FCC Catalyst Evaluation

FCC Catalyst Evaluation

1.0Introduction

Catalyst management is a very important aspect of the FCC process. Selection and management of the catalyst, as well as how the unit is operated, are largely responsible for achieving the desired products. Proper choice of a catalyst will go along way toward achieving a successful cat cracker operation.

Catalyst change-out is a relatively simple process and allows a refiner to select the catalyst that maximizes the profit margin. Although catalyst change-out is physically simple, it requires a lot of homework.

As many catalyst formulations are available, catalyst evaluation should be an ongoing process; however, it is not an easy task to evaluate the performance of an FCC catalyst in a commercial unit because of continual changes in feedstocks and operating conditions in addition to inaccuracies in measurements. Because of these limitations, refiners sometimes switch catalysts without identifying the objectives and limitations of their cat crackers. To ensure that a proper catalyst is selected, each refiner should establish a methodology that allows identification of ‘real’ objectives and constraints and ensures that the choice of the catalyst is based on well-thought-out technical and business merits.

In today’s market, there are over 120 different formulations of FCC catalysts. Refiners should evaluate catalysts mianly to maximize profit opportunity and to minimize risk. The right catalyst for one refiner may not necessarily be right for another.

2.0Catalyst Selection Methodology

One of the most important parameters that specify the competitiveness of a refinery FCC unit is the proper selection of catalyst, since the catalyst type determines both quantity and quality of the catalytic cracking products. Laboratory of Environmental Fuels and Hydrocarbons evaluates FCC catalysts, through MAT tests, specifying each catalyst activity and selectivity.

For the above purpose a Short Contact Time Microactivity Test unit (SCT-MAT) was constructed in CPERI, at the beginning of 1999, in order to replace the conventional MAT unit, as an attempt to follow the worldwide inclination of short residence times during the FCC reaction. The unit's excellent performance along with the compatible results derived by comparing it with the FCC pilot plant soon lead to the construction of an identical unit (January, 2001).

Catalysts are evaluated following a standard FCC evaluation protocol. Initially the catalysts are deactivated; either by metal deposition or by steaming sieved and finally tested in one of CPERI's MAT units. At least eight different tests are carried out for a specific catalyst and for each test detailed experimental and normalised mass balances are quoted. The individual product yields are plotted vs. conversion and catalysts evaluation is completed by comparing their product yields at a constant conversion level (65%wt).

The microactivity test (MAT) unit was originally designed to determine the activity and selectivity of either equilibrium or laboratory deactivated fluid catalytic cracking (FCC) catalysts. Currently, the MAT unit is accepted as a tool to perform general laboratory scale FCC research and testing because of its simple operation and cost effectiveness. The unit only requires small quantities of catalyst and gas oil for each MAT test, compared with barrels of materials needed for a pilot-scale riser run.

A comprehensive catalyst selection methodology will have the following elements:

1. Optimize unit operation with current catalyst and vendor.

a. Conduct test run.

b. Incorporate the test run results into an FCC kinetic model.

c. Identify opportunities for operational improvements.

d. Identify unit’s constraints.

e. Optimize incumbent catalyst with vendor.

2. Issue technical inquiry to catalyst vendors.

a. Provide Test run results.

b. Provide E-cat sample.

c. Provide Processing objectives.

d. Provide Unit Limitations.

3. Obtain vendor responses.

a. Obtain catalyst recommendation.

b. Obtain alternate recommendation.

c. Obtain comparative yield projections.

4. Obtain current product price projections.

a. For present and future four quarters.

5. Perform economic evaluations for vendor yields.

a. Select catalyst for MAT evaluations.

6. Conduct MAT of selected list.

a. Perform physical and chemical analyses.

b. Determine steam deactivation conditions.

c. Deactivate incumbent fresh catalysts to match incumbent

E-cat

d. Use same deactivation steps for each candidate catalyst.

7. Perform economic analysis of alternatives.

a. Estimate commercial yield from MAT evaluations.

8. Request commercial proposals.

a. Consult at least two vendors.

b. Obtain references.

c. Check references.

