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Generic Approach of Using Dynamic Simulation for Industrial Emission Reduction under Abnormal Operations: Scenario Study of an Ethylene Plant Start-up Ha Dinh,† Shujing Zhang,† Yiling Xu,† Qiang Xu,*,† Fadwa Eljack,‡ and Mahmoud El-Halwagi§ †

Dan F. Smith Department of Chemical Engineering, Lamar University, Beaumont, Texas 77710, United States Department of Chemical Engineering, Qatar University, Doha, Qatar § The Artie McFerrin Department of Chemical Engineering, Texas A&M University, College Station, Texas 77843, United States ‡

ABSTRACT: Flare minimization under normal and abnormal operating conditions in large-scale industrial processes, especially for refineries and petrochemical plants, is a double-win practice, which simultaneously benefits industrial and environmental sustainability. Unfortunately, proactive and cost-effective flare minimization (PCFM) approaches under abnormal situations are still lacking. In this study, a PCFM approach for an ethylene plant is presented for its start-up operations. This approach employs rigorous steady-state and dynamic models of a front-end deethanizer ethylene plant to serve as a foundation to explore flare root causes during plant start-ups and subsequently a test bed to support both design and operational strategies for start-up flare minimizations. It has been demonstrated that the charge gas compressor (CGC) start-up is the most critical operation, which results in the largest amount of flaring sources. Several start-up strategies at different CGC feed rates and compositions, including scenarios recycling off-spec C 2s and C 3s from downstream recovery units to the CGC inlet as a substitute of a portion of charge gas, are virtually examined to identify the least flaring one. This work contributes to evaluating new start-up strategies, predicting abnormal process dynamic behaviors, and acquiring more precise estimation of flare emission sources. The study shows that the plant flaring can be significantly reduced, while the start-up time is shortened.

1. INTRODUCTION Flaring is a safety measure for large-scale chemical process industry (CPI) plants to protect personnel, equipment, and their surrounding environment under normal and abnormal operating conditions. For safety considerations, flaring occurs when relief valves automatically direct gaseous substances to the flare system when equipment is overpressurized; for process considerations, flaring may also occur when a large quantity of off-spec gas streams needs to be destroyed until a process system reaches its normal conditions. Flaring has been recognized as a worldwide environmental concern with multiple implications. Flaring results in economic losses such as waste of limited material and energy resources as well as emission of large amounts of carbon dioxide (CO2), contributing to global warming. Yearly, over 150 billion cubic meters of natural gas (NG) is flared globally, the equivalent of 400 million tons of CO2 emissions. The numbers seem large in magnitude, but their impact is even larger considering that 400 million tons of CO2 emissions per year equals to the annual emission rate of 77 million cars. In terms of economic losses, it amounts to $10−15 billion in losses at the current $2−3 per MMBTU gas prices.1 Besides CO2 emissions, flared gases also contain many primary or secondary pollutants, such as volatile organic compounds (VOCs), highly reactive VOCs (HRVOCs), and, in some cases, carbon monoxide (CO), which affects people’s quality of life and health.2 For example, in the reported studies of HRVOC emissions and controls on the refineries and chemical plants in Harris County of Texas, a special emission inventory is collected from data within a 1-year period from more than 51 sites including refineries, chemical and polymer plants, and terminals.3 © 2014 American Chemical Society

It states that almost 61% of reported HRVOC emissions is from flaring, in which olefins and polymer manufacturing contribute more than half. The released VOCs and HRVOCs can react with nitrous oxides (NOx) in the presence of sunlight to generate ground-level ozone pollution or smog, which particularly upsets the local communities near these industrial areas. Hence, flare minimization (FM) in such large-scale industrial processes is a double-win practice, which simultaneously benefits both industrial and environmental sustainability. Thermal pyrolysis for olefin production is the most complicated and important CPI process. Olefins like ethylene, propylene, and butadiene are first produced by pyrolysis of hydrocarbons in cracking furnaces. The charge gas from furnaces is subsequently cooled in an oil and/or water quench system to separate gasoline, aromatics, and C 5s and heavier components from the lighter ones. The separation of the lighter ends requires high pressure and low temperature. This is acquired by a multistage charge gas compressor (CGC), a chilling train, a series of distillation columns, and some other units like a caustic wash tower and catalytic reactors to keep the products within specifications by removing impurities. It should be noted that ethylene plants are significant flare emitters. As reported, a typical start-up of an ethylene plant flares approximately 5.0 million lbs. of raw materials and generates at least 15.4 million lbs. of CO2, 40.0 klbs. of CO, 7.4 klbs. of NOx, 15.1 klbs. of hydrocarbons, and Received: Revised: Accepted: Published: 15089

April 4, 2014 September 2, 2014 September 4, 2014 September 4, 2014 dx.doi.org/10.1021/ie501414f | Ind. Eng. Chem. Res. 2014, 53, 15089−15100

Industrial & Engineering Chemistry Research

Article

Figure 1. General PCFM methodology framework.

