Dew-point of natural gas

Figure 4-6. Dew point of natural gas. (From Gas Processors Suppliers Association, Engineering Data Bookj 10th Edition.)...

HC/HC Dew pointing of natural gas Being tested Reverse selectivity can be lost due to plugging... 

Onshore or offshore gas plants are designed for either LNG rejection and gas injection, or LNG rejection and transmission for sale. In the case of offshore plants, onshore facilities further process the natural gas before transmission for sale. In either case, natural gas must be treated and then refrigerated to make rejection of heavy hydrocarbons possible. In plants where natural gas is treated for sale purposes, water and hydrocarbon dew points of the gas must also be controlled. 

The gas processing options described in the previous section were designed primarily to meet on-site usage or evacuation specifications. Before delivery to the customer further processing would normally be carried out at dedicated gas processing plants, which may receive gas from many different gas and oil fields. Gas piped to such plants is normally treated to prevent liquid drop out under pipeline conditions (dew point control) but may still contain considerable volumes of natural gas liquids (NGL) and also contaminants. 

The need to obtain greater recoveries of the C9, C3, and C4S in natural gas has resulted in the expanded use of low-temperature processing of these streams. The majority of the natural gas processing at low temperatures to recover light hydrocarbons is now accomphshed using the turboexpander cycle. Feed gas is normally available from 1 to 10 MPa. The gas is first dehydrated to a dew point of 200 K and lower. After dehydration the feed is cooled with cold residue gas. Liquid produced at this point is separated before entering the expander and sent to the condensate stabilizer. The gas from the separator is... 

The temperature at which water condenses from natural gas is called its dew point. If the gas is saturated with water vapor, it is by definition at its dew point. The amount of water vapor saturated in the gas is given... 

The economic value of natural gas is primarily determined by the thermal energy it contains, which is expressed in British thermal units (Btu) or calorific value (CV). Other important physical properties comprise the liquid content, the burning characteristics, the dew point and the compressibility. In order to enable the calculation of these properties from its composition, a natural gas analysis should contain a detailed determination of all of the individual components, even in the low-concentration range. 

All of these equations suffer from at least one common deficiency- -they require that the critical properties of all components in the system be defined. This requirement extends to any undefined component (C6+, crude oil, heavy tar fractions, etc.) which may be present in the system. Prediction of the critical properties of these compounds is at best an art. Changing the critical temperature of an undefined fraction present in quantities less than one mol percent by 10°C can change the predicted dew point of a natural gas system by 35 bar. 

Sihca gel Drying of gases, refrigerants, organic solvents, transformer oils desiccant in packings and double glazing dew point control of natural gas. 

Water Dew Point For flue gas, the water dew point is that temperature at which the actual water vapor pressure equals the water saturation vapor pressure. Cooling the flue gas below this temperature will result in the formation of liquid water [or ice, below 273 K (0°C)]. For example, burning natural gas with 3 percent excess oxygen (15 percent excess air), the flue gas water dew point would be (Fig. 24-57) 330K(56.7°C). 

Dehydration is applied to natural gas, air for feed to air separation units, cracked gas (e.g., ethylene), various other hydrocarbon streams just to name a few. Zeolite molecular sieves play a major role in dehydration because of their abihty to reduce moisture content to very low levels attaining dew points of —100°C or lower. 

Natural gas usually contains varying amounts of ethane, propane, butane, and higher hydrocarbons. The gas is often close to its saturation point with respect to some of these hydrocarbons, which means liquids will condense from the gas at cold spots in the pipeline transmission system. To avoid the problems caused by condensation of liquids, the dew point of US natural gas is lowered to about —20 °C before delivery to the pipeline by removing portions of the propane and butane and higher hydrocarbons. For safety reasons the Btu rating of the pipeline gas is also usually controlled within a narrow range, typically... 

Also calculate the dew point of the exit flue gas and the air-to-fuel ratio in.lb/lb. Formaldehyde can be made by the partial oxidation of natural gas using pure oxygen made industrially from liquid air. The natural gas must be in large excess. 

Increased pressure drop. Could require conversion to FD burners, or installation of an ID fan. Precipitation can still occur in the catalyst bed because of capillary action that raises the dew point of the ammonium salts. This is negligible for natural gas and refinery fuel gas fired systems but can be significant for applications with high sulfur in the fuel. 

Another industrial application of gas-separation membranes is the removal of carbon dioxide from natural gas. The CO2/CH4, selectivity is about 20 to 30 for polycarbonate, polysulfone, and cellulose acetate membranes at 35°C and 40 atm. A selectivity of over 60 can be obtained with Kapton , but this polymer is much less permeable than the others. Increasing the temperature raises the permeability of most polymers but generally causes a. slight decrease in selectivity. The operating temperature is chosen to be somewhat above the dew point of the residue gas. There is considerable COj absorbed in the membranes at high CO2 partial pressures, and the plasticization effect of CO2 increases the effective diffusion coefficients for all gases and makes the selectivity less than that based on pure-gas data. Methods of allowing for such nonlinear effects have been presented. ... 

Currently, PRISM membranes provide an attractive alternative to traditional glycol dehydration systems (Figure 14.8) based on simple process designs, lower costs, and the other benefits listed below. These benefits become even more pronounced as the industry produces natural gas from very remote locations [55]. An offshore membrane system for Shell Nigeria was designed to dry 600000NmVh of natural gas from an inlet dew-point of 41 °C to an outlet dew-point of 0°C at 38 bar. It was designed and built by Petreco, an Air Products PRISM Membranes licensed partner. Other plants were installed in Italy and Holland. 

The accurate prediction of thermodynamic properties of natural gas systems is of interest for gas industry. Compressibility factors are used in energy and flow metering. It is also used in calculations of gas pressure gradient in tubing and pipelines. When large volumes of gas are traded between produeers, distributors, and consumers, error in the estimation of the amount of involved are of real economic significance. In gas condensate reservoirs, well-productivity often declines rapidly when pressure drops below the dew point pressure near-wellbore. Therefore, it is very important to accurately determine the dew point pressure. The pressure and temperature of most natural gas mixtures can be found up to 150 MPa and 500 K, respectively (Nasrifar and Boland, 2006). At these eonditions, methane, ethane, and nitrogen are almost always supercritical while other hydroearbons are subcritical. Thus, the equation of state of natural gas mixture must be aeeurate at supercritical and subcritical behavior of methane and heavy hydrocarbons, respectively. 

The dew-point of a natural gas is very sensitive to the heavy ends characterization. Consider a natural gas stream with the following composition ... 

The different characterizations of the heavy ends, which make up less than 1% of the entire mixture, were considered. In Case 1 normal butane was selected to represent the heavy ends, in Case 2 normal pentane, and in Case 3 normal hexane. The phase envelopes for the three cases were predicted with the BWR—11 equation of state. The results are given in Figure 14. The bubble-point curves almost coincide, and the true critical points are not very sensitive to composition. However, as shown, the dew-point is greatly affected by the characterization of the heavy ends composition, and there is about a 70 F difference in the maximum dew-points of the three mixtures. Therefore, when working with a natural gas type system, if the breakdown of the C+ fraction into additional compounds is available, it should be used, especially in flash calculations, to get an accurate representation of the phase behavior. 

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