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Pneumatic transport

Pneumatic transport. Basic definition – using gas to transport a particulate solid through a pipeline Ex: grain, flour, plastic, pulverized coal Two modes

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Pneumatic transport

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  1. Pneumatic transport • Basic definition – using gas to transport a particulate solid through a pipeline • Ex: grain, flour, plastic, pulverized coal • Two modes • Dilute phase – particles are fully suspended, like entrainment in FB but deliberate, solids less than 1 % by volume, lots of pumping req’d • Dense phase – particles not suspended, loading > 30 % by volume, lots of interparticle interactions

  2. Phase diagram for dilute phase vertical pneu. transport(after Rhodes, p. 141, Fig 6.1) Static head of solids dominates p/L G = G2 > G1 Friction resistance dominates G = G1 B G = 0 A Uch for G1 Superficial gas velocity U Uch, lowest velocity at which dilute phase transport line can be operated if solids feed rate is G1

  3. Phase diagram for dilute phase horizontal pneu. transport(after Rhodes, p. 142, Fig 6.2) Saltation, solids begin to settle out in the bottom of the pipe p/L G = G2 > G1 G = G1 B G = 0 A Superficial gas velocity U Usalt for G1

  4. Definitions • Superficial gas velocity Ufs = Qf (gas volumetric flow) /A (cross sectional area of pipe) • Superficial solids velocity Ups = Qp/A (Qp = volumetric flow of solids) • Actual gas velocity Uf = Qf/Ae (void fraction) • Actual particle velocity Up = Qp/[A(1-e)]

  5. Important relationships • Mass flow rate of particles • Mass flow of fluid • Solids loading = Mp/Mf

  6. Pressure drop in pneumatic transport Contributors to pressure drop • Gas acceleration (gas acting on gas) • Particle acceleration (gas acting on particles) • Gas/pipe friction wall friction • Solids/pipe friction “ • Static head of solids fighting gravity • Static head of gas “ Not considered: interparticle forces

  7. Force balance on pipe rate of increase in momentum of contents Net force acting on pipe contents = Pressure - gas/wall - solids wall - gravity = rate of increase in force friction force friction force momentum of gas + rate of increase in momentum solids Ffw and Fpw are gas to wall and solids to wall friction force respectively, L = pipe length,  = angle of pipe with horizontal What happens for steady state? Horizontal flow?

  8. Terms and physical meaning (you match them up): • Total pressure drop • Gas acceleration (gas acting on gas) • Particle acceleration (gas acting on particles) • Gas/pipe friction wall friction • Solids/pipe friction wall friction • Static head of solids fighting gravity • Static head of gas fighting gravity

  9. Tools to calculate pressure drop Correlations for Fpw For vertical transport [G = solids mass flux, mass particles/(area x time)] Horizontal transport where and For gas/wall friction pressure drop, calculate with friction factor assuming it is independent of presence of particles.

  10. Simple method for s.s. horizontal From Particle Technology by Orr (1966) Ratio of total pressure loss due to solids/air system (DPt) to total pressure loss due to only air flowing (DPa) R = mass of solid material mass of air k is an empirically derived coefficient

  11. Bends Generally problematic. Solids that may be in suspension in vert/horiz transport may salt out as they go around bends. Worst case: vertical going to horizontal blinded tees recommended if bends are unavoidable No reliable correlations exist for bend pressure drops. Only a rough rule of thumb: Bend DP = DP for 7.5 m of vertical pipe under same flow conditions

  12. For more info (and movies!) • http://www.erpt.org/014Q/rhoe-00.htm

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