C
C
rude methanol (MeOH) distillation is an energy intensiverude methanol (MeOH) distillation is an energy intensive separation process and contributes significantly to the total separation process and contributes significantly to the total production cost of this alcohol. It is very important to choose production cost of this alcohol. It is very important to choose the right distillation configuration columns for MeOH purification. the right distillation configuration columns for MeOH purification. In the presented study, a two-column configuration is compared In the presented study, a two-column configuration is compared withwith thrthree-cee-columolumn n confconfiguraiguration tion with with forwaforward- rd- and and backbackwardward-hea-heatt integration schemes. Reduction of approximately 64% in integration schemes. Reduction of approximately 64% in low-pressure (LP) steam consumption is observed in a
pressure (LP) steam consumption is observed in a three-columthree-columnn configuration case as compared to the base case of two-column case configuration case as compared to the base case of two-column case for a small capacity plant (about 23,000 metric tpy). Further for a small capacity plant (about 23,000 metric tpy). Further reduc-tion in specific energy consumpreduc-tion for a
tion in specific energy consumption for a three-column configura-three-column configura-tion is possible with a backward-heat integraconfigura-tion scheme.
tion is possible with a backward-heat integration scheme.
KEY PETROCHEMICAL
KEY PETROCHEMICAL
Methanol is one of the most important petrochemicals Methanol is one of the most important petrochemicals pro-duced globally. It is extensively used as feedstock in the
duced globally. It is extensively used as feedstock in the productionproduction of chemicals such as formaldehyde, methyl tertiary-butyl ether of chemicals such as formaldehyde, methyl tertiary-butyl ether (MTBE), tertiary amyl methyl ether (
(MTBE), tertiary amyl methyl ether (TAME) and acetic acid, andTAME) and acetic acid, and also as a hydrogen source in the fuel cells used in automobiles. The also as a hydrogen source in the fuel cells used in automobiles. The majority of MEOH is produced via natural gas through steam majority of MEOH is produced via natural gas through steam reforming; other processing methods include use of petroleum reforming; other processing methods include use of petroleum fraction and process offgas. The MeOH-manufacturing process fraction and process offgas. The MeOH-manufacturing process can be divided into three major sections: feedstock purification can be divided into three major sections: feedstock purification and syngas generation, compression and MeOH synthesis, and and syngas generation, compression and MeOH synthesis, and MeOH purification. Fig. 1 is a general flow diagram of a MeOH MeOH purification. Fig. 1 is a general flow diagram of a MeOH facility using natural gas as the feedstock.
facility using natural gas as the feedstock.
In this design, three process sections may be considered In this design, three process sections may be considered inde-pendently
pendently, and , and the technology may be selected and the technology may be selected and optimizedoptimized separately for each section. The normal criteria for technology separately for each section. The normal criteria for technology selection are capital cost
selection are capital cost and plant and plant efficiencyefficiency.. In a conventional natural gas-based In a conventional natural gas-based MeOH plant with a capacity of 2,500 MeOH plant with a capacity of 2,500++ metric tpd, syngas generation accounts for metric tpd, syngas generation accounts for 55%, distillation accounts for 12%, 55%, distillation accounts for 12%, com-pression and MeOH synthesis accounts pression and MeOH synthesis accounts for 12% and utilities and other services for 12% and utilities and other services account for 24% of the total capital cost. account for 24% of the total capital cost.
Methaol
Methaol priicatiopriicatio.. CrudeCrude MeOH, as removed from the MeOH MeOH, as removed from the MeOH synthesis section, contains water, higher synthesis section, contains water, higher alcohols, impurities and light ends. Table 1 alcohols, impurities and light ends. Table 1
summarizes a typical composition of the
summarizes a typical composition of the crude MeOH obtainedcrude MeOH obtained through commercial processes. US federal-grade specification through commercial processes. US federal-grade specification OM-232e identifies three grades of MeOH. Grade “C” is for OM-232e identifies three grades of MeOH. Grade “C” is for wood
wood alcohoalcohol usl used ied in den denaturnaturing. ing. Grade Grade ““A” cA” coverovers mes methanothanol gel gen- n-erally used as a
erally used as a solvent. Federal-grade “solvent. Federal-grade “AA” is the purest productAA” is the purest product and it is used for petrochemical/chemical applications in which and it is used for petrochemical/chemical applications in which high-purity and low-ethanol content are required, such as for high-purity and low-ethanol content are required, such as for MTBE, methyl amines manufacture, etc. The general standard MTBE, methyl amines manufacture, etc. The general standard observed by the chemical industry for MeOH product purity is observed by the chemical industry for MeOH product purity is US federal-grade “AA”. Another known methanol grade is the US federal-grade “AA”. Another known methanol grade is the fuel-grade; it is used as a
fuel-grade; it is used as a blending component for gasoline.blending component for gasoline.
