Author / Liang Jyh Chang, Adjunct Professor, Department of Chemical Engineering, National Taiwan University and Tsing Hua University 

In the first part of this series^[1] it was mentioned that the measures to reduce carbon emission in the petrochemical industry can be summarized in an acronym SCUR which represents 4 words: Save, Capture, Utilization, and Renewable. The first article in this series introduces Carbon Capture and Storage (CCS). In this article, the Save part will be discussed. The objective is to introduce a variety of ways and technologies energy saving can be done in a petrochemical plant.

In a typical petrochemical plant, the majority of carbon dioxide emission is due to burning of hydrocarbon fuels to generate energy. Thus, energy saving is essentially equivalent to carbon reduction. Of course, energy saving has significant economic incentives by itself given that energy cost is usually the second largest operating expense besides feed cost.

Considering that the audience of this platform has a wide variety of backgrounds an effort is made to keep the discussion on the conceptual level and avoid getting into technical details. Besides traditional energy-saving measures recent technology development will be introduced. For traditional energy-saving opportunities US Energy Star program's Energy Guide for Petrochemical Industry^[2] is the best resource. It was published in 2008 and updated in 2016.

Energy is typically used in 3 forms in a petrochemical plant: 1. Direct heating through fired heaters, 2. Steam generation. Steam is then used in heating or driving machinery, 3. Electricity is used in many ways including electric machinery e.g. pumps.

For example, in a typical refinery, the breakdown of energy consumption which is closely related to CO₂ emission is roughly the following: 50% for heating, 30% for utilities like steam generation, 16% from H₂ plant, and 4% from Fluid Catalytic Cracking (FCC).

Most chemical reactions and separations are conducted at elevated temperatures. That's why the feed material needs to be heated up to the desired temperature. Besides high temperatures, certain processes may require very low temperatures (cryogenic process), very high pressure, or very low pressure (vacuum). All these operation conditions need energy to create. For example, naphtha cracking typically happens at 850°C. The process to make butyl rubber, an important material for making automobile tires, usually runs at -100°C. Hydrotreating of heavy hydrocarbons requires pressure as high as 200 bars. On the other extreme, a vacuum distillation tower's pressure is as low as 0.05 bar.

Besides the heating, chilling, high or low pressure usages another important contributor of energy consumption is hydrogen production. Hydrogen is an important element to make many chemicals e.g. ammonia (NH₃) which is a basic ingredient of fertilizers. It is also increasingly important for refineries that need more and more hydrogen to meet tightened sulfur specifications of all fuel products (gasoline, diesel, marine fuel oil, etc) and to upgrade heavy hydrocarbons in crude oil. Today hydrogen is also projected to have much broader roles because of its potential in transportation, energy storage, and power production to reduce carbon emissions. Its importance will grow significantly in the near future. For this reason, a section is devoted to discussing the latest hydrogen production technologies.

Energy saving in a petrochemical plant requires a lot of attention to details and, if capital investment is needed, careful analysis and planning. Let's start with a typical heater in Figure 1 as an example to illustrate the points.

A typical heater's function is to raise the feed temperature T_in in Figure 1 to a desired temperature T_out. Fuel gas (or other fuels), mixed with air, is burned to generate the heat. The feed, transported in pipes, passes through the heater and its temperature is raised to the desired temperature. The amount of energy absorbed by the feed divided by the amount of energy released by the fuel represents the efficiency of the heater. How many different ways we can reduce the carbon emission of the heater through energy saving while still achieving its function?

