From the Dream Reaction to the Real Process the Implemented HPPO Process

The HPPO process has been implemented recently by BASF and The Dow Chemical Company; the first plant has been in operation since November 2008 in Antwerp, Belgium. Dow acquired its PO technology from its 2001 purchase of EniChem's polyurethane business. BASF had been exploring HP-based routes to PO since the mid-1990s; Dow and BASF began collaborating in 2003. The two companies have established a long-term partnership with Solvay SA (supplier of HP to the new facility). The HPPO plant is fed with HP from a new mega-sized plant next to it.

334 | 6 Synthesis of Propene Oxide: A Successful Example of Sustainable Industrial Chemistry

334 | 6 Synthesis of Propene Oxide: A Successful Example of Sustainable Industrial Chemistry

Hppo Plant Process

RAHQ solution

Figure 6.4 Simplified flow-sheet ofthe integrated in situ HPPO (a) and HPPO (b) processes for propene epoxidation with generation of HP, developed by EniChem.

HP solution

RAHQ solution

Figure 6.4 Simplified flow-sheet ofthe integrated in situ HPPO (a) and HPPO (b) processes for propene epoxidation with generation of HP, developed by EniChem.

The HP plant, built jointly by Dow, BASF and Solvay, has a capacity of 230 000 tons yr~\ based on the RAQ/RAHQ route for HP production. The 300 000 metric-tons per year HPPO single-train plant, built jointly by BASF and Dow, started up in 2008. In 2007, Dow and Solvay also announced an agreement to create a joint venture for the construction of a HP plant in Map Ta Phut, Thailand. The plant will be the largest

6.2 PO-only Routes: Several Approaches for Sustainable Alternatives j 335 crude PO

6.2 PO-only Routes: Several Approaches for Sustainable Alternatives j 335 crude PO

Basf Prozess
Figure 6.5 Simplified flow-sheet of the BASF/Dow process for PO production [16].

in the world, with a capacity of over 330 000 ton yr-1 of HP at 100% concentration. Scheduled to be operational by 2010, it will serve as a raw material source for the manufacture of PO. Dow and BASF are negotiating the construction of a world-scale, 390 000 metric-ton per year PO manufacturing facility in Thailand. Propene will be supplied from the liquids cracker that Dow announced it was building together with The Siam Cement Group in October 2006, which is expected to be fully operational by 2010.

Figure 6.5 shows a simplified flow-sheet of the BASF/Dow PO process [16]. Even though the scheme does not show any propene recycling, it is possible that the process also includes the recycle of the unconverted alkene.

The dilute HP-alcohol solution (HP concentration less than 10%) is introduced in a fixed-bed epoxidation reactor. ATi silicalite (TS-1) catalyst is used also in this case, to produce PO from propene and HP. The reaction is carried out at 40 °C and 20 atm pressure. Process PO yield is estimated to be around 95 mol.%; by-products are 1,2-propandiol and the ethers formed by the methanolysis of the oxirane ring (1-methoxy-2-propanol and 2-methoxy-1-propanol), which may further react with PO to yield dipropeneglycol monomethylethers. PO may also form propanol hydroperoxides (1-hydroperoxy-2-propanol and 2-hydroperoxy-1-propanol). Other side-reactions such as the decomposition of HP normally occur to a very low extent.

The following achievements are claimed for the newly developed HPPO process in the patents jointly issued by BASF and Dow [17]:

1. An optimal performance in terms of PO yield is obtained when the reaction of PO synthesis is carried out in two steps, with intermediate separation of the PO produced [17b]. The output from the first reactor consists of methanol, water, PO, by-products, unreacted propene and HP. The yield achieved is 85% with a PO selectivity based on HP of 95%. The output is depressurized into a column operating at atmospheric pressure, at a bottom temperature of 69 ° C. PO is distilled offtogether with propene and some methanol, while the bottom product is fed to a second reactor operating under the same conditions as the first reactor, together with fresh propene. In the second step, the HP conversion achieved is 96%, and the PO selectivity is 96%. The overall HP conversion is 99.4%, and the overall PO selectivity is 95-96%; the PO yield based on HP is 94-95%. When the reaction is carried out in a single reactor, the HP conversion achieved is similar, that is, 98.4%, butthe PO selectivity based on HP is 80.3%, with a PO yield of 79%. Overall, the higher propene-to-HP ratio in each reactor leads to a lower propene conversion, but the selectivity for PO with respect to HP is improved.

