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FCC Catalyst Evaluation

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					                          FCC Catalyst Evaluation

1.0 Introduction

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




                                     1
profit opportunity and to minimize risk. The right catalyst for one
refiner may not necessarily be right for another.


2.0 Catalyst 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



                                     2
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.



                                    3
     b.   Obtain references.
     c.   Check references.

9.   Test the selected catalyst in a pilot plant.
     a. Calibrate the pilot plant steaming conditions using
         incumbent E-cat.
     b. Deactivate the incumbent and other candidate catalysts.
     c. Collect at least two or three data points on each by
         varying catalyst-to-oil ratio.

10. Evaluate pilot plant results.
     a. Translate the pilot data.
     b. Use the kinetic model to heat-balance the data.
     c. Identify limitations and constraints.

11. Make the catalyst selection.
     a. Perform economic evaluation.
     b. Consider intangibles-research, quality control, price,
         steady supply, manufacturing location.
     c. Make the recommendations.

12. Post selection.
     a. Monitoring transitions-% changeover.
     b. Post transition test run.
     c. Confirm computer model.

13. Issue the final report.
     a. Analyze benefits.
     b. Evaluate selection methodology.




                               4
3.0 Reactors 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


                                         5
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.




                                  6
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-) The operation 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,




                                       7
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




                                    8
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 test industrial 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,




                                     9
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
                                                      o
temperatures ranging from 550 to 930                      C for 2 to 24 hr.       The
primary objective is to deactivate a fresh catalyst such that its


                                          10
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.




                                                           Y-Zeolite



                                                            M atrix

                                                              SiO ·A l O
                                                                  2    2   3




                           0
                     520-71 µm


                 Figure 3.1. Particles of FCC catalyst.




                                    11
   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 710 oC, respectively.




                                   12
4.0 CATALYST 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.


4.1    Experimental 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.




                                  13
Figure 4.1: Schematic for the MAT unit.




                  14
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).




                                   15
Figure 4.2 MAT reactor



          16
Figure 4.3: Liquid receiver.




            17
   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.


   Gaseous product was analyzed by another GC to determine its
composition as hydrogen, and C1 to C5 hydrocarbon.          A thermal

conductivity detector (TCD) was used. The analytical columns were:

Reference column:     20 inch, 2% OV-101         on    100/120   mesh,
                      Chromosorb W-HP.

Analytical columns: 1A. 5 ft, 35% DC-200          on    80/100   mesh,
                        Chromosorb P-AW

                      1B. 24 ft, 20% bis(2-methoxyethyl) adibate on
                          80/100 mesh, Chromosorb P-AW

                      2.   6 ft. Porapak Q, 80/100 mesh

                      3.   10 ft, molecular sieve 13X, 45/60 mesh

All columns were 1/8-inch OD stainless steel.


   The carbon analyzer used was CS244 (LeCo Corp.). Oxygen was

supplied to the unit directly.




                                     18
   Demands on the MAT lab involve more than the simple rating of

catalyst activities. At the very least, there is sufficient interest in

characterizing the coke and hydrogen producing properties of a

catalyst to require collection and analysis of the gas and to determine

the carbon on the discharged catalyst.        Calculation of a weight

balance is another reason for obtaining samples for gas and coke

analyses.   Most MAT laboratories are capable of obtaining mass

balance of around 95% in studies using VGO feedstocks. A major

interest for the MAT is in obtaining product selectivity data because it

is recognized that this inexpensive laboratory test can provide good

replication of plant yields if suitable chromatographic technology is

used with both the liquid and gaseous products .



   MAT operating conditions are shown in Table 4.1. The commercial

vacuum gas oil (VGO) was obtained from Neghishi Refinery and its

properties are shown in Table 4.2.



