Petrochemical DeveloPments SpecialRepoRt

eplacing the ethylene and/or prop‑ ylene refrigerant compressors and their steam turbine drives is a major

cost component in the revamp of olefin plants. Sometimes, the capacity expansion goal may require new, larger compressor cas‑ings and/or new compressor foundations. In some cases, these modifications can be avoided by including a thorough evaluation of process heat integration, quantification of equipment actual performance and investi‑gation of process redesign opportunities dur‑ing the revamp study. Also early involvement and iteration with the compressor and tur‑bine vendors is crucial for project success.

A process‑heat integration study (pinch analysis) can identify process design changes that reduce refrigerant compressor shaft‑work power, i.e., stage volumetric flowrates. Equipment performance analysis defines actual capability of heat exchangers, com‑pressors, turbines and distillation columns. This information can be used to decide how to optimize plant operating conditions and determine the best options to invest capital in revamped or new equipment.

This case history describes a recent eth‑ylene plant revamp. The ethylene producer, consultant and vendors worked together during the feasibility study to change the original design and establish new process

operating conditions that avoided replacing the ethylene and propylene compressors.

Life cycle of an olefins plant. Ethyl‑ene plants generally go through at least one revamp during their service life to increase operating capacity and take advantage of

market opportunities. The revamp scope depends on the desired increase in produc‑tion capacity. “Creep” revamps (up to 15% increase in capacity) include using excess capacity (over design margins) by adopt‑ing more aggressive plant operation using advance process control or real‑time optimi‑

Reduce revamp costs by optimizing design and operationscombining process and equipment analysis with vendor solutions can minimize capital spending

J. J. Lee, B. H. Ye and H. Y. Jeong, Korea petrochemical Industries Co., Ulsan Metropolitan City, South Korea, and F. J. ALAnis, i. sincLAir and n. s. PArk, Aspentech UK Ltd., Warrington, UK

R

Understanding plant limits

Working withequipment vendors

Pinch and column analysisTest run data andP&ID analysis

Vendor iteration:Compressors

steam turbinescolumn internals

Cost estimates

REVAMP capacity selectionstrategy (phased approach?)

project development

Equipmentrating

Equipment revamplist and ranking

Understanding plantheat integrationUnderstanding plant operation

Preliminary projectsPredictive

simulation model

Test runsimulation model

High loadtest run

Capacity and revampoptions modifications list

equipment capacity checks

Revamp package

Work flow for ethylene plant revamp using integrated approach.Fig. 1

april 2007 issue, pgs 77–81Used with permission.www.hydrocarbonProcessing.com

originally appeared in:Article copyright © 2007 by Gulf Publishing Company. All rights reserved. Not to be distributed in electronic or printed form, or posted on a Website, without express written permission of copyright holder.

HYDROCARBON PROCESSING ApRil 2007

SpecialRepoRt Petrochemical DeveloPments

zation techniques together with a minor update of equipment.1,2

After creep expansions are implemented, a much higher increase in capacity (“leap” revamps, typically 40% or higher increase in capacity) is usually required to produce an accept‑able return on investment.2 Major capital investment can be required when a conven‑tional revamp strategy separately addresses each equipment item that limits capacity, and modifies or replaces them by larger equip‑ment. Often, the resulting high investment cost and/or lengthy downtime required for project implementation can render the proposed revamp as unattractive.1,4

Traditional feasibility studies done to define the scope and capital cost of the revamp do not simultaneously include pro‑cess design, equipment design and oper‑ating changes that can reduce investment and operating costs. An alternative revamp approach was developed to capture these missing opportunities for investment and operating cost reductions.3

This alternative approach, also called the integrated approach (IA), has been applied to more than 35 ethylene revamp projects and its economic benefits fully demonstrated.4,6 Such applications have ranged from creep to leap revamps. However, applying the IA method fits better in the capacity creep projects where minor process design modi‑fications and operating changes can yield production capacity increases. In all cases, the IA has been instrumental in:

• Defining the optimum capacity, scope, investment cost, design and operating changes

• Reducing investment cost ($/lb) and energy usage (Btu/lb)

• Improving the likelihood of revamp approval and capital sanction by minimizing risks and improving financial justification.

iA to low-cost revamps. The IA is based on a deep understanding of the plant and its limitations. Such understanding is provided by a thorough evaluation of plant operation, process heat integration and equipment performance. A typical workflow for the integrated approach is summarized in Fig. 1.

