Flare Minimization Strategy for Ethylene Plants

Pollution prevention

Xiongtao Yang1 Qiang Xu1

Review

Kuyen Li1

Flare Minimization Strategy for Ethylene Plants

1

Dan F. Smith Department of Chemical Engineering, Lamar University, Beaumont, TX, USA.

1059

Flaring is an important but passive method in ethylene plants to protect plant personnel, facilities, and the ambient environment. However, excessive flaring emits huge amounts of CO, CO2, NOx, and hazardous volatile organic compounds (VOCs), which may cause locally transient air pollution problems and negative societal impacts. Flaring may also cause high losses of raw material and energy that could generate more desired products for the industry. Thus, flare minimization has great benefits to environmental, societal, and industrial sustainability. Based on current industrial practices, a general strategy for flare minimization under various ethylene plant events is presented. Keywords: Dynamic simulation, Ethylene plants, Flare minimization, Pollution prevention, Steady-state simulation Received: December 3, 2009; revised: April 28, 2010; accepted: May 7, 2010 DOI: 10.1002/ceat.200900588

1

Introduction

Ethylene plant activities such as start-ups, shutdowns, maintenance, upsets, and routine operations will generate off-spec product streams that are typically sent to flares for destruction. This flaring activity emits large amounts of carbon monoxide (CO), nitrogen oxides (NOx), volatile organic compounds (VOCs), highly reactive VOCs (HRVOCs) (defined in the Texas air quality regulation as ethylene, propylene, isomers of butene, and 1,3-butadiene), and partially oxygenated hydrocarbons (e.g., formaldehyde), which will cause severe air pollution problems and negative societal impacts. Flaring also results in tremendous raw material and energy losses that could generate much needed products for the industry. For instance, an ethylene plant with a capacity of 1.2 billion pounds of ethylene production per year may send five million pounds of ethylene for flaring during an ordinary start-up [1]. Based on the flaring efficiency (destruction efficiency) of 98 % for estimation, the total emission during the start-up generates 7.5 Klbs of NOx, 40.0 Klbs of CO, and 100.0 Klbs of HRVOCs. This is just a normal scenario of the flaring emission for the ethylene stream. If all the other flaring species are included, such as ethane, propylene, and propane, huge amounts of air emissions will be produced through flaring emission. It should also be noted that flaring may cause localized and transient air pollution events, which are harmful to people’s

– Correspondence: Prof. Q. Xu ([email protected]), Dan F. Smith Department of Chemical Engineering, Lamar University, 4400 MLK Blvd, Beaumont, TX 77710, USA.

Chem. Eng. Technol. 2010, 33, No. 7, 1059–1065

health. For instance, the industrial flare emission of HRVOCs and NOx has been identified with associating high concentrations of ozone as observed in the Texas Houston/Galveston area, which violates the National Ambient Air Quality Standards (NAAQS) for ozone [2, 3]. Thus, flare minimization (FM) has great benefits to environmental, societal, and industrial sustainability. In the last two decades, ethylene producers and researchers have been enthusiastically implementing approaches to reduce flaring. Loring and Smith [4] reduced the flare load to ensure a smokeless start-up. Shaikh and Lee [5] used natural gas during commissioning of an ethylene plant to diminish off-spec material generation. The Westlake Petrochemicals Carlyss plant reduced start-up flaring through a recycle method [6]. The Nova Chemicals Joffre site minimized flaring through procedural changes during ethylene plant start-ups and shutdowns [7]. The Dow Chemical Freeport site implemented the Six Sigma methodology for FM during plant upsets [8]. Shell Chemical’s Deer Park OP-III olefins unit developed a parking mode to reduce feed rates to the unit when an unanticipated flaring occurs [9]. Lyondell Chemicals implemented several FM projects within its olefins sites [10, 11]. Xu and Li’s research groups at the Lamar University systematically addressed FM methodologies in chemical plants through dynamic simulations [12–16]. Based on the current literature survey, comprehensive studies on major FM opportunities at ethylene plants are still lacking. As ethylene plants are important flaring contributions, which will experience numerous flaring events in the lifetime, a comprehensive FM study is necessary. Based on current industrial practices, a general FM strategy for ethylene plants is presented. In this strategy, various flaring activities are classi-

© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

www.cet-journal.com

1060

X. Yang et al.

fied and many possible FM measures are presented and discussed. The significance of modeling and simulation for helping ethylene plants to conduct FM is also highlighted.

