Catalytic Naphtha Reforming: A Novel Control System for the Bench

Article

Catalytic naphtha reforming – a novel control system for the bench-scale evaluation of commercial continuous catalytic regeneration catalysts. Enrico Caricato, John Herman Hoffeldt, P. D. Riekert Kotze, Jared A. Lloyd, Nicolaas Marthinus Prinsloo, and Carel Johannes Swanepoel Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 18 May 2017 Downloaded from http://pubs.acs.org on May 27, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Industrial & Engineering Chemistry Research is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Industrial & Engineering Chemistry Research

Catalytic naphtha reforming – a novel control system for the bench-scale evaluation of commercial continuous catalytic regeneration catalysts. Enrico Caricato, John H. Hoffeldt, P. D. Riekert Kotze, Jared A. Lloyd, Nicolaas M. Prinsloo* and Carel J. Swanepoel. Sasol Group Technology, Research & Technology, 1 Klasie Havenga Road, Sasolburg 1947, South Africa. KEYWORDS: Catalytic naphtha reforming, paraffin dehydrocyclization, bench-scale piloting, octane number measurement, process monitoring and on-line NIR spectrometry.

ABSTRACT: An automated control system was developed for the bench-scale evaluation of continuous catalytic regeneration (CCR) naphtha reforming catalysts. Deactivation of these catalysts is too rapid for fixed bed operation and regenerating the catalyst continually, is not feasible on bench scale. To emulate the commercial process, a solution was required which would allow catalyst evaluation at a constant research octane number (iso-RON). Because changes are too fast for manual adjustment (especially when using Fischer-Tropsch naphtha), automated adjustment of process conditions was required. This was achieved through the use of simple matrix algebra, semi-automated gas chromatography and on-line NIR (Near Infrared) analyses. Commercial catalysts with very small activity and reformate yield differences could be

ACS Paragon Plus Environment

1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 34

compared, and their RON vs. yield correlations determined. Product samples large enough for engine research octane number (RON) analyses, could also be collected at steady state. Through the use of multidisciplinary analytical and advanced process control techniques, substantial improvement of bench-scale piloting methodology in naphtha reforming, was obtained.

1. Introduction Catalytic naphtha reforming is possibly one of the most researched topics in the petrochemical and transportation fuel industries, with journal articles, books and reviews appearing frequently 1−9

. It is known to be a complex chemical process but nevertheless a true industry workhorse as

almost every refinery in the world has such a processing unit 1. In the chemical industry it is an important source of aromatic hydrocarbons more specifically BTX’s (benzene, toluene and xylene) 9. In fuels processing, its sole purpose is to upgrade low octane naphtha feed-stocks (rich in paraffins and naphthenes) into high octane gasoline by increasing the aromatic content and isomeric paraffin content of the product 1. The catalyst is typically platinum supported on chlorinated alumina, and the main product is referred to as “reformate” whereas LPG and hydrogen are very important side-products. A simplified reaction scheme is shown in Figure 1.


2

Page 3 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. Reaction scheme of catalytic naphtha reforming; a=acid catalyzed; m= metal catalyzed Through the years, this indispensable refining technology evolved from a relatively simple fixed bed reactor system where the catalyst was regenerated ex-situ, to a highly sophisticated system where the catalyst is continuously removed from the reaction zone and regenerated in a separate section 2. In this regeneration section, the catalyst is purged to remove hydrocarbons and hydrogen, coke is burned off in a mixture of nitrogen and air, metals are re-dispersed through oxychlorination, followed by the reduction of the metals 1. The regenerated catalyst is then returned to the reaction zone through a dedicated hopper system. This process is referred to as continuous catalytic regeneration (CCR) and is currently used in Sasol’s Synfuels Complex as the largest source of high octane fuel blending material

10

. Mostly due to the feed’s low N+2A

value (N=naphthenes, A=aromatics), catalytic reforming is not the technology of choice for upgrading feed-stocks produced by a Fischer Tropsch (FT) based refinery complex 10. To obtain the required RON from this lean feedstock, it requires high severity operation of the catalytic reformers and the associated loss in reformate yield. It has however been in commercial practice for more than 35 years and pushing catalyst and processing boundaries, have always been an


3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 34

incentive. This has become especially true when targeting a reformate with a RON above 95, whilst guarding against excessive volume loss and adhering to gasoline fuel specifications with regards to benzene content.