100.0 klbs. of HRVOCs.4 Therefore, FM practice in an ethylene plant is very essential. FM can be accomplished in both normal and abnormal conditions. In this study, the abnormal conditions are defined as when situations such as start-ups, shutdowns, routine maintenance, or other process upsets (plant trips) happen. For instance, Loring and Smith5 introduced smokeless start-up by attempting to reduce the flaring load, while Shaikh and Lee6 used NG during commissioning. The Westlake Petrochemicals Carlyss plant reduced start-up flaring through a recycle method.7 Several successful cases have been reported from ethylene producers to reduce flaring such as applying procedural changes at the Nova Chemicals Joffre site during start-ups and shutdowns8 and implementation of a Six Sigma methodology at the Dow Chemical Freeport site9 during plant upsets. Kagay and colleagues10 also developed a parking mode to reduce the feed rates to an operation unit for unexpected flaring at Shell Chemical’s Deer Park OP-III olefins unit. Besides, Lyondell Chemicals purportedly executed several FM procedures at its olefin sites.11,12 Turnaround operations are usually modified by coupling some new FM plans; hence, detailed FM operation strategies are better to be virtually examined in advance to ensure operational safety and feasibility. In the past decade, dynamic simulations are largely employed to investigate the unit performance13−15 or serve as a platform to refine or optimize FM operation strategies during ethylene plant turnaround operations.16−25 It should be noted that although lots of effort has been spent on ethylene plant FM, there is still a lack of proactive and costeffective flare minimization (PCFM) approaches under abnormal situations. In this study, a PCFM approach for an

ethylene plant is presented for its start-up operation. This approach employs rigorous steady-state and dynamic models of a front-end deethanizer ethylene plant to serve as a foundation to explore flare root causes during plant start-ups and subsequently a test bed to support both design and operational strategies for start-up FMs. It has been demonstrated that the CGC start-up is the most critical operation, which results in the largest amount of flaring sources. Several start-up strategies at different CGC feed rates and compositions, including scenarios that recycle off-spec C 2s and C 3s from downstream recovery units to the CGC inlet as a substitute for a portion of charge gas, are virtually examined to identify the least flaring one. This work contributes to evaluating new start-up strategies, predicting abnormal process dynamic behaviors, and acquiring more precise estimation of flare emission sources. The study shows that the plant flaring can be significantly reduced while the start-up time is shortened.

2. METHODOLOGY FRAMEWORK A general PCFM approach extended based on the methodology from Xu et al.22 is presented in Figure 1. The first stage of work is developing a steady-state model of the whole plant and entering geometrical and sizing information. This steady-state model is constructed in detail based on the literature, process flow diagram (PFD), and plant design data (if available) in order to offer more realistic insights for dynamic simulation of process upsets. The thermodynamic properties are also assigned at this stage, which remains unchanged throughout the PCFM study. The steady-state validation is usually proceeded by a comparison of model outputs with a specific plant status or fine-tuning of the whole system to match the design data. Simplifications and modifications are finalized at this stage because alternating the 15090

dx.doi.org/10.1021/ie501414f | Ind. Eng. Chem. Res. 2014, 53, 15089−15100


Article

Figure 2. Sketch of flow diagram of an ethylene plant with SRUs.

