Natural gas and associated condensate are often produced from the reservoir saturated (in equilibrium) with water. In addition, the gas and condensate often contain CO2 and H2S which might require removal. This is frequently accomplished with aqueous solutions such as amines, potassium carbonate, etc. which saturate the gas or condensate with water. Liquid hydrocarbonsmay also contain water downstream of product treaters or upon removal from underground storage. Dehydration is the process used to remove water from natural gas and natural gas liquids (NGLs), and is required to: • prevent formation of hydrates and condensation of free water in processing and transportation facilities, • meet a water content specification, and • prevent corrosion
FIG. 20-1Nomenclature A B C Cp Cg Cs Css CT D d EOS Fs G H ∆H Kvs L Lg LMTZ Ls . m MTZ MW MWI N P ∆P q Q Qc Qhl Qr Qs Qsi = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = area, ft2 constant in Equation 20-11 constant in Equation 20-11 heat capacity, Btu/(lb • °F) gravity correction factor for water content salinity correction factor for water content saturation correction factor for sievetemperature correction factor diameter, ft depression of the water dewpoint or the gas hydrate freezing point, °F Equation of State lb / (ft • sec) sizing parameter for packed towers, √ 2 mass velocity, lb /(ft • hr) enthalpy, BTU/lb latent heat of vaporization, Btu/lb vapor/solid equilibrium K-value length of packed bed, ft glycol flow rate, U.S. gal./hr length of packed bed masstransfer zone, ft length of packed bed saturation zone, ft mass flow rate, lb/hr mass transfer zone molecular weight molecular weight of inhibitor0 number of theoretical stages pressure, psia pressure drop, psi actual gas flow rate, ft3/min heat duty, Btu/hr reflux condensing heat duty, Btu/gal. regeneration heat loss duty, Btu total regeneration heat duty, Btu/gal. sensible heat, Btu/gal. dutyrequired to heat mole sieve to regeneration temperature, Btu duty required to heat vessel and piping to regeneration temperature, Btu Qtr = total regeneration heat duty, Btu Qv = vaporization of water heat duty, Btu/gal. Qw = desorption of water heat duty, Btu Ss = amount molecular sieve req’d in saturation zone, lb t = thickness of the vessel wall, in. T = temperature, °F Trg = regenerationtemperature, °F v = vapor velocity, ft/sec V = superficial vapor velocity, ft/min . w = water rate, lb/hr W = water content of gas, lb/MMscf Wbbl = water content of gas, bbl/MMSCF Wr = water removed per cycle, lb x = mole fraction in the liquid phase X = mass fraction in the liquid phase y = mole fraction in the gas phase z = compressibility factor γ = specific gravity µ = viscosity, cp ρ = density, lb/ft3Subscripts i = inlet o = outlet l = liquid v = vapor t = total CO2 = carbon dioxide H2S = hydrogen sulfide HC = hydrocarbon s = solid phase L = lean inhibitor R = rich inhibitor I = inhibitor H2O = water H = hydrate rg = regeneration f = feed p = permeate
Techniques for dehydrating natural gas, associated gas condensate and NGLs include: • Absorption using liquid desiccants, •Adsorption using solid desiccants, • Dehydration with CaCl2, • Dehydration by refrigeration, • Dehydration by membrane permeation, • Dehydration by gas stripping, and • Dehydration by distillation
Cg = 0.98 W = (0.98)(220) = 216 lb/MMscf For a gas in equilibrium with a 3% brine, Cs = 0.93 W = (0.93)(220) = 205 lb/MMscf
Water Content of High CO2/H2S Gases
BothH2S and CO2 contain more water at saturation than methane or sweet natural gas mixtures. The relative amounts vary considerably with temperature and pressure. Fig. 20-67 and 20-76 display saturated water content of pure CO2 and H2S, respectively, at various temperatures and pressures. Fig. 20-8 shows the saturated water contents of pure CH4, CO2 and selected mixtures vs. pressure at 100°F. Fig....