Energy consumption and heat recovery of an industrial fluidized catalytic cracking process based on cost savings
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Abstract
Energy consumption is a significant issue in operation design for lowcost sustainable production and is accomplished by heat integration giving overall environmental advantages via reducing carbon emissions. Heat recovery is a beneficial tool that determines the minimum cooling and heating demand through recovery and reuse of energy within the process. Thus in this study, process of heat recovery and energy consumption of the fluidized catalytic cracking (FCC) is investigated to recover most of the external energy and reducing the environmental effect in addition to maximizing the productivity with minimum overall cost of the process. Where the performance of the FCC units plays a major role on the overall economics of refinery plants and improvement in operation or control of FCC units, it will result in dramatic economic benefits. The heat integration process is done based on experimental information from pilot scale, mathematical modeling developed and commercial process reported in our earlier study.
Keywords
FCC Heat integration Energy consumption Heat recovery Cost savingsIntroduction
One of the assignments with which chemical engineers constantly tend to be involved is scaling up of pilot plant tests to fullscale production. Because of the high operating cost of a pilot plant, this progression is beginning to be outperformed in different cases by generating the fullscale unit depending on the operation of a smallscale plant named a microplant. With a specific end goal making such manner effectively, an exhaustive comprehension of the chemical kinetics and transport restrictions is essential [1, 8]. The evaluation of the optimal cooling and heating prerequisites (the minimum) uncover imperative energy savings. For example, Union Carbide in the United States of America and Imperial Chemical Industries in the United Kingdom have both detailed the consequences of various case examinations that reference 30–50% energy savings in contrast with traditional practice [5]. CO_{2} contents in the air have risen from 270 to 380 ppm by 2006. The essential human source of CO_{2} in the air is generated by consuming petroleum derivatives towards energy generation and transportation opportunities. To keep away from or lessen global warming, an important concentration in all CO_{2} emissions should be accomplished (25%–40% by 1990 and 80%–95% by 2050) [6, 8]. Nevertheless, more productive usage of energy consumption reduces the negative effects of CO_{2} discharges into the environment.
Industrial petroleum refining units generate huge amount of heat, which is normally ejected to the environment using either air or cooling water frameworks. Some processes are employed to recover such energy as a part of integration process system as well as for heating in local and commercial operations via hot water network [12].
Energy saving is very significant in process design, and estimating minimum cooling and heating requirement is very important in energy savings issues. Heat integration is a very beneficial tool used in calculating the cost of initial design and waste heat recovery provides environmental benefits for handling unit operators. A wide range of units, such as oil refinery and other industrial processes generate a large amount of heat that is discarded to the environment without reusing in other processes. Such behavior takes into account the recovery of some energy and a part of it is used in process integration. The cooling and heating units are called utility units involving hot utility and cold utility. Hot utility includes furnace, boilers, hot water, steam, and generators. Cold utility includes cold water from external source. In recovery system, the process streams exchange heat so as to reduce the cold and hot utility requirements of the heat exchangers, which are the only units in a heat recovery system. Heat exchanger is a unit in which heat is transferred from the hot fluid to the cold fluid. Conventional outline strategies start by planning the reactor, the separation system, the heat exchanger and lastly end by using utilities for providing residual requirements [2, 5, 11, 13].
More recently, we [7] employed the FCC reactor to optimize the process conditions of the operation for the purpose of maximizing the conversion as well as the octane number while minimizing the coke amount in the regenerator based on pilot plant experiments at a very high temperature (460 °C–540 °C) requiring high energy consumption in the process. Also, energy consumption for the lab scale is an unimportant issue (ignored) and no extra utility was needed as the quantities of reactants and products were little at laboratory scale, hence recovery issues were not a point in the laboratoryscale operation. However, the design when scaledup to a commercial level [7] offers the chance of energy savings via appropriate process integration. In commercial operation, heat recovery and energy consumption should be taken into account to decrease environmental effect in addition to reducing the treatment operation cost. Therefore, the main focus of this study is to maximize heat recovery of an industrial fluidized catalytic cracking reactor utilizing the mathematical model developed earlier [7] and then analyze the heat integration process while minimizing overall annual cost of such process.