3. EMISSION REDUCTION OPPORTUNITIES IN AN ETHYLENE PLANT START-UP In this paper, rigorous steady-state and dynamic models of a front-end deethanizer ethylene plant are developed to explore flare root causes during plant start-ups and subsequently a test bed to support both design and operational strategies for its PCFM. As shown in Figure 2, there are four major process sections: (i) desulfurization section including an acid gas recovery unit (AGRU), a sulfur recovery unit (SRU), and a tail-gas-treating unit (TGTU); (ii) cracking section including furnaces and quench towers; (iii) CGC section; (iv) recovery system including a front-end deethanizer (DeC2), a chilling train and demethanizer (DeC1), a C2 splitter, and a depropanizer (DeC3). This ethylene plant is designed to process sour ethane gas and produce polymer-grade ethylene while cogenerating high-purity hydrogen and liquid fuel. The first part of the whole process consists of the desulfurization units. The ethane feed with high contents of H2S and CO2 is first treated by two sour gas removal units, each of which uses an “amine” solvent to selectively absorb sour gas. After the AGRU, the removed H2S and CO2 are sent to the SRU to recover sulfur compounds in the feed, generating a side product as elemental liquid sulfur by a conventional method such as the Claus process. Next, the tail gas from the SRU goes through the TGTU to convert all of the remaining sulfur compounds into H2S, which is removed by amine absorption. The purpose of this desulfurization section is to pretreat sour ethane gas, so that it meets the feed specifications for its downstream ethylene production. This ethylene production is called a front-end DeC2 system because C 2s and lighter components of the charge gas are separated first. The system contains six major processing subsystems: furnace and quench, CGC and caustic wash tower, DeC2 and acetylene converter, DeC3, chilling train and DeC1, and C2 splitter. During normal operations, all of generated ethane and C 3s are recycled to

model structure is not desirable when the dynamic model is finished. Second, dynamic inputs are entered to this model either from the provided equipment geometries or from vessel sizing based on the steady-state/normal operating conditions. A large-scale dynamic model of the whole process is simulated with control strategies, and their parameters are implemented in each subunit. Dynamic response tests such as the open-loop method are employed to ensure convergence and flexibility of this model, especially when the process is simulated under extreme settings. After each modeling step, model validation is necessary to ensure performance precision and closeness to reality. Root-cause analysis and historian data provide the foundation for the basis of simulation. Flare mapping, targeting PCFM opportunities, is initially done from the design standpoint, such as identifying the roots of waste gas production. Evaluation from both design engineers and operators needs to be taken into account before the model is tuned to meet process operation requirements in normal conditions. Third, abnormal operating conditions are simulated once the dynamic model gains adequate credibility and reliability. This validated dynamic model should be led to the initial status of an investigated abnormal event before PCFM studies can be carried out. Model responses are collected and evaluated in order to generate several milestones and guidance to real-life practice. The unexpected behaviors at a certain equipment or excessive emissions can be identified and reduced/prevented, and a stepby-step guideline can be generated to guarantee effective and safe operations. During simulations, the abundance of communication and feedbacks from the plant operators and engineers is again very beneficial in constructing credible PCFM studies. 15091



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Figure 3. Ethylene plant start-up with total recycle and makeup feeds.

Figure 4. Flare mapping and start-up procedure with small recycle in an ethylene plant.

product flow rate of DeC1 exceeds pipe limits to the fuel gas network), DeC2/DeC3 overheads (to prevent catalysis deactivation when overhead products do not meet the feed requirements of C2/C3 reactors), C2/C3 reactor outlets (most of the SSM flaring happens here because these reactors are very sensitive and their unqualified products can contaminate the C2/ C3 splitter if sent to the downstream), and C2/C3 splitter overheads (when products do not meet customer purity requirements). During the usual start-ups, chemical plants input a large flow rate of feedstock in order to commission downstream units. The off-spec products are directed to flare until completion. Actually, these off-spec products can be rerouted to other units during

furnaces for extinction. As cogenerated products, high-purity hydrogen is produced from a pressure swing absorption unit to supply to both the TGTU and acetylene converter, and liquid fuel is produced from the bottom of DeC3. 3.1. Flare Mapping during Plant Start-ups. The flaremapping procedure requires tracking and understanding of the locations and conditions in which waste gases are produced. This can be done by surveying the literature, combining design and operating feedbacks, and analyzing the simulation model. Yang et al.23 summarize eight usual flaring locations in an ethylene plant during start-ups, shutdowns, and malfunctions (SSM) including CGC suction (when the compressors are unable to receive the charge gas), a chilling train tail-gas outlet (for safety when the top 15092



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Figure 5. CGC feed compositions of the studied cases/scenarios.

