Predictive Modeling and Optimization for an Industrial Penex

Subscriber access provided by UNIV OF UTAH

Article

Predictive Modeling and Optimization for an Industrial Penex Isomerization Unit - A Case Study Mohanad M. Said, Tamer Samir Ahmed, and Tarek M. Moustafa Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 23 Nov 2014 Downloaded from http://pubs.acs.org on December 1, 2014

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.

Energy & Fuels 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 56

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

Energy & Fuels

Predictive Modeling and Optimization for an Industrial Penex Isomerization Unit - A Case Study Mohanad M. Saida, Tamer S. Ahmeda,*, Tarek M. Moustafaa a

Chemical Engineering Department, Faculty of Engineering, Cairo University Giza 12613, Egypt * Corresponding author: Tel.: +20 114 292 4407; E-mail address: [email protected]

Abstract. This work presents a model for UOP Hydrogen Once Through (HOT) Penex Process using Aspen HYSYS Petroleum Refining module. The model relies on routinely taken industrial data of process streams during normal operating conditions. Acquired data sets have been tested and screened to ensure data validity for building the model and avoiding erroneous results. A reaction network with 20 reactions and 19 components has been used for the reactors model. The reactors model has been validated using 4 months of industrial plant data. In addition, rigorous tray-to-tray simulation of isomerate stabilizer has been utilized to match the performance of plant stabilizer. The model validated has been used for studying the effects of each process variable on plant performance. In addition, the model has been used in optimizing the operating conditions of the process. This optimization showed a potential for notable fuel savings in the process.

1 `

ACS Paragon Plus Environment

Energy & Fuels

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. Introduction Since the 70’s, elimination of lead compounds from gasoline pool has increased interest into the isomerization of light straight run naphtha so as to preserve the octane number of produced gasoline1. Recent and future regulations put strict limits on benzene and aromatic levels in motor gasoline. For example, the U.S. Environmental Protection Agency MSATII (Mobile Sources Air Toxics Phase 2) regulations requires decreasing the average benzene content of U.S. gasoline pool to 0.62 vol.%2,3. This urged refiners to search for other sources of gasoline with a lower aromatic content than catalytic reforming. Isomerization is thought to be one of the effective solutions to produce motor gasoline compatible with environmental regulations. Isomerate (isomerization product) is highly desirable with respect to environmental regulations due to its zero benzene content and high octane number. Isomerization reactions are exothermic equilibrium-limited reactions. As conversion is far from equilibrium conversion, an increase in reactor temperature leads to increase in reaction velocity and subsequent increase in conversion. However, once equilibrium is approached, increasing temperature decreases conversion due to the decrease of reaction equilibrium constant4. Figure 1 shows the change of conversion (iso-paraffins yield) with temperature. As indicated in Figure 1, conversion increases with temperature until a certain temperature (optimum temperature) is reached, then iso-paraffins yield decreases.

INSERT FIGURE 1

Currently, three catalyst types are used commercially for naphtha isomerization (Figure 2). All of them are platinum containing catalysts6: 1) Zeolite catalysts: zeolite catalysts have the lowest activity among isomerization catalysts, hence they are used at higher temperatures that are unfavorable with respect to isomers yield. However, these 2 `


Page 2 of 56

Page 3 of 56

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

Energy & Fuels

catalysts have high resistance to feed impurities and can be regenerated. Zeolite catalyst units use fired heaters for feed and hydrogen heating up to the reaction temperature. The process also employs a high ratio of hydrogen to hydrocarbon for hydrotreating and dearomatization of feedstock. Accordingly, a recycle gas compressor and a product separator are used; 2) Chlorinated alumina catalysts: These are the most active isomerization catalysts providing the highest octane number and isomerate yield. These catalysts require continuous injection of a chlorine compound (CCl4) to maintain catalyst activity. In addition, they are very sensible to impurities (oxygen, sulfur and nitrogen compounds). Therefore, feed hydrotreating and drying is a mandatory. Low hydrogen to hydrocarbon ratio is required. Hence, neither a recycle gas compressor nor a recycle gas is needed; 3) Sulfated zirconia: These catalysts have the advantages of both previous types. They are more active than zeolite catalysts, hence favoring higher isomers conversions. In addition, they are resistant to impurities and regenerable. However, units using sulfated zirconia need a recycle gas compressor and a product separator.

INSERT FIGURE 2

As seen in Figure 2, chlorinated alumina catalysts provide the highest octane number and isomerate yield. By good operation of upstream hydrotreating unit and feed dryers, a long service life (more than 10 years) could be achieved2. UOP and Axens are the licensors of processes using chlorinated alumina-based catalysts. UOP licenses the process under the name of "Penex Process". The first Penex process was brought on stream at Borger Texas Refinery using I-3 catalyst7. Most of the literature concentrates on developing new types of catalysts8. A few research work paid attention to kinetic modeling of isomerization reactions and a fewer paid 3 `


Energy & Fuels

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

attention to modeling isomerization reactions over chlorinated alumina catalysts. Consequently, optimization for industrial isomerization units is very scarce in the open literature since the rigorous kinetic modeling for the reactor is the heart of the optimization process. Besl et al.9 discussed briefly the optimization of Penex process in a German refinery. On the other hand, Dudley and Malloy10 developed a simple kinetic model based only on isomerization and cracking reactions that was used in optimizing a process that uses AlCl3 liquid catalyst. Ahari and coworkers11 investigated the effects of methyl cyclopentane in the feed of isomerization feed using a process model developed by HYSYS. In addition, they studied experimentally the hydrogen partial pressure effect on Pt mordenite zeolite catalyst activity and conversion of n-paraffins and proposed kinetic equations for n-C5 and n-C6 conversion12. Brito et al.13 studied experimentally the performance of Pt-Ni/mordenite zeolite catalysts with different metal proportions and kinetic model for catalyst deactivation was proposed. Koncsag et al.14 proposed a kinetic model for C5/C6 isomerization over Pt/Hzeolite at industrial conditions. Surla et al.15 used a single event methodology to establish a kinetic model for C5/C6 isomerization over chlorinated alumina catalyst. Finally, a detailed kinetic model that is suitable for the three major catalyst types was proposed by Chekantsev et al.8. The reaction network they proposed contained 36 reactions. The authors illustrated that the differences in reaction rates over different catalysts are modest except for few isomerization reactions. The model they proposed agreed well with experimental results of the three catalyst types. Very little attention was paid in the literature to the application of kinetic modeling and optimization to an existing industrial unit. In this context, we present here kinetic modeling for an existing industrial Penex isomerization unit using Aspen HYSYS Petroleum Refining® isomerization reactor model. In addition, the model has been used for studying the effect of different process variables on process performance and for process optimization. 4 `