   Before testing the prepared catalysts the MAT unit was examined

by running a commercial catalyst. The same catalyst, conditions and

almost the same catalyst amount was tested twice to investigate the

reproducibility of the unit. The MAT data for both runs is shown in

Table 4.3. Hence, it can be said that the unit was ready to examine

the prepared catalysts.




                                   19
          Table 4.1: Mat operating conditions


       Temperature                          520 oC

       Feed rate                            1 g/30 sec

       Amount of catalyst                   0.5–3.0 g

       Feed type                            VGO




           Table 4.2: MAT feed oil properties



Specific gravity (15/4 _C)         0.8821
Sulfur (wt %)                      0.18
Conradson carbon (wt %)            0.09
Refractive index (15 _C)           1.4719
Bromine number                     3.2
Ni (ppm)                           <1
V (ppm)                            <1

Distillation data (vol. %)         ASTMD-1160 Temp. (_oC)
   10                              376 oC
   50                              437 oC
   90                              518 oC




                             20
            Table 4.3: The yield reproducibility of MAT unit


Test                     1                         2
cat/oil ratio            2.78                      2.82
Conv. (wt%)            68.3                       67.5
Component                          Yield (wt%)

H2                       0.70                      0.60

C1                       0.26                      0.28

C2                       0.20                      0.21

C2                       0.36                      0.39

C3                       0.54                      0.55

C3                       4.86                      4.82

iC4                      4.00                      3.87

nC4                      0.60                      0.59

t2C4                     2.08                      1.97

1C4                      1.58                      1.52

iC4                      1.98                      1.91

c2C4                     1.58                      1.50

Total C4                 7.23                      6.91

C5 + Gasoline          48.41                      47.97

LCO                    19.12                      18.37

HCO                    12.61                      14.17

Coke                     1.66                      1.74

Total                  99.93                      99.93




                                   21
4.2     Procedure


     In the MAT test, 3 g of catalyst was packed in the glass reactor

(Fig 4.2). The reactor was installed in a vertical tube furnace and

purged with nitrogen until it attained the required temperature (520
o
    C). Vacuum gas oil (VGO) was pumped using the syringe pump at a

controlled rate to deliver 1 g over 30 sec. The feed passed through a

pre-heater before it contacted the catalyst bed.


       In the MAT setup, as shown in Fig. 4.1, the reactor outlet was

connected to the liquid product receiver immersed in an ice bath. The

outlet of the receiver was connected to a gas holder from which

water was displaced. Following the injection of the VGO, the MAT

reactor was swept with an inert gas (usually nitrogen) for a period of

time sufficient to sweep all vapors from the reactor and to transfer all

non-condensed material into the gas holder. This stage was called

stripping and it usually lasted around 25 min.       At this point the

products of the reaction were collected in three locations: the coke

and a small amount of liquid residue are in the reactor; most of the

liquid are in the receiver; and the gaseous products are in the gas

holder.    After the stripping stage, the liquid receiver was removed

from the ice bath and warmed to 25 oC in order to remove the liquid

product easily from the receiver.      Nitrogen should flow to the gas

collector to remove the volatile materials that would otherwise be

lost during handling. Following the gas sweep, the liquid receiver was

disconnected, sealed and weighed. The volume of gas product and

flush nitrogen was equal to the volume of liquid displaced from the

gas holder. Then the furnace was switched off and the reactor

removed and cooled by air. Catalyst was removed from the cooled

reactor after the run. The quartz wool which was placed above the


                                  22
catalyst bed was removed so that the spend catalyst could be

analyzed for carbon individually. Liquid holdup in the bottom of the

reactor was typically measured by using filter paper and weighing it

after wiping. Collecting this liquid was important for accurate mass

balance.


4.3      Product Analysis

A.       Coke


     Coke deposition on spent catalysts was determined by a common

combustion method. In this method, a carbon analyzer Cs 244 (LeCo

Corp.) was used. Oxygen was supplied to the unit directly. A small

amount of spent catalyst (0.25 g) was used for the desired analysis.