2

Ethylene Process and Flare Source Description

To reduce flaring emission sources, the ethylene plant process should be analyzed first. Fig. 1 illustrates a typical flow diagram for an ethylene plant.

2.1

Ethylene Production Process

The feedstock (e.g., LPG, naphtha, and light diesel) is sent to furnaces for cracking. The furnace effluent, called cracked gas, is then forwarded to the quench tower where the cracked gas is cooled and partially condensed. The quench tower overhead vapor is then sent to cracked gas compressors (CGCs). After being compressed and dried, the cracked gas is fed to the chilling train, where the cracked gas becomes chilled and then is directed to flash drums. Hydrogen is separated from the top of the flash drums, which will be used as a cold stream in the chil-

ling train and the reactants thereafter for C2 and C3 reactors. Liquid accumulated in the bottom of flash drums is fed to the demethanizer (DeC1). In DeC1, methane is separated from the cracked gas as the overhead product stream mainly used as the fuel gas. The bottom stream consisting of C2 and heavier components is sent to the recovery section, which includes deethanizer (DeC2), depropanizer (DeC3), and debutanizer (DeC4) to separate the C2, C3, and C4 components, respectively. The C2 and C3 reactors are used to convert acetylene to ethylene and convert MAPD (methyl acetylene/propadiene) to propylene, respectively. The main products of ethylene and propylene are separated from the tops of the C2 and C3 splitters, respectively, while the bottom streams from the C2 and C3 splitters, containing ethane and propane distinctively, are circulated to the upstream as the feedstock.

2.2

Identification of Flare Sources

Major flare activities in ethylene plants are start-ups, shutdowns, process upsets, and plant trips. During these activities, eight flaring locations are identified, which include: – CGC suction. Cracked gas will be diverted to the plant flaring system from CGC suction during situations that com-

Figure 1. Typical flow sheet for an ethylene plant.

www.cet-journal.com

© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Chem. Eng. Technol. 2010, 33, No. 7, 1059–1065

Pollution prevention

–

–

–

–

pressors trip, compressors are unable to receive cracked gas during start-ups, or compressors are already shut down while the cracked gas still keeps coming. Chilling train tail gas outlet. When the top product flow rate of DeC1 overpasses the pipe limits to the fuel gas network, the excessive products will be directed to flare in order to protect the systems. DeC2/DeC3 overheads. When DeC2/DeC3 overhead products do not meet the feed requirements to the C2/C3 reactor distinctively, the products will be diverted to the flare system to prevent catalysis deactivation. C2/C3 reactor outlets. The majority of flaring during ethylene plant start-ups and process upsets occurs at the C2/C3 reactor outlets. Since very low concentrations of acetylene/ MAPD are specified at their outlets, C2/C3 reactors easily suffer upsets. Once the unqualified outlet streams are sent to the downstream units, the C2/C3 splitters will be contaminated and require at least several hours to recover. Under this situation, the unqualified streams from the C2/C3 reactor outlets should flow to the flare system directly. C2/C3 splitter overheads. The final products are high-purity ethylene from the C2 splitter overhead and high-purity propylene from the C3 splitter. The customer requirements are around 99.95 vol.-% and 99.60 vol.-%, respectively. Therefore, off-spec products with lower purity will be directed to the flare system.

tions [12–16]. Their methodology can be generally divided into three steps: (i) develop and validate steady-state simulation (SS) models; (ii) upgrade the SS models to dynamic simulation (DS) models with real validations; (iii) use the validated DS models to examine FM procedures for their operational safety and operability [17]. Fig. 2 presents this methodology framework. Note that based on the methodology framework, FM strategies can also be optimized. The plant expertise-assisted dynamic simulation will be a cost-effective and promising way to examine FM opportunities in the future.

Figure 2. General methodology framework.

3.3

3

General FM Strategy

Based on the introduction, it is apparent that FM at ethylene plants is a very challenging task. It deals with such a complex system that synergistic effects should be sought and utilized whenever possible and at all possible scales. The developed strategy contains three steps as explained in the following sections.

3.1

FM Classification and FM Opportunity Identification

Flares occur when off-spec products are generated or operational emergencies are met. Generally, they can be classified by planned and unplanned categories, both of which involve different scenarios. Because the conditions and root causes for each flaring scenario are generally different, FM measures for these scenarios should be identified in advance. Examining efforts should be conducted in advance instead of during the flaring. The potential FM opportunities can be identified based on the cooperation of researchers and site expertise.

