Patent Application
20120142973
1. Field of the Invention
The present invention relates to a distillation tower used for preparing dimethyl ether (DME), especially a dual-bed catalytic distillation tower. The present invention also relates to a method for preparing dimethyl ether using the same.
2. Description of the Prior Arts
Dimethyl ether (DME) is prepared by using different raw materials such as coal, nature gas, petroleum coke and biochar. The methanol is firstly prepared from the raw materials, and the methanol is dehydrated to obtain DME. In the dehydration reaction, catalysts are needed to carry out the reaction. Solid-state acid catalysts such as zeolite, silicone, aluminum oxide, resin etc. (Spivey, J. J., 1991) or modified acid ion exchange resin (U.S. Pat. No. 6,740,783) are commonly used in the dehydration. Resin catalysts are low temperature catalysts with reaction temperature ranging from 70 to 150° C. and almost 100% conversion of methanol can be obtained. However, the low temperature catalysts cannot bear high temperature. Other catalysts are high temperature catalysts with reaction temperature ranging from 200 to 350° C. When the reaction temperature is getting higher, however, methanol and DME will be further dehydrated to C2-C4 olefin and results in the activity of catalysts decay and DME yield reduce.
Further, to reduce the energy consumption during the distillation process, the catalysts are put in the distillation tower so the heat generated from the methanol dehydration reaction can be used for distillation. The requirement for this application is the catalytic reaction and distillation can happen at the same tower pressure and temperature range.
US 2007/0066855 A1 discloses a method for production of DME using catalytic distillation tower so the heat generated from dehydration can be fully used and reduce the production cost. However, the tower temperature and pressure used for dehydration reaction cause methanol and water in the stripping column cannot be separated easily.
WO 2007/014534 provides high temperature dehydration in the catalytic distillation tower by use of high temperature dehydration catalysts at reaction temperature from 160 to 180° C. and under tower pressure from 1.8 to 2.3 bar. However, the energy consumption of the condenser and reboiler increases in order to drive the catalytic reaction under high temperature.
U.S. Pat. No. 5,684,213 mentions high temperature dehydration reaction at reaction temperature from 350 to 400° C. and under 600 psi tower pressure. When dehydration is carried out at such high temperature, olefin side products increase and cause the activity of catalysts decays and the DME yield decreases. Hydrogen is added to the catalytic distillation tower to inhibit undesired side products generated. However, adding hydrogen increases the production cost and the reaction complexity.
The technique of using low temperature dehydration catalysts in the catalytic distillation tower is also disclosed. For example, WO 2007/014534 provides that dehydration reaction is carried at reaction temperature from 130 to 158° C. and under tower pressure from 10 to 18 bar by using low temperature acid ion exchange catalyst. However, the low temperature catalysts only can be used at temperature lower than 140° C. When the reaction is carried out at low temperature and pressure, the dew point of high purity DME on the top of the tower will decrease. For example, the dew point of DME becomes 40.3° C. when the tower pressure is low to 9 bar and therefore using 20° C. industrial water to condense DME will be impossible. When the tower pressure is adjusted to 12 bar, the temperature at the catalytic column will be higher than 150° C., and the low temperature catalysts will be damaged.
All the above-mentioned methods use single-bed catalytic system in the distillation tower, i.e. either high temperature or low temperature dehydration reaction is carried out. In the conventional methods, the distillation tower cannot be easily adjusted depending on the grade of methanol feeding and DME to be obtained because none of the conventional method can provide suitable and complementary tower temperature and pressure conditions.
Therefore, the purpose of the present invention is to provide a catalytic distillation method for preparing dimethyl ether (DME) in which the tower temperature and pressure can be proper adjusted depending on the relation among DME, methanol and water in the distillation and the dehydration catalysts selected.
Specifically, the present invention relates to a dual-bed catalytic distillation tower and the method for preparing DME using the same.