The complexity of catalytic reforming chemistry is well known with several reactions taking place simultaneously i.e. dehydrogenation, dehydrocyclization, isomerization, hydro-cracking, alkylation, dealkylation and coke formation 5. The fundamental understanding of the chemistry and catalysis related to FT derived feeds is however still lacking, as most papers refer to crude oil derived feed-stocks 1-9. This also becomes problematic when modeling approaches are used to estimate yield and/or activity benefits for new commercial catalysts. Apart from the fact that suppliers use different models for these estimates, the FT feed and the associated operating envelope typically lay outside these models’ validity. As a result, proposals for alternative catalysts for the Sasol Synfuels Refinery are treated with caution and an empirical/piloting approach is followed to confirm suppliers’ recommendations. Performance testing is normally performed on bench scale using representative FT derived feed, and at conditions best approximating that of the commercial units.

Performance testing of reforming catalysts is significantly constrained as characterization of commercial catalysts is rarely allowed. Even basic analyses, important for catalysts evaluations e.g. the carbon and chloride contents of spent catalyst, as well as phase changes of the alumina support, are contractually prohibited. Furthermore, the typical CCR-type catalysts deactivate significantly within weeks in a fixed bed piloting system when commercially relevant conditions are used 11. This presents special challenges when different catalysts are to be compared: Firstly,


4

Page 5 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


meaningful yield vs. selectivity data (C5+ yield vs. RON) needs to be collected while large enough samples for engine analyses (described later) are retained. This translates to performing tests at constant octane (iso-RON) of the resulting reformate while the catalyst is deactivating 11. Secondly, significant care has to be taken for accurate control of the catalyst bed temperature profiles, especially when adiabatic testing is not feasible. This is due to the variety of endothermic and exothermic reactions taking place simultaneously where temperature profile differences can introduce significant variation in the product composition

12

. Commercially,

adjustment of the furnace temperature is used to control the reformate RON at a desired value 1. Although the RON of the reformate is also determined by feed composition, total pressure, liquid hourly space velocity hydrogen to oil ratio etc., catalyst bed temperature is deemed the most effective parameter to vary 11. Other process variables are to be kept constant when comparing commercial catalysts at iso-RON conditions. Therefore to achieve iso-RON operation on bench scale, the RON of the product has to be measured continuously and the heat flux over the catalyst bed controlled very effectively. By adjusting the average catalyst bed temperature based on a real-time RON analysis of the reformate, means that the control loop can be closed and automated iso-RON operation enabled. Similar to the commercial process, heat input to the reactor is manipulated to control the average catalyst bed temperature through the heating elements around the reactor tube. A feedback loop is used to manipulate this heat input based on the real-time RON analyses of the reformate, controlling the octane number at the desired value.

Performing octane number analyses suitable for control purposes, present special challenges when doing bench scale piloting. The octane numbers of reformate samples can be measured with a calibrated Cooperative Fuel Research (CFR) engine using ASTM (American Society for


5


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 34

Testing and Materials) methods D2699 & D2700 respectively 1. However, relatively large samples are required for testing (500-1000 mL) making it impractical for research and/or piloting projects. Alternatives to CFR engine analyses based on detailed compositional analyses and spectroscopic techniques, have been developed over several decades 1. Special chromatographic systems (sometimes referred to as “Reformalizers”) are routinely used in process laboratories from which the octane number of reformate samples can be estimated

13,14

. These systems are

variants of the classic PIONA (Paraffins, Isoparaffins, Olefins, Naphthenes, and Aromatics) analyzers employing multiple GC columns and detectors. Although the chromatographic analyses of products of the naphtha reforming process is widely used in laboratory, pilot and industrial plants, the aforementioned equipment require specialized knowledge and maintenance procedures as well as significant capital investment. A cost effective alternative was applied in this work employing a classic one dimensional GC technique for component separation, but combined with more sophisticated calculations/modeling to arrive at the estimated RON and MON (motor octane number)

15

. The approach followed here was to construct proprietary

models for octane number estimation (RON & MON) which are variants of those given in open literature

15,16

but differ in the way components are grouped together. To arrive at meaningful

octane results, large samples still have to be collected periodically to correlate the estimated GC RON and MON results with actual CFR engine analyses. Simple linear correlations are used to go from “GC estimated RON & MON” to “estimated engine RON & MON”. For every new feedstock used, these correlations are updated and the GC estimates corrected accordingly.

