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1、Methanol Distillation System: Process Analysis and Column Design Sun J insheng , Tian Yu f eng , Xu Shimin , Ding Hui , Wang Tao , Li Xingang , Zheng Yanmei (1 Shool of Chemical Engineering and Technology, Tianjin University, Tianjin 300072; 2 National Engineering Research Center for Distillation Te

2、chnology, Tianjin 300072)Abstract Base on industrial research and experience, the process of methanol distillation is analyzed, and above all, a new concept of high pressure flowsheet and low pressure flowsheet is defined. The new configuration helps to handle problems encountered in many factories

3、in China. The inter influence between process and column internal pattern is also pointed out. Recommendation of new column internal designs is given. Finally, industrial examples tell the how the new concept works and the possibility of combining process to give more opens to solve engineering prob

4、lems.Keywords methanol distillation, two - effect distillation, high pressure flowsheet and low pressure flowsheet, flowsheet analysis, simulation with hydraulic, column internal design A very important material in organic chemical industry, methanol is widely involved in the manufacture of plastic

5、material, synthetic fiber and rubber, dyestuff, covering material, spice, medical field, pesticide and etc. Also, it is not only a fine solvent for many organic compounds but a sound substitute for gasoline in many countries as green energy. With the quick development of the national economy, the de

6、mand (actual or potential) of methanol has sharply increased, which accelerates the development of the methanol industry and increment of the methanol output. Presently, methanol distillation technology has been developed to be large - scale, low energy consuming and more environmental. The main pat

7、ent dealers in this industry of the world have the equipments designed for 5 000 7 500 t/dEz. But China (mainland) has hardly no running units with capability over 200kt/a of high quality (such as O - M - 232E US). Recently, National Engineering Research Center for Distillation Technology at Tianjin

8、 University(NERCDT) has done much on technical and market research, including the simulation of all the familiar flowsheets on industrial and lab data, especially studying the 2 - effect heat exchange between the 2 methanol refining columns. As a summary of our work, the high pressure process and th

9、e low pressure process for 3 + 1 column process was put forward.1 PROCESS DESCRIPTION Methanol can be synthesized from many materials, for instance, natural gas, gas oil, fuel oil, coal, etc. Except for the difference in catalysts, reaction mechanics and the concentration of methanol in raw product,

10、 the subsequent distillation process and equipments are similar, falling into three main assortments: 2 column process, 3 -column process and 3+1 column process. A 2 -column process is composed of light ends column and methanol refining column or main column which is found in factories under 100 kt/

11、a, offering simple process, equipments and low investment. 3 -column process is built up by light ends column, pressured methanol refining column (PMRC) and atmospheric methanol refining column (AMRC), forming a 2 -effects distillation with the mixture of the distillates from the 2 methanol refining

12、 columns the methanol product. The characteristic of this process is the 2 effect distillation pattern between PMRC and AMRC, meaning heat exchange between the top of PMRC and the bottom of AMRC. This pattern saves the energy (by 30% 40) and the recycle rate of cooling water. If a methanol recovery

13、column, or stripper, or water column involves after the low pressure methanol refining column to improve the quality of the wastewater, the process will turn into 3+1-column process.1.1 Light Ends Column No matter in 2 - column process, 3column process or 3+1-column process, the Light End Column (LE

14、C) plays the same Role. Some of the light component in raw methanol fatal to the product are removed in this column, such as methyl formate (MFOR), dimethyl ether (DME), C1 C6, as well as acetone and methyl ethyl ketone (MEK). Some publications define MFOR,DME and C1 C6 as the alcoholic soluble impu

15、rities, while DME and MEK the aquatic- soluble impurities. The standard of the division can be found in Table 1. Industrial investigation shows that the basic measures of the removal operation include large reflux ratio and process water inlet or both taken. The principles respectively employ the di

16、fference of boiling point or relative volatility and solubility of the components. As a routine method, large reflux ratio; say 0. 70. 9 ( reflux to column feed), requires high condenser temperature to remove light impurities of high boiling point, for example acetone. Obviously, this method is acco