The present invention provides a dual-bed catalytic distillation tower comprising a catalytic column. The catalytic column from the top down comprises an upper catalytic bed and a lower catalytic bed. The upper catalytic bed is filled with low temperature dehydration catalysts and the lower catalytic bed is filled with high temperature dehydration catalysts. At least one feed port is set on the top of the upper catalytic bed, between the upper catalytic bed and the lower catalytic bed and on the bottom of the lower catalytic bed.
Preferably, the low temperature dehydration catalyst is Amberlyst® 15 acid ion exchange resin or Amberlyst® 35 acid ion exchange resin.
Preferably, the high temperature dehydration catalyst is fluoridized transition metal oxide, sulfate transition metal oxide, βzeolite and HZSM-5. All high temperature dehydration catalysts are covered by Teflon to enhance hydrophobic property of the catalysts.
The present invention also provides a method for preparing dimethyl ether by using aforesaid dual-bed catalytic distillation tower wherein the catalytic column is used for dehydration of methanol. The feeding stream containing methanol is fed from the feed port located on the top of the upper catalytic bed, between the upper catalytic bed and the lower catalytic bed or on the bottom of the lower catalytic bed at the tower pressure from 6 to 30 bar for dehydration to obtain DME.
The temperature of the catalytic distillation tower of the present invention is at the range from 60 to 250° C. When the tower temperature is at the range from 60 to 180° C., the low temperature dehydration catalysts are used for dehydration. When the tower temperature is at the range from 110 to 250° C., the high temperature dehydration catalysts are used for dehydration.
Preferably, the tower pressure of the catalytic distillation tower is ranging from 8 to 14 bar.
Preferably, the catalytic distillation tower further comprises at least one flash zone on the on the top of the upper catalytic bed, between the upper catalytic bed and the lower catalytic bed and on the bottom of the lower catalytic bed is reserved for heat exchange and reflux.
When anhydrous methanol is used as the feeding, it is fed to the tower from the feed port located on the top of the upper catalytic bed.
When syngas is used as the feeding, it is firstly converted to a mixture containing DME, methanol and water, and the mixture is fed to the tower from the feed port located on the top of the upper catalytic bed or between the upper catalytic bed and the lower catalytic bed.
When crude methanol is used as the feeding, it is feed to the tower from the feed port located on the bottom of the lower catalytic bed or between the upper catalytic bed and the lower catalytic bed.
Comparing to the conventional single-bed catalytic distillation tower, the dual-bed catalytic distillation tower of the present invention has advantages of flexible set up depending on various feedings such as anhydrous or crude methanol and on different grade of DME to be obtained.
The above and other technical features and advantages of the present invention will be described in greater detail with reference to the drawings.
Based on the substance properties as described above, the present invention provides a catalytic distillation tower where different catalysts can be placed in different temperature zones to carry out dehydration of methanol efficiently and prevent the catalysts from being damaged due to high temperature. As shown in
Generally, the temperature of the catalytic distillation tower of the present invention is from 60° C. to 250° C. When the tower temperature is from 60° C. to 180° C., the low temperature dehydration catalyst is used in Z1 shown in
Preferably, the low temperature dehydration catalyst is used in the upper catalytic bed 7. For example, but not limited to, Amberlyst® 15 acid ion exchange resin is used at tower temperature from 85 to 110° C. and Amberlyst® 35 acid ion exchange resin is used at tower temperature from 110 to 135° C. The high temperature dehydration catalyst is used in the lower catalytic bed 11. For example, fluoridized transition metal oxide such as F-alumina, sulfate transition metal oxide such as sulfate zirconium dioxide (SO42−/ZrO2), βzeolite and HZSM-5 are suitable high temperature dehydration catalyst. The high temperature dehydration catalyst is covered by Teflon to enhance hydrophobic property of the catalyst and prevent the mass transfer resistance caused by the liquid molecular on the surface of the catalyst.
The tower pressure varies depending on the environment. The pressure is usually at the range from 6 bar to 30 bar, preferably from 8 bar to 14 bar. If the distillation tower is used at high latitudes, considering using 20° C. industrial water to condense the DME with dew point 25.4° C., the tower pressure can be low to 6 bar. When the tower pressure is up to 18 bar, the relative volatility of methanol and water should be considered.