The sample was burned completely, which converted all carbon to

carbon dioxide. Carbon dioxide was removed by Kolt adsorption, so

that by re-measuring the volume of gaseous products, carbon

dioxide, and thus carbon, could be determined.


B.      Liquid Product Yields


       Simulated distillation by ASTM method D-2887 was used to

determine the boiling range distribution of the liquid receiver

contents. This method was used to give three different boiling ranges

as follows:


     36–221 oC    gasoline wt%

     221–343 oC   light cycle oil (LCO) wt%

     343–650 oC   heavy cycle oil (HCO) wt%




                                  23
C.               Gaseous Products


      A sample of gaseous product was analyzed by a GC to determine
its     composition          such   as,    nitrogen,   hydrogen,   and   C1–C5

hydrocarbons. The amount of C5 found in the gas must be added to
the    C
           
                (gasoline range) in the liquid.


4.4         CHARACTERIZATION OF CATALYSTS


      The catalyst properties, such as zeolite content, the unit cell size

of the zeolite, and the surface area determine the activity and

selectivity of the catalyst.              Instrumental methods were used to

characterize the FCC catalyst particle both fresh and steamed.

Instrumental methods were used to characterize the changes that

occur in the catalyst during the FCC process. These changes were

related to desirable or undesirable changes in the selectivity and

activity of the catalyst.


      In most advanced technology catalysts, the zeolite is designed to

hydrothermally dealuminate in a controlled and stable way to the

intended unit cell size and surface area. Catalyst characterization is

essential in order to define key features and to understand

variabilities in catalyst performance. For FCC catalysts, the important

characteristics are surface area, acidity, and unit cell size.



4.5         Catalyst Evaluation

            The catalytic reaction experiments are carried out, in the
present study, using a novel Riser Simulator unit. The Riser
Simulator is basically a mini fluidized bed reactor operating in the
batch mode with intense gas recirculation. The following section




                                            24
describes the experimental setup, and the experimental procedure
adopted during this study.




4.5.1        Experimental Apparatus

        Experimental catalytic cracking runs were carried out in a Riser
Simulator reactor in operation at CREC-UWO laboratory. The reactor
was connected to a vacuum box through a four-port valve. The
cracked products were removed from the Riser Simulator at the end
of the pre set reaction period. A time/actuator assembly linked to the
feed injection system controlled the four-port valve. The vacuum
system was connected to a manually operated six-port sampling
valve. This sampling valve was connected on-line to the gas
chromatograph. Furthermore, the Riser Simulator reactor and the
vacuum box were equipped with pressure transducers to monitor the
pressure during and after the reaction periods. Both the reactor and
the vacuum system were supplied by separated heating systems and
both were well insulated.

        The feed injecting system includes a gas tight syringe
connected to switches to control the timer/actuator assembly on the
four port valve and the data acquisition system. The data acquisition
system allowed monitoring the change of pressure with time from
both the reactor and the vacuum box. A schematic diagram of the
experimental setup is given in Fig. 4.4. All main parts of the set-up
will be discussed in detail in the following section




                                    25
4.5.1.1     Riser Simulator

      The novel Riser Simulator is the center of the experimental
setup for catalytic cracking testing. This reactor was designed and
manufactured at CREC-UWO. The Riser Simulator was made out from
lnconel, which is a high temperature nickel alloy. The reactor consists
of four main components: the reactor shells, the catalyst basket, the
impeller, and the impeller drive-housing unit.


      The reactor is composed of two shells, the top and bottom
sections. While the top shell is fixed on the steel reactor support
frame, the lower shell is removable. The lower shell is attached to
the upper shell section by means of a series of eight bolts and nuts.
The upper shell also includes the impeller, which is operated by an
electric belt-driven motor. A manual motor controller adjusts the
speed of the impeller. The top shell also contains three ports, two of
which are connected to the four ports valve. The third port is hooked
up to the reactor pressure transducer.




