April 2017

Special Focus: Petrochemical Developments

An investigative study for technological innovation for ethylene plants

The development of an industrial process like an ethylene (C2H4) plant requires skills and knowledge in two main subject areas: synthesis and analysis. Optimal configuration through process synthesis (PS) with selected task selection (STS) is the main theme of this work.

Abdel-Aal, H. K., National Research Center (NRC)

The development of an industrial process like an ethylene (C2H4) plant requires skills and knowledge in two main subject areas: synthesis and analysis. Synthesis deals with the creation of artificial things that have desired properties, while analysis deals with the understanding of how things are constructed and how they work. Optimal configuration through process synthesis (PS) with selected task selection (STS) is the main theme of this work.

The search for innovation in ethylene production, whether in the end product or in the core manufacturing process, is targeted and carried out by first presenting the “road map” of the entire process, including the four “pillar” operations in ethylene production: steam cracking, quench operation, fractionation and product refrigeration. Consideration is also given to recent upward trends in energy costs, combined with a focus on greenhouse gas (GHG) emissions. Research work to optimize energy consumption in olefin plants in areas related to pyrolysis, furnaces, compressors, refrigeration, fractionation trains and others is highlighted.

Three case studies for ethylene plants are presented here. Design aspects that lead to reduced energy consumption are considered for relevant unit operations. An important process design problem is separation sequencing, which is concerned with the selection of the optimal method and the sequence of a given configuration. The formula for estimating the cost of fractionation (separation) as a function of the load for given conditions of reflux ratio is outlined. The integration of advanced technology in ethylene plants is also considered.

Typical process features of an ethylene process include:

  • Short residence time in the furnace
  • High selectivity
  • Feedstock flexibility
  • Operational reliability and safety
  • Easy startup
  • Energy efficiency.

The framework for understanding the evolution of technological innovations in ethylene manufacturing is measured by two main aspects: how the existing process works, and how the energy is used.

While the fundamentals of the ethylene manufacturing process are well known, the search for innovations—both in the end product or the core manufacturing process—continues. The quest for productivity improvements inevitably focuses on cost reductions.1

Significant reductions in energy consumption and environmental emissions can be achieved by the application of innovative equipment technologies and new process design concepts.2

The latest process synthesis techniques, which combine the advantages of mathematical programming, thermodynamic analysis and sound engineering judgment, will be incorporated into a new methodology for systematically screening different process schemes. The resulting optimized olefin separations configuration could significantly reduce energy consumption and production costs, and lower GHG emissions. The separation train of an olefins plant includes multiple distillation columns, together with an enormous compressor-driven refrigeration system, numerous flash drums, heat exchangers and pumps, and a reactor system to reduce acetylene (C2H2).

FIG. 1 illustrates a simplified representation of the three main steps in petrochemical manufacturing, focusing on process development through PS and STS. The third step is shown in FIG. 2.

Fig. 1. A simple representation of the main three steps in petrochemical manufacturing.
Fig. 1. A simple representation of the main three steps in petrochemical manufacturing.
Fig. 2. By using them as fuel to produce steam and/or power, waste streams are abated.
Fig. 2. By using them as fuel to produce steam and/or power, waste streams are abated.

The four pillars of ethylene production

It is well-established in chemical engineering that process development/design usually begins with a process scheme, or flowsheet, that serves as a guide for a given process. This diagrammatic model describes process steps in their proper sequence.

For process development in ethylene plants, ethylene production also undergoes a process/flow diagram (FIG. 3). Four primary steps (pillars) have been identified:

  • Steam cracking
  • Quench process
  • Fractionation
  • Ethylene refrigeration.
Fig. 3. A flow diagram for ethylene production.
Fig. 3. A flow diagram for ethylene production.


The two primary feedstocks for ethylene production are naphtha and natural gas (ethane, propane, butane, etc.).

