The permeation of hydrogen through palladium membranes can be described by Sieverts' law. In a membrane separator, hydrogen flow rates and partial pressures may vary along the flow directions on both sides of the separator. Sieverts' law itself cannot be directly used for the design of membrane separators. Seven flow scenarios are considered in which the flow pattern, flow direction and the existence of a sweep gas in a membrane separator are considered. Maximum potential hydrogen production rates are obtained. The relationship between the hydrogen production rate and the membrane permeation capacity is derived. A procedure is recommended for the design of membrane separators, and a typical design case is presented. Concepts of economical permeation capacity and hydrogen production rate are proposed and defined. Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.