We revisit the evolution model of grain size distribution in a galaxy for the ultimate purpose of implementing it in hydrodynamical simulations. We simplify the previous model in such a way that some model-dependent assumptions are replaced with simpler functional forms. For the first test of the developed framework, we apply it to a one-zone chemical evolution model of a galaxy, confirming that our new model satisfactorily reproduces the previous results and that efficient coagulation of small grains produced by shattering and accretion is essential in reproducing the so-called MRN grain size distribution. For the next step, in order to test if our model can be treated together with the hydrodynamical evolution of the interstellar medium (ISM), we post-process a hydrodynamical simulation of an isolated disc galaxy using the new grain evolution model. We sample hydrodynamical particles representing each of the dense and diffuse ISM phases. By this post-processing, we find that the processes occurring in the dense gas (grain growth by accretion and coagulation) are important in reproducing the grain size distribution consistent with the Milky Way extinction curve. In our model, the grain size distributions are similar between the dense and diffuse ISM, although we observe a larger dispersion in the dense ISM. Moreover, we also show that even if we degrade the grain radius resolution (with 16 grid points), the overall shape of grain size distribution (and of resulting extinction curve) can be captured.
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