A PDE formulation is proposed, referred to as a heat equation with order-respecting absorption, aimed at characterizing hydrodynamic limits of a class of particle systems on the line with topological interaction that have so far been described by free boundary problems. It consists of the heat equation with measure-valued injection and absorption terms, where the absorption measure respects the usual order on R in the sense that, for all r ∈ R, it charges (−∞, r) only at times when the solution vanishes on (r, ∞). The formulation is used to obtain new hydrodynamic limit results for two models. One is a variant of the main model studied by Carinci, De Masi, Giardinà and Presutti [5] where Brownian particles undergo injection according to a general injection measure, and removal that is restricted to the rightmost particle of the configuration. This partially addresses a conjecture of [5]. Next a Brownian particle system is considered where the Q-quantile member of the population is removed until extinction, where Q is a given [0, 1]-valued continuous function of time. Here, unlike in earlier work on the subject, the removal mechanism acts on particles that are 'at the boundary' but are not rightmost or leftmost. Finally, further potential uses of order-respecting absorption are mentioned.