Cryogenic Air Separation Process and Apparatus

Abstract

A cryogenic process for the production of oxygen by cryogenic distillation using an air separation unit comprising a double column, said double column comprising a high pressure column and a low pressure column comprising the steps of: sending compressed, cooled and purified air to the high pressure column in gaseous form, sending oxygen enriched fluid from the bottom of the high pressure column to the low pressure column, removing nitrogen enriched gas from the top of the high pressure column, sending a first portion of nitrogen enriched gas to a lower reboiler in the low pressure column following compression in a cold compressor having a cryogenic inlet temperature, sending a second portion of nitrogen enriched gas to an upper reboiler in the low pressure column, sending nitrogen enriched liquid from at least one of the first and second reboilers to at least one of the high pressure column and the low pressure column, expanding a stream of nitrogen enriched gas from the high pressure column in an expander following a warming step, driving the cold compressor using a motor and producing oxygen rich gas from the low pressure column by at least one of the steps of removing a gaseous stream and by vaporizing a liquid stream.

Description

BACKGROUND OF THE INVENTION[0001]The present invention relates to a cryogenic air separation process and apparatus.[0002]All pressures listed in the document are absolute pressures.[0003]Because of the global warming effect caused by the increased release of CO2 (carbon dioxide) generated by combustion processes, efforts have been made by utility companies and governments worldwide to reduce and minimize the CO2 emission. One major source of CO2 emission is the power generation plant's combustion process. There are mainly two types of power plants based on combustion processes: coal combustion and natural gas combustion. Both of these processes produce CO2 when generating power. The most efficient approach to reduce or minimize the CO2 emission is to capture most of the CO2 emitted by the power plants. For this effort to be efficient, it must also target the existing coal combustion plants that represent a large portion of the power generation plants worldwide. The oxy-combustion te...

AI Technical Summary

Benefits of technology

[0038]iv) increasing the flowrate of the first air stream, feeding the air separation unit to a value greater than the first flowrate.

Problems solved by technology

The diluted composition of CO2 increases the size and the power consumption of the CO2 recovery unit.

Because of this dilution, it becomes very costly and difficult to recover and capture the CO2 especially with the low pressure of the flue gas.

However, significant amount of heat is needed to regenerate the amine and to extract the CO2 such that the amine process is not cost effective.

For retrofitted oxycombustion coal plants, it is clear that the effort to capture CO2 is hindered by the cost of the oxygen plant.

Furthermore, the power consumption of the oxygen plant, which can be about 10% of the power plant output, also introduces additional cost issues: part of the power generated by the power plant must be diverted to supply the oxygen plant.

Therefore less power will be available to supply the grid, especially during peak demand when power is scarce and power costs are premium, resulting in reduction of power plant's revenue.

In this situation, the economics of CO2 capture and disposal by oxycombustion technique depend strongly on the cost and power consumption of the oxygen plant.

Without an efficient setup for the oxygen generation, the cost penalty would be such that it would become uneconomical to operate the clean and CO2-free oxycombustion power plants.

Since the CO of the fuel gas is combusted to yield CO2 in the burner of the gas turbine, the resulting CO2 is also mixed with the nitrogen of the gas turbine feed air such that the CO2 recovery is also a costly and difficult task.

The production of oxygen requires additional capital investment and consumes significant power to drive the compression equipment.

In addition to the oxygen plant, the CO2 recovery from flue gas of an oxycombustion or from the fuel gas of an IGCC also consumes power and requires significant investment since the CO2 must be further concentrated to about 95 mol % and then compressed to about 100 bar or higher for disposal.

It is well known that power demand varies during the day, there are “peak” periods of high demand, hence high power cost, and there are “off-peak” periods of low demand, low power cost.

There are also some intermediate demands and costs.

Because of the variable demand, power plants usually run at or near its design capacity during peaks, but must idle at very low output during off-peaks.

Power cost is high during peaks and sometimes the utility companies must purchase additional power from other suppliers to satisfy demand.

The addition of an oxygen plant to the power generation plant worsens the power cost structure especially for peak periods.

This additional power consumption is quite costly because it deprives the utility companies from having the available kW to sell on the grid at the premium value.

During off-peaks, the power output is at the minimum level; the consumption of the oxygen plant is also at its lowest level and cannot take advantage of the lower power cost.

This high power consumption creates a power demand and keeps the power generating equipment running above it minimum rate, thus potentially avoid costly equipment shutdown.

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