1. Can we increase the inlet temperature T_in before the feed enters the heater?
2. Since the combustion air also needs to be heated in this process how do we make sure that only the minimum amount of air is used?
3. Can the air's temperature be higher before it enters the heater? 
4. The heat exchange effectiveness between the burning fuel gas and the feed in the tubes is affected by the cleanness of the tube surfaces. How do we monitor and perform periodic cleaning?
5. After burning, the flue gas generated by the combustion typically has very high temperature. Instead of letting it be released to atmosphere can we make use of this heat?
6. Is it better to use pure oxygen instead of air? (The oxyfuel approach mentioned in the first article^[1].)
7. Instead of the traditional heaters, are there heater innovations that offer better efficiency and reliability?
8. Can we use electricity from renewable energy to heat instead of fired heaters?

 

As can you see, just in this relatively simple equipment there are already many potential areas of carbon reduction and energy saving. Some require diligent monitoring and timely action; some require capital spending and planning. Occasionally you hear stories of AI's role in energy saving e.g. a data center energy project by Google's DeepMind^[3] but typically in petrochemical plant's energy saving there is no short cut nor magic bullet.

 

Figure 1. A Typical Fired Heater

Source: Jim Cahill

 

Another major energy user is the utility area especially steam system. A simplified depiction of the system is shown in Figure 2. On this figure the blue lines contain water and the yellow lines contain vapor. Steam is produced in a boiler which is not much different from a home water boiler where fuel is burned to heat up the treated water to generate steam.  The steam is then distributed to the various processes that consume the steam. Typically, a petrochemical plant has at least 2-3 pressure levels of steam users. After the steam serves its purpose, the condensed water then passes through a device called steam trap and then be recycled back to the boilers. A steam system can be thought of as two main parts: the steam production part and the steam distribution part. Each part has many potential opportunities of energy saving.

 

Figure 2. Simplified View of a Steam System

Source: Neelis et al.^[2]

 

The third area to consider is hydrogen production. Hydrogen is an important element to make many kinds of chemicals. In refineries it is used in hydroprocessing to remove sulfur and make heavy molecules easier to be cracked to more useful molecules or chemicals.

At present hydrogen is produced predominantly by Steam Methane Reforming (SMR). In this process Methane (CH₄) from natural gas reacts with steam (H₂O) to form Hydrogen (H₂) and CO₂. It takes approximately 6 MWh of energy to make one ton of hydrogen. Unfortunately, SMR also produces 9 to 11 tons of CO₂ for each ton of hydrogen. Energy is consumed both in the endothermic reaction (~760°C) and also the gas separation of H₂ and CO₂. Figure 3 is a simplified depiction of the SMR process^[4] with the main reactions imposed on the corresponding unit.

In the H₂ economy discussion hydrogen produced by SMR is called gray hydrogen. Gray hydrogen, when combined with CCS, is called blue hydrogen^[5]. One may ask: why don't we just use water electrolysis which does not produce CO2? Most people do not realize that electrolysis requires 55-60 MWh of energy to make one ton of hydrogen. That is more than 9 times of the SMR process! If hydrogen is used in fuel cells there is a better chance to recover the high electrolysis energy but in chemicals manufacturing and hydroprocessing in the petrochemical industry electrolysis is far from being economical. We will discuss a few latest developments in hydrogen production later.

 

Figure 3. Hydrogen Production Using Steam Methane Reforming (SMR)

 

In the following sections, we will discuss various practices and technologies that can contribute to carbon reduction via energy saving in a petrochemical plant.

 

I. Monitoring and Review

In most processes, the best achievable indicators can be established and monitored by the operation staff. For example, the percentage of O₂ in the heater flue gas is an important indicator whether too much air is being used (if all the oxygen is used O₂ should be zero) and the flue gas temperature ("stack temperature") is an indicator of the heat exchange efficiency.

These indicators should be displayed and reviewed regularly so that appropriate actions can be taken. For example, a 2% high limit of O₂ can be set to trigger operator action to reduce air input and save energy if the limit is exceeded.

Many such indicators can be defined in a plant based on safety limit, past experience, or engineering calculations. In many cases, the implementation of action can also be done through automatic control.

