Section Five
WASTE TREATMENT PROCESSES
B. SUPERCRITICAL FLUID EXTRACTION
39    CATALYST EVALUATION: SUPERCRITICAL WATER
OXIDATION PROCESS
Michael A. Frisch, Graduate Research Assistant
Lixiong Li, Research Scientist
Earnest F. Gloyna, Bettie Margaret Smith Chair
Environmental Health Engineering
The University of Texas at Austin
Austin, Texas 78758
INTRODUCTION
Supercritical water oxidation (SCWO) exhibits rapid destruction of dimethyl methylphosphonate
(DMMP), pyridine and 2,4-dichlorophenol (2,4-DCP). Relatively stable transformation products that
result from the SCWO of these compounds are methylphosphonic acid (MPA), ammonia and acetic
acid, and mono- and di-chlorinated phenols, respectively. To destroy these by-products it is necessary
to provide higher temperatures, longer residence times, or catalyst enhancement. Preliminary tests
have indicated that some commercially available catalysts, including metals used in reactor vessels,
reduce the required temperature and residence time. Higher temperatures and reactor residence times
require both increased capital and operational costs. For example, a decrease in required process
temperature from 600°C to 450°C may translate into a considerable reduction in corrosion and
capital costs. This corrosion problem is of particular importance because many hazardous wastes
contain heteroatoms (such as CI, F, P, S and N) which are eventually oxidized to form corrosive acids.
Catalyst and support stability is critical as it will govern the useful lifetime of a catalyst. While
catalysts may increase the oxidation rate and thereby reduce reactor volume requirements, aggressive
SCWO environments may deactivate some catalysts and/or supports. Consequently, catalysts operating within a SCWO process must maintain physical stability and activity over a practical lifetime.
The objective of this research was to evaluate the performance of selected catalysts for use in
SCWO environments. A three-step procedure was used to evaluate catalyst performance. First, commercially available metal oxide supports were tested for physical stability in SCWO environments.
Stability was determined by examination of the supports and dissolved species in the liquid phase
before and after exposure to SCWO in laboratory-scale, batch reactors. Second, promising supports
were coated with platinum (platinum-coated supports were then referred to as platinum catalysts).
These platinum catalysts and supports were each packed in a continuous-flow, fixed bed reactor in
order to evaluate the catalytic oxidation of DMMP, acetic acid, pyridine and 2,4-DCP and their
corresponding byproducts. Third, selected platinum catalysts and supports were exposed to longevity
tests in order to evaluate temporal stability and activity in the catalytic oxidation of acetic acid.
Longevity tests also were used to evaluate the effect of sulfuric and hydrochloric acid additions to the
feed.
BACKGROUND
Properties of supercritical water (SCW) give the SCWO process an advantage over biological
oxidation and wet air oxidation processes.1,2 A sharp decrease in hydrogen bonding near the critical
point renders SCW a nonpolar solvent, which dissolves most organic substances in all proportions.3
SCW also dissolves most gases in all proportions.4-6 Intimate mixing of reactants and high temperatures permit 99.99999 percent destruction of many organic compounds in seconds. Another important
49th Purdue Industrial Waste Conference Proceedings, 1994 Lewis Publishers, Chelsea, Michigan 48118. Printed in
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