Dehydrogenation process at reduced hydrogen to hydrocarbon ratios

Abstract

Processes for dehydrogenation of a hydrocarbon feedstock are described. The process can be run at lower H.sub.2/HC ratios and lower RITs while maintaining coke production at the same level as operation at higher H.sub.2/HC ratios and higher RITs without decreasing the yield per pass. Acceptable levels of coke were achieved when operating the process at low hydrogen to hydrocarbon molar ratio in the range of 0.01 to 0.40 and reactor inlet temperatures in the range of 500-645 C.

Claims

1. A process for dehydrogenation of a hydrocarbon feedstock, the process comprising: passing a feed stream comprising hydrogen and paraffins into a dehydrogenation zone comprising at least one reactor containing a dehydrogenation catalyst maintained at dehydrogenation conditions to produce a dehydrogenation zone product stream comprising hydrogen, light ends, paraffins, and olefins; wherein the dehydrogenation conditions in the at least one reactor include a hydrogen to hydrocarbon molar ratio, a reactor inlet temperature, and a coke index, wherein the hydrogen to hydrocarbon molar ratio is in a range of 0.01 to 0.4, and wherein one dehydrogenation condition is adjusted based on the other two dehydrogenation conditions; wherein the coke index is determined by measuring coking for a range of reactor inlet temperature and hydrogen to hydrocarbon molar ratio combinations and determining a correlation between the reactor inlet temperature and the hydrogen to hydrocarbon molar ratio; and wherein a desired reactor inlet temperature is determined using the correlation and the selected hydrogen to hydrocarbon molar ratio and wherein the reactor inlet temperature is adjusted to the determined reactor inlet temperature; or wherein a desired hydrogen to hydrocarbon molar ratio is determined using the correlation and the selected reactor inlet temperature and wherein the hydrogen to hydrocarbon molar ratio is adjusted to the determined hydrogen to hydrocarbon molar ratio.

2. The process of claim 1 wherein either the reactor inlet temperature is adjusted based on a selected hydrogen to hydrocarbon molar ratio and the coke index or the hydrogen to hydrocarbon molar ratio is adjusted based on a selected reactor inlet temperature and the coke index.

3. The process of claim 1 wherein the coke index is in a range of 0-250.

4. The process of claim 1 wherein the hydrocarbon feed comprises at least one paraffin having 2 to 30 carbon atoms.

5. The process of claim 1 wherein the hydrocarbon feed comprises at least one paraffin having 2 to 6 carbon atoms.

6. The process of claim 1 wherein the hydrocarbon feed comprises at least one paraffin having 3 to 4 carbon atoms.

7. The process of claim 1 further comprising separating the dehydrogenation zone product stream into a hydrocarbon rich product stream and hydrogen rich product stream.

8. The process of claim 7 further comprising passing a portion of the hydrogen rich stream to the dehydrogenation zone.

Description

EXAMPLE 1

(1) Development of the Coke Index for Propane Dehydrogenation:

(2) A series of catalyst coking experiments were conducted covering a temperature range of 490-650 C. and a H.sub.2 to HC ratio range of 0.05 to 0.80. The feed to the catalyst beds comprised mixtures of hydrocarbon and hydrogen.

(3) After each test the catalyst samples were sent for carbon analysis, which was reported as a wt % of the total catalyst sample. Table 1 provides a few illustrative examples of these experiments. As the results in Table 1 show, catalyst coking is a strong function of both temperature and the H.sub.2/HC ratio. Thus, a coke index can be created using these two key effects.

(4) Equation 1 summarizes the coke index created using the experiments described above. First, the experiments were sorted into groups of constant H.sub.2/HC ratios as a function of temperature. Next, the LN(coke) versus 1/T was plotted for each H.sub.2/HC ratio. This provided a unique linear relationship for each H.sub.2/HC ratio. Next, the slopes and intercepts of each of these unique linear relationships were plotted as a function of the H.sub.2/HC ratio to yield a second set of linear relationships. Finally, these two sets of linear relationships were combined to yield the coke index shown in Equation 1.

