Methanol synthesis process layout for large production capacity

Abstract

A process layout for large scale methanol synthesis comprises one or more boiling water reactors and one or more radial flow reactors in series, the boiling water reactor(s) being fed with approximately fresh make-up syngas. The methanol synthesis loop comprises a make-up gas compressor K1, a recycle gas compressor K2, two or more boiling water converters for methanol synthesis (A1, A2, . . . ), a radial flow converter (B) for methanol synthesis, a steam drum (V1), a high pressure separator (V2), a low pressure separator (V3), feed effluent heat exchangers (E1 and E2), a wash column (C), an air cooler (E3) and a water cooler (E4).

Claims

1. A process layout for a methanol synthesis loop comprising; a first gas compressor for pressurizing a make-up gas, two or more boiling water converters for receiving the pressurized make-up gas and outputting a product gas, a first heat exchanger for cooling the product gas and pre-heating the make-up gas, a high pressure separator for separating the cooled product gas into a liquid stream and a gas stream, a second gas compressor for compressing the gas stream, a second heat exchanger for pre-heating the compressed gas stream, a radial flow reactor for receiving pre-heated compressed gas stream and outputting an effluent product gas, the effluent product gas being partly cooled in the second heat exchanger and added, in part, to the product gas from the boiling water converters, wherein a second part of the effluent product gas is drawn as a purge gas, a wash column for washing the purge gas with water to remove methanol, resulting in a methanol-free gas and a washed product, and a low pressure separator for receiving the washed product and the cooled liquid stream from the high pressure separator, separating out gases, and outputting a crude methanol product.

2. A process layout for a methanol synthesis loop according to claim 1, wherein the purge gas is split from the effluent product gas as wet gas (including methanol) and washed with water in the wash column to recover methanol at approximately the synthesis loop pressure.

3. A process layout for a methanol synthesis loop according to claim 1, wherein the radial flow reactor temperature is controlled by adjusting the purge gas and hence the level of inert gas in the reactor inlet.

4. A process layout for a methanol synthesis loop according to claim 1, wherein the radial flow reactor has a structure which requires no cooling device.

5. A process layout for a methanol synthesis loop according to claim 1, wherein only one train of cooling equipment is used.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The sole FIGURE shows the synthesis loop layout of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(2) In the following, the process layout according to the invention will be described with reference to the appended FIGURE. The synthesis loop layout in the FIGURE. consists of make-up gas (MUG) (1), which is pressurized in K1, mixed with a fraction of recycle gas (2) if it is needed (for example during the start-of-run period when the catalysts in the BWRs are extremely active) and pre-heated in El. The pre-heated flow (3) is introduced into the two (or more) BWCs A1, A2 . . . , from which a product gas (4) is withdrawn and subjected to feed-effluent (F/E) heat exchange in E1. The partly cooled stream (5) from the heat exchanger is mixed with the effluent (6) from the radial flow converter B and further cooled in the air cooler E3. The outlet gas (7) from E3 is water cooled in E4, and the resulting two-phase stream (8) is split into two streams, a liquid stream (9) and a gas stream (10), of which the latter is compressed in K2 to a stream (11).

(3) The pressurized stream (11) is divided into two streams (12 and 2). Stream 2 is a smaller fraction of stream 11 and might be used if it is needed to control the catalyst peak temperature, and consequently the formation of synthesis by-products in the BWRs. Stream 12 is heat exchanged (pre-heated) in the feed-effluent (F/E) heat exchanger E2. The pre-heated gas is introduced into the radial flow converter B, resulting in the effluent product gas 13 which is cooled partly in E2 and added (as stream 6) to the inlet gas to E3. A part of the E2 outlet is drawn as purge gas 17. The purge gas is washed with water 21 in the wash column C to remove mainly methanol from the stream. The methanol-free gas 18 is purged and can be used as fuel.

(4) The washed product 16 is introduced into the low pressure separator V3 along with the crude methanol stream 9 from the high pressure separator V2. As the separator V3 is operating at a low pressure, gases dissolved in crude methanol are released as stream 14. The crude methanol product is sent to a distillation unit for further purification.

(5) The radial flow converter B is an outward radial flow converter with a methanol synthesis catalyst located between the converter shell and the center tube, which is used for gas distribution over the catalyst bed. In this radial flow converter, no cooling device is used. The catalyst temperature from the synthesis reactions heat is merely controlled by adjusting the purge gas flow, i.e. stream 18. The concentration of inert gases is increased in the converter B inlet by reducing the purge gas flow. Due to insignificant pressure drop in converter B, it is possible to run the synthesis loop with a relatively high recycle flow.