On a higher level, the energy consumption of each process unit and the whole plant should be calculated, monitored, and reviewed regularly. If you wonder how your unit's energy performance compares with competitors there are services such as Solomon Survey^[6] that establishes a benchmark energy intensity index (EII) for each type of process units (e.g. FCC) . A plant can calculate its value and compare with this benchmark. This process uses the energy utilization, through put and output to determine the Solomon EII of the plant.

For example, a newly constructed unit with the latest technology and energy saving practices may have EII around 60% while older plant's EII may be around 90% or even over 100%. The EII value for a unit and for the overall plant give the plant a goal post for continuous monitoring and improvement.

 

II. Equipment Maintenance

Equipment in energy service needs periodic maintenance. For example, fouling of heat exchangers, heaters, and boilers require the heat exchange surfaces be cleaned.

Figure 4 is an example of a heater cleaning performed by CPC^[7]. It clearly demonstrated that, as a result of cleaning, the flue gas temperature decreased and the heater efficiency increased.

 

Figure 4. Effects of Cleaning on Flue Gas Temperature and Efficiency

Source: CPC^[7]

 

For a plant, to decide which equipment to clean, when and how to do the cleaning with minimum down time is an important issue. For example, in a crude oil preheat train there are typically more than a dozen heat exchangers in one or two trains before reaching T_in in Figure 1. How to prioritize which exchanger to clean, how to predict the benefit the plant can gain with cleaning, and how to do it as fast as possible with minimum down time are all areas that can be improved upon. Are there technologies that can help? In the author's experience a model of heat exchanger trains can be set up to address the first two questions and several practices can be established to shorten the time the selected heat exchanger is taken offline. For example, when the author was in Singapore a robotic arm was considered to speed up the process of removing and re-installing the heat exchanger. Also considered was the use of chemical additives to reduce fouling caused by crude oil.

 

III. Steam System

A steam system consists of many components and thus many opportunities to save energy. Resources such as^[2] and information from steam specialist company like TLV^[8][17] give detailed ideas of where to look and how to tackle the opportunities. As examples Table 1 and 2 highlight some key areas and the reported fuel saving percentages^[2].

 

Table 1. Energy Saving Areas in Steam Production

Source: Neelis et al.^[2]

 

Table 2. Energy Saving Areas in Steam Distribution

Source: Neelis et al.^[2]

 

Here I just want to bring up two extra points:

1. It is important to take an overview approach to the steam system first. Often steam is generated at higher pressures than needed or in larger volumes than needed at a particular time thus consumes more energy than necessary. These inefficiencies may lead steam systems to let down steam to a lower pressure or to vent steam to the atmosphere. Hence, it is recommended to evaluate the steam system on the use of appropriate pressure levels and production schedules As a result of this kind of study overall steam pressure and volume reduction have been reported in many cases^[1][9].
2. Cogeneration, which combines heat and power generation, is a much more efficient way to produce steam (and electricity) than water boilers. Cogeneration produces heat and electricity simultaneously in a single plant, powered by just one primary energy source. In this way, nearly all the thermal energy produced by combustion is not dissipated into the environment but is recovered and reused. Figure 5 is a depiction of a typical cogeneration process. The fuel is combusted to drive the gas turbine. Its hot exhaust gas, instead of being released to atmosphere, is then used to generate steam. This is because the steam pressure required by power turbines is much higher than that of industrial usages. After going through the turbines the steam pressure drops significantly but still sufficiently high for petrochemical plants.
   Cogeneration plant requires significant capital investment. So usually only large plant that uses significant amount of both steam and electricity would build such a plant. Formosa Plastics' Mailiao Power Station is one large cogeneration plant that supplies its many plants in Mailiao complex. Unfortunately, despite its improved efficiency this power station burns large amount of coal and, according to ClimateTrace.org^[10], a new CO₂ tracking website unveiled in 2022's COP27, Mailiao Power Station's CO₂ emission ranks number 6 in the world among all the power stations. By the way Taichung Power Station is number 1 in the world.

