documentclass{book}

%PAGES
usepackage[
    vcentering,
    hcentering,
    inner=16mm,
    outer=16mm,
    top=22.0mm,
    bottom=15.0mm,
    headsep=9.3mm,
    headheight=5mm,
%   showframe
    ]
    {geometry} % Extended page layout customization

geometry{papersize={189mm,246mm}} % Possible dimensions of textbook pages
%usepackage[cross,a4,center]{crop} %Package to print on A4 with crop signs.
usepackage{multicol}% handling of multicolumn in documents
usepackage[toc]{multitoc}%enables table of contents with multiple columns
    renewcommand*{multicolumntoc}{2}% Set two columns for the table of contents
usepackage{emptypage} % suppresses page numbers and headings on empty pages
usepackage{xcolor}[2007/01/21] % Driver-independent color extensions

%SKETCHES AND PLOTS
usepackage{tikz} % To create graphics programmatically. TikZ is build on top of PGF.
    usetikzlibrary{decorations.markings,arrows,backgrounds}
    usetikzlibrary{patterns}
    usetikzlibrary{shapes.misc, positioning}
    usetikzlibrary{calc}

usepackage{xcolor}[2007/01/21] % Driver-independent color extensions
    definecolor{BlueGreen}{RGB}{7,116,159}
    definecolor{LightBlue}{RGB}{185,229,251}
    definecolor{LightGreen}{RGB}{203,231,211}

%FONTS
usepackage{fontspec}%LuaLaTex fonts
    setmainfont[Ligatures=TeX,ItalicFont={Apollo MT Std Italic}]{Apollo MT}
usepackage{titletoc,xpatch}%
usepackage[explicit]{titlesec}% An in­ter­face to sec­tion­ing com­mands for se­lec­tion from var­i­ous ti­tle styles.

%Font families
newfontfamilyAvenir{Avenir}
newfontfamilyAvenirLight{Avenir Light}
newfontfamilyAvenirHeavy{Avenir Heavy}


%Chapters in TOC
newcommand{chaptertocfont}        {fontsize{10}{12}color{red}AvenirHeavy}
newcommand{chaptertocpagefont}        {fontsize{10}{12}color{red}AvenirHeavy}

%Chapters
newcommand{chapterfont}       {fontsize{24}{27}color{black}AvenirLight}
newcommand{chapternrfont}     {fontsize{24}{27}color{BlueGreen}AvenirHeavy}


titlecontents{chapter}% section-type
    [1.5em]% <- changed
    {vspace*{0.6cm}}
    {chaptertocfontcontentslabel{1.5em}}% <- changed
    {hspace*{-1.5em}}% <- changed
    {hfillchaptertocpagefontcontentspage}[]

%CHAPTER HEADINGS
titlespacing{chapter}{2cm}{-0.7cm}{1.7cm} % set spacing: left, before, after

titleformat{chapter}[hang]% command to be modified
    {chapterfont} %format
    {hspace*{-2.7cm}
        tikz [anchor=base, baseline,remember picture, overlay]
        {% colorbox for the chapter number
        node[
        line width = 1pt,
        draw=BlueGreen,
        fill =LightBlue,
        anchor=base,
        rectangle,
        text width=2cm,
        align=right,
        rounded rectangle,
        rounded rectangle west arc=5pt,
         minimum height=15mm,
        inner sep=11pt
        ] (A)
        {
        chapternrfontthechapter};
        draw[line width = 1pt, red] let p1=(A.south east), p2=(current page.east) in  ([xshift=2.6ex]A.south east) -- +(x2,0);
        }
    }% label
  {dimexpr2emrelax}% sep
  {#1} % before-code
  [] % after-code

begin{document}

tableofcontents

chapter{Energy Conversion Systems}

chapter[Thermodynamic Properties of \ Multicomponent Fluids]{Thermodynamic Properties of \ Multicomponent Fluids}

end{document}