The experimental data
The experimental results have been taken from literature [16]. A brief description of the materials, apparatus and experimental procedure used for getting the experimental results are as follows.
Energy consumption and heat recovery of the FCC unit
In the pilot plantscale process, heat recovery was not an issue and the energy consumption was negligible, while in industrial processes, energy consumption will be a big issue and heat recovery must be taken into account. A heat integrated system was considered for reducing overall energy consumption (hence reducing environmental effect). However, it is important to incorporate for heat exchangers in the process system. Evaluating the required design is an important issue to minimize the energy consumption and to maximize the energy recovery hence consequently minimizing the capital investment.
In general, heat exchanging is working in series with heating and cooling systems. The heater controls the final temperature of the cold liquid to the needed reaction temperature, and the cooler alters the final temperature of the hot liquid to prerequisites of the subsequent stage of the procedure. It is shown in Fig. 2 that the feed stock is pumped by a pump (PU) into a heat exchanger 1 (H.E.1) and heated from T_{in} to T_{in1} it is fed into a heat exchanger 2 (H.E.2) to further heating from T_{in1} to T_{in2} to the required temperature of the reaction T_{R} then it is fed to the furnace. The reaction occurs inside a reactor (R), and after completion the hot product stream leaves the reactor and goes to the fractionators (FR). Then it is cooled from T_{out} to T_{out1} via heat exchanger 1 (H.E.1) by contacting with the main feed stock and further cooling via heat exchanger 2 (H.E.2) from T_{out2} to T_{out3}. The final product temperature is cooled from T_{out,1} to T_{F1} via cooler (C1) by contacting with a cold water stream at temperature T_{W,1} that is heated into T_{W,2}. The final product of the other stream temperature is cooled from T_{out,3} to 2 via cooler (C2) by contacting with the cold water stream at temperature T_{W,3}, which is heated into T_{W,4}.
Process model equations
The main concern of this study is to minimize energy consumption and maximize heat recovery of an industrial fluidized catalytic cracking unit. More recently, we [7] have developed a new mathematical model for the fluidized catalytic cracking operation taking into consideration the complex hydrodynamics of the reactor regenerator system based on a new sixlump kinetic model for the riser. The best kinetic parameters of the relevant reactions have been evaluated employing the optimization technique depending upon the experimental results taken from literature. The optimal operating conditions (mainly, reaction temp (T), catalysttooil ratio (CTO) and weight hourly space velocity (WHSV) on the product composition were investigated and the optimal kinetic parameters obtained from the pilot plant scale were used to develop an industrial fluidized catalytic cracking process. The optimal operating conditions based on maximum conversion of vacuum gas oil with minimum cost in addition to maximizing the octane number of gasoline and coke content deposited on the catalyst within the regenerator, have also been studied. Thus, mass balance equations, energy balance equations, reaction rate equations, catalyst deactivation with the characteristics of the catalyst bed utilized, riser hydrodynamics, regenerator model, dense bed modelling, dilute phase modelling, physical and chemical properties of the reactants and products, equipment and procedure, scaleup of FCC reactor with their optimization processes and results related to the optimal design and operation of such unit can be found with more detail in Jarullah et al. [7]. Therefore, the equations related to the heat exchangers of an industrial FCC reactor utilizing the optimal results obtained previously [7] can be stated as follows:
Heat exchanger 1 (H.E.1)
Heat exchanger 2 (H.E.2)
Cooler 1 (C1)
Cooler 2 (C2)
Furnace 1 (F1)
Optimization problem formulation

Heating cost (\(C_{\text{Heating}}\)) ($/yr):