Monitoring the process in this fashion can lead to process development in terms of PS and STS. Some fundamental concepts must be presented regarding PS—using the rules of heuristics—and STS—using the separation factor (SF)—to be defined later.

The role of process synthesis

Process synthesis methods and tools must evolve in response to the challenges the ethylene industry is likely to face. These challenges may involve the development of methods for the generation of heat exchanger networks, separation trains, reactor systems and complete flow sheets, as well as the role of paradigm, representation, heuristics, search and mathematical programming.

Two important alternatives can be considered:

  1. Raw materials options—Different raw materials, of course, bring different chemical routes to the desired products. Many of these routes are known today, and many more will be invented. Almost all will be facilitated by catalysts or, perhaps, biocatalysts.
  2. Different approaches in plant design—Chemical process design is a complex task.3,4 All previous systematic generation approaches have decomposed the design into a hierarchical series of sub-problems, such as reaction subsystems, basic material input-output recycle structure, separation and purification subsystems, energy and power subsystems, and environmental protection subsystems, among others.

Using heuristics for separation selection

As one of the most energy-intensive operations, distillation is the most common unit operation for separating liquid mixtures into valuable and/or high-purity products, particularly in the petrochemicals industry. Therefore, optimization of distillation column design and operation should take high priority. Numerous distillation heuristics for quick optimization have emerged. Heuristic is defined as, “to reveal or guide: valuable for empirical research, but unproved.” Essentially, all reported heuristic rules for separation sequencing can be broadly classified into these “rules of thumb” (FIG. 4):

  1. All other things being equal, aim to separate the more plentiful components early.
  2. Difficult separations are best saved for last.
  3. When using distillation or similar schemes, a sequence that will remove the most valuable species or desired product as a distillate should be selected.
  4. During distillation, sequences that remove
    the components one by one in column overheads should be favored.
  5. Excursions in temperature and pressure should be avoided, but aim high rather than low.
Fig. 4. Four broadly classified categories of heuristic rules  for separation sequencing.
Fig. 4. Four broadly classified categories of heuristic rules for separation sequencing.

Cost of separation, fractionation

The operation of fractionation columns, at a given feed input (load), typically involves a tradeoff between energy usage and product recovery.5,6 Setting the proper target involves evaluating the relative economic values of these two factors. The factors influencing the cost of separation by distillation include:

  • The quantity or feed input to be processed (load, or L)
  • The desired separation between two components (SF)
  • The corresponding reflux ratio (RR) and the heat input (Q) by reboiler.

Eq. 1 and Eq. 2 detail the cost of separation:

= f [(L, RR, Q)/SF]           (1)

= k [(L, RR, Q)/ SF], where k is a proportionality constant               (2)

For a given separation, the SF is constant, and separation is carried out at selected optimum conditions of RR and Q. Therefore, the total cost of separation is simply a direct function of the load.

Conversely, the total cost (TC) of separation can be computed in absolute dollars by using the empirical power series equation (Eq. 3):

TC = (L)a × (RR)b × (Q)c × (1/SF)d                     (3)

Eq. 3 can be easily expressed in a linear format by taking “ln” of both sides of the equation (Eq. 4):

ln TC = a lnL + b ln RR + c ln Q + d ln (1/SF)      (4)

Now, the values of the power constants (a, b, c and d) can be evaluated from previously published data—e.g., given TC, L, RR, Q and SF, calculate the values of these power constants (a, b, c and d).

For simplification, the effect of some of these variables can be ignored. For example (Eq. 5), if:

ln TC = a ln L + b ln RR                                     (5)

then the TC of separation as a function of the load and the reflux ratio can be calculated, which is common.

In this case, the main objective is to select an optimal arrangement of separation sequence that will minimize L, leading to a reduction in the cost of separation.

Difficult separation, however, is identified by:

  • A higher number of distillation trays, which affects column size
  • A higher reflux ratio, which influences pump size and power consumption
  • Additional reboiler heat duty, which influences reboiler size and energy consumption.