9. Test the selected catalyst in a pilot plant.

a. Calibrate the pilot plant steaming conditions using

incumbentE-cat.

1. Deactivate the incumbent and other candidate catalysts.
2. Collect at least two or three data points on each by varying catalyst-to-oil ratio.

1. Evaluate pilot plant results.
2. Translate the pilot data.
3. Use the kinetic model to heat-balance the data.
4. Identify limitations and constraints.

1. Make the catalyst selection.
2. Perform economic evaluation.
3. Consider intangibles-research, quality control, price, steady supply, manufacturing location.
4. Make the recommendations.

1. Post selection.
2. Monitoring transitions-% changeover.
3. Post transition test run.
4. Confirm computer model.

1. Issue the final report.
2. Analyze benefits.
3. Evaluate selection methodology.

3.0Reactors Used for FCC Studies

Catalytic cracking catalyst development requires the adequate evaluation of catalyst performance. Different kinds of laboratory reactors are available to evaluate catalyst performance. These reactors include fixed bed, fluidized bed, stirred batch, differential, recycle, and pulse reactors (Weekman, 1974: Sunderland, 1976).

The testing of catalyst at the laboratory scale can serve many purposes. One possibility is the need of improving catalyst formulation or altogether to develop a new catalyst (Mooreheed et al., 1993). However, a common task for a bench scale unit is to compare the relative performance of two or more catalysts (Mooreheed et al., 1993).

Regarding the specific approach used for FCC catalysts, very frequently catalyst evaluations are done on the basis of a microactivity test (MAT). MAT studies are hindered by mismatching of industrial operating conditions. Thus, MAT studies with long catalyst time-on-stream, low hydrocarbon partial pressures, and cumulative coke content do not represent industrial operation.

It is our view that to represent, in a laboratory scale unit, the reaction environment of a commercial riser, the operation of this unit has to be carefully controlled. The present dissertation considers in this respect, a novel CREC Riser Simulator invented by de Lasa (1992) at the University of Western Ontario.

3.1-Microactivity Test (MAT)

The Micro Activity Test (MAT) has been a main tool for basic FCC research, and this includes catalyst selection and feedstock evaluation (O’Connor and Hartkamp, 1988; Campagna et al., 1986). This test was developed due to its simplicity, reproducibility, and quickness of evaluation in comparison to tests in a continuous pilot plant.

The MAT technique is an ASTM procedure (ASTM D-3907-88) which was developed on the basis of using a fixed bed of 4 grams of catalyst, operated with a continuous oil vapour feed for 75 seconds at a temperature range of 480-550C and using an average catalyst/oil ratio of about 3. The standard MAT has had limited success predicting commercial unit performance and has provided limiting information about product selectivity (Mauleon and Courcelle, 1985; O’Connor and Hartkamp, 1988; Mooreheed et al, 1993). There are important warnings in the technical literature about the value of the data obtained in the MAT for catalyst selection. Some authors claim, without fundamentally based arguments, that the MAT could provide some kind of relative comparison on catalyst activity and coke make selectivity (Humphries and Wilcox, 1990).

Although the MAT unit can provide some data for catalyst screening, several important differences exist between MAT and the commercial FCC unit (Mooreheed et al, 1993) as follows;

a-) The MAT reactor is based on a cylindrical (ASTM design) catalyst fixed bed with a flow of feedstock flowing through a bed of catalyst. A commercial riser uses instead an upflow of oil and catalyst circulating together (Mooreheed et al, 1993).

b-) The MAT uses a cumulative catalyst time on stream of 75 second while a commercial riser uses a short contact time of 3-5 second.

c-) The MAT employs a reactant partial pressure much lower than the one of the commercial riser: 0.05 atm for MAT and 1.5 atm for the commercial riser.

d-) Coke profiles develop in the 150 mm long catalyst bed of the MAT and the catalyst deactivates at different rates. On the other hand, in the riser all catalyst particles experience the same feed exposure having at the riser outlet uniform coke concentration.

e-) Theoperation of the MAT provides average results over a 75 second period. These results are by nature different than those taken after 3-5 seconds contact time in the riser. For instance, this difference explains the low olefinicity of the MAT products (Mooreheed et al, 1993).

f-) The MAT cannot provide information about catalyst attrition since it is a fixed bed unit.

