Chemistry: fischer-tropsch processes; or purification or recover – Liquid phase fischer-tropsch reaction
Reexamination Certificate
2001-11-07
2002-10-01
Parsa, J. (Department: 1621)
Chemistry: fischer-tropsch processes; or purification or recover
Liquid phase fischer-tropsch reaction
C518S724000, C518S725000, C568S671000, C568S698000, C568S699000
Reexamination Certificate
active
06458856
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a separation process for the one-step production of dimethyl ether (DME) from synthesis gas (syngas), a mixture of hydrogen (H
2
) and carbon monoxide (CO). In the one-step syngas-to-DME process, syngas is converted in a single reactor (DME reactor) to methanol and DME over a catalyst system with methanol synthesis (2H
2
+CO→CH
3
OH), methanol dehydration (2CH
3
OH→DME+H
2
O), and water gas shift (H
2
O+CO→CO
2
+H
2
) activities. Due to a chemical synergy among these three reactions, the single pass syngas conversion in the DME reactor, or productivity, is significantly greater than that in a methanol synthesis reactor, wherein the methanol synthesis reaction primarily takes place. Since reactors for syngas conversion are expensive equipment for high-pressure operation at elevated temperatures, greater conversion or productivity means smaller DME reactors, associated equipment, and operation. This can reduce the cost in the syngas conversion part of the process, and possibly lead to a more economic process for DME production than the traditional two-step process, namely, methanol synthesis followed by methanol dehydration in two separate reactors.
However, the downstream separation for the one-step process could be costly because of the high volatility of two reaction products, DME and CO
2
. CO
2
is especially a problem. In general, there are three ways to deal with the CO
2
problem. First, scrub out DME and methanol from the reactor effluent and let CO
2
remain in the unconverted syngas and build up in the DME reactor loop. CO
2
formation in the reactor will be suppressed when the CO
2
concentration in the reactor loop reaches a certain level. In this approach, the product stream entering the downstream separation process is CO
2
free, therefore it can be conducted at reasonable cost. However, this approach adversely affects the productivity of the DME reactor. Unless the H
2
:CO ratio in the syngas feed to the DME reactor, is very high (e.g., >5, as in the WO Patent 96/23755 shown below), the final equilibrium CO
2
concentration in the DME reactor loop will be large. The presence of a large amount of CO
2
dilutes the reactants and hampers the synergy for the reaction system, resulting in a large decrease in the reactor productivity. One can avoid the build-up of a large amount CO
2
by operating in very H
2
-rich (e.g., H
2
:CO>5) regime. However, the productivity of the DME reactor in this regime is much lower than that when the DME reactor is operated in the optimal regime (H
2
:CO around 1). It could even be lower than the productivity of a syngas-to-methanol reactor at its best-feed composition (H
2
:CO=2) on a methanol equivalent basis. In brief, this suppressing-CO
2
-formation approach can minimize the CO
2
handling cost; but it also takes away the very advantage of the one-step syngas-to-DME process—its high reactor productivity.
The second approach is to remove CO
2
from the unconverted syngas in the above approach before it is recycled to the DME reactor, thereby preserving the high productivity of the DME reactor. A commercially available CO
2
separation technology (physical or chemical absorption) can be used. However, since this requires an independent CO
2
separation system, the cost could be so high that it may negate all the cost saving from the high reactor productivity. Furthermore, in the natural gas-based syngas-to-DME process, CO
2
needs to be recycled to the syngas generation equipment to maintain a desired hydrogen:CO ratio. Since the pressure of the recovered CO
2
from these absorption units is low, the compressing cost for returning CO
2
to high-pressure syngas generation units will be high.
The third way to deal with the CO
2
problem is to make it an integral part of the downstream product separation process. CO
2
is removed, along with DME and methanol, from the reactor effluent. This would maintain the high productivity of the DME reactor, the very source of cost saving against the two-step DME process. The cost of the downstream separation will be higher than that of the two-step DME process due to the presence of CO
2
. However, one may be able to develop optimized separation schemes so that the CO
2
-induced cost will be much smaller than the cost saving by the high reactor productivity, therefore, warranting an economic one-step DME process. The objective of the current invention is to develop such an optimized separation scheme.
A number of separation schemes have been disclosed in the prior art for the one-step syngas-to-DME process. WO Patent 96/23755 and its equivalent U.S. Pat. No. 5,908,963 choose to avoid the CO
2
problem by operating a fixed bed syngas-to-DME reactor in a H
2
-rich regime (H
2
:CO>5). The reactor effluent is cooled in a condenser. The condensed reaction products, methanol, water and dissolved DME, are sent to two distillation columns for DME-methanol/water separation and methanol-water separation, respectively. Part of the gaseous stream from the condenser, containing unconverted syngas, DME and a small amount of CO
2
, is recycled back to the DME reactor; the rest is sent to a scrubbing column to recover DME. Methanol, from the water-methanol column, is used as the scrubbing solvent. The DME-methanol mixture from the scrubbing column is fed to a methanol dehydration reactor. Due to the high H
2
:CO ratio in the reactor feed, CO
2
formation is suppressed with a small amount of CO
2
(e.g., 3 mol. %) in the reactor loop. However, the reactor is operated in a regime far away from the optimal conditions.
Methanol is also used as the scrubbing solvent in separation scheme disclosed in a paper by Bhatt, Toseland, Peng and Heydorn, 17
th
International Pittsburgh Coal Conference, Pittsburgh (September, 2000), for a 10 tons/day one step syngas-to-DME pilot plant (referred to as Bhatt's paper hereafter). In this separation scheme, the effluent from a slurry phase syngas-to-DME reactor is first cooled to condense methanol and water out. The rest of the effluent is fed to a scrubbing column which uses methanol as a solvent. All DME, methanol and CO
2
are removed from the unconverted syngas in the scrubbing column. The bottom stream from the scrubber is sent to a distillation column to regenerate methanol from DME and CO
2
. Due to the trial nature of the work, the DME and CO
2
mixture was sent to flare without further separation.
A paper by Xie and Niu (Tianranqi Huagong, 24 (1999) p.28) examines different scrubbing solvents for DME separation, including methanol, water and methanol/water mixture. Methanol and 50/50 methanol/water mixture exhibited similar solubility to DME; both are better than pure water.
Chinese patent application No.1085824A to Guangyu et al. describes a downstream separation scheme for a one-step syngas-to-DME process. The water and methanol in the effluent from a fixed bed syngas-to-DME reactor are removed through a condenser and an absorption column, respectively. The rest of the reactor effluent enters into an extraction column. The unconverted syngas leaves the column from the top and is recycled to the DME reactor. A solvent is used in the extraction column to remove DME from the recycle stream. Water and ethanol are two solvents taught in the patent. When water is used as the extraction solvent, 5% of the CO
2
in the effluent gas is also dissolved in the water. The water solution is sent to a stripping-distillation column to recover product DME and regenerate water. When ethanol is used as the solvent, considerable amount of CO
2
(40%) is dissolved in the ethanol along with DME. The CO
2
from the bottom of the extraction column is first removed by some method (not specified). The rest is sent to a stripping-distillation column for DME-ethanol separation.
A downstream CO
2
separation scheme for a one-step syngas-to-DME process is described in a paper by Ohno, Ogawa, Shikada, Inoue, Ohyma, Yao and Kamijo, International DME Workshop, Japan (Sept. 7, 2000), (referr
Bhatt Bharat Lajjaram
Diamond Barry W.
Peng Xiang-Dong
Tsao Tsun-Chiu Robert
Air Products and Chemicals Inc.
Parsa J.
Wolff Robert J.
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