(5) $\begin{array}{cc}\mathrm{Coke}\mathrm{Index}=e^{\left[\left(-13923\frac{H2}{\mathrm{HC}}-20201\right)\left(\frac{1}{T}\right)\left(18.63\frac{H2}{\mathrm{HC}}+38.12\right)\right]}& \mathrm{Equation}1\left(C_3\right)\end{array}$

(6) Table 2 illustrates how to make use of this coke index. For the purposes of this example, it is assumed that an operating unit is running at 635 C. and an H.sub.2/HC of 0.50, and the amount of coke being formed on the catalyst is within the acceptable range. The plant operator wants to reduce H.sub.2/HC to 0.40 and needs to determine how much to lower the temperature such that coke on catalyst is expected to remain essentially the same as in the base case. First, equation 1 is used to calculate the coke index for the base case, which is 105.5 at 635 C. and an H.sub.2/HC of 0.50. Next, equation 1 is used again, but this time the coke index and temperature are known and instead the temperature (T) must be solved for to achieve the same coke index as in the base case, with the resulting temperature being approximately 630 C.

(7) TABLE-US-00001 TABLE 1 Hydrocarbon Feed H.sub.2/HC Temp., deg C. Coke, wt % Sample 1 Propane 0.5 590 0.40 Sample 2 Propane 0.2 590 1.34 Sample 3 Propane 0.2 560 0.05

(8) TABLE-US-00002 TABLE 2 H.sub.2/HC Temp., deg C. Coke Index Base Case 0.50 635 105.5 Target Operation 0.40 630 106.2

EXAMPLE 2

(9) Use of the Coke Index for Propane Dehydrogenation Process:

(10) A case study was rigorously simulated using commercially available process simulator (such as Aspen or Unisim) to demonstrate how the coke index can be used to select reactor inlet temperatures for a new H.sub.2/HC ratio target. The results of the simulation are shown in Table 3.

(11) TABLE-US-00003 TABLE 3 H.sub.2/HC Rx Inlet Temps Selectivity Yield Per Pass Molar ratio C. wt % wt % Base Case 0.5 Base Base Base Coke Index 0.4 Base 5 Base + 1% Base 1% Results

(12) The results shown in Table 3 have been normalized such that all comparisons are made on a relative basis to the base case. In this example the objective was to reduce H.sub.2/HC from 0.5 to 0.4. Reactor inlet temperatures were selected according to the methodology outlined in Example 1. The coke index indicated that reactor inlet temperatures needed to be reduced by 5 C. After obtaining the reactor inlet temperature settings for the lower H.sub.2/HC case, a second simulation at 0.4 H.sub.2/HC with the updated reactor inlet temperatures was performed. As the results of the table show, the yield per pass remained approximately constant and selectivity to propylene improved. In propane dehydrogenation it is common to have 3-4 reactors in series. Thus, the reactor inlet temperatures for all the reactors were reduced by 5 C. to produce the results shown in the table (i.e., if all reactors were operating at 600 then they all would need to be reduced to 595 C.). In summary, coke on catalyst would not be expected to increase, yield per pass remained essentially the same and selectivity to propylene increased, which improves the profitability of the dehydrogenation process.

EXAMPLE 3

(13) Use of the Coke Index for Isobutane and n-Butane Process:

(14) Use of a coke index is not exclusive to propane dehydrogenation. The same approach can be extended to isobutane and n-butane dehydrogenation. The process simulation disclosed in Example 2 contains a rigorous catalyst coking model used to predict the amount of coke on catalyst expected at the exit of the last reactor. It requires similar inputs as the coke index (i.e., H.sub.2/HC ratio, reactor inlet temperature, hydrocarbon feed composition). The process simulation also contains a dehydrogenation model that runs concurrently with the catalyst coking model. The dehydrogenation model is used to predict the temperature within the catalyst bed and the amount of hydrogen and olefin produced across the catalyst bed. Both models were developed using appropriate kinetic expressions known to those skilled in art of chemical reactor engineering. When combined, these two models can be used to estimate the amount of coke produced, selectivity and yield per pass for a set of proposed operating conditions. The results of the process simulation for three different feeds, with the same RITs and number of reactors are shown in Table 4.