                                  26
Fig 4.4: Schematic diagram for the experimental setup.




                              27
The lower reactor shell includes both the injection port and the
catalyst basket. The catalyst basket is designed to fit inside the
annular space of the bottom shell. The catalyst basket contains top
and bottom porous inconel disks, and this prevents the catalyst from
being entrained out of the basket into the other sections of the
reactor. Furthermore, this design allows free gas motion through the
basket. The porous disks are kept in place in the catalyst basket by
two snap rings. The two shells are tightly secured using a flexitallic
gasket pressure seal manufactured out of an inconel graphite
composite material.

     Each of the shells has its own sets of heaters. The bottom
section contains four cartridge heaters each heaters having a
resistance of around 29 . Because of the high temperature involved
in the system, the top section of the Riser Simulator, including the
impeller shaft and packing gland assembly, needs heat dissipation.
With this end a cooling system is implemented utilizing cold tap
water as the coolant.

    Figure 4.5 shows a cross-sectional view of the Riser Simulator. A
comprehensive description of the construction and operation of this
novel unit are given by Kraemer (1987) and Pruski (1996).



4.5.1.2    Injector system

     The injector system consists of a gas tight glass syringe, which
is fitted with a parallel threaded support rod and a threaded disc
placed between two nuts limiting the motion of the syringe plunger.
The disc is fixed at the support rod, allowing the syringe to intake,
for every injection, a fixed amount of feedstock from the reservoir
chamber.

    The injector system contained two electrically actuated micro-
switches fixed at opposite ends of the sliding support rod. While one



                                  28
of the switches controls the data acquisition system, the other
controls the timer/actuator assembly of the four-port valve.




        Fig 4.5: A schematic diagram for the Riser Simulator.




                                  29
      In the sample position of the three-way valve, the syringe is
attached   to   the   feedstock (gas oil   and/or   model   compound)
container. The syringe fills the required feedstock amount when the
plunger is pulled all the way back. Meanwhile, the plunger presses
against one of the switches, preventing data acquisition      pressure
signals.

      The feed syringe is connected to its needle. Thus, when the
plunger is pushed all the way forward, the feed sample is delivered to
the reactor. At this point, the micro-switch is released, hence
initiating the data acquisition program. Moreover, when the plunger
is fully pushed forward, presses against another switch connected to
the timer/actuator assembly of the 4-port valve. Consequently, the
timer starts to count down the pre-set reaction time, upon which the
timer would activate the actuator. The 4-port valve is then opened by
the actuator, equalizing the pressure between the reactor and the
vacuum box, thereby terminating the reaction.




4.5.1.3     4-Port and 6-port valves

      The reactor is connected to the air/argon supply through a 1/8
inch 4-port valve. The other end of this valve is used to connect the
reactor with the vacuum box. In both positions, there are always two
paths available through the valve for the reaction products to move
along. In the open position, the reaction products are transfered
through the valve, into the reactor through an inlet port. Then, they
move out of the reactor through an outlet port, back into the valve
and finally they reach the vacuum box. In the closed position,
however, the reactor is completely isolated from the rest of the
system and connected to itself through two of the four ports of the
valve.




                                  30
     The sample injection valve (1/8” 6-port chromatographic valve)
is installed between the vacuum box and the GC. For both positions,
there are always two independent loops for the gases to pass
through.   While   one   path   connects   the   vacuum   box   to   the
vent/vacuum pump, the other joints the helium carrier gas with the
GC detector. The position of the valve determines the path which
includes the sample loop.

4.5.1.4    Vacuum System

      The vacuum system consists of a 485 cm3 stainless steel
cylinder fixed between the 4-port and the 6-port valves. These
components together with two on-off valves, two three- way valves,
and two position selector valves are placed inside a heated box. The
V1 valve (Fig 4.3) is connected between the air/argon gas supply
bottles and the first “on-off” valve. This valve connects the gas bottle
to the reactor and to the gas system. In addition, V1 connects to the
4-port valve through V2 (Fig 4.4). Finally, V2 allows the separation of
the entire system from the gas supply.