Although this methodology has been shown to be quite effective, it is still dependent on a fairly lengthy GC method

16

and in our case, not available as an on-line method. It has been


6

Page 7 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


proposed earlier that on-line NIR spectrometry could be a suitable process analytical tool to monitor reformate properties

17

. In fact, one of the very first applications of on-line NIR

spectrometry in fuels refining, was that of estimating gasoline octane number and other fuel properties

18

. Combined with classic chemometric techniques, NIR spectrometry has also been

shown to be robust and very powerful in the research environment 19,20. Despite being one of the classic applications, no reference could be found where NIR results are actually used for controlling the octane number of reformate product on a pilot scale, apart from monitoring it 21. This concept was pursued here i.e. using off-line GC RON estimates to calibrate an on-line NIR analyzer, whereupon the on-line NIR analyses is then used to control the reformate RON.

An on-line chemometric product property control system was developed here for an existing bench scale piloting unit to facilitate performance testing of CCR-type naphtha reforming catalysts. The approach followed firstly involved improving the control system by which the temperature profiles of the catalyst beds are more accurately controlled. Subsequently, the analytical system consisting of an RGA (refinery gas analyzer) and off-line GC, was supplemented with an on-line FT-NIR (Fourier Transform Near Infrared) process analyzer which was fully integrated with the piloting unit’s control system. A procedure was then developed to calibrate the NIR analyzer using the off-line GC analyses and corrected/adjusted octane number estimates of the reformate. Closing the loop between the analyzer output i.e. the reformate octane number and the temperature control system, was the final step to enable octane number control of the product. The resulting iso-RON control system was validated through the comparison of various commercial catalysts, as well as using different FT derived feeds and performing engine RON analyses of reformate samples collected.


7


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 34

2. Experimental Section 2.1. Bench scale reactor configuration and operation The bench scale reactor system comprises of a feed system, a feed pre-heater, a single fixed bed reactor and a product work-up system which includes three phase separators and a product vessel (Figure 2). The system is capable of liquid flow rates between 10-100 mL/h, operation at pressures up to 10 Mpa and temperatures up to 550 °C. The reactor (i.d. 18.9 mm; length 1306 mm) is equipped with four heating elements. Undiluted, uncrushed catalyst is loaded in two segments/beds of equal volume (21.8 mL each; bed length ca. 80 mm), with carborundum (SiC 24 gritt) filling the space above and below the catalyst beds.

N2 Supply Bronkhorst Flow Meter

Electrical Preheater

Off Gas Knock Out Vessel

H2 Supply

Ritter Gas Meter

Bronkhorst Flow Meter Online RGA

Pressure Control Valve

Reactor Vent

Liquid Sampling System

Hydrocarbon Feed Vessel Selector Valve

Metering Valve

Vent

PIC Pressure Control

Manual Sampling Point HPLC Pump

Feed Mass Flow Meter

Air Cooler

Cl Feed Vessel

Dosing Pump

Product Vessel

HP Phase Separator

LP Phase Separator

Mass Flow Meter

Level Control

Flow Cell Level Control Mass Flow Meter

FT-NIR

Fiber Optics

Figure 2. Simplified schematic representation of the catalytic reforming pilot unit


8

Page 9 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The catalyst is not diluted to improve reproducibility during loading as the internal bed temperatures are measured with an internal thermocouple with measuring points at six fixed positions. Prior to the introduction of liquid feed, the catalyst is reduced in situ using standard protocol prescribed by the catalyst supplier. The liquid feed consists of hydrogenated naphtha (typical properties given in Table 1) collected from a commercial FT syncrude refinery. It is predried using 3A molecular sieves and manually doping the feed with a chlorine containing compound (tert-butylchloride – 1wppm Cl) to keep the catalyst at the desired chlorination level (1 weight%). Hydrogen is co-fed with the naphtha, passed through the preheater and the naphtha vaporized in the top of the reactor tube before contact with the catalyst (top-down flow). After contact with the catalyst, the gas is condensed at 20 °C while the hydrogen is stripped off and the liquid (referred to as reformate) collected in the product vessel. The liquid product continuously flows through a high pressure NIR flow cell and then through a special sampling system (described later) situated just before the product pot. An automated sampling valve switches at pre-set intervals (typically hourly) collecting just enough liquid to fill a GC vial. Product samples are intermittently transferred from this sampling system to GC vials and introduced to an off-line gas chromatograph. The off gas is continuously characterized via an on-line RGA analyzer set to sample every hour. The NIR flow cell is connected to an on-line near-infrared analyzer via fiber optics and collects an absorbance spectrum every two minutes.


