1 May 2005

Real time chromatograph control

By Al Kania

Opportunities exist for process gas chromatographs to better cracker's performance.

In a refinery, one of the most important processes for converting heavy oils into more valuable gasoline and light products is the fluid catalytic cracking (FCC) unit.

More than 50% of a refinery's heavy petroleum goes through the FCC unit for processing and the unit's optimum operation is essential.

Use of a process gas chromatograph in the FCC unit provides such benefits as regulating the temperature in the regenerator, minimizing the loss of naphtha and gasoline components, and monitoring the C4 and C5 olefins in the reactor.

Degrees of oil separation

Heavy gas oils from the crude unit and vacuum gas oils from the vacuum crude unit feed the fluid catalytic cracking unit in a refinery. The feed can also come from other units that generate heavy petroleum streams such as the coker or deasphalter.

The feed stream heats to more than 600º F and then mixes with freshly regenerated catalyst before it enters the main FCC reactor. The chemical reactions begin immediately upon contact with the oil in the pipe leading to the reactor.

During the cracking process of the oil's large molecules into smaller molecules, carbon forms on the surface of the catalyst and quickly deactivates the catalyst.

Once inside the reactor, injected steam strips off any oil clinging to the catalyst, causing the catalyst in the reactor to move in a manner similar to a fluid-hence the name fluid catalytic cracking. The cracked oil vapors flow out the top of the reactor with the spent catalyst flowing out the bottom.

Fluid catalytic cracking analysis

As the catalyst leaves the bottom of the reactor, it mixes with air and enters the regenerator vessel where the carbon on the surface of the catalyst burns off. The regenerated catalyst leaves the bottom of the regenerator to mix with the feed, completing the catalyst flow path. Exiting the top of the regenerator is a very hot flue gas stream that heats other boilers in the plant.

While the catalyst regenerates, the hot petroleum vapors leaving the top of the reactor enter the main fractionator. This fractionator acts like a miniature crude tower by separating the "cracked" petroleum stream into various petroleum cuts such as gasoline, gas oils, and others. The heaviest fraction leaving the bottom of the main fractionator is typically recycled back to the feed stream for reprocessing.

Exiting the overhead of the main fractionator is a gas vapor stream rich in olefin compounds. This stream often travels to the vapor recovery unit, which recovers the olefins for use in other processes like the alkylation unit.

Process gas chromatograph applications in a fluid catalytic cracking unit.

Gas chromatograph purposes

Several opportunities exist for process gas chromatographs to improve the fluid catalytic cracking unit's performance. The first process gas chromatograph (AX 1) monitors the CO to CO2 ratio in the flue gas leaving the top of the regenerator. This ratio is critical to regulating the temperature in the regenerator since high temperatures damage the catalyst.

The second gas chromatograph (AX 2) monitors the overhead vapors of the main fractionator. This gas chromatograph typically serves for two purposes. The first is to minimize the loss of naphtha/gasoline components in the overhead stream by keeping the C5 concentration low. The second is to monitor the C4 and C5 olefins generated in the reactor.

These olefins are important feed components to other processes in the refinery such as the alkylation unit.

Behind the byline

Al Kania (Al.Kania@EmersonProcess.com) has a chemical engineering degree and is a published author on the use of online analyzers. His 20-plus years of experience with process analyzers serve him as a manager at Rosemount Analytical.

The online chromatograph is a different animal

By Raymond Annino

Analysis must be faster than the system time to achieve control action.

Gas chromatography (GC) is a method for separating the components of a sample that contains a mixture of volatile compounds.

The separations determine the quantity of each of the sample components of interest. It has become one of the most often used procedures in analytical chemistry for separation and analysis.

The reasons for its popularity are its ease of use for the separation of complex mixtures, its high sensitivity, and the small sample required for analysis.

Separations occur because the sample components have different solubilities in the liquid stationary phase or different adsorbtivities on a solid stationary phase. One of the two phases exist in and operate as part of the GC.

Each sample component is retarded a different amount by the stationary phase and is carried down the column by the mobile phase at a different rate. By choosing a stationary phase that maximizes the solubility or adsorbtivity differences of the sample components, each component will emerge from the column at a different time.

A detector that responds to some property difference between the carrier gas and the sample components is at the end of the column. It yields a signal that, when recorded as a function of time, produces a chromatogram.

Chromatogram showing fully and partially resolved peaks.

This time to emerge is the retention time. The magnitude of the detector signal (peak height or the area under the peak) relates to the amount of the compound present in the sample.

The instrumentation necessary to accomplish chromatography consists of a supply of carrier gas (commonly helium, hydrogen, or nitrogen), a sample introduction device to inject a fixed volume of sample into the carrier gas stream, a column containing the stationary phase, and a detector.

Basic elements of a chromatograph.

Although they employ the same principles, the process gas chromatograph (PGC) is quite different than its laboratory counterpart. These differences appear as the result of a number of factors, not the least of which includes:

The need of continuous, reliable operation of the unit
The need for the cycle time of the analysis to be shorter than the time required for the process control system to achieve proper control action
The nature of the environment in which the PGC is placed
The diverse user attitudes regarding maintenance and their capability regarding the operation of the unit
The necessity to interface with state of the art computer controlled systems

So, there's a premium on simple, reliable design to ensure a low mean failure rate, but straightforward, rapid repair when required, and the strategic placement of appropriate sensors to aid in diagnosing problems and to provide alarm with regard to the operation of the unit.

Basic elements of a multistream process gas chromatograph system.

The PGC has these attributes:

Located in the plant as close as possible to the sample point to minimize transport time
Dedicated to monitor one or more components in one or more process streams
Designed for continuous, unattended operation
Designed for operation in hazardous environments
Designed to withstand exposure to weather, humidity, dust, and corrosive atmospheres
Contains integral hardware and software to allow communication with the process control system as well as remote maintenance stations
Contains alarms and diagnostic aids to continuously monitor the health of the instrument and aid in diagnosing problem

A process gas chromatographic system consists of both the PGC and a sample handling system.

Behind the byline

Raymond Annino is a retired professor and researcher. He formerly worked for The Foxboro Company. This piece came from his contribution to Instrument Engineers' Handbook, Fourth Edition, CRC Press and ISA.