(15) TABLE-US-00004 TABLE 4 Coke on H.sub.2/HC Rx Inlet Temps Catalyst Yield Per Pass FEED Molar ratio C. wt % wt % C3 0.4 Base Base Base iC4 0.4 Base Base 0.6 Base + 16% nC4 0.4 Base Base 2.0 Base + 16%

(16) The results shown in Table 4 have been normalized relative to a propane dehydrogenation process. An isobutane dehydrogenation process operating under the same conditions as a propane dehydrogenation process would be expected to make less coke at significantly higher yield per pass. The higher yield per pass is expected as it becomes progressively easier to dehydrogenate hydrocarbons with a higher number of carbon atoms. Comparatively, an n-butane dehydrogenation process operating under the same conditions as a propane dehydrogenation process would be expected to make more coke at the same yield per pass as the isobutane dehydrogenation process.

(17) Coke index equations can additionally be developed using the rigorous coking model embedded with process simulation disclosed in Example 2. First, the process model is used to simulate the experiments performed in Example 1. Next, the same procedures for sorting and plotting the data are followed in order to yield three new coking indexes shown in Equations 2-4.

(18) $\begin{array}{cc}\mathrm{Coke}\mathrm{Index}=e^{\left[\left(-1140\frac{H2}{\mathrm{HC}}-19841\right)\left(\frac{1}{T}\right)+\left(-1.11\frac{H2}{\mathrm{HC}}+35.30\right)\right]}& \mathrm{Equation}2\left(C_3\right)\\ \mathrm{Coke}\mathrm{Index}=e^{\left[\left(215\frac{H2}{\mathrm{HC}}-19660\right)\left(\frac{1}{T}\right)+\left(-2.88\frac{H2}{\mathrm{HC}}+34.57\right)\right]}& \mathrm{Equation}3\left({\mathrm{iC}}_4\right)\\ \mathrm{Coke}\mathrm{Index}=e^{\left[\left(-1125\frac{H2}{\mathrm{HC}}-24496\right)\left(\frac{1}{T}\right)+\left(0.06\frac{H2}{\mathrm{HC}}+43.82\right)\right]}& \mathrm{Equation}4\left({\mathrm{nC}}_4\right)\end{array}$

(19) Table 5 summarizes the results of using the coke index estimated from the process simulator (Equation 2) instead of the coke index obtained from experimental data (Equation 1). The coke index for the same scenario (635 C. and an H.sub.2/HC of 0.50) is 13.5. In order to obtain the same coke index at a H.sub.2/HC of 0.40, the temperature must be reduced to approximately 629 C. Thus, the resulting temperature adjustments were the same, within the margin of equipment error, even though the coke index equations were different.

(20) TABLE-US-00005 TABLE 5 H.sub.2/HC Temp., deg C. Coke Index Base Case 0.50 635 13.5 Target Operation 0.40 629 13.3

(21) In summary, the advantage of the process simulation model is that it can provide predictions over a broad range of proposed operating conditions and feed compositions. The advantage of the coke index is that it only requires the reactor inlet temperature and H.sub.2/HC ratio. However, each hydrocarbon feed composition may require a customized coke index. Finally, the process simulation model and/or coke index can be used to obtain suitable pairs of H.sub.2/HC ratios and reactor inlet temperatures that result in a profitable dehydrogenation process operating without excessive catalyst coking, as summarized in Table 6.

(22) TABLE-US-00006 TABLE 6 H.sub.2/HC Rx Inlet Temps, deg C. Molar Ratio C.sub.3 & iC.sub.4 nC.sub.4 Range 0.01-0.40 525-645 0.35-0.40 585-645 555-625 0.25-0.35 575-640 545-620 0.15-0.25 565-630 535-610 0.01-0.15 555-620 525-600

Specific Embodiments

(23) While the following is described in conjunction with specific embodiments, it will be understood that this description is intended to illustrate and not limit the scope of the preceding description and the appended claims.