depressurization to decrease flaring and emission, which suggests several PCFM opportunities. On the other hand, olefins plants normally supply the products to their customers through a system of pipeline; therefore, adding appropriate recycle lines and including temporary storage to be available during SSM provide off-specs a holdup place in contrast to flaring.26 This can be carried out at each unit or from one subsection to another. In an ethylene plant start-up, an off-spec overhead stream from DeC2 can be fed to both CGC and this very column simultaneously until commissioning is finished. Besides, some production sites include parallel production lines that can support each other in SSM by either receiving a flaring stream or providing an in-spec feed, especially within the time period when the furnaces are not at their full capacity yet and the distillation columns are still accumulating liquid holdups. Figure 3 presents a start-up-on-total-recycle procedure in which all of the off-spec materials (hydrogen and tail gas from the chilling train and C 3s and lighters from commissioned distillation columns) are directed toward either the furnace feed or the CGC inlet to partially substitute charge gas. Besides, an olefin complex usually has several twin plants; hence, it is also possible to borrow an external feed between them to ensure smooth start-up. Even though this approach potentially requires no flaring and shorter time, it also has higher risks and requires more process retrofitting and constructing of additional pipelines. On the basis of the above discussions, a conservative start-up design is introduced in Figure 4 and demonstrated in the following case studies. The proposed PCFM strategies employ recycle loops to partially substitute for the furnace feed in order to shorten the start-up time. The off-spec C 2s from a C2 splitter top with/without the off-spec C 3s from the DeC3 top is fed back to the CGC inlet, where it is combined with charge gas from the fired furnaces and the external NG feed to achieve the minimum flow rate to start CGC. As aforementioned, the proposed PCFM strategies can be evaluated by dynamic simulation and industrial expertise opinions to select the most suitable one in reality. 3.2. General Start-up Strategies. 3.2.1. Case 1 (Base Case): Ordinary Start-up Strategy. Before start-up officially takes place, most of the operation units are filled by nitrogen while compressors are set at a very low rotation speed. Thus, the first start-up operation is commissioning, in which nitrogen is purged and replaced by hydrocarbons from either external sources or off-spec storage tanks. Second, start-up liquids are injected and accumulated in these vessels to buildup the initial vapor−liquid balance; meanwhile, columns are geared to be

operated in a full-reflux mode, while a reboiler and a condenser are working. Associated with this, a precooling chilling train is also completed under consideration of equipment safety. Third, cracking furnaces are fired gradually by a fresh feed to acquire enough flow rate in order to drive the compressor shaft. Fourth, the CGC section is started as the shaft rotation speed is increased to its regular value. Once the compressors are running at its desired rotation speed and the chilling train and DeC1 section are chilled down enough to guarantee methane and hydrogen separation, the intermediate subsection (the integrated subsection of DeC2 and C2 converter) between these two sections can be connected to the CGC outlet. When DeC2 product streams and the reactor effluent are in specifications, DeC1 and the chilling train is brought online. Finally, each column is transferred from full-reflux mode to their normal operation status. 3.2.2. Case 2: PCFM Start-up Strategy with Simultaneous Recycles of C 2s and C 3s. During the start-up time period, the off-spec products and intermediate streams between subsequent sections will be flared. The total flaring amount of such a start-up can reach millions of pounds.12 One of the major flaring sources is at the CGC inlet, where the system has to gather enough feed flow rate to start the CGC; before reaching the required flow-rate threshold, the CGC inlet feed has to be flared. For PCFM practice, an alternative way should be used to lessen the waiting time for gathering enough of this feed flow rate. Here, foreign feed such as NG and off-spec products from storage are employed as substitutes for the furnace effluent such that fewer furnaces need to be fired up to provide enough material quickly at the CGC inlet. Therefore, case 2 combines charge gas from cracking furnaces, NG feed, and the off-specs from the overheads of DeC3 and C2 splitter to make up a feed substitute mixture to the CGC inlet, also named the feed cocktail in this paper. In case 2, two different scenarios can be further evaluated: case 2.A employs one furnace, while case 2.B employs two furnaces before CGC start-up takes place. 3.2.3. Case 3: PCFM Start-up Strategy with Recycle of C 2s Only. Case 3 employs charge gas from cracking furnaces, NG feed, and the off-specs from the overhead of a C2 splitter only to make up the feed cocktail to the CGC inlet. Similar to case 2, it also has two different scenarios that can be further evaluated: case 3.A employs one furnace for start-up, while case 3.B employs two furnaces. Figure 5 summarize the CGC inlet compositions under all three cases and their associated scenarios. 15093



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Figure 6. Dynamic model of a front-end DeC2 ethylene plant with a SRU.

