CO2 Compressor type 1

[22854]

I'm interested to know opinions on which compressor type (alternative , centrifugal, ecc.) is mostly used or reccomended from different points of view (operational, maintenance, initial cost, ec.) nowadays.-

Service is to compress low pressure (0,5/0,6 bar g) gaseous CO2 coming from MEA or MDEA purification unit up to 160 bar abs. to feed into an urea reactor.

Temp: 40 deg. C
Normal flow: 15500 Kg/hr
Max flow: 21500 Kg/hr

Power: approx. 2800 kW

 

[Montemayor]

The big question on feeding CO2 to the Urea reactor is how do you plan to carrying this out? Presumably, you intend to feed the Low Pressure CO2 from the recovery system to the proposed compressor and on to the Urea reactor. If you check your Thermo data or the CO2 T-S diagram, you will become aware that at 160 bar(a) and from 25 to 40 oC you are going from the liquid phase to the supercritical phase - both of which you don't want to put through any compression stage. My point here is that you passed the critical point at 82.5 bar & 32 oC. Any pressure above 82 bar will cause condensation at 32 oC or lower. It would be hell trying to make sure the CO2 doesn't cool below 32 oC at pressures beyond 82 bar (usually, it takes 3 stages to compress the CO2 to 82 bar).

What you can do is produce the CO2 (after the compressor) as either HP liquid (82+ bar) or LP liquid (usually 14 bar - in 2-stages of compression). This is the key decision because it sets the compression requirements - which can be quite different - and the type of compressor you can use. The classic machine for this has been the reciprocating type for relative smaller flows while the centrifugal was used for larger, steady and constant flows. Today, you can also apply the lubricated screw compressor as a possible choice. However, the process downstream (urea reactor) also affects the possible decision. Oil contamination may not be tolerated due to quality or catalyst problems, so the situation has to be carefully analyzed from the process perspective and trade-offs.

Note that you don't have to compress all the way up to 160 bars. You can liquify at any of the two levels I mentioned above and subsequently feed the Urea reactor with positive, metering pumps fed from intermediate CO2 storage. It all depends on how you want to configure it for maximum economy. Certainly, it is cheaper to pump liquid than to compress gas - and you can absorb a lot of process variance, turndowns, and fluctuations. You haven't given details on how you are handling the NH3 production or how that is split or not.

From what little I know of your process, I would opt for multiple reciprocating compressors, each driven by synchronous electric motors - or steam turbine through a gear drive. I favor these machines in sites other than those where you have excellent infrastructure and technology backup available. They are much simpler to operate, repair, control, and instrument. They can be non-lubed in the compression cylinders as well. Centrifugal machines are great - but they are technically more dependent due to surging characteristics and the high speed bearings. Instrumentation is critical and your turn-down ratio is critical. Today, reciprocating compressors are probably more expensive since so few of them are being built and installed.

Summarizing, you have to be more specific and detailed on your application for a specific recommendation on the type. Certainly, your process must be frozen and pre-determined on the scope. Hope this helps you on the main points.