Flow-electrode Capacitive Deionization (FCDI) is a membrane based electrically driven desalination method combining ion-exchange membranes and carbon based flow electrodes. The flow electrodes function as a pumpable ion storage and enable a fully continuous operation of FCDI. In contrast, classical capacitive deionization processes are operated in cycles. Yet another advantage of FCDI is the ability to treat high salinity water streams. Until now most FCDI related works are of experimental nature, a process model is needed to obtain a better understanding of the physical dependencies. We report a process model for continuous, steady state FCDI processes operated at constant voltage. Processes such as a continuous two-module FCDI process or processes based on a single FCDI desalination module operated in single-pass mode can be simulated. The storage of ions in the pores of the carbon particles is modeled using the modified Donnan model. Simulations performed with this model match experimental results well and were fitted using an apparent Stern capacity as the only fitting parameter. Influences of the module geometry and process parameters can be investigated, such as feed flow rates and flow electrode composition. The model predicts correctly a strong influence of the flow electrode flow rate on the desalination performance.