4. MODELING AND DYNAMIC SIMULATION FOR AN ETHYLENE PLANT START-UP The entire plant and sectional models are constructed in Aspen Plus Dynamics.27 The plant is assumed to have an annual ethylene production capacity of 900000 tons/year. The sour gas feed contains 6% H2S, 17% CO2, and 77% ethane at 37 °C and 25 bar. The AGRU section and furnaces are simplified with constant effluent yields at their outlets. The desulfurization section is modeled as two separators to purge out all of the sour gas components (H2S and CO2), as shown in Figure 6. The start-up conditions are specified as follows: (i) firing and stabilizing each furnace at its 80% normal capacity requires 1 h; (ii) the compressor requires at least 50% of the normal feed flow rate to start up (approximately 150 tons of charge gas/h); (iii) CGC feed cocktails share the same average molecular weight of 19.8 and the same heating value as the normal charge gas from furnaces; (iv) compressors require 0.5 h for stabilization after reaching the regular shaft speed; (v) the ethylene and propylene refrigeration cycles are already in operation at the beginning of CGC start-up; (vi) the precooling process of the chilling train and DeC1 takes approximately 5 h, and its outlet is not flared but redirected to the furnace as a fuel gas. 4.1. Preparation of the Cocktail Feedstock. In cases 2 and 3, the overhead streams from the C2 splitter with/without DeC3 are recycled to the CGC inlet as part of its feed cocktail. Individual dynamic models of both columns are employed to determine whether the columns’ holdups are enough to sustain the required withdrawing time duration and flow rate during CGC start-up. In this section, the C2 splitter is taken as an example for demonstration. Similar simulations have also been conducted for DeC3 to confirm its availability of feed cocktail sources under all FM cases/scenarios. 4.1.1. Modeling of Full-Reflux Distillation Columns throughout CGC Start-up. A Radfrac column unit from Aspen Plus 7.3 with a built-in constant-duty partial condenser and a reboiler is utilized to examine the C2 splitter in this step, as

illustrated in Figure 7. The control scheme of this column has four controllers. A top-tray pressure controller (PC) is placed by

Figure 7. PFD of the simplified C2 splitter with a control scheme.

manipulating the vapor outlet flow rate, while a temperature controller (Tray TC) maintains the most sensitive tray temperature (at the 65th tray) by manipulating the reboiler duty. Each of reflux drums and column sumps has a level controller (Reflux LC and Sump LC), which is very essential in start-up simulations. The column’s initial status is full of nitrogen vapor, and its temperature and pressure are slightly above the ambient conditions. Dynamic simulation for a supplier column involves three steps: commissioning (nitrogen purge by off-specs C 2s plus start-up liquid injection to build up full-reflux status), recycling overhead withdrawal for CGC start-up, and resetting full-reflux after CGC start-up. 4.1.2. Optimization of the Operation Procedure for FullReflux Columns. According to Figure 5, the feed cocktail in case 3.A has a flow rate made up by 33% charge gas from one furnace, 46% NG, and 21% C 2s recycle, which has the largest C 2s 15094



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Table 1. Summary of Control Actions of a C2 Splitter during CGC Start-up initial full-reflux status

after recycle withdrawal

reset full-reflux status

controller

mode

SP/PV (%)

OP (%)

mode

SP/PV (%)

OP (%)

mode

SP (%)

PV (%)

OP (%)

PC Tray TC Reflux LC Sump LC

manual manual manual manual

25.67 18.39 2.37 59.17

0.00 53.00 0.00 0.00

manual manual manual manual

34.29 33.28 2.42 20.21

82.16 53.00 0.00 0.00

automatic automatic manual manual

40.00 18.39 2.36 24.71

24.69 18.39 2.36 24.71

0.00 49.68 0.00 0.00

Figure 8. Dynamic response of the C2 splitter during CGC start-up in case 3.A.

Figure 9. CGC model with controller information for three case studies.

demand from a C2 splitter overhead among all cases/scenarios. The CGC start-up takes approximately 0.56 h to reach normal status and another 0.5 h for stabilization. Thus, it takes at least 1.06 h for CGC start-up. A dynamic simulation of withdrawing 15.7 tons/h of C 2s from column overhead (equivalent to 21% feed cocktail in case 3.A) is carried out in 1.4 h, given about 20 min of more backup time in case upset occurs during CGC startup. Table 1 indicates all controllers’ actions of a C2 splitter including their set points (SPs), process variables (PVs), and outputs (OPs) after commissioning, after CGC start-up, and after the column is reset to its full-reflux mode. When a controller is in the manual mode, the SPs and PVs are identical and the

operators can assign the OP at a desired value. In the third step, PC and Tray TC controllers are returned to automatic mode. The dynamic profiles of a C2 splitter throughout CGC start-up are shown in Figure 8. The simulation predicts that the C2 splitter sump level still remains at approximately 25%, while the reflux drum still contains nearly 3% of liquid level after CGC start-up (Figure 8b), when the overhead withdrawal is discontinued. This suggests that the liquid holdup before CGC start-up in the column is adequate to provide recycle supply for a longer time period than needed (1.4 h instead of 1.06 h for CGC start-up) and enough material is left at the start of stage 3. Figure 8b shows that stage 3 takes approximately 36 min for the 15095



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Figure 10. Compressor outlet pressure settings during CGC start-up simulation.