3.2

FM Feasibility and Reliability Examination

Once possible FM opportunities (measures) are identified, they need to be examined and validated to ensure their feasibility and reliability. Previously, many plants used a trial-and-error approach to gain expensive and sometimes painful experiences [12]. Recently, Xu and Li’s research groups systematically studied FM problems in chemical plants through dynamic simula-

Chem. Eng. Technol. 2010, 33, No. 7, 1059–1065

1061

FM Project Management

When validated FM measures are put into practice, their execution should be well managed. This needs experienced operators, engineers, and administration, along with effective planning, scheduling, and training. For instance, many planned FM projects involve plant operational and design modifications. Thus, the affected facilities or even the entire plant should be in shutdown mode to protect the personnel and equipment safety. To reduce the downtime loss, the modifications should be scheduled during plant turnarounds. Since a typical turnaround takes only 20–50 days, advanced planning is critical in order to avoid excess downtime [18, 19]. A special team should be organized and equipments and pipelines must be ordered well in advance of the date required onsite [19]. Qualified personnel is required to execute the projects. The project should also be scheduled at stages, and various tasks of each stage should be well-monitored to ensure that the tasks are accomplished with quality on time.

4

Major FM Activities

As aforementioned, the major flares that an ethylene plant experiences can be classified into planned and unplanned categories. In this section, FM activities related to these two categories will be extensively addressed.

4.1

Planned Plant Events

Planned events provide enough time for orderly and controlled actions in plants. These events mainly include plant shut-

© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

www.cet-journal.com

1062

X. Yang et al.

downs, maintenances, and start-ups. As the flaring emissions during shutdowns and start-ups account for a significant portion of the whole plant flaring, this section will focus on these two planned events.

4.1.1 Plant Shutdown Shutdown is a transient operation plants experience from normal operation to an idle mode. During this period, the raw feed to the furnace section is worked offline, the units are depressurized, and the process is brought to an ambient temperature. The operation transition generates off-spec products, which are released to the flare system. Historical practices during shutdowns would firstly reduce the feed flow rates to the lowest point of plant stability, then trip out CGC, and pull furnaces off as quickly as possible. This process results in a huge number of flaring from the CGC suction. For such a shutdown, flaring emission could be as high as millions of pounds [4]. In order to reduce flaring and recover the high-value gas, the following measures are identified [7, 10, 11]: – Replace all plant-produced hydrogen to the C2/C3 reactors with imported hydrogen to maintain reactor effluents on specification. Circulate plant-produced hydrogen and tail gas to the CGC suction. – For an ethane/propane feedstock, reduce the furnace coil outlet temperatures (COTs) to suppress cracking reactions, while pushing high-value components out of facilities as product with uncracked ethane/propane. – Use ethane and propane cycles to gradually replace the fresh feedstock until the plant runs in full recycle. – Recycle the C2/C3 reactor effluents back to the CGC suction when product streams from the C2/C3 splitters are no longer on-spec. – Recover ethane from the C2 splitter and direct it to the fuel system until the C2 splitter bottom is drained out. Then take the furnaces offline. – Slowly take refrigeration systems offline, shut down the CGC operation, and depressurize the left hydrocarbons in the process to the flare system.

4.1.2 Plant Start-up Start-up involves operating the process from its initial state to its normal operation condition. During this period fresh feeds increase gradually and equipments are pressurized; fluids are heated up or cooled down to normal operating temperatures. Historical start-up procedures would ramp up furnace feeds to provide the CGC feed and help generating super high-pressure steam to drive the compressors’ turbine. It will then start up the CGC section, start up the chilling train, start up DeC1 through DeC4, bring the C2/C3 reactors on specification, and bring ethylene and propylene products on specification. During the start-up, from the activating CGC to the normalizing product quality, off-spec product or intermediate product streams will be generated and flared. In detail, the feed stream to the CGC suction will be flared off until the require-