(6) Radial flow converters (RFCs) and boiling water converters (BWCs) are well-known pieces of equipment in the chemical industry. The disclosed synthesis loop configuration uses these well-known unit operations in an innovative way, thereby offering a more effective process for methanol synthesis from syngas.

(7) By using the novel process layout for a methanol synthesis loop according to the present invention, a number of advantages over what was previously known are obtained. The main advantages are that: only two BWCs, instead of three or even four BWCs, are needed for a typical 5000 MTPD methanol synthesis loop; a potentially low CAPEX (capital expenditure, which is the cost of developing or providing non-consumable parts for the product or system) is obtained compared to a standard synthesis loop with only BWCs; a high carbon efficiency is seen in the synthesis loop according to the present invention; a lower pressure drop is observed across the converters, the layout is simple and practical for industrial implementation and only one train of cooling and condensation is needed for two set of converters.

(8) The invention is illustrated further by the example which follows.

EXAMPLE

(9) A natural gas (NG) based methanol synthesis loop according to the invention with a capacity of 5000 MTPD methanol is used. A front-end stand-alone ATR gives a flow of hydrogen enriched (from the hydrogen recovery unit from purge gas) make-up gas (MUG) of 510.000 Nm.sup.3/h with the following composition: 69% H.sub.2, 21% CO, 8.5% CO.sub.2, 1% CH.sub.4 and 0.5% N.sub.2.

(10) The total volume of methanol catalyst is 174 m.sup.3, more specifically split into 108 m.sup.3 in the two BWCs and 66 m.sup.3 in the RFC. The two BWCs include 11000 tubes in total, each with an inner diameter of 40.3 mm, an outer diameter of 44.5 mm and a length of 7.7 m. In the RFC, the inner diameter of the center tube is 1.0 m, the shell diameter is 3.6 m and the bed height is 7 m.

(11) A synthesis loop operating pressure of 80 kg/cm.sup.2 is kept constant from the start-of-run (SOR) to the end-of-run (EOR). The BWT (boiling water temperature) is varied from 225 C. to 260 C. from SOR to EOR.

(12) The catalyst activity loss is assumed to be 60% for the RFC and 65% for the BWCs over an operation time of 4 years.

(13) At the end-of-run (EOR), i.e. after an operation time of 4 years, the stream composition results (in mole %) shown in the following Table 1 were calculated (the stream numbers (S. no) refer to the FIGURE):

(14) TABLE-US-00001 TABLE 1 Stream compositions after 4 years of operation S. no H.sub.2 CO CO.sub.2 N.sub.2 CH.sub.4 MeOH H.sub.2O 1 69.0 21 8.5 0.5 1 0 0 3 66.9 11.3 5.8 5.5 10.2 0.25 0.02 4 61.4 6.9 5.6 6.3 11.6 7.2 1 10 65.7 5.6 4.1 8.6 15.6 0.4 0.03 13 63.5 4.8 3.4 8.9 16.1 2.3 0.9 18 65.1 4.9 3.5 9.2 16.6 0 0.7

(15) The product stream 15 from the low pressure separator V3 consisted of 85.7 weight percent crude methanol (corresponding to 5009 MTPD pure methanol). The stream 15 contained 1120 ppmw ethanol and 9 ppm methyl ethyl ketone.

(16) The flow (f) of the individual streams (S) is indicated in Table 2.

(17) TABLE-US-00002 TABLE 2 Flow of individual streams* 1 2 3 4 6 10 12 13 14 18 510 867 1377 1210 3315 4335 3468 3342 3 26 *Upper row: Stream no., lower row: Flow (1000 Nm.sup.3/h)

(18) The power and duty of respectively compressors and heat exchangers used in this production unit are listed as follows:

(19) Compressors

(20) K1: 39.7 MWe K2: 12.5 MWe (both 65% efficiency)

(21) Heat exchangers

(22) E1: 75 MW E2: 256 MW E3: 143 MW E4: 50 MW

(23) The synthesis loop carbon efficiency drops slightly from 98.6% at SOR to 97% at EOR (after 4 years of operation). The pressure drop of the catalyst beds in RFC and BWCs increase from 0.1 and 0.9 bar to 0.3 and 1.8 bar, respectively.