Department of Chemical & Biomolecular Engineering, University of Notre Dame

Project 8: Urea Production

Shawn Coleman Erin Heck Matthew Napierski Jessica Stouffer

AbstractA 2.9 metric tonne, $425 million urea plant was designed that utilizes a yield reactor to convert 70 mol% of reacting carbon dioxide to urea. A system of two distillation columns separates the unconverted reactants and water from the urea product stream. The reactant stream and water are then separated in the second column, and the remaining ammonia and carbon dioxide is compressed to reaction conditions and recycled back to the reactor. An Aspen Plus simulation modeling the urea plant appropriately reproduces the process. Negligible discrepancies in energy and mass balances support the accuracy of the simulation, with errors of no more than 0.04%, despite some estimation of the thermodynamic parameters for urea by Aspen. Overall the process is not profitable, resulting in a discounted rate of return on investment of -10.9 percent. This is caused by expensive, corrosion-resistant materials of construction, and multiple high- pressure reactors. In its current state, this process is not highly recommended as its discounted rate of return on investment is -10.9 percent. It is recommended that the project be retired. The narrow margin between product and reactant costs does not justify the insurmountable grass roots cost of the project.

Process Flow Diagram

S.W.O.T.Strengths Elimination of Carbon Dioxide - Removed carbon dioxide from plant processes is routed to the Urea Unit. The main reaction in the unit yields urea, a desired fertilizer, and carbon dioxide and ammonia at a high quality. Location The plant location is ideal for Urea sales. Mobile, Alabama is one of the countries largest producers of cotton. Cotton is a crop that consumes copious amounts of Urea. In addition, the land is very cheap when compared to other options nationwide. Recycle Stream The liquid recycle stream that maintains the process at 19 bar drastically reduces plant cost. Most urea plants drop the process pressure to atmospheric before recycling. This allows our plant to avoid the cost of a 9 figure, 3 stage compressor. Weaknesses Fluctuating Government Policies Whether it is tax holidays or free Carbon Dioxide feed, a fluctuating government policy makes it difficult to predict the future profit margins and success of the Urea Plant. High Pressure The reactor runs at 240 bar. The recycle stream and both separation columns operate at 19 bar. As a result, the reactors are very expensive and the process is very hazardous. Safety failure could lead to catastrophic events. Corrosive Materials Ammonia and ammonia-carbamate necessitate special materials of construction. Contact with ammonia require stainless steel and ammonia-carbamate require nickel alloy. In addition, safety failure needs special consideration as the materials cannot be vented to atmosphere, flared, or spilled to grade. Opportunities Expanding Market In order to produce more urea for fertilizer and other uses and increase income, there exists potential to expand the current unit and add units in similar plants. Sources of Reacting Materials Carbon Dioxide and Ammonia create opportunities to expand the current unit. Instead of purchasing these reactant streams they can be produced within existing plant through coal gasification or with natural gas. Exporting Market values for Urea fluctuate throughout the year and internationally. The opportunity exists to export urea to markets with higher demand and purchase prices for Urea Threats Carbon Dioxide Competition - Competition for free carbon dioxide exists. Carbon Dioxide sequestration and consumption through solar power are two current projects attempting to harness carbon dioxide and convert it into a less harmful material. Seasonal Market The demand for urea fluctuates continually throughout the year. Since fertilizer is a seasonal commodity, storage of Urea might be required. Ammonia Price Ammonia is a direct product of Natural Gas. As a result, this reactant stream fluctuates in price drastically and is hard to predict.

Introduction

Carbon Dioxide Liquefaction Gaseous carbon dioxide is compressed, cooled to a liquid, and pumped up to process pressure. Reaction The liquid ammonia, carbon dioxide, and recycle streams are pumped to 240 bar and reacted at 200C in R-101A/B/C. Valve V-101 drops the pressure to 19 bar for separation. Separation Two separation columns separate out aqueous urea product and waste water for treatment. The distillate is recycled back as carbon dioxide in liquid ammonia at 19 bar. Recycle The recycle stream takes liquid distillate from T-102 and pumps it to reaction pressure at 240 bar. Heat Exchange Network The bottoms from T-102 are used to cool the compressed carbon dioxide stream and the reactor before entering the reboiler.