www.cet-journal.com

ment is met. After the CGC has started, the tail gas from the chilling train and the DeC1 top will be flared until the chilling train and the DeC1 systems finally chill down, allowing that methane and hydrogen separations are guaranteed. Similarly, before C3s contained in the DeC2 overhead stream meet the composition requirement of the C2 reactor feed, the feed is directed to flare [5, 6, 10]. At that time when C3s meet the requirement, the flow is routed forward through the C2 reactor. Then, hydrogen is added to the reactor to start catalytic reactions. After that, the flaring source will be the offspec effluent (acetylene is higher than the limit) from the reactor. Once the reactor effluent is on-spec, the reactor effluent will be fed into the C2 splitter for the separation of ethylene and ethane. Before ethylene from the C2 splitter overhead meets all customer specifications, the off-spec product still needs to be flared. This principle is also used in the C3s separation. After products are on-spec, furnace feed rates begin to increase. For such a start-up, the total flaring emission could be as high as millions of pounds [11]. The following measures have been summarized to minimize flares [4–8, 10–16, 20]: – Establish appropriate recycles to recover off-spec products. – Pre-inventory and operate distillation towers on total reflux. – Start up the CGC section with artificial gas before start up the cracking furnaces. – Start up furnaces one by one and control the feed ramping rate carefully. – Speed up the chilling-down rate while maintaining the chilling train within equipment constraints. – Ensure that the normal operating conditions of the DeC1 through the DeC4 are almost achieved before directing the bottom stream of one tower to the downstream of the next. – Add imported high-purity ethylene into the reflux drum of the C2 splitter to reduce its settling time. Very recently, DS-based studies for start-up FMs have made a significant progress. It is a cost-effective way to help plants identify and improve their start-up strategies. Two reported case studies are employed to demonstrate the DS application significance. The first case is about a DS-assisted flare minimization project [12], which studied the OP1 plant of Equistar Chemicals Channelview complex. The DS was performed to cover the DeC2, DeC3, and C2 reactors and the C2 splitter on the basis of process flow scheme, equipment data, and instrument data. With the help of simulations, the DeC2 control tray temperature set point was tuned from 62.2 /C (144 /F) to 57.2 /C (135 /F), which improved the temperature distribution of DeC2 from Figs. 3a to 3b, and thus enhanced its operation stability during the start-up. As reported, the actual start-up flaring load in OP1 plant was significantly reduced by 75 % compared with the earlier startup of the OP2 plant (OP1 and OP2 plants have the same design and production scheme) on that site. The second case is about an ethylene plant start-up flare minimization via plant-wide dynamic simulation [15]. This study covers the CGC, chilling train, DeC1, and recover sections. To avoid excessive flaring, multiple recycles were established including the major recycles from the DeC1/chilling train, DeC2, and DeC3 to the CGC suction, and a short recycle from the DeC4 top to the DeC2 bottom. These recycles were

© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Chem. Eng. Technol. 2010, 33, No. 7, 1059–1065

Pollution prevention

1063

37.8 °C 65.6°C (100 °F) (150 °F) -45.5 °C -17.8 °C 10 °C (-50 °F) (0 °F) (50 °F)

-45.5 °C -17.8 °C 10 °C 37.8 °C 65.6°C (-50 °F) (0 °F) (50 °F) (100 °F) (150 °F)