Understanding plant operation. The revamp feasibility study starts with a high load test run and data gathering. Analysis and review of this data with plant operators

together with a review of the plant’s piping and instrumentation diagrams (P&ID) help to develop a clear understanding of plant operation and how operating conditions have changed since plant design. It also iden‑tifies, early on in the study, whether potential operational or design changes can improve plant performance. This is important since the plant owner does not have to wait to the end of the feasibility study to start imple‑menting beneficial operating changes.

For example, in a recent revamp study, it was found that a heat exchanger hot‑side bypass had been added to one of the demethanizer reboilers. A third‑stage eth‑ylene refrigerant chiller was downstream of this reboiler. A second reboiler on the demethanizer was subcooling fourth‑stage (warmest level) ethylene refrigerant. Closing the bypass reduced heat load at the lower refrigeration temperature levels and reduced the ethylene‑refrigerant compressor speed and shaftwork, which provided additional capacity to both compressor and its steam‑turbine driver.

Understanding plant limitations. The second step in the approach is to develop a rigorous simulation model of the plant. The model is tuned to match test run data and then converted from a point solution to a scalable model. So, this model can react correctly to changes in capacity, feedstock, yield and utilities such as cooling water tem‑perature.

This is a critical step in the IA since the model must mimic current plant perfor‑mance as it is the basis for the equipment rating. Equipment rating defines the per‑formance and quantifies limiting or spare capacity of major process equipment items. Ranking of all equipment items based on spare capacity can visualize and understand plant limitations and opportunities. Fig. 2

shows an example of equipment rating results and ranking.

In this particular example, the cracked gas compressor (CGC) turbine, propylene refrigerant compressor and its steam turbine are maxed out and therefore lim‑ited plant capacity. Conversely, the ethylene refrigerant compres‑sor and its steam turbine have some spare capacity (compres‑sor speed is below the maximum continuous speed and brake‑horse power is below the turbine’s design rated power).

Note: Assessing available spare equipment capacity is only possi‑ble by including sufficient details

in the simulation model. For example, pro‑cess and refrigerant compressors are set up with performance curves to predict head, speed and efficiency at higher loads and/or different suction pressure. The performance curves used are design performance curves “tuned” to test run data. This allows a more realistic prediction of performance at differ‑ent loads and/or process conditions.

Understanding plant heat integration. A major challenge driving the selection of a minimum capital cost process revamp scheme is identifying design and operating changes that will help shift plant bottlenecks from expensive to cheaper equipment items and thus minimize the total capital invest‑ment costs.

Ethylene plant designs are highly inte‑grated, particularly in the cold‑end sepa‑ration section of the plant where process streams, distillation columns and refrigera‑tion systems are heat integrated. Design fea‑tures can include distillation column side exchangers and reboilers and recuperation or sub‑cooling of refrigerant. There may also be process‑to‑process heat recovery to reduce refrigeration usage and the provision of adia‑batic or mechanical expansion to generate low temperatures for product recovery.

This high level of integration results in complex interactions within the process, between the process and utility systems and with refrigeration systems. Column operat‑ing pressure affects separation performance, column hydraulics, compressor performance and condenser/reboiler required heat trans‑fer area. The choice of refrigerant level for sub‑cooling and recuperation affects com‑pressor performance, capacity and required heat exchanger size.

Compressor suction pressure affects com‑pressor capacity and pressure profile over the stages and influences the required heat

Compressors andturbines rating results

0

10

20

30

40

50

Processgas com-pressorturbine

C3=

com-pressor

C3=

com-pressorturbine

C2=

com-pressor

C2=

com-pressorturbine

Processgas com-pressor

C1com-

pressor

Incr

ease

C2=

pro

duct

ion,

%

example of limiting or available spare capacity of rotating equipment items for plant revamp.

Fig. 2

HYDROCARBON PROCESSING ApRil 2007

Petrochemical DeveloPments SpecialRepoRt

transfer area. These are a few examples of such interactions and they highlight some of the degrees of freedom available to the designer. As a result, the complexity and number of possible combinations at first sight may appear overwhelming, and the task of finding improvements within the limited time available by means of inspec‑tion or trial and error may seem daunting and time consuming.

Thermal pinch and distillation column analysis make it easier to understand the design interactions within the process and between the process and utility systems. These techniques are already well established in the literature for heat exchanger networks, distillation columns and utility systems.5

Pinch and column analyses have some interesting features. An important one is the ability to exploit the fact that the process and the utility systems interact. By capital‑izing on these interactions, it is possible to more effectively use existing or new equip‑ment. A good example of this is distillation column feed conditioning. Column analy‑sis may find that thermal conditioning of a column feed is beneficial. Feed preheat recovers refrigeration at a colder level than the reboiler, or, conversely, feed cooling shifts colder utility use from the condenser to a warmer level in the feed chiller. In both cases, adding a new feed exchanger finds more capacity in the refrigeration compres‑sor, beneficially reduces the vapor and/or liquid traffic within the column, and may unload the condenser and/or reboiler.

