July 2016

Special Report: Refinery of the Future

Teaching an old plant new tricks: The rise of the methanol plant revamp

The last time we saw such marked activity in the methanol industry was back in the 1990s and early 2000s, when methanol production in North America plummeted by almost two-thirds.

Stanbridge, S., Jacobs Consultancy

The last time we saw such marked activity in the methanol industry was back in the 1990s and early 2000s, when methanol production in North America (NA) plummeted by almost two-thirds. High gas prices forced the mothballing of plants, with many never restarting. The shale gas bonanza has shaken the industry awake, and there is a rush to market. Restarts and relocations are hoping to beat newbuilds to the finish line. The ripple effect of this newfound competition is ruffling feathers around the world, with a push for efficiency now at the top of the to-do list.

One of the world’s largest and most diverse providers of technical professional and construction services is now helping clients to not only build new plants, but also to teach their old plants new tricks and coax some out of retirement, all of which present significant and complex challenges.

These old plants take some convincing, but it can be surprising what motivation the offer of a relocation package or revamp can bring. For the operators that succeed in getting their old plants up and running, the benefits are numerous with increased asset realization and improved project schedules, environmental performance and plant life extension, to name a few.

This article presents key process feasibility aspects that should be assessed prior to tackling the enormous challenges faced by the mechanical, civil, structural, piping, instrumentation, electrical and construction disciplines.

The relocation package

Methanol plants are sensitive but stubborn beasts, and they do not take kindly to having their gas supply significantly reduced or even switched off. Some plants just give up altogether and long for demolition: the more adventurous ones can be convinced otherwise. The thought of upping roots and moving to new pastures is never the most attractive prospect. However, the lure of warmer weather and a secure/reasonably priced gas supply is hard to beat. From an operator’s perspective, moving an existing plant from Chile to Louisiana presented schedule benefits over building a new facility, not to mention further utilization of non-running assets.

To sell the entire relocation prospect to the plant owners, operators must be fully aware of the following important aspects:

  •  Feedstock
  •  Climate
  •  Available utilities
  •  Regulatory upgrades, including gas and liquid effluents—e.g., nitrogen oxides (NOx), etc.

Good-quality, local feedstock is the most important consideration. Any increase in feed gas carbon dioxide (CO2) or C2+ content will increase methanol production and raise more steam in the reformed gas train, with reductions in carbon having the opposite effect. Inert content is also important. Higher nitrogen (N2) content in the feed gas results in larger, inert concentration within the methanol synthesis loop, reducing reactant partial pressures and dropping production. This can have an additional effect of increasing methane slip in the reformer, compounding the loop inert content and increasing NOx volumes from the reformer stack.

Considering the relocation challenges

It is a fairly easy sell to move the plant from a cold Chilean coastal location up to the warmer climes of the US Gulf Coast (Fig. 1), but planning must ensure that it will be prepared for the heat. That is, how are air coolers going to behave in sunny Louisiana compared to chillier Chile? Atmospheric temperatures have increased with the move, thereby reducing the cooling/condensing capacity of the air exchangers. Each cooler needs to be considered on an individual basis, and additional cooling/condensing provisions should be made by introducing new water-cooled exchangers, or by adding a bundle or two to the existing air exchangers. The extra cooling equipment can present complex and extensive piping and stress challenges.

Fig. 1. An aerial view of the new methanol plant site in Louisiana. Moving an existing plant presented schedule benefits over building a new facility, as well as further utilization of non-running assets.
Fig. 1. An aerial view of the new methanol plant site in Louisiana. Moving an existing plant presented schedule benefits over building a new facility, as well as further utilization of non-running assets.

One additional concern with the climate relates to seawater. If a plant is located by the ocean, it makes sense to utilize such a huge heat sink. The plants in Chile used seawater for the larger-duty methanol loop and turbine vacuum condensers. The impacts of the reduced vacuum resulting from the higher cooling temperatures—associated with the new cooling water tower loop needed in Louisiana—required due attention to ensure that the turbine performance was not restricted.