Temperature

Temperature

other significant project requirements [19]. In this stage, everyone who participates in the project should be in agreement with the concept [7]. Detailed engineering execution begins once the PFDs, P&IDs, equipment specifications, and the overall scope of the project are completed [19]. Planning and scheduling helps ensure project quality and efficiency [19]. The project for upgrading start-up and shutdown operations to reduce flaring is a nontrivial job, and it not only involves a lot of experienced staff and various stages of work, but also consumes much time. Ideally, the project time should fall within the scheduled downtime. Advance planning and realistic scheduling are the keys to make this ideal a reality. On the contrary, poorscheduled and/or extended shutdowns can result in excessive costs, business losses, and safety or environmental 0 5 10 15 20 25 30 35 40 45 50 incidents [21, 22]. To plan, schedule, and execute the project, it is very Tray Index essential to organize a team to support and direct the pro(a) ject. This team will be responsible to develop strategic planning, marshal internal and external sources, manage DeC2 Temperature Profile human power, and monitor work progress at stages. When necessary it is charged with removing any barriers so as to make sure the project is accomplished smoothly. Therefore, the team should include representatives from the operators, process engineers, maintenance staff, rotating equipment specialists, and plant leaders [7]. Once a collaborative environment is established, the team goal is then set up: minimize flaring during plant start-up and/or shutdown. The objectives connected to this goal are usually as follows: (i) reduce volume and duration of flaring; (ii) install all project-required equipments and pipelines; (iii) minimize project duration; (iv) ensure carrying out the project smoothly. Within these objectives, the steps to be done are: plan activities, set up jobs, define work scope, schedule assignments, execute tasks and monitor work progress, and eval0 5 10 15 20 25 30 35 40 45 50 uate project quality. According to workload at stages, perTray Index sonnel in specified numbers should be brought in to (b) accomplish the work at each stage and released when their jobs are completed. This personnel includes project engiFigure 3. DeC2 tower temperature profiles [12]. neers, construction engineers, cost control specialists, recording and file keeping supervisors, purchasing staff, used to recover off-spec materials during start-up operations. welding engineers, material engineers, piping supervisors, The plant-wide DS was performed to test the feasibility and excavation and foundation supervisors, transportation and delivery experts, and field construction supervisors [19]. The key operability of the plant start-up procedure when setting up these recycles. It was reported that the DS successfully helped to execution is good communication, organization, supervithe plant start-up operation, and the flaring emission was sion, and a contingency plan. Communication ensures that the right people receive the required information to execute their reduced by over 50 % from the estimation. work; organization ensures that roles and responsibilities are met; supervision ensures that daily and weekly schedules are 4.1.3 Execute a Planned FM Project executed; the contingency plan ensures that unforeseen issues can be handled promptly and effectively [23]. Upon the completion of construction, the plant will be Once a planned FM project is approved, it should be considcommissioned. During this period, operators conduct the ered from the systems’ engineering point of view and impleimproved procedures, collect information, and provide commented in a systematic way. It starts with marking up an existments and suggestions for further improving the procedures. ing process flow diagram and providing a project description Process engineers measure the success of the modified processthat defines (i) which modifications are to be made, (ii) which es/procedures and test operators’ suggestions, and further equipments are required, and (iii) what is needed to meet DeC2 Temperature Profile

Chem. Eng. Technol. 2010, 33, No. 7, 1059–1065

© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

www.cet-journal.com

1064

X. Yang et al.

improve the process. Operation during this period is for troubleshooting and ensures that the plant start-up, daily operation, and shutdown are performed safely, reliably, and efficiently. The team leader needs to conduct meetings to evaluate the project, update documents, conclude the project, assign responsibility for further improvements, etc.

4.2

Unplanned Plant Events

Unplanned events occur in plant operation as a result of fault operations, equipment malfunctions, power outage, etc. Such events require immediate and synergistic actions to reject process disturbances and suppress process upsets, so as to protect plant personnel and facilities. For unplanned events, plant safety is always the first priority. Flaring is one of the most frequent choices during these unplanned events, because it is a quick and effective way to protect the plant personnel, facilities, and the ambient environment. FM activities should not jeopardize plant safety and should be very carefully implemented. In practice, two types of unplanned events contribute to a significant part of flaring: process upsets and plant trips. FM details of these two events are discussed below.

4.2.1 Process Upsets Process upsets mean that facilities run in an unstable or an undesirable situation that may jeopardize product quality and/ or plant safety. The most significant flaring scenarios during process upsets occur when off-spec products are generated and have to be burned. To reduce the flaring caused by process upsets, recycling offspec materials to the upstream process for reprocessing presents a good choice. Two major options are available to route the off-spec components: one is to line the recycle to the CGC suction, and the other is to tie it to the furnace feed [6, 8]. Both options have advantages and disadvantages. For the former option, there are limits for the recycled flow rate. Too big flow rates will cause further unstable operations as a result of the snowball effect. The latter option, because ethylene and/or propylene are brought to cracking again, will generate more acetylene and MAPD and subsequently take a longer time for product streams to come back on-spec. Under this scenario, the furnace COT set point and the flow rate of raw feed should be considered to decrease to some extent. Another choice for handling the off-spec propylene stream is to store it temporarily because of its noncryogenic temperature for the liquid phase. To determine the best FM way, plant situations and upset root cause should be investigated case by case. Under this situation, dynamic simulation-based decision making can be helpful.

