Methanol Production from Synthesis Gas derived from Municipal Solid Waste
Department of Chemical Engineering
University of Illinois at Chicago
ChE. 397
Group Charlie: Priya Chetty, Scott Morgan, Brian Mottel, Daniyal Qamar, Sukhjinder Singh
Mentor: Dennis O’Brien
Instructor: Dr. Jeffery Perl
3/3/11
Table of Contents
I. Abstract
II. Executive Summary
III. Introduction
IV. Process Description
V. Process Control
VI. Environmental Concerns and their mitigation
VII. Economics
VIII. Recommendations
IX. Appendices
1. Design Basis
2. Block Flow Diagram
3. Process Flow Diagram
4. Material and Energy balances
5. Calculations
6. Process Simulation in ASPEN
7. Annotated Equipment List
8. Economic Evaluation
9. Utilities
10. Conceptual Control Scheme
11. General Arrangement
12. Distribution and End-use Issues review
13. Constraints Review
1. Feedstock definition
2. Conversion technology description
3. Separation technology description
4. Product description
5. Location sensitivity Analysis
6. ESH law compliance
7. Laws of physics compliance
8. Turndown ratio
14. Applicable Standards
15. Project Communications File
16. Information Sources and References
I. Abstract
Methanol is a chemical which is used in many diverse applications. Methanol is commonly used as a fuel, as antifreeze, and as a solvent, but can also be utilized to produce plastics, adhesives, and dyes. The most common method for producing methanol involves combining methane (natural gas) and water to produce synthesis gas, which is carbon monoxide and diatomic hydrogen.
This synthesis gas is then used to produce methanol. The issue with this process is that methane is very useful as a heating fuel which is why it is used to fulfill residential heating needs. This method of methanol production is essentially using a fuel to produce another fuel as the product. Our goal is to make Methanol by using a sustainable source of fuel. The source of our synthesis gas is Chicago Land Municipal Solid Waste. We are utilizing a waste stream rather than a fossil sourced feed stock. This will be the source of our synthesis gas. We will then compress the synthesis gas, and feed it into a multiple feed reactor which contains Cu/ZnO/Al2O3 catalysts to produce methanol.
Unused synthesis gas will be separated from the initial product stream and recycled back into the reactor. The initial product stream will contain methanol, along with water, as well as other alcohols such as ethanol, and propanol. To ensure that the methanol produced will meet the grade AA industry standard, which states that it must be 99.85% pure, the initial product will go through a flash separator followed by two distillation columns.
II. Executive Summary
By utilizing the process outlined in this report, 99.85% pure methanol can be produced at a rate of 1060 tons per day from a feed stock of 1700 tons per day of synthesis gas which will be produced by gasification of 5,000 tons per day of municipal solid waste. Conversion of synthesis gas to methanol is conducted in a Lurgi reactor which will utilize a Cu/ZnO/Al2O3 catalyst, and a recycle stream. The desired product will then be separated from byproducts via flash separation, and purified using two distillation columns.
The capital cost needed to build a plant of this magnitude is $116.8 million. The internal rate of return for this plant would be 25.60%, and the payback period would be 5 years.
III. Introduction
The objective of the syngas to methanol process is to produce 1060 tons/day of grade AA methanol for use in the chemical, automotive, and plastics industries. The syngas that is procured for this process will have been produced by gasification of municipal sold waste. There are many advantages to utilizing syngas produced via this method, such as; it eliminates the need for methane, it is less expensive due to the solid waste tipping fee, and it is a much more green technology which is important to consumers.
Currently there are 90 methanol plants worldwide with an annual production of 11 billion gallons, generating $12 billion per year in revenue. The market outlook for the future of the methanol industry is good, with global demand expected to rise in the years to come.
The gasification plant will be located in the Chicagoland area because ample municipal solid waste is produced there; in fact the average production is 25,000 tons per day. The methanol plant will be located near the gasification plant, because geographic proximity will reduce transportation costs, which also reduces the consumer price of the product.