$$C_{\text{Heating}} \left( {\$ /{\text{year}}} \right) = \left( {Q_{F} ({\text{kW}}) } \right)\left( {\frac{0.06\$ }{\text{kWh}}} \right)\left( {\frac{{24{\text{h}}}}{{1{\text{day}}}}} \right)\left( {\frac{{342{\text{days}}}}{{1{\text{year}}}}} \right)$$(38) 
Pumping cost (\(C_{\text{Pumping}}\))($/yr):
$$C_{\text{pumping}} \left( {\$ /{\text{year}}} \right) = \left( {Q_{p} ({\text{kW}}) } \right)\left( {\frac{0.06\$ }{\text{kWh}}} \right)\left( {\frac{{24{\text{h}}}}{{1{\text{day}}}}} \right)\left( {\frac{{342{\text{days}}}}{\text{year}}} \right)$$(39) 
Cooling cost (C_{cooling})($/year) can be estimated by the following relationship with a price of cooling water (0.00375 $/kg) [8]:
$$C_{{\text{cooling}}} \left( {\$ /{\text{year}}} \right) = \left( {{m_{\text{W}}} \left( {\frac{\text{kg}}{{\text{h}}}} \right)} \right)\left( {\frac{0.00375\$}{\text{kg}}} \right)\left( {\frac{{24{\text{h}}}}{1{\text{day}}}} \right) \left({\frac{342{{\text{days}}}}{\text{year}}} \right).$$(40)

Reactor cost (C_{r}) ($):
$$C_{\text{r}} \left( \$ \right) = \left( {\frac{M\& S}{280}} \right) 10 1. 9D_{\text{r}}^{ 1.0 6 6} L_{\text{r}}^{0. 80 2} \left( { 2. 1 8+ F_{c} } \right)$$(41)
D_{r} and L_{r} are the reactor diameter and length, respectively. \(M\,\& \,S\) is Marshal and Swift index for cost escalation (\({\text{M }}\& S\) = 1536.5) [3].

Heat exchanger cost (\(C_{{{\text{heatexch}}.}}\))($):
$$C_{{{\text{heatexch}}.}} \left( \$ \right) = \left( {\frac{M\& S}{280}} \right) 2 10. 7 8 { }A_{t}^{0.65} \left( { 2. 2 9+ F_{c} } \right),$$(43)$$F_{c} = F_{m} \left( {F_{d} + F_{p} } \right).$$(44) 
Pump cost (\(C_{\text{Pump}}\))($):
$$C_{\text{Pump}} \left( \$ \right) = \left( {\frac{M\& S}{280}} \right) 9. 8 4\times 10^{3} F_{c} \left( {\frac{{Q_{p} }}{4}} \right)^{0.55} ,$$(45)$$F_{c} = F_{m} F_{p } F_{T} .$$(46) 
Furnace cost (\(C_{{{\text{Furn}}.}}\))($):
$$C_{{{\text{Furn}}.}} \left( \$ \right) = \left( {\frac{M\& S}{280}} \right) 5. 5 2\times 10 3Q_{F}^{0.85} \left( { 1. 2 7+ F_{c} } \right),$$(47)where QF is the heat duty of the furnace, W; F_{m}, F_{p}, F_{c},F_{d} and F_{T} are dimensionless factors that are functions of the construction material, operating pressure and temperature in addition to the design type.$$F_{c} = F_{m} + F_{p} + F_{d} ,$$(48)
Given  Inlet temperature of feed stock T_{in,0}, outlet product mixture 1 T_{out1}, outlet product mixture2 \(T_{{{\text{out}}2}}\) reaction temperature T_{R}, inlet water temperature T_{W,1}, T_{W,3} volumetric flow rates of feed stock (Q_{VGO}) 
Optimize  T_{F}, T_{W,2} 
So as to minimize  The total annual cost of the process (OAPC) 
Subjected to  Process constraints and linear bounds on all decision variables 
The optimization problem can mathematically be represented as follows:
Min OAPC
\(T_{\text{F}}\), T_{W,2}
s.t. f(x(z), u(z), v) (model, equality constraints)
\(T_{\text{F}}^{L} < T_{\text{F}} < T_{\text{F}}^{U}\) (inequality constraints)
T _{W,2} ^{L} < T_{W,2} < T _{W,2} ^{ U} (inequality constraints)
\(\Delta T_{{{\text{W}},2}}^{L} < \Delta T_{{{\text{W}},2}} < \Delta T_{{{\text{W}},2}}^{U}\) (inequality constraints)
\(\Delta T_{\text{F}}^{L} < \Delta T_{\text{F}} < \Delta T_{\text{F}}^{U}\) (inequality constraints)
T_{F} = T_{F}* (equality constraints),
where \(\Delta T_{{{\text{W}},2}}\) is the temperature difference between inlet and outlet temperature of water in the cooler. Practically, the best temperature difference between inlet and outlet water in the cooler is 5–25 \(^\circ {\text{C}}\). \(\Delta T_{\text{F}}\) is the temperature difference between inlet and outlet temperature of feed stock in the furnace. T_{F}* is the target final temperature of the product. The optimization solution method used by gPROMS is a twostep method known as feasible path approach. The first step performs the simulation to converge all the equality constraints (described by f) and to satisfy the inequality constraints. The second step performs the optimization (updates the values of the decision variables such as the kinetic parameters) [10]. The optimization problem is posed as a nonlinear programming (NLP) problem and is solved using a successive quadratic programming (SQP) method within gPROMS software.
