A numerical model for membrane-based liquid desiccant dehumidifiers is presented. The model uses the conjugate method and calculates a more realistic boundary condition on the membrane, and the separating walls. This boundary condition is calculated by the simultaneous solution of momentum, energy, and mass transfer equations of flows. To increase accuracy, flows are considered hydrodynamically, thermally, and in concentration to be developing. The validation results showed that the model can accurately predict the humidity ratio change of the supply air with less than 3.22% error. Afterward, four membrane-based liquid desiccant dehumidifiers including cross-flow, counter-flow, internally cooled and developed internally cooled are modeled and investigated. The dehumidifiers performances are compared at different inlet air conditions, and the results indicate that the developed internally cooled dehumidifier has the highest effectiveness.