      The stainless steel cylinder works as a sink for the reaction
products. It has a large volume. In addition, an important pressure
difference, with respect to the reactor, facilitates a quick and easy
removal of reaction products as well as unreacted hydrocarbons. This
rapid evacuation is needed to prevent further progress of cracking
reactions after the pre-set time.

      The second isolation valve V3 is essential to control product
sampling (Fig 4.3). This valve is set in closed position during post
reaction evacuation period keeping the reaction products within a
volume of set dimensions.




                                    31
      The second three-way valve (V4) is connected between V3 and
the vacuum pump/vent line. This valve allows to incorporate or
remove the vacuum pump in the path of the exhaust gases going to
the fume hood. The main function of vacuum pump is to reduce the
pressure within the vacuum box to around 0.5 psia (almost vacuum)
prior to the reaction test. Between the second isolation valve (V3)
and the vent line/vacuum pump, there is a glass bottle. This glass
bottle provides a lower pressure than the one in the vacuum box and
an extra driving force for filling the sample loop of the 6-port valve.




4.5.1.5     Heating and Insulation

     Heating tapes and insulation cover all of the connecting lines

between the vacuum box and the two chromatographic valves. There

are six heating tapes, each of them is connected to a Variac- type

power supply. This system helps to keep the lines at high

temperature, preventing hydrocarbon condensation in the lines and

valves. Furthermore, the reactor is insulated to maintain close to

isothermal operating conditions. Maximum temperature deviation

during experiments is only of a few degrees centigrade.


4.5.1.6   Control Devices

4.5.1.6.1    Temperature control

     There    are   two   independently   powered    controlled   heater

systems. One controller keeps the reaction temperature constant at

around 525°C by means of four heating rods. These heaters are




                                   32
inserted in the bottom shell of the Riser Simulator. However, the top

section of the Riser Simulator is directly heated using two smaller

insertion rod heaters directly powered by Variacs.




4.5.1.6.2   Pressure Transducers

      The reactor and the vacuum box are provided by two identical

Omega pressure transducers, series PX-303. Figure 4.3 shows P1

and P2 which represent the location of the reactor and the vacuum

box transducers respectively. Each transducer is powered by its own

power supply. Furthermore, these transducers have a calibrated span

of 0-50 psia with 0.25% accuracy, 1 ms response time and a 0.5-5.5

Volt output signal range. The transducers are also equipped with

protective pressure snubbers to take care of any sudden pressure

spikes or fluctuations.




4.5.1.6.3   Thermocouples



      Several thermocouples are mounted around the Riser Simulator
reactor to accurately monitor the temperature. Two thermocouples
are   connected   to      the   reactor    and   the   valve   block.   Other
thermocouples are fixed at the following places:



a)- Impeller shaft cooling jacket (20°C-40°C)

b)- Upper reactor shell section (425°C-475°C)



                                      33
c)- Lines from the reactor to the 4-port valve (275°C-300°C)

d)- 4-port valve body (225°C-250°C)

e)- Vacuum box (350°C-390°C)

f)- Lines between 6-port valve and vacuum box (225°C-250°C)

g)- 6-port valve sampling loop (250°C-275°C)

h)- Line from 6-port valve to GC (275°C-300°C)

i)- Gas oil reservoir (50°C-75°C)



     Note that the values in brackets indicate typical temperature
ranges using in the various Riser Simulator ranges.




                                    34
4.6- Analytical Equipment

4.6.1        Gas Chromatograph System

        The GC system, used in the present study, consists of a
HP5890 gas chromatograph, a HP3392A integrator, gas supply
bottles and connecting lines, valves and associated wiring.

