and temperature of a gas from Aidj. JP x i to B(()2. J 2) may be traced on the humidity chart as shown in Figure 13.11. The first method consists of saturating the air by water artificially maintained at the dew point of air of humidity (line AC) and then heating at constant humidity to 82 (line CB). In the second method, the air is heated (line AD) so that its adiabatic saturation temperature corresponds with the dew point of air of humidity JP2- It is then saturated by water at the adiabatic saturation temperature (line DC) and heated at constant humidity to 82 (line CB). In this second method, an additional operation — the preliminary heating—is carried out on the air, hut the water temperature automatically adjusts itself to the required value. 

Either the temperatures or flows could be adjusted first. The common choice is to correct the temperature. Correction of temperatures is usually done through either bubble-point or dew-point determinations on the calculated stage compositions. After correcting the stage temperatures, the liquid and vapor enthalpies may be obtained from the calculated compositions, and the flows corrected by solution of the now linear heat balance equations of Table I. 

Six pound moles of ethane, three lb moles of propane and one lb mole of n-butane are mixed in a closed container and the temperature is adjusted to 75°F. What is the bubble-point pressure What is the dew-point pressure Calculate the composi-... 

The theta method. This method has been primarily applied to the Thiele-Geddes equations but a form of the theta method equation has also been applied to the equations of the Lewis-Matheson method. The main independent variable of the method is a convergence promoter, theta (or 6). The convergence promoter 0 is used to force an overall component and total material balance and to adjust the compositions on each stage. These new compositions are then used to calculate new stage temperatures by an approximation of the dew- or bubble-point equation called the Kb method. The power of the Kb method is that it directly calculates a new temperature without the sort of failures that occur when iteratively solving the bubble- or dew-point equations. 

A scanning mobility particle sizer (SMPS) was applied to analyze nanoparticles [2][4]. Size distributions and total number concentrations (TNC) of particles in the range from 0.01-0.7 pm are determined by analysis of particle mobility. An impactor with a cut-off size of 1 pm is used to withdraw the coarse particle fraction. Exhaust gas is taken with a probe, which is also fed with particle free air. The resulting dilution factor is adjusted by the flow rate of the diluting air and the total flow. To prevent condensation of water onto the particle surface, the dilution factor is chosen high enough, to achieve a dew point below ambient temperature. The analytical set-up is shown in Figure I. 

These relationships can be used to assess spray rate requirements for aqueous coating systems. For example, 72 g/min water sprayed into a process at 300 fE/min process air, lO C dew point, and 30°C exhaust temperature would be equivalent to 40% of the available drying capacity (0.60 g/min capacity x 0.4 x 300 = 72 g/min). A total spray rate of 90 g/min would apply if the coat suspension consists of only 80% water (0.72/0.8 = 90). If the coater were a two gun configuration, each gun would be set to deliver 45 g/min spray rate. Depending on the critical parameters of the process, a spray rate range can be set to allow spray adjustment to actual drying conditions across the applicable... 

The production of chips with a final moisture content of 50 ppm typically requires a residence time of 2 h using heated gas with a dew point of -40°C. Whereas this moisture level is satisfactory for normal yams and staple fibers, microfilament quality requires a moisture level of 20 ppm, which is achieved by corresponding adjustments to the retention time and gas dew point. 

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