Urea synthesis with improved heat recovery and conversion

Claim 1

In a urea synthesis process wherein fluid NH 3 and fluid CO 2 are contacted and reacted at elevated pressure and temperature to form ammonium carbamate, and ammonium carbamate is converted to urea in a vertical cylindrical urea synthesis reactor, the improvement which comprises (a) contacting from about 5 mol percent to less than 100 mol percent of a fluid NH 3 reactor feed stream and a fluid CO 2 reactor feed stream at substantially reactor pressure in a heat exchanger external (1) to the urea synthesis reactor to form ammonium carbamate in said heat exchanger and simultaneously removing from said external heat exchanger (1) heat of reaction evolved from reaction of NH 3 and CO 2 from said external heat exchanger (1), the NH 3 to CO 2 molar ratio inside said external heat exchanger (1) being from about 2.5 to about 3.5 to 1, (b) charging the resulting reaction mixture into the lower portion of a reactor having an indirect heat exchanger (2) positioned therein which extends from the top portion to the lower portion of the reactor, said heat exchanger (2) including a heat exchange medium therein and having its inlet and its outlet located at the upper portion of the reactor, (c) passing heated heat exchange medium from said inlet to said lower portion of said reactor thereby transferring heat therefrom to the total reaction mixture formed in (d) as the total reaction mixture passes upwardly in said reactor, and removing heat exchange medium from said outlet, said heat exchange medium being substantially cooler at said outlet than at said inlet, and (d) charging remaining NH 3 reactor feed stream to said lower portion of said reactor, and mixing said remaining NH 3 with said resulting reaction mixture of (a) discharged from said external heat exchanger (1) into the lower portion of said reactor to form a total urea synthesis mixture having a NH 3 to CO 2 molar ratio of from about 2.8 to about 7 to 1.