Animal Guts as Ideal Chemical Reactors: Maximizing Absorption Rates

I solved equations that describe coupled hydrolysis in and absorption from a continuously stirred tank reactor (CSTR), a plug flow reactor (PFR), and a batch reactor (BR) for the rate of ingestion and/or the throughput time that maximizes the rate of absorption (=gross rate of gain from digestion). Predictions are that foods requiring a single hydrolytic step (e.g., disaccharides) yield ingestion rates that vary inversely with the concentration of food substrate ingested, whereas foods that require multiple hydrolytic and absorptive reactions proceeding in parallel (e.g., proteins) yield maximal ingestion rates at intermediate substrate concentrations. Counterintuitively, then, animals acting to maximize their absorption rates should show compensatory ingestion (more rapid feeding on food of lower concentration), except for the lower range of diet quality fur complex diets and except for animals that show purely linear (passive) uptake. At their respective maxima in absorption rates, the PFR and BR yield only modestly higher rates of gain than the CSTR but do so at substantially lower rates of ingestion. All three ideal reactors show milder than linear reduction in rate of absorption when throughput or holding time in the gut is increased (e.g., by scarcity or predation hazard); higher efficiency of hydrolysis and extraction offset lower intake. Hence adding feeding costs and hazards of predation is likely to slow ingestion rates and raise absorption efficiencies substantially over the cost-free optima found here.

Kinetics and Mechanism of Oxygen Delignification

Considerable research has been conducted into the kinetics and selectivity of the oxygen delignification process to overcome limitation in its use. However most studies were performed in a batch reactor whereby the hydroxide and dissolved oxygen concentrations are changing during the reaction time in an effort to simulate tower performance in pulp mills. This makes it difficult to determine the reaction order of the different reactants in the rate expressions. Also the lignin content and cellulose degradation of the pulp are only established at the end of the experiment when the sample is removed from the batch reactor. To overcome these deficiencies, we have adopted a differential reactor system used frequently for fluid-solid rate studies (so-called Berty reactor) for measurement of oxygen delignification kinetics. In this reactor, the dissolved oxygen concentration and the alkali concentration in the feed are kept constant, and the rate of lignin removal is determined from the dissolved lignin content in the outflow stream measured by UV absorption. The mass of lignin removed is verified by analyzing the pulp at several time intervals. Experiments were performed at different temperatures, oxygen pressures and caustic concentrations. The delignification rate was found to be first order in HexA-free residual lignin content. The delignification rate reaction order in caustic concentration and oxygen pressure were determined to be 0.42 and 0.44 respectively. The activation energy was found to be 53kJ/mol. The carbohydrate degradation during oxygen delignification can be described by two contributions: one due to radicals produced by phenolic delignification, and a much smaller contribution due to alkaline hydrolysis. From the first order of the reaction and the pKa of the active lignin site, a new oxygen delignification mechanism is proposed. The number 3 carbon atom in the aromatic ring with the attached methoxyl group forms the lignin active site for oxygen adsorption and subsequent electrophic reaction to form a hydroperoxide with a pKa value similar to that of the present delignification kinetics. The uniform presence of the aromatic methoxyl groups in residual lignin further support the first order in lignin kinetics.