Methanol Distillation
Short Description
Methanol Distillation...
Description
Petrochemical DeveloPments
Proof ony. Copyrhtd mtr. My not rpr rproducd oducd wthout prmsson.
Use new economics for purification on a small scale F u, w dg b gy g p f g- pfby K. PATwARdHAn, G. SATISHbAbu, S. RAjYALASHMI and P. bALARAMKRISHnA, Larsen and Toubro, Powai, Mumbai, India
C
rude methanol (MeOH) distillation is an energy intensive separation process and contributes significantly to the total production cost of this alcohol. It is very important to choose the right distillation configuration columns for MeOH purification. In the presented study, a two-column configuration is compared with thr three-c ee-colum olumn n conf configura iguration tion with forwa forwardrd- and back backward ward-hea -heatt integration schemes. Reduction of approximately 64% in lowpressure (LP) steam consumption is observed in a three-colum three-column n configuration case as compared to the base case of two-column case for a small capacity plant (about 23,000 metric tpy). Further reduction in specific energy consumption for a three-column configuration is possible with a backward-heat integration scheme.
summarizes a typical composition of the crude MeOH obtained through commercial processes. US federal-grade specification OM-232e identifies three grades of MeOH. Grade “C” is for wood alcoho alcoholl used used in in denatur denaturing. ing. Grade “A” cover coverss methano methanoll gengenerally used as a solvent. Federal-grade “AA” “AA” is the purest product and it is used for petrochemical/chemical applications in which high-purity and low-ethanol content are required, such as for MTBE, methyl amines manufacture, etc. The general standard observed by the chemical industry for MeOH product purity is US federal-grade “AA”. Another known methanol grade is the fuel-grade; it is used as a blending component for gasoline. Priicatio schemes. Crude MeOH is purified by distil-
KEY PETROCHEMICAL
lation with one- or two- or three- or four-column configuration. Methanol is one of the most important petrochemicals proFuel-grade methanol methanol is normally produced with a single distillation duced globally. It is extensively used as feedstock in the production tower.. But to produce federal-grade AA methanol, two-, three-, tower of chemicals such as formaldehyde, methyl tertiary-butyl ether and sometimes, even four -tower distillation systems sy stems are used. The (MTBE), tertiary amyl methyl ether (TAME) ( TAME) and acetic acid, and amount of distillation required depends on the byproduct formaalso as a hydrogen source in the fuel cells used in automobiles. The tion of the MeOH synthesis catalyst and plant capacity. majority of MEOH is produced via natural gas through steam The economics of the purification scheme involves the complex reforming; other processing methods include use of petroleum relationship of plant capacity, capacity, heat available in the plant, the energy fraction and process offgas. The MeOH-manufacturing process export requirement and customer requirements, etc. For example, can be divided into three major sections: feedstock purification the four-column configuration configuration is justified only at large capacities and syngas generation, compression and MeOH synthesis, and such as 5,000 metric tpd of MeOH production where as choice MeOH purification. Fig. 1 is a general flow diagram of a MeOH of two- or three-column configuration depends very much on facility using natural gas as the feedstock. customer’s requirements and energy availability in the front end. In this design, three process sections may be considered independently,, and the technology may be selected and optimized pendently Sigle-colm Sigle-col m coigratio coigratio. . For fuel-grade MeOH as a separately for each section. The normal criteria for technology blending component (for gasoline), the major demands regardselection are capital cost and plant efficiency efficiency.. ing quality are the water content and dissolved gases. g ases. Fuel-grade In a conventional natural gas-based MeOH plant with a capacity of 2,500 + Natural MeOH metric tpd, syngas generation accounts for gas Syngas MeOH MeOH Desulfurization Compression production synthesis distillation 55%, distillation accounts for 12%, compression and MeOH synthesis accounts for 12% and utilities and other services Reactor Distillation Reforming account for 24% of the total capital cost. technologies technologies technologies 1. Steam 2. Combined 3. Autothermal
Methaol priicatio priicatio. . Crude
MeOH, as removed from the MeOH synthesis section, contains water, higher alcohols, impurities and light ends. Table 1
Fig. 1
1. Isothermal 2. Adiabatic
1. Single column 2. Multicolumn
G fw dg f u-g bd moh fy.