Results and discussion
Kinetic parameters estimation
Optimal kinetic parameters and operating conditions obtained for industrial FCC process
Parameter  Symbol  Unit  Value 

Order of vacuum gas oil concentration  n _{1}  (−)  0.925367 
Order of light cycle oil concentration  n _{2}  (−)  1.000001 
Order of gasoline concentration  n _{3}  (−)  0.999785 
Order of liquefied petroleum gases concentration  n _{4}  (−)  0.999413 
Activation energy for reaction VGO → LCO  E _{1}  kJ/mol  20431.1 
Activation energy for reaction VGO → GLN  E _{2}  kJ/mol  23082.6 
Activation energy for reaction VGO → LPG  E _{3}  kJ/mol  23082.6 
Activation energy for reaction VGO → DG  E _{4}  kJ/mol  22271.8 
Activation energy for reaction VGO → CK  E _{5}  kJ/mol  9006.57 
Activation energy for reaction LCO → GLN  E _{6}  kJ/mol  49215.6 
Activation energy for reaction LCO → CK  E _{7}  kJ/mol  19854.4 
Activation energy for reaction GLN → LPG  E _{8}  kJ/mol  70463.8 
Activation energy for reaction GLN → DG  E _{9}  kJ/mol  88051.1 
Activation energy for reaction LPG → DG  E _{10}  kJ/mol  65992.4 
Preexponential factor for reaction VGO → LCO  A _{1}  (cm^{3}/g) 0.074633 s^{−1}  8.15295 × 106 
Preexponential factor for reaction VGO → GLN  A _{2}  (cm^{3}/g) 0.074633 s^{−1}  391.828 
Preexponential factor for reaction VGO → LPG  A _{3}  (cm^{3}/g) 0.074633 s^{−1}  1276.72 
Preexponential factor for reaction VGO → DG  A _{4}  (cm^{3}/g) 0.074633 s^{−1}  1656.55 
Preexponential factor for reaction VGO → Ck  A5  (cm^{3}/g) 0.074633 s^{−1}  1204.11 
Preexponential factor for reaction LCO → GLN  A _{6}  s^{−1}  598.233 
Preexponential factor for reaction LCO → CK  A _{7}  s^{−1}  20986.8 
Preexponential factor for reaction GLN → LPG  A _{8}  (cm^{3}g) 0.000215 s^{−1}  3.0214 × 107 
Preexponential factor for reaction VGO → DG  A _{9}  (cm^{3}g) 0.000215 s^{−1}  1.46191 × 107 
Preexponential factor for reaction LPG → DG  A _{10}  (cm^{3}g) 0.000215 s^{−1}  28090.8 
Sum of Square Errors  SSE  (−)  4.42621 × 10^{−7} 
Operating conditions  Symbol  Unit  Values 

Optimal operating conditions obtained for industrial FCC process  
Reaction temperature  TR  K  820.012 
Weight Hourly space velocity  WHSV  hr ^{−1}  2.002 
Catalysttooil ratio  CTO  (−)  10.00 
Conversion  CV  (−)  87.6075 
Octane number  Octane number  (−)  97.5722 
Energy recovery and cost savings
Here, an industrial fluidized catalytic cracking operation with energy consumption and heat recovery choice is regarded to reduce overall energy consumption (subsequently lessening environmental impact). Notwithstanding, various heat exchangers are added to the process. The goal is to provide a retrofit outline for the purpose of reducing the energy consumption and maximizing the heat recovery leading to reduce the capital investment. The heater controls the final temperature of the cold liquid to the required reaction temperature, and the cooler sets out the final temperature of the hot liquid to necessities of the following stage of the operation.