     The GC contains a 25 m long capillary column, an FID-type
detector and a temperature controlled oven. While helium is used as
the sample carrier gas, air and hydrogen are used as the gases for
the FID detector. Furthermore, liquid nitrogen is used to facilitate the
initial cryogenic operation of the GC temperature program. The liquid
nitrogen cools the GC oven to –30°C. The flow of liquid nitrogen is
administered by a solenoid valve actuated from the GCs‟ internal
oven temperature controller.

     The HP3392A integrator allows strip chart recording as well
integration of the GC detector signal. The integrator is connected to
the GC via the HP-IL instrument network cabling system.

        A Mettler balance is used to accurately weigh the catalyst
sample. A Hamilton gas tight syringe was calibrated for the different
feedstock used in the present study. Analytical weights are of
precision grade or calibrated against a set of certified standard
weights. Availability of this balance is of major importance for good
mass balance calculations.



4.5.2-Coke analyzer

        Coke deposited on spent catalysts is determined, in the present
study, by a common combustion method. In this method, a carbon
analyzer Cs-244 (Leco Corp.) is used. Oxygen is supplied to the unit
directly. A small amount of the spent catalyst (0.25 g) is used for the
analysis.   The   coke   laid   out   on   the   sample   during   reaction



                                      35
experiments is burned completely converting the carbonaceous
deposit into carbon dioxide. The moles of carbon dioxide formed are
measured, and thus the coke formed is determined.


4.6- Procedure


      Both CAT-LC and CAT-SC were used in the present study. The
reaction conditions adopted during the present study are close to
those used in an industrial FCC unit. Both catalysts were tested at
four different contacts times (3, 5, 7,and 10sec). In the case of
cumene, four different reaction temperature levels were used: 400,
450, 500, and 550C. However, in the case of 1,3,5 TIPB six different
temperatures were considered 350, 400, 450, 500, 525 and 550 C.
In addition, for all experiments, one catalyst to oil ratio of C/O=5
was employed (feed weight =0.16g and catalyst weight=0.81g).
More than three repeat runs were conducted at each experimental
condition.

      Regarding the experimental procedure in the Riser Simulator,
every experimental run uses 0.81g of catalyst in the Riser Simulator
basket. The system is sealed and tested for any pressure leaks by
applying special liquids around the reactor and vacuum box and
monitoring any pressure changes in the system. The reactor is then
heated to the reaction temperature. The vacuum box is heated to
around 250C and is evacuated at around 0.5 psi to prevent any
condensation of hydrocarbons inside the box. The heating of the
Riser Simulator is conducted under continuous flow of inert gases
(argon) and the process usually takes around 3 hours until reaching
thermal equilibrium. At this point the GC is started and its
temperature lowered to –30°C. This temperature is kept for 3
minute, then increased at a rate of 15°C/min up to 240°C. The GC is
left at 240°C for 1 minute, and then the temperature is increased at



                                 36
a rate of 40°C/min up to 300°C. Then, the temperature is left at
300°C for 20 minutes to ensure that all the hydrocarbons present in
the reacted gases are eluted from the capillary column.




                                 37
References

Boock, L.T., and Zhao, X., “Recent Advances in FCC Catalyst
Evalutions: MAT VS DCR Pilot Plant Results”,        In Fluid Catalytic
Cracking, Edited by Occelli, M.L., and O‟Connor, P. pp131-141
(1997).


Campagna, R.J., Brady, M.F., Fort, D.L. and Wick, J.P., ”Fresh FCC
Catalyst Tests Predict Performance”, Oil and Gas J., March 24,85
(1986).


Corella, J., Fernandez, A. and Vidal, J.M., “Pilot Plant for the Fluid
Catalytic Cracking Process: Determination of the Kinetic Parameters
of Deactivation of the Catalyst”, Ind. Eng. Chem. Proc. Des. Dev., 25,
pp554-562 (1986).