The RR is a key variable, affecting both the capital cost and the operating cost of a column. Distillation columns are best operated at what is called optimum reflux ratio (R0), which is defined as the value at which the total annual cost of the distillation is a minimum, which is the sum of the capital cost of the column (function of the number of theoretical plates) and operating running cost, depending on the RR (FIG. 5).

Fig. 5. The total annual cost of distillation is the sum of the capital cost of the column (function of the number of theoretical plates)  and the operating running cost, which is dependant on the RR.
Fig. 5. The total annual cost of distillation is the sum of the capital cost of the column (function of the number of theoretical plates) and the operating running cost, which is dependant on the RR.

The SF must also be considered. As an equilibration separation process, SF should be much greater than unity for separation to occur and to avoid difficult separation. SF is a measure of the ease of separation of one component (A) from another (B), and is defined by Eq. 6:

           (6)

where C is the concentration of a species.

A typical example of easy separation is the evaporation of salt from seawater. Here, the value of SF is found, by intuition, to be infinity, due to the separation of water (volatile component A) from salt (nonvolatile component B).

The stream in TABLE 1 is to be separated by distillation. For this separation, the scheme in FIG. 6 is proposed.

Fig. 6. As the most plentiful species, <i>M</i> is separted first. <i>O</i> is the next most plentiful and is processed next. <i>B</i> and <i>T,</i> the least plentiful species, are separated last. Note: Δ = 30C° for <i>B/T</i> and Δ = 34C° for <i>T/O</i> are basically the same. The amounts to be processed might be the determing factor in establishing the separtion sequence.
Fig. 6. As the most plentiful species, M is separted first. O is the next most plentiful and is processed next. B and T, the least plentiful species, are separated last. Note: Δ = 30C° for B/T and Δ = 34C° for T/O are basically the same. The amounts to be processed might be the determing factor in establishing the separtion sequence.

CASE STUDIES

Case 1: Process synthesis in ethylene and propylene production

This case is a direct industrial application of some of the heuristics presented. The plant shown in FIG. 7 describes a sequence of distillation separation processes for a gas mixture produced by the catalytic cracking of natural gas. The plant produces 250 Mtpy of ethylene. Ethylene and propylene are formed by catalytic cracking of the hydrocarbons found in natural gas.

The following heuristics are applied for process synthesis,7,8 as illustrated in FIG. 7:

  1. Carry out difficult separation last—Due to the close boiling points of propane and propylene, separation between them is delayed to the very last phase (splitter). The next most difficult separation is between ethane and ethylene, which is also held until the splitter.
  2. Favor overhead removal in plentiful quantities—In the first distillation column, the demethanizer separates the volatile components hydrogen and methane (18% and 15%, respectively).
  3. Remove valuable products as distillates—Both ethylene and propylene are separated as top products, ensuring that the materials do not experience discoloration and separate in pure form.
Fig. 7. The sequence of distillation separation processes for a gas mixture produced by the catalytic cracking of natural gas.
Fig. 7. The sequence of distillation separation processes for a gas mixture produced by the catalytic cracking of natural gas.

Case 2: Ethylene production by oxidative coupling of methane

Oxidative coupling of methane (OCM), in which the reaction directly converts methane into ethylene, seems to be a sustainable solution for both short-term oil price and long-term environment preservation. Cost estimation in OCM revealed that cryogenic distillation is a major bottleneck in this process.9 Different substitutes were reviewed, and a new configuration for process synthesis based on adsorptive separation was proposed.

In another work, a separation process, such as pressure swing adsorption (PSA), is used to remove hydrogen (H2) and methane (CH4) from a demethanizer overhead stream comprising H2, CH4 and C2 hydrocarbons, and subsequently return the recovered C2 hydrocarbons to be admixed with the effluent from the oxygenate conversion process. This integration of a separation zone with a fractionation scheme in an ethylene recovery scheme, using an initial demethanizer zone, resulted in significant capital and operating cost savings by eliminating cryogenic ethylene-based refrigeration from the overall recovery scheme.

