1 Introduction

Over the years, the demand for non-renewable energy sources such as coal, natural gas, and oil, has skyrocketed. Non-renewable resources are generally used to expand economies because they supply the energy needed to manufacture essential products that can be used to sustain human life as well as convey goods and people [1]. If these resources are not utilized sustainably, future generations may be left without supplies. As a result, new technologies must be developed, or existing technologies must be improved to focus on the use of renewable and sustainable energy resources. These new or enhanced technologies must be ecologically beneficial and capable of replacing non-renewable resources [2, 3]. From 2008 through 2035, global energy-linked CO2 emission is predicted to increase by 43% if petroleum-based resources are used [4]. This could lead to global warming, which could lead to climate change.

A biorefinery is a promising approach for utilizing renewable biomass resources in a sustainable manner [5, 6]. Sarma et al. [7] defined biorefinery as a processing facility to convert biomass into valuable biofuels (such as biomethane and biodiesel), heat, power, and other value-added compounds. Biomass-derived energy sources are thought to be more environmentally friendly, particularly in the transportation sector [8]. Biodiesel is the fatty alkyl ester formed when a fatty acid is transesterified with alcohol (usually methanol or ethanol) in the presence of an alkali catalyst which could either be KOH or NaOH [9, 10]. This biofuel can be used to replace petroleum-based diesel, which is non-renewable. Apart from the widely known transesterification process, biodiesel could also be produced by other methods such as the microemulsion process, pyrolysis, as well as direct use and blending process [11]. Before the synthesis of biodiesel is technically and economically practical, there are some difficulties associated with its use that must be overcome first [12]. The generation of huge amounts of crude glycerol is one of the issues related to the usage of biodiesel. A mole of crude glycerol is generated for every three moles of biodiesel produced during the transesterification process [13]. This process accounts for about 10% of glycerol's overall output [14,15,16]. Since the glycerol synthesized during the biodiesel production process has a very low purity (usually between 60–80 wt.%), its application is limited. Depending on its purity, glycerol has been used as a raw element in the manufacture of cosmetics, toiletries, medicinal formulations, and foodstuffs [17].

The cost of refining crude glycerol is high due to the high cost of the processing equipment needed to purify it [18]. Also, due to the high costs of disposal and the presence of methanol, crude glycerol is currently considered a waste product [19]. Processing crude glycerol to produce value-added platform chemicals could be one answer to this challenge [16, 20]. Reforming glycerol to hydrogen, syngas, acrolein, propylene glycol, methanol, and other valuable compounds is an example of a possible conversion process [16, 17]. Converting glycerol to methanol is an appealing solution to the problem of excess glycerol. Methanol is a key ingredient in the production of biodiesel and other high-value compounds [21]. The 1992 Energy Policy Act also classified methanol as an alternative fuel since, when utilized as motor fuel, it is physically and chemically identical to ethanol. Around 90% of methanol now comes from fossil sources [22]. A promising and environmentally friendly source for renewable methanol is glycerol [22]. Thus, researchers have recently focused on the manufacture of methanol from glycerol [21]. The crude glycerol to methanol (CGtM) process can take several alternative courses, but the most common one involves two steps: glycerol reforming to syngas and methanol synthesis from syngas [3, 23]. Aqueous reforming, partial oxidation, pyrolysis, supercritical water reforming, steam reforming, and dry reforming reactions are all glycerol reforming techniques that can be utilized to obtain syngas or hydrogen gas from glycerol [24, 25]. Another approach for producing methanol from glycerol without reforming it to syngas has been proposed, and this process may be carried out under moderate conditions but requires the introduction of hydrogen [26].

Large-scale biodiesel producers can purify crude glycerol for other industrial use, unfortunately, the process of purifying crude glycerol is costly for small-scale producers [27]. However, the varying price and the increase in production of glycerol indicate that it is no longer economically feasible to use glycerol as an end product in a situation where supply and demand are balanced [28]. Therefore, transforming crude glycerol is essential to maintaining the biodiesel business. While crude glycerol may appear to be a disadvantage for certain biodiesel producers, it has enormous potential to be used as a feedstock for value-added biochemicals from a biorefining standpoint [29]. Converting glycerol into an energy derivative product is a promising and practical strategy that can be used to boost the profit earned by the biodiesel industry [18]. As an illustration, crude glycerol can be used to create hydrogen and syngas, which are frequently used as primary feedstocks for the synthesis of valuable compounds like methanol [18, 28]. The forecast for glycerol production for the year 2020 was estimated to be around 41.9 billion liters (as shown in Fig. 1) with an accompanying market price of 80% purity costs between $0.09 and $0.20 per kilogram of crude glycerol [27, 30]. Also, since an average of 1000 kilotons of glycerol are generated globally each year, there are limited market information available for it [28]. This necessitates the development of a practical and affordable method for transforming crude glycerol into valuable compounds. In addition, previous techno-economic studies [31,32,33] investigated the minimum selling price of synthetic methanol to be around 0.5—1.5$/kgMeOH which is more than twice the market price of methanol. However, none of these studies considered methanol production from the syngas derived from crude glycerol.

Fig. 1
figure 1

The estimated global production of crude glycerol from 2006 – 2020 [30]

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Most of the critical studies [34,35,36,37,38] on glycerol/crude glycerol steam reforming have mainly focused on hydrogen production. There are limited studies that investigates the methanol production through steam reforming of crude glycerol, hence, this study aims at the technical and economic evaluation of methanol production from the syngas produced from the steam reforming (STR) of crude glycerol. To achieve this, a detailed sensitivity analysis on effects of the key operating conditions such as STR temperature, steam-to-glycerol ratio (SGR), methanol synthesis pressure, and methanol synthesis temperature on syngas composition and product yield were investigated.