result of a power outage, instrument air failure, equipment malfunction, lightening weather, etc. Plant trips usually cause the partial or total shutdown of a plant immediately, allowing very little time for specific planning. Under such situations, piping and equipments in the system should be depressurized to flare to protect personnel, equipments, and facilities. The ambient environment is usually polluted, because it is often impossible to make adjustments necessary to minimize flaring to the same extent as what should be done during a scheduled shutdown. Despite this situation, there are still actions that can be taken to reduce flaring. As the flaring source is the furnace sector, reducing the furnace load as quickly as possible is the best option [8, 9]. Like Shell Chemicals at Deer Park, plants can develop an emergency shutdown sequence for furnaces based on their flaring impact [9]. The following rules may be useful: (i) shut down large-capacity furnaces before small-capacity furnaces; (ii) shut down cracking furnaces with lighter feed first. Note that when shutting down furnaces, the feed flow rates should be well controlled within equipment constraints. The simulation can be helpful to identify which way is the most effective and which shutdown rate is the safest and fastest for flaring reduction. There are some situations that furnaces just need to be cut down instead of shut down. In these cases, the cut rates can be optimally determined with the help of a simulator. Also note that operators’ faultless handling of emergencies definitely helps plants accomplish flare minimization. As some emergencies happen occasionally, operators seldom have experience to handle them. Simulations can virtually create these situations so that operators can have a better training. Simulations can also be used to develop a standard operation procedure (SOP) and train operators to improve their operation when facing these emergencies.

5

Concluding Remarks

Flaring is an important but passive method in ethylene plants to protect the personnel, plant, and the environment. Excessive flaring causes air pollution problems and negative societal impacts, as well as tremendous raw material and energy losses. Thus, flare minimization has great benefits to environmental, societal, and industrial sustainability. Based on various industrial practices, a general solution strategy is presented to minimize ethylene plant flaring. In this strategy, various flaring activities are classified and major possible flare minimization measures are presented and discussed. The significance of modeling and simulation for helping plant flare minimization in terms of examining operation feasibility and safety and developing training programs and SOP to improve operators’ performance is also highlighted.

4.2.2 Plant Trips

Acknowledgements

Plant trips usually bring plants to emergency situations, under which quick and effective actions are a must. Most plant trips are CGC or refrigeration compressor tripping, primarily as a

This work was supported in part by the Texas Commission on Environmental Quality (TCEQ) and the Texas Air Research Center (TARC) in the United States.

www.cet-journal.com

© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Chem. Eng. Technol. 2010, 33, No. 7, 1059–1065

Pollution prevention

References [1] Q. Xu, K. Y. Li, Integrated Environmental Management Consortium Meeting, Houston, TX, June 2008. [2] J. Nam et al., Atmos. Environ. 2008, 42, 4198. [3] W. Vizuete et al., Atmos. Environ. 2008, 42, 7641. [4] M. P. Loring, S. K. Smith, AIChE Spring National Meeting, Houston, TX, March 1995. [5] A. Shaikh, C. J. Lee, Hydrocarbon Processing 1995, July, 89. [6] M. Chellino, AIChE Spring National Meeting, Houston, TX, April 2001. [7] M. Williamson, J. Dennehy, AIChE Spring National Meeting, Houston, TX, April 2001. [8] S. Krientenstein, AIChE Spring National Meeting, Atlanta, GA, April 2005. [9] E. B. Kagay, M. Taylor, N. Genty, AIChE Spring National Meeting, Atlanta, GA, April 2005. [10] D. Chenevert et al., Oil Gas J. 2005, 103, 54.

Chem. Eng. Technol. 2010, 33, No. 7, 1059–1065

1065

[11] S. Cullen, J. Saionz, AIChE Spring National Meeting, Orlando, FL, April 2006. [12] A. Singh et al., Int. J. Environ. Pollut. 2007, 29, 19. [13] Q. Xu et al., Proc. FOCAPO 2008, 247. [14] Q. Xu et al., Proc. FOCAPD 2009, 603. [15] Q. Xu et al., Ind. Eng. Chem. Res. 2009, 48 (7), 3505. [16] X. Yang et al., Ind. Eng. Chem. Res. 2009, 48 (20), 9195. [17] P. Rutherford, W. Persad, M. Lauritsin, Hydrocarbon Processing 2003, October, 75. [18] K. Y. Cheung et al., Comput. Chem. Eng. 2004, 28, 91. [19] R. Elshout, D. Garcia, Chem. Eng. 2009, July, 34. [20] A. Bernard, AIChE Spring National Meeting, Houston, TX, April 2007. [21] J. Thompson, C. Etter, AIChE Spring National Meeting, Houston, TX, March 2001. [22] A. Bonton, M. Thomas, AIChE Spring National Meeting, New Orleans, LA, March 2003. [23] K. Lewton, www.metdemand.com. December 2005.

© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

www.cet-journal.com