17、mpanied by high energy consumption .Because the above method can not reduce the loss of methanol, process water is added to the column to change the concentration of methanol and water in order to weaken the solubility of the liquid mixture in the column to the alcoholic- soluble impurities. The pro

18、cess water is normally fed to the column in 3 ways: into the 2nd condenser, at the top of the column and along with the raw methanol feed.To get the same methanol and water concentration at the inlet position, process water in the first way amounts approximately 5 % that of the feed, while the other

19、s, 10%20%. In many units, an alkaline solution (15% NaOH) is added to the exhaustion section of the column to balance the acids for a moderate pH value of the refined raw methanol.12 Refining ColumnMain Column Flowsheet This column，packed or trayed，have the product methanol by side draw near the top

20、 of the column in the distillation section, taking up 85 effective length of the column. At the middle of the exhaustion section, a side draw removes methanol, ethanol, isopropanol and propanol , etcTo assure high purity of the wastewater (100 PPM methanol) . When high quality methanol product requi

21、red，another side draw，called ethanol side draw in factories，is opened at the bottom of the distillation section and just a little higher than the feed inlet，to help reduce the concentration of ethanol in methanol product13 Refining Columns in 3-column Flowsheet In the 3 multiple-effect distillation

22、types, feed splitting(FS)，light spiltforward (LSF) and 1igh (splitreverse (LSR)， the LSF 2 -effectpattern by Lurgi is the most popular in China As showed in Figure 2 the entire feed stream (refined raw methanol) is pumped to PMRCAbout half of the 1ight key component (methanol) is removed inthe disti

23、llate at relatively high purityThe bottom product，containing remainder of the key light component is fed to AMRC and is purified in this column. One or two Side lines Will be drawn to help the purification of wastewater and the distillateIn this case the heat integration is in the direction of mass

24、flowAt high pressure，the relative volatility is decreased. This contributes to explain the reason why in many factories the methanol from the PMRCContains more impurities than that from AMRC, though the reflux ratio in PMPC is 1arger than that of the AMRC The other reasons are bobble point reflux te

25、mperature and the existence of remainder light ends The two products can be sold at different prices or in mixtureBesides this， the shortage includes the increased pumping costs to increase the feed pressure, the purchase cost for thicker column wall needed for high pressure and operation cost for t

26、he more complicated flowsheet But in many cases, these increases are small when compared with the large savings in the utilities in boiling and condensing Since in 3-column flowsheet, purity of both the methanol at the top and waste water at the bottom is required， a minimum T between the flows in t

27、he reboilier of AMRC and condenser of PMRC to give enough heat duty for these services. This, usually 20, is determined by the operation pressure of PMRC, and the pressure drop of the AMRC, while the latter is determined by the internals of the column at a certain column I. D.1.4 The High Pressure F

28、lowsheet (HPF) In methanol distillation units of large scale (say, 300 kt/a, usually in 3 - column flowsheet), methanol lost within the side draw (s) of the AMRC, as well as high pressure it imposes on the wastewater treatment, can not be omitted. The solution is feeding the side draw from exhaustio

29、n section of the AMRC to a methanol recovery column (or stripper) on the basis of 3- column process. Most of the purified wastewater, in this case, drains from the AMRC bottom, another little portion of it from that of the forth column. This requires much higher bottom temperature in order to ripe o

30、ff methanol and other light organic impurities in wastewater. Because the thermal resource of reboiler for this column is the overhead methanol vapor in the PMRC, to maintain normal operation, the operation pressure of the high pressure methanol refining column must be high enough to offer the tempe

31、rature gradient in heat exchange (condenser/reboiler) .1.5 The Low Pressure Flowsheet (LPF) The configuration of low pressure process comes from the comparison with the high pressure flowsheet, differing in the feed pattern of the methanol recovery column. This defines a flowsheet with AMRC to aband

32、on its requirement to the bottom wastewater and the bottom product (with 5%10% wt methanol in it) as the feed of the forth column. The variation saves the side draws form the exhaustion section of AMRC, making the column structure simpler, and, of the most importance, obtains a lower reboiling tempe