2 Literature review

2.1 Glycerol reforming

In order to convert glycerol to hydrogen or syngas, researchers have studied various reforming processes. Crude glycerol has a high variety of liquid soluble intermediates, making glycerol reforming more difficult than methanol-reforming. Although technically possible, most reforming methods for converting glycerol to methanol are not yet financially appealing [39].

2.2 Supercritical water reforming of glycerol

Supercritical water reforming (SCWR) is the most recently researched process that appears to be a favourable unconventional route that could be used to manufacture syngas from liquid biomass. Supercritical water is defined as water that has been compressed and heated beyond its critical pressure (22.1 × 10–5 bar) and temperature (374 ℃) [18]. The benefits of this method are due to its distinctive thermo-physical characteristics, which include high water reactivity beyond its critical conditions, and high ability of solubilizing gases (such as CO, CO2, CH4, and CH2), and difficulty in solubilizing polar molecules [30]. As a result, the crude glycerol in the product makes it easier to separate these compounds, including KCl, NaCl, and several others. Because of the hydrothermal conditions of SCWR, organic compounds that cannot react in water except in the presence powerful base or acid catalyst can easily react. This is a result of SCWR producing a considerable amount of ions ([H+] or [OH–]), which causes it to behave as an acid or base during the process [13]. Since reactions in supercritical water can be carried out in a single fluid phase, the SWR process has been explored both with and without a catalyst. This would imply that the process may still be carried out without the catalyst [18]. According to Markoi et al. [40], the presence of a catalyst allows for the reduction of SCWR operating expenses as well as process conditions including temperature and energy requirements. Transition metal catalysts appear to be the ideal catalyst to use [1].

2.3 Steam reforming of glycerol

Steam reforming (STR) is a well-established process approach that may be easily run at atmospheric pressures without the use of expensive advanced control equipment [41]. Theoretically, in glycerol STR, steam combines with glycerol to produce mainly H2, CO, and CO2 [24]. With STR, complete glycerol conversion can be accomplished with a high syngas and hydrogen yield [42]. Glycerol could be used as a renewable feedstock in STR process without changing the nature of the procedure or any process equipment [38]. Since glycerol is a carbohydrate (and not a hydrocarbon), reforming it with steam is more difficult. This is because of the more complex reactions of the STR of glycerol relative to the STR of hydrocarbons [38]. Glycerol STR process comprises of a combination of glycerol decomposition into H2 and CO and water–gas-shift (WGS) reaction as shown in Eq. 1 and Eq. 2 respectively [27]. More H2 is produced as a result of the process's excessive steam oxidizing CO into CO2 [43]. The overall representation of the STR of glycerol is shown in Eq. 3.

$$\begin{array}{cc}{C}_{3}{H}_{8}{O}_{3}\leftrightarrow 3CO+4{H}_{2}& \Delta {{H}^{o}}_{298K}=+250 kJ/mol\end{array}$$
(1)
$$\begin{array}{cc}CO+{H}_{2}O\leftrightarrow {CO}_{2}+{H}_{2}& \Delta {{H}^{o}}_{298K}=-41 kJ/mol\end{array}$$
(2)
$${C}_{3}{H}_{8}{O}_{3}+x{H}_{2}O\leftrightarrow \left(3-x\right)CO+x{CO}_{2}+(4+x){H}_{2}$$
(3)

The amount of steam required during STR is denoted by x which varies depending on the target product(s) [44]. Methane STR, methanation, methane dry reforming, and several other parallel reactions could take place during the STR of glycerol depending on the type of catalyst being employed [41]. The following variables must be carefully regulated in order for the STR process to be successful: pressure, steam-to-glycerol ratio (SGR), temperature, reactants-to-inert gas ratio, feed gas rate, and catalyst loading [44]. Most investigations [37, 45, 46] have employed SGR of 9:1 and 6:1. In addition, STR temperatures between 525–725 ℃ provide high yields of hydrogen and syngas in general [46].

2.4 Autothermal reforming of glycerol

An efficient method of making the ideal syngas composition for the production of methanol is autothermal reforming (ATR). It combines the partial oxidation (POX) process with the STR process [28]. ATR is the process of co-feeding glycerol and water under POX conditions to increase the output of hydrogen or syngas as the case may be. Equation 6, which combines the STR equation (Eq. 4), and the POX equation (Eq. 5) represents the total reaction for ATR. The temperature of the process affects the stoichiometric coefficients.

$$\begin{array}{cc}{C}_{3}{H}_{8}{O}_{3}+3{H}_{2}O\leftrightarrow 3{CO}_{2}+7{H}_{2}& \Delta {{H}^{o}}_{298K}=127.67 kJ/mol\end{array}$$
(4)
$$\begin{array}{cc}2{C}_{3}{H}_{8}{O}_{3}+\frac{1}{2}{O}_{2}\leftrightarrow 2CO+{CO}_{2}+4{H}_{2}& \Delta {{H}^{o}}_{298K}=-31.79 kJ/mol\end{array}$$
(5)
$${C}_{3}{H}_{8}{O}_{3}+a{O}_{2}+b{H}_{2}O\leftrightarrow cCO+d{CO}_{2}+e{H}_{2}+f{CH}_{4}$$
(6)

This process takes place at higher temperatures (900–1150 ℃) and a wide pressure range (1–80 bar), hence a reactor that can handle these conditions is required [47]. For autothermal reactors to endure the high temperatures, pressure, and high partial pressure of hydrogen, refractories are typically used as the reactor's outer shell [47]. The start-up time for ATR is significantly faster than that of STR because POX is an exothermic process that takes place on the surface of the catalyst [42]. According to Ali [48], the best working conditions were determined to be a high ATR temperature of 862 ℃, an SGR of 4.5, and a carbon-to-oxygen ratio (COR) of 0.9 for completely converting glycerol and attaining a hydrogen selectivity of 79%. Their result also revealed that under these ideal conditions, the selectivity of undesirable products, such as CH4, was inhibited to less than 2% [48].