Having decent utilities can be a good lure for a methanol plant looking to relocate. Consider obtaining a local supply of water, N2, medium-pressure (MP) steam and, perhaps, effluent treatment. These items reduce initial capital expenditure (CAPEX) burdens, and also benefit from the economies of scale offered by larger-scale utility complexes. It is important to assess the capabilities of these supporting systems that provide safety-critical functions. For example, MP steam must be available immediately during a reformer trip, which requires boilers with large turndown capabilities, or flexibility within the external steam users to have supplies reduced accordingly.

If offered decent feedstock, weather and utilities, it is time for the methanol plant to give something back. In relocating to a new jurisdiction, the most up-to-date and location-specific environmental and operating regulations must be adhered to. The most prevalent in the new Louisiana location were those for NOx and volatile organic compounds (VOCs). The plant had to undergo several modifications to adhere to NOx criteria of around 5 ppmv. There are several options available, including burner replacement, selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR). Each option represents various challenges to the plant with respect to performance, CAPEX and process impact.

Fig. 2. A methanol reformer crossing a temporary road bridge.
Fig. 2. A methanol reformer crossing a temporary road bridge.

Giving the green light

Once all parties are agreed and satisfied with the location, and all relevant regulations have been identified and modifications agreed upon, the owner is finally ready to commence the relocation. This is the cue to begin the herculean undertakings by the deconstruction and construction teams. These teams are required to grind, blind, chop, crop, plug and lug this plant some 7,500 mi up the east coast of the Americas using caterpillars (Fig. 2) and massive vessels (Fig. 3)—an epic endeavor in every sense.

One word of warning: care must be taken when packing up these plants. It was only during the dismantling of a Canadian methanol plant for shipment to China several years ago that personnel noticed a lorry-sized hole in the fence. It was then discovered that they were 21 one-ton exchanger heads short. It is safe to say that all equipment sent from Chile arrived in Louisiana, and the odds of making a getaway with an 800-ton reformer radiant box at 2 mph are pretty slim.

Fig. 3. The herculean task of deconstructing and relocating the methanol plant some 7,500 mi involved extensive land and sea transportation.
Fig. 3. The herculean task of deconstructing and relocating the methanol plant some 7,500 mi involved extensive land and sea transportation.

The makeover

After being shut down in 2004, the methanol plant in Channelview, Texas was buttoned up and kept in good shape under a maintenance program for many years. However, after being out of action for a while, it was in need of a good makeover, and several things needed addressing prior to startup:

  •  Mechanical equipment assessment and refurbishment/replacement
  •  Finite life equipment analysis
  •  Gap analysis and detailed engineering.

Mechanical equipment assessment and replacement

The plant’s operators are beginning to think about methanol again, but there is a problem. As time has passed, the general borrowing of parts that befalls idle mechanical equipment means that a few heat exchangers and a distillation column are missing. Returning borrowed equipment back to service can take some time, especially if it has had a “career change.”

A distillation column will have very different vapor/liquid traffic and tray efficiencies in an isobutylene application, as compared to a methanol refining application. The column may need retraying; liquid and vapor distributors might require replacement and, most likely, a very good scrub. Equipment replacement can be slightly more involved. A like-for-like exchange would be the ideal situation, but changes in mechanical design codes are always developing. Presenting a redrafted mechanical datasheet to a vendor may end up with a raft of modifications being sent back, as well as equipment that no longer fits into the piping ends where its predecessor was placed. These challenges are not insurmountable, but they will add complexity and additional engineering time to the project. The most pressing matter with equipment replacement is always lead time on delivery. It is essential to specify the equipment as soon as possible and contact procurement and vendor contacts.

Finite-life equipment analysis

Admittedly, the physical presence of equipment is only slightly more important than its good material condition. Maintenance programs can help ensure this, but certain aspects of a methanol plant have components with a finite life that will need detailed assessment to determine their condition. Certain sections of the standard methanol plant are exposed to aggressive process conditions: they often operate at the edge of their mechanical design envelope, and some sections within the creep region.

The reforming section is one example. The reforming of natural gas at 880°C and 20 barg calls for the use of exotic microalloy steel tubing. These reformer tubes are operating within the creep region and are, therefore, designed for an operating life of approximately 100,000 hr at specific conditions. Operating in the creep region can present significant challenges in the quantification of remaining tube life, especially if tubes have been exposed to periods of high temperatures as a result of trips and/or catalyst degradation. A plant in an idle state is well placed to undertake mechanical inspections and nondestructive testing (NDT) to determine any wall thickness losses/extent of creep and translate this data into a remaining tube life through mechanical calculations (e.g., API Standard 530).