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Petrochemical DeveloPments MeOH should be dissolved-gas free and preferably should not contain more than 500 wt-ppm of water. The limitation on water content is due to its immiscibility with gasoline (Fig. 2). Mlti-colm coigratio. The condensate from the syn-
thesis loop is generally purified in two stages using conventional distillation columns operating at pressures slightly above atmospheric pressure. The first distillation stage is for light ends removal, and is carried out in a single-distillation column known as the topping column. This column acts as a refluxed stripper. The liquid feed enters near the top stage and MeOH vapor generated in the reboiler strips the light ends—such as di-methyl ether (DME), methyl formate and acetone—and residual dissolved gases from the crude MeOH. The main area of investigation is the second stage of MeOH purification. This is the MeOH refining stage, where MeOH is recovered as the overhead product from one or more distillation columns. Water is withdrawn as the bottoms product. Middle boiling impurities (principally ethanol, but also higher alcohols, ketones and esters), referred to as fusel oil are withdrawn as a side stream below the feed stage. Provision of this side stream enables the MeOH production to US federal specification O-M- 232K Grade ‘‘AA’’. In typical two-column MeOH purification scheme, as shown in Fig. 3, about 20% of the total heat for purification is associated with the topping column. The remainder is required to separate methanol from ethanol and water. This basic arrangement is widely reported in the literature. 1,2 With the sharp rise in energy costs, MeOH technology licensors and operators have focused considerable attention on alternaTable 1. Typical crude MeOH composition to MeOH purification section cp
W%
CO, CO2, H2, CH4, N2, DME, aldehydes, ketones
0.5–0.8
Methanol
88–90
Ethanol, higher alcohols (propanol, butanol, etc.)
0.1–0.6
Water
9–11
tives to this standard two-column arrangement. 2–8 A double-effect three-column scheme was developed and it is widely applied in industry.4 A number of these alternative schemes involve splitting the refining column into two separate columns operating at different pressures, such that the overheads of the higher pressure column can be used to reboil the lower pressure column. Several novel energy-saving three-column distillation configurations have been explored in the literature. 9 The capital cost of the three-column schemes is significantly greater than the standard two-column arrangement. The threecolumn distillation unit consists of a topping column and two refining columns. Refining column II operates at normal pressure. Refining column I operates at a higher pressure, thus utilizing the condensation duty of this column as the reboiler duty of refining column II. This substantially reduces the LP steam consumption of the distillation section. Another configuration of three-column systems is operating refining column I at atmospheric pressure and refining column II at high pressure (HP). Federal-grade “AA” MeOH is withdrawn close to the top of both refining columns. Although the three-column system is more costly, it can reduce the required distillation heat input by 30%–40%. Multi-column systems (three or more columns) can generally only be justified when energy costs are prohibitively high. The design of the MeOH distillation unit primarily depends on the energy situation in the front end. The two-column distillation unit represents the low-cost unit, and the three-column distillation unit is the low-energy system. Multi-column design maximizes the yield and minimizes LP steam consumption. The four-column design (Fig. 4) includes the three columns described previously as well as an additional recovery column. The fusel oil purge from refining column II is processed in the recovery column to minimize MeOH losses even further. The distillation unit can be designed to limit the MeOH content in the process water to a maximum of 10 wt-ppm. Furthermore, the heat available from the front end (syngas generation) at a low temperature is efficiently used to minimize steam consumption. As we go higher up in the column configuration, MeOH recovery increases but specific steam consumption decreases. In
Tail gas
Condenser 1
Condenser 2 Stripped gas
Fuel-grade product
Reflux drum 1
Liquid off steam
Crude MeOH
Raw MeOH
Fig. 2
2
I
Concentration column
Process gas
LP steam
LP steam Recycle water
Stabilizer MeoH pump
sg-u fgu f moh p.
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Fig. 3
Product MeOH
Higher alcohols Stabilizer column
Process gas
Reflux drum 2
tw-u fgu f moh p.
Petrochemical DeveloPments four-column configurations, as high as 60% savings in the steam consumption can be achieved when compared to the base case of a two-column configuration. SIMuLATIOn STudY
An analysis was conducted for purifying “AA” grade MeOH from crude MeOH through a two-column and three-column configuration using a commercially available process simulator. The results were validated with the reference data available for the two-column scheme. The simulations were extended for the three-column configuration. As in three-column configuration, due to higher degree of freedom, one extra case is generated for the reboiler coupling. In forward heat integration, out of the three columns, the first column is the topping column, as in the twocolumn case; the second is a HP refining column; and the third is LP refining column. Total heat required for the HP-column reboilers is provided by LP steam. Instead of using a cooling water heat exchanger to chill overheads of the HP column, heat is used to run the LP column reboiler. This is called the forward-heat integration because heat integration is in the direction of material flow. The HP column is operated at a pressure of 7 to 10 atmospheres depending on the feed composition. The LP column is operated near to atmospheric pressure.