2.5 Comparison between SCWR, STR, and ATR of glycerol

At low temperatures, SCWR reforming has been investigated to produce the highest target products (syngas or hydrogen) yield, however, the process requires high pressure which would increase the production cost [46]. The SCWR process has the disadvantage that substantial hydrogen yields are only obtained at reaction temperatures above 600 ℃, whereas temperatures below 450 ℃ favoured the generation of methane [49]. The STR method allows for the simultaneous production of additional amounts of target products from the surplus water, which increases the yield of the reaction [46]. Also, given that the STR method is now the most popular method used to produce hydrogen and syngas globally, utilising this method for glycerol reforming is advantageous because it is simple to modify equipment that is already in use by the industry [46]. Although the ATR processes can take place at atmospheric pressure, it requires oxygen and higher temperatures to produce effective results [46]. In comparison to STR, coke formation is reduced in ATR. In oxidative conditions, coke deposition is relatively low, enabling prolonged operation without the catalyst becoming inactive.

2.6 Methanol production from syngas derived from glycerol reforming

Syngas produced from different materials, including biomass, natural gas, and petroleum, varies in quality. Biomass-derived syngas usually has a poor hydrogen-to-carbon ratio due to their high CO2 content and low H2 content [8]. The synthesis of methanol cannot be done with the syngas in this state. Methanol cannot be produced when only H2 and CO are present in syngas, hence, Lücking [50] asserts that the hydrogenation of CO and catalytic hydrogenation of CO2 are necessary for its production. Because of this, the allowable level of CO2 during the processing of methanol typically ranges from 2—8%, with the vapor phase typically falling between 2—4% and the liquid phase typically falling between 4—8% [51]. When syngas containing 28 mol%, 70 mol%, and 2 mol% of CO, H2, and CO2 correspondingly are supplied to a reactor, methanol synthesis may typically occur [3]. An estimate of syngas (CO, H2, CO2, and CH4) conditions was given as 24 vol.%, 67 vol.%, 4 vol.%, and 5 vol.% respectively in the study by Van Bennekom et al. [3].

The syngas-to-methanol (StM) reaction is typically conducted in a fixed bed reactor at temperatures between 250—280 ℃ and pressures between 50—100 bar over a Cu–ZnO-based catalyst [52]. The optimum yield of methanol from CO2 has been found to be significantly less than 40% at 200 ℃ and 50 bar, but the yield from both CO2 and CO under the same conditions is higher than 80% [38]. The hydrogenation of CO (Eq. 7), the reverse WGS process (Eq. 8), and the hydrogenation of CO2 (Eq. 9) are the three major reactions that make up the synthesis of methanol [3]. Equation 7 and Eq. 8 are independent processes that are adequate to determine the ratio of reactants to products at equilibrium [3].

$$\begin{array}{cc}CO+2{H}_{2}\leftrightarrow {CH}_{3}OH& \Delta {{H}^{o}}_{298K}=-90.64 kJ/mol\end{array}$$
(7)
$$\begin{array}{cc}{CO}_{2}+{H}_{2}\leftrightarrow CO+{H}_{2}O& \Delta {{H}^{o}}_{298K}=+41.17 kJ/mol\end{array}$$
(8)
$$\begin{array}{cc}{CO}_{2}+3{H}_{2}\leftrightarrow {CH}_{3}OH+{H}_{2}O& \Delta {{H}^{o}}_{298K}=-49.47 kJ/mol\end{array}$$
(9)

Chemical equilibrium is what regulates the production of methanol from syngas, thus Eq. 10 and Eq. 11 can be used to obtain the equilibrium constants based on fugacity [50]. According to Eq. 9, the entire methanol production process is exothermic. The effectiveness of methanol synthesis is hence severely constrained by thermodynamics [17]. Due to the exothermic nature of the two processes, the volume drops when CO and CO2 are hydrogenated. When this occurs, it is possible to maximize the production of methanol by using Le Chatelier's principle to favor the two reactions by raising the pressure and lowering the temperature. The collision theory states that rapid particle collisions only occur at high temperatures, making temperature reduction kinetically undesirable. Additionally, it is not a good idea to lower the temperature during this procedure because the catalysts required for methanol synthesis are only effective at temperatures above 200 ℃ [53].

$$\mathrm{log}{K}_{{p}_{Co}}=\frac{5139}{T}-12.621$$
(10)
$$\mathrm{log}{K}_{{p}_{RWGS}}=-\frac{2073}{T}+2029$$
(11)

where \({K}_{{p}_{Co}}\) is in \({bar}^{-2}\) and \({K}_{{p}_{RWGS}}\) has no units. The most ideal syngas for the synthesis of methanol has a stoichiometric number (\({S}_{N}\)) of 2 (see Eq. 12), which corresponds to the stoichiometric ratio for the synthesis of methanol [2]. All the reactants react to generate methanol when \({S}_{N}\) is equal to 2, only CO and CO2 react when \({S}_{N}\) is higher than 2, and hydrogen is the limiting reagent when \({S}_{N}\) is less than 2 [50, 53].

$${S}_{N}=\frac{{H}_{2}-{CO}_{2}}{CO+{CO}_{2}}=2$$
(12)

3 Methodology

3.1 Crude glycerol composition, proximate, and ultimate analysis

Table 1 shows the crude glycerol composition, proximate, and ultimate analysis obtained from Tamošiūnas et al. [54]. The crude glycerol employed in the study was a by-product obtained from rapeseed biodiesel production process.