Page 4 of 56

Page 5 of 56

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

Energy & Fuels

2. Process Description Figure 3 shows a process flow diagram for Penex process. Feed naphtha and make-up hydrogen first pass through driers to eliminate any traces of water because water is a permanent poison of Penex catalyst. Then, naphtha and hydrogen are mixed prior to heating the mixture up to reaction temperature. Maintaining a proper hydrogen partial pressure is required inside the reactor to prevent coke deposition on catalyst. The reactor charge mixture is heated by exchanging heat with the second and first reactor effluent, respectively. A chlorine compound "CCl4" is injected into the reactor charge to provide acid sites on catalyst's surface that is required for isomerization reaction. The feed is brought up to the reaction temperature through a fired heater. The effluent of the first reactor is then cooled through exchangers prior to entering the second reactor to remove heat generated by exothermic reactions in the first reactor bed so that to favor equilibrium limited isomerization in the second reactor bed. The reactors' effluent is then fed to a stabilizer to separate light gases (C4- and hydrogen) from the product stream. The overhead gases is sent to a packed bed scrubber that employs a caustic wash to neutralize hydrogen chloride formed from the decomposition of the chlorine compound. Finally, the produced gases are sent to vapor recovery for LPG production. The stabilized isomerate may be sent directly to gasoline pool or may undergo fractionation to maximize the octane number of the isomerate.

INSERT FIGURE 3

3. Process Chemistry Praffin Isomerization. Paraffin isomerization is the main reaction in the process. As mentioned before, paraffin isomerization is an exothermic equilibrium limited reaction so that 5 `


Energy & Fuels

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

a higher conversion is favored at low temperatures. Table 1 lists the research octane numbers (RON) of n-C5 , n-C6 and their isomers. It is clear that multi branched isomers such as 2,2-dimethyl butane (2,2 DMB) and 2,3-dimethyl butane (2,3 DMB) have higher octane numbers than single branched isomers such as 2-Methyl Pentane (2MP) and 3-Methyl Pentane (3MP). Thus, formation of multi-branched isomers is highly desirable.

INSERT TABLE 1

Naphthenes isomerization. Naphthene isomerization reaction is also an equilibriumlimited reaction. Cyclohexane (CH) and methylcyclopentane (MCP) exist in equilibrium at reaction conditions but formation of MCP increases as temperature is increased.

Benzene Saturation. Benzene saturation to cyclohexane is a highly exothermic rapid reaction. The high heat of reaction of benzene saturation affects the conversion of the exothermic isomerization reaction that is favored at low temperature. This puts a limit on the amount of benzene that can be tolerated in reactor feed. All benzene in the feed is saturated completely in the lead reactor. Actually, the lead reactor temperature is always adjusted to maximize isomer ratios. If the benzene content of feed increases then a lower reactor inlet temperature is used to maximize isomers' conversion attained at lead reactor outlet and vice versa.

6 `


Page 6 of 56

Page 7 of 56

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

Energy & Fuels

Naphthene ring opening. Naphthenes may undergo ring opening and form paraffins at reactor conditions. Ring opening increases with increasing reactor bed temperature. Hydrocracking. As reactor temperature increases, hydrocracking rate increases. Large molecules (C7) are easier to crack than smaller molecules. As C5/C6 isomerization approaches equilibrium, the extent of hydrocracking increases leading to lower liquid yield and an increase in stabilizer overhead gas (C4-).

4. Reactor Model Aspen HYSYS v. 7.3 Petroleum Refining isomerization reactor module was used in developing the process model. The module contains a detailed kinetic model of reactions that occur in isomerization process. The reactor model contains rate equations for isomerization, benzene saturation, ring opening, hydrocracking, and heavy (C7+) reactions (Figure 4). The rate expression for each reaction class is coded to match literature data. All reactions are irreversible except for isomerization and benzene saturation. Typically, the reaction network consists of 20 reactions and 8 of them are reversible. Each reaction class is first order with respect to the primary reactant and reaction-class rate equation has a denominator following Langmiur-Hinshelwood-Hougen-Watson

mechanism.

The

reaction

scheme

contains

hydrocarbons up to C7. Higher carbon components are mapped into six ring C8 naphthenes. In reality, isomerization feed usually contains trace amounts of C7+ components. Reaction rate equations are expressed in the model as:

Reaction Rate = Global activity ⨯ Reaction-class Activity ⨯ Heterogeneous

(1)

Reaction Rate Reactor model can be tuned to match plant reactor performance via two schemes: basic tuning and advanced tuning schemes. Basic tuning includes the tuning of the activity parameters of the reactor such as global activity (overall reactor activity) and specific 7 `


Energy & Fuels

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

reaction-class activity parameters (activity of each type of reactions, e,g,: hydrocracking class reactions). Global activity parameter affects the rates of all the reactions. On the other hand, the activity of each specific reaction-class can be adjusted via the specific reaction-class activity parameter (e.g.: hydrocracking activity parameter for hydrocracking-class reactions). If basic tuning is not sufficient, kinetic parameters of each individual reaction may be tuned to match plant performance using the advanced tuning. The Penex unit under investigation is a HOT (Hydrogen-Once-Through unit). The process flow diagram of the unit is identical to that shown in Figure 3. The current work is only concerned with the isomerization reactors and the downstream stabilizer. Dryers and gas scrubber modeling are beyond the scope of this study. No changes or special procedures were applied for building the model and the model was calibrated according to samples that are routinely taken during normal operation.

INSERT FIGURE 4

Industrial Data. Data from the industrial unit under investigation were gathered and were organized into data sets. Each data set represents an operating day. Each data set includes a component analysis of all input and output streams (make-up hydrogen, feed naphtha, isomerate and stabilizer off-gas) and component analysis of the lead reactor product. In addition, each data set includes the operating conditions of the reactors and the stabilizing column. C7+ de-lumping. Isomerization feed usually contains a little amount of C7+ hydrocarbons and this amount could be controlled via the upstream naphtha splitter. Figure 5 shows the varaiation of the amount of C7+ in feed during study. The C7+ is mostly around 1 8 `


Page 8 of 56

Page 9 of 56

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

Energy & Fuels

wt%. The C7+ lump in isomerization feed is expected to be mostly normal heptane, heptane isomers and C7 naphthenes, with almost no toluene since the boiling point of toluene is 110°C, which is far from that existing in the isomerization feed. For accurate representation of C7+ lump, the de-lumping procedure developed by Riazi18 was used for the C7+ lump in the current study. The procedure uses correlations for estimating the paraffins, naphthenes and romatics (PNA) composition of petroleum fractions using only bulk properities such as specific gravity (SG) and molecular weight (MW). The results of PNA composition calculations for C7+ in feed are presented in Table 2. The results reinforce the postulation that toluene is negligable in the isomerization feed. Therefore, the C7+ fraction is de-lumped into C7 paraffins and C7 naphthenes with known percentage of each hydrocarbon class.

