Petrochemical DeveloPments

HYDROCARBON PROCESSING January 2012 I 1

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 three-column configuration with forward- and backward-heat integration schemes. Reduction of approximately 64% in low-pressure (LP) steam consumption is observed in a three-column configuration case as compared to the base case of two-column case for a small capacity plant (about 23,000 metric tpy). Further reduc-tion in specific energy consumption for a three-column configura-tion is possible with a backward-heat integration scheme.

KEY PETROCHEMICALMethanol is one of the most important petrochemicals pro-

duced globally. It is extensively used as feedstock in the production of chemicals such as formaldehyde, methyl tertiary-butyl ether (MTBE), tertiary amyl methyl ether (TAME) and acetic acid, and also as a hydrogen source in the fuel cells used in automobiles. The majority of MEOH is produced via natural gas through steam reforming; other processing methods include use of petroleum fraction and process offgas. The MeOH-manufacturing process can be divided into three major sections: feedstock purification and syngas generation, compression and MeOH synthesis, and MeOH purification. Fig. 1 is a general flow diagram of a MeOH facility using natural gas as the feedstock.

In this design, three process sections may be considered inde-pendently, and the technology may be selected and optimized separately for each section. The normal criteria for technology selection are capital cost and plant efficiency.

In a conventional natural gas-based MeOH plant with a capacity of 2,500+ metric tpd, syngas generation accounts for 55%, distillation accounts for 12%, com-pression and MeOH synthesis accounts for 12% and utilities and other services account for 24% of the total capital cost.

Methanol purification. Crude MeOH, as removed from the MeOH synthesis section, contains water, higher alcohols, impurities and light ends. Table 1

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 alcohol used in denaturing. Grade “A” covers methanol gen-erally used as a solvent. Federal-grade “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.

Purification schemes. Crude MeOH is purified by distil-lation with one- or two- or three- or four-column configuration. Fuel-grade methanol is normally produced with a single distillation tower. But to produce federal-grade AA methanol, two-, three-, and sometimes, even four -tower distillation systems are used. The amount of distillation required depends on the byproduct forma-tion of the MeOH synthesis catalyst and plant capacity.

The economics of the purification scheme involves the complex relationship of plant capacity, heat available in the plant, the energy export requirement and customer requirements, etc. For example, the four-column configuration is justified only at large capacities such as 5,000 metric tpd of MeOH production where as choice of two- or three-column configuration depends very much on customer’s requirements and energy availability in the front end.

Single-column configuration. For fuel-grade MeOH as a blending component (for gasoline), the major demands regard-ing quality are the water content and dissolved gases. Fuel-grade

Use new economics for purification on a small scaleFor smaller methanol units, new designs balance energy cost against capital cost for long-term profitability

K. PATwARdHAn, G. SATISHbAbu, S. RAjYALASHMI and P. bALARAMKRISHnA, Larsen and Toubro, Powai, Mumbai, India

Reformingtechnologies1. Steam2. Combined3. Autothermal

Reactortechnologies1. Isothermal2. Adiabatic

Distillationtechnologies1. Single column2. Multicolumn

Desulfurization Syngasproduction Compression MeOH

synthesisMeOH

distillation

Naturalgas MeOH

General flow diagram for a natural-gas based meoh facility.Fig. 1

Proof only. Copyrighted material. May not be reproduced without permission.

Petrochemical DeveloPments

2 I January 2012 HydrocarbonProcessing.com

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).

Multi-column configuration. 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.2–8 A double-effect three-column scheme was developed and it is widely applied in industry.4 A number of these alternative schemes involve split-ting 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 three-column 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 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-gradeproduct

Tail gas

single-column configuration for an meoh plant.Fig. 2

LPsteam

Recycle water

ProductMeOH

Liquid offsteam Reflux

drum 2

Processgas

Stabilizer MeoH pump

Higher alcohols

Refluxdrum 1

CrudeMeOH

Stabilizercolumn

Concentrationcolumn

Condenser 1 Condenser 2Stripped gas

two-column configuration for an meoh plant.Fig. 3

Table 1. Typical crude MeOH composition to MeOH purification section

component Wt%

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

Petrochemical DeveloPments

HYDROCARBON PROCESSING January 2012 I 3

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 STudYAn 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 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

two-column scheme

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)

three-column (forward integration) scheme

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)

three-column (backward integration) scheme

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)

Processgas

StabilizerMeOH pump

Liquidoff

steam

Refluxdrum 1

Toppingcolumn

Condenser 1 Condenser 2

Strippedgas

LPsteam

Refluxdrum 2

HPcolumn

Reboiler3

Recycle water

ProductMeOH

Reboiler2

Reboiler1

Higher alcohols

Refluxdrum 3

LPcolumn

CrudeMeOH

three-column configuration (forward integration) for an meoh plant.

Fig. 4a

Processgas

Liquidoff

steam

Refluxdrum 1

Toppingcolumn

Condenser 1 Condenser 2

Strippedgas

LPsteam

Refluxdrum 2

HPcolumn

Reboiler3

Recycle water

Product MeOH

Reboiler2

Reboiler1

Higheralcohols

Refluxdrum 3

LPcolumn

CrudeMeOH

three-column configuration (forward integration) for an meoh plant.

Fig. 4b

Petrochemical DeveloPments

4 I January 2012 HydrocarbonProcessing.com

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 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 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 dISTILLATIOnFor 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.

Operating 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 Cpw TB −Tref( )+ λ( )

LHVNG( )×ηBoiler

⎛

⎝

⎜⎜⎜⎜⎜⎜

⎞

⎠

⎟⎟⎟⎟⎟⎟⎟NGunit price( )

ColumnReboilerCondenser drum

CondenserPumpInstrumentation

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)

cost contribution to the capital cost of equipments for various configuration—a: two-column configuration, B: three-column forward integration configuration and c: three-column forward integration configuration.

Fig. 5

0 20 40 60 80 100 120 140

2-columnconfiguration

3F-columnconfiguration

3B-columnconfiguration

Relative cost

Operating costCapital cost

relative capital/operating cost for column configuration.Fig. 6

0 20 40 60 80 100 120

2-columnconfiguration

3F-columnconfiguration

3B-columnconfiguration

Relative cost

LP steamCW

operating cost contributions.Fig. 7

Petrochemical DeveloPments

HYDROCARBON PROCESSING January 2012 I 5

The 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.

new thinking. A techno-commercial comparison 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 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. 2 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,”

IMTOF, London, 1993. 5 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 inte-gration 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.