Table 1 Crude glycerol mass composition, proximate and ultimate analysis [54]
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3.2 Process description

The Aspen Plus process flowsheet for the crude glycerol-to-methanol (CGtM) is shown in Fig. 2. The physicochemical properties of fuels were determined by using the Peng-Robinson equation of state as proposed in the work of Mousavi Ehteshami & Chan [55]. The CGtM process configuration was divided into four key sections which include the syngas production (from STR of crude glycerol) section, the gas treatment section (using pressured swing adsorption (PSA) system), and the methanol synthesis section, and finally, the crude methanol purification section. A comprehensive description of the method of operation of each unit is explained in the following subsection.

Fig. 2
figure 2

Process flowsheet for methanol production from crude glycerol

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3.3 Steam reforming of crude glycerol section

As illustrated in Fig. 2, the raw feed streams (i.e., water and crude glycerol) were introduced at room temperature (25 ℃), and a pressure of 1 bar into the system. The water stream is heated to 100 °C to make the steam required for the STR reactor. For crude glycerol, a flow basis of 100 kmol/hr was assumed. The SGR employed was 9 which is in line with Hunpinyo & Narataruksa [37]. Other investigations have employed SGRs of 6 as well, however, a high SGR is considered to increase H2 yield [46]. Nonetheless, to support this assertion, the flowrate of the water required for the STR process in this investigation is varied from 300 to 1200 kmol/hr. Aspen plus RGIBBS reactor was employed to simulate the steam reformer because RGIBBS can produce phase equilibrium without involving a chemical reaction and it also minimizes the Gibbs free energy within the constraints of atom balancing [48]. The steam reformer was fixed at 650 ℃ because it has been established in previous works that the methanation rate is negligible at temperatures above 600 ℃ [45]. The stream designated SYNGAS (see Fig. 2) leaves the reformer at 650 °C and 1 bar and is composed of syngas (H2, CO, CO2, and CH4), alkali, and a very small quantity of glycerol, and methanol. Because of the low CO content, the syngas ratio was also low. The syngas ratio was increased by means of a gas treatment process (see next section) before they could be used to synthesize methanol.

3.4 Gas treatment (Pressure swing adsorption system) section

The syngas stream was cooled down using two coolers (E-102 and E-103) connected in series, from 650 ℃ to 482.2 ℃ and from 482.2 ℃ to 35 ℃, respectively [13]. This was carried out to condense water out of the system. Water was eliminated from the process since it would increase the energy cost due to vaporization, and also cause the catalyst as well as the sorbent to lose their effectiveness due to clogged pores [36]. Complete condensation was accomplished with a streamlined separator. A two-stage compressor with intercoolers operating at a pressure ratio of 5.4 at 35 ℃ in each stage was used to compress the gases in stream S-4. The three PSA units of the pressure swing adsorption (PSA) system were initiated from stream S-5. The use of ideal component separators operating at 35 ℃ and 30 bar resulted in the simplification of all PSA units [18]. In the first PSA unit, 95% pure H2 was recovered using the PSA-1. According to Ortiz et al. [18], some of the purified H2 stream can be used to run the furnace or routed to a proton-exchange membrane fuel cell for power production. The second PSA unit received the re-compressed gas that the PSA-1 was unable to recover. As seen in Table 2, the purity of CO improved in the PSA-2, where 98% of CO would be recovered. The residual gas from the PSA-2 was treated further in the PSA-3 after exiting at the bottom. The PSA-3 was employed to raise the syngas stoichiometric ratio to around 2 by reducing the CO2 content of the gas stream. The excess CO2 was collected for prospective sequestration or further utilization. The syngas ratio (using Eq. 12) was increased after mixing the streams of H2, CO, and CO2 from the three PSA units. The conditioned syngas in stream S-8 was compressed to 80 bar using a compressor (C-104). The methanol synthesis section is where methanol is produced after the compressed syngas has been transferred there. The procedure for simulating the generation of methanol from processed syngas is explained in the next section.

Table 2 Component recovery from the three PSA units
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3.5 Methanol production section

The compressed gas (S-11) from the gas treatment section was heated to 250 ℃ in the methanol synthesis section using a heat-exchanger (E-104) in that section. Three reactions involved in the formation of methanol are Eqs. 7–9 and Eq. 8 is the primary methanol synthesis reaction. The methanol production was simulated in Aspen REquil reactor based on previous related study by Ortiz et al. [18]. The reactor was designed to run at a minimal pressure drop under isothermal circumstances. The crude methanol was then condensed by cooling the product stream (S-13) to 45 ℃. After cooling, the exit stream was fed to a flash drum (V-101) where the gas and liquid components were separated at 45 ℃ and 38 bar, respectively [56]. In order to increase the overall conversion of CO to methanol, the gas was compressed further with a multi-stage compressor and then recycled. In addition, to prevent the build-up of inert gases, mostly methane, 1% of the recompressed gas stream was purged off. The stream carrying the crude methanol (S-16) was introduced into the purification stage, where it is removed from dissolved gas and water.

