Chapter 6 Gas-Liquid Mass Transfer 6. 1 Oxygen Transfer in Metabolism Process of Microorganism Oxygen solubility in a fermentation medium is very low, while its demand for the growth of aerobic microorganisms is high For example, when the oxygen is provided from air, the typical maximum concentration of oxygen in aqueous solution is on the order of 6 to 8 mg /L Oxygen requirement of cells is, although it can vary widely depending on microorganisms, on the order of I g /L h Even though a fermentation medium is fully
Chapter 6 Gas-Liquid Mass Transfer 6.1 Oxygen Transfer in Metabolism Process of Microorganism Oxygen solubility in a fermentation medium is very low, while its demand for the growth of aerobic microorganisms is high. For example,when the oxygen is provided from air,the typical maximum concentration of oxygen in aqueous solution is on the order of 6 to 8 mg /L.Oxygen requirement of cells is,although it can vary widely depending on microorganisms,on the order of 1 g /L h. Even though a fermentation medium is fully
saturated with oxygen, the dissolved oxygen will be consumed in less than one minute by organisms if not provided continuously. Adequate oxygen supply to cells is often critical in aerobic fermentation Even temporary depletion of oxygen can damage cells irreversibly. Therefore, gaseous oxygen must be supplied continuously to meet the requirements for nigh oxygen needs of microorganisms, and the oxygen transfer can be a major limiting step for cell growth and metabolism Mass-Transfer Path: The path of gaseous substrate from a gas bubble to an organelle in a 的;mm1Ai microorganism can be divided into several steps
saturated with oxygen,the dissolved oxygen will be consumed in less than one minute by organisms if not provided continuously. Adequate oxygen supply to cells is often critical in aerobic fermentation. Even temporary depletion of oxygen can damage cells irreversibly.Therefore,gaseous oxygen must be supplied continuously to meet the requirements for high oxygen needs of microorganisms, and the oxygen transfer can be a major limiting step for cell growth and metabolism. Mass-Transfer Path:The path of gaseous substrate from a gas bubble to an organelle in a microorganism can be divided into several steps
(Figure 6.1), as follows )Transfer from bulk gas in a bubble to a ′ relativ rely unmixed gas layer (2)Diffusion through the relatively unmixed gas ayer B)Diffusion through the relatively unmixed liquid layer surrounding the bubble (4 ) Transfer from the relatively unmixed liquid layer to the bulk liquid (5)Transfer from the bulk liquid to the unmixed liquid layer surrounding a microorganism
(Figure 6.1), as follows: (1)Transfer from bulk gas in a bubble to a relatively unmixed gas layer (2)Diffusion through the relatively unmixed gas layer (3) Diffusion through the relatively unmixed liquid layer surrounding the bubble (4)Transfer from the relatively unmixed liquid layer to the bulk liquid (5)Transfer from the bulk liquid to the relatively unmixed liquid layer surrounding a microorganism
(6)Diffusion through the relatively unmixed liquid layer (7)Diffusion from the surface of a microorganism to an organelle in which oxygen is consumed 4 Gas bubble Microorganism ig 6.1 Schematic diagram of the path of a gaseous substrate to an organenelle in a cell
(6)Diffusion through the relatively unmixed liquid layer (7)Diffusion from the surface of a microorganism to an organelle in which oxygen is consumed 4 1 Gas bubble Microorganism Fig.6.1 Schematic diagram of the path of a gaseous substrate to an organenelle in a cell 2 3 5 6
6.2 Basic Mass-Transfer Concepts 6. 2. 1 Molecular Diffusion in Liquids When the concentration of a com ponent varies from one point to another, the component has a tendency to flow in the direction that will reduce the local differences in concentration. Molar flux of a component A relative to the average molal velocity of all constituent JA is proportional tol the concentration gradient dCA/dz as J,=-D dc AB Which is fick's first law written for the z-direction The PaB in Eq. (9.1) is the diffusivity of
6.2 Basic Mass-Transfer Concepts 6.2.1 Molecular Diffusion in Liquids When the concentration of a component varies from one point to another, the component has a tendency to flow in the direction that will reduce the local differences in concentration. Molar flux of a component A relative to the average molal velocity of all constituent JA is proportional to the concentration gradient dCA/dz as Which is Fick’s first law written for the z-direction. The DAB in Eq. (9.1) is the diffusivity of Z A A AB d dC J = −D (6.1)