The objective of this project was to model an industrial plant that converts carbon dioxide and ammonia to urea. This process consists of a two step reaction converting ammonia and carbon dioxide to ammonium carbamate, which in turn is dehydrated to form urea via the following reactions:2NH3 + CO2 NH2COONH4 (1) ammonium carbamate NH2COONH4 H2O + NH2CONH2 (2) ammonium carbamate urea

Safety and EnvironmentHazardous Materials: Ammonia Strong odor. Vaporization if leaked (processed as a liquid). NIOSH REL: 25 ppm Toxic if consumed in aqueous form (Deadly for aquatic organisms) Carbon Dioxide Odorless gas. Common in atmosphere. NIOSH REL: 5000 ppm (long term exposure) NIOSH REL: 30000 ppm (short term exposure) Ammonia Carbamate Mild irritant. Breaks down immediately to gaseous ammonia and carbon dioxide in the event of a leak. Only present at small concentrations in unit. Urea skin exposure and inhalant irritant. No OSHA or NIOSH REL values. No immediate exposure threat. Hazards Operating Conditions Reactor High Pressure - 240 bar operating condition. High Temperature 200C with an exothermic reaction Corrosive and irritating materials cannot be spilled to grade or vented to atmosphere Evacuate T-101 and T-102 and dump extra reactor materials to corrosive resistant separation towers (up 15% of reactor volume)

RecommendationsIt is not recommended to pursue the production of urea through this method. The cost of buying ammonia is too close to the selling price of urea to turn a profit when such an expensive plant is required. Profits maybe found if ammonia and carbon dioxide are produced within the same facility. Two viable alternative forms of ammonia and carbon dioxide are from natural gas or coal gasification. If the production or urea is sought this way, it is recommended that tests be performed to achieve better reaction data.

These reactions are carried out at temperatures between 180C and 280 C, and at a pressure of 240 barg1,2. At these conditions, a flow of excess of ammonia prevents an undesirable side reaction forming buiret, which is toxic to plants and would decrease the purity of the product urea stream. Urea has several different profitable uses including its role as a component in fertilizer (hence the undesirability of biuret in the product), a stabilizer in explosives, a raw material for plastics, and an ingredient in personal hygiene products (i.e. hair conditioner, soap, lotion)3. This plant will focus on the production of urea prills and solutions, but there are other common marketable forms including granules, flakes, and pellets4.

Base Case EconomicsAssuming a four year construction period with capital investments of 10, 20, 40, and 30 percent, and the plant running at half capacity for the first year of production, the Net Present Value (NPV) of the grass roots project after twenty years is -$350.37 million. This project has an estimated discounted cash flow rate of return -10.9%, well under the of 15% benchmark required for a profitable project, and the discounted payback period is nonexistent. These values stem primarily from a calculated $193.7 million and $207.4 million annual revenue and cost of manufacturing respectively, using a 35% tax rate, a 10% interest rate, a salvage value of $42.52 million, and a working capital of $52.6 million.Discounted Cash Flow Diagram0.0

ConclusionA model successfully simulated a urea production plant taking ammonia and carbon dioxide to urea and waste water has been developed. The economic analysis of this plant concludes that the process will not be profitable, operating at a discounted rate of return of -10.9 percent. This is caused by expensive, corrosion-resistant materials of construction, and multiple high-pressure reactors. The largest source of fixed capital investment is the three 240 bar, nickel alloy reactors. Deeper investigations on the profitability potential of the project prove that urea production in this fashion is not economically feasible.

Design ProcedureAspen: Mass and Energy Balances Aspen Plus simulates all equipment balances The simulation was corroborated by hand calculated mass and energy balances Sizing of Equipment Equipment was sized using data from the Aspen simulation and heuristics Pinch Technology The heat exchange network was designed using HENSAD and yielded a utilities savings of $15.8 Million/year.B1

Project Value (millions of dollars)

-50.0 -100.0 -150.0 -200.0 -250.0 -300.0 -350.0 -400.0 -1 4 9 Project Life (Years) 14 19

References1) Copplestone, J. C., C. M. Kirk, S. L. Death, N. G. Betteridge, and S. M. Fellows. "Ammonia and Urea Production." Ed. Heather Wansbrough. 2) Mennen, Johannes H., and Kees Jonckers. Process For The Preparation Of Urea. DSM N.V., assignee. Patent 6392096. 2002. 3) Urea." Stamicarbon - pure knowledge. 04 Mar. 2009 . 4) Ondrey, Gerald. "Spotlight On Ammonia And Urea." Chemical Engineering Oct. 2008: 28-31.