Figure 11. Feed profiles at the CGC inlet during start-up for three cases.

compressors, their relative heat exchangers, suction drums, and a caustic wash tower is employed to study process behaviors during CGC start-up (see Figure 9). The compressor start-up is modeled by setting the pressure increment (ΔP) between the

temperature and pressure within the C2 splitter to stabilize and regain the full-reflux status. 4.2. Dynamic Simulation of CGC Start-up. Similar to a C2 splitter, a sectional dynamic model including the first three-stage 15096



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time is reduced compared with other start-up scenarios. Consequently, less material is flared. It should be noted that, although each approach (A or B) results in the same flare amount between cases 2 and 3, the compositions of those flaring streams are different because of their relative CGC feed cocktails. Besides having the least flare amount, case 3.A is also more practical and economically beneficial because only the C2 recycle line needs to be built. 4.3. Dynamic Simulation of Recovery Section Start-up. After case 3.A is identified as the most suitable scenario for PCFM purposes in section 4.2, further study is carried out to validate the feasibility of the start-up strategy for the recovery section. The intermediate streams between the CGC and DeC2 sections are connected at once after the CGC start-up is completed. It should be noted that three simultaneous tasks are carried out at this time point of various locations within the whole plant: (i) firing of the rest of the furnace(s) (in cases 2 and 3); (ii) start-up of the following recovery section; (iii) gradual shutdown of the makeup feed (which includes both NG and recycle lines from full-reflux columns). The sketch of the recovery section with its controller information is shown in Figure 12. Note that there are combinations of heat exchangers acting as preheaters/coolers for the reactor inlet and effluent in this section. These heat-transfer units operate with utilities available on-site prior to the start-up time, such as low-pressure steam, cooling water, ethylene, and propylene refrigerants. After nitrogen is purged out and liquid levels are gained for all columns and drums, there are enough hydrocarbons in the DeC1 first separator and DeC2 to drive the fourth-stage compressor, as well as build column full-reflux status while waiting for the feed from the third stage of CGC. Figure 13 shows simulation results of dynamic concentration profiles of overhead and bottom streams for DeC2, DeC3, and C2 splitter in case 3.A (the optimal case for CGC start-up). The zero hour in Figure 13 means the time instant when each individual column is connected to its upstream and is brought out from its full-reflux mode. Table 3 provides a summary of outlet stream specifications for these columns, which are used to determine whether the streams need to be flared. It should be noted that each column requires a different transition time to reach specifications. From Figure 13a, DeC2 needs less than 1.5 h to reach its steady operation because the fourth-stage CGC and subsequent units are already started when the DeC2 full-reflux mode is implemented. Hence, the DeC2 subsystem could reach their desired states within a short time. Consequently, the separation within DeC2 is only off-spec for the first 0.5 h when its reflux stream from the DeC1 first separator is gradually adjusted. Thus, flaring from DeC2 cannot be avoided in the first 0.5 h. Similar observations are made in the simulation models of other columns. C 3s in DeC3 bottoms exceeds the requirement (less than 2.6%) for the first 0.3 h, and its overhead stream does not satisfy specification until 1.25 h (see Figure 13b); hence, flaring occurs until these concentrations are lowered. In Figure 13c, the quality of the ethylene product in the C2 splitter top stream and ethane recycle stream at column bottoms is monitored. Start-up of the C2 splitter takes more than 6 h, during which the first 4 h are dedicated to bringing its condenser and reboiler from the full-reflux mode to their full loads and accumulating liquid levels. Because the C2 splitter serves as a supplier for the CGC feed cocktail, it has lower levels from the remaining hydrocarbons after CGC start-up. Although the top product is within its specification, the bottoms is off-spec for 2 h, which leads to flaring. At the sixth hour, both product

compressor inlet and discharge at each stage, according to the rotation speed (N) increase based on Affinity Laws and Bernoulli’s equation.28 Equation 1 shows the relationship between ΔP and N. (ΔP /N 2)start‐up = (ΔP /N 2)normal = constant

(1)