(24) A first embodiment of the invention is a process for dehydrogenation of a hydrocarbon feedstock, the process comprising passing a feed stream comprising hydrogen and paraffins into a dehydrogenation zone comprising at least one reactor containing a dehydrogenation catalyst maintained at dehydrogenation conditions to produce a dehydrogenation zone product stream comprising hydrogen, paraffins, and olefins; wherein the dehydrogenation conditions in the at least one reactor include a hydrogen to hydrocarbon molar ratio in a range of 0.01 to 0.40 and a reactor inlet temperature in a range of 500-645 C. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph wherein the hydrogen to hydrocarbon molar ratio is in the range of 0.01-0.35 and the reactor inlet temperature is in the range of 500-640 C. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph wherein the hydrogen to hydrocarbon molar ratio is in the range of 0.01-0.25 and the reactor inlet temperature is in the range of 500-630 C. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph wherein the hydrogen to hydrocarbon molar ratio is in the range of 0.01-0.15 and the reactor inlet temperature is in the range of 500-620 C. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph further comprising separating the dehydrogenation zone product stream into a hydrocarbon rich product stream and hydrogen rich product stream. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph further comprising passing a portion of the hydrogen rich stream to the dehydrogenation zone. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph wherein the hydrocarbon feed comprises at least one paraffin having 2 to 30 carbon atoms. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph wherein the hydrocarbon feed comprises at least one paraffin having 2 to 6 carbon atoms. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph wherein the hydrocarbon feed comprises at least one paraffin having 3 to 4 carbon atoms. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph, further comprising at least one of sensing at least one parameter of the process and generating a signal or data from the sensing; generating and transmitting a signal; or generating and transmitting data.

(25) A second embodiment of the invention is a process for dehydrogenation of a hydrocarbon feedstock, the process comprising passing a feed stream comprising hydrogen and paraffins into a dehydrogenation zone comprising at least one reactor containing a dehydrogenation catalyst maintained at dehydrogenation conditions to produce a dehydrogenation zone product stream comprising hydrogen, light ends, paraffins, and olefins; wherein the dehydrogenation conditions in the at least one reactor include a hydrogen to hydrocarbon molar ratio, a reactor inlet temperature, and a coke index, wherein the hydrogen to hydrocarbon molar ratio is in a range of 0.01 to 0.4, and wherein one dehydrogenation condition is adjusted based on the other two dehydrogenation conditions. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the second embodiment in this paragraph wherein either the reactor inlet temperature is adjusted based on a selected hydrogen to hydrocarbon molar ratio and the coke index or the hydrogen to hydrocarbon molar ratio is adjusted based on a selected reactor inlet temperature and the coke index. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the second embodiment in this paragraph wherein the coke index is in a range of 0-250. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the second embodiment in this paragraph wherein the coke index is determined by measuring coking for a range of reactor inlet temperature and hydrogen to hydrocarbon molar ratio combinations and determining a correlation between the reactor inlet temperature and the hydrogen to hydrocarbon molar ratio; and wherein a desired reactor inlet temperature is determined using the correlation and the selected hydrogen to hydrocarbon molar ratio and wherein the reactor inlet temperature is adjusted to the determined reactor inlet temperature; or wherein a desired hydrogen to hydrocarbon molar ratio is determined using the correlation and the selected reactor inlet temperature and wherein the hydrogen to hydrocarbon molar ratio is adjusted to the determined hydrogen to hydrocarbon molar ratio. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the second embodiment in this paragraph wherein the hydrocarbon feed comprises at least one paraffin having 2 to 30 carbon atoms. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the second embodiment in this paragraph wherein the hydrocarbon feed comprises at least one paraffin having 2 to 6 carbon atoms. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the second embodiment in this paragraph wherein the hydrocarbon feed comprises at least one paraffin having 3 to 4 carbon atoms. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the second embodiment in this paragraph further comprising separating the dehydrogenation zone product stream into a hydrocarbon rich product stream and hydrogen rich product stream. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the second embodiment in this paragraph further comprising passing a portion of the hydrogen rich stream to the dehydrogenation zone. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the second embodiment in this paragraph, further comprising at least one of sensing at least one parameter of the process and generating a signal or data from the sensing; generating and transmitting a signal; or generating and transmitting data.

(26) Without further elaboration, it is believed that using the preceding description that one skilled in the art can utilize the present invention to its fullest extent and easily ascertain the essential characteristics of this invention, without departing from the spirit and scope thereof, to make various changes and modifications of the invention and to adapt it to various usages and conditions. The preceding preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limiting the remainder of the disclosure in any way whatsoever, and that it is intended to cover various modifications and equivalent arrangements included within the scope of the appended claims.

(27) In the foregoing, all temperatures are set forth in degrees Celsius and, all parts and percentages are by weight, unless otherwise indicated.