The feeding can be feed to the distillation tower from different feed ports 1, 2 and 3. When anhydrous methanol is used as the feeding, it is usually dehydrated in liquid phase so it is feed to the tower from feed port 1 located on the top of the upper catalytic bed 7. When syngas is used as the feeding, it firstly converts to a mixture containing dimethyl either, methanol and water, and the mixture is usually feed to the tower from feed port 1 located on the top of the upper catalytic bed 7 or feed port 2 located between the upper catalytic bed 7 and the lower catalytic bed 11. When crude methanol is used as the feeding, it is firstly heated to saturated vapor for further reaction so it is usually feed to the tower from feed port 3 located on the bottom of the lower catalytic bed 11 or feed port 2 between the upper catalytic bed 7 and the lower catalytic bed 11. The heat exchanger and/or reflux condenser 12 located between the upper catalytic bed 7 and the lower catalytic bed 11 is used to control the temperatures of the two catalytic beds and simultaneously protect the low temperature dehydration catalysts in the upper catalytic bed 7 from being damaged due to high temperature.
As shown in
In the following examples, low temperature dehydration rate (An et al., 2004), thermodynamic NRTL-RK equation, the thermodynamic data for high temperature dehydration reaction (Lin et al., 1981; Hayashi, 1982) are used for theoretical calculation of the DME catalytic distillation tower by the simulation software “Aspen Plus” to illustrate the feasibility of the present invention.
The catalytic distillation tower from the top down contains an overhead condenser, rectification column, catalytic column, stripping column and a reboiler at tower bottom.
The conditions for the conventional single-bed catalytic distillation tower are set as follows. The theoretical plate numbers of the rectification column and the stripping column are respectively set to 7 (including part or all of the condenser and reboiler). The feed containing methanol mixture is fed to the distillation tower at 30° C., and performs heat exchange with the hot water comes from the bottom of the distillation tower. The feed temperature is about 40° C. after the heat exchange step. The amount of catalyst and the tower height are adjusted, i.e. changing the plate number of the catalytic column and the tolerated flow rate for liquid in the catalytic column (24 inch height/plate) to be 24.4 m/s. Afterwards, the reflux ratio and D/F are adjusted so the concentration of DME and water produced from the distillation tower are respectively 99.9 wt %.
In the dual-bed catalytic distillation tower, the catalytic column includes an upper catalytic bed filled with low temperature dehydration catalysts and a lower catalytic bed filled with high temperature dehydration catalysts. The position in the tower having temperature at 135° C. is set as the demarcation point for upper and lower catalytic beds. The plate number of the catalytic column, the reflux ratio and D/F are adjusted so the concentration of DME and water produced from the distillation tower are respectively 99.9 wt %. When the fuel grade DME (93 wt % of DME and 7 wt % of methanol) is desired under the same distillation design, the amount of catalysts will be too much. In this situation, the feeding increases so the conversion of methanol decreases and the fuel grade DME is obtained from the top of the tower and the concentration of the water obtained from the bottom of the distillation tower is still 99.9 wt %.
The conditions of the present dual-bed catalytic distillation tower are assumed as below. The unit volume of the upper catalytic bed equals to 0.6 unit volume of the theoretical plate and the unit volume of the lower catalytic bed equals to 0.85 unit volume of the theoretical plate (referring to U.S. Pat. No. 6,045,762 and US 2007/U.S. Pat. No. 0,095,646). The dehydration rate of the hydrophobic and strong acidic catalysts is five times more than the rate disclosed in the literature Lin et al., 1981.
The simulation for the production of DME in US 2007/U.S. Pat. No. 0,066,855 is indicated below.
The practice result shows that the DME produced is 3598 tpd (99.61 weight %) and the temperatures on the top and bottom of the tower is respectively 52° C. and 190° C. Although the tower temperature distribution does not disclosed in the prior art, the temperature is assumed having 4° C. inaccuracy compared to the simulated tower temperature.
