《Alkylation Pretreatment.pdf》由会员分享,可在线阅读,更多相关《Alkylation Pretreatment.pdf(17页珍藏版)》请在三一文库上搜索。
1、1 Alkylation Pretreatment CDTECH Introduction Alkylation Unit performance can be significantly affected by the operation of upstream units. MTBE (Methyl-t-butyl Ether) Units and Selective Hydrogenation units are often upstream of Alkylation Units and may be thought of as “feed pretreatment units” fo
2、r the downstream Alkylation Units. This paper discusses and the factors that affect the oxygenate content of Alkylation Unit feeds derived from MTBE Units. This paper also discusses other effects of MTBE Units on downstream Alkylation Unit operations. Finally, this paper discusses selective hydrogen
3、ation, including the types of Selective Hydrogenation Units, the chemistry involved, and the benefits of feed selective hydrogenation to Alkylation Units. Various feed impurities have different detrimental effects on Alkylation Unit performance. The following table shows the effects, in terms of dil
4、ution factor, of several of the impurities that can be found in the product of some MTBE and Selective Hydrogenation Units. Sulfuric Acid Dilution Factor Compound# Acid / # Contaminant MTBE10.0 DME13.5 Methanol28.3 TBA10.6 1,3 Butadiene9.0 Water10.6 (Data taken from MTBE/TAME AND ALKYLATION presente
5、d by Randy Peterson, Process Engineering Supervisor, September 1993, Copyright 1993 STRATCO, Inc. Updated by personal correspondence with Randy Peterson 1998) This data shows that it is quite beneficial to minimize the presence of these compounds in the feed to a sulfuric acid Alkylation Unit. 2 Oxy
6、genates In Alkylation Unit Feeds Typical Values and How to Reduce Them Four oxygenates are commonly found in Alkylation Unit feeds when there is an MTBE Unit upstream of the Alkylation Unit: MTBE (Methyl-t-butyl Ether), Dimethyl Ether (DME), Methanol, and Water. MTBE is present only in low concentra
7、tions, typically between 5 ppmw and 50 ppmw. DME is another oxygenate which is found in the product stream from an MTBE Unit which is sent to an Alkylation Unit. Its concentration can range from approximately 300 ppmw to higher than 1500 ppmw depending on how the MTBE Unit was designed and operated.
8、 Methanol is also found in Alkylation Unit feeds from MTBE Units and its range is approximately 25 ppmw to 50 ppmw. Water is also present. Typically the raffinate product from an MTBE Unit which is sent to an Alkylation Unit is saturated with Water (approximately 300 ppmw). OxygenateExpected Concent
9、ration in Alkylate Feed (wppm) MTBE 5 50 DME30 1500 Methanol25 50 WaterSaturated TBA (Note 1)None MSBE (Note 1)None Note:1. The presence of either of these oxygenates indicates a major problem in the MTBE Unit. Other oxygenates may on rare occasions be found in Alkylation Unit feeds, but their prese
10、nce indicates a major problem in the upstream unit. The most important of these is Tertiary-butyl Alcohol (TBA). Depending on which ether is found in the Alkylation Unit feed, there are different steps which should be taken in the upstream MTBE Unit. 3 A higher level of MTBE in the Alkylation Unit f
11、eed is usually an indication that the MTBE Units debutanizer column (CD Reaction Column) is not being operated properly. This column should separate the vast majority of the MTBE product from the components that end in the raffinate stream. One way in which an improperly operated MTBE Unit debutaniz
12、er can lead to MTBE in the Alkylation Unit feed is when the MTBE debutanizer is operating with too low a reflux ratio. Another cause of MTBE in the MTBE Unit raffinate is too much Methanol in the feed to the debutanizer (CD Reaction Column). The increased Methanol can not exit the bottom of the towe
13、r, due to the temperature control, so it must leave out the top. Because the concentration of Methanol in the overhead exceeds the concentration of Methanol in the C4/Methanol azeotrope, another azeotrope is formed. This one is between MTBE and Methanol. This can be understood as the Methanol draggi
14、ng the MTBE up the debutanizer column. The presence of DME in the raffinate is not as easy to control as the presence of MTBE. The three factors that cause the formation of DME are 1) temperature, 2) time on the catalyst, and 3) excess methanol. These are also the requirements for MTBE production as
15、 well, so controlling DME production can be complex. In some cases it may be possible to decrease the reactor or CD Reaction Column temperature without significantly affecting MTBE production. In other cases, based on the economics of the individual site, it might be more economical to decrease the
16、amount of excess methanol in the MTBE Unit. This will decrease the DME concentration in the Alkylation Unit feed, but it will also decrease the conversion of Isobutylene to MTBE in the MTBE Unit. This trade off will need to be evaluated in each site. The presence of Methanol in the Alkylation Unit f
