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Carbon dioxide freezing in a cryogenic system can result in plugging and other operational problems. This article offers insights into the CO2 freezing phenomenon.
CRYOGENIC PROCESSES ARE USED IN natural gas plants, petroleum refineries, ethylene plants and elsewhere in the process industries to recover and purify products that would normally be gaseous at ambient temperature and pressure. Carbon dioxide can freeze at the low temperatures encountered in cryogenic plants, leading to plugged equipment and other operating problems. Accurate and reliable predictions of CO2 freeze points are needed for the design of cryogenic systems to ensure that freeze conditions are avoided. CO2 freeze-out prevention may dictate the type of cryogenic recovery process utilized, the maximum achievable recovery of products, or the amount of CO2 recovered from the feed gas.
During the revamp of a cryogenic natural gas plant, several commercial process simulators inaccurately predicted CO2 freeze points. Table 1 compares commercial simulator predictions with experimental liquid/solid equilibrium (LSE) freeze point data for the methaneCO2 binary system found in GPA Research Report RR-10 (1). It is evident that the process simulator results do not reliably match the experimental data for even this simple system.
This article reviews existing experimental data and the thermodynamics of solid CO2 formation in both liquids and vapors, presents calculation methods tailored to equipment commonly encountered in cryogenic processes, and tests our predictions against the freeze points observed in several commercial-scale cryogenic plants known to be constrained by CO2 solid formation. While the focus is on CO2 freeze point predictions for natural gas plant applications, the methodology can be readily extended to other solutes and other applications.
There are two basic modes for formation of solid CO2. Where the CO2 content of a liquid exceeds its solubility limit, CO2 precipitates or crystallizes from the liquid solution, as described by the thermodynamics of liquid/solid equilibria (LSE). Where the CO2 content of a vapor exceeds the solubility limit, CO2 is formed by desublimation or frosting, which is described by the thermodynamics of vapor/solid equilibria (VSE).
Liquid/solid equilibria
The GPA Research Report RR-10 (1) and Knapp, et al. (2) are good resources for many of the original papers containing experimental data for the liquid/solid systems of interest. The data presented in GPA RR-10 are of...