IV. Process Description
The overall process starts as syngas from group Delta enters the very first heat exchanger. The syngas, now cool enough to enter a compressor, goes through a two stage compressor with cooling between stages. The syngas at a high enough pressure to react under catalytic conditions now enters a reactor. A catalyst based on Cu/Zn/Al oxides is used and it gives a conversion of Carbon Monoxide to methanol of 0.5 per pass (based on reaction 1 below). As reactions 1 through 4 (listed below) take place in the reactor all the products are discharged towards a heat exchanger. Methanol synthesis involves highly exothermic reactions and the heat accumulates in the product stream. This heat must be taken out so it can be utilized in reboilers for distillation columns. The product stream is also highly pressurized thus it goes through a turbo expander so we can run it through a turbine and make power to use in air coolers and put into the grid. After the turbo expander the products go through heat exchangers and finally into a flash drum. The flash drum operates at conditions where the non condensables are separated easily and sent back to the compressor in the beginning of the process as a recycle stream. This recycle stream contains a high concentration of CO2 that accumulates as the process goes on. A purge from this recycle stream works to minimize the accumulation of this CO2. The condensables, ethanol, butanol, methanol, and water proceed to the separations train where they go through a couple distillation columns. The first column gets rid of any left over non condensables, CO, CO2, H2 and the second one gives AA grade methanol separated from ethanol, butanol, and water.
Detailed descriptions of units are given below:
Compressor
The syngas from the gasifier will reach the methanol synthesis plant at 300 psi. But this pressure is too low for the reactor and needs to be raised. There will be a multistage compressor to raise the pressure to the needed value of 1200 psi. Since it is a multistage compressor there is some pressure drop between each of the stages, due to the fact that when the gas is heated in the compressor it will need to be cooled, and this cooling will decrease the pressure. So instead of having an overall compression ratio of 2:1, the total compression will need to be around 2.5:1 to overcome the pressure drop from the inter-stage coolers. The cooling water in this part of the process will be at 90 °C, because this will eliminate the need for a cooling tower which is fairly expensive to install.
Reactor
Methanol synthesis is a very complex process since it is very equilibrium limited, and quite exothermic; therefore it is imperative that the correct type of reactor is chosen to obtain the desired results. There are many diverse types of reactors which may be used in the synthesis of methanol, which utilize various methods of catalysis, and removal of heat from the reaction. A significant amount of time was spent evaluating the different types of reactors to ensure that the correct one was chosen. Below is an example of some of the reactors that were considered along with information about each:
ICI Multiple Feed Quench Reactor
The ICI reactor uses multiple feeds to bring the reaction temperature down to the desired value. This however requires a very high flow of air into the reactor which reduces the effectiveness of the compressor. The reactor utilizes multiple catalyst beds to reach a conversion of 35-40%. The maximum production rate of the ICI multiple feed quench reactor is 5000 tons per day. The amount of syngas required to remove the heat of the reaction is in excess of that which is able to be procured, therefore this reactor did not meet the necessary requirements for this process.
Lurgi Combination Converter Reactor
The Lurgi reactor works as an ICI reactor but it also combines water cooling to control the reaction temperature. The reactor works as a heat exchanger with tube and shell sides. The reactions take place in the shell side while coolants go through the tube side. There are essentially two reactor vessels the first one has two feeds to it. The first feed (cooling stream) is the cold fresh syngas; the second feed (reactant stream) is the hotter product gas from the second reactor vessel. This hot product stream has leftover CO and H2 that can react again and it does so in the first vessel. This essentially gives two reactors in series and therefore a higher conversion. The cold fresh syngas is used to control the temperature of the first reactor and this colder gas proceeds to the second reactor as the reactant stream. To control the temperature of the second reactor, steam is used. Water at its saturation point (428 F and 420.61 psi) is kept in a steam drum and as this water goes through the reactor it removes energy from the product stream in the reactor and evaporates, the steam is sent back to the drum where new cooling water controls the saturation point. This loop continues until the process reaches steady state, and before that a light up heater is used to bring the reactant temperature up to the desired value.