In backward-heat integration, the second and third columns are exchanged. In this scheme, the overheads from third column (HP) supply heat for the second-column reboiler. The material and heat flows in the opposite direction. The basic assumptions made are: • All trays behave ideally (tray efficiency is 100%). • Liquid reflux from the condenser is saturated at calculated conditions. • Pressure drop/ tray is 0.01 kg/cm2. • Negligible pressure drop in reboiler and condenser. • Reductions or increases in the pressure between the columns are achieved by the reduction valve and pump respectively. • A 15°C approach (∆ temperature difference) is maintained between LP column reboiling liquid and HP column overheads. Table 2 summarizes the simulation results for the base case of two-column, three-column schemes with forward- and backwardheat integration configuration. The LP steam consumption in the two-column configuration is much greater than the three-column configuration. This is because Condenser 1
tw-u Stabilizer column No. of stages
Concentration column
38
80
Reboiler duty, Gcal/hr
5.20
25.53
Condenser duty, Gcal/hr
6.26
25.22
Diameter, m
1.84
4.10
Reflux ratio
132
2.21
Boil-up ratio
0.64
13.27
Process gas
LP column
58
53
Reboiler duty, Gcal/hr
5.20
19.47
17.98
Condenser duty, Gcal/hr
6.26
17.98
19.09
Diameter, m
1.84
2.61
3.51
Reflux ratio
132
5.64
2.96
Boil-up ratio
0.64
3.45
9.44
Stabilizer column No. of stages
Condenser 1
HP column
LP column
55
58
Reboiler duty, Gcal/hr
5.20
17.46
17.85
Condenser duty, Gcal/hr
6.26
17.67
17.46
Diameter, m
1.84
3.36
2.62
Reflux ratio
132
2.70
5.00
Boil-up ratio
0.64
3.83
9.92
Reboiler 3 Recycle water
Reflux drum 1
Condenser 2 Stripped gas
Reflux drum 2
Liquid off steam
Crude MeOH
38
LP steam consumption 0.8265 (metric ton/metric ton of MeOH)
Reboiler 2
Fig. 4a t-u fgu (fwd g) f moh p.
0.934
t-u (bkwd g)
LP column
LP steam
Reboiler 1
38
LP steam consumption (metric ton/metric ton of MeOH)
HP column
Stabilizer MeOH pump
HP column
Reflux drum 3
Higher alcohols
Topping column
t-u (fwd g) No. of stages
Reflux drum 2
Liquid off steam
Crude MeOH
LP steam consumption 1.3384 (metric ton/metric ton of MeOH)
Stabilizer column
Stripped gas
Reflux drum 1
Table 2. Simulation results for column schemes
Condenser 2 Product MeOH
Topping column Process gas Reboiler 1
HP column
LP steam Reboiler 2
Product MeOH Reflux drum 3
Higher alcohols LP column
Reboiler 3 Recycle water
Fig. 4b t-u fgu (fwd g) f moh p. HYDROCARBON PROCESSING
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Petrochemical DeveloPments the heat required for the concentration column is supplied by LP steam. In a three-column configuration, there is a possibility to couple the reboiler of one column with the condenser of another. Temperature differences between utility (LP steam) and reboiler temperature decrease with increasing column pressure. Thus, the reboiler requires a higher area for the same duty when compared to base two-column configuration. In the backward-heat integration scheme, due to altered column sequencing (i.e., LP column preceding the HP column), around 60% of MeOH product is recovered in the first stage. This offers advantages in two ways: 1) Ease of separation (characterized by the relative volatilities) increases with decreasing operating pressure for a constant feed composition 2) Altered composition as compared to a forward-heat integrated scheme distillation can be done at lower pressure in HP column. This reduces the heat duty on the HP column reboiler. The reverse heat integration results in more energy savings. ECOnOMICS Of METHAnOL dISTILLATIOn
For capital cost, an MeOH distillation complex involves distillation column, reboiler, condenser, reflux tank, pump and associated column controls. The cost for each units depends on various operating and design parameters. Fig. 5 summarizes the contribution of the individual costs to the total cost for the distil2.56%
0.34% 2.58%
4.23%
