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New physical model for the control of distillation columns in air separation units

IP.com Disclosure Number: IPCOM000197372D
Publication Date: 2010-Jul-05
Document File: 50 page(s) / 3M

Publishing Venue

The IP.com Prior Art Database

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New physical model for the control of distillation columns in air separation units

Cryogenic distillation columns are operated at extremely low temperature to separate air into nitrogen, oxygen and argon in Air Separation Units (ASU). The major operating cost of air distillation is electricity. AL ASU electricity consumption represents 600 M€ each year. Therefore, small improvements in process control would lead to significant savings.

Current control technology is based on linear dynamic models and linear model predictive control. Until now this linear control technology had given satisfaction over a small range of production rates. Namely, it efficiently stabilizes steady-state. However, deregulation of the electric utility will lead to frequent changes in the cost of electricity. The production of air separation units will change as well and steady-state will not represent the most common range of an ASU. With these new operating conditions come new control needs. It should be noticed that the main part to control in an ASU is the distillation column. This invention proposes a new physical model for the control of distillation columns in ASU. This model is more accurate than most of the other existing ones and it can be used to implement nonlinear purity control.

The state-of-the-art of distillation column model is presented here. Many papers about distillation dynamic modeling have been published since 2001. As far as we know, nothing concerning global ASU optimization has been published yet.

It should be noticed that:

-      At the end of every model presentation, a conclusion recalls the hypotheses and the specificities.

-      In the remaining, vertical axis is oriented up to down.

1.1. Stage-by-stage model

Symbols used in the following.

 

Theoretical tray height

 

 

Tray efficiency

-

 

Liquid component molar fraction

-

 

Vapor component molar fraction

-

 

Liquid molar flow, Nm3/h

 

 

Vapor molar flow, Nm3/h

 

 

Flow rates velocity

 

 

Liquid molar hold-up

 

 

Vapor molar hold-up

 

 

Relative volatility

-

O2

As index, relative to oxygen

-

AR

As index, relative to argon

-

N2

As index, relative to nitrogen

-

i

As index, relative to stage i

-

 

Pressure

 

 

Temperature

 

 

Saturation pressure

 

 

 

 

 

 

 

Distillation columns used to be composed of a superposition of trays. The liquid was dropping from tray to tray and the vapor was going up the column through little holes in the tray, which insured the liquid/vapor contact necessary to the distillation process. Columns are now made of packings. Liquid and vapor are in contact all along the column and the distillation is more efficient.

 

Figure 1 Packing

 

However, distillation simulation models have kept the shape of a tray column with 2 empiric parameters: theoretical tray height  and tray efficiency . For example if , then a 6 meters high column is made of 20 theoretical stages.

AL

engineering group currently uses Aspen Custom Modeler to simulate an entire ASU dynamic behavior. The distillation part of the model relies on a stage-by-stage model. The...