 

Figure 5. Cogeneration System

Source: CENTRAX GAS TURBINES

 

IV. Improved Heat Integration

A petrochemical plant often has many hot sources and cold points that potentially can be paired so that the heat is not wasted. Once heating is done the energy is rarely used completely and the remaining heat can often be used to preheat its own feed or be used in other units. Figure 6 demonstrates a very simple example of using a heater's flue gas to preheat the air ("preheat" means heating up before entering the fired heater). This example has two heaters that share the flue gas duct that preheats the inlet air. A typical heater's efficiency could increase 15-20% just to install such a preheater.

For more complicated integration a study that looks for potential heat sources and sinks can be conducted to see whether pairings are economically feasible. For example, if the two candidate units are far apart in the plant it would be difficult to make the pairing both physically and economically. Note that capital investments are usually needed and the timing of execution is usually during plant turnaround when both sides of equipment become available.

In plants that have multiple heating and cooling demands, the use of a techniques called "pinch analysis"  may significantly improve efficiencies. Developed in the early 1970's, pinch technology is  now a well-established methodology for continuous processes^[11]. The methodology involves the linking of hot and cold streams in a process in a thermodynamically optimal way. Process integration is the art of ensuring that the components are well suited and matched in terms of size, function and capability.

 

Figure 6. Simple Heat Integration Example: Heaters with Air Preheat using Flue Gas

 

V. Distillation

Distillation is used to separate a mixture of hydrocarbons based on their boiling points. Distillation columns are ubiquitous in petrochemical plants. Most reactors are followed by a couple of distillation towers to separate products. A naphtha steam cracker can be followed by over 10 distillation columns. Each distillation column has its defined purposes and target purity requirements of its product. Besides preheating its feed, a column usually has a reboiler to heat up the bottom of the tower to generate vapor that moves up and makes contact with the liquid that comes down to achieve the separation. The reboiler consumes energy by using steam or a heater.

Distillation columns can consume 30% of the energy in a plant^[9]. Whether a column spends too much energy than necessary may not be obvious. A simulation model is usually very helpful to adjust the operation conditions to minimize energy while still achieving its objectives. Consulting experience with the petrochemical industry indicates that there are many such opportunities to save energy in distillation columns in Taiwan.  

A systematic approach using a so-called column grand composite curve can also be used to guide operation parameters and column modifications^[9].

 

VI. New Hydrogen Production Methods 

Low-carbon and low-cost hydrogen production is an area of very active research around the world. This of course is not just for petrochemical but because of hydrogen's potential as a low-carbon transportation and even power generation fuel.

As mentioned earlier splitting water (H₂O) using electrolysis is highly energy-consuming. Despite many research in improving its efficiency the inherent energy demand is difficult to overcome.

Methane Pyrolysis is an emerging alternative for clean hydrogen production. The word "pyrolysis" means the chemical decomposition of carbon-based materials through the application of heat. In a pyrolysis reactor, the feedstock of methane is heated in the absence of oxygen to pyrolysis conditions (1,000 – 1,500°C), at which point the methane molecules (CH₄) dissociate into solid carbon (C) and hydrogen (H₂). Since solid carbon, not CO₂, is the principal by-product, greenhouse gas emissions are significantly reduced when compared with a conventional SMR process.  Moreover, since methane pyrolysis does not require CO₂ sequestration or water as feedstock, it can be flexibly sited wherever natural gas infrastructure exists. Since hydrogens that are produced through different pathways are identified by colors some call the pyrolysis hydrogen the "turquoise hydrogen".

Splitting CH₄ molecules requires less energy than splitting H₂O molecules. BASF's estimate is that pyrolysis needs 10 MWh to make one ton of hydrogen^[12]. This would be about 1.6 times of the energy used in SMR but only 18% of what's required in electrolysis.

