Urea Ullmann

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spesifikasi, sintesa mengenai urea

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  • c 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim10.1002/14356007.a27 333

    Urea 1

    UreaJozef H.Meessen, DSM Stamicarbon, Geleen, The Netherlands (Chaps. 1 7)Harro Petersen, BASF Aktiengesellschaft, Ludwigshafen, Federal Republic of Germany (Chap. 8)

    1. Physical Properties . . . . . . . . . . 22. Chemical Properties . . . . . . . . . 33. Production . . . . . . . . . . . . . . . 43.1. Principles . . . . . . . . . . . . . . . . 43.1.1. Chemical Equilibrium . . . . . . . . 43.1.2. Physical Phase Equilibria . . . . . . 73.2. Challenges in Urea Production

    Process Design . . . . . . . . . . . . 83.2.1. Recycle of Nonconverted Ammonia

    and Carbon Dioxide . . . . . . . . . . 93.2.2. Corrosion . . . . . . . . . . . . . . . . 113.2.3. Side Reactions . . . . . . . . . . . . . 123.3. Description of Processes . . . . . . 133.3.1. Conventional Processes . . . . . . . . 133.3.2. Stripping Processes . . . . . . . . . . 133.3.2.1. Stamicarbon CO2-Stripping Process 133.3.2.2. Snamprogetti Ammonia- and

    Self-Stripping Processes . . . . . . . 163.3.2.3. ACES Process . . . . . . . . . . . . . 183.3.2.4. Isobaric Double-Recycle Process . 193.3.3. Other Processes . . . . . . . . . . . . 193.4. Efuents and Efuent Reduction 203.5. Product-Shaping Technology . . . 214. Forms Supplied, Storage, and

    Transportation . . . . . . . . . . . . 225. Quality Specications and

    Analysis . . . . . . . . . . . . . . . . . 23

    6. Uses . . . . . . . . . . . . . . . . . . . 247. Economic Aspects . . . . . . . . . . 248. Urea Derivatives . . . . . . . . . . . 258.1. Thermal Condensation Products

    of Urea . . . . . . . . . . . . . . . . . . 258.2. Alkyl- and Arylureas . . . . . . . . 258.2.1. Transamidation of Urea with

    Amines . . . . . . . . . . . . . . . . . . 258.2.2. Alkylation of Urea with Tertiary

    Alcohols . . . . . . . . . . . . . . . . . 268.2.3. Phosgenation of Amines . . . . . . . 268.2.4. Reaction of Amines with Cyanates

    (Salts) . . . . . . . . . . . . . . . . . . 278.2.5. Reaction with Isocyanates . . . . . . 278.2.6. Acylation of Ammonia or Amines

    with Carbamoyl Chlorides . . . . . . 278.2.7. Aminolysis of Esters of Carbonic

    and Carbamic Acids . . . . . . . . . . 278.3. Reaction of Urea and Its

    Derivatives with Aldehydes . . . . 288.3.1. -Hydroxyalkylureas . . . . . . . . . 288.3.2. -Alkoxyalkylureas . . . . . . . . . . 298.3.3. ,-Alkyleneureas . . . . . . . . . . 308.3.4. Cyclic Urea Aldehyde

    Condensation Products . . . . . . . . 319. References . . . . . . . . . . . . . . . 33

    Abbreviations:CRH, % critical relative humidityHS, kJ/mol integral heat of solutionm, mol/kg urea molality, moles of urea per

    kilogram of waterPO(s)H2 , Pa water vapor pressure of a satu-

    rated urea solutionPv , Pa vapor pressure

    Urea [57-13-6], CO(NH2)2, Mr 60.056,plays an important role in many biological pro-cesses, among others in decomposition of pro-teins. The human body produces 20 30 g ofurea per day.

    In 1828, Wohler discovered [1] that ureacan be produced from ammonia and cyanic acidin aqueous solution. Since then, research on the

    preparation of urea has continuously progressed.The starting point for the present industrial pro-duction of urea is the synthesis of Basaroff [2],in which urea is obtained by dehydration of am-moniumcarbamate at increased temperature andpressure:

    NH2COONH4CO(NH2)2 +H2O

    In the beginning of this century, urea was pro-duced on an industrial scale by hydration ofcyanamide, which was obtained from calciumcyanamide:

    CaCN2 +H2O+CO2CaCO3 +CNNH2

    CNNH2 +H2OCO(NH2)2

  • 2 Urea

    After development of the NH3 process (Haberand Bosch, 1913, Ammonia, Chap. 2.Ammonia, Chap. 3. Ammonia, Chap. 4.),the production of urea from NH3 and CO2,which are both formed in the NH3 synthesis,developed rapidly:

    2NH3 +CO2NH2COONH4

    NH2COONH4CO(NH2)2 +H2O

    At present, urea is prepared on an industrialscale exclusively by reactions based on this re-action mechanism.

    1. Physical Properties [3], [4]

    Pure urea forms white, odorless, long, thinneedles, but it can also appear in theform of rhomboid prisms. The crystal lat-tice is tetragonal scalenohedral; the axis ratioa : c=1 : 0.833. The urea crystal is anisotropic(noncubic) and thus shows birefringence. At20 C the refractive indices are 1.484 and1.602. Urea has an mp of 132.6 C; its heat offusion is 13.61 kJ/mol.