2. After the epoxidation reactor(s), a first column separates the lights, mainly propene, propane, nitrogen (used as ballast to avoid the formation of flammable mixtures in the epoxidation reactor) and oxygen, the latter deriving from the decomposition of HP. Specifically, the off-gas stream is compressed (16atm) with cooling (35 °C) and fed to a stripping column under pressure, to obtain an olefin-containing bottom stream (potentially also containing propane) recycled to the epoxidation, and an off-gas stream with a low hydrocarbon content, containing nitrogen, oxygen and further volatile by-products [17c]. The following is an example of the stream outlet composition from the epoxidation reactor with the subsequent separation of the light-boiling compounds [17d]: unreacted propene 0.01 mass%, formaldehyde 0.01%, acetaldehyde 0.03%, PO 9.45%, methanol 71.97%, water 17.54%, glycol ethers 0.43%, propene glycol 0.05%, heavy boilers remainder.

3. PO is separated from methanol by extractive distillation, using water (or in another embodiment, propene glycol) as the extractor. PO is distilled overhead from the extractive column as top stream, while the bottom stream contains methanol and water [17d, f]. The energy integration of the column is one important issue of the patents; the vapors of the top stream are compressed, and the condensation heat is returned to the vaporizer employed in the extractive distillation column.

Degussa-Evonik, currently the world's second largest producer of HP after Solvay (both use the anthraquinone route), with approximately 600000 metric tons, has investigated both the direct synthesis of HP and PO production with HP. The HPPO process, developed in cooperation with Krupp-Uhde, is available for licensing. To meet the growing demand for PO in the Asian market, the Seoul-based Korean company SKC acquired a license for the Degussa/Uhde process; an HPPO plant with an annual capacity of 100 000 metric tons per year at Ulsan was successfully started up in 2009 [18]. The process uses a titanium silicalite catalyst developed by Degussa, and will be supplied with HP from a conventional anthraquinone plant belonging to Degussa/Headwaters. The joint venture has also acquired a HP plant in Ulsan from the Helsinki-based Kemira, more than doubling its annual capacity from the previous 34000 metric-tons per year to supply HP to the SKC plant.

In the Degussa/Uhde process [19], the reactor works at 60 °C. The raw PO is purified by extractive distillation using propene glycol that effectively removes impurities such as acetaldehyde. The final purity of PO is 99.97 wt%. The main by-product is propene glycol. The catalyst deactivates due to PO oligomers formation and is regenerated by calcining or treatment with HP solutions. The high selectivity is

methanol, by-products, water

Figure 6.6 Simplified flow-sheet of the Degussa/Uhde process [19].

methanol, by-products, water

Figure 6.6 Simplified flow-sheet of the Degussa/Uhde process [19].

reached thanks to the moderate reaction temperature; the continuous removal from circulation of substances that can deactivate the catalyst brings about a considerable improvement in the performance and catalyst longevity.

Figure 6.6 shows a simplified flow-sheet of the Degussa/Uhde process [19]. In this process configuration, after the fixed-bed reactor the product mixture, consisting of methanol, water, propene and PO is depressurized; the resulting propene-rich gas phase is compressed, condensed and after separation of propane, it is returned to the reactor. Propane enters the process because chemical-grade propene can be used as the feedstock; to avoid the build-up of the alkane in circulation, which would behave as an inert gas in the reaction, propane is separated from propene. The expanded liquid phase is split into a raw PO stream and a stream consisting of methanol and water. The raw PO is fed to the purification section; purification is performed by extractive distillation using propene glycol that effectively removes impurities such as acetaldehyde. The final purity of PO is 99.97 wt%. The methanol-water mixture from the pre-separation section and the PO purification section is fed to the methanol processing unit, in which methanol is separated and then returned to the reaction section; this separation requires a large amount of energy. The bottom product of this section consists of water and by-products.