Working with equipment vendors. Selective process redesign and correctly applied pinch projects can reduce sub‑ambi‑ent utility requirements. This will unload some or all of the stages of the refrigerant compressors. It is normally desirable, if pos‑sible, to evenly unload each of the stages of the refrigerant compressors to a point where no modifications to the compressor and its driver are required. This involves assessing the revised load on the compressors for each process modification project and selecting an optimum combination of projects that will free up most refrigeration and process equipment capacity.

In most instances, the compressors’ spare capacity gained through process design improvements is lower than the spare capac‑ity available in other items of equipment. There is then the opportunity to introduce refrigerant compressor modifications or replacement to bring their capacity into line with that of all other equipment items in the plant to achieve the revamp capacity target.

Modifying the compressor (re‑rotor) in its existing casing is the lowest cost option.

Rather than scaling compressor loads up and requesting a solution from the compres‑sor vendor, the integrated approach includes working and iterating with the compressor and turbine vendors to define maximum stage flows and inter‑stage operating con‑ditions that are needed to avoid expensive compressor/turbine replacement. With this information, the optimum combination of process modification projects is re‑assessed to meet the compressor’s modified performance as defined by the compressor vendor.

Since the refrigerant compressors are heat integrated with the process (via refrigerant subcooling and recuperation), a few itera‑tions are needed to define the new refrig‑erant compressor suction, intermediate and discharge pressures, as well as process operating conditions that will result in maxi‑mum revamp capacity at lowest investment cost and with lowest implementation time. Interaction with column internal vendors and special heat exchanger vendors (core‑in kettle and high‑flux exchangers, for instance) is also included in the work flow of the inte‑grated approach.

case history. The application of the IA as described here was very instrumental in identifying cost‑effective solutions in sev‑eral ethylene revamp projects. The Korea Petrochemical Ind. Co. (KPIC) operates a 390,000‑tpy (390‑Mtpy) ethylene plant at Ulsan, South Korea. The plant was commis‑sioned in 1991. Design feedstock includes light naphtha, gasoil and C4 LPG with eth‑ane recycle. The plant original design ethyl‑ene production capacity is 300 Mtpy. Plant capacity was increased to 390 Mtpy after a +30% revamp in the year 2000. Furnace coils, distillation tower internals and some heat exchangers were modified at this time.

KPIC engineers carried out this revamp. A previous conventional feasibility study had concluded that, among other modifications, a new cracking heater and associated equip‑ment, replacing of the low pressure (LP) cas‑ing of the CGC compressor, re‑rotoring of the CGC turbine, a new depropanizer and associated equipment, and re‑rotoring of the ethylene and propylene compressors/tur‑bines would have been required to achieve a new production capacity of 400 Mtpy.

KPIC decided to apply the IA method to develop a new revamp plan for its Ulsan site. KPIC initially planned to carry out a two‑stage capacity revamp to take ethylene production capacity to 470 Mtpy (+20% of current capacity), determined by maximum

furnace capacity. According to this plan, the Stage 1 revamp (+10%) was going to take place during the 2005 turnaround; the exact timing of the Stage 2 revamp had not been decided. KPIC’s initial plan was set on the assumed basis that no rotating machinery modifications were needed to achieve Stage 1 capacity. However, it was found early that the charge‑gas compressor turbine limited capacity at current rates and needed to be modified to achieve higher production capacity. Increasing CGC suction pressure was an option to reduce turbine’s power. This, however, would only achieve a mod‑est increase in plant capacity. The turbine vendor indicated that a 13‑month lead time was needed to install the required turbine modifications. As a result, KPIC rescheduled the turnaround to late 2005.

Due to a change in business opportunity with strong regional demand for ethylene, KPIC preferred to maximize ethylene pro‑duction and decided not to adopt the original phased revamp strategy. KPIC’s new strategy was to revamp the ethylene and propylene refrigeration compressors and to push plant capacity to +20% (set by cracking furnace limi‑tations) for the 2005 turnaround.

Operating data and equipment rat‑ing showed that the propylene compres‑sor and its turbine were also at their limit at test run conditions and had no addi‑tional spare capacity. A predictive process simulation model was used to estimate the compressor loading at the increased production rate of +20%. At this stage, the compressor manufacturer was con‑tacted to assess the required compressor modifications.