INSERT FIGURE 5 INSERT TABLE 2

Properities of a petroleum fraction of known composition could be calculated from the properities of constituent pure components by applying the proper mixing role:

= ⨯

(2)

Using this principle, an optimization algorithm can be used to estimate the composition of a petroleum fraction by minimizing the error between the known properities of that fraction and the calculated properities. In this work, the following objective function

was minimized:

9 `


Energy & Fuels

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

=

∥ − ∥ 100

Page 10 of 56

(3)

This is the same principle that was used in the literature to estimate the composition of catalytic reformer feed19,20. However, in the current work, a limited number of properties of C7+ are available (SG, MW, Reid Vapor Pressure (RVP) and RON). This limits the number of compositions that can be estimated. Using the preliminary de-lumping obtained through PNA composition calculation, two additional equations were added for compositions estimate. In this work, the C7+ fraction

was de-lumped into five pesudo components: multi-branched heptane paraffins (MBP7), single branched heptane paraffins (SBP7), normal heptane (NP7), five ring C7 naphthenes (5N7), and six ring C7 naphthenes (6N7). Properties of these pesudo components were obtained from Aspen properities data bank. Mole fractions of pesudo components were used for the calculation of RVP and MW and volume fractions were used for SG and RON. Table 3 shows a comparison between available properities and calculated properities for C7+ in feed and product streams. Estimated compositions of C7+ are shown in Table 4.

INSERT TABLE 3 INSERT TABLE 4

Data Screening. Model quality depends mainly on the data used. As indictaed earlier, data sets used for developing this process model were obtained during normal operation .Obtaining consistent data from industrial units may sometimes be an extreme difficult mission due to frequent changes in feed and process conditions. Data sets in days that 10 `


Page 11 of 56

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

Energy & Fuels

witnessed any up-normal changes in process operation were excluded. In addition, a mass balance test was applied to all data sets and any data sets having a mass balance error more than 2% were excluded. In order to ensure data verification, a hydrogen balance was applied also to data sets by calculating the hydrogen weight in each stream through summing the hydrogen contribution of each component and any sets having an error more than 3% were omitted. Taskar and Riggs20 used the following formula to calculate the weight fraction of hydrogen in component CiHj (H factor):

=

⨯ ⨯ + ⨯

(4)

The weight of hydrogen in each stream could be calculated by:

!"#$ $ %#& ' = &%% ( # %#& ' × ⨯

(5)

Apart from data sets in days that witnessed capacity changes, most data sets were quite consistent having an average mass balance and hydrogen balance error of -0.969% and -1.481%, respectively. Model Calibration and Parameters Estimation. Parameter estimation is the most critical step in model building. A well-calibrated model produces significant and repeatable predictions over a wide range of operating conditions. Improper calibration of reactor model may lead to an over calibrated model with a poor predictive power. Aspen HYSYS isomerization reactor model enables user to match plant performance through basic tuning and advanced tuning by adjusting kinetic parameters for each reaction. Model developers claim that basic tuning of reactor model is sufficient to match plant data. In this work, a basic tuning scheme was applied in order to avoid over calibration of reactor model. Advanced

11 `


Energy & Fuels

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 56

tuning may require extremely accurate plant data that were not feasible in our case. For accurate representation of plant reactor, the upcoming steps were followed:

A- Generating streams: Each data set was used to generate a stream representing the lead reactor charge at the day that each data set represents.

B- Running the model: The reactor model requires some mechanical data to run. These data include reactor dimensions and catalyst properties (catalyst bed porosity and bulk density).Then the model is allowed to run generating an output stream.

C- Model calibration: Reactor model calibration is mainly adjusting the activity parameters to match plant data. In this work, the reactor model was calibrated with the aid of Aspen HYSYS Optimizer® by minimizing the following objective function:

4

f*+ = * ⨯ *,-./0 − 1/230 +4 + + 5∆7,-./ − ∆71/23 8

(6)

Table 5 lists the adjustment factors used for reactor model calibration and applied bonds for each parameter. It was found that acceptable model performance is reached when using those bonds during calibration. It is important not to include yields of all components in reactor calibration. Pashikanti and Liu21 showed that including all measurements in reactor calibration may result in a poor calibrated model that responds wildly even to small changes to input variables. Table 6 shows the reactor measurements that were included in optimization function and weighting factors used with them.

12 `


Page 13 of 56

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

Energy & Fuels

INSERT TABLE 5 INSERT TABLE 6

Larger weighting factors were applied for terms where a closer fit is required. For example, the reactor model is required to fit the plant isomer ratios. Therefore, larger weighting factors were applied to normal and iso-paraffins. On the contrary, the lowest weighting factor was given to reactor temperature rise since the lead reactor temperature is frequently adjusted to maximize isomer ratios in the reactor effluent and hence the reactor temperature rise usually fluctuates during operation. In the available data sets, an average of reactor inlet and outlet temperature is only given. Therefore, temperature rise may be the least reliable data point in each data set and applying a high weighting factor for temperature rise may result in poor calibration. The three previous steps were repeated for each data set and activity parameters were calculated for each data set individually. Almost all activity parameters were found to vary within a narrow range for different data sets. Consequently, the average values of calculated activity parameters are expected to be quite satisfactory for model calibration. The same procedure was used for lag reactor calibration. The composition of the effluent of the calibrated lead reactor was used to simulate the composition of lag reactor feed at plant conditions. The lag reactor was calibrated by minimizing error between model and plant data using the same objective function. Routine sampling in the industrial unit does not include a sample for lag reactor effluent. Thus, the composition of lag reactor effluent was calculated through back-mixing of isomerate and stabilizer off-gas streams. The same activity parameters were used for model calibration and same bonds were used except for global activity parameter. In isomerization units, the lead reactor catalyst loses activity before lag 13 `


Energy & Fuels

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

reactor. When lead reactor performance becomes unsatisfactory, catalyst in the lead reactor is replaced with fresh catalyst and the order of reactors is reversed that the new loaded lead reactor lags the existing lag reactor (which is now the lead reactor). Therefore, lag reactor catalyst is always more active than lead reactor catalyst. Due to this fact, a higher upper bond was given to global activity parameter during lag reactor calibration. Table 7 shows the average activity parameters estimated through calibration process for both reactors.

INSERT TABLE 7

Model validation and testing. Figures 6 and 7 show the model performance versus plant yields for the 23 data sets used in reactor model calibration process for lead and lag reactors, respectively. The model predications are satisfactory for both reactor. It should be noted that a closer fit may be achieved with advanced tuning of kinetic parameters (especially the rate constants of ring opening reactions). However, advanced tuning requires strict plant measurements, which were not available.

INSERT FIGURE 6 INSERT FIGURE 7

In order to ensure model capability of predicting plant performance, the model yields were compared with plant yields for the next 4 months after calibration. Chlorinated alumina catalysts lose activity very slowly during normal operation (the catalyst used in the industrial unit under investigation was loaded from about 13 years and is still being used with good

14 `


Page 14 of 56

Page 15 of 56

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

Energy & Fuels

activity). Thus, changes in catalyst activity during the 4 months are negligible and recalibration of reactors' models is not required. Figures 8 and 9 show that the model was useful in tracking plant performance for the 4 months after the calibration.