As a result of the above described inadequacies, some modifications have been suggested to the MAT to provide a more reliable method for catalyst testing (O’Connor and Hartkamp, 1988; McElhiney, 1988, Mott, 1987; Tasi et al., 1989). However, and despite the proposed modifications the MAT still allows coke profiles and temperature differences. Consequently, the kinetic modeling of catalytic cracking reactions using the standard MAT test is rather unreliable, and a number of strong approximations are needed (Froissier and Bernard, 1989).

Corma et al., (1994) highlighted the limitations and the inadequacies of MAT unit to compare different FCC catalysts made from different materials. These authors pointed out that when two different FCC catalysts, one made from ultrastable Y-zeolite and the other was made of SAPO-37, which had a faujasite structure with different framework composition, were used in the MAT, the tests performed were not reliable. It was recommended, by these authors,to use different tools with short contact times and based on mini-fluidized beds.

3.2- Pilot plant unit.

A successful scale up procedure is essential for further advancement of any chemical technology. Usually, if the tested catalyst passes the bench scale reactor test (like the MAT), the following level of demonstration is the pilot plant unit. In this respect, it is extremely important to bridge the differences between the lab-scale and commercial FCC units. According to Carter and McElhiney (1989), circulating riser pilot plants can provide the best small-scale simulation of commercial FCC yields.

Several pilot plants are available for the FCC process, with the favored ones being those with a riser reactor and continuous catalyst regeneration (Yang and Weatherbee, 1989). Davison Circulating Riser (DCR) unit is one of the most effective FCC pilot plants. It includes an adiabatic riser reactor where the reactor temperature is maintained by controlling the circulating rate of the hot regenerated catalyst. This process is identical to the commercial unit. This unit can work in the isothermal mode for certain kinetic studies. It is reported that this unit can be used to process heavy oils and it can be also used for catalyst studies. The DCR unit is 12 feet in height and it has a catalyst and vapor residence time of about 6 and 3 sec respectively (Yang and Weatherbee, 1989).

While, these pilot plant units provide, in principle, good simulation for commercial FCC units, they are expensive, difficult and costly to operate, and they are not suited to test large number of catalyst samples. Furthermore, there is an intrinsic difficulty to operate these pilot plants isothermally, showing some limitations in catalyst/oil ratios and contact times (Corella et al., 1986).

3.3- CREC Riser Simulator

As stated, one of the most important challenges for FCC catalyst development has been the one of simulating catalyst performance under commercial conditions and in this respect, a laboratory scale unit is needed (Book and Zhao, 1997).

The Riser Simulator is a novel unit invented by de Lasa (1987) to overcome the technical difficulties of MAT units. This unit can be used for several purposes:a) to testindustrial catalysts at commercial conditions (Kraemer, 1990), b) to carry out kinetic and modeling studies for certain reactions, c) to develop adsorption studies (Pruski, 1996). d) to use the data of this unit for assessing the enthalpy of cracking reactions.

The different characteristics and advantages of the CREC Riser Simulator can be summarized as follows:

a-) Temperature, reaction time, cat/oil can be varied in a wide range,

b-) Different feedstocks (VGO, gas oil, and model compounds) can be tested,

c-) Different chemical reactions such as alkylation, hydrogen transfer, transalkylation, and coke formation can be investigated,

d-) Catalyst regeneration is simple and can be conducted at typical regeneration conditions,

e-) For testing a catalyst, only a small catalyst sample (0.8 g) can be used throughout many runs at different temperatures, contact times, and cat/oil ratios,

f-) For testing a feedstock, only a small amount of feed (0.16 g) is needed,

g-) The Riser Simulator can be operated in a broad range of total pressures,

h-) The Riser Simulator can be used in the fluidized bed mode with active mixing of catalyst particles. In this respect, perfect mixing with the absence of coke profiles and gas channeling can be obtained with all catalyst particles being exposed to the same reaction environment.

