Design and Assessment of a Hybrid Membrane-Absorption

As previously discussed in Chapter 2.2.1, a hybrid CO₂ removal section for the NG-OCM had already been developed and tested at mini-plant scale accom-panied by the creation of process models for its simulation and optimization at TUB (Esche, 2015; Song et al., 2013; Stünkel, 2013). However, a clear pic-ture on its techno-economic feasibility at industrial scale production was still

Table 4.1: Dierent reactor gas outlet compositions considered as feed for the design of the hybrid CO2 removal process in the previous studies

Mole Fractions I II III IV

CO₂ 0.110 0.090 0.245 0.380 C₂H₄ 0.060 0.090 0.045 0.050 N2 0.330 0.167 0.080 0.100 CH₄ 0.500 0.653 0.630 0.470

Total 1 1 1 1

missing. In (Penteado et al., 2016c) and in greater detail in (Penteado et al., 2016b), a simulation-based design and economic assessment of the system for a NG-OCM plant producing 100 ktC2H4year⁻¹ is performed.

The hybrid CO2 removal system has been designed considering four dierent feed compositions listed in Table 4.1. They represent the NG-OCM reactor product gas using four dierent operating scenarios, i.e., nitrogen dilution (feed composition I), high methane to oxygen ratio (feed composition II), and two dierent levels of CO₂ dilution (feed compositions III and IV). The standalone absorption/desorption process with a 30 wt%aqueous Monoethanolamine (IU-PAC: 2-aminoethan-1-ol) (MEA) solution is taken as the benchmark for eval-uating the hybrid process. The two considered membrane materials are Poly-imide Membrane (PIM) and Poly-(ethylene oxide) Membrane (PEOM). The Gas-Separation Membraness (GSMs) can be employed in a single module, as previously depicted in the process ow diagram in Figure 1.5, or in dierent cascade congurations, as illustrated in Figure 4.1. A simulation-based design is performed for each system ensuring a xed CO2 removal ratio (Eq. 4.1) of 97 % and the Total Annualized Cost (TAC) of each conguration is computed and compared.

ηCO2 = Ṅⁱⁿ

CO2 −Ṅ^out

CO2

Ṅⁱⁿ

CO2

(4.1) The operation pressure range of 10 bar to 32 bar has been adopted based on (Salerno-Paredes, 2012; Stünkel, 2013) in order to reduce the amine re-generation energy required, i.e., reduce steam consumption as heating utility.

However, it has been shown that higher pressures lead to higher electricity (compression) cost and product loss due to the higher ethylene solubility in the absorption solution (Penteado et al., 2016b). Based on ethylene's market

4.1 Previous Studies

SINGLE MEMBRANE WITH PERMEATE RECYCLE

PURGE CO2

C₂H₄

TWO-STAGE STRIPPING CASCADE WITH PERMEATE RECYCLE

TWO-STAGE RECTIFICATION CASCADE WITH RETENTATE RECYCLE

Figure 4.1: Membrane cascade congurations considered in the previous stud-ies. Adapted from (Penteado et al., 2016b)

0 100 200 300 400 500 600 700 800 900

10.0 15.5 21.0 26.5 32.0

Operating costs (USD/h)

Absorption Pressure (bar)

Total Cost Compression Steam Ethylene Loss

Figure 4.2: Sensitivity study for the inuence of the absorption pressure on the operating costs for feed composition II and standalone absorption.

Reproduced from (Penteado et al., 2016b))

value and Aspen Plus' default steam and electricity cost, the parametric study in Figure 4.2 is performed to show that absorption pressures lower than 10 bar should also be considered.

In terms of OPEX, the hybrid process only outperforms the standalone ab-sorption for the CO2 dilution scenarios. For the other scenarios, the CO2 par-tial pressure in the gas is too low to justify a permeation-based separation. No conguration utilizing the PEOM material outperformed the standalone ab-sorption. The main advantages of the PEOM are the high CO2 permeance, which leads to low membrane areas, and its high selectivity towards H₂ and N₂ (8.42 and 45.93 at 303 K respectively), but selectivities towards hydrocarbons are lower than in polyimide or cellulose acetate membranes and only 3.14 for C₂H₄ at 303 K (Brinkmann et al., 2015). This translates into high ethylene loss, which is critical to the OCM process. Therefore, the use PEOM is not considered further.

The hybrid process congurations using a single membrane module or a two-stage rectication cascade (see Figure 4.1) with PIM material had a comparable performance to standalone absorption in terms of OPEX. The equipment cost is then estimated and the TAC of each conguration is compared. For feed composition III, the TAC of the hybrid process employing a single PIM module

4.1 Previous Studies is only slightly lower than that of the standalone absorption, so that a clear advantage is not attainable. For feed composition IV, the hybrid process em-ploying two-stage rectication cascade yields the lowest TAC in despite of its higher capital investment cost, due to its higher ethylene recovery.

Another study in a similar fashion also considered the utilization of a37 wt%

aqueous N-Methyl Diethanolamine (MDEA) solution with 3 wt% content of activated Piperazine (PZ) as absorption uid to remove the CO₂ from reactor outlet gas composition III (Penteado et al., 2016a). The standalone MDEA+PZ absorption performed only slightly better than the standalone MEA absorption, providing 4.6 % reduction in the TAC. On the other hand, the hybrid PIM module and MDEA+PZ absorption process provided 20.5 % reduction in the TAC. This mixed amine solution has not been considered further, because the model has not been tted nor validated to the same extent as the MEA model described in Chapter 2.2.2.

These three studies concluded that the addition of GSM in the upstream of the absorption unit can only bring economic advantages if a high amount of CO2, i.e., above 20 %, is present in the feed gas. This is the case if CO2 is added as dilution gas in a natural gas-fed OCM reactor or if biogas is used.

Also, absorption pressures lower than10 barmust be considered. There is the need to further develop and validate models for the equilibrium of the OCM component system with other amine solutions such as MDEA+PZ for a more rigorous comparison.

Limited process optimization was applied within these preliminary studies, mainly due to the diculties related to the size, complexity, and bad conver-gence behavior of the model. This has been achieved in a more recent study by applying the Bbop framework described in Chapter 3 (Penteado et al., 2018a).

The TAC for the CO₂ removal section of a BG-OCM process consisting of stan-dalone absorption with MEA is minimized. The resulting process conguration is depicted in Figure 4.3, wherein bypassed equipment/lines are shown in blank and dashed respectively. The most important cost savings have been achieved by reducing the absorption pressure from10 bar to 3.7 barand by eliminating one compression stage (Penteado et al., 2018a). Since a large amount of CO2 is present if biogas is used as a feed, it is sensible to reduce the absorption pres-sure and shift most of the compression duty to the downstream compressors, i.e., after CO2 is removed and the total gas ow is smaller. In Chapter 4.3, this study is extended to include Gas-Separation Membranes (GSM), which typi-cally require a higher operating pressure to increase separation driving force.

CO2

CO2 OCM Gas

from reaction

Water Out

CO2-Free Gas to distillation

Water Out

Figure 4.3: Superstructure and optimal process conguration for the gas quenching, rst compression, amine-based CO₂ removal, and sec-ond compression steps of the BG-OCM process. Blank equipment and dashed line streams are not employed in the optimal process conguration. Adapted from (Penteado et al., 2018a).