How to optimize hydrogen plant designs

Abstract

In a typical hydrogen plant of the type which will be discussed, methane or higher hydrocarbons are reformed with steam in a steam hydrocarbon reformer operating at a pressure of 250–400 psig, a temperature of 1500–1600°F, and with a ratio of steam to carbon in the feed of about 3.0. Following the reformer and cooling, there is a single stage of high temperature carbon monoxide shift conversion. Optionally, after further cooling, this may be followed by a second stage of carbon monoxide shift conversion operating at a lower temperature to obtain a more favourable equilibrium; this is called low temperature shift conversion. After cooling to ambient temperature, and separation of the condensate, the gas is passed through a Pressure Swing Adsorption (PSA) unit which removes all the impurities along with a small amount of hydrogen. The waste gas from the PSA unit containing all the impurities is used as fuel to the reformer. Heat is recovered from the reformer flue gas, reformer product, high temperature shift converter product and low temperature shift converter product. This paper discusses some of the process variables and design variables which must be considered in arriving at an optimized design. Seven different flow schemes are discussed in the light of the objectives they are designed for. The seven schemes and their objectives are: • Flow Scheme 1—lowest first cost, moderate efficiency • Flow Scheme 2—high efficiency, low cost • Flow Scheme 3—low feed plus fuel, moderately high efficiency • Flow Scheme 4—lowest feed plus fuel • Flow Scheme 5—lowest feed, low fuel • Flow Scheme 6—lowest feed, highest efficiency • Flow Scheme 7—lowest feed plus fuel, export electric power instead of export steam. Flow Schemes 4, 5 and 7, all of which use preheated combustion air, are attractive only if there is no use for export steam and hence no credit for export steam. Scheme 7 is a new concept which employs an organic Rankine Cycle to generate electric power from low level heat. Economic figures on the Rankine Cycle are presented.