The main features of the process, as inferred from the patent literature [20], include:

1. PO synthesis is carried out in a methanol solvent, propene and 40% HP adjusted with ammonia to pH 4.5; pressure is 25 atm [20a]. The feed stream contains 21.5 wt% propene, 57 wt% methanol and 9.4 wt% HP. In some patents, a different feed composition is reported, containing 43 wt% propene, 43 wt% methanol and 8.4 wt% HP (from the feeding ofa 60% solution of HP in water, adjusted to pH 4.5 with 1100 ppm ammonia) [20c]. The effect of temperature is shown in Table 6.3, for an upflow feed. By-products are 1-methoxy-2-propanol, 2-methoxy-1-propanol (propene glycol monomethyl ethers) and propene glycol (1,2-propandiol). How-

Table6.3 Catalytic performance in propene epoxidation with HP in the Degussa/Uhde process [20].

Temperature

Flow rate

HP conversion

PO selectivity

PO yield based

(C)

(kg h1)

(%)

(%)

on HP (%)

30

0.35

81

95

77

40

0.55

96

93

89

60

1.8

92

85

78

60

4.1

87

70

61

ever, when the reactor feed is downflow, at 400 C, P = 25 atm and with a flow rate of 0.55 kg h~\ the PO selectivity is 96% and the PO yield with respect to HP is 92%. The maximum recommended temperature in the catalytic bed is 60 0C, whereas the average cooling temperature is 400 C. The optimum values reported are 97% selectivity for PO and 93% yield, both calculated with respect to HP, with a 96% HP conversion [20d]. The content of alkali metal ions and of bases is another key issue of the process [20e]. The catalyst is regenerated by washing with methanol at a temperature of at least 100 0C [20c].

2. A more detailed configuration of products separation and recovery section can be inferred from patents [20]. The stream exiting the reactor consists of a gas and a liquid phase. The gaseous stream, containing PO, propene, propane, oxygen and the inert gas is first directed to a condensation unit, wherein condensable components like propene, PO, propane and the solvent are partially condensed and recycled to the reactor. In an absorption unit, the gas stream from the phaseseparator is subsequently put into contact with the same solvent used in the reaction stage, that is, methanol. The solvent stream loaded with propene, propane and PO is removed from the absorption unit and recycled to the reactor, with the gas stream containing oxygen and the inert gas. The presence of the inert gas prevents the formation of flammable mixtures [20f,h]. The liquid stream from the reactor (Figure 6.7), containing water, the water soluble solvent (methanol), PO, propane and propene, is first directed to a pressure release unit, and then separated into an overhead gas stream containing propene, PO, propane and the organic solvent, whereas the bottom stream contains the remainder of the organic solvent, water and by-products [20i,j]. The methanol-water mixture from the pre-separation section and the PO purification section is fed to the methanol processing unit, in which methanol is separated and then returned to the reaction section; this separation requires a large amount of energy. The bottom product of this section consists of water and by-products.

An alternative scheme for the separation of products is reported in [20j], in which the gas stream leaving the pressure release unit is directly recycled to the reactor. The liquid stream, which contains PO, methanol, water, high-boiling compounds and unconverted HP, as well as some propene and propane, is treated in a pre-evaporation column to obtain an overhead stream and a bottom stream. The latter, containing methanol, water and by-products, is subjected to subsequent purification steps,

Uhde Methanol
to waste treatment Figure 6.7 Block diagram of the liquid stream separation section of the Degussa/Uhde process for PO production [20].

whereas the former is further treated in a condenser and in a C3 stripping unit, to finally obtain a hydrocarbon-rich stream that is recycled to the reactor, and a PO-in-methanol-water-rich stream that is subjected to further purification. The following is an example of the composition of the liquid stream exiting the reactor: 20.4 wt% propene, 2wt% propane, 13.4wt% PO, 0.1wt% low-boilers, 47.7wt% methanol, 15.5 wt% water, 0.5 wt% HP and 1.1 wt% high-boilers.

The bottom product of the pre-evaporation stage (Figure 6.7) can eventually be subjected to hydrogenation in a trickle bed reactor, to purify the solvent recycle stream by eliminating impurities in the form of formaldehyde and acetaldehyde, reducing them to methanol and ethanol, and also to eliminate traces of unconverted HP. Moreover, traces ofhydroperoxypropanol and hydroxyacetone are converted into 1,2-propanediol. This allows a considerable decrease in catalyst deactivation in the epoxidation reactor and the improvement of product quality [20k].

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