9


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 34

Table 1. Typical naphtha feed properties (detail composition is proprietary information) Parameter

Value

Total aromatics (GC analysis)

8.6 m/m%

Total paraffins (GC analysis)

79.7 m/m%

Total naphthenes (GC analysis)

11.5 m/m%

N+2A

28.8 m/m%

Distillation (IBP/50/FBP)

53.0/125.9/172.5 °C

Density

0.72 g/mL

Molecular weight

117.4 g/mol

Sulfur

< 0.1 wppm

Nitrogen

< 0.3 wppm

Water (by Karl Fischer Titration)

< 10 wppm

2.2. Product analyses: Sampling system Since tests are performed at commercially relevant conditions and the CCR-type catalyst deactivates relatively fast

11

, it is necessary to determine the chemical properties of the product

regularly, accurately and reproducibly. The unit is therefore equipped with an automated sampling system to allow for frequent on-line gas analysis and essentially operator-free liquid sampling. Automation, via scheduler systems, allows samples to be taken as frequently as required and allows for unattended operation. The frequency of liquid sampling is not dependent on the GC method as would have been the case for full on-line GC analyses. This means that samples are taken only when required. This flexibility in the sampling frequency is used to confirm steady state at a chemical level and significantly reduces manual operator control. An inline valve has been installed after the liquid sampling system (Figure 2) which allows for the


10

Page 11 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


collection of a 500-1000 mL sample for CFR engine tests. A more detailed description of the gas and liquid sampling system is given in the Supporting Information.

2.3. Product analyses: Chromatographic methods and octane number estimations Using the sampling system described above, the off gas was analyzed hourly by an Agilent Technologies 7890A RGA gas chromatograph and the liquid feed and product samples using an Agilent Technologies 6850 GC-FID gas chromatograph. The parameters and methods used for the RGA and liquid GC methods are given in the Supporting Information. Verification of the mass balance takes into account that the feed and gas samples are analyzed separately. Due to limitations of the separation unit, the gas formed (denoted as LPG) still contains a small amount of C5+ material. This is quantified and incorporated in the C5+ yield calculation. The octane number of the reformate product is estimated from the resulting compositional data using Sasol’s proprietary “Synfuels LFCM model”. This model is able to estimate various properties of complex organic mixtures from Sasol’s refineries and specifically developed for Fischer Tropsch derived feed-stocks. Similar to the methods described in literature

16

, this model uses the

properties of the single compounds to estimate/predict properties such as RON, MON, RVP (Reid Vapor Pressure), density, flashpoint, cloud point of any given fuel mixture. The model estimates RON values for reformate over quite a wide range. Depending on the naphtha feed composition, however, predicted values from this particular model often deviate significantly from the actual CFR engine RON values. This discrepancy is on average around 1.5 RON units but can be as large as 4.4 RON units (standard error of prediction = 1.827 RON units). This is corrected effectively using a simple linear correlation with CFR engine analyses.


11


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 34

2.4. Product analyses: On-line NIR The NIR spectra of the reformate were collected with an ABB model TALYSTM FT-NIR Analyzer equipped with a high pressure transmission flow cell (ABB model ACC113) with a 2 mm path-length. The flow cell is interfaced with the analyzer via a fiber-optic cable pair (10 m; 300 µm). The temperature of the flow cell is controlled at 15 °C (±0.1 °C) with a chiller unit. The position of the flow cell is indicated in Figure 2. The ABB analyzer was configured to collect NIR absorbance spectra every two minutes (170 scans at 8 cm-1 resolution) throughout the entire evaluation. The empty cell spectrum is used as background. The analyzer was calibrated using the synchronized liquid GC data (estimated octane numbers and individual components) and the NIR spectral data from 5000 to 9500 cm-1. A standard multivariate approach was followed for the quantitative NIR analysis where the “full spectrum” is correlated with the desired product properties as opposed to using specific peaks or spectral features

17

. The analyzer allows for

automated quantitative analyses through partial least squares (PLS) models created using the GRAMS IQ software. The analyzer was configured to log/export the properties of the reformate product to the DeltaV distributed control system (DCS) that is used to control the piloting equipment. These analyses were used for the advanced process control as discussed in the following section. The calibration of the NIR was initially updated regularly (typically after each pilot run), but as more and more spectra/samples were added to the calibration, this only became necessary when a new naphtha feed was introduced or when significant outliers were detected by the NIR analyzer (Mahalanobis distance > 2).