During CGC start-up, the change of the shaft rotation speed is known. On the basis of eq 1 and the normal operation status, the discharge pressure at each CGC start-up operation can be estimated. A simulation task is written in Aspen Plus Dynamics to automate the CGC activation procedure with respect to an N increase. The corresponding profiles of discharge pressures at each stage during CGC start-up are shown Figure 10. The zero hour indicates the moment the compressor shaft speed starts to increase. Prior to this time point, compressors are considered at rest while rotating at a very low speed. The first slope in Figure 10 represents the first increment of rotation speed N to 700 rpm (revolutions per minute) within approximately 186 s. Subsequently, CGC is idling for 814 s; then the second increment from 700 to 2000 rpm takes place within 137 s. After the compressors rest for about 750 s, the third N increment to reach the required normal operating speed is divided into three steps: 2200, 6000, and finally 6400 rpm. This action takes 44 s and is also the steepest incline in the whole CGC start-up process. As indicated in Figure 11, the CGC shaft speed does not start to increase until the total feed flow rate achieves 75 tons/h, equivalent to 50% feed at normal operations. Figure 11 expresses the dynamic feed profiles of the CGC inlet during the entire start-up for three cases. The start-up process takes approximately 4 h for case 1, among which 1 h is spent for each of the three furnaces, including 30 min for ramping up the fresh feed and 30 min for stabilization; the fourth hour is dedicated to the CGC start-up procedure. It should be noted that an antisurge controller and a “3-to-1 recycle” stream from the third to the first stage of CGC are included in our modeling to avoid the potential surge risk. Meanwhile, each suction drum has a PC and a LC. During the start-up, PCs are set to manual mode to buffer short-time pressure upsets when the rotation speed is ramped up. After liquid accumulation, all of LCs remain in automatic mode throughout the whole start-up time. Note that flare occurs from firing of the first furnace until the CGC is successfully started up. Quantitative feeding information and flaring statistics during CGC start-up for all of three cases are summarized in Table 2. The results show that cases 2.A and 3.A have the minimum amount of flaring. In these two start-up scenarios, because the recycled feed substitutes for two furnaces, the standby time to generate enough feed flow rate to start CGC is shortened (i.e., it only needs to wait for firing one furnace). Thus, the total start-up Table 2. Major CGC Start-up Features of All Case Studies

charged gas to CGC (tons/h) NG (tons/h) off-spec C2 (tons/h) off-spec C3 (tons/h) no. of fired furnaces start-up time (h) CGC flaring amount (tons)

case 1

case 2.A

case 2.B

case 3.A

case 3.B

74.40

24.80

49.60

24.80

49.60

17.84 6.2 0.76 2 3.06 133.31

33.93 15.67

16.96 7.84

3 4.05 205.06

35.69 12.4 1.51 1 2.06 92.56

1 2.06 92.56

2 3.06 133.31

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Figure 12. Recovery section model with controller information.

concentration profiles start flattening out, indicating that the column is stabilized and brought to normal operations. The whole plant model is also employed to observe smooth connection and test additional control strategies when needed. Once all of subsections are linked, the rest of the furnaces are fired to gradually bring the system from the current 50% to full capacity. Note that there is no necessary flaring in this period if the process is operated with sufficient care because all of the distillation columns and their supporting units are already brought to normal operation statuses and their outlets are all within specifications.

one furnace to start-up CGC, which needs less standby time. Between cases 2.A and 3.A, case 3.A is more preferred because only one recycle line from the C2 splitter needs to be constructed. As aforementioned, flaring during CGC start-up contributes the most in each of the studied scenarios, varying from about 40% (cases 2.A and 3.A) to almost 70% of the total flare amount from the entire plant (case 1). Hence, more attention should be paid to this section in PCFM studies. It should be noted that flaring during the vessel commissioning step is not included in this paper because all if these cases are assumed to have an identical flare amount during this commissioning step, which does not affect the comparison among these studied cases. Through various scenarios, the developed dynamic models show adequate flexibility in handling different feed compositions and start-up strategies, either conservative (as case 1) or aggressive (as case 3.A). The amount of flare saving is relatively significant, from 17.60% (case 2.B) to 22.74% (case 3.A) of the total plant start-up flaring. It should be highlighted that dynamic simulations in PCFM build a solid foundation for future studies including emission reduction, conscious guidance on plant design, operation, production planning and scheduling under normal and abnormal operation conditions. From the estimated flaring sources, HRVOC and NOx emissions and greenhouse gas (i.e., CO2) releases can be further predicted to study the impact of flare