INSERT FIGURE 8 INSERT FIGURE 9

Stabilizer Model. Although this study is concerned with the optimization of process variables of reaction section only, a precise stabilizer model is also required for accurate prediction of isomerate yield. The standard inside-out method was used for stabilizer model22. The inside-out method converges quickly with a wide variety of specifications. Since the isomerate stabilizer is used to adjust the RVP of isomerate product by limiting the amount of C4- in the isomerate product, it functions very similar to the function of a de-butanizer column. Data provided by Kaes23 show that the overall efficiency of a de-butanizer is 8590%. Since actual plant stabilizer has 30 trays, the model stabilizer should contain about 26 theoretical trays. According to the guidelines provided by Kaes23, stabilizer specifications were adopted. The function of isomerate stabilizer is to stabilize isomerate product by separating light ends from it. Therefore, the RVP of isomerate is an indication of the recovery of light ends and degree of separation achieved. Since the stabilizer operates with full reflux, another specification was needed for building stabilizer model. Therefore, the overhead condenser temperature was specified as a performance specification since the condenser operates with significant vapor product flows23. Figure 10 shows a complete process flow diagram of the process model. 15 `


Energy & Fuels

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 56

INSERT FIGURE 10

5. Process Variables Having established a process model for Penex unit, the next step is studying the effects of each process variable to predict the process performance for a given change in a process variable. Studying the effects of each process variable is essential for process optimization. The following study was conducted using the average composition of industrial unit feed during the study period (6 months). Lead Reactor Inlet Temperature. Isomerization reactions are equilibrium limited reactions; hence there is a maximum conversion (equilibrium conversion) that could be attained at each temperature (Figure 1). Because isomerization reactions are exothermic, the equilibrium conversion decreases as temperature increases. Whenever the reaction is far from equilibrium conversion, an increase in temperature leads to increase in reaction velocity and an increase in conversion (isomers yield and hence RON). However, once equilibrium is reached, increasing temperature increases backward reaction velocity and reduces conversion. This is reflected in Figure 11A, which shows the effect of lead reactor temperature on RON. Actually, i-pentane and 2,2-DMB are the components with the greatest effect on RON of isomerate. Therefore, the effect of temperature on (I-C5/C5)% and (2,2-DMB/C6) are identical with that of RON (Figure 11B and 11C). Although, the isomerization of 2,3-DMB is still far from equilibrium (Figure 11D), but it has little effect on RON. In addition, increasing temperature increases the rate of other reactions. Increased hydrocracking and ring opening reactions leads to increased hydrogen consumption (Figure 11E). Finally, the increase in hydrocraking leads to the decrease of of

16 `


Page 17 of 56

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

Energy & Fuels

isomerate yield (Figure 11F). Isomerate wt.% was defined in this study as the wt% of isomerate to the total reactor effluent.

INSERT FIGURE 11

Lag Reactor Inlet Temperature. For the lag reactor, the effect of the inlet temperature is similar in trend to that of the lead reactor (Figures 12 A-F).

INSERT FIGURE 12

Hydrogen to hydrocarbon mole ratio. Hydrogen is required for completing the reactions and decreasing coke deposition on catalyst surface. Generally, increasing inlet hydrogen leads to higher hydrogen partial pressure inside the reactor which increases hydrocracking. Consequently, isomerate yield (Figure 13A) and the RON of isomerate (Figure 13B) decrease due to increased hydrocracking of paraffins isomers (lower paraffin Isomerization Number (PIN = i-C5/C5 + (2,2-DMB+2,3-DMB)/C6)) (Figure 13C)). Therefore, the reactors should be operated at the lowest possible hydrogen to Hydrocarbon (H2:HC) ratio. However, at any instance, the H2:HC ratio should not be lower than 0.05 mole H2/mole Hydrocarbon as claimed by process licensor. The H2:HC mole ratio is usually measured at reactor outlet for Hydrogen Once Through Penex units.

17 `


Energy & Fuels

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 56

INSERT FIGURE 13

Feed rate (LHSV). At the same reactor inlet temperature, the lower the liquid hourly space velocity (LHSV), the higher the PIN (Figure 14A). On the contrary, the higher the LHSV, the higher the yield of isomerate due to lower hydrocracking (Figure 14B). This is expected since increasing feed rate decreases contact time with catalyst, which results in lower reactions rates.

INSERT FIGURE 14

Feed composition. A) Methylcyclopentane and cyclohexane: Generally, cyclic compounds adsorb on catalyst surface reducing active sites available for other reactants. Hence, an increase in methylcyclopentane (MCP) and cyclohexane (CH) % causes slightly lower isomer ratios in isomerate (Figures 15A and 15B). On the other hand, the RON of isomerate will be higher due to the increase of MCP or CH in isomerate, which has a relative high RON (Figures 15C and 15D). This contradicts what was reported by Ahari et al.11. They investigated the effects of MCP in isomerization feed and claimed that increasing the amount of MCP in feed results in reduction of isomerate RON. However, the authors carried out that study using a mixture of nhexane and MCP as a feed on a different catalyst operating at higher temperature. Therefore, at these conditions, the naphthene isomerization reaction is driven strongly in the direction of forming CH, which has a relatively lower RON compared to MCP and multi-branched hexane isomers. In addition, the higher operating temperature leads to increased ring opening of MCP

18 `


Page 19 of 56

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

Energy & Fuels

and CH, which lead to lower RON. Accordingly, the differences in feed composition and operating conditions may be the reason for the contradicting results. Isomerate yield increases with the increase of MCP or CH content in feed. Both cyclo components do not undergo hydrocracking as paraffins, however ring opening reactions may occur. Generally, ring opening reaction is favored slightly compared to hydrocracking with increasing temperature, because of its higher activation energy (e.g. MCP ring opening has a higher activation energy compared to C6 paraffins hydrocracking). Moreover, reduction of active sites available for paraffins reduces paraffins hydrocracking, and hence increases liquid yield (Figures 15E and 15F). Methylcyclopentane and cyclohexane are also major hydrogen consumers through ring opening. Therefore, hydrogen consumption increases as the amount of MCP or CH in the feed increases. It should be noted that the overall hydrogen consumption may not increase (Figures 15G and 15H) due to that the increase of MCP or CH content in feed is on the account of other components including benzene that is the major hydrogen consumer. However, the hydrogen consumption in the lag reactor shows an increase with the increase in MCP or CH content (Figures 15I and 15J).