3.6 Methanol purification section

Fuel grade and AA grade methanol are the two different forms of methanol available [57]. Three distillation setups can be employed for grade AA methanol. Among these are two-column distillation, which is a low-cost unit, three-column distillation, which is utilized in low-energy systems, and four-column distillation, which combines the three-column distillation with a recovery column. One-column distillation is required for methanol of the fuel grade. In both grades of methanol, there cannot be any dissolved gases [57]. Dissolved gases were discovered to be present in S-16, hence, the two-column distillation was employed in this study. To remove dissolved gas, the first distillation (T-101) was employed. The bottom product, which comprises mainly methanol and water, was then further processed in the second column. The partial-vapor condenser in the first column had reflux and boil-up ratios of 1.1 and 0.6, respectively. In the first stage of the column, the condenser pressure was estimated to be 1.24 bar while in the first stage of the second column, a total condenser was chosen. In the second column, the reflux was taken to be 1.1 and a boil-up ratio of 0.8 was used. In line with a related study [57], 30 stages were selected for both T-101 and T-102, and the feed was introduced in the middle (stage 15). The refined methanol was extracted from the gas distillate and collected using a component separator (S-105). The S-105 methanol was mixed with the T-102 exit stream in the final exit stream.

4 Results and discussions

4.1 STR and PSA section material balance

The material balance around the steam reforming (STR) and pressure swing adsorption (PSA) section is presented in Table 3. At an STR temperature of 650 ℃, SGR of 9, and pressure of 1 bar, the conversion of glycerol was found to be almost 100% when Eq. 13 is used [43]. The same operating conditions were used in this study since these variables (temperature, SGR, and pressure) have been identified as the most beneficial for glycerol STR by several authors, including Ali [48] and Dang et al. [35]. The SN value derived from this process is found to be 1.34 using the flowrates of the syngas components (H2, CO, and CO2). It can be shown that the PSA system has improved SN value to 2 using the stream S-8 syngas component (see Table 3). The ideal syngas stoichiometric ratio for the synthesis of methanol is within the range of this SN value [2, 18]. It is designed for a ratio of 2 to reduce the molar proportion of water, thereby limiting its adsorption onto the catalyst, thus, improving the efficiency of methanol synthesis [58].

Table 3 Balance around the STR and PSA sections
Full size table
$${{C}_{3}{H}_{8}{O}_{3}}_{conversion}=\frac{{{C}_{3}{H}_{8}{O}_{3}}_{in}-{{C}_{3}{H}_{8}{O}_{3}}_{out}}{ {{C}_{3}{H}_{8}{O}_{3}}_{in} }\times 100\%$$
(13)

Table 3 shows that glycerol, methyl oleate, and methanol were detected in very small amounts in the output stream (SYNGAS), indicating that these substances were the main compounds involved in the glycerol STR reaction. The output syngas stream has flow rates of 418.6 kmol/hr, 40.6 kmol/hr, 155 kmol/hr and 0.7 kmol/hr which is equivalent to 30.1 mol%, 2.9 mol%, 11.2 mol%, and 0.05 mol% of H2, CO, CO2, and CH4, respectively. Asides from the impurities, the water exiting the STR section was around 55.7 mol% which was condensed out of the system before introducing the gas stream into the PSA section. The detailed material and energy balance around all the sections can be found in Tables S2-S5 (see supplementary information).

4.2 Methanol reactor syngas component conversion and methanol yield

As earlier mentioned, Aspen Plus REquil reactor was employed in simulation the methanol production process. The conversion per pass of the participating syngas components (H2, CO, and CO2) were investigated and presented in Table 4. In order to produce methanol, CO hydrogenation was observed to be the main mechanism at higher temperatures, whilst CO2 hydrogenation seems to be the main channel at lower temperatures. Therefore, at the reactor’s designated temperature, significant CO conversion is required. The standard operating conditions for the methanol synthesis are between 220—280 ℃ and 50.7—101.3 bar [18, 31, 59,60,61]. Beyond 280 ℃, the catalyst would be prone to sintering and fusion [18], which would permanently damage the catalyst. In contrast, lowering the reaction temperature favours the right shift in equilibrium even though it significantly lowers the reaction rate. Consequently, the operating temperature is a compromise, hence, 250 ℃ would be employed in this study to investigate the conversion of the syngas components. Higher pressures result in equilibrium conversion- benefit for methanol synthesis, however, the increase above 80 bar is not significant [18, 31]. Nonetheless, the effect of varying the synthesis temperature and pressure was also investigated in the subsequent section.

Table 4 Reactant conversion per pass and product yield
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From the result presented in Table 4, the CO to methanol conversion per pass was found to be 81% while CO2 to methanol was around 25%. Since methanol synthesis is a reversible reaction, Ortiz et al. [18] claimed that these conversions are caused by thermodynamic constraints. The methanol produced is around 2007 kg/hr which is equivalent to 0.29 kgMEOH/kgCG. It can also be observed in Table 4 that at high conversions, the recycling rate tends to be very low. Low amounts of these unreacted components were recycled back in the equilibrium reactor when compared to stoichiometry reactor observed by Ortiz et al. [18] with 20% and 3% CO and CO2 conversions per pass respectively. In addition, Lücking [50] and Leonzio [53] both observed from their experiments that all the reactants react to generate methanol when \({S}_{N}\) is equal to 2, this was also the case in this study where all the syngas components participated in the methanol synthesis process.

4.3 Sensitivity analysis results

Critical operational parameters can be determined with the help of sensitivity analysis. In this section, sensitivity analysis was used to assess how two factors affected the reforming of crude glycerol and two other variables that affected the synthesis of methanol. While crude glycerol flow was maintained constant at 100 kmol/hr, the flowrate of water for the STR process varied from 300 to 1200 kmol/hr. In the STR section, the effects of SGR and STR temperatures syngas composition were investigated between 3—12 and 600—725 °C respectively. The reaction temperature and pressure were the two variables employed in the methanol synthesis section. Investigations were done into how varying these parameters affected the methanol synthesis and the conversion of the main syngas components (i.e., H2, CO, CO2, and CH4).

