Butane and propane only create soot along with visible yellow tips if the burners are not getting enough air i.e clogged air intake or incorrectly set gas/air mix, or a natural gas appliance is run straight on butane/propane without conversion to LPG first using the proper jets etc.

Check your density value for propane - it should be, according to Wikipedia, 2.0098mg per mL @ 0 celcius (or 2kg per cubic meter).

i have used both and propane produces less soot. that is why it is used for soldering and brazeing

Look up Energy Additives online for Blue Moon filtration systems and additives for a cleaner propane, Filters can be used in a butane application also!

Your density values are inconsistent. You seem to have quoted a density for butane in its gaseous state and a density for propane in its liquid state, possibly because of the reference temperature used and differences in boiling point (or just because you blindly took some numbers from a source such as wikipedia). For densities to have any meaning in a comparison, they should be at the same reference temperature at least - when talking about LPGs this would of course lead to a different pressure. If you wish to quote gas densities then they should be at the same reference temperature and pressure.