In conclusion, and in order to obtain reliable cracking results, the appropriate tools have to be used in conducting reaction runs. For example, it is well known that to measure catalyst activity and selectivity of FCC catalysts a number of conditions have to be met: a) a short contact time, b) fluidized bed conditions, c) appropriate temperatures, d) adequate hydrocarbon partial pressure, e) representative cat/oil ratio. The CREC Riser Simulator, experimental tool employed in the present study, allows to study FCC catalyst performance under relavant conditions used in commercial units and this secure the value.

1.steaming

The evaluation of fresh catalysts normally includes a deactivation step that precedes the actual activity test. This deactivation typically involves the steaming of a catalyst sample at temperatures ranging from 550 to 930 oC for 2 to 24 hr. The primary objective is to deactivate a fresh catalyst such that its performance in the activity test is representative of what is observed when testing a commercially deactivated sample of the same catalyst. In this way, prediction of commercial performance for new catalysts can be made. In addition, the steaming was used in this study to vary the unit cell size.

Figure 3.1.Particles of FCC catalyst.

Laboratory steaming of fresh FCC catalysts is generally done in the presence of 100 percent steam in fluidizing nitrogen while temperature is increased to the desired target. Steam, obtained by vaporization of injected water, was introduced and the nitrogen flow was stopped. After a specified period of time (6 hr), the water injection was stopped, the nitrogen was introduced again and the temperature was set back to an ambient level. Then the catalyst was unloaded and screened to remove fines, if necessary. The steaming temperature was varied in order to change the unit cell size and hence, a large range of unit cell sizes was obtained.

For all runs, the catalyst was steamed at constant temperature for 6 hr. For example, a part of fresh catalyst B was steamed at 810 oC for 6 hr, while other parts of fresh catalyst B were steamed for 6 hr at 760 and 710oC, respectively.

4.0catalyst evaluation

The catalytic experiments were carried out in a microactivity test (MAT) unit which is basically a fixed bed reactor, which has been designed according to ASTM D-3907 method. The following section describes the experimental setup, and the experimental procedures.

4.1Experimental Apparatus

A schematic diagram of the MAT unit used in this study is shown in Fig. 4.1. The main parts of the unit are:

•Syringe (used for feed addition)
•Syringe heater
•Syringe pump
•Furnace
•Glass reactor
•Liquid product collection system
•Gas product collection system
•Analytical balance and weights
•Chromatographic equipment
•Carbon analyzer

The syringe was 2.5 ml and used for VGO addition. The syringe should be equipped with a multiport, high-pressure valve to allow nitrogen and VGO entry to the reactor through a common feed line.

The syringe heater was used to heat the syringe to 40±5 oC using a heat lamp. The syringe pump has to be able to deliver uniform flow of 1±0.03 g of VGO in 30 sec.

Figure 4.1:Schematic for the MAT unit.

A three-zone furnace was used – middle zone of 150 mm length and top and bottom zones of 75 mm length each. The temperature controllers of the three zones were calibrated to achieve a constant temperature 520±1 oC over the whole length of the catalyst bed (actual bed temperature).

A glass reactor of 15.6 mm internal diameter was used. Dimensions and details of the reactor are given in Figure 4.2. Quartz wool is usually put beneath and above the catalyst bed. The liquid product was collected in a glass receiver (Fig. 4.3).

Figure 4.2MAT reactor

Figure 4.3:Liquid receiver.

The balance was used to weigh the catalyst sample, liquid receiver before and after the reaction, and the syringe before and after the reaction. Analytical weights were of precision grade or calibrated against a set of certified standard weights. An accurate balance was very significant for mass balance.

Liquid product was analyzed by GC to determine the boiling range distribution by simulated distillation. The gasoline boiling range was from 0 to 221 oC, light cycle oil (LCO) from 221 to 343 oC, and heavy cycle oil (HCO) from 343 to 650 oC. The GC was equipped with flame ionization detector (FID). The column for simulated distillation is 1/8 x 20 inches stainless steel, 10% UC-W982 on 80/100 mesh Chromosorb PAW. This column was attached to the FID with a 0.030 inch jet.