Figure 7, a diagram from the company Monolith^[13][14], a leading methane pyrolysis developer, illustrates the process in a simplified way. Basically, natural gas which consists of mainly methane enters the reactor which is heated to over 1000°C. The high temperature breaks up the methane molecules ("pyrolysis") into solid carbon and H₂ and they are then separated.

Figure 8, also from Monolith^[13][14], is a comparison of carbon intensity of different hydrogen production methods. It is assumed that the heating of pyrolysis reactor furnace uses electricity from renewable energy. The rightmost method which shows negative carbon intensity uses renewable natural gas (RNG)^[26] as its feed. As mentioned above some call the pyrolysis hydrogen the turquoise hydrogen and thus the background color of the Monolith figures is turquoise.

The following are three companies at various stages of commercializing pyrolysis:

1. Monolith has been running a demonstration plant producing 5,000 tons of hydrogen per year (also 15,000 tons of carbon) in Nebraska, USA since 2020. It is expanding the plant by 12 times by adding additional furnaces^[13].
2. BASF has been developing its own methane pyrolysis technology. Details are not disclosed. A pilot reactor has been constructed in Ludwigshafen, Germany, and is being started up^[12]. The development is funded by German government. BASF targets 2030 for large scale production if the process proves its economic feasibility.
3. A very different approach is taken by the company SG H2 ENERGY. It uses plasma torch to generate a much higher temperatures of over 3,500°C and thus can process more complex solid waste as feed^[15]. The process produces hydrogen and syngas (CO+H₂) as products. The gasification process is described as "plasma-enhanced thermal catalytic conversion optimized with oxygen-enriched gas". After years of pilot plants and marketing SG H2 ENERGY is building a commercial waste treatment plant for the City of Lancaster in California. The plant is supposed to start up in the first quarter of 2023^[16]. It would be very interesting to see how this project turns out. My feeling is that the complex feed and the very high operation temperature would make long term operation and reliability a challenge.

Note that methane pyrolysis also produces large amount of solid carbon as a byproduct. Traditional carbon black markets for rubbers, tires and specialty plastics are significantly smaller than the hydrogen markets and is extremely demanding in terms of grade and morphology for carbon delivery. These considerations present a challenge to methane pyrolysis companies that are seeking to sell carbon as a means for offsetting production cost in their operations. The availability of low-cost by-product carbon from hydrogen production presents new opportunities for large-scale, bulk carbon utilization in ubiquitous markets such as biochar replacement for agriculture and application in construction materials, such as asphalt and concrete. Developing these markets is a key step to maximizing economic value for methane pyrolysis^[18] .

 

Figure 7. Monolith's Methane Pyrolysis Reactor

Source: Monolith^[14]

 

Figure 8. Carbon Intensity Comparison of Different Hydrogen Production Processes

Source: Monolith

 

VII. Real-Time Energy Management System

In the sections above we treat the heaters, steam systems, and electricity separately and mostly at each equipment's level. At higher level we could integrate the energy related systems together and optimize the energy use as a whole. If data are available in real-time the decision can also be made and implemented in real-time. This kind of system is called Real-Time Energy Management System. For example, the software Visual Mesa allows a plant to take advantage of the tradeoffs among fuel, steam, electrical, chilled water systems, and greenhouse gas emission in the most economical ways^[19]. Yokogawa, the owner company of Visual Mesa, claims that overall benefit in the range of 2% to 5%of the total energy cost can be achieved^[20]. Such a system can also help to plan and manage energy transition to renewables^[21].

 

VIII. Real-Time Optimization (RTO) and Advanced Control

Modern control systems are often not solely designed for energy efficiency, but rather at improving productivity, product quality and efficiency of a production line. Applications of advanced control and energy management systems are in varying development stages and can be found in all industrial sectors. Control systems result in reduced downtime, reduced maintenance costs, reduced processing time, and increased resource and energy efficiency, as well as improved emissions control. Many modern energy-efficient technologies depend heavily on precise control of process variables, and applications of process control systems are growing rapidly. Modern process control systems exist for virtually any industrial process.

