New Diammonium Phosphate Technology – Powdered or Granular DAP

Keywords: Ammonium phosphate, DAP, MAP, Pipe reactor, Process revamp.

PART I

Powdered or Granular DAP:

Summary:

The manufacture of ammonium phosphates by the ERT-ESPINDESA process is based on the use of a special reactor of proprietary design. What differentiates the ERT-ESPINDESA pipe reactor from other pipe reactors is that the reactions of phosphoric acid and ammonia to make mono and diammonium phosphate take place to completion in it, and it is thus the only pipe reactor system to date in which total reaction of ammonia and phosphoric acid to DAP is achieved in a single piece of equipment.

The development of the ERT-ESPINDESA pipe reactor started in 1979 when ERT decided to use the TVA pipe reactor for the manufacture of NPK fertilisers. The reactor was located inside the granulation drum and encouraging results were obtained and by 1980, phosphoric acid substituted MAP as raw material in some plants for the manufacture of NPK fertilisers.

The next steps were the modification of the pipe-reactor design to enable the manufacture of powdered MAP in a 450 tpd plant and the manufacture of granular DAP in a 600 tpd plant located at Huelva. A second 600 tpd plant is being started up in Turkey.

Phosphoric acid made from Morocco and Florida rock have been used in the manufacture of DAP and even Florida sludge acid has been tested with excellent results.

The last studies have led to the development of a new fertiliser: powdered DAP which can economically substitute powdered MAP as an intermediate to the manufacture of NPK fertilisers. Extensive pilot plant research has been carried out and at the time of writing this paper (1986) the above mentioned 450 tpd MAP tower is being retrofitted to the manufacture of powdered DAP. Pay-back time of this retrofit is expected to be less than 4 months.

L M Marzo, Espindesa, Madrid Spain.

J L Lopez-Nino, ERT, Spain.

13 Pages, 3 Figures, 1 Table.

PART II:

Dual Pipe Reactor Process for DAP, NP and NPK Processes.

Introduction:

When reviewing how most of the granulated fertilisers containing ammonium phosphate are now produced in the world, it becomes obvious that:

– the production processes are generally very classical,

– few (and minor) improvements have been achieved during the last decades.

Most of these processes include a reactor vessel of high capacity (neutralizer) where ammonia (often gas) reacts with phosphoric acid up to a molar ratio N/P of 1.35 to 1.5 when DAP is to be produced. The phosphoric acid, prior to its introduction in the neutralizer, generally goes through a washing section designed to recover the ammonia losses from the neutralizer and the granulator. If needed for the production of fertilisers some sulphuric acid may also be added in the neutraliser.

The neutraliser may or may not be stirred; most of the time it is operated at atmospheric pressure or under a slight pressure.

The ammonium phosphate slurry flowing from the neutraliser must be fluid enough to flow by gravity, or to be pumped, into the granulator where more ammoniation takes place. A minimum water content is thus needed, between 12% and 25% according to the design. As a consequence a high recycle ratio (often higher than 5) and a high drying fuel consumption are unavoidable.

It is somewhat surprising to ascertain that the powerful USA fertilisers industry still works along these lines for DAP production and that relatively recent investments have been made using this process.

It is all the more surprising that the first industrial uses of a pipe reactor, a possible substitute for the neutralizer, took place in the USA. It is in the USA too, that much information on the pipe reactor has been made available through TVA publications and demonstrations as well as papers given in various conventions (IFA, Round Table, AIChE, etc.).

An interesting historical point on pipe reactors, including a significant bibliography, is included in a paper given in 1982 at the IFA meeting in Kalithea (Greece) by MM. Ph. Moraillon and Y. Cotonea (Saving on Energy in Granulation of Fertilisers by a New Method of Using Pipe Reactors).

How could the lack of success of this new technology be explained? It is probably related to the operating problems the first pipe reactors were faced with (scaling in the pipe, plugging inside the granulator) and the disappointment of producers considering the few improvements of granulation brought in by the use of a pipe reactor.

The timorous use of promising, but still imperfect, new technologies without the will to promote them may well jeopardise their future for a long time. In the same field the risky choice of a brand new process, without any industrial background could, if it fails, induce a move back to a technology proven in the past. Maybe it happened in India when the failure of a new fluidised bed granulation process appears to have been the main reason for the choice of a traditional process for new units.

Nevertheless conclusive improvements have been made in the production of granulated fertilisers based on ammonium phosphates when using a pipe reactor process and specially the AZF Dual Pipe Reactor system. This process has now been widely proven as 48 pipe reactors are now operating in 11 different countries and represent together more than 12 million tons of fertiliser per year.

P Chinal, Y Cotonea, C Debayeux and J F Priat, CDF Chimie, AZF, France.

22 Pages, 14 Figures, 2 Tables, 9 References.