2 replies to this topic

[DUmesh]

Is there any relation between void volume and bulk density.

If any please share with me.

Thanks

[breizh]

Hi ,
Did you try google ?
http://en.wikipedia.org/wiki/Porosity

Hope this helps

Breizh

[kkala]

Web reference given by Breizh complies with examples presented below for clarification & specific indications.
Α. Consider a pile of rock pebbles, e.g. granite, having no internal pores. It has a volume of 1.0 m3 and weighs 1500 kg. Its bulk (or apparent) density is ρb=1500/1.0 = 1500 kg/m3, or 1.5 g/cm3.
We transport these pebbles into a 1.0 m3 container, then fill it with water to cover the air space between pebbles until the container gets full. The pebbles & water as a total weighs 1900 kg now; so mass of water =1900-1500=400 kg, volume of water = 400/1000 = 0.4 m3. The rock material has a volume of 1-0.4 = 0.6 m3, so its true (or real) density is ρt=1500/0.6 = 2500 kg/m3, or 2.5 g/cm3. The void fraction of the initial pile is e = 0.4/1.0 = 0.4 or 40%, representing the fraction of total pile volume not occupied by the rock.
Considering volumes per unit mass instead of densities, above can be expressed as 1/ρb - e*(1/ρb = 1/ρt, from where ρb = (1-e)*ρt. See also http://en.wikipedia....article_density.
Β. Suppose now that light stone pebbles are used instead of granite. Light stone has numerous internal pores and water will come into them, at least partially. So water will not fill only space between pebbles but also (at least part of) internal pores. Resulting void fraction will represent external volume not occupied by pebbles, plus (at least part of) pebble volume of internal pores.
Γ. For the specific case of catalyst pellets in granular beds, resembling light stone pebbles, following assumptions are made. Advise / comments on them would be gladly welcomed.
1. Internal void fraction of a catalyst pellet can be estimated or given by its supplier. This represents catalyst pores, where reactants are diffused into. Practically all conversion occurs in these pores, but in kinetics calculations this is usually simplified by considering catalyst effectiveness E and reactant concentration(s) Cs at surface of the catalyst. Active catalyst surface can be specified by BET apparatus or by its supplier.
2. Books express reaction rate (per volume of catalytic bed) as k*(1-e)*Cs*E, where k=reaction rate per catalyst volume, e=external void fraction of catalyst pellets, which could be specified through formula of para A above with ρt = density of one pellet with its pores = mass of one pellet / geometric volume of one pellet.
3. As an example, Perry (7th ed, Table 23-1, p 23-7) refers a residence time T = 33 s for fixed bed NH3 synthesis (at specific conditions). This T=33 s concerns synthesis gas passing through e*VR, where e=external void fraction, VR=total volume of catalytic bed.
Δ. Above tries to distinguish between bulk density, real density, void fraction (also called porosity), hoping it can be helpful.

Edited by kkala, 25 November 2011 - 03:10 PM.