Compressibility factors gases

The above equation is valid at low pressures where the assumptions hold. However, at typical reservoir temperatures and pressures, the assumptions are no longer valid, and the behaviour of hydrocarbon reservoir gases deviate from the ideal gas law. In practice, it is convenient to represent the behaviour of these real gases by introducing a correction factor known as the gas deviation factor, (also called the dimensionless compressibility factor, or z-factor) into the ideal gas law ... 

The virial equation of state, first advocated by Kamerlingh Oimes in 1901, expresses the compressibility factor of a gas as a power series in die number density ... 

When the mobile phase is a gas, a compressibility factor j must be applied to the adjusted retention volume to give the net retention volume ... 

Natural gas upgra ding economics may be affected by additional factors. The increasing use of compressed natural gas (CNG) directiy as fuel in vehicles provides an alternative market which affects both gas price and value (see Gasoline and other motor fuels Gas, natural). The hostility of the remote site environment where the natural gas is located may contribute to additional costs, eg, offshore sites require platforms and submarine pipelines. 

Natural Gas. Natural gas, an abundant fuel resource in the United States, has sufficient reserves to fuel over 10 x 10 U.S. vehicles per year for the next 50 years (122). Natural gas is used in two forms as a transportation fuel compressed or Hquefied at low temperatures. Tanks for the storage of compressed natural gas are heavy and larger in volume than for Hquid fuels. However, the added cost is offset by an expected lower pump price compared to gasoline (123). Whereas the lack of pubHc natural gas fueling stations and other factors make natural gas more attractive for fleet vehicles in the United... 

For isothermal compressible flow of a gas with constant compressibility factor Z through a packed bed of granular solids, an equation similar to Eq. (6-114) for pipe flow may be derived ... 

Compressibility of Natural Gas All gases deviate from the perfect gas law at some combinations of temperature and pressure, the extent depending on the gas. This behavior is described by a dimensionless compressibility factor Z that corrects the perfect gas law for real-gas behavior, FV = ZRT. Any consistent units may be used. Z is unity for an ideal gas, but for a real gas, Z has values ranging from less than 1 to greater than 1, depending on temperature and pressure. The compressibihty faclor is described further in Secs. 2 and 4 of this handbook. 

Ideal gas obeys the equation of state PV = MRT or P/p = MRT, where P denotes the pressure, V the volume, p the density, M the mass, T the temperature of the gas, and R the gas constant per unit mass independent of pressure and temperature. In most cases the ideal gas laws are sufficient to describe the flow within 5% of actual conditions. When the perfect gas laws do not apply, the gas compressibility factor Z can be introduced ... 

L = line length, miles T = gas temperature, °R Z = gas compressibility factor D = pipe inside diameter, in. 

Pipecalc 2.0, Gulf Publishing Company, Houston, Texas. Note Pipecalc 2.0 will calculate the compressibility factor, minimum pipe ID, upstream pressure, downstream pressure, and flow rate for Panhandle A, Panhandle B, Weymouth, AGA, and Colebrook-White equations. The flow rates calculated in the above sample calculations will differ slightly from those calculated with Pipecalc 2.0 since the viscosity used in the examples was extracted from Figure 5, p. 147. Pipecalc uses the Dranchuk et al. method for calculating gas compressibility. 

Z = Gas compressibility factor T = Absolute temperature, °R Mw = Gas molecular weight... 

The following analysis enables one to calculate the diameter of a pipeline transporting any compressible fluid. The required inputs are volumetric flow rate, the specific gravity of the gas relative to air, flow conditions, compressibility factor Z where Z is defined by nZRT = PV, the pressure at the point of origin and the destination, the pipe length, and pipe constants such as effective roughness. The working equations have been obtained from the literature. Since the friction factor... 

Since non-ideal gases do not obey the ideal gas law (i.e., PV = nRT), corrections for nonideality must be made using an equation of state such as the Van der Waals or Redlich-Kwong equations. This process involves complex analytical expressions. Another method for a nonideal gas situation is the use of the compressibility factor Z, where Z equals PV/nRT. Of the analytical methods available for calculation of Z, the most compact one is obtained from the Redlich-Kwong equation of state. The working equations are listed below ... 

Z = gas compressibility factor R = gas/liquid ratio, ft /bbl T = operating temperature, °R P = pressure, psia Qi = liquid flow rate, bbl/day V = maximum allowable velocity, ft/sec... 

Qg = gas flow rate, MMscfd T - operating temperature, °R Z = compressibility factor P = operating pressure, psia... 

Pi = flowing pressure, psia (set pressure + overpressure + 14.7) Overpressure is normally 10% of set pressure. MW = molecular weight of gas Z = compressibility factor... 

W = gas rate, Ib/hr Z = gas compressibility factor Pc = Pcriv critical pressure, psia... 

M = molecular weight of flowing fluid Z = compressibility factor for deviation from perfect gas if known, otherwise use Z = 1.0 for pressures below 250 psia, at inlet conditions. 

Yo = initial concentration of component (oxidant) under low pressure, mol fraction Z = compressibility factor, deviation of actual gas from perfect gas law. Usually Z = 1.0 at low pressure below 300 psig. 

Gas volumes are corrected at the intake conditions on the first and each succeeding stage of the compression step, and compressibility factors are calculated or evaluated at these individual intake conditions. Some manufacturers use the average value between intake and discharge conditions. 

T = absolute temperatures, °R (Rankine) = °F + 460 R = universal gas constant = 10.729 for units noted here Z = compressibility factor N = number of lb-moles of gas... 

Meters are accurate within close limits as legislation demands. However, gas is metered on a volume basis rather than a mass basis and is thus subject to variation with temperature and pressure. The Imperial Standard Conditions are 60°F, 30inHg, saturated (15.56°C, 1913.7405 mbar, saturated). Gas Tariff sales are not normally corrected, but sales on a contract basis are. Correction may be for pressure only on a fixed factor basis based on Boyle s Law or, for larger loads, over 190,000 therms per annum for both temperature and pressure using electronic (formerly mechanical) correctors. For high pressures, the compressibility factor Z may also be relevant. The current generation of correctors corrects for pressure on an absolute basis taking into account barometric pressure. 

Compressibility is experimentally derived from data about the actual behavior of a particular gas under pVT changes. The compressibility factor, Z, is a multiplier in the basic formula. It is the ratio of the actual volume at a given pT condition to ideal volume at the same pT condition. The ideal gas equation is therefore modified to ... 

T = flowing temperature, R Z = gas deviation, compressibility factor T = base temperature, (520 R)... 

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