5. FLARE AMOUNT STATISTICS AND DISCUSSIONS Table 4 summarizes the total flaring amount during the plant start-up for all three cases. It can be seen that case 1 (base case) has the largest amount of flaring sources (off-spec hydrocarbons before flare combustion), which is 293.6 tons. Among these flare sources, CGC flaring contributes the majority, while DeC1 and the C2 splitter supply nearly the rest of the amounts accounted for. Flaring from DeC2 and DeC3 is comparably very small. Compared with the base case, cases 2 and 3 have significant flaring source reduction. This is because they both employ a feed cocktail at the CGC inlet to partially replace charge gas from furnaces and consequently decrease both the start-up time and flaring quantity throughout CGC start-up. Cases 2.A and 3.A are better than cases 2.B and 3.B, respectively, because they only fire 15098



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Table 4. Flaring Source Statistics for All Case Studies section CGC DeC1 DeC2 DeC3 C2 splitter total flare saving (%)

case 1 (tons)

case 2.A (tons)

case 2.B (tons)

case 3.A (tons)

case 3.B (tons)

205.06 55.08 0.33 1.17 31.91 293.55

92.56 55.08 49.93 3.16 26.66 227.39 22.54

133.31 55.08 25.13 2.53 25.83 241.87 17.60

92.60 55.08 49.93 1.17 28.02 226.80 22.74

133.31 55.08 25.13 1.17 24.63 239.32 18.47

operations. This approach employs rigorous steady-state and dynamic models of a front-end DeC2 ethylene plant to serve as a foundation to explore flare root causes during plant start-ups and subsequently as a test bed to support both design and operational strategies for start-up FMs. Case studies suggests that flare emission sources during CGC start-up can be reduced by more than 50% by recycling off-specs products from downstream distillation units on standby plus an external NG feed during start-up; meanwhile, the total plant-wide start-up flaring can be saved by more than 24%. The study helps promote the doublewin FM practice in the CPI, which simultaneously benefits industrial and environmental sustainability.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 409-880-7818. Fax: 409-880-2197. E-mail: Qiang.xu@ lamar.edu. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported, in part, by the Qatar National Research Fund (NPRP 5-351-2-136) and Texas Air Research Center headquartered at Lamar University.

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Figure 13. Dynamic concentration profiles of overhead and bottom streams for (a) DeC2, (b) DeC3, and (c) C2 splitter in case 3.A.

(1) Farina, M. F. GE Gas Flaring Report − Recent global trends and policy considerations. Accessed online at www.ge-energy.com (2011). (2) Flare Efficiency Study; U.S. Environmental Protection Agency: Cincinnati, OH, 1983; EPA-600/2-83-052. (3) Final Report for TCEQ Projects 2009-52 and 2009-53; prepared for work order 582-07-84005-FY09-15; ENVIRON International Corp.: Houston, TX, 2009; http://www.tceq.texas.gov/assets/public/ implementation/air/rules/contracts/2009-hrvoc-cost-analysis-final.pdf (accessed on Sept 12, 2013). (4) Liu, C.; Xu, Q. Emission Source Characterization for Proactive Flare Minimization during Ethylene Plant Start-ups. Ind. Eng. Chem. Res. 2010, 49, 5734−5741. (5) Loring, M. P.; Smith, S. K. Flare load reduction in ethylene plants can ensure smokeless start-ups. AIChE Spring National Meeting, Houston, TX, Mar 19, 1995. (6) Shaikh, A.; Lee, C. J. Minimize flaring during ethylene plant startup. Hydrocarbon Process. 1995, No. July, 89. (7) Chellino, M. Recycle method to reduce ethylene plant start-up flaring. AIChE Spring National Meeting, Houston, TX, Apr 22, 2001. (8) Williamson, M.; Dennehy, J. Procedural Changes to Minimize Flaring during Ethylene Plant Start-ups and Shutdowns. AIChE Spring National Meeting, Houston, TX, Apr 22, 2001. (9) Krientenstein, S. Flare minimization strategy during plant start-up. AIChE Spring National Meeting, Atlanta, GA, Apr 12, 2005. (10) Kagay, E. B.; Taylor, M.; Genty, N. Development of a parking mode at Shell Chemical’s Deer Park Plant Olefin Unit OP-III. AIChE Spring National Meeting, Atlanta, GA, Apr 12, 2005.

Table 3. Composition Specifications of the Main Columns during Production type

location

specifications

C 3s components C 2s components C 4s components C 3s components ethylene ethylene

DeC2 overhead DeC2 bottoms DeC3 overhead DeC3 bottoms C2 splitter overhead C2 splitter bottoms

Generic Approach of Using Dynamic Simulation for ... - ACS Publications

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