lation setup under consideration. The cost contribution is higher for instrumentation in three-column backward configuration than for a forward design due to the complex control system. The capital cost in the case of the three-column configuration is more (12%–17%) than that of two-column configuration due to the additional column and associated equipment. It is very important that before adopting any of the listed schemes, a balance between the fixed and operating cost is done. Operatig cost. The operating cost for the distillation column
scheme under consideration includes cost for cooling water in the overhead condenser and steam in the reboiler. The operating cost of cooling water is governed by various factors such as ambient conditions, electrical consumption in fans and cooling water pumps, water cost and chemical treatment. The cost of cooling water is taken as $0.2/m3. The three-column configuration saves energy consumption in terms of LP steam supplying heat to the reboiler. The steam required is the operating cost, and it can be expressed in terms of natural gas consumption. The steam costs can be determined assuming water at available temperature is heated in boiler by burning natural gas, and it can be expressed by:
⎛ M Cp (T −T ) + λ ⎞⎟ )⎟⎟ NG unit price ⎜ ( w B ref Cost of steam, $ = ⎜⎜⎜ ) ⎟( ⎜⎜⎝ ( LHV NG )×ηBoiler ⎟⎟⎠
84.69%
5.61%
3B-column configuration Operating cost Capital cost
(a) 3F-column configuration
3.15%
0.28% 4.27%
7.75%
77.52%
7.04%
2-column configuration 0
20
40
60
(b)
Fig. 6
7.18%
80
100
120
140
Relative cost
0.27%
3.36%
9.58%
r p/pg f u fgu.
76.68%
2.93%
3B-column configuration LP steam CW
(c) 3F-column configuration
Column Reboiler Condenser drum
Condenser Pump Instrumentation
2-column configuration 0
Fig. 5
4
I
c bu p f qup f u fgu—a: w-u fgu, B: -u fwd g fgu d c: -u fwd g fgu.
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20
40
60
Relative cost
Fig. 7
opg bu.
80
100
120
Petrochemical DeveloPments The three-column configuration saves energy. Thus, less natural gas is consumed via lesser steam demand by the reboiler. Almost 30%–40% savings can be realized by adopting either three-column forward configuration or three-column backward configuration. But a higher coolant flowrate is required in the additional condenser in the three-column configuration; accordingly operating costs increased. Fig. 6 illustrates the combined effect, where it can be seen that operating cost is high for a threecolumn configuration with forward integration, while, in others, marginal savings can be seen. Fig. 7 shows the split.
IMTOF, London, 1993. Chiang, T. P. and W. L. Luyben, Comparison of energy consumption in five integrated distillation column configurations, Industrial Engineering Chemical Process Des. Dev., No. 22, 1983, pp. 175–179. 6 Wu, J. and L. Chen, Simulation of novel process of distillation with heat integration and water integration for purification of synthetic methanol, Journal Chemical Industrial Engineering, China, No. 58, 2007, pp. 3210–3214. 7 Liu, B. Z., Y. C. Zhang, P. Chen, and K. J. Yao, Research on energy sav ing process of methanol distillation, Chemical Industry Engineering Progress, China, Vol. 27, 2008, pp. 1659–1662. 8 Douglas, A. P. and A. F. A. Hoadley, A process integration approach to the design of the two- and three- column methanol distillation schemes, Applied Thermodynamics Engineering 26 , 2006, pp. 338–349. 5
ne thikig. A techno-commercial comp arison of the
two-column and three-column schemes for medium capacity MeOH plant is presented here. The three-column scheme with backward-heat integration offers approximately 60% saving in LP steam as compared to two-column scheme. It can provide as an option where LP steam costs are higher compared to cooling water. Although, in the three-column scheme, backward integration offers higher savings as compared to forward integration scheme the column control will be complicated, and it needs to be provided more attention during operation. HP LITERATURE CITED 1
Pinto, “Methanol distillation process,” US patent 4,210,495, 1980. Fiedler, E., G. Grossmann, D. B. Kersebohm, G. Weiss, and C. White, Ullmann’s Encyclopedia of Industrial Chemistry, Wiley-VCH Verlag/ GMbH & Co., Weinheim, 2002. 3 Meyers, R. A., Handbook of SynfuelsTechnology, McGraw Hill, New York, 1984. 4 M. Harvey, “Methanol Distillation-Two and Three Column Schemes,” 2
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