However, the direct conversion of CH4 requires fewer steps and, therefore, lower capital cost. The simplest reaction is thermal dehydrogenation, but the high stability of the CH4 molecule makes the process difficult.10,11,12

Case 3: Increased production and profitability with gas cooling injectors

An ethylene producer in China needed to spray quenching oil into a gas stream to cool the temperature from 750°F to 340°F (400°C to 170°C). The producer was using eight 8-in., full-cone nozzles for cooling in the original process line. To increase total ethylene production from 600 Mtpy to 1.2 MMtpy, the producer was adding two more steam cracking furnaces. Because the gas flow in the new cracking furnaces was significantly lower, a new gas cooling system was needed for each.

Specific nozzle selection, co-current spray direction and insertion points were determined using computational fluid dynamics (CFD) modeling. This advanced modeling technique analyzed several criteria, including gas velocity, spray concentration, drop residence time and temperature, and tested various nozzle configurations for the gas cooling system. The nozzles were successfully installed, and the plant has achieved its goal of doubling ethylene output.

The newly designed quenching system also helped increase the plant’s output of Level “A” ethylene, which sells at a 20% price premium above lower grades. Increasing the percentage of this top-grade product helped generate a payback period of approximately one month for the total investment in CFD modeling and nozzle lances.13

Integration of advanced technology in separation processes

Petrochemical plants consume large amounts of energy, much of which goes into separation processes, particularly distillation. Distillation columns are used in ethylene production where high volumes of finished products are made with a minimal amount of waste. High energy consumption, combined with large processing volumes, make the distillation columns a prime target for development. By using appropriate control instruments and logic, column capacity can be increased, and/or energy consumption can be significantly reduced.

Some important design innovations in distillation columns14,15,16,17,18,19,20 to be considered include:

  • Heat integration. Significant savings in utilities may be achieved by using heat-integrated distillation in innovative configurations. Processes that use a lower-level source of heat or preheat configurations, such as feed/bottom exchangers, are desired.
  • Heat pumps. Significant heat savings (up to 90%) can be obtained by compressing the overhead vapors from a distillation tower to a T and P sufficiently higher than the bottom’s T; and then using this Q in the column’s reboiler. C3 splitters are frequently designed with vapor-recompression heat pumps (FIG. 8).
Fig. 8. C<sub>3</sub> splitter with a vapor compression heat pump.
Fig. 8. C3 splitter with a vapor compression heat pump.

Following the road map

The process flow diagram, which represents the road map for ethylene production, emphasizes the four pillars underlying this industry: steam cracking, quench process, fractionation and ethylene refrigeration. Monitoring the process in this fashion leads to process development in terms of PS and STS.

The three cases presented here can inspire cyber solutions to some of the problems associated with the ethylene production. Case 1 (PS in the manufacture of ethylene and propylene) addresses the energetic and economic optimization for the configuration of distillation-based ethylene production by applying the rules of heuristics. A common practice is to reduce separation load by using stream splitting and blending, rather than using separation by distillation.

Case 2 (ethylene production by OCM cost estimation) revealed that cryogenic distillation is a major process bottleneck. Different substitutes were reviewed and a new configuration based on adsorptive separation was proposed. Case 2 reinforces the role of STS in ethylene production.

Case 3 described a newly designed oil spray quenching system. To increase total ethylene production from 600 Mtpy to 1.2 MMtpy, two steam cracking furnaces were required. Since the gas flow in the new cracking furnaces was significantly lower, a new gas cooling system was needed for each. The newly designed quenching systems doubled the plant’s output and increased the production of Level “A” ethylene.

These case studies in process development for the ethylene industry illustrate some emerging technologies, such as PS and STS, for olefin production. Reasonable progress exists in the synthesis of separation systems based on recently published works on distillation technology. HP

LITERATURE CITED

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