Example of Lurgi Methanol Reactor courtesy of http://www.lurgi.com/
In the production of methanol from syngas, which will have a H2 to CO ratio of 2:1, there are several reactions that will have to be considered:
CO + 2H2 ↔ CH3OH (1)
CO2 + H2 ↔ CO + H2O (2)
CO2 + 3H2 ↔ CH3OH + H2O (3)
2CO + 4H2 ↔ C2H5OH + H2O (4)
These reactions occur simultaneously inside of the reactor, during the methanol synthesis step.
The kinetics of the above reactions can be found in literature. From all of the reaction kinetics, it is possible to find the production yield of each of the components in the syngas. The following reactions are for the kinetics of methanol synthesis:
Catalyst
In the reactor there will be a Cu/Zn/Al catalyst to drive the overall reaction towards methanol. This catalyst is used in many other reactors. The poisons of this specific catalyst are sulfur, chlorine, CO2, and high temperatures. The amount of CO2 activates the catalyst and it also deactivates it, so an adequate amount of CO2 must be allowed into the reactor. So in order to ensure that the catalyst life can stay at a reasonable time, the syngas that is fed into the reactor must have none of the above chemicals, and the temperature of the reactor will need to stay at 518 F. The conversions are 50% for reaction 1, 13% for reaction 2, 2 % for reaction 3, and 2% for reaction 4.
After the syngas has been reacted it will be at 1185 psi and 518 F. The reacted gas will then go through several pieces of equipment until it reaches the distillation columns for purification of the product. The gas will need to be split for a recycle feed and remaining gas. The remaining gas will have a high amount of crude methanol and fewer impurities than before. After this the stream will be put through a condenser and heat exchanger in order to have the methanol in the liquid phase and to be at a much lower temperature. Then the crude methanol is put through a flash tank and finally to the distillation unit.
Heat Exchangers
Since many of the different pieces of equipment throughout the process need the syngas or methanol at different temperatures, there will be three heat exchangers throughout the entire process. These particular heat exchangers will be a shell and tube heat exchangers, which consists of a series of tubes. One set of these tubes contains the fluid that must be either heated or cooled. The second fluid runs over the tubes that are being heated or cooled so that it can either provide the heat or absorb the heat required. The fluid used in this case will be water. A heat exchanger will be present before and after the multi-stage compressor in order to utilize the energy from a high compression process. The third heat exchanger will be present after the reactor. The reactor will be producing heat at an amount of -12,891,1520 Btu/hr, which can be utilized with this heat exchanger. The first heat exchanger will have an area of 64.18 ft2, with 6 inner tubes having an inner and outer diameter of 0.33ft and 0.34ft respectively. The second heat exchanger will have an area of 1200 ft2, with 105 inner tubes having an inner and outer diameter of 0.11ft and .12ft respectively. The third heat exchanger will have an area of 425 ft2, with 15 inner tubes having an inner and outer diameter of 0.33ft and .34ft respectively. The energy balances around the heat exchangers can be found in the appendix, and they show how much energy is needed to run each of them.
There will be several non condensables in the product stream coming from the reactor. Since these gasses do not dissolve very readily in the crude product they are very easy to separate. A flash drum is used to do so.
Flash Drum
A flash drum with a diameter of 4.63 ft and a height of 7.79 ft is being used for a simple separation of CO, H2, and CO2 from methanol, water, n-butane, and ethanol. The drum operates at a temperature of 118 °F and a pressure of 638 psi (Grue et. al.). The energy lost in the flash drum is minimized by using a heat exchanger before and lowering the product temperature to 118 °F, this heat is then used in the distillation columns. High separation percentages of non condensables were achieved using this flash drum. 97.5% of CO, 76.44% of CO2, 3.9% methanol, 99.99% of H2, and 2.3% of Ethanol are the percentages of feed components to the flash drum that are sent to the vapor stream, the rest was sent into the liquid stream which is distilled twice to obtain grade AA methanol. It can be noted that the lesser CO2 than CO goes into the vapor stream and this can be attributed to the fact that CO2 readily dissolves in the H20 and methanol rich stream that goes off as liquid to the distillation column.