INSERT FIGURE 15

B) Benzene: Figure 16A shows the actual variation of benzene content in the feed. As mentioned earlier, benzene is hydrogenated completely to CH in the lead reactor. Benzene saturation is highly exothermic reaction that is unfavorable by the equilibrium limited

19 `


Energy & Fuels

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 56

isomerization. Figure 16B shows the effect of feed benzene content on the lead reactor temperature rise. Small changes in feed benzene content greatly affect the lead reactor temperature rise. However, this does not affect isomerate RON much (Figure 16C) since benzene is converted to CH that has a moderate RON or may undergo isomerization to form MCP that has higher RON. On the other hand, the exothermic heat of benzene saturation affects PIN significantly (Figure 16D). In actual operation, the lead reactor inlet temperature is always varied in order to maintain a constant temperature rise through the reactor bed. The allowable temperature rise is increased with feed rate in order to obtain a reasonable conversion of normal paraffins. This operating scheme may be effective but it is very tedious, especially when wide variations in feed benzene content occur. Moreover, the increase in feed benzene content leads to an increase in isomerate yield (Figure 16E) since the produced CH or MCP undergo slow ring opening and adsorb on catalyst surface reducing active sites available for praffins hydrocracking. Finally, benzene saturation is the major hydrogen consuming reaction in Penex process. Figure 16F shows the increase in hydrogen consumption with increase of benzene in feed.

INSERT FIGURE 16

6. Model application to process optimization From previous analysis, it can be concluded that feed composition and feed rate are the dominant process variables. Benzene composition may be the most important variable. As seen 20 `


Page 21 of 56

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

Energy & Fuels

in Figure 16D, at constant inlet temperature to the lead reactor, the PIN is greatly affected by variation of feed benzene content. Thus, continuous monitoring of feed benzene content is always required. Although, feed benzene content does not change widely (Figure 16A), even a small change in benzene has a great impact on isomerization degree. Refiners offset this impact by continuous variation of lead reactor inlet temperature so as to obtain a fixed temperature rise through lead reactor. The amount of allowable temperature rise is increased as feed rate increases to increase reactions rates. This operating technique may be effective, however if the feed witnesses frequent variations in benzene level, this operating scheme will be nearly impossible. For more effective operation and more profits, some refiners developed rigorous models for real time optimization of penex process9. This shows the necessity of developing rigorous models in modern refineries. Another important variable is the H2:HC mole ratio inside the reactor. As indicated before, hydrogen is required for completing isomerization and reducing coke lay-down on catalyst. However, an increase in hydrogen partial pressure inside the reactors leads to an increase in hydrocracking and reduction of isomerate yield. Therefore, the reactors should be operated with the lowest possible hydrogen partial pressure. However, as a rule of thumb to avoid coke deposition, the reactors should not be operated at H2:H.C ratio less than 0.05 measured at lag reactor outlet at any time. Refiners face two operational scenarios in optimizing refinery processes. The first scenario is "what-if" scenario where the refiners want to predict the process performance if a change occurred to one or more of the process variables. This scenario has been covered in the previous analysis of operating variables. The other scenario is the "how-to" scenario. Refiners

21 `


Energy & Fuels

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

usually aim at maximizing profits. Hence, the frequent question in refiners' minds is "How to increase profit?" In order to optimize the process, the notes obtained from the first scenario should be implemented. Table 8 presents the base operating conditions of the investigated industrial unit and the limit bounds induced by process licensor. It is clear that the reactors are operated with H2:HC ratio much higher than required. Then, the first step was reducing hydrogen partial pressure to the lowest possible value with the same reactors' inlet temperature. Actually, the lowest possible H2:HC ratio at the reactors' outlet was 0.0865 which is still far from the allowable minimum.

INSERT TABLE 8

Lower H2:HC ratios could be obtained but either at lower lead reactor temperature or higher lag reactor inlet temperature because the amount of make-up hydrogen used affects heat transfer coefficients in both the hot and cold combined exchangers and consequently affects the outlet temperatures. Having obtained the lowest possible hydrogen partial pressure in reactors, the next step was varying the lead and lag reactor inlet temperature simultaneously so as to obtain the optimum operating point. Figures 17, 18 and 19 show the obtained results. From Figure 17, the lower the reactors inlet temperatures, the higher the isomerate yield. Therefore, it is always the refiner's decision to raise the reactors temperature to increase the isomerization degree (Figure 18) to obtain a higher RON (Figure 19) with the sacrifice of

22 `


Page 22 of 56

Page 23 of 56

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

Energy & Fuels

isomerate yield. Table 9 shows a comparison between plant yields at base point and optimum operating point.

INSERT FIGURE 17 INSERT FIGURE 18 INSERT FIGURE 19 INSERT TABLE 9

Results shown in Table 9 indicate that about 210 kg/hr can be added to the total regular isomerate yield by following the optimization scheme. Savings in isomerate yield are mainly attributed to decreased hydrocracking by reducing hydrogen partial pressure inside reactors. In addition, it is shown that the reactor charge heater duty has declined due to reduced inlet temperature of lead reactor and reduced load on heater by eliminating a part of make-up hydrogen that was used originally. However, the decrease in reactor charge-heater duty is overcome by the increase in stabilizer bottom reboiler, resulting in apparent energy deficiency of about 96,635 kJ/hr. However, following the optimized operating scheme will not only improve isomerate yield and RON, but it will also reduce the consumption of make-up gas. Make-up gas from naphtha reformer may be directed to fuel gas system in the refinery. The new operating scheme will add about 161 kg/hr of make-up gas to the fuel gas system. This will provide approximately 11.3 x 106 kJ/hr. This may not only cover the increase in stabilizer bottom reboiler duty, but also it may cover the total energy consumption of the fired heaters in the Penex unit. It should be noted here that savings in energy and product yield are proportional to plant capacity. 23 `


Energy & Fuels

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 56

Finally, the model may be used to predict the plant performance at various operating points. The model is extremely useful for predicting suitable operating conditions, especially when plant experiences variations in feed benzene content. This can be achieved by incorporating the model into a real time optimization (RTO) scheme. In this case, it is recommended that the model be finely tuned to match plant performance closely.

7. Conclusions A process model for an industrial Penex process was developed using Aspen HYSYS Petroleum Refining isomerization reactor module. The model could track the plant performance satisfactorily. In addition, the model was used for studying the effect of each process variable on process performance. Among all process variables, benzene feed content and H2:HC ratio were the most prominent factors to affect the process performance. Finally, the model was used for optimizing process at steady state conditions. Results obtained from the model showed that considerable savings in product yield can be achieved by lowering hydrogen partial pressure inside reactors to the lowest possible practical value. The surplus in make-up gas may be directed to fuel gas resulting in significant fuel savings. The model may also be incorporated in a real time optimization (RTO) scheme. In this case, the model should be finely tuned to match plant performance closely.