4.4 Effect of Steam Reforming (STR) Temperature on syngas composition

In the STR section, the temperature was the most examined parameter, and its effect on syngas composition are determined. This effect was investigated at two SGRs (6 and 9) that have been established in literature [37, 45, 46] as the best for syngas production for methanol synthesis via steam reforming. However, the effect of other SGRs values is also investigated in subsequent sections. The effect of STR temperature is illustrated in Fig. 3.

Fig. 3
figure 3

Effect of STR temperature on the syngas composition at (a) SGR = 6 (b) SGR = 9

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From Fig. 3a., it can be seen that there was slight change in H2 composition as the temperature increases from 600 ℃ and 625 ℃ which is consistent with the assertions made by Silva et al. [36]. An additional increase in the STR temperature from 625 °C to 725 °C leads to an insignificant reduction in the H2 composition. Adeniyi and Ighalo [43] also verified that H2 production was favoured as the STR temperature increased up to a certain point. However, according to Remón et al. [34], the best temperature for producing H2 was found to be between 600—650 ℃ (the peak operating temperature) which was also confirmed in this study. As a result, when the temperature was raised above 650 ℃, less H2 was produced. The H2 combustion process could possibly be responsible this decline at high temperatures [49]. In order to ascertain this, varying the temperature was also carried on at an SGR of 9 as shown in Fig. 3b. At 650 ℃, the same scenario also occurred where H2 production peaked, however, it was less pronounced here. According to Yus et al. [62], the molar proportion of CH4 drops as temperature increases. This was also seen in this investigation. There were insignificant CH4 produced at STR temperatures above 600 ℃. This is also confirmed by Picou [45] and Silva et al. [36], who both reported that the exothermic nature of the methanation reaction makes the CH4 production insignificant at temperatures above 600 ℃. Thus, in order to prevent the creation of CH4 and coke, the STR of crude glycerol must be carried out at high temperatures. From both Fig. 3a & b, it can also be observed that while the composition of CO2 declines as the STR temperature increases. Contrary to the CO2 composition, the CO composition in the syngas stream increased as the STR temperature increases from 600 ℃—725 CO2. Ismaila et al. [38] confirmed the observation as well, the authors confirmed in their study that increasing temperature resulted in low CO2 selectivity and high H2 and CO selectivity. Further research by Cheng [41] revealed that CO production is accelerated at high temperatures. The improved syngas stoichiometric ratio, brought on by the high yields of H2 and CO, may be the source of the increase in the rate of methanol synthesis. Being highly endothermic, glycerol STR requires high temperatures, moderate pressures, and partially high SGR in order to achieve high conversion.

4.5 Effect of Steam-to-glycerol Ratio (SGR) on syngas composition

From the previous section, we observed that H2 production peaked at 650 ℃, hence, the effect SGR on syngas composition was determined at STR temperatures of 625 ℃ and 650 ℃. According to Ali [48], high SGR glycerol STR is required to get high conversion. Since STR is an endothermic and reversible reaction, it indicates that there is little product production since the products are changed back to reactants after attaining equilibrium. Le Chatelier's principle was used by Ravuru and Patel [63] in their investigation in an effort to boost the conversion of glycerol by raising the concentration of one of the reactants.

Figure 4a and b shows the syngas component (H2, CO, CO2, and CH4) all reduced with an increase in the SGR. However, high SGR help the methane reforming and water–gas-shift reactions progress toward the formation of H2 [45]. One explanation for the drop in syngas composition at higher SGR could be that beyond equilibrium, the reverse reaction was favoured (see Eq. 1 and Eq. 4). Silva et al. [36] reported similar findings when examining how SGR affected syngas composition, noting a decline in the syngas composition. On the other hand, lower SGRs are less problematic economically because the glycerol produced from the biodiesel synthesis process contains a small amount of water [36]. Even though H2 is not the target product in this study, it is required in syngas production which ultimately participates in the methanol production process, hence, H2 peak production STR temperature (i.e., 650 ℃) was taken as the reference point. In addition, since the objective of this methanol production, an SGR of 9 were adopted in this study. At SGR of 9, all the syngas components are in moderate quantities hence, using this value, the cost of excess CO2 cleaning is moderate, and almost all other components were used all in the methanol production process after recycling.

Fig. 4
figure 4

Effect of SGR on the syngas composition at (a) STR = 625 ℃ (b) STR = 650 ℃

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4.6 Effect of methanol synthesis temperature on syngas conversion and product yield

The temperature and pressure of the methanol synthesis reaction, the type of catalyst employed, the type of reactor, and many other factors can all have an impact on the production of methanol [1, 43]. This section examines the effect of changing the temperature of the methanol synthesis reaction while maintaining the base pressure of 80 bar and also a commonly used pressure of 75 bar for effective comparison. The entire process of the methanol production process is exothermic. Laitinen [61] asserts that the production of methanol takes place between 220—280 ℃. As a result, a 5 ℃ increment was used in the sensitivity analysis. The conversion of CO2 and CO as well as the production of methanol as a function of temperature during the methanol synthesis process are shown in Fig. 5a and b.

Fig. 5
figure 5

Effect of temperature on syngas conversion and methanol production (a) at 75 bar (b) at 80 bar

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Since the production of methanol takes place in the gaseous phase, it stands to reason that if the reaction temperature is raised, the rate of reaction will also increase, leading to a high amount of products being produced. This is due to the fact that rising temperatures make gas particles more kinetically energetic, which causes them to travel more quickly and collide with the container's wall more frequently, producing a massive volume. Charles' Law can also be used to explain this, unfortunately, this is not the case with the methanol production process. Low amounts of methanol are synthesized at high temperatures because the conversion of the syngas components (H2, CO and CO2) reduced. Izbassarov et al. [64] claimed that the trend seen can be explained by the fact that methanol synthesis is predominantly an exothermic process, which implies that increasing the temperature reduces the equilibrium and consequently methanol production drops.