Priicatio schemes.
Priicatio schemes. Crude MeOH is purified by distil-Crude MeOH is purified by distil-lation with one- or two- or three- or four-column configuration. lation with one- or two- or three- or four-column configuration. Fuel-grade methano
Fuel-grade methanol is l is normally produced with a single normally produced with a single distillationdistillation tower
tower. But to . But to produce federal-grade AA produce federal-grade AA methanol, two-, three-,methanol, two-, three-, and sometimes, even four -tower distillation sy
and sometimes, even four -tower distillation systems are used. Thestems are used. The amount of distillation required depends on the byproduct amount of distillation required depends on the byproduct forma-tion of the MeOH synthesis catalyst and plant capacity.
tion of the MeOH synthesis catalyst and plant capacity.
The economics of the purification scheme involves the complex The economics of the purification scheme involves the complex relationship of plant capacity
relationship of plant capacity, heat available , heat available in the plant, in the plant, the energy the energy export requirement and customer requirements, etc. For example, export requirement and customer requirements, etc. For example, the four-column configuratio
the four-column configuration is n is justified only at justified only at large capacitieslarge capacities such as 5,000 metric tpd of MeOH production where as choice such as 5,000 metric tpd of MeOH production where as choice of two- or three-column configuration depends very much on of two- or three-column configuration depends very much on customer’s requirements and energy availability in the front end. customer’s requirements and energy availability in the front end.
Sigle-col
Sigle-colm m coigratiocoigratio..For fuel-grade MeOH as a For fuel-grade MeOH as a blending component (for gasoline), the major demands blending component (for gasoline), the major demands regard-ing quality are the water content and dissolved g
ing quality are the water content and dissolved gases. Fuel-gradeases. Fuel-grade
Use new economics for
Use new economics for
purification on a small scale
purification on a small scale
F u, w dg b gy
F u, w dg b gy
g p f g- pfby
g p f g- pfby
K. PATwARdHAn, G. SATISHbAbu, S. RAjYALASHMI
K. PATwARdHAn, G. SATISHbAbu, S. RAjYALASHMI andand P. bALARAMKRISHnA,P. bALARAMKRISHnA,
Larsen and Toubro, Powai, Mumbai, India
Larsen and Toubro, Powai, Mumbai, India
Reforming Reforming technologies technologies 1. Steam 1. Steam 2. Combined 2. Combined 3. Autothermal 3. Autothermal Reactor Reactor technologies technologies 1. Isothermal 1. Isothermal 2. Adiabatic 2. Adiabatic Distillation Distillation technologies technologies 1. Single column 1. Single column 2. Multicolumn 2. Multicolumn Desulfurization
Desulfurization productionproductionSyngasSyngas CompressionCompression synthesissynthesisMeOHMeOH distillationdistillationMeOHMeOH Natural
Natural gas
gas MeOHMeOH
G fw dg f
G fw dg f u-g bd moh fy.u-g bd moh fy.
Fig. 1
Fig. 1
My not
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 dis-tillation 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 col-umn. 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 licen-sors and operators have focused considerable attention on
alterna-tives to this standard two-column arrangement.2–8A double-effect
three-column scheme was developed and it is widely applied in industry.4A number of these alternative schemes involve
split-ting the refining column into two separate columns operasplit-ting 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 three-column distillation unit consists of a topping three-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 distil-lation 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 consump-tion. As we go higher up in the column configuration, MeOH recovery increases but specific steam consumption decreases. In
Raw MeOH LP steam Process gas Fuel-grade product Tail gas
sg-u fgu f moh p. Fig. 2 LP steam Recycle water Product MeOH Liquid off steam Reflux drum 2 Process gas
Stabilizer MeoH pump
Higher alcohols Reflux drum 1 Crude MeOH Stabilizer column Concentration column Condenser 1 Condenser 2 Stripped gas
tw-u fgu f moh p. Fig. 3
Table 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
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 two-column case; the second is a HP refining two-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 backward-heat integration configuration.