Nomenclature A6

Benzene

C1

Methane

24 `


Page 25 of 56

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

Energy & Fuels

C2

Ethane

C3

Propane

i-C4

Iso butane

n-C4

Normal butane

i-C5

Iso pentane

n-C5

Normal pentane

n-C6

Normal hexane

n-C7

Normal heptane

CH

Cyclo hexane

2,2-DMB

2,2 Dimethyl butane

2,3-DMB

2,3 Dimethyl butane

H factori

factor of component i

MCH

Methyl cyclo hexane

MCP

Methyl cyclopentane

MBP7

Multi branched heptanes

2MP

2-Methyl pentane

3MP

3-Methyl pentane

5N5

Cyclo pentane

5N7

Five ring, seven carbon naphthene

25 `


Energy & Fuels

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

6N7

Six ring, seven carbon naphthene

NP7

Normal heptane

SBP7

Single branched heptanes

∆7,-./

Temperature rise in reactor model

∆71/23

Temperature rise in plant reactor

Property of a petroleum fraction

Property of a pure component i

Mole , volume or mass fraction of component i

Known property of C7+ fraction

Weighting factor for component i

,-./0

Mass fraction of component i in model outlet stream

1/230

Mass fraction of component i in plant reactor outlet stream

PIN

Paraffin Isomerization Number = i-C5/C5 + (2,2-DMB+2,3-DMB)/C6

References (1)

Moulijn, J. A.; Makkee, M.; van Diepen, A. E. Chemical Process Technology; 2nd ed.; Wiley, 2013.

(2)

Deak, V. G.; Rosin, R. R.; Sullivan, D. K. In AICHE 2008 Spring National Meeting; New Orleans, LA, USA, 2008.

(3)

Laredo, G. C.; Castillo, J.; Cano, J. L. Fuel 2014, 135, 459–467.

26 `


Page 26 of 56

Page 27 of 56

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

Energy & Fuels

(4)

Smith, J. M.; van Ness, H. C.; Abbott, M. M. Introduction to Chemical Engineering Thermodynamics; 6th ed.; McGraw-Hill, 2001.

(5)

Yasakova, E. A.; Sitdikova, A. V; Achmetov, A. F. Oil Gas Bus. 2010.

(6)

Meyers, R. A. Handbook of Petroleum Refining Processes; 3rd ed.; McGraw-Hill, 2004.

(7)

Dean, L. E.; Harris, H. R.; Belden, D. H.; Haensel, V. Platin. Met. Rev. 1959, 3, 9–11.

(8)

Chekantsev, N. V.; Gyngazova, M. S.; Ivanchina, E. D. Chem. Eng. J. 2014, 238, 120– 128.

(9)

Besl, H.; Kossman, W.; Crowe, T. J.; Caracotsios, M. Oil Gas J. 1998, 96, 61–64.

(10)

Dudley, R. E.; Malloy, J. B. Ind. Eng. Chem. Process Des. Dev. 1963, 2, 239–244.

(11)

Ahari, J. S.; Ahmadpanah, S. J.; Khaleghinasab, A.; Kakavand, M. Pet. Coal 2005, 47, 26–31.

(12)

Ahari, J. S.; Khorsand, K.; Hosseini, A. A.; Farshi, A. Pet. Coal 2006, 48, 42–50.

(13)

Brito, K. D.; Sousa, B. V.; Rodrigues, M. G. F.; Alves, J. J. N. Brazilian J. Pet. Gas 2008, 2, 1–8.

(14)

Koncsag, C. I.; Tutun, I. A.; SAFTA, C. Ovidius Univ. Ann. Chem. 2011, 22, 102–106.

(15)

Surla, K.; Guillaume, D.; Verstraete, J. J.; Galtier, P. Oil Gas Sci. Technol. 2011, 66, 343– 365.

(16)

Ghosh, P.; Hickey, K. J.; Jaffe, S. B. Ind. Eng. Chem. Res. 2006, 45, 337–345.

(17)

Perdih, A.; Perdih, F. Acta Chim. Slov. 2006, 53, 306–315.

(18)

Riazi, M. R. Characterization and Properties of Petroleum Fractions; 1st ed.; ASTM International, 2005.

(19)

Mahdavian, M.; Fatemi, S.; Fazeli, A. Int. J. Chem. React. Eng. 2010, 8, A8.

(20)

Taskar, U.; Riggs, J. B. AIChE J. 1997, 43, 740–753.

(21)

Pashikanti, K.; Liu, Y. A. Energy & Fuels 2011, 25, 5320–5344.

(22)

Russell, R. A. Chem. Eng. - New York 1983, 90, 53–59.

(23)

Kaes, G. L. Refinery Process Modeling: A Practical Guide to Steady State Modeling of Petroleum Processes; 1st ed.; Athens Printing Company: Athens, GA, USA, 2000. 27

`


Energy & Fuels

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 28 of 56

Tables

Table 1: RON of normal and iso paraffins16,17 RON

Component

62

n-pentane

92

i-pentane 2-Methyl Pentane (2MP) 3-Methyl Pentane (3MP)

74.5

2,2 dimethyl butane (2,2-DMB)

91.8

2,3 dimethyl butane (2,3-DMB)

105.8

n-hexane

24.8

28 `

73.4


Page 29 of 56

Energy & Fuels

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

Table 2: PNA composition of C7+ fraction Hydrocarbon class

Vol.%

Paraffins

74.24

Naphthenes

23.53

Aromatics

2.22

29 `


Energy & Fuels

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 30 of 56

Table 3: Available and calculated Properties of C7+ fraction in feed and product streams Property

C7+ (Feed)

C7+ (Product)

Given

Calc.

Given

Calc.

SG

0.6915

0.7042

0.683

0.709

MW

100.198 99.666

100.198

99.70

RVP (psi)

1.97

1.97

2.1

2.187

RON

55

55

82

82

30 `


Page 31 of 56

Energy & Fuels

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

Table 4: Estimated composition of C7+ fraction in feed and product streams Compnent

Feed (wt.%)

Product (wt.%)

MBP7

≈0

48.2

SBP7

52.7

27.3

N-C7

21.2

≈0

5N7

10.1

12.2

6N7

16

12.3

31 `


Energy & Fuels

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 32 of 56

Table 5: Adjustment factors used for reactor model calibration Range of deviation Parameter from the base Global activity

0.1-1

Isomerization activity

0.1-1.1

Hydrocracking activity

0.1-1.1

Hydrogenation activity

0.1-1.1

Ring opening activity

0.1-1.1

Heavy activity

0.1-1.1

32 `


Page 33 of 56

Energy & Fuels

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

Table 6: Terms included in objective function for reactor model calibration and applied weighing factors Term

Applied weighing factor

Temperature Rise (°C)

1

I-Pentane (wt%)

5

N-Pentane (wt%)

5

Cyclo Pentane (wt%)

2

2 Methyl Pentane (wt%)

5

3 Methyl Pentane (wt%)

5

2,2 Di-Methyl Butane (wt%)

5

2,2 Di-Methyl Butane (wt%)

5

N-Hexane (wt%)

5

Methyl Cyclo Pentane (wt%)

2

Benzene (wt%)

2

Cyclo Hexane (wt%)

2

C7+ (wt%)

2

33 `


Energy & Fuels

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 34 of 56

Table 7: Estimated average activity parameters for lead and lag reactors Parameter

Lead Reactor

Lag reactor

Global activity

0.912

2.402

Isomerization activity

0.827

1.092

Hydrocracking activity

1.023

0.968

Hydrogenation activity

0.939

0.995

Ring opening activity

1.054

1.042

Heavy activity

1.07

1.059

34 `


Page 35 of 56

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

Energy & Fuels

Table 8: Operating conditions of the unit at the base operating point and limit bounds induced by process licensor Process Variable