Generally, due to thermodynamic constraints, the efficiency of methanol production is reduced at high temperatures [54]. As a result, low temperatures are more favourable for the synthesis of methanol, and this is observed at both 75 bar and 80 bar as shown in Fig. 5. Chemical equilibrium can be compromised with temperature [61]. Since lower temperatures result in a higher equilibrium yield for methanol and vice-versa, optimum temperature control is essential for the proper operation of a methanol synthesis reactor due to the overall severity of the temperature effect [65].

4.7 Effect of methanol synthesis pressure on syngas conversion and product yield

At the adopted temperature (250 ℃) and also at the widely used temperature (265 ℃) in methanol production, the effect of pressure on syngas component conversion and methanol synthesis was investigated. As shown in Fig. 6a and b, pressure significantly affects the rate of methanol production as well as the conversion of the syngas components. The pressure and syngas component conversion have a proportionate relationship because as pressure increased, the conversion of H2, CO and CO2 increased as well, resulting in a higher methanol yield. A similar result was reported by Adji et al. [66], who observed that the production profile of methanol increased with an increase in the reactor pressure. The hydrogenation of CO and CO2 reaction continues at higher pressures, according to Kiss et al. [67], which is favourable for methanol synthesis. This explains the observations when the pressure was varied from 50—110 bar. At 250 ℃, the conversion of CO goes from 62%—89% and that of CO2 goes from 16%—34% however, at 265 ℃ there was reduction in both the CO and CO2 46%—81% and 13%—29% respectively as shown in Fig. 6. Methanol production was more pronounced at high pressure, but it should be emphasized that these pressures also increase energy demand investment costs, and safety concerns for the reactor [35, 64]. Samimi et al. [68] found out from their experiment that even though high pressures are favourable for the synthesis of methanol, significant amounts of water are also produced as catalyst poisoning during the procedure [68]. Adji et al. [66], emphasized that the operating pressure must be kept as low as feasible to reduce capital expenditures. Hence, production of methanol at any moderate pressure is recommended.

Fig. 6
figure 6

Effect of pressure on syngas conversion and methanol production (a) STR = 250 ℃ (b) STR = 265 ℃

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4.8 Methanol yield at different temperatures and pressures

The effects of pressure and temperature on the synthesis of methanol were examined in the preceding subsections. Due to thermodynamic restrictions, it was shown that methanol production is favourable at low temperatures and high pressures. Figure 7 shows the effect of changing the temperature and pressure of the methanol synthesis reaction on methanol yield. As mentioned earlier, the standard operating conditions for the methanol synthesis are between 220—280 ℃ and 50.7—101.3 bar [18, 31, 59,60,61]. However, at temperatures above 280 ℃, the catalyst would be prone to sintering and fusion, which would permanently damage the catalyst [18, 68].

Fig. 7
figure 7

Effect of both temperature and pressure on methanol production

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4.9 Commercial viability

To ascertain the viability of the process commercially, three different purities of crude glycerol from different experimental studies were employed using the same process condition. As the base case, Purity A obtained from Tamošiūnas et al. [54], was employed. Purity B and Purity C were obtained from Odoom [69] and Remón [34] experimental studies respectively. The yield from the reactors as well as overall yield of methanol for each purity of crude glycerol were computed as shown in Table 5. Purity C produced the highest methanol yield with 40.1 wt.% which is equivalent to 0.34kgMeOH/kgCG. The methanol achieved in purities A and B were found to be 34.6 wt. % and 36.4 wt.% respectively. In addition, Purities A and B have the highest methanol concentration in the final products stream as shown in Table 5 with 98% and 99% respectively. However, looking at the methanol concentration in the initial concentration of methanol in the crude glycerol and final methanol produced, the process of steam reforming will not be viable for crude glycerol with high methanol concentration.

Table 5 Effect of different crude glycerol purities on the commercial viability of the methanol synthesis process
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4.10 Economic analysis

An economic analysis was carried out to determine the profitability of the refined key product (methanol). The water condensed out of the syngas as well as the hydrogen fuel produced from the PSA process were also considered as by-products from the process. A plant capacity of 6.8 tonnes/hr of crude glycerol feed for a 20-year plant lifetime was investigated in this study. The overall economic analysis was broken down into three key areas which include cost assessment, profitability analysis using the key economic performance indicators (i.e., the net present value (NPV), return on investment (ROI), discounted payback period (DPBP), and the net production cost (NPC)) and finally sensitivity analysis. The data for the analysis are available in the supporting documents (see costing and economics supplementary information).

4.11 Cost assessment

The equipment cost estimation as well as the raw materials and product cost assessment were all carried out using Aspen Plus v11 software. The cost of the main reactors (steam reformer and methanol synthesis reactor) were estimated from actual commercial quotations and costs present in literature [1, 70]. These costs were updated to present value using the widely used chemical engineering plant cost index (CEPCI) [71]. In addition, the main equipment costs were adjusted for the required capacity to estimate the present values of the equipment as shown in Eq. 14.

$${C}_{ya}={C}_{yb}\left(\frac{C{I}_{ya}}{C{I}_{yb}}\right)\times {\left(\frac{{A}_{ya}}{{A}_{yb}}\right)}^{n}$$
(14)

where CI is the cost index, yb and ya are the base year (2018 in this case) and current year (2022 in this case), C is the equipment cost, A is the equipment capacity, n is the cost exponent. A plant cost index of 806.3 was employed for the year 2022 [72].

