  We study the dust concentration and emission in protoplanetary disks
vortices. We extend the Lyra-Lin solution for the dust concentration of a
single grain size to a power-law distribution of grain sizes $n(a) \propto
a^{-p}$. Assuming dust conservation in the disk, we find an analytic dust
surface density as a function of the grain radius. We calculate the increase of
the dust to gas mass ratio $\epsilon$ and the slope $p$ of the dust size
distribution due to grain segregation within the vortex. We apply this model to
a numerical simulation of a disk containing a persistent vortex. Due to the
accumulation of large grains towards the vortex center, $\epsilon$ increases by
a factor of 10 from the background disk value, and $p$ decreases from 3.5 to
3.0. We find the disk emission at millimeter wavelengths corresponding to
synthetic observations with ALMA and VLA. The simulated maps at 7 mm and 1 cm
show a strong azimuthal asymmetry. This happens because, at these wavelengths,
the disk becomes optically thin while the vortex remains optically thick. The
large vortex opacity is mainly due to an increase in the dust to gas mass
ratio. In addition, the change in the slope of the dust size distribution
increases the opacity by a factor of 2. We also show that the inclusion of the
dust scattering opacity substantially changes the disks images.