Base Operating Point

Bounds

H2:HC ratio

0.1565

min. 0.05

Lead Reactor Inlet T

124 °C

105 - 204°C

Lag Reactor Inlet T

120°C

105 - 204°C

35 `


Energy & Fuels

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 36 of 56

Table 9: Process performance at base and optimum operating point Reactor Lead

Lag

Make-up

Reactor

Reactor

Hydrogen

Inlet T

Inlet T

Flow

(°C)

(°C)

(kg/hr)

Stabilizer Charge

Isomerate Reboiler

Heater

Yield

PIN

Duty Duty

(kg/hr) (kJ/hr)

(kJ/hr) Base Operating

124

120

614.2

1,018,275

9,452,829

31,693

1.18

116

117

453

721,645

9,846,094

31,903

1.184

Point Optimum Operating Point

36 `


Page 37 of 56

Energy & Fuels

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

Figures

Figure 1: Effect of temperature on isomers yield5

37 `


Energy & Fuels

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 2: Comparison of the activity of different isomerization catalysts5

38 `


Page 38 of 56

Page 39 of 56

Energy & Fuels

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 3: UOP Penex process6

39 `


Energy & Fuels

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 4: Isomerization model reaction network

40 `


Page 40 of 56

Page 41 of 56

Energy & Fuels

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

6 5

C7+ wt.%

4 3 2 1 0 25-Feb

16-Apr

5-Jun

25-Jul

13-Sep

Figure 5: Variation of C7+ wt% in the feed

41 `


Energy & Fuels

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 42 of 56

40 35 i-C-5% 30

n-C5 5N5

Model (wt%)

25

2,2DMB 20

2,3 DMB 2MP

15

3MP 10

N-C6 5N6

5 0 0

5

10

15

20 25 Plant (wt.%)

30

35

40

Figure 6: Plant versus model yields with model calibration data sets forlead reactor

42 `


Page 43 of 56

Energy & Fuels

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

45 40 i-C-5% 35

n-C5

30 Model (wt%)

5N5

25

2,2DMB

20

2,3 DMB 2MP

15

3MP

10

N-C6

5

5N6

0 0

5

10

15

20 25 Plant (wt.%)

30

35

40

45

Figure 7: Plant versus model yields with model calibration data sets for lag reactor

43 `


Energy & Fuels

40 35 i-C-5% 30

Model (wt%)

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 44 of 56

n-C5 5N5

25

2,2DMB 20 2,3 DMB 2MP

15

3MP

10

N-C6 5 5N6 0 0

5

10

15

20

25

30

35

40

Plant (wt%)

Figure 8: Plant versus model yields for the 4 months after calibration for lead reactor

44 `


Page 45 of 56

Energy & Fuels

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

40 35

i-C-5% n-C5

30

5N5 Model (wt%)

25 2,2DMB

20

2,3 DMB 2MP

15

3MP 10

N-C6 5N6

5 0 0

5

10

15

20 25 Plant (wt%)

30

35

40

Figure 9: Plant versus model yields for the 4 months after calibration for lag reactor

45 `


Energy & Fuels

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 10: Penex isomerization unit model

46 `


Page 46 of 56

Page 47 of 56

(B)

(A)

74 73.8

(I-C5/C5)%

RON

84.15 84.14 84.13 84.12 84.11 84.1 84.09 84.08 84.07 84.06

73.6 73.4 73.2 73 72.8

105

110 115 120 Lead Reactor Inlet Temp. °C

72.6

125

105

110

115

120

125

Lead Reactor Inlet Temp. °C

(D) 8.55

(2,3DMB/C6)%

(2,2DMB/C6)%

(C) 27.8 27.6 27.4 27.2 27 26.8 26.6 26.4 26.2 26 25.8

8.5 8.45 8.4 8.35 8.3

105

110 115 120 Lead Reactor Inlet Temp. °C

105

125

110

115

120

125


(E)

(F) 95.28

908

95.27

906

Isomerate wt.%

Hydrogen consumption (STD_m3/hr)

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

Energy & Fuels

904 902 900 898 896

95.26 95.25 95.24 95.23 95.22 95.21 95.2 95.19

894 892 105

110

115

120

95.18

125

105


110

115

120

125


Figure 11: A- Effect of Lead reactor inlet temperature on RON; B- Variation of (I-C5/C5)% with lead reactor inlet temperature; C- Variation of (2,2DMB/C6)% with lead reactor inlet temperature; D- Variation of (2,3DMB/C6)% with lead reactor inlet temperature; E- Effect of lead inlet reactor temperature on hydrogen consumption in lead reactor; F- Effect of lead reactor inlet temperature on isomerate yield. Lag reactor inlet temperature = 120 °C, H2:HC = 0.1565, A6 = 2.99 wt.%, LHSV = 1.15 hr-1

47 `


Energy & Fuels

(A)

(B)

84.12

76.5

84.1

76.45

RON

(I-C5/C5)%

84.14

76.55

84.08 84.06

76.4 76.35

84.04

76.3

84.02

76.25 106

111 116 121 Lag Reactor Inlet Temp. (°C)

126

106


126

(D) 32.9

8.62 (2,3DMB/C6)%

32.8 (2,2DMB/C6)%

8.64

(C)

32.7 32.6 32.5

8.6 8.58 8.56 8.54 8.52

32.4

8.5

32.3

8.48 106

111

116

121

106

126

Lag Reactor Inlet Temp. (°C)


(E) 4 3.5 3 2.5 2 1.5 1 0.5 0 106


126

95.22 95.215 95.21 95.205 95.2 95.195 95.19 95.185 95.18 95.175 110

115 120 Lag Reactor Inlet Temp. (°C)

Figure 12: A- Effect of Lag reactor inlet temperature on RON; B- Variation of (I-C5/C5)% with lag reactor inlet temperature; C- Variation of (2,2DMB/C6)% with lag reactor inlet temperature; D- Variation of (2,3DMB/C6)% with lag reactor inlet temperature; E- Effect of lead inlet reactor temperature on hydrogen consumption in lag reactor; F- Effect of lag reactor inlet temperature on isomerate yield. Lead reactor inlet temperature = 124 °C, H2:HC = 0.1565, A6 = 2.99 wt%, LHSV = 1.15 hr-1

48 `

126

(F) Isomerate wt.%

Hydrogen consumption (STD_m3/hr)

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 48 of 56


125

Page 49 of 56

(A)

96.4

Isomerate wt.%

96.2 96 95.8 95.6 95.4 95.2 95 0.07

0.09

0.11

0.13

0.15

0.17

H2:HC

RON

(B) 84.22 84.21 84.2 84.19 84.18 84.17 84.16 84.15 84.14 84.13 84.12 0.07

0.09

0.11 0.13 H2:HC

0.15

0.17

(C)

