Isfahan Oil Refinery (ZLD and MLD for WWTP)

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1 General Overview and Process Description

This part describes the process of MLD + ZLD plant, based on a softening, an ultrafiltration, two Units of reverse osmosis (RO1+ Batch High Pressure RO), and a ZLD unit.

Our treatment process for Isfahan Oil Refinery Project is planned to receive RO brines from water treatment plants with an average capacity of 9600 m3/d.

2 MLD Plant

MLD (Minimum Liquid Discharge) consists of the main units:

Prefiltration; Consists of softening reactor, HPD (High-Performance Decanter) with Lammella, and Self-cleaning filter. Softening unit has been considered to reduce iones which increase scaling in RO units or cause the need for larger RO unit. To reach these aims an innovative cold softening has been designed with lower chemical/ energy consumption.

In the Isfahan MLD project, the most important challenging point of RO brine treatment was a high concentration of calcium (1200 ppm) and silica (120 ppm). These parameters can affect RO recovery and they cause scaling problems. So we designed a softening unit to reduce the above parameters, considering the lowest energy and chemical consumption.

Feed Specification:

Isfahan MLD Feed Specification

Stream

Flow(m3/hr)

TDS (ppm)

Density(kg/m3)

Flow(Ton/hr)

RO Reject Water

400

~12500

~1000

400

Ultrafiltration process; After the softening step, water passes through 3 self-cleaning filters (100 microns) then Filtered water comes to 3 UF trains. Each train has 40 modules of dead-end membranes. UF physically separates solids from liquid streams based on the principle of size exclusion. The effluent of UF is the feed of advanced treatment. Reverse osmosis process; Consists of multi-stage RO and Batch RO for removal of dissolved inorganic solids from water. Batch RO is a new type of closed-loop reverse osmosis that works semi-Batch to take advantage of dead-end filtration. In this batch RO the feed is continuous while the reject stream or concentrate is recirculated internally to provide turbulent flow and prevent salt precipitation; the BARO concentrate is recirculated to the feed until one of the 3 objectives is met:
  • conductivity reaches a preset set point.
  • the maximum working pressure, as defined above, is reached
  • the expected conversion is obtained

One of the most important advantages of Batch RO is the performance capability in high concentrations of silica without any scaling risk. Normally in conventional RO, the concentration of silica can increase to 200 ppm while in Batch RO, we increase it to 400 ppm. Considering this superiority, recovery in Batch RO is so much more than conventional RO.

MLD Discharge Specification:

Isfahan MLD Discharge Specification

Stream

% Solid(ppm)

Density (kg/m3)

Flow(Ton/hr)

RO Permeate Water

<200

~1000

~280

Batch RO Permeate Water

<550

~1000

~62

Batch RO Concentrate

~95000

~1000

~50

3 ZLD Plant

Zero liquid discharge can be defined broadly as a process for completely eliminating wastewater completely from a system. The final discharges of a ZLD plant are treated water and salt. This treated water is beneficially reused, and the salts are disposed of in a landfill, generally. In some cases, recovered salt also can be reused as a valuable salt.

 

Isfahan ZLD feed is the concentrated water of upstream Batch RO as below specification:

Feed Specification:

Isfahan ZLD Feed Specification

Stream

% Solid(ppm)

Flow(Ton/hr)

Batch RO Concentrate

~95000

~50.0

ZLD consists of below main units:

Degasser system: As Batch RO Concentrate water contains ammonia content, before entering this stream to the MEE unit, they have to be degassed. So degasser unit was designed and installed. Products of this unit are condensate water and degassed wastewater. Finally, the degassed wastewater is discharged to storage tank.

The Multiple Effect Evaporator (MEE): The Multiple Effect Evaporators are vertical shell and tube heat exchangers with a recirculation pump at the bottom. The calandria is connected to a vapor liquid separator. The recirculation pump pumps the effluent through the tubes of the calandria, and into the vapor liquid separator. The water gets separated from the effluent due to flash evaporation in the vapor liquid separator. Products of this unit are condensate water and slury wastewater.

Pusher Centrifuge & Dryer System:

By the time the concentrated wastewater reaches the last evaporator stage, the concentration of the salts increases beyond their solubility limit. This concentrated wastewater is named Slurry. Now it is needed to separate salts from Slurry. Due to this aim Pusher centrifuge and SST dryer systems are used. Products of this unit are salt and condensate water.

ZLD Discharge Specification:

Isfahan ZLD Products

Stream

Flow (kg/h)

Specification

Crystal clear water

~45000

TDS< 100 ppm

Salt

~5000

Mositure< 7%

Required Utility:

  • Cooling water
  • Low-Pressure Steam
  • Electricity
  • Instrument air

4 Schematic Block Diagram of MLD + ZLD process is shown as below: