Identification of alternatives, selection and design of process steps

The feedstock in the proposed biorefinery is hemicelluose prehydrolysate extracted from a receptor Kraft dissolving pulp mill. The prehydrolysate composition was obtained from pilot plant experiments at FPinnovations (Canada) using steam and hot water hydrolysis at 170ºC /800 kPa and dry wood chips to water weight ratio of 1:4 and [25]. The composition of the prehydrolysate considered is shown in Table 5.2

Table 5.2: Composition of hemicelluloses prehydrolysate

Components Mass Flow (t/d)

Water 4800 Acetic Acid 17 Furfural 3 Lignin 15 Pentose 83 Hexose 17

The prehydrolysate had a low sugar concentration (<2 % wt), to reduce the energy requirement and size of the downstream biorefinery process units (hemicelluloses conversion and furfural purification), the implementation of a concentration step is essential. A concentration method that retains the acetic acid fraction is also a means for lowering the pH and thus reducing the organic acid requirement.

Different state of the art alternatives were identified for the following biorefinery process steps:

 prehydrolysate concentration,

 sugars conversion

Alternatives that would lead to an energy efficient and environmentally friendly process were selected.

5.5.1.1 Prehydrolysate concentration

Two alternatives for prehydrolysate concentration were compared: the use of nanofiltration membranes or multi effect evaporators. The feasibility of concentration with membranes has been reported in an earlier publication [9]. It was shown that a reduction in permeate flux during concentration can be compensated for by increasing the pressure. Furthermore, a permeate flux of 12 L/m2h which is feasible at an industrial scale can be obtained during a volumetric concentration by a factor of 5. The energy demand of the membrane system comes mainly from pumping to supply the pressure gradient between the feed and permeate sides and can be calculated by equation 3.

𝑊 = 𝑄𝑓𝑃𝑓

(𝜂−𝐿) ( 19 )

The power consumption (W) of the pump is related to the feed volumetric flow rate (Qf), feed

pressure (Pf), efficiency of the pump (η) and pressure loss adjustment (L). The power requirement

for the pump is 300 kW at the design conditions, a prehydrolysate volumetric flow rate of 0.06 m3/s, feed pressure of 3500 kPa, total pump efficiency of 80% and pressure losses of 10%. Another well-known alternative for concentrating the prehydrolysate is the use of multi-effect evaporators. The amount of water to be evaporated to reach a volumetric concentration factor of 5 can be calculated from equation 4.

𝑚̇𝑒𝑣𝑎𝑝𝑜𝑟𝑎𝑡𝑒𝑑 =𝑚̇𝑓𝑒𝑒𝑑(1 −

𝑖𝑛𝑖𝑡𝑖𝑎𝑙 𝑠𝑢𝑔𝑎𝑟 𝑐𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛

𝑓𝑖𝑛𝑎𝑙 𝑠𝑢𝑔𝑎𝑟 𝑐𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛) ( 20 )

The amount of water that must be evaporated (45 kg/s) can be further used in equation 5 to calculate the minimum amount of energy (Qevap) that must be supplied to a multi-effect evaporator with the number of effects Nt,eff. of 4. The latent heat of evaporation (λ ) of water is 2,270 kJ/kg.

𝑄𝑒𝑣𝑎𝑝 =

.𝑚̇𝑒𝑣𝑎𝑝𝑜𝑟𝑎𝑡𝑒𝑑

A comparison of both alternatives was made given that the membrane cleaning frequency is 7 days with a lifetime of 2 years while the price of steam is 3.3 $/ ton. The results are shown in Table 5.3.

Table 5.3: Comparison of nanofiltration membranes and multi-effect evaporators Membrane

System

Multi-effect Evaporator

Purchased Equipment Cost (M$) 0.42 0.67

Installed Equipment Cost (M$) 3.3 0.94

Energy requirement * (MW) 0.3 26

Yearly Operating cost (M$/a) 0.8 2.7

* Electricity for membranes system and steam for evaporator

A detailed economic analysis of the membrane filtration system and the sensitivity of the concentration costs to the membrane cleaning frequency and useful life are presented in section 5.5.5.