17、eed is an indicator of one of several different problems in the Methanol recovery section of the MTBE Unit operation. First, the Water flow to the Methanol extraction column may not be high enough and should be adjusted. Second, the Methanol recovery column may not be removing enough Methanol from t
18、he Water that is recycled to the extraction column. When the recycle Water contains too much Methanol, it is not possible to reduce the 4 Methanol level in the hydrocarbon to the low ppmw levels. A third reason may be that the temperature of the extraction column is too high. At higher temperatures,
19、 more methanol is soluble in the hydrocarbon phase. One other potential cause of increased Methanol in the MTBE Unit raffinate to the Alkylation Unit is that the Methanol recovery section may be overloaded. There may be too much methanol in the debutanizer (CD Reaction Column) overhead for the Metha
20、nol recovery section to remove. Water is also present in MTBE Unit raffinate product sent to the Alkylation Unit, but it should not form a separate phase. The presence of a separate phase indicates that there is some problem with the top of the extraction column or a problem with a coalescer downstr
21、eam of the extraction column. TBA in the MTBE Unit raffinate product indicates a major problem in the MTBE Unit debutanizer. If the debutanizer is operating properly, all of the TBA will distill out the bottom of the column with the MTBE. In some cases refineries and chemical plants have found that
22、it is economical to install an Oxygenate Removal Unit between the MTBE Unit and the Alkylation Unit. There are three types of these units. First, there are chillers. These are the least expensive type of unit, and they remove water but do not affect the concentration of MTBE, DME or Methanol. The ne
23、xt most expensive type of unit is a distillation column. These remove the light oxygenates: DME, Water, and Methanol. The third type of Oxygenate Removal Unit is both the most costly and the most effective. This is an adsorbent process such as an Alcoa Alumina bed or a Mol Seive process. These adsor
24、bents remove MTBE, DME, Methanol, and Water to the extremely low ppm levels. However, their use is limited to sites where a low olefin, low oxygenate stream is available for regenerating the adsorbent bed. Troubleshooting by Using an Outside Lab Specializing in Oxygenates It can sometimes be quite d
25、ifficult for a plants analytical laboratory to measure the quantity of oxygenates in these hydrocarbon streams because their concentrations are so low. The analytical laboratory may not be familiar with the different 5 procedures and techniques required to measure these compounds. Another problem ma
26、y be that the analyzer equipment is not available in the plants quality control. For these reasons, it is often beneficial to use the services of an outside company that regularly measures these components at such low concentrations. One such company that has this expertise is CDTECH, which licenses
27、 MTBE and Selective Hydrogenation Technologies. Companies such as CDTECH have the equipment, have the procedures, and provide reliable results. In some cases, they even have technical support teams with mobile laboratories that can come to the chemical plant site, take the samples, and provide the r
28、esults to the unit personnel almost immediately. Depending on the companies involved there may or may not be a charge for these types of services. Another way to utilize an outside laboratory is through the use of a “sample kit.” This “sample kit,” allows the operating units engineers to take the sa
29、mples and ship them to the laboratory for analysis. In this way, the use of a “sample kit,” minimizes the presence of outside engineers on the operating site, thereby ensuring that the samples are taken in compliance with the plants safety and environmental procedures. Technical Support for Ethers U
30、nits The technical support available for MTBE Units is not limited to analyzing samples. Technology licensors also have experienced personnel who visit operating plants sites, and can train operating company engineers, laboratory technicians, and operators. These same technical support people from t
31、he licensors also troubleshoot operating units. Another advantage to using technical services provided by the MTBE Unit licensor is that the people involved have extensive experience with those particular units. They know what common problems can be. For example, insufficient Methanol removal in the
32、 Methanol recovery column can lead to ineffective Methanol extraction column operation, resulting in high levels of Methanol in the raffinate stream sent to the Alkylation Unit. The technical services personnel may also be familiar with 6 obscure feed poisons that deactivate the catalyst by unusual
33、routes leading to decreased Isobutylene conversion and increased isoolefin concentration in the Alkylation Unit feed. For all of these reasons, the technical support services available from a licensor can be an important tool for solving operating problems. ISOBUTYLENE REMOVAL USING AN MTBE UNIT Ben