12

Page 13 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.5. Advanced process control of the piloting reactor bed temperatures Since the chemistry prevalent in the catalyst bed is a mixture of endothermic and exothermic reactions, controlling the weighted average bed temperature (WABT) of the catalyst bed is not trivial. For previous iso-thermal piloting test, the catalyst bed temperatures were controlled by manually setting the heater temperatures to the most appropriate values. While this approach was adequate for evaluating stable catalysts, the fast deactivation of CCR-type catalyst causes the bed reaction profile to change continuously, rendering the control ineffective. Due to the offset positions of the internal thermocouples and that of the heaters, it is extremely difficult to manually select reactor wall controller set-points so that a desired reactor bed profile is obtained. What complicates the matter is that the dynamics of the reactor change over time and that different zones in the reactor may be endothermic and/or exothermic. The challenge here is that while there are six measuring points inside the reactor, there are only four temperature control points on the middle of each of the heaters (Figure 3).

Relatively simple matrix algebra was therefore applied in the DCS which enabled automated control as opposed to the “temperature adjustment by hand”. Here a multivariate model between heater temperatures and catalyst bed temperatures was constructed and an advanced process control (APC) loop was designed for setting desired catalyst bed temperatures (WABT) during the tests. An algorithm was developed for selecting the most appropriate heater set-points to obtain the desired bed profile. Details of this algorithm are given in the Supporting Information.


13


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 34

tw1

tb1

Heating element

tb2

Reactor wall

tb3

Thermocouple

tw2

T measurement tb4 SiC inert

tw3 tb5

Catalyst bed tb6

tw4

Figure 3. Catalyst loading diagram for test runs with CCR-type reforming catalysts – tb & tw denotes catalyst bed and reactor wall thermocouple positions respectively 2.6. On-line NIR Product Property Control System For effective CCR catalysts and/or feed comparisons, evaluations need to be performed not only at isothermal conditions, but also at iso-RON conditions. In principle the RON value of the product can be steered to a predetermined set-point through manipulating the process parameters


14

Page 15 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


i.e. catalyst bed temperatures, H2/feed ratio’s, LHSV (liquid hour space velocity) and pressure. The CCR catalyst deactivates within two weeks and the product research octane number is maintained by adjusting the catalyst bed temperatures upwards over time. In order to perform iso-RON evaluations, the RON of the product will consequently have to be measured and controlled continuously. This is classically performed through off-line engine knock test and manual catalyst bed temperature adjustments. For this work, this was achieved through controlling the reactor bed temperatures with the APC system described above using the product’s RON estimation from the on-line NIR analyzer, as input. For any given experiment, the H2/feed ratio’s, LHSV and pressure were kept constant.

The steps for achieving iso-RON operation were: a) Installation of a GC sampling system and developing a method that RON estimates could be derived from the chromatogram (described previously). b) The installation of an on-line NIR analyser and calibration thereof for RON estimates by using GC data. c) Design of the heater temperature control loop for semi-automated operation. d) Establishing data transfer protocol between analyser and the reformer DeltaV DCS. Only the five most important product properties (GC-RON, GC-MON, Engine RON, total paraffins and benzene content) are logged onto the DeltaV system and allow for real time trending of the product properties. e) Design of an automated temperature control loop with online RON as input. This was done by simply linking the output of the analyzer to the temperature control system and using a standard Proportional-Integral-Derivative (PD&I) control loop. It is foreseen that apart from RON, the control philosophy could include multiple product properties e.g. total aromatics, benzene content, iso/normal paraffin ratios and other properties like RVP that can be derived from the NIR spectra.


15


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 34

2.5 Description of catalysts evaluated The catalysts used in this study were all commercial versions of CCR-type catalytic reforming catalysts and the composition and identity thereof is proprietary and therefore only referred to as catalyst A to C. They differ very slightly in terms of their activity/yield performances often within the statistical limits of determination.