The LP steam consumption in the two-column configuration is much greater than the three-column configuration. This is because
Table 2. Simulation results for column schemes tw-u
Stabilizer column Concentration column
No. of stages 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
LP steam consumption 1.3384 (metric ton/metric ton of MeOH)
t-u (fwd g)
Stabilizer column HP column LP column
No. of stages 38 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
LP steam consumption 0.934 (metric ton/metric ton of MeOH)
t-u (bkwd g)
Stabilizer column HP column LP column
No. of stages 38 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
LP steam consumption 0.8265 (metric ton/metric ton of MeOH)
Process gas Stabilizer MeOH pump Liquid off steam Reflux drum 1 Topping column Condenser 1 Condenser 2 Stripped gas LP steam Reflux drum 2 HP column Reboiler 3 Recycle water Product MeOH Reboiler 2 Reboiler 1 Higher alcohols Reflux drum 3 LP column Crude MeOH t-u fgu (fwd g) f moh p. Fig. 4a Process gas Liquid off steam Reflux drum 1 Topping column Condenser 1 Condenser 2 Stripped gas LP steam Reflux drum 2 HP column Reboiler 3 Recycle water Product MeOH Reboiler 2 Reboiler 1 Higher alcohols Reflux drum 3 LP column Crude MeOH t-u fgu (fwd g) f moh p. Fig. 4b
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 col-umn sequencing (i.e., LP colcol-umn preceding the HP colcol-umn), 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 inte-grated 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 dis-tillation 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
distil-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 bal-ance 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 ambi-ent 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:
Cost of steam, $= M Cp
(
w(
T B −T ref)
+λ)
LHV NG(
)
×ηBoiler ⎛ ⎝ ⎜⎜ ⎜⎜ ⎜⎜ ⎞ ⎠ ⎟⎟⎟ ⎟⎟ ⎟⎟(
NG unit price)
Column Reboiler Condenser drum Condenser Pump Instrumentation 76.68% 9.58% 3.36% 0.27% 7.18% 2.93% 77.52% 7.75% 4.27% 0.28% 3.15% 7.04% 84.69% 4.23% 2.58% 0.34% 2.56% 5.61% (a) (b) (c) c bu p f qup f u fgu—a: w-u fgu, B: -u fwd g fgu d c: -u fwd g fgu. Fig. 5 0 20 40 60 80 100 120 140 2-column configuration 3F-column configuration 3B-column configuration Relative cost Operating cost Capital cost r p/pg f u fgu. Fig. 6 0 20 40 60 80 100 120 2-column configuration 3F-column configuration 3B-column configuration Relative cost LP steam CW opg bu. Fig. 7The three-column configuration saves energy. Thus, less nat-ural 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; accord-ingly operating costs increased. Fig. 6 illustrates the combined effect, where it can be seen that operating cost is high for a three-column configuration with forward integration, while, in others, marginal savings can be seen. Fig. 7 shows the split.
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
integra-tion offers higher savings as compared to forward integraintegra-tion scheme the column control will be complicated, and it needs to be provided more attention during operation. HP
LITERATURE CITED
1Pinto, “Methanol distillation process,” US patent 4,210,495, 1980.
2Fiedler, E., G. Grossmann, D. B. Kersebohm, G. Weiss, and C. White,
Ullmann’s Encyclopedia of Industrial Chemistry, Wiley-VCH Verlag/ GMbH & Co., Weinheim, 2002.
3Meyers, R. A., Handbook of SynfuelsTechnology, McGraw Hill, New York,
1984.
4M. Harvey, “Methanol Distillation-Two and Three Column Schemes,”
5Chiang, 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.
6Wu, J. and L. Chen, Simulation of novel process of distillation with heat
inte-gration and water inteinte-gration for purification of synthetic methanol, Journal Chemical Industrial Engineering, China, No. 58, 2007, pp. 3210–3214.
7Liu, B. Z., Y. C. Zhang, P. Chen, and K. J. Yao, Research on energy
sav -ing process of methanol distillation, Chemical Industry Engineersav -ing Progress, China, Vol. 27, 2008, pp. 1659–1662.
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