34、efits There are several benefits that MTBE Units provide both alone and in combination with downstream Alkylation Units. The etherification units upgrade Isobutylene to MTBE. This increases the octane of the gasoline pool and while reducing its vapor pressure. Of course it also allows the refiner to
35、 meet oxygenate content specification set by state or federal governments. MTBE Units unit remove isoolefins which allows a downstream Alkylation Unit to upgrade other materials Summary Description of a “catalytic distillation” type MTBE Unit The attached simplified PFD shows a typical Catalytic Dis
36、tillation MTBE Unit. The first operation of this unit is a Water wash to remove any poisons present in the feed. This wash may be a mixer valve followed by a coalescer drum, or it may be a liquid /liquid extraction column. A multistage Water wash is significantly better than a single stage coalescer
37、 to remove feed poisons. Some of these poisons will recycled back to the front end of the unit with the recycle Methanol unless they are removed in the feed Water wash. The next processing step is a small fixed bed reactor. This reactor is not large enough to achieve an equilibrium product mixture,
38、but it does achieve approximately 90 wt% Isobutylene conversion. The remaining conversion occurs within the next processing step, the catalytic distillation column. The conversion is not limited to the equilibrium conversion at a given temperature and pressure because the products of the reaction ar
39、e continuously removed in the column. A catalytic distillation unit can be designed for any conversion desired. There are units in operation that continuously achieve in excess of 99.5% conversion. In one case the conversion is continuously greater than 99.9% conversion of the Isobutylene in the fee
40、d. 7 There is a Methanol recovery section downstream of the debutanizer column. The first operation in the Methanol recovery section is a counter current liquid-liquid extraction of the Methanol with Water. This Water/ Methanol mixture, and a small amount of MTBE, is sent to the Methanol Recovery Co
41、lumn where the Methanol, and MTBE, distill overhead while the Water distills down. The Methanol is recycled to the front of the unit while the Water is recycled to the Methanol extraction column. There are other types of MTBE Units, but the differences in terms of oxygenates in the feed to the downs
42、tream Alkylation Unit are minimal. Chemistry including reaction conditions The chemical reactions that occur in an MTBE Unit are the same in all types of MTBE Units. MTBE is produced by the reaction of Isobutylene with Methanol over an acidic catalyst. The reaction is exothermic and can occur outsid
43、e the range of temperatures and pressures found within an MTBE Unit, but higher temperatures and lower pressures tend to favor the reactants as predicted by Le Chatlirs principal. The by-products (DME, TBA, and Diisobutylene) that are also produced are all gasoline compatible. TBA and Diisobutylene
44、distill with the MTBE, usually into gasoline blending. The DME is volatile and is sent with the MTBE Unit raffinate to the Alkylation Unit. 8 The following diagrams show the main reaction and the principal side reactions. All reactions are liquid phase. In the catalytic distillation units, the produ
45、cts are continuously removed while the reaction occurs. This allows the reaction to proceed beyond equilibrium limitations to high conversion levels. For reference I here give the equivalent information for TAME Units. All of the TAME Units in the United States that are used for fuels production use
46、 the catalytic distillation process. Main Reaction: CH3 - C = CH2 + CH3OH CH3 CH3 - C - O - CH3 CH3 CH3 IsobutyleneMethanol MTBE Isobutylene + WaterTBA Methanol + MethanolWater + DME Isobutylene + IsobutyleneDIB Side Reactions: 9 TAME is produced by the reaction of isoamylene with Methanol over an a
47、cidic catalyst. Because the reaction is exothermic and decreases the number of moles, Le Chatlirs principal dictates that higher temperatures and lower pressures favor the feedstocks. The byproducts, TAA (Tertiary Amyl Achohol), DIA (Diisoamylene), and DME are analogous to the byproducts produced in
48、 an MTBE Unit. The by- products are all gasoline compatible and TAA and DIA distill with the TAME, usually into gasoline blending. The DME distills overhead and is sent with the TAME Unit raffinate to the downstream unit. The main reactions and principal side reactions are shown in the following dia
49、grams. or + CH3OH Methanol 2MB2 (2-methyl-2-butene) CH3 - C = CH - CH3 CH3 CH3 - C - O - CH3 CH3 CH2 CH3 TAME 2MB1 (2-methyl-1-butene) CH2 = C - CH2 - CH3 CH3 Main Reaction: 10 Diene and Sulfur Removal Using Selective Hydrogenation MTBE Units are not the only “pretreatment” units found upstream of Alkylation Units. The second major type of unit is the Selective Hydrogenation Unit. Such a unit can remove dienes (and sulfur) from the feed to the Alkylation Unit. They are called “Selective” Hydrogenation Units because they are usually designed to “s
链接地址:https://www.31doc.com/p-3579567.html