Alexandre Mendonça Teixeira

Exergy Analysis of Monoethylene Glycol (MEG) Recovery Systems

Abstract Hydrates are ice-like compounds comprised of water and light hydrocarbons whose formation in natural gas pipelines can lead to several problems such as pipeline blockage. To avoid these problems, the injection of Thermodynamic Hydrate Inhibitors (THI) in well-heads is widely employed. There are several options of THIs. In the present work, Monoethylene Glycol (MEG) was chosen due to its comparative advantages. As MEG can be reused, it must be re-concentrated and stripped of salts before recirculation through subsea pipelines. MEG Recovery Units (MRU) can be divided into three types: Traditional Process (TP), Full-Stream Process (FS) and Slip-Stream Process (SS). TP, FS and SS were discussed and compared via Energy and Exergy Analyses. The Exergy Analysis was based on two different approaches: firstly allocating the Reference Environmental Reservoir (RER) as the atmosphere at sea level; secondly allocating the RER as sea level atmospheric air saturated with water in equilibrium with liquid water containing MEG at infinite dilution. Those two approaches led to different results, but, in both cases TP shows the highest exergy efficiency and lowest energy consumption, while FS exhibits the lowest exergy efficiency and highest energy consumption. SS occupied an intermediate position.

Thermodynamics of Glycol Systems

Thermodynamic calculations using the Twu-Sim-Tassone EOS provide accurate glycol–water modeling (including MEG, TEG, and DEG), as well as reliable methods for phase equilibrium, surface tension prediction, and liquid density prediction. Twu, Sim, and Tassone have developed an excess Gibbs free energy function G E that allows both zero-pressure and infinite-pressure cubic equations of state/A E (CSEOS/A E) mixing rules to transition smoothly to the conventional van der Waals one-fluid mixing rules. The alpha function of TST-EOS is generalized as a linear function of acentric factor at a constant reduced temperature, guaranteeing a very accurate prediction of hydrocarbon vapor pressure from the triple point to the critical point. Thus, the EOS can handle nonpolar systems as well as nonideal systems with precise calculation of high pressure and high temperature phase equilibria. This chapter details the Twu–Sim–Tassone equation of state used in this work for simulation of binary systems of monoethylene glycol and water.

Exergy Analysis of MRU Processes in Offshore Platforms

The theory of exergy analysis (ExA) of the previous chapter is now specifically applied to the three main types of MRU commercially available to offshore oil and gas platforms. The first point to address is the definition of the reference environment reservoir (RER). The RER definition has a great influence on the ExA results, therefore two kinds of RER approaches are considered in this work giving very different exergy efficiencies: RER Approach #1 and RER Approach #2. In this chapter, it is shown that only one of them makes sense to carry out useful ExA of MRUs. RER Approach #2 allows much better discrimination of exergy efficiencies and better identification of exergy sinks in all studied MRUs. RER Approach #1, although consistent and correct, failed to provide discrimination of exergy performances and realistic results, whereas the proposed novel RER Approach #2 is able to provide much more realistic and meaningful exergy efficiency values. Under RER Approach #2 the existing irreversibilities are more easily revealed and, thus, affect with more impact the calculation of exergy efficiencies, better discriminating them.

Energy Performance Versus Exergy Performance of MRU Processes

The interrelationship between energy performance and exergy performance of a chemical process is somewhat subtle and commonly not well interpreted in general. For instance, it is possible to keep the same level of energy performance of a given operating process, but adopting some modifications—characteristically based on investing some capital into the process—to operate with a better exergy performance, as demonstrated in this chapter. In other words, an upgrade of the exergy performance does not necessarily mean that the process now uses less energy, albeit a better exergy performance usually leads to better energy usage and less energy expenditures. Another obvious fact is that making the process more costly does not necessarily imply in better exergy performance. The truth is that the achievement of a better exergy performance of a process always implies, on the one hand, that some monetary investment has to be injected into the process increasing its capital expenditure (CAPEX), size and possibly operational complexity, but, on the other hand, with the counterpart that some benefit can be expected in one or more of the following contexts related to the process performance as a whole: (1) energy degradation and energy usage; (2) energy consumption; (3) health, safety and environmental (HSE) impacts; (4) durability of equipment; (5) maintenance costs of equipment; (6) energy costs; (7) operation costs (OPEX); (8) product degradation costs; (9) waste production costs; (10) pollutant emissions; and (11) atmospheric emissions of CO2.

Hydrate Formation and Inhibition in Offshore Natural Gas Processing

Natural gas hydrates are crystalline water-based solids physically resembling ice, with a crystalline structure comprised of water and light hydrocarbon molecules (mainly CH4). Such solids can be formed above the freezing temperature of water, and, for this reason, represent a major flow assurance concern, especially at high pressures. Gas hydrate structures are characterized by repetitive crystal units composed of asymmetric, spherical-like “cages” of hydrogen-bonded water molecules, each cage typically containing one (or more) guest molecule(s) held in its interior by dispersion forces. A lot of shortcomings might occur if gas hydrates accumulates severely in subsea flowlines. Their remediation is costly and risky and could mean production stoppages, causing economic losses, and posing hazards to the security and integrity of the pipelines. To thermodynamically inhibit hydrate formation in continuous gas pipeline systems, the most common prevention method is to continuously displace the hydrate forming boundary such that the operational temperature and pressure of the system lie on the outside of the hydrate boundary. This is accomplished by continuous injection of a certain flow rate of a THI compound, which must be proportional to the flow rate of water carried by the stream. This chapter covers the main aspects of hydrates and their structures, hydrate remediation and the mechanism of thermodynamic inhibition of hydrate formation.

