Control of temperature profiles in a tubular reactor of a supercritical water oxidation pilot plant

Resumen

Supercritical Water Oxidation is a powerful process to remove wastewaters in a clean and efficient way. Since oxidation reactions are strongly exothermic. the temperarure profile in the reactor may increase very quicky and it is necessary control that situation to improve tbe efficiency of the process and to avoid security risks. With the aim to control the temperature along the reactor, both cooling water injections and split oxidant addiction have been installed in a 25 kg/h pilot plant. Experimental tests bave been carried out to compare operarion of the pilot plant before and after those two improvements.Supercritical water oxidation (SCWO) is a high temperature and pressure process whoseoperational conditions are above the critical point of the pure water (Tc=374°C and Pc=221 bar). In theseconditions. water exhibits unique physical chemical properties that make it an effective reaction mediumfor the oxidation of organic and inorganic compounds. Oxidation reactions occur in a single reactionphase (no mass transfer limitations), with very high reaction rates (removal efficiencies >99.99) and nonharmful products, allowing the effective treatment of a wide variety of industrial wastes. In conventionalSCWO reactors, the wastewater feed and the oxidant are mixed at the reactor entrance, so there is not adirect control of the energy released inside the reactor. Since oxidation reactions are strongly exothermic,the temperature profile in tbe reactor may increase very quickly, so the initial concentration of thewastewater to be treated is limited by the maximum temperature supported by the material of the reactor(stainless steel, Hastelloy, Inconel, etc).To control that situation, two injections of cooling water to optimize the reactor performance have been installed, both increasing the concentration of wastewater to be treat and avoiding segurityrisks. These injections are situated in the first step of the reactor, when the wastewater is mixed with air,at temperature around 400ºC and the majority of the reactions take place, leading to temperature peaks of550°C in a few seconds. The injection of cooling water in this step achieved a quickly decline of thetemperature ensuring that the maximum temperarure in the reactor did not exceed 550°C. In this way, it ispossible to control the temperarure profile in the reactor automatically, establishing a set point in whichthe cooling water is injected. The flow rate of cooling water also increases the total flow rate of hoteffluent in the plant. Therefore. the available energy to preheat the feed or even to recover energy fromthe SCWO process increases.Other improvement ro control the temperature profile is the split oxidant addiction in the reactorusing two different lines. In this way, it is possible to distribute the air addiction in different areas of thereactor and therefore control the reaction. In a first step, deficiency of air is introduced to ensure that apart of the waste remains unreacted and control the heat produced by reaction. In a second step, theremaining organics in the wastewater react with the new amount of oxidant added in the second line. Inthese two steps, it is possible to remove completely the organics present in the wastewater and to controlthe reaction, and consequently the remperarure profile.This study shows how these improvements make possible to increase the feed concentration tobe treated and the energy recovery in a SCWO process, improving the control security of the pilot plant,maintaining the same removal efficiency (<96%).