The practice result shows that the temperature at the bottom two plate, i.e. plate number 17 and 18, is respectively 151.5° C. and 156.4° C. which exceed the limit of temperature resistance of acid ion exchange resin catalysts such as Amberlyst® 35 having temperature resistance at 140° C. Therefore, the low temperature dehydration catalysts cannot be used in the conventional distillation tower under certain tower pressure such as at the pressure ranging from 8 to 12 bar.
The simulation for the production of DME in the dual-bed distillation tower of the present invention and in the conventional single-bed distillation tower is indicated below.
The practice results show in Table 1, Table 2 and
As shown in Table 2, 4 more plates are needed in the dual-bed distillation tower compared to the single-bed distillation tower in order to increase methanol conversion rate. This is because the reaction rate of the high temperature dehydration catalyst is lower than the rate of the low temperature dehydration catalyst so the amount of high temperature dehydration catalyst is more than the amount of low temperature dehydration catalyst. In the present invention, high temperature dehydration catalysts (from plate 17 to 24) are used to replace part of the low temperature dehydration catalysts (from plate number 17 to 20) in the conventional distillation tower so the temperature increases moderately at the lower catalytic bed. Therefore, the temperature at the border between the upper catalytic bed and the lower catalytic bed would not be too high and damage the low temperature dehydration catalysts in the upper catalytic bed.
The feeding comes from the process for preparing methanol from syngas (with reference to U.S. Pat. No. 5,750,799). The produced methanol contains 10-20 mole % of water. The produced methanol is fed to the catalytic distillation tower of the present invention directly for dehydration without being purified. The simulation for the production of DME in the dual-bed distillation tower of the present invention and in the conventional single-bed distillation tower is indicated below.
The results of using dual-bed distillation tower of the present invention and using conventional single-bed distillation tower are shown in Table 3, Table 4 and
When the methanol is fed from feed port 1, the liquid flow amount is more than the amount in Example 1 because the methanol fed containing water. Therefore, the tower diameter in this example is larger than the tower diameter in Example 1. The theoretical calculated result of the temperature distribution in the tower is similar to the tower temperature distribution shown in
When the upper catalytic bed is moved a plate upper and methanol is fed from feed port 2, the reflux rate is raised to increase the conversion rate of methanol. The concentrations of DME on the tower top and water on the tower bottom respectively decrease to 99.3 wt % and 98.9 wt %.
When methanol is fed from feed port 3, only high temperature dehydration catalysts can be used in the single-bed distillation system because low temperature dehydration catalysts will be damaged when the column temperature is higher than 160° C.
The feeding comes from the process for preparing methanol from syngas (with reference to U.S. Pat. No. 5,908,963). After separation, the concentration of carbon dioxide, DME, methanol and water in the product from the reactor are respectively 2.8 wt %, 49.7 wt %, 31.0 wt % and 16.5 wt %. The mixture can be fed from the top catalytic column or between the upper and lower catalytic beds. The reflux ratios are calculated to be 0.378 and 1.22. The practice results are shown in Table 5, Table 6 and
The feeding comes from the process for preparing methanol from syngas (with reference to U.S. Pat. No. 5,908,963). After separation, the concentration of carbon dioxide, DME, methanol and water in the product from the reactor are respectively 2.8 wt %, 49.7 wt %, 31.0 wt % and 16.5 wt %. The carbon dioxide can not be liquidized because of the limitation of the tower pressure. Thus, an overhead condenser is equipped on the top of the distillation tower. The results are shown in Table 7. The DME obtained contains 1.36 wt % of carbon dioxide so it will flow back to the distillation tower for purify. The gas phase carbon dioxide and DME are respectively 25 wt % and 75 wt %. They also need to be re-purified in the distillation tower.
What stated above is only preferred embodiments of the present invention, which is illustrative only and not restrictive. Many changes, modifications, or the equivalents may be made by those skilled in the art without departing from the spirits and scope of the present invention as defined by the claims, but will fall within the scope of protection of the present invention.