33、rature because of the change in bottom composition. This situation, as the logic of the high pressure flowsheet, calls for a lower operation temperature of the top vapor flow at PMRC and a lower operation pressure accordingly.1.6 Comparison of the HPF and LPF The operation pressure of PMRC in HPF is

34、 around 850900 kPa(A), while that of LPF 640700 kPa(A). Obviously rise up at the first glance the differences in costs for feed pumping and requirement for the condition of heating medium. And, to obtain both the aim of high purity methanol and wastewater, HPF permits an optional methanol recovery c

35、olumn,meaning intermittent shut down in large units or cancellation of the 4th column in small ones where theside draw flow rate is ignorable, whereas LPF must have the 4th column keep running in the operation duration. Another slight variation may be the purity of PMRC methanol at the same reflux r

36、atio caused by dissimilar relative volatility at two pressures. There is also a fact that, due to the flow rate and composition of the feed to the 4th column, the difficulty in fractionation distillation under the same product demand to the 4th column differs. HPF, with a feed of smaller flow and hi

37、gher methanol concentration, requires small reflux ratio and, consequently, a small column in diameter the counterpart, low methanol concentration, larger reflux ratio and especially large liquid flow in exhaustion section make a bigger column unavoidable.1.7 The Recovery Column and the type of retu

38、rn In running units, several patterns of methanol recovery columns or stripper can be found. As the simplest one, stripper receives its feed at the top and a steam at the bottom, with or without side draw. The main service is to get rid of methanol in the bottom stream, and spontaneously, with the t

39、op stream in low methanol concentration. This operation also directly loses steam and turns some of it to wastewater. A refluxed stripper is fed at the lower part of the column, able to recover methanol in top product in a concentration of over 90%, with optional side draw(s), makes the flowsheet a

40、little more complex. Apparently it has the same problems as the stripper. In many cases, the recovery column is miniature of the AMRC in HPF, with the exhaustion section larger in portion to the distillation section, one or two optional side draws and a top product of 90 % methanol or nearly qualifi

41、ed methanol. The nearly qualified methanol, when merged in those from PMRC and AMRC, causes almost no difference to the composition due to its relatively small mount and slight composition difference, contributes to lift the total recovery of the whole unit up to 99.2% 99. 5%. In this case, the retu

42、rn of the top stream to raw methanol drums or other columns in the unit is unnecessary. In HPF, the wastewater from the 4th column, though contains more organic impurities, cannot make any difference to the unit wastewater when submerged into that from AMRC. For the top stream with methanol concentr

43、ation far apart from qualified, returning it to the raw methanol, PMRC or AMRC is a better method to increase the total methanol recovery of the unit. The position of this feed back is determined by the composition and condition of the returned stream. Although back to AMRC seems better, most factor

44、ies in China prefer to the raw methanol drum.PROCESS SIMULATION For designing, unit analyzing or troubleshooting, process simulation is one of the most effect way. The understanding to the detail of the flowsheet facilitates the establishment of the mirror units in the software and keeps the result

45、reasonable. From 2001, NERCDT began to relative progresses in methanol distillation and gradually establish a serious of simulation frameworks on the base of Proll with PROVISION. The information included came from lab research, industrial investigation and research of process packages form dealers

46、within China. The process framework, or a simplified process package, involves not only the process aims, parts of the heat exchange equipments and pumps relative to distillation operation, but simultaneous column hydraulic calculation of the designated internals. The results also help to optimum ar

47、rangement or design of internals to the columns. In contrast with the on - site data from running units, the process framework gives sound uniformity. The running time, under continuous improvement, decreased from 3 days in 2001 (artificial adjustment) to 30 minutes now.2. 1 Selection of the Softwar

48、e The factors determining the selection of software includes the familiarity of the user to it, and more importantly, the accuracy of description to the thermodynamic behavior of the components involved, especially the trace components but fatal the quality of refined methanol, such as ethanol, ceto

49、ne and methyl ethyl ketone, etc. Since the alcohol package in Porll offers the smallest adjustment work and habituation of most of our faculty, it was finally locked.2.2 Selection of the thermodynamic package for the columns in the flowsheets Although we found that the Alcohol Package describe the main column or PMRC