The concept of single pellet string reactors (SPSR), where particles are packed in tubes of only slightly larger diameter has been subject of recent studies which demonstrate a renewed interest in this reactor concept, including its use for catalyst testing. However, there remains the need for a thorough characterisation of the reactor performance. In this study, an attempt is made to systematically investigate the flow and conversion behaviour of SPSRs consisting of spherical, non-porous particles in cylindrical confining walls using computational fluid dynamics (CFD). A parameter study with variations on reactor geometry is conducted considering laminar flow and an isothermal, irreversible hypothetical first order reaction taking place at the catalytic particle surface. In addition to reaction simulation, the residence time behaviour is investigated based on numerical tracer experiments. The results showed that conversion close to plug flow can be achieved with SPSRs using favourable parameter settings. Conversion was found to be enhanced in reactors at larger scale, with smaller cylinder-to-particle diameter ratio and with a larger number of catalytic particles. The applicability of existing design criteria for conventional fixed-bed reactors is evaluated. Based on the observations a design criterion for SPSRs is proposed which allows prediction of the deviation to plug flow conversion based on reactor geometry, fluid properties and flow velocity.