For the cost analysis, the total capital investment (TCI) and the annual production cost (APC) were all investigated. The TCI is estimated by summing the fixed capital investment (FCI) (see Fig. 8 for FCI breakdown) and working capital (WC) and this was computed to be $43.3 million. The main contributors to the TCI are the compressors, steam reforming reactor, and the methanol purification distillation columns (see Fig. 9a.). Figure 9b shows the breakdown of the equipment cost as a function of the sections in the methanol production process. The syngas cleaning (PSA) process is the most expensive, it represents 49% of the cost of the processes involved in the methanol production process. Meanwhile, the cost of methanol purification was the least expensive and it represents just 6% of the whole process. The steam reforming as well as the methanol synthesis processes represents 22% and 23% respectively as shown in Fig. 9b.

Fig. 8
figure 8

Fixed capital investment breakdown

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Fig. 9
figure 9

Equipment cost breakdown by (a) main equipment (b) main sections

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The APC on the other hand includes the fixed cost (FC), the variable cost (VC), research and development, sales expenses, and general overheads costs. The APC was estimated to be around $38.9 million. The estimated FC was around $11 million and the breakdown include maintenance, salaries, insurance, taxes, supervision, capital charges, royalties, and laboratory cost. The VC was estimated to $17.8 million while the other costs involved in the APC was estimated to $10.1 million. The breakdown of the APC components is shown in Fig. 10. The cost of the raw materials (crude glycerol and water), the assumed cost of the product (methanol) and byproducts (hydrogen and water), and the hazardous and non-hazardous wastes employed in this study are available in the supporting document (Table S10). In addition, since only a very small quantity of methane was recovered from the PSA, it was considered to be used up in the reactor furnace [18].

Fig. 10
figure 10

Annual production cost breakdown

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4.12 Profitability analysis using the Key Economic Performance Indicators (KPI)

As mentioned earlier, the KPIs considered in this study include the ROI, NPV, DPBP, and NPC. The economic options employed include a 28% tax rate, 6% average annual inflation rate and a 3.5% annual loan interest rate over a 20-year project life (including the construction period) using South African rates as a base scenario. Other economic parameters employed in this analysis can be found in Table S19 (see supporting document). For the process to be profitable the minimum selling price of methanol was set at $846/ton while the hydrogen was set at $5000/ton. The MSP of both methanol and hydrogen produced from steam reforming process are within the observed MSP from previous related studies [31,32,33, 73]. The NPV, ROI, DPBP, and NPC from the process were estimated to be $74.2 million, 17%, 4.59 years, and 85₵/kgMeOH respectively. In addition, the hydrogen produced contributed to 48% of the total revenue from the process, if the revenue from hydrogen is excluded, the NPC of methanol from the process is around $2.1/kgMeOH. Considering the TCI and other economic trade-offs, the production of methanol from steam reforming of crude glycerol is proven to be economically feasible.

4.13 Sensitivity analysis using the Key Economic Performance Indicators (KPI)

Parameters such as the fixed capital investment (FCI), cost of manufacturing (COM), cost of raw materials, the revenue, the working capital, the income tax rate, the interest rate, the current inflation rate, the discount rate, operating labour cost, and finally, the utilities and heat exchanger network cost were all analyzed over a ± 20% variation in the KPIs. From the analysis, the revenue from the sales of the product and by-products, the cost of manufacturing (COM), the cost of raw materials, as well as FCI were the most sensitive variables as shown in Fig. 11. It was observed that the selling price of methanol must not go below the estimated price (85₵/kgMeOH). From the simulation, prices below this will have a significant effect on the profitability/economic feasibility of the project. The least sensitive variables are the working capital, loan interest rate and the discount rate.

Fig. 11
figure 11

Effect of ± 20% variation on the KPIs

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5 Conclusion

Aspen Plus was able to simulate the production of methanol from crude glycerol, proving the technical viability of the process. To determine the effects of the key operating conditions which include: STR temperature, steam-to-glycerol ratio (SGR), methanol synthesis pressure, and methanol synthesis temperature, a sensitivity analysis was carried out on the reforming and methanol synthesis section. It was observed that high methanol yield was obtained as a result of high H2 and CO conversion. Despite the economic performance of the limited number of commercial biorefineries as well as the global technological developments, biorefineries still face substantial uncertainty due to the fluctuations in market prices. Another key observation from this process is that for every 100 kmol of crude glycerol processed through STR, 0.21—0.34 kgMeOH/kgCG can be obtained along with high quantity of hydrogen (48%) and a little quantity of methane (1%) which could either be used to fuel the furnace or sold to obtain further revenue. The commercial viability study also shows that the final methanol produced is a function of the both the initial glycerol and methanol concentration in the crude glycerol. Crude glycerol with glycerol composition above 80 wt.% and methanol concentration above 20 wt.% were observed to produced more methanol from the STR process.

From the economic assessment carried out, the net present value (NPV), the return on investment (ROI), the discounted payback period (DPBP) and the net production cost (NPC) show that methanol produced from the syngas derived from steam STR process combined with PSA process is economically feasible. The sensitivity analysis shows that the revenue is the most sensitive variable. Hence, a minimum selling price of 85₵ per kg of methanol was the most feasible for the production of methanol from crude glycerol STR process. In addition, since the production of by-products hydrogen and methane from the syngas cleaning process are inevitable, further investigation from the STR process should be investigated. Also, future studies could also consider the trade-offs between producing methanol from other renewable sources.