Other finite-life equipment, such as reformed gas boilers and reformer convection section coils, can be reviewed in a similar fashion to determine remaining equipment life. It may be the case that startup can be permitted with sufficient life remaining, with future equipment replacement penciled into future turnarounds of the restarted plant.

Gap analysis and detailed engineering

With major equipment replaced and assessed, the finishing touches to the makeover can progress. As with most plant revamp projects, attention to detail is key. Starting an old plant within a new regulatory environment may demand upgrades of control and instrument systems or electrical systems that fall under ATEX/HAZLOC directives. Validating and managing the interfaces with older systems can require significant engineering effort and drain resources when the original as-built documentation is inaccurate.

Other detailed engineering efforts, such as the replacement of valve internals and refurbishment of older rotating equipment, require early planning to ensure the availability of relevant parts and their delivery to the required schedule. Piping design should be reassessed when introducing new equipment into the system to ensure adequate protection and support to any new nozzles and interconnecting pipework.

The personal trainer

If the offered relocation is met with some consternation, a “personal trainer” can sometimes swing the balance. Some old plants require training sessions to improve methanol production performance. Under increasing efficiency pressures (primarily as a knock-on effect from site gas cap restrictions), the focus has been on improving a plant’s overall fitness and efficiency, enabling it to produce more methanol for a given or reduced feedstock volume.

The trainer puts the plant through its paces and will complete a benchmark assessment to gauge its current performance. This is reflected by the specific gas efficiency (Eq. 1):

(1)

For a typical steam methane reformer (SMR)-based methanol plant, this value is 35 MMBtu/metric t to 37 MMBtu/metric t (HHV). Based on this assessment, and after discussion with the operator, a strict dietary and exercise regime will be developed. At this stage, it is important to keep the goals realistic and achievable. It is worth noting that with significant plant modifications, efficiency improvements of about 5%–10% can be expected.

This regime has several common aspects and is interspersed with specifics depending on the individual circumstances of the plant. Essentially, the plant must reduce consumption and consume a varied diet.

Reducing consumption

Natural gas is supplied to a typical SMR methanol plant for both process feedstock (typically 80%) and utility fuel (20%). In the syngas generation section, efficiency gains are all about reducing the methane slip and making more syngas for a given natural gas feed flow.

Increasing the reforming reaction temperature has by far the most significant effect on methane conversion, but improvements in SMR plants are restricted by the mechanical design conditions of the reformer tubes and outlet system. Increasing the steam-to-carbon ratio would reduce the methane slip, but this may result in increased reforming furnace and boiler loads, using more natural gas. In one consultant’s experience, process side efficiency improvements in the SMR section are limited, and the best improvements are often found in the operation of the reformer (i.e., reducing tube temperature scatter).

The retrofit of a prereformer can be attractive in certain applications to reduce overall reforming duty. Using some of the sensible heat normally provided to steam raising/superheating in the reforming duty increases syngas production at the expense of decreasing steam. This presents benefits for debottlenecking, but if gas supply limitations are present, then the increased CAPEX associated with a fired heater or reformer inlet system upgrade can hamper its economic viability.

After ruling out any major improvements in the syngas section, attention quickly moves to the methanol synthesis section. Fundamentally, the more methanol produced from a given syngas quantity, the better the overall plant performance. Methanol synthesis performance is defined by the carbon efficiency (Eq. 2):

   (2)

Typical methanol plant carbon efficiencies can range from 89%–95%. Several opportunities exist for carbon efficiency improvement, including the adjustment of methanol loop process conditions, improved catalysts and changing the reactor operation or addition of CO2 if it is available. Of these, one of the largest gains in improving efficiency is through changing the reactor operation.

Fig. 4. Three competing methanol synthesis reactor designs, each with a different heat transfer mechanism.
Fig. 4. Three competing methanol synthesis reactor designs, each with a different heat transfer mechanism.

As illustrated in Fig. 4, three methanol synthesis reactor designs are dominant in the market, each with a different heat transfer mechanism: quench, tube-cooled convertor (TCC) and steam-raising convertor (SRC).