    Physical properties of the melt at 135 C fol-low: 1247 kg/m3Molecular volume 48.16m3/kmol 3.018mPa sKinematic viscosity 2.42106 m2/sMolar heat capacity, Cp 135.2 Jmol1 K1Specic heat capacity, cp 2.25 kJ kg1 K1Surface tension 66.3103 N/m

    In the temperature range 133 150 C, den-sity and dynamic viscosity of a urea melt can becalculated as follows:

    r= 1638.5 0.96T

    ln= 6700/T 15.311

    The density of the solid phase at 20 C is1335 kg/m3; the temperature dependence of thedensity is given by 0.208 kgm3 K1.

    At 240 400K, themolar heat capacity of thesolid phase is [5]

    Cp= 38.43 + 4.98 102T +7.05 104T 2

    8.61 107T 3

    The vapor pressure of the solid phase between56 and 130 C [6] can be calculated from

    lnPv= 32.472 11755/T

    Hygroscopicity. The water vapor pressure ofa saturated solution of urea in water PO(s)H2 inthe temperature range 10 80 C is given by therelation [7]

    lnP(s)H2O= 175.766 11552/T 22.679lnT

    By starting from the vapor pressure of purewaterPOH2 , the critical relative humidity (CRH) thencan be calculated as

    CRH =(P(s)H2O/PH2O

    )100

    The CRH is a threshold value, above which ureastarts absorbing moisture from ambient air. Itshows the following dependence on tempera-ture:

    25 C 76.5%30 C 74.3%40 C 69.2%

    At 25 C, in the range of 0 20mol of ureaper kilogram of water, the integral heat of solu-tion of urea crystals in waterHs as a functionof molality m is given by [8]:

    Hs= 15.351 0.3523m+2.327 102m2

    1.0106 103m3+1.8853 105m4

    Urea forms a eutectic mixture with 67.5wt%of water with a eutectic point at 11.5 C.

    The solubility of urea in a number of solvents,as a function of temperature is summarized inTable 1 [9], [10].

    2. Chemical Properties

    Upon heating, urea decomposes primarily toammonia and isocyanic acid. As a result, thegas phase above a urea solution contains a con-siderable amount of HNCO, if the isomerizationreaction in the liquid phase

    CO(NH2)2NH4NCONH3 +HNCO

  • Urea 3Table 1. Solubility of urea in various solvents (solubility in wt% of urea)

    Temperature, C

    Solvent 0 20 40 60 80 100

    Water 39.5 51.8 62.3 71.7 80.2 88.1Ammonia 34.9 48.6 67.2 78.7 84.5 90.4Methanol 13.0 18.0 26.1 38.6Ethanol 2.5 5.1 8.5 13.1

    has come to equilibrium [11]. In diluteaqueous solution, the HNCO formed hy-drolyzes mainly to NH3 and CO2. In amore concentrated solution or in a ureamelt, the isocyanic acid reacts further withurea, at relatively low temperature, to formbiuret (NH2 CONHCONH2), tri-uret (NH2 CONHCONHCONH2),and cyanuric acid (HNCO)3 [12]. Athigher temperature, guanidine [CNH(NH2)2],ammelide [C3N3(OH)2NH2], amme-line [C3N3OH(NH2)2], and melamine[C3N3(NH2)3] are also formed [13], [14].

    Melamine can also be produced from ureaby a catalytic reaction in the gas phase. To thisend, urea is decomposed into NH3 andHNCO atlow pressure, and subsequently transformed cat-alytically to melamine (Melamine and Gua-namines, Chap. 4.).

    Urea reacts with NOx, both in the gas phaseat 800 1150 C and in the liquid phase at lowertemperature, to form N2, CO2, and H2O. Thisreaction is used industrially for the removal ofNOx from combustion gases [15], [16].

    Reactions with Formaldehyde. Under acidconditions, urea reacts with formaldehyde toform among others, methyleneurea, as wellas dimethylene-, trimethylene-, tetramethy-lene-, and polymethyleneureas. These prod-ucts are used as slow-release fertilizer underthe generic name ureaform [17] (Fertilizers,Chap. 4.4.2.1.). The reaction scheme for theformation of methyleneurea is given below:

    Methyleneurea reacts with additional moleculesof formaldehyde to yield dimethyleneurea andother homologous products.

    The reactions of urea with formaldehyde un-der basic conditions are used widely for the pro-duction of synthetic resins (Amino Resins,Chap. 7.1.). As a rst step, methylolurea insteadof methyleneurea is formed:

    This product subsequently reacts with formal-dehyde to dimethylol urea, CO(NHCH2OH)2,and further polymerization products. Since ureais also the raw material for the production ofmelamine, from which melamine formalde-hyde resins are produced, it is the most impor-tant building block in the production of aminoresins.

    When urea is applied as fertilizer to soil, ithydrolyzes in the presence of the enzyme ureaseto NH3 and CO2, after which NH3 is bacteri-ologically converted into nitrate and, as such,absorbed by crops [17].

    3. Production

    3.1. Principles

    3.1.1. Chemical Equilibrium

    In all commercial processes, urea is producedby reacting ammonia and carbon dioxide at ele-vated temperature and pressure according to theBasaroff reactions:

  • 4 Urea

    2NH3 (l) + CO2 (l)NH2COONH4H = 117 kJ/mol (1)

    NH2COONH4NH2CONH2 +H2OH =+ 15.5 kJ/mol (2)

    A schematic of the overall process and the phys-ical and chemical equilibria involved is shownin Figure 1. In the rst reaction, carbon dioxideand ammonia are converted to ammonium carb-amate; the reaction is fast and exothermic. Inthe second rection, which is slow and endother-mic, ammonium carbamate dehydrates to pro-duce urea andwater. Sincemore heat is producedin the rst reaction than consumed in the second,the overall reaction is exothermic.

    Figure 1. Physical and chemical equilibria in urea produc-tion

    Processes differ mainly in the conditions(composition, tempe