Corma, A., Miguel, P.J., Orchilles, A.V., and Koermer, G., “ Zeolite
Effect on the Cracking of Long-Chain Akyl Aromatics”.Journal of
Catalysis, 145,pp181-186, (1994).


Corma, A., and Martinez-Triguero, J. “ Kintics of Gas Oil Cracking and
Catalyst Decay on SAPO-37 and USY Molecular Sieves”, App Catal,
118,pp.153-162 (1994).


Corma, A., Miguel, P.J., and Orchilles, A.V. “ Kintics of the Catalytic
Cracking of Paraffins at Very Short Times on Stream”.Journal of
Catalysis, 145,pp.58-64 (1994).


Corma, A, Martinez, A, and Martinez-Triguero, J. “Limitation of the
Microactivity Test for Comparing New potential Cracking Catalysts
with Actual Ultrastable-Y-Based Samples” In “Fluid Catalytic Cracking



                                  38
III;   materials    and     Processes”        edited   by    Occelli,   M.L.,    and
O‟Connor,P., American Chemical Sciety, 1994.


de Lasa, H.I. “ Us Patent 5,102,628, (1991).


de Lasa, H.I., “Fluidized Bed Catalytic Cracking Technology”, Lat. Am.
J. Chem. Eng. Appl. Chem., 12, 171-184 (1982).


Forissier, M. and Bernard, J.R., “Modelling the Microactivity Test of
FCC Catalysts to Compute Kinetic Parameters. AIChE Meeting,
Houston, TX, (1989).


Humphries,    A., Wilcox,        J.R., “Zeolite        Components and Matrix
Composition Determine FCC Catalysts Performance”, Oil & Gas
Journal, Feb. 6, 45-51, (1989).


Kraemer, D.W., Sedran, U., and de Lasa, H.I, “Catalytic Cracking in a
Novel Riser Simulator”. Chem. Eng. Sci. 45(8), pp2447-2452,
(1990).


Kraemer, D.W., Larocca, M., and de Lasa, H.I, “Deactivation of
Cracking Catalysts in Short Contact Time Reactors: Alternative
Models”. Can. J. Chem. Eng., (1990).


McElhiney,    G.,      “FCC     Catalyst       Selectivity    Determined        From
Microactivity Tests”, Oil and Gas J., Feb. 8, pp35-38 (1988).


Moorhead, E.L., Mclean,J.B., and Cronkright, W.A. “Microactivity
Evaluation of FCC Catalysts in the Laboratory: Principles, Approachs
and    Applications”      In:   Fluid   Catalytic      Cracking:    Science      and
Technology, Magee, J.S., and Mitchell, M.M. (editors), Amesterdam:




                                         39
Elsevier, 1993.


Mott, R.W., “New Concept Measures Catalyst Performance”, Oil and
Gas J., Jan. 26, pp73-77 (1987).


O‟Connor, P. and Hartkamp, M.B. “A Microscale Simulation Test for
FCC Development”, Paper Presented at the Symposium on the
Preparation and Characterization of catalysts before the Division of
Petroleum Chemistry, Inc , American Chemical Society Meeting, Los
Angeless, Sep 25-30 (1988).


Sunderland,   P.,   “An   Assessment    of   Laboratory   Reactors   for
Heterogeneously Catalyzed Vapor Phase Reactions”, Trans. Instn.
Chem. Engrs., 54, 135 (1976).


Tasi, T.C., Pan, W.P., Leu, L.J. and Yu, S.T., “A Procedure for
Evaluation of Commercial FCC Catalyst”, Chem. Eng. Comm., 78,
pp97-109 (1989).


Weekman, V.W., “Laboratory Reactors and Their Limitations”, AIChE,
20(5), pp833-840, (1974).


Young, G.W. and Weatherbee, G.D., “FCCU Studies with an Adiabatic
Circulating Pilot Plant Unit”, Paper Presented at the AIChE Annual
Meeting, San Francisco, November (1989).




                                   40

				
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