5.5.1.2 Sugars conversion

Existing furfural production processes generally require the fractionation of the biomass feedstock before furfural production. An overview of some furfural processes (pilot or commercial scale) is given in Table 5.1. Although the processes are not directly comparable due to the difference in feedstock, operating conditions and substrate concentration, it can be concluded that the conversion of the pentoses in the prehydrolysate into furfural will require a shorter resident time because the sugars have been broken down into a mixture of oligomers and monomers, during the prehydrolysis step. Furthermore, the kinetics of oligomeric sugar conversion into monomers is much faster than the transformation of the monomers into furfural. For the conceptual design in this case study, it was taken that temperatures between 170 – 210 °C and a pressure of 1800 – 2250 kPa would be sufficient to obtain conversion as high as 75 % of the theoretical yield and a short residence time which would minimize the furfural loss reactions. Upon concentration of the prehydrolysate, the pH is reduced from 3.5 to 2.9 due to the acetic acid

concentration but additional acid catalyst is required to further reduce the pH in the reactor from 2.9 to ~1.5. Sulphuric acid was selected as the catalyst because its use and efficiency have been validated in several publications. The conditions used for the design in this case study are 170°C, 2100 kPa and sulfuric acid concentration of 1.8% wt.

5.5.1.3 Furfural purification

The product stream from the sugars conversion step will contain less than 6% wt of furfural. This is in the same range as all known conversion processes [18]. It must be purified to obtain high quality furfural. The use of a simple distillation for the recovery of furfural is however impossible because furfural forms an azeotrope with water at 35 % wt. The azeotrope formed is a positive azeotrope with its boiling point (97.8ºC) lower than that of water (100ºC) and furfural (126 ºC). A common practice in existing furfural processes is to make use of two distillation columns with a decantation step between the two columns as shown in Figure 5–5A. Furfural is recovered at just below its azeotropic point at the top of the first column and then fed into a decanter where the mixture spontaneously splits into furfural rich and furfural lean phases. The furfural rich phase is withdrawn and sent to the secondary distillation column where furfural at 99% purity can be recovered from the bottom of the column. An alternative method for the purification of azeotropic mixtures is extractive distillation shown in Figure 5–5B. It requires the introduction of a solvent which can alter the relative volatility of the furfural-water mixture. Water and the acid from the product stream can be recovered from the first column while furfural is recovered with the second column and the solvent recycled for use in the first column.

P-12 P-20 E-11 E-1 FURFURAL Dist Dist Decanter Water, Acids Furfural, Water, Acids FURFURAL Dist Water, Acids P-40 Furfural, Water, Acids Dist Solvent Recycle Makeup Solvent A) B)

Figure 5–5. Process diagrams illustrating A) conventional furfural distillation B) extractive distillation

Aspen Plus® _{V7.2 azeotrope search function was used to evaluate the possibility of utilizing}

several entrainer solvents available in published literature for extractive distillation [26]. It was discovered that most of the solvents are not suitable; they all result in the formation of other azeotropes (in addition to water – furfural) except for trichlorobenzene.

Table 5.4. Entrainer solvents evaluated for the extractive distillation of furfural

Entrainer Solvent BP (ºC) Azeotropes*

Trichloroethane 73.9 1 Diethylbenzene 180.9 3 Dipropylbenzene 210.5 3 N-Butylbenzene 182.9 2 Tetraline 207.6 2 Isophorone 215.2 2 Dichlorobenzene 173.1 2 Dibromobenzene 218.0 1 Trichlorobenzene 213.0 0 Carbon-Tetrachloride 76.6 1 Ethyl-Acetate 77.1 1 N-Octyl-Acetate 211.3 1 * Azeotropes formed other than water - furfural

It was determined that the energy requirement for furfural distillation can be reduced by as much as 30% with the use of trichlorobenzene. However, it is not a sustainable entrainer from an environmental standpoint since trichlorobenzene is currently classified as a hazardous chemical with restricted use [27, 28]. The design of the furfural purification step in this work was based on Figure 5–5A.