1. A computer based method for determining the cavity size in packed bed systems using correlation or mathematical model, said method comprising the steps of: a) obtaining data related to material properties of the packed bed system; b) calculating the cavity radius for both increasing gas velocity and decreasing gas velocity using mathematical model incorporating the stresses/frictional forces as: 2 ⁢ nR 2 - 2 ⁢ nHR + pn ⁢ ⁢ β ⁢ ⁢ v b 2 ⁢ D T 2 2 ⁢ π 2 ⁢ M ⁢ { ln ⁢ ⁢ W 2 ⁢ π - ln ( R - D T 2 ⁢ π } + ( 2 ⁢ r o M ⁢ ⁢ π ⁢ ( α + β ⁢ ⁢ v H ) ⁢ v H ⁡ ( H - r o ) - F wd M ⁢ ⁢ π ) = 0 ⁢ ⁢ and ( 29 ) 2 ⁢ nR 2 - 2 ⁢ nHR + pn ⁢ ⁢ β ⁢ ⁢ v b 2 ⁢ D T 2 2 ⁢ π 2 ⁢ M ⁢ { ln ⁢ ⁢ W 2 ⁢ π - ln ( R - D T 2 ⁢ π } + ( 2 ⁢ r o M ⁢ ⁢ π ⁢ ( α + β ⁢ ⁢ v H ) ⁢ v H ⁡ ( H - r o ) + F wd M ⁢ ⁢ π ) = 0 ( 28 ) respectively; or calculating the cavity radius for both increasing gas velocity and decreasing gas velocity using mathematical equations based on correlation as: D r D T = 4.2 ⁢ ( ρ g ⁢ v b 2 ⁢ D T ρ eff ⁢ g ⁢ ⁢ d eff ⁢ W ) 0.6 ⁢ ( D T H ) - 0.12 ⁢ ( μ w ) - 0.24 ( 36 ) D r D T = 164 ⁢ ( ρ g ⁢ v b 2 ⁢ D T 2 ρ eff ⁢ g ⁢ ⁢ d eff ⁢ H ⁢ ⁢ W ) 0.80 ⁢ ( μ w ) - 0.25 ( 33 ) respectively, and c) calculating the cavity size using the cavity radius obtained in step (b).