Wilcox home ×
ChE design home
× Profession ×
General ×
Properties ×
Equipment ×
Separation ×
HYSYS ×
Costs ×
Safety ×
Case studies
× Excel ×
MATLAB
Oxygen costs
The following is extracted from emails by a Clarkson alum at Praxair (www.praxair.com) on 10/05/2004 and 11/29/04 in response to a query by W.R. Wilcox regarding the cost of an oxygen feed to an ethylene oxide plant and to a coal gasification plant.
The usual commercial arrangement in the industrial gas business is to
supply product over the fence. That is, Praxair will own and operate the
cryogenic oxygen plant and insure high reliability by providing liquid back-up
reserves from a nationwide system of plants. The customer pays a fixed monthly
facility fee. As you can imagine there are a number of variables in the
cost of oxygen including the volume required, purity, pressure, power cost and
co-product credits for nitrogen, argon and rare gases. To keep it simple,
assume 1000 short tons per day of contained oxygen at 97% purity. For
this exercise use a facility fee of 2.75% of the capital (see below) per month,
which includes a capital charge, all maintenance and repair costs, back-up
systems and a back-up supply of liquid. Power costs, which are
proportional to production rate, are passed through to the customer. A
typical power cost in the US is $0.04/kw-hr for an industrial gas company.
On-stream factors for cryoplants are typically above 99%, which is far
more reliable than a gasifier!
For the gasifier application, use $27MM in capital for 1000 TPD of contained
oxygen at pressure. Power consumption will be about 16 kw-h/1000cf-NTP
(70oF and 14.696psia). For the EO application use $26MM in
capital for 1000 TPD of O2. Power will be about 15
kw-h/1000cf-NTP. This capital includes tankage for liquid back-up.
Typical factors can be used to scale capital with capacity. Unit
power won't change much with scale.
The students may want to think about the effect of oxygen purity on EO process
economics. Higher purity oxygen costs more up front but getting rid of
the inerts downstream may impose a larger economic penalty. There are
less costly oxygen options if the requirement is for less than 500 TPD of oxygen
and purity can be relaxed to 90%.
Gaseous oxygen is typically supplied around ambient temperature as it
is used to cool feed air. For the few
applications that use hot oxygen, the oxygen is heated as part of the
application. If cold oxygen or nitrogen (as for food freezing) were
required, the product would be taken from the plant as cryogenic liquid around
70K. Nitrogen is typically supplied at low pressure. A nitrogen
compressor is added if necessary. Gaseous oxygen can be supplied at any
pressure but costs escalate rapidly above 500 psia due to safety concerns.
Safety concerns are somewhat mitigated when oxygen purity is less than
98% or so, but it's still dangerous!
Following is the
recommended procedure:
1. Calculate how much oxygen feed you need and convert to short tons[1] per day (TPD) and to cubic feet per year at NTP[2] (CFY).
2. Calculate the capital required, CR = $26,500,000 x (TPD/1000)0.6 updated from October 2004 to today using CEPCI. (Note that CR is paid by the oxygen supplier and is not to be included in the estimated cost of the chemical plant.)
3. Calculate the annual facility fee, AFF = 12 x 0.0275 x CR.
4. Determine the cost of electricity (CE) in $/kWh[3], e.g. through the U.S. Government survey.
5. Calculate the annual cost of electricity, ACE = 15.5 kWh x (CFY/1000) x CE.
6. The annual cost of the oxygen is AFF + ACE. Add this to your cost of manufacturing.
7. Note that the oxygen is only 97% pure, so that you will have to deal with the remaining impurities, probably nitrogen. Assume that the supply pressure can be as high as 50 bar and the temperature as low as desired.
Please contact Professor
Wilcox with comments, corrections and suggestions. Last changed on 19 December 2005.
Wilcox home ×
ChE design home
× Profession ×
General ×
Properties ×
Equipment ×
Separation ×
HYSYS ×
Costs ×
Safety ×
Case studies
× Excel ×
MATLAB