1.183 1.1825 1.182 PIN

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

Energy & Fuels

1.1815 1.181 1.1805 1.18 1.1795 0.08

0.1

0.12 H2:HC

0.14

0.16

Figure 13: A- Effect of H2:HC ratio on isomerate yield; B- Effect of H2:HC ratio on RON of isomerate; C- Effect of H2:HC ratio on PIN. Lead reactor inlet temperature = 124°C, lag reactor inlet temperature = 120 °C, A6 = 2.99 wt%, LHSV = 1.15 hr-1

49 `


Energy & Fuels

(A)

1.11

(B) 95.3 95.28 Isomerate wt.%

1.1 1.09 PIN

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 50 of 56

1.08 1.07 1.06

95.26 95.24 95.22 95.2 95.18 95.16

1.05

95.14 105

110

115

120

125

105

Lead Reactor inlet T 1.03 hr-1

1.15 hr-1

110

115

120

1.3 hr-1

1.03hr-1

1.15hr-1

1.3 hr-1

Figure 14: A- Effect of feed rate (LHSV) on PIN; B- Effect of feed rate (LHSV) on isomerate yield. Lag reactor inlet temperature = 120 °C, H2:HC = 0.1565, A6 = 2.99 wt%

50 `

125

Lead Reactor T


Page 51 of 56

(A) 1.181 PIN

1.18 1.1795 1.179 0

0

2 4 6 8 Feed MCP content, wt% (base 4.08%)

0

2

4

6

0.5

1

1.5

(D)

84.18 84.17 84.16 84.15 84.14 84.13 84.12 84.11 84.1 84.09

8

0

Feed MCP content, wt% (base 4.08%)

0.5

1

1.5

2

Feed CH content, wt% (base 1.18%)

(E)

(F) 95.26

95.4

95.24 Isomerate wt.%

95.5

95.3 95.2 95.1

95.22 95.2 95.18 95.16 95.14

95

95.12

94.9 0

95.1


0

0.5 1 1.5 Feed CH content, wt% (base 1.18%)

51 `

2

Feed CH content, wt% (base 1.18%)

(C)

84.3 84.25 84.2 84.15 84.1 84.05 84 83.95

(B)

1.18015 1.1801 1.18005 1.18 1.17995 1.1799 1.17985 1.1798 1.17975

RON

RON

PIN

1.1805

Isomerate wt.%

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

Energy & Fuels


2

Energy & Fuels

(H) Total Hydrogen Consumption (STD_m3/hr)


8

914 912 910 908 906 904 902 0 0.5 1 1.5 Feed CH content, wt% (base 1.18%)

(I)

3.2 3 2.8 2.6 2.4 2.2 2 0

2

4

6

8

2.85 2.8 2.75 2.7 2.65 2.6 0

Feed MCP content, wt%(base 4.08%)

0.5 1 1.5 2 Feed CH content, wt% (base 1.18%)

Figure 15: A- Effect of feed MCP content on PIN; B- Effect of feed CH content on PIN; CEffect of feed MCP content on RON; D- Effect of feed CH content on RON; E- Effect of feed MCP content on isomerate yield; F- Effect of feed CH content on isomerate yield; GEffect of feed MCP content on total hydrogen consumption; H- Effect of feed CH content on total hydrogen consumption; I:- Effect of feed MCP content on hydrogen consumption in the lag reactor; J- Effect of feed CH content on hydrogen consumption in the lag reactor. Lead reactor inlet temperature = 124 °C, lag reactor Inlet temperature = 120°C, LHSV = 1.15hr-1

52 `

2

(J) Hydrogen Consumption in lag reactor (STD_m3/hr)

Total Hydrogen Consumption (STD_m3/hr)

(G) 930 925 920 915 910 905 900 895 890

Hydrogen Consumption in lag reactor (STD_m3/hr)

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 52 of 56


Temperature Rise in lead reactor

(A)

25-Jun 14-Aug

0

3-Oct

(C)

84.25 84.2 84.15 84.1 84.05 84 83.95 83.9 83.85

(B)

45 40 35 30 25 20 15 10 5 0

0

(D)

1.1 1.095 1.09 1.085 1.08 1.075 1.07 1.065 1.06

2

4

6

0

Feed benzene, wt% (base 2,99%)

2 4 6 Feed benzene content, wt% (base 2.99%)

(E)

(F) Hydrogen Consumption in lead reactor (STD_m3/hr)

96 95.8 95.6 95.4 95.2 95 94.8 94.6 94.4

2 4 6 Feed benzene content, wt% (base 2.99%)

PIN

RON

5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 26-Jan 17-Mar 6-May

Isomerate wt%

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

Energy & Fuels

A6 wt%

Page 53 of 56

0 2 4 6 Feed benzene content, wt% (base 2.99%)

1600 1400 1200 1000 800 600 400 200 0 0 2 4 6 Feed benzene content, wt% (base 2.99%)

Figure 16: A- Variation of feed benzene content; B- Effect of feed benzene content on lead reactor temperature rise; C- Effect of feed benzene content on isomerate RON; D- Effect of Feed benzene content on PIN in the lead reactor; E- Effect of feed benzene content on isomerate yield; F- Effect of feed benzene content on hydrogen consumption in lead reactor. Lead reactor inlet temperature = 124 °C, lag reactor inlet temperature = 120°C, LHSV = 1.15 hr-1

53 `


Energy & Fuels

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 54 of 56

96.49 96.49-96.5

Isomerate wt %

96.47

96.47-96.49

96.45

96.45-96.47 96.43

96.43-96.45

96.41

96.41-96.43 96.39-96.41

96.39

96.37-96.39 96.37 117 115 Lead Reactor T, oC

113

111

109

107112

114

116

118

120

124 122

Lag reactor T, oC

Figure 17: Variation of isomerate yield with reactors inlet temperatures

54 `


Page 55 of 56

Energy & Fuels

PIN

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

1.186

1.184-1.186

1.184

1.182-1.184

1.182

1.18-1.182

1.18

1.178-1.18

1.178

1.176-1.178

1.176

1.174-1.176

1.174

1.172-1.174

1.172

1.17-1.172

1.17

124 117

115

Lead Reactor Inlet T, oC

121 113

111

118 109

107 112

115

Lag Reactor Inlet T, oC

Figure 18: Variation of Paraffin Isomerization Number with reactors inlet temperatures

55 `


Energy & Fuels

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 56 of 56

84.24 84.22 84.24-84.25 84.22-84.24 84.2-84.22 84.18-84.2 84.16-84.18 84.14-84.16 84.12-84.14 84.1-84.12 84.08-84.1

RON

84.2 84.18 84.16 84.14 84.12 84.1 84.08 117 114 Lead reactor inlet T, oC

111 108

112

114

116

118

120

122

124

Lag reactor inlet T, oC

Figure 19: Variation of isomerate RON with reactors inlet temperatures

56 `


Predictive Modeling and Optimization for an Industrial Penex

Recommend Documents