16

Page 17 of 34

3. Results and Discussion 3.1. Isothermal studies (activity comparison) For CCR catalytic reforming studies, two modes of operation can be applied depending on the type of data required. One involves collecting data at a constant WABT (stability test) and the other at constant octane (accelerated stability test). During a constant WABT operation the CCR catalyst deactivates rapidly (mainly due to coking) which leads to a linear decline in the reformate research octane value (RON) over time 11.

30

540

535 25 530 20

15 520

515

10

510 5

Average Bed Temperature (°C)

525

Bed delta Temperature (°C)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


505 0 500 -5

dT Top bed 520 °C case

dT Bottom bed 520 °C case

dT Top bed 500 °C case

dT Bottom bed 500 °C case

Weighted average bed temperature = 520 °C

Weighted average bed temperature = 500 °C

495

-10

490 0

10

20

30

40

50

60 70 80 Time on line (hours)

90

100

110

120

130

140

Figure 4. Temperature measurement and profiles for catalyst A over two consecutive iso-thermal runs. LHSV = 2.5 h-1, H2/oil molar ratio = 3, pressure = 9 barg.


17


For such a RON decline study effective control of the temperature profile and WABT across the catalyst beds is crucial. Here the APC control system was applied as described previously to control the temperature profile. Figure 4 shows the change in temperature profile for two subsequent isothermal studies (500 & 520 °C). Although the bed temperature differences “drift” over time, the control system is still able to control the WABT within 1 °C. Figure 5 is an example of an iso-thermal run at a WABT of 510 °C.

100 95

Estimated Engine Octane Number

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 34

90 Sample drawn for engine analysis (Actual RON = 95.7)

85 80 75 70 65 60 55 50 0

10

20

30

40

50

60

70

80

90

100

110

120

130

140

Time on line (hours)

Figure 5. Example of an iso-thermal study (510 °C) indicating decline in estimated reformate octane number with time on line. LHSV = 2.5 h-1, H2/oil molar ratio = 3, pressure = 9 barg Legend: (■) Offline GC; (─) On-line NIR The initial (time zero) RON shown by the NIR is due to the product from a previous run still in the product vessel and sample lines. As the product is replaced with essentially unconverted feed, the observed RON drops to values typically below 60 RON units. As the process temperature rises, the calculated octane number rises to a maximum around 20 hours after feed has been


18

Page 19 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


introduced. At this maximum, large samples (500 mL) are drawn for the CFR engine analyses. These initial maxima were found to be reproducible for any given set of conditions and/or catalyst evaluated. It was initially thought that the estimated RON decline could be used as a measure to quantify the stability (rate of deactivation) of the catalyst, but results were inconclusive (example given in the Supporting Information). This however provided the impetus to automate the temperature ramping for the iso-RON studies (described subsequently) provided that the feed composition and other process parameters are kept constant. The initial octane values were found to be a good indication of the activity of the catalyst and correlated linearly with the octane number values estimated from the GC analyses (Figure 6).

Figure 6. GC RON estimates correlated to actual CFR engine analyses. LHSV = 3.0 h-1, H2/oil molar ratio = 3, pressure = 9 barg.


19


Using multiple iso-thermal runs at different WABT’s, the maximum initial estimated engine RON values are subsequently used to firstly compare catalysts in terms of its activity (Figure 7), and secondly to estimate the WABT at which a specific engine octane can be achieved. This is important for the iso-RON evaluations described in the following section. Note, although large enough samples are collected at this maximum and the CFR engine RON is determined, the product is still a composite of the 6 hour period used to produce the sample. The appropriate data to use for the activity comparison is therefore the GC estimated RON of the samples at the turning point. This is effectively and precisely determined with the on-line NIR analyzer.

