3. INDUSTRIAL APPLICATION OF NUOVO PIGNONE CENTRIFUGAL COMPRESSORS

Centrifugal compressors have a large number of applications in numerous sectors of industry (see figure. 3.1) and particularly in many processes that call for very wide performance ranges.

 Table 3.1 - Typical centrifugal compressor applications

The last thirty years in particular have seen continuous expansion in the field of application which has grown to encompass some services traditionally covered by other types of compressors. This is also due to a big rise in individual plant capacities as well as to introduction of new processes.

In fact, centrifugal compressors were initially used mainly as atmospheric air blowers for blast furnaces, mines, etc. They had flow rates that varied greatly from case to case but always low pressure ratios. Later, as they began to become more common in chemical plants these machines had to change considerably. The gases compressed changed from atmospheric air to different gases and then to gas and/or vapor mixtures, all with different characteristics from air. Also the compression flows and pressure ratios increased remarkably to meet process requirements.

Some of the main processes requiring centrifugal compressors are outlined herebelow along with the principal features of the Nuovo Pignone machines used for them.

3 .1 REFINERIES

Figure. 3.2 shows a schematic diagram of a refinery with the main process lines.

The heart of a refinery is a fractionating column which separates the various crude components that may be used as they are Of further treated as illustrated herebelow:

3.1.1 Reforming

In reforming plants hydrocarbon molecular structures are changed from an open-chain to a cyclic structure which translates into a inproved octane rating for reformed substances. This reaction which involves evolution of hydrogen takes place in a reactor with a catalyst which is almost always platinum, hence the term platforming generally used to indicate this process.

The compressor's task (see figure. 3.3) is to recycle the gas mixture in the reactor with a high percentage of hydrogen which minimizes side reactions that tend to

form carbon deposits on the catalyst eliminating its porosity and consequently deactivating it. The extra pressure the compressor gives the gas is therefore used essentially to make up for the pressure losses in the system ensuring a constant gas flow.

As a rule pressure ratios are not very high and operating pressure vary from 20 to 40 bars. It is interesting to note that the molecular weight of compressed gas may vary in time in relation to the condition of the reactor. This is because the catalyst tends to become more and more heavily coated with carbon deposits and therefore the compressor has to recycle gas which has more and more hydrogen in it.

Beyond a certain limit it is necessary for the economy of operation to regenerate the catalyst.

3 .1. 2 Cracking

The heavy distillates produced by the fractionating column can be treated by a cracking process in order to break the heavy molecules into lighter ones to get premium grade fuel.

There are several types of cracking processes, one of which is Fluid Catalytic Cracking (FCC) which requires also centrifugal compressors. This process gets its name from the fact that the reactor has a fluid catalytic bed. This feature offers several advantages such as an even distribution of temperature, a very large area of contact and good heat transmission.

As regards the machines air compressors are required with delivery pressures which may vary from 2 to 4 bars depending on the process. Also cracking gas compressors with a relatively high molecular weight (30-40) and delivery pressures around 15 to 17 bars are necessary.

Axial compressors are sometimes used to compress the air as low pressure ratios and relatively high flows are required.

3.1 .3 Lubricant Broduction

In these plants which utilize the heavy distillates produced by the fractionating column centrifugal compressors are chiefly used in the oil dewaxing process. Since paraffin wax crystallization takes place at low temperatures and separation at low pressures, it follows that two different types of compressors are necessary:

• refrigerating compressors, which generally handle propane with design temperatures around -20°C;
• compressors that maintain a high vacuum level (suction pressure is usually 2 bars) inside the large rotary filters where the crystallized paraffin wax is deposited.

Both machines have low operating pressures and therefore they normally have horizontally split casings (MCL compressors).

3.1.4 Bitumen and asphalt production

Bitumen is obtained by oxidating the heavy distiilate produced by the fractionating column in a tower with air flowing through it.

A low pressure air compressor is therefore required, generally on-line without intercooling

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