Design parameters for automation control system for ammonia process plant, front end

Computer-control process system is advantages than manually operated plants in terms of monitoring, alarming and operator messaging about the crucial and bottleneck variables in the process. Implementation of automation means utilizing a modern computer to monitor the process to control it by interaction mode between the loops and able to perform the optimization to achieve the profit objective function of the overall plant. Ammonia plants around the world are one of the earliest process industries established to meet the demand for fertilizers. At initial stages, some of these plants are operated manually and later by the introduction of computers and instrumentation technology, every process plant was automizied.  Ammonia produced by using either natural gas or naphtha as a raw material is the efficient rout until now. Equipment used for this scheme are identical around the world with similar operation due to the same chemistry route.

Most important equipment to be considered in the front end of ammonia plant are:

  • Primary and secondary reformers
  • Synthesis gas compressor
  • Synthesis converter
  • Primary and secondary shift converters
  • Refrigeration, purge and recycle systems

Some of the typical optimizing variables of front-end ammonia process plant in offline and online:

  • H2/N2 ratio to converter feed
  • H2 makeup
  • The temperature of primary reformer exit
  • Steam to carbon ratio in primary reformer feed
  • Intel temperature of primary and secondary reformers
  • Syngas compressor speed
  • Cooling water flow rate
  • Bypass of syngas from the preheater

By the fact of equilibrium, we observe that any system tries to attain equilibrium and it is strongly affected by the state parameters. In this condition of process controlling temperature, pressure and flow rate play in governing the system. In a typical ammonia plant, we find the operational effect of the variables during the controlling.

To control some of the important parameters, manipulated variable, interacting variable and disturbance variables used for automation control system for the ammonia process plant are,

  • H2 make up rate: a manipulated variable process gas is controlled.
  • H2:N2 make up the ratio:   manipulated variable is air flow rate and an interacting variable is processed gas rate.
  • Primary reformer exit temperature: manipulated variable is fuel flow rate, the interacting variable is processed gas and steam flow rate and disturbance variable would be purge gas.
  • CH4 make-up system: Two manipulating variables are used for this, one is steam to shift converter and second its feed temperature, interacting variables are secondary reformer CH4 exist percentage and exit temperature of primary and secondary reformers. The disturbance variables to be monitored are carbon monoxide percentage from shift converter outlet and carbon dioxide concentration in MEA absorber.
Front end control important parameters of ammonia process plant

Controls flow diagram of an ammonia plant