100 99 98

Calcualted Engine RON

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 34

97 96 95 94 93 92 91 497.6 °C

498.7 °C

504.2 °C

90 490

492

494

496

498

500 502 WABT (°C)

504

506

508

510

512

Figure 7. Iso-thermal studies used to compare catalysts in terms of “activity”. LHSV = 3.0 h-1, H2/oil molar ratio = 3, pressure = 9 barg. Legend: (•) Catalyst A; (■) Catalyst B; (▲) Catalyst C Due to the product octane continuously changing, it is meaningless to do reformate (or LPG) yield estimates using the iso-thermal studies. As can be observed from Figure 7, catalyst B and C


20

Page 21 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


clearly have an “activity” benefit over catalyst A, of around 5 °C. If this benefit could be sustained over the catalyst lifetime, it is a significant advantage when the commercial units are operated close to or at the maximum design temperature. Catalyst C had the highest initial activity at a WABT of 510 °C reaching a calculated reformate RON value of 99.9. Catalyst A and B had RON values of 97.7 and 99.1, respectively.

3.2 Iso-RON Studies (yield confirmation at constant octane) The second mode of operation for CCR work involves an enforced ‘steady-state’ scenario where a property of the reformate product is kept at a constant value during the process. The property of choice to keep constant is normally RON, since catalytic reformers are usually operated in such a manner to produce reformate with a specific RON value. As such the yields and selectivities obtained for that particular RON value, are most important in practice. This is commonly referred to as accelerated stability testing or simply iso-RON operation. During this operation mass flows and the process chemistry are allowed to reach a stable state or equilibrium at which a set of reformate and gas yield/selectivity data can be collected at a particular reformate RON. The most effective method for establishing such a scenario is to apply a constant increase in the bed temperatures to nullify the effect of the deactivation of the catalyst. Activity (in terms of conversion of species) of the catalyst is therefore maintained at a particular constant level whereupon data can be collected over a long period of time. An example of such iso-RON evaluations are presented in Figures 8 & 9. Here the full property control system was applied as described in the experimental section. This study was specifically aimed at collecting yield data at an engine octane value of 95 RON. Here separate iso-thermal studies were used to estimate at which WABT the maximum RON would be 95, which in this particular case was 497 °C. The


21


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 34

RON control system is only activated once this maximum is roughly achieved (27 hours in this case).

Figure 8. Example of an iso-RON 95 study for reformate yield determination at “steady state”. LHSV = 3.0 h-1, H2/oil molar ratio = 3, pressure = 9 barg. Legend: (─) NIR Engine RON; (■) GC Engine RON; (•) C5+ yield; (----) WABT At a set-point of 95, the P&ID controller is then allowed to take control over the catalyst bed temperatures (WABT) to achieve and maintain the desired reformate octane value. This results in the WABT being ramped up very slowly and from around 55 hours, the estimated octane number (NIR and GC) is steady and yield data can be collected. During this “pseudo steady state” period a large enough product sample is also collected for confirmation of the actual CFR engine RON. In this particular case the CFR engine result was 94.7 RON which is within the reproducibility of the ASTM method referred to earlier. Several of these iso-RON 95 studies were performed using the methodology described and results for two such studies are exemplified in Figure 9 mainly to


22

Page 23 of 34

indicate reproducibility and effectiveness of RON control. In this case the measured engine RON values for the two catalysts with slightly different activities, were 94.6 and 95.3, respectively. These values were again within the reproducibility error of the engine analysis. More information on the effectiveness of the RON control can be found in the Supporting Information.

502

96

94 501 92 500 90

88

499

86 498 84 497 82

80

496 10

15

20

25

30


50

55

60

65

70

Figure 9. Fully automated reformate octane number control at RON 95 for two catalysts of different activity. LHSV = 3.0 h-1, H2/oil molar ratio = 3, pressure = 9 barg. Legend: (─) Catalyst A; (----) Catalyst B In Table 2 the PIONA analysis of the feed is compared with that of the reformate (C5+) product for catalysts A & B. Despite the differences in activity, no significant differences in the respective product compositions were observed. This was also true for individual components like benzene. The conversion from paraffins and naphthenes to aromatics and the associated increase in octane number is clear, but no obvious differences in isomerization propensity of the


23

Temperature (°C)

Estimated engine octane number

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 34

two catalysts could be observed. For the catalysts evaluated thus far, differences in the benzene content of the products indicate that this is feed related rather than catalyst related. This is unfortunate since any catalyst that results in a lower benzene content together with a higher paraffin isomerization propensity, would be the catalyst of choice.

