Читать книгу Agitator Design for Gas-Liquid Fermenters and Bioreactors - Gregory T. Benz - Страница 17

In principle, many tank shapes can be used. That can include cylindrical, rectangular, and spherical tanks. However, odd‐shaped tanks may be hard to baffle and agitate properly. Rectangular tanks may be harder to clean and sterilize if they have sharp corners. Most tanks used in this industry are cylindrical. Most are mounted with their axis vertical. However, the author knows of at least one installation where a multitude of horizontal cylindrical tanks were used. This is decidedly not recommended, for a host of reasons. Just a few worth mentioning: Multiple gear drives per tank are required, increasing agitator cost. Very random hydraulic forces occur, causing more frequent mechanical failure. Low absolute liquid height fails to take advantage of higher oxygen solubility at the bottom of the tank due to liquid head. Harmonic flows in the sparge system can occur.

For the purposes of this book, we will stick to vertical cylindrical tanks. With that restriction, the geometry to be decided is the ratio of liquid height to tank diameter (Z/T), often referred to as aspect ratio. Fermenters have been built with a wide variety of aspect ratios, for various reasons. The most common or popular designs normally have aspect ratios between 2 and 3, but that may be more related to tradition than because it is optimum for a particular set of circumstances.

Chapter 19 discusses aspect ratio in detail, in terms of its effect on capital and power costs, which provides an opportunity for optimization. However, we have to begin somewhere to go through the rest of this flow chart. In the absence of any restrictions on geometry, an aspect ratio between 2 and 3 seems reasonable to start with.

There can be restrictions on aspect ratio due to building constraints. If the vessel must fit within a given floor space, that may place restrictions on the diameter, forcing a certain minimum aspect ratio. Sometimes, local building codes carry height restrictions. So, the allowable aspect ratio range may be bound by such constraints, among others.