When we model any piece of process equipment, some of the data we supply to the computer to create the model are derived from the observed performance of existing equipment in similar service. For instance:
• Heat exchangers—Fouling factors
• Compressors—Adiabatic compression efficiency
• Centrifugal pumps—Clearance between impeller and wear ring
• Distillation towers—Tray efficiency and relative volatility
The relative volatility is a measure of the ease of separation between the light and heavy components. For ideal components it is the ratio of the vapor pressure of the more volatile component divided by the vapor pressure of the less volatile component. For nonideal components, the relative volatility is calculated from the "equation of state."
The equation of state is a set of empirically derived equations. When you set up a computer model for any process involving vaporliquid equilibrium, you must select an equation of state from perhaps a dozen choices. Experience is the only real guide in making the most
accurate selection.
When you set up a computer model for any distillation process equipment, you must select a tray efficiency. Tray efficiencies (for mechanically intact trays) vary from 30 to 90 percent. Experience is the only guide in making the least wrong selection.
42.1.3 Establishing a Firm Design Basis
The design engineer for the new propylene splitter had used the following values in his model:
• Relative volatility = 1.18
• Tray efficiency = 65 percent
I had used in my computer model:
• Relative volatility = 1.21
• Tray efficiency = 75 percent
I had arbitrarily manipulated tray efficiency and relative volatility to force my computer model to match the observed plant data. It might seem that by arbitrarily selecting both the relative volatility and tray efficiency for my computer model, my calculations would be little better than a guess.
• Heat exchangers—Fouling factors
• Compressors—Adiabatic compression efficiency
• Centrifugal pumps—Clearance between impeller and wear ring
• Distillation towers—Tray efficiency and relative volatility
The relative volatility is a measure of the ease of separation between the light and heavy components. For ideal components it is the ratio of the vapor pressure of the more volatile component divided by the vapor pressure of the less volatile component. For nonideal components, the relative volatility is calculated from the "equation of state."
The equation of state is a set of empirically derived equations. When you set up a computer model for any process involving vaporliquid equilibrium, you must select an equation of state from perhaps a dozen choices. Experience is the only real guide in making the most
accurate selection.
When you set up a computer model for any distillation process equipment, you must select a tray efficiency. Tray efficiencies (for mechanically intact trays) vary from 30 to 90 percent. Experience is the only guide in making the least wrong selection.
42.1.3 Establishing a Firm Design Basis
The design engineer for the new propylene splitter had used the following values in his model:
• Relative volatility = 1.18
• Tray efficiency = 65 percent
I had used in my computer model:
• Relative volatility = 1.21
• Tray efficiency = 75 percent
I had arbitrarily manipulated tray efficiency and relative volatility to force my computer model to match the observed plant data. It might seem that by arbitrarily selecting both the relative volatility and tray efficiency for my computer model, my calculations would be little better than a guess.