Values of parameters employed in this model
Parameter  Unit  Value 

T _{in,0}  °C  24 
T _{out}  °C  347 
T _{out2}  °C  192 
T _{W,1}  °C  20 
U _{1}  W/m^{2} K  150 
U _{2}  W/m^{2} K  900 
Dimensionless constants used in this model
Dimensionless parameters  Furnace  Pump  Heat exchanger 

F _{ m}  0.75  1  3.75 
F _{ p}  0.15  1.9  0.625 
F _{ d}  1  0  1 
F _{ T}  0  1  0 
Optimization results for heat integration system
Variables  Without heating integration  With heating integration  Decision variable type  Optimized value 

A_{t} (m^{2})  15.852884  68.744644  T_{Out,1} (°C)  231.22226 
C_{t} ($/year)  2.42532704E8  1.57962688E8  T_{Out,3} (°C)  125.18642 
m_{w} (kg/hr)  1928.32956  1806.57972  T_{w1} (°C)  115.0 
\(C_{s} ({\text{\% )}}\)  –  34.87  T_{w2} (°C)  120 
Q1_{VGO} (W)  –  67098.45  T_{F1} (°C)  28 
Q2_{VGO} (W)  –  43591.605  T_{F}* (°C)  26 
Q3_{w} (W)  –  138432.78  –  – 
Q4w (W)  –  85681.945  –  – 
Q_{t} (W)  –  3453183.5  –  – 
ES (%)  –  48  –  – 
Based on the results presented in Table 4, it is observed that the minimum total cost (C_{t}) and amounts of cooling water (m_{w}) with heating integration of the fluidized catalytic cracking process are less than those obtained without heating integration. Also, it is noticed that the cost saving is (34.87%) in comparison with the cost obtained without heating integration to reach reaction temperature and to minimize the final product temperature. To achieve the target final temperature of the product (26 °C), it is noticed that the amount of cooling water needed to reach the final temperature without heat integration is larger than that used with heating integration due to the heat recovery. The energy requirement is also taken into account in this work and shown in Table 4. It has been noted that the energy saving obtained here is about 48% compared with those without heat integration. Such result gives a clear indication that the CO_{2} emissions will be reduced by 48%, which has the added benefit of significantly reducing environmental impact.
The performance of an industrial FCC reactor
Fluidized catalytic cracking is considered one of the most significant operations where the large molecules (undesirable compound) are converted into more valuable compounds leading to achieve the market request via increasing the middle distillates yields [particularly car fuel (gasoline) and diesel fuel (light cycle oil)] in addition to light gases obtained by this process. Therefore, an increase in productivity of such cuts is considered a major issue in the FCC operations and plays a significant role in coping with these issues.
Conclusions
Process of heat integration of the FCC unit was investigated to recover most of the external energy and reducing the environmental effect in addition to maximizing the productivity with minimum cost of the process. The energy recovery by heat integration between hot and cold streams from a largescale fluidized catalytic cracking process was investigated and optimized. The energy consumption and heat recovery process was performed based on experimental information from pilot scale, mathematical modeling and commercial process and the minimum energy requirement and heat recovery were optimized. It was found that the heat integration process is very useful and efficient in commercial processes which maximizes production with minimum cost of the process. The cost savings and energy savings have been calculated to be 35% and 48%, respectively, in comparison with the process of without heat integration. Also, applying the optimal results obtained, the yield of product of fluidized catalytic cracking processes has clearly been increased.
Notes
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