Heterogeneous flow-cell detectors packed with scintillator granules were simulated with the Monte Carlo code, PENetration and Energy LOss of Positrons and Electrons (PENELOPE) (Version 2000), for the detection of beta particles in aqueous media. The simulations were run for monoenergetic electrons at the average and endpoint energies of common beta-emitting radionuclides. PENELOPE was used to simulate the electron interactions as a means of investigating the effects of: packing geometry of scintillator granules, scintillator granule size, and the energy and initial location of electrons within the flow-cell on the fraction of energy deposited in the scintillator and the geometrical detection efficiency. The simulation results showed that higher packing density (lower porosity) and smaller scintillator granule size would result in a higher fraction of energy deposited in the scintillator, but the effect is more pronounced for low-energy electrons (180 keV or lower) than for higher-energy ones (550 keV or higher). The simulation results also showed that the initial location of electrons within the flow-cell has a significant effect on the fraction of energy deposited in the scintillator for low-energy electrons (180 keV or lower), but gradually diminishes as the electron energy increases (above 550 keV). The geometrical detection efficiency as simulated using PENELOPE was compared to experimental absolute detection efficiency found in the literature. The former was generally higher than the latter as expected and their ratio was used to estimate the light collection efficiency, whereas the reasons for the difference at electron energy of 53 keV and lower were discussed.