Advanced Control systems (e.g. model-predictive controller) have brought huge amount of benefits to the petrochemical industry. The technology directly and indirectly contributes to energy savings to all kind of petrochemical processes.

Another related technology is the RTO system that maximize the economic benefit of the overall process and the economics include the energy cost calculation. Unlike the Real-Time Energy Management System discussed above this kind of system uses detailed process models to optimize the operation parameters to maximize its total profit by considering feed, product, and energy values. This kind of system avoids the potential problem of minimizing energy use but inadvertently losing profit because of the loss of product values^[22].

 

IX. Energy Management System in Organization

As I alluded to earlier energy saving is an accumulation of many small efforts and it is easy for an organization to lose focus after the initial attention period. Continuous improvements to energy efficiency typically only occur when a strong organizational commitment exists. A sound energy management program is required to create a foundation for positive change and to provide guidance for managing energy throughout an organization. Energy management programs help to ensure that energy efficiency improvements do not just happen on a one-time basis, but rather are continuously identified and implemented in an ongoing process of continuous improvement. Without the backing of a sound energy management program, energy efficiency improvements might not reach their full potential due to lack of a systems perspective and/or proper maintenance and follow-up.

An important aspect for ensuring the success of the action plan is involving personnel throughout the organization. Personnel at all levels should be aware of energy use and goals for efficiency. Changing how energy is managed  by implementing an organization-wide energy management program is one of the most successful and cost-effective ways to bring about energy efficiency improvements. I won't discuss in details but there are some organizational suggestions in [2] if interested.

 

X. New Equipment, Process, and Catalyst

The petrochemical industry and its many vendors continuously innovate and improve equipment and processes. For example, the application of Heat Pipe technology^[23] to replace fired heaters to improve efficiency and reliability has reportedly been used in many plants, especially in China. New processes that are more efficient, use less raw material, and reduces waste continue to be developed. A report by Dow Chemicals^[24] provided some very good examples how various kinds of innovations beyond the traditional energy saving methods can indirectly contribute to energy saving through better process, better catalyst, recycling waste, investing in renewable raw materials, and creating products that enables energy savings.

Recently a most notable case is BASF's effort to use as much renewable energy as possible. In September 2022, BASF, SABIC and Linde have started construction of the world's first demonstration plant for large-scale electrically heated steam cracker furnaces. By using electricity from renewable sources instead of natural gas, the new technology has the potential to reduce CO₂ emissions of one of the most energy-intensive production processes in the chemical industry by at least 90% compared to technologies commonly used today^[12]. However, as I noted in the first article, it is a hard-sell to convince the petrochemical industry to rely on public renewable energy due to its intermittent nature. BASF uses direct investment in wind power farms and also long-term contract with renewable suppliers to alleviate this concern.

 

XI. Concluding Remarks

Figure 9 from KBC^[25], a major consulting firm in the industry, shows the operating cost breakdown of industry average (red bars) and the best in class (blue bars). Energy is represented by the rightmost bars and the largest item. How much can a petrochemical plant expect to save with a well-executed energy management program? The data shows about 10-15% (the difference between the red and blue bars) and this is also consistent with the author's experience as ExxonMobil's energy advisor in Asia Pacific.

Energy saving is a no-brainer for petrochemical industry because of its economic incentives and now it has the additional driver of carbon reduction. As far as I know many Taiwan's companies have embarked on energy saving programs but recent consulting experience also indicates that more can be done. It is my hope that this article can inspire renewed interest in both traditional practices and latest technology development.

 

Figure 9. Petrochemical Plants' Operating Cost Breakdown

Source: Allan Rudman^[25]

 

Related article:

Petrochemical Industry's Pathways to Carbon Reduction: I. Carbon Capture and Storage