This is probably way too late of a response, but to convert SCFD to lb/day, you would use the Ideal Gas Law. First we would need to know what the "standard" temperature and pressure are. Probably either 0, 20, or 25 Celsius and 1 atmosphere. Then plug values into ideal gas law (P*V=n*R*T) to get "n" (moles/time), then use the molecular weight to convert moles into mass. For instance: assuming standard temp = 0 C (273K) and standard pressure is 1 atm: the calculation would be:
(1 atm) * (1000 scf/day) = n * 0.002898 cf-atm/mol-K) * (273 K)
n = 1264 mol/day
mass = n * MW = 1264 mol/day* 20.63 g/mol = 26,076 g/day, then convert to 57 lb/day
This calculation also assumes that the volumetric measurement is "dry" (moisture has been subtracted). If it is wet, then you would have to subtract out the volume of water. Also remember you can't use the ideal gas law under extreme temperatures or pressures, but most pressures natural gas would be under are probably OK.
This estimate lines up pretty close with the commonly accepted natural gas density of 0.04-0.06 lb/cf (standard conditions)