The compressor manufacturer’s ini‑tial study concluded that the compressor required a new, larger casing. At the project team’s request, the compressor manufac‑turer advised that the maximum increase in capacity attainable with internal modi‑fications while retaining the existing case was between +10% and +15%. Also, at the project team’s request, the compressor man‑ufacturer advised and confirmed the fea‑sible operating region of the compressor’s second‑ and third‑stage discharge pressures of the re‑rotored compressor.

A number of process design and operat‑ing changes were identified to reduce the loading of the compressor. A few iterations with the compressor manufacturer were needed to define the optimum set of design and operating changes required to achieve the target production rate of +20% without replacing the propylene compressor.

Other major modifications in the final

Petrochemical DeveloPments SpecialRepoRt

revamp package include the re‑traying of some towers, rerotoring the ethylene compressor, rerotoring the ethylene and propylene compressor turbines, and install‑ing two new heat exchangers and three new pieces of equipment as part of process modi‑fications that reduced the propylene com‑pressor loading. Most of the modifications were done during the 2005 turnaround. Tie‑ins were also put in place to install remaining modifications early in 2006 without a plant shutdown. The plant is now fully operating at the target production rate of 470 Mtpy.

The economic success has also been very satisfying. KPIC has estimated total

annual profits from production increase and energy reduction at $16.1 million with a 15.7 months payback. HP

LITERATURE CITED1 Walz, R. and R. Zeppenfeld, “Steam Cracker Revamp

Projects: Challenges and Technologies,” Fourth European Petrochemicals Technology Conference, June 26–27, 2002, Budapest, Hungary.

2 McDonald, R. V. and C. P. Bowen, “Recovery sys‑tem to increase ethylene plant capacity,” Petroleum Technology Quarterly, Summer 2001.

3 Alanis, F. J. and I. J. C. Sinclair, “Understanding process and design interactions: The key to effi‑ciency improvements and low cost revamps in ethylene plants,” Fourth European Petrochemicals Technology Conference, June 26–27, 2002, Budapest, Hungary.

4 Choi, B.H., “Asset optimization: A better approach for energy savings and capacity increase,” AspenWorld 2002, Washington, D.C., October 2002.

5 Trivedi, K. K., et al., “Optimize a licensor’s design using pinch technology,” Hydrocarbon Processing, May 1996.

6 US Department of Energy, Office of Industrial Technologies, Energy Efficiency and Renewable Energy, Project fact sheet “Ethylene process design optimization,” September 2001.

ACkNowLEDgmENTRevised and upgraded from an earlier presentation

to the ERTC Petrochemical Conference, Dusseldorf, Germany, Oct. 9–11, 2006.

Byeonghee Ye is an R&D team manager at Korea Petrochemical Co., Ltd., in Ulsan, South Korea. He is responsible for researching new beneficial projects and developing energy-saving ideas for his company.

His industrial experience has focused on simulation, basic and detail design of petrochemical plants. He graduated with a BS degree in chemical engineering from Yeungnam University (Korea) and holds an MS degree in chemical engineering from Pusan University (Korea). E-mail: ybhee@kpic.co.kr.

H.Y. Jeong is an R&D team mem-ber at Korea Petrochemical Co., Ltd., in Ulsan, South Korea. He has 11 years of experience in process engineering, process modeling and process revamps. He graduated with

a degree in polymer science and technology from INHA University, South Korea. E-mail: jhy@kpic.co.kr.

Francisco J. Alanis is a princi-pal consultant in AspenTech’s EMEA services organization. He is based in the UK and is responsible for manag-ing and executing process modeling, energy improvement and low-cost

revamp projects. Dr. Alanis graduated in chemical engi-neering from UMSNH University (Mexico) and holds MSc and PhD degrees from UMIST (UK). E-mail: fran-cisco.alanis@aspentech.com.

iain sinclair is a senior advi-sor for Chemicals and Ethylene in AspenTech’s services organization. He is based in the UK and is respon-sible for managing and delivering process consulting services. Mr. Sin-

clair is a Fellow of the IChemE and has wide experi-ence of many industrial processes and technologies. E-mail: iain.sinclair@aspentech.com.

n. s. Park is a senior business consultant in the AspenTech Sales organization based in Seoul, South Korea. He has supported several energy improvement projects and debottlenecking projects in Korea.

Mr. Park holds a BS degree in chemical engineering from Seoul National University (Korea) and holds an MS degree in chemical engineering from SNU. Before join-ing AspenTech, he worked with SK Corp. as a process engineer. E-mail: ns.park@aspentech.com.

Article copyright © 2007 by Gulf Publishing Company. All rights reserved. Printed in U.S.A.Not to be distributed in electronic or printed form, or posted on a Website, without express written permission of copyright holder.