Table 2. PIONA comparison of feed and C5+ reformate composition for two catalysts Parameter

Feed

Catalyst A

Catalyst B

Total n-paraffins (m/m%)

36.48

8.75

8.87

Total i-paraffins (m/m%)

37.92

24.06

24.63

Total olefins (m/m%)

15.62

0.92

0.91

Total naphthenes (m/m%)

0.23

1.48

1.45

Total aromatics (m/m%)

9.75

64.79

64.14

Benzene content (m/m%)

0.34

3.05

3.06

CFR Engine RON

c.a. 62*

95.3

94.6

CFR Engine MON

c.a. 53*

85.7

85.3

*NIR estimates 3.3 Stepped Iso-RON Studies (RON vs. C5+ yield comparisons) There are clear indications that catalyst A would be the choice if yield was prioritized and catalyst B if octane number was the priority. It is however essential to show that the yield benefit of catalyst A is also valid across a reasonable reformate octane number range and not just at RON 95. Another protocol was developed to address this issue - referred to here as Stepped isoRON studies. It is typically started at a WABT where a particular catalyst feed combination would result in a high octane reformate product (c.a.100 RON). As soon as the octane is at the initial maximum, the automated RON control is activated and the temperature allowed to ramp


24

Page 25 of 34

upwards. Once the pseudo steady state sample has been collected and a steady C5+ yield confirmed, the WABT is dropped to a lower level (typically 10 °C lower) and the process repeated. Figure 10 represents an example from such a test. 102

520

101

518

100 99

516

98

514

97 512

96 95

510

94

508

93 92

506

91

504

90 502

89

500

88 10

20

30

40

50

60

70


110

120

130

140

150

160

170

Figure 10. Stepped iso-RON study with multiple constant octane segments. LHSV = 3.0 h-1, H2/oil molar ratio = 3, pressure = 9 barg. Legend: (─) NIR Engine RON; (•) GC Engine RON; (■) CFR Engine RON; (----) WABT After establishing the Stepped iso-RON operation, a comparison was done to verify that it can discern yield vs. activity differences between commercial CCR reforming catalysts over a wider octane number range. This involved comparing two commercial CCR catalysts expected to differ in terms of yield and activity. Separate runs were conducted again at similar operating conditions and feed, and results are depicted in Figure 11. As would be expected, each catalyst showed a different behavior during catalytic reforming. Catalyst A resulted in higher yields across the reformate octane range as compared to catalyst B. In accordance to the C5+ yield benefit


25

WABT (°C)

Octane Number

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 34

discussed above, the higher yielding catalyst provides the benefit of producing less LPG compared to the other catalyst. Hydrogen yields were found to be comparable and within expectations.

Figure 11. Catalyst comparisons over a RON range. LHSV = 3.0 h-1, H2/oil molar ratio = 3, pressure = 9 barg. Legend: (•) Catalyst A; (■) Catalyst B It is intriguing that very small, but significant differences can be observed in the selectivities between the various catalysts. The high yield catalyst shows a higher concentration of aromatic species in the reformate product across the reformate octane range. Overall it has been shown that activity, yield and selectivity differences between catalysts can be distinguished. A direct comparison was made between the observed yields here and yield estimates which were provided by the technology supplier (proprietary information). The experimental results from the catalyst comparison study closely represented these yield estimates.


26

Page 27 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3.4 Importance of NIR calibration maintenance The calibration maintenance of the NIR analyzer, to produce meaningful estimates of the engine octane number, is a key element in this study. This warrants further discussion as to its effective application. Throughout this study several reformate samples were analyzed and “paired” or synchronized with the NIR spectra. A cross validation correlation graph for the (adjusted) engine octane number is given for the 1156 samples collected to date (Figure 12). Using the PLSA algorithm from the GRAMS IQ package, a cross-validated R2 value of 0.986 and a standard error of cross validation of 0.355 octane units was obtained with 12 factors.

Figure 12. NIR vs. GC correlation of estimated engine octane for 1156 reformate samples The effective on-line engine octane analyses of the reformate to enable octane number control, was already indicated in previous sections.


27


Surprisingly, calibration maintenance of the NIR analyzer turned out to be less cumbersome than expected

20

with recalibrations only required when going far outside the range of the

existing models and/or when a new feed was introduced. Recalibration was initiated only when the Mahalanobis distance as automatically indicated by the NIR analyzer, was larger than 2. Normally, adding a small number of (

Catalytic Naphtha Reforming: A Novel Control System for the Bench

Recommend Documents