Energy Consumption and CO 2 Emission of MRU Processes

In order to quantitatively evaluate MRU processes in terms of heat and power consumptions, CO2 atmospheric emissions and exergy performance, traditional, full-stream, and slip-stream processes are first implemented in a professional process simulator, with the same inlet and outlet conditions for a comparative study. Process conditions and relevant parameters are first defined and steady-state flowcharts of MRU processes are installed as process flow diagrams (PFD) in process simulator in order to solve the respective mass and energy balances. Values of electric energy (EE) and heat consumptions of MRU processes are assessed via simulations, as well as all thermodynamic properties of the relevant material, thermal and mechanical energy streams. Heat streams are used to represent heating and cooling effects associated to a contact between two (or more) material streams in a heat exchanger. Hence, some assumptions for the simulations were adopted and they are listed in this chapter. Further, the implementation in simulation environment and the main results, as well as electrical energy (EE) and heat consumptions, required flow rate of utilities and CO2 emissions for the processes are also presented in this chapter.

Exergy Analysis of Chemical Processes

Exergy analysis (ExA) has been gaining relevance in the field of energy efficiency as a powerful tool to assess degradation of energy quality. ExA quantifies the percentage of destroyed exergy via process irreversibilities, as well as the percentage of lost exergy via process deficiencies when handling waste (material and energy) streams. ExA also assesses the primary sinks responsible for exergy destruction and/or exergy losses by process inefficiencies and/or design limitations. Moreover, ExA might also be used as design criteria for optimization of process in order to minimize energy requirements, energy degradation and waste (material and energy) streams. It is believed that a complete picture of the thermodynamic performance of a process is achieved and best evaluated by performing an exergy analysis in place of or in addition to conventional energy analysis. This is due to the fact that ExA can clearly indicate the components or blocks that destroy or lose exergy the most, i.e., the most thermodynamically or materially inefficient components. In this chapter, formulae for exergy flow rate of streams are obtained from application of first and second Laws of Thermodynamics together with conservation equations for the studied system (general steady-state open system and its reference environment).

Thermodynamic Efficiency of Steady State Operations of MRUs

Distillation columns and evaporation equipment are the main energy-consuming components utilized by offshore MRUs, besides several heating and cooling operations. One may ask about what would be the range of expected values of thermodynamic efficiencies for the main MRU operations. A first point to be realized beforehand is that thermodynamic efficiency and exergy efficiency are not in general the same thing, but both have a direct relationship and vary in the same direction. When only energy-consuming processes are considered, as in the case of MRUs, the thermodynamic efficiency expresses the ratio between the minimum consumption of equivalent power to accomplish a given task and the actual consumption of equivalent power for the same task. On the other hand, exergy efficiency is based on the fact that the difference between the outlet usable exergy flow rate of streams and the inlet usable exergy flow rate of streams is related to the minimum consumption (maximum production) of power to accomplish the process task under reversible conditions. The definition of exergy efficiency is similar to the thermodynamic efficiency, but its exergy version can give, in general, different results from the classical thermodynamic efficiency, as exergy counterpart is very dependent on the definition of the environment. This chapter initially approaches the thermodynamic efficiency of a binary idealized distillation column solved by the approximate McCabe–Thiele method, then a real commercial multicomponent distillation is assessed in terms of thermodynamic efficiency, being solved via rigorous distillation package of commercial process simulators with typical specification of products.

Influence of Design Parameters on Exergy Efficiencies of MRU Processes

Certain equipment design parameters have direct influence on the degree of irreversibility associated with the operation of the equipment in question, concomitantly with inverse influence on the respective size and capital cost. Typical examples are the temperature approach (TAPP) of heat exchangers and the reflux ratio (RR) of distillation columns. From the standpoint of the perspective of the exergy methodology presented in this work, it is worthwhile to assess (and verify) the chains of influences of such design parameters—like TAPP and RR—closely related to the degree of irreversibility of processes. Such analysis is made in this chapter, which shows that the greater the TAPP, the smaller the exchangers and the greater the degree of irreversibility of exchangers; the greater the degree of irreversibility of exchangers, the greater the consumption of thermal utilities and the greater the rate of exergy destruction; the greater the rate of exergy destruction, the lower the exergy efficiency of exchangers and the lower the exergy efficiency of the plant. The chain of influences is similar for the RR of a given column: the greater the RR, the smaller the distillation column and the greater its consumption of thermal utilities; the greater the column consumption of thermal utilities, the greater the column degree of irreversibility, the greater its rate of exergy destruction, the lower the exergy efficiency of the column and the lower the exergy efficiency of the plant.

MEG Loops in Offshore Natural Gas Fields

As a thermodynamic hydrate inhibitor (THI), MEG must be injected at certain points of the natural gas production systems to be thermodynamically effective against hydrate formation. The most appropriate points for injection of THIs are the warm wet points in the system like “heads” of production wells (well-heads) upstream the production choke, subsea transmission lines and flowlines that will be subjected to cooling at high pressures. The flow rate of MEG must be dosed proportionally to the expected flow rate of water in the system according to a proportion at least 1:1 in molar basis. After injection, MEG circulates through the production system and emerges at high pressure as Rich MEG (i.e., MEG solution rich in water and possibly having dissolved salts) on the production platform. At this point, after depressurization and separation of the gas and condensate phases, the treatment of Rich MEG stream is necessary in order to perform the recovery and regeneration of lean MEG for reuse, together with the disposal of water and salts. This step, known as MEG recovery (regeneration), is accomplished in MEG Recovery Units (MRUs). The MEG loop is a critical subsystem in the gas production system, because the MEG maintained in a closed circuit can become gradually degraded and contaminated with its continued utilization. This chapter describes MEG loops with MRU either located onshore or offshore, as well as presents some examples of MRUs.