  •  Quench—Inter-bed direct cool feed gas injection
  •  TCC—Counter-current gas exchange
  •  SRC—Steam-raising, isothermal bed temperatures.

The TCC has the greatest methanol production and carbon efficiency per unit volume of all the reactor types. In the context of improving the gas efficiency of the plant, the cost and risks associated to justify the changing out of a quench for a TCC would be difficult. A more widely available option is to convert the quench reactor into an isothermal steam-raising type and increase the interchanger bypasses. Quench-type reactor systems often have the largest required catalyst volumes. These large vessels are prime candidates for a retrofit with isothermal type internals. As a demonstrated method of improving carbon efficiency, this retrofit may be on the order of 1 MMBtu/metric ton of methanol.

Converter upgrades of this type, although providing a way to improve capital efficiency of existing equipment, can incur a significant downtime penalty when compared with other potential plant modifications. Both the once-through reactor (OTR) and purge gas reactor (PGR) will increase the plant’s carbon efficiency with only standard downtime requirements (due to the requirement of only pipework tie-ins on the main plant). Both reactor arrangements enable extra methanol synthesis catalyst to be introduced into the process, thus enabling improved methanol production.

The OTR can be installed between the stages of the syngas compressor and the suction of the circulator, debottlenecking sections of the compressor train and synthesis loop while producing more methanol. The OTR is often an isothermal type, due to the reactive nature of the fresh syngas, no recirculation and the low equilibrium methanol composition. However, due consideration must be given to the steam balance. The PGR is generally installed on the purge gas outlet from the main synthesis loop. The purge gas is less reactive and possesses higher inerts than the fresh syngas feed, so a TCC is often a good choice for installation, as the lower heat transfer requirements can be more easily managed with this design. A loop or once-through arrangement can be considered for a PGR, depending on the nature and range of operation of the main loop. Both options can result in efficiency improvements of 0.5 MMBtu/metric t to 1 MMBtu/metric t methanol.

Varying diet

A balanced and varied diet includes natural gas, water (H2O), oxygen (O2) and carbon oxides (CO and CO2). Eq. 3 represents the optimum syngas composition for methanol synthesis, known as the “R” number:

R = (H2 mol – CO2 mol) / (CO2 mol + CO mol)          (3)

The syngas mixture produced in an SMR has an R ratio of 3:1; however, the stoichiometric requirement for methanol synthesis is only 2:1. At its simplest, this reveals a significant amount of excess hydrogen. The easiest option to reduce excess H2 is the injection of carbon oxides. Methanol plants are often located near sources of available carbon, often CO2. If the plant is fortunate, there might be a nearby ammonia or even an ethylene oxide complex that will allow it to “purchase” its potential waste stream at a reasonable price, reducing the R ratio to about 2.1—the realistic optimum for methanol synthesis.

Injection of CO2 can be carried out either upstream of the reformer or into the synthesis loop, and there are important considerations for each location. Injection upstream of the reformer requires control system modifications, heat transfer and hydraulic limitation checks, and produces a more reactive syngas that requires further reactor modifications. This is best carried out on a newbuild plant where these design aspects can be addressed. On an existing plant that already has fixed pipework, injection into the front end is not always the most suitable location.

CO2 injection to the methanol synthesis section requires slightly less plant modification, but results in a relatively smaller increase in methanol production. Both options result in increased power requirements of the syngas and circulating compressors, and may lead to capacity limitations due to machine performance.

The introduction of O2 into the diet is slightly more involved, and, while making syngas more stoichiometric, it is often associated with capacity increase applications. It also presents significant challenges for retrofitting into an SMR-based methanol plant.

The introduction of CO2 and associated equipment modifications will save 0.5 MMBtu/metric t, and the introduction of O2 saves 1 MMBtu/metric t, albeit without considering the energy associated with O2 generation.

Rise of the revamp

Many interesting and challenging projects have contributed to the rise of methanol plant revamps. Working alongside different methanol plant operators has shown that, given the right motivation and incentives, our stalwart and retired methanol plants can be relocated, “made over” or “trained” to compete with higher efficiencies and profitability. HP

The Author

Related Articles

From the Archive

Comments

Comments

{{ error }}
{{ comment.